JPH07307527A - Optical semiconductor device, method of driving light source for optical communication, optical communication system using the same, and optical communication system - Google Patents

Abstract

(57) [Abstract] [Purpose] An optical semiconductor device capable of constructing a light source for optical communication using a direct polarization modulation system with small dynamic wavelength fluctuation, a driving method of the light source for optical communication, and an optical communication system using the same , And an optical communication system. [Structure] The semiconductor laser 1 is used as a light source for optical communication. The semiconductor laser 1 has a structure in which a current flowing through a part of an optical waveguide is modulated and two oscillation polarization modes having different polarization planes are switched. Means 2 for branching two polarization modes of the semiconductor laser 1 having different polarization planes of the modulated output light into different propagation directions, and means for coupling one light of the two polarization modes to the optical transmission line 9. , A photodiode 3 for converting the remaining one light into an electric signal. The electric current supplied to the semiconductor laser 1 is controlled based on the electric signal so that the modulation state of the light coupled to the optical transmission line 9 is stabilized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical semiconductor device, a method of driving a light source for optical communication, an optical communication system using the same, and an optical communication system, and more particularly to suppressing dynamic wavelength fluctuation even during high speed modulation. The present invention relates to a driving method of a light source device for optical communication for stably realizing high-density wavelength-division multiplexed optical communication, an optical communication system using the same, an optical communication system, and the like.

[0002]

2. Description of the Related Art In recent years, it has been desired to expand the transmission capacity in the field of optical communication, and optical frequency multiplexing (optical F) in which a plurality of wavelengths or optical frequencies are multiplexed in one optical fiber.
DM) transmission is being developed.

Optical FDM technology can be roughly classified into two types according to the receiving method. The two methods are coherent optical communication in which a beat with a local light source is taken to obtain and detect an intermediate frequency, and a method in which only a light of a desired wavelength (frequency) is transmitted by a wavelength tunable filter and detected.

Here, the latter optical communication system using a wavelength tunable filter will be a problem. The wavelength tunable filter includes a Mach-Zehnder type, a fiber Fabry-Perot type, an AO (acousto-optic) modulator type, a semiconductor type, and the like, and their development is proceeding.

Since the Mach-Zehnder type and the fiber Fabry-Perot type can be designed with a relatively free transmission bandwidth and a narrow band of several Å can be obtained, there is an advantage that the frequency multiplicity of the optical FDM communication can be increased. Further, there is a great merit that it is not affected by the polarization state of the signal. Known examples of the Mach-Zehnder type include K.K. Oda et al.
"Channel selection characteristics of filters for optical FDM", OCS
89-65 1989, and known examples of the fiber Fabry-Perot type include I.I. P. Kamin
ow et al. "FDMA-FSK Star
Network with a Tunable Opt
iCal Filter Demultiplexe
r ″, IEEE J. Lightwave Tec
hnol. , Vol. 6, NO. 9, p. 140
6, September, 1988 and the like. However, these have drawbacks such as loss of light, the inability to integrate the semiconductor photodetector and the filter, and the difficulty in downsizing the receiving device.

In the case of the AO modulator type, since the transmission bandwidth is large and is about several tens of Å, reception control is easy, but the number of wavelength division multiplexing cannot be increased. As a conventional example, "N.
Shimosaka et al. Wavelength division / time division multiplexing multiplex optical network in broadcasting station using acousto-optic filter ", OCS 91-83 1991. The disadvantages of this are loss of light, non-integrable signal It is necessary to control the polarization because it is affected by the polarization state.

On the other hand, in the case of the semiconductor type, for example, a DFB filter provided with a diffraction grating in an optical guide layer for realizing a single longitudinal mode can have a narrow transmission band width (several Å) and an amplification effect of light (20 dB). The advantages are that the wavelength multiplicity can be increased and the minimum receiving sensitivity can be reduced. A known example thereof is T.W. Numai et al.
"Semiconductor variable wavelength filter", OQE 88-65
1988. Since it can be made of the same material as the semiconductor photodetector, it can be integrated and miniaturized.

From the above, it can be said that the semiconductor DFB type is an optical filter having characteristics suitable for optical FDM communication.

The current state of the signal modulation system is as follows. Currently, the most widely used method in a transmission system using an optical filter is a digital intensity modulation method (AS).
K; Amplitude Shift Keying)
Is. For this modulation, there are a method of directly modulating the injection current of the LD and a method of using an external intensity modulator. The former is not suitable for high-density wavelength multiplexing because of LD wavelength chirping (about several Å), and the latter has a problem of light loss in the modulator. Therefore, a signal with a small amplitude is superimposed on the injection current of the LD to perform digital frequency modulation (FSK; Frequency).
Cy Shift Keying) and a method of demodulating by utilizing the wavelength discrimination characteristic of the optical filter have also been developed (for example, MJ Chawski et al.
"1.56 Gbit / s FSK Transmis
sion System Using Two Ele
ctode DFB Laser As A Tun
Able FSK Discriminator / Ph
Oto detector ", Electron. L
ett. Vol. 26 No. 26. 15 1990
reference). As another method, it is proposed to switch the polarization mode of the oscillation light of the DFB laser and select only one mode of TE and TM.

In order to increase the transmission capacity as much as possible, it is important to reduce the wavelength (frequency) interval between multiplexed signals. For that purpose, it is desirable that the wavelength tunable filter has a small transmission bandwidth and the laser serving as a light source has a small occupied frequency band or spectral line width. For example, in a semiconductor distributed feedback (DFB) filter with a wavelength variable width of 3 nm, the transmission bandwidth is about 0.03 nm, so ideally 100 channels can be multiplexed. However, in this case, the spectral line width of the light source is required to be 0.03 nm or less. At present, even a DFB laser known as a semiconductor laser that oscillates a dynamic single mode directly
When SK modulation is performed, a dynamic wavelength variation occurs and the spectral line width expands to about 0.3 nm. Therefore, this laser is not suitable for such wavelength division multiplexing transmission.

Therefore, in order to suppress such wavelength fluctuation, as described above, a method using an external intensity modulator (for example, Suzuki et al .; “4 / λ shift DFB laser / absorption optical modulator integrated light source”, electronic) Proceedings of IEICE Technical Committee, O
QE90-45, p. 99, 1990), a direct FSK modulation method, a direct polarization modulation method (for example, Japanese Patent Laid-Open No. 2-159).
781) and the like have been proposed.

The three proposed examples will be compared. In the case of the external intensity modulator, the wavelength variation is about 0.03 nm, which is a performance marginal to the specifications, and the number of devices increases, which is not preferable in terms of cost. Further, in the case of the FSK system, the filter on the receiving side needs to function as a wavelength discriminating device, which requires a complicated control technique.

On the other hand, the direct polarization modulation method is an ordinary DFB.
The number of devices does not increase only by using multiple electrodes for the laser, and the wavelength fluctuation is smaller than that of the external modulation method. Further, since the transmission signal is ASK, there is an advantage that the load of the filter on the receiving side is small.

[0014]

As described above, the polarization modulation system is a modulation system suitable for wavelength division multiplex transmission and the like, but in the conventional proposal example, there is no mention of the polarization modulation driving method. In order to put it into practical use, the driving method must first be established.

In polarization modulation, a rectangular signal is applied to one electrode of a multi-electrode DFB laser, TE polarization and TM polarization are switched, and a polarizer is arranged at the exit end to take out either polarization. Although the ASK modulation is performed, the extinction ratio or an amount proportional thereto (hereinafter, simply referred to as extinction ratio) is sensitive to the bias current of the laser. For example, the extinction ratio may be halved even if the bias current deviates by 0.1 mA. Therefore, it is necessary to precisely control the direct current and the element temperature, which is a practical problem.

Therefore, an object of the present invention is to provide an optical semiconductor device, a method or device for driving a light source device for optical communication, an optical communication system using the same, an optical communication system and the like, which solve the above problems. is there.

[0017]

In order to solve the above problems and to provide a practical polarization modulation driving method, one is a driving method or device when a semiconductor laser is used as a light source for optical communication. The semiconductor laser has a structure in which two oscillation polarization modes having different polarization planes are switched by modulating (modulating signal) a current flowing through a part of the optical waveguide. Of means for splitting the two polarization modes having different polarization planes into different propagation directions, means for coupling one light of the two polarization modes to an optical transmission line, and the remaining one light as an electrical signal. And a means for converting the electric current to the semiconductor laser so as to stabilize the modulation state of the light coupled to the optical transmission line. The driving method of the light source for It is the apparatus. That is, specifically, using a polarization beam splitter instead of a polarizer,
It is divided into two, TE polarized wave and TM polarized wave. One is used as a transmission line and the other is used for laser feedback control.

A detailed description will be given with reference to FIG. A digital signal of small amplitude is superimposed on the bias current I ₁ flowing through one electrode of the DFB semiconductor laser 1 having a two-electrode structure, and TE /
Performs TM polarization switching modulation. Polarization modulation is shown in FIG.
As shown in, when the bias current I ₂ is fixed and I ₁ is changed, the phenomenon that the oscillation mode switches from the TE polarized wave to the TM polarized wave at a certain bias point is used to respond to the rectangular signal of I _1. 17 is a technique for temporally modulating the plane of polarization as shown in FIG. FIG. 27 shows a graph in which the relationship between the extinction ratio for the TE polarized light after passing through the polarization beam splitter and the modulation current amplitude at that time is plotted with the bias current I ₁ as a parameter. It can be seen that when I ₁ is 27.6 mA, the extinction ratio becomes large at the smallest current amplitude, which is the optimum point. It can be seen that smaller I ₁ requires a larger current amplitude to obtain the same extinction ratio.
Further, it is found that when I ₁ becomes large, the extinction ratio cannot be sufficiently obtained even if the current amplitude is increased. Thus, I
_An optimum point exists for ₁ because the extinction ratio differs depending on whether the bias point is near the TE polarization or the TM polarization. Therefore, feedback control is performed by monitoring the amount of deviation of the bias point of the polarization-modulated light that is not used for transmission (here, TM polarization). As shown in FIG. 1, the light from the laser is divided into TE and TM by the polarization beam splitter 2, and the TM light is received by the photodiode 3. From the electric signal obtained here, a low-frequency component is extracted by the low-pass filter 4, and the bias current I ₁ is extracted by the current source 5 for driving the laser.
To control. As shown in FIG. 4, when the bias point shifts to the TE polarization side, the monitor signal becomes small and when it shifts to TM, the monitor signal becomes large. Therefore, the shift amount can be detected by this signal and a feedback control system can be constructed. By the driving method as described above, the TE light sent to the transmission path can maintain a stable extinction ratio.

More specifically, the following configuration can be adopted. The current controlled in the semiconductor laser is a current flowing in the same part as the part of the optical waveguide through which the modulation signal flows or a different part. The semiconductor laser is a distributed feedback type semiconductor laser provided with a diffraction grating in the vicinity of an optical waveguide including a light emitting layer, the light emitting layer is composed of quantum wells, and the light hole order as the hole order and the electron base order. The pitch of the diffraction grating is set so that the Bragg wavelength is near the wavelength corresponding to the energy band gap between
A light source is a semiconductor laser configured so that the threshold gain at the Bragg wavelength is substantially equal in the two polarization modes. A semiconductor in which the light emitting layer of the distributed feedback semiconductor laser is composed of multiple quantum wells with tensile strain introduced, and the hole order is equal to the heavy hole order or the light hole order, or the light hole order is higher. Use a laser as a light source. An output obtained by comparing the electric signal with a reference voltage by a differential amplifier through a low pass filter,
The current supplied to the semiconductor laser is controlled by feeding back to the direct current source for driving the semiconductor laser with an appropriate feedback ratio by a proportional and integrating circuit. It has means for modulating a current flowing through the semiconductor laser with a low frequency sine wave, and means for multiplying the electric signal by the low frequency sine wave, and the output of the multiplying means is differentially passed through a low pass filter. The current is supplied to the semiconductor laser by comparing the output with a reference voltage by an amplifier and feeding back the output to a direct current source for driving the semiconductor laser with a proper feedback ratio by a proportional and integrating circuit. The oscillation wavelength of the semiconductor laser is variable by changing the ratio of the currents flowing in at least two parts of the waveguide, and the fluctuation of the wavelength is converted into the fluctuation of the intensity of the transmitted light immediately before the means for converting into an electric signal. A wavelength discriminator is provided. An output obtained by comparing the electric signal of light input to the means for converting the electric signal through the wavelength discriminator with a reference voltage through a low-pass filter by a differential amplifier, and a proper feedback ratio by a proportional and integrating circuit,
The oscillation wavelength is stabilized by a control method of controlling a current flowing through the semiconductor laser by feeding back to a direct current source that drives the semiconductor laser. Means for modulating a low frequency sine wave of a current flowing through the semiconductor laser, and means for multiplying the low frequency sine wave by an electric signal of light inputted to the means for converting the electric signal through the wavelength discriminator. The output of the multiplying means is compared with a reference voltage by a differential amplifier through a low-pass filter, and its output is converted to a direct current source for driving the semiconductor laser with an appropriate feedback ratio by a proportional and integrating circuit. The oscillation wavelength is stabilized by a control method that controls the current flowing through the semiconductor laser by feedback. The oscillation wavelength of the semiconductor laser is variable by changing the ratio of the currents flowing in at least two parts of the waveguide, and the fluctuation of the wavelength is converted into the fluctuation of the intensity of the transmitted light immediately before the means for converting into an electric signal. A wavelength discriminator, the output of the electrical signal of the light input to the means for converting the electrical signal via the wavelength discriminator is compared with a reference voltage by a differential amplifier through a low-pass filter,
The oscillation wavelength is also stabilized at the same time by the control method of controlling the current flowing through the semiconductor laser by feeding back to the direct current source for driving the semiconductor laser with an appropriate feedback rate by the integrating circuit. The oscillation wavelength of the semiconductor laser is variable by changing the ratio of the currents flowing in at least two parts of the waveguide, and the fluctuation of the wavelength is converted into the fluctuation of the intensity of the transmitted light immediately before the means for converting into an electric signal. Means for modulating a current flowing through the semiconductor laser with a low frequency sine wave, and an electric signal of light input to the means for converting the electric signal through the wavelength discriminator and the low frequency Means for multiplying the sine wave of the semiconductor laser by comparing the output of the multiplying means with a reference voltage by a differential amplifier through a low-pass filter, and by proportionally integrating the output of the semiconductor laser with an appropriate feedback ratio. The oscillation wavelength is also stabilized at the same time by the control method of controlling the current flowing through the semiconductor laser by feeding back to the direct current source for driving.

In order to solve the above-mentioned problems, in the integrated optical semiconductor device of the second structure of the present invention, the current flowing through a part of the optical waveguide including the light emitting layer is modulated (modulated signal) so as to be polarized. A semiconductor laser having a structure in which two oscillation polarization modes having different wavefronts are switched, and a waveguide-type polarization for branching the two polarization modes having different polarization planes of the modulated output light of the semiconductor laser into different propagation directions. The wave mode splitter and a photodetector for converting light of at least one of the two polarization modes into an electric signal are integrated on the same substrate. That is, a semiconductor laser capable of direct polarization modulation, a polarization beam splitter, and a photodiode are integrated using an optical waveguide.

Specifically, as shown in FIG. 9, a Y-branch type polarization beam splitter and a three-electrode type DFB laser are integrated,
At one end of the beam splitter, a photodiode is monolithically integrated on the same semiconductor substrate. A polarization beam splitter can be manufactured by utilizing the effect that the refractive index decreases with respect to TE polarization and increases with TM polarization due to mixing of superlattices (Suzuki et al .; “Semiconductor Superlattice Polarization mode filter / splitter by mixed crystal ”, IEICE Technical Committee Report OQE91-160, p.5
5, 1991). In addition, the Y-branch polarization mode splitter utilizes a fact that a metal-loaded waveguide has a large TM mode loss (Y. Yamamoto et.
al. J. Q. Electron. , QE-11,
p. 729,1975), LiNbO ₃ as shown in FIG. 11
A polarization mode splitter using a substrate may be hybrid-mounted with a semiconductor laser and a photodiode.
There is also a method of integrating a vertical directional coupler, a semiconductor laser and a photodiode as shown in FIG.

More specifically, the following configuration is also possible. The semiconductor laser has a structure in which two oscillation polarization modes having different polarization planes are switched by modulating (modulating signal) a current flowing through a part of an optical waveguide. Means for splitting two polarization modes of the output light having different polarization planes into different propagation directions, means for coupling one light of the two polarization modes to an optical transmission line, and one remaining light And a means for converting the electric signal into an electric signal, and a current supplied to the semiconductor laser based on the electric signal so that at least one of the modulation state and the oscillation wavelength of the light coupled to the optical transmission line is stabilized. To control. A semiconductor laser having a structure in which two oscillation polarization modes having different polarization planes are switched by modulating (modulating signal) a current flowing through a part of an optical waveguide including a light emitting layer, and a modulated output light of the semiconductor laser. A waveguide-type polarization mode splitter that splits two polarization modes having different polarization planes in different propagation directions, and light that converts at least one of the two polarization modes into an electrical signal The detector and the detector are integrated on the same substrate. The polarization mode splitter is formed of a Y-branch waveguide layer on the same plane, the waveguide layer includes a semiconductor superlattice, and only one waveguide layer of the Y-branch waveguide layer is a mixed crystal of the semiconductor superlattice. It has been transformed. The polarization mode splitter is formed of a Y-branch waveguide layer on the same plane, and only one of the Y-branch waveguide layers is loaded with metal. The polarization mode splitter is formed of at least two waveguide layers in the longitudinal direction,
A diffraction grating is provided near any of the waveguide layers.
Two polarization mode splitters are provided on both sides of the resonator of the semiconductor laser. Only one photodetector is provided for detecting only the light in one waveguide layer of the polarization mode splitter. Two photodetectors are provided for detecting the light in both waveguide layers of the polarization mode splitter. A current flowing through the semiconductor laser based on an electric signal obtained by the photodetector so that at least one of the modulation state and the oscillation wavelength of the light emitted to the outside of the integrated optical semiconductor device is stabilized. To control.

Further, in the optical communication system of the present invention which achieves the above object, the light source used in the communication system modulates the electric current flowing through a part of the optical waveguide (modulation signal) so that the polarization plane and the wavelength are different. A semiconductor laser having a structure of switching between two oscillation modes, wherein one of the two oscillation modes is selected from the modulated output light of the semiconductor laser by a wavelength discriminating means, and an optical transmission line is selected. It is characterized in that the output light is ASK-modulated by being coupled to the optical path and used as a signal for optical communication.

More specifically, the following configuration is also possible.
As means for discriminating the wavelength, a fixed filter that couples one of two oscillation modes emitted from the semiconductor laser to an optical transmission line is used, and
SK modulation. The ASK-modulated signal light is transmitted through an optical fiber and directly detected by an optical receiver. The wavelength discriminating means is composed of a Fabry-Perot etalon. The oscillation wavelength of the two oscillation modes of the semiconductor laser is variable by changing the ratio of the currents flowing in at least two or more portions of the optical waveguide. As a means for discriminating the wavelength, the oscillation wavelength of the semiconductor laser is used. The signal light is ASK-modulated by using a wavelength tunable filter that tunes to one of the two oscillation modes and couples it to the optical transmission line. Connect multiple light sources for optical communication to one optical fiber,
Wavelength division multiplexing transmission is performed so that light of a plurality of wavelengths is modulated and transmitted, and an optical receiver equipped with an optical filter extracts only the signal on the light of a desired wavelength. The fixed optical filter is a DFB filter. The wavelength tunable filter is a DFB filter. As the means for discriminating the wavelength, an element that spatially separates a light wave of a specific wavelength and a light wave other than that is used, one of the two oscillation modes is coupled to an optical transmission line, and the remaining one mode is an electric signal. Is equipped with a means for converting
At least one of the modulation state and the oscillation wavelength of the light wave coupled to the optical transmission line is stabilized by setting the driving condition of the semiconductor laser serving as the light source based on the electric signal.

Further, the optical communication system for achieving the above object is characterized in that the light source for optical communication is directly modulated by the above-mentioned driving method, transmitted through an optical fiber, and directly detected by an optical receiver.

Further, in the optical communication system which achieves the above object, a plurality of optical communication light sources which directly modulate by the above-mentioned driving method are connected to one optical fiber, and lights of a plurality of wavelengths are respectively modulated and transmitted. Then, to extract only the signal on the light of the desired wavelength by the optical receiver equipped with the optical filter,
It is characterized by wavelength division multiplexing transmission.

In the optical-electrical conversion device or the wavelength division multiplexing optical transmission system that achieves the above object, the light source for optical communication and the optical receiver are combined into one to perform optical transmission / reception by the above optical communication system. Is characterized by.

In the optical CATV system that achieves the above object, a broadcasting center is provided with an optical communication light source that directly modulates by the above driving method, and an optical receiver is provided with an optical filter for an optical subscriber. It is characterized by performing the above-mentioned optical communication system.

[0029]

[Embodiment 1] A first embodiment of the present invention will be described. FIG. 2A is a sectional perspective view of the semiconductor DFB laser used in this embodiment. In the figure, 101 is a substrate n
-InP, 102 is an n-InP buffer layer in which a diffraction grating g having a depth of 0.05 μm is formed, and 103 is 0.2 μm in thickness.
m n-In _0.82 Ga _0.18 As _0.4 P _0.6
Lower light guide layer, 104 is i-In _0.28 Ga
_0.72 As well layer (thickness 10 nm), i-In
_0.82 Ga _0.18 As _0.4 P _0.6 Barrier layer (thickness 10 nm) Active layer having a strained superlattice structure consisting of 10 layers, 1
Reference numeral 05 is a p-InP clad layer, 106 is a p-In
_0.53 Ga _0.47 As contact layer, 107 is a high resistance InP buried layer, 108 is an electrode isolation region where the contact layer 106 is removed, 109 is a Cr / AuZnNi / Au layer which is an electrode on the light emitting side, and 110 is Cr / AuZnNi / Au layer, which is an electrode for passing a current with superimposed signals, 1
11 is an AuGeNi / Au layer which is a substrate side electrode, 112
Is a SiO film that serves as an antireflection film. Where this DF
B lasers have become multi-quantum well layer having a tensile strain active layer _104, E lh0 _{-E e0} and _E hh0 -E
_Since the transition energies of _e0 (discussed immediately below) are designed to be equal, TM polarization (transition energy between ground level of light hole and electron (E
₍ corresponding to _{1h0- Ee0} )), the oscillation threshold is low, and the polarization can be switched efficiently. In the DFB laser 1 shown in FIG. 1, the grating g
Is formed on the active layer and is drawn differently from the structure of FIG. 2A, but the operation is substantially the same.

FIG. 2B shows an emission spectrum of the above structure when a bias current is passed through the electrodes 109 and 110 to bring about a state immediately before laser oscillation. Wavelength corresponding to the light holes and electrons ground levels between the transition energy _{(E lh0 -E e0)} is 1.56 .mu.m, even wavelengths corresponding to the heavy holes and electrons ground levels between the transition energy _(E hh0 _{-E e0)} It becomes 1.56 μm. In addition, TE mode (solid line) and T
Emission spectrum of the M-mode (dashed line) is substantially overlap, the distributed feedback wavelength by the diffraction grating g is the pitch of the diffraction grating g such that the vicinity of the wavelength corresponding to the _E lh0 -E _e0 24
It is set to 0 nm and has a Bragg wavelength of 1.562 μm in the TE mode and 1.558 μm in the TM mode.

Here, a DC bias of 52 m is applied to the electrode 109.
A. When a DC bias of 27.6 mA is applied to the electrode 110 and a digital signal having an amplitude of 5 mA is superimposed on the electrode 110, polarization switching occurs between TE and TM as described above. This laser light is split into TE polarized light and TM polarized light by a polarization beam splitter as shown in FIG. The time waveform at this time is shown in FIG. In FIG. 3, (1)
Is the waveform of the modulation current ΔI ₁ from the current source 7, (2) is the output light of the laser, (3) is the optical output of TE polarization, and (4) is TM.
It represents the optical output of polarized waves. As shown in FIG. 3, the laser output light does not largely change due to the modulation (FIG. 3 (2)),
It can be seen that after polarization separation, they are modulated in opposite phases (Figs. 3 (3) and 3 (4)). At this time, the modulation band is 200
It was kHz to 5 GHz. Poor low-frequency characteristics are due to the influence of heat.

An actual driving method will be described with reference to FIG. The TE light split by the polarization beam splitter 2 is an isolator 10 (having a configuration in which a Faraday rotator that rotates a polarization plane by 45 degrees is sandwiched between Glan-Thompson prisms).
Is transmitted through the optical fiber 9 through the through. Here, the isolator 10 not only prevents the reflected light from the transmission line 9 and the like from entering the semiconductor laser 1 and disturbs the oscillation, but also uses the Glan-Thompson prisms provided on both sides of the isolator 10 to prevent TE polarization and TM polarization. It also has the function of improving the extinction ratio of polarized waves. At this time, the extinction ratio of TE and TM is 2
It is possible to achieve ASK transmission with this extinction ratio by obtaining 0 dB or more.

On the other hand, TM light is detected and monitored by the photodiode 3. When this signal is converted into a signal having a band of 10 kHz or less by the low-pass filter 4, 100 Mb
The components modulated by high-speed signals of ps or more are averaged,
As shown in FIG. 4, the output voltage changes according to the amount of deviation of the bias point of the current. Therefore, after comparison with the reference voltage by the differential amplifier 8, the current source 5 that applies a DC bias to the electrode 110.
By feeding back to, it is possible to automatically stabilize the bias point. In addition, the electrode 10 does not modulate the feed back.
A current source 6 for supplying a DC bias to 9 may be used. This automatic stabilization was performed by PI (proportional, integral) control, but it was possible to keep the change of the extinction ratio within 1% over a long period of time by optimizing the gain, the feedback ratio, and the integration time. . As a result, it was possible to improve the long-term reduction in error rate, which was a problem with conventional polarization-modulated transmission.

[0034]

Second Embodiment A second embodiment according to the present invention will be described with reference to FIG. 5A is a perspective view of the semiconductor DFB laser, which has the same structure as that of the first embodiment, but has a three-electrode structure, and the central electrode 210 has a phase adjustment region in which the active layer is removed. Yes, the controllability of polarization switching is further improved.

In FIG. 5, 201 is an n-I serving as a substrate.
nP, 202 is an n-InP buffer layer in which a diffraction grating g having a depth of 0.05 μm is formed, 203 is an n-In _0.82 Ga _0.18 As _0.4 P _0.6 lower optical guide layer having a thickness of 0.2 μm, 2
04 is a well layer i-In _0.28 Ga _0.72 As (thickness 10 n
m), barrier layer i-In _0.82 Ga _0.18 As _0.4 P _0.6 (thickness 10 nm), active layer having a strained superlattice structure consisting of 10 layers, 2
Reference numeral 05 is a p-InP clad layer, 206 is a p-In _0.53 G
a _0.47 As contact layer, 207 is a high resistance InP buried layer, 208 and 208 ′ are electrode isolation regions where the contact layer 206 is removed, 209 and 209 ′ are Cr / AuZnNi / Au layers which are electrodes on the light emitting side, 210 Is an electrode for passing a current with superimposed signals Cr / AuZnNi /
Au layer, 211 is AuGeNi / Au which is the substrate side electrode
The layers 212 and 212 'are SiO films that serve as antireflection films.

In the central portion where the active layer is removed, the contact layer, the clad layer and the active layer are etched, and then the i-In
_0.82 Ga _0.18 As _0.4 P _0.6 Optical guide layer 213, p-In
A P clad layer 214 and a p-In _0.53 Ga _0.47 As contact layer 215 are formed by selective regrowth. The pitch of the grating g and the like are the same as in the first embodiment.

The current driving method is as follows.
The same DC bias I ₂ is applied to 9 ′ from the current source 217, and the DC bias I ₁ and the modulation current ΔI ₁ from the current source 216 are applied to the center electrode 210. I ₂ = 60 mA, I ₁ =
At 20 mA, TE / TM polarization switching could be performed by superimposing a digital signal of ΔI ₁ = 2 mA.

Compared with the first embodiment, only the phase can be controlled without changing the gain in the phase adjusting region of the center electrode 210, and the influence of heat is small, and the refractive index is changed only by changing the carrier density to control the phase. Therefore, it is possible to achieve a wide band (with respect to the modulation band) with high efficiency. Therefore, compared with the first embodiment, the amplitude of the modulation current ΔI ₁ is as small as 2 mA, and the modulation band is also improved to 10 kHz to 5 GHz.

The method of stabilizing the bias point can be performed in the same manner as in the first embodiment, but an embodiment of the method of stabilizing the bias point with higher accuracy will be described with reference to FIG. The DC bias I ₁ is modulated by the sine wave oscillator 218 with a sine wave having an amplitude of about 0.2 mA at which polarization switching of 20 kHz does not occur, and the TM light extracted by the polarization beam splitter 220 is detected by the photodiode 221. A balanced modulator or multiplier 222 mixes the same signal as the sine wave. This output is cut off frequency 10
When passing through the low-pass filter 223 of kHz, the output thereof has the differential waveform of FIG. 4 as shown in FIG. 6, and the slope characteristic on the left side of the peak is used in this waveform. This output is compared with the reference voltage by the differential amplifier 224, output, and fed back to the current source 216 of I ₁ . As a result, fluctuations in the power of TM polarized light can be detected more accurately than in the first embodiment. If the isolator 219 is inserted before the TM light is detected, the TE light as noise can be further removed, and the returning light to the laser can be suppressed. When the photodiode 221 is tilted from the incident direction of light to suppress the returning light to the laser, only the polarizing plate may be used instead of the isolator.

[0040]

[Third Embodiment] A third embodiment of the present invention will be described with reference to FIG. In this embodiment, since the laser is used as a light source for wavelength division multiplexing transmission, the DFB described in the first and second embodiments is used.
The laser oscillation wavelength is changed by changing the DC bias current to stabilize the wavelength.

FIG. 8 shows the wavelength variable characteristic of the three-electrode type DFB laser described in the second embodiment. Here, the electrodes 2 on both sides
While controlling the currents injected into 09 and 209 ′ independently of I ₂ and I ₂ ′ and keeping the value of I ₂ + I ₂ ′ constant at 60 mA,
The ratio is changed. However, I ₁ = 20m
The value is fixed to A and is measured in the range where single mode oscillation occurs in the TE mode. As shown in this figure, the current ratio is 0.1 to 0.
By changing to 4, the single mode is maintained and about 3.
A variable width of 0 nm is obtained.

The wavelength stabilization is performed in the same system as in the second embodiment as shown in FIG. However, light is detected by passing through a wavelength discriminator that detects a change in wavelength, for example, a Fabry-Perot etalon 303. When the wavelength is on the shorter wavelength side than the transmission peak of the Fabry-Perot etalon 303, the change in the oscillation wavelength of the DFB laser 301 is in opposite phase to the oscillator 308,
Conversely, when it is on the long wavelength side, it is in phase, and when it is just at the peak, a signal is detected by the photodiode 304 at a double frequency, and the outputs of the balanced modulator 305 are negative, 0, and 0, respectively.
Be positive. Therefore, by performing feedback control, the oscillation wavelength of the DFB laser 301 is stabilized at the transmission peak of the Fabry-Perot etalon 303. The Fabry-Perot etalon 303 used here has a free spectral range of 10 GHz and a finesse of 10.
The oscillation wavelength of the B laser 301 is 10 GHz (about 0.05
nm) intervals and wavelength stability of 0.1 GHz or less. Of course, the wavelength interval and the wavelength stability can be set to desired values by changing the design value of the Fabry-Perot etalon 303. The S / N between the amount of wavelength fluctuation detected by the Fabry-Perot etalon and the amount of fluctuation of the optical power of TM polarization is 10 or more, which is an S / N sufficient for control.
N has been obtained.

If the wavelength stabilization and the bias point stabilization described in the second embodiment are simultaneously performed in parallel (the configurations of the third embodiment and the second embodiment may be provided in parallel), This is an optimal light source driving method for wavelength division multiplexing transmission.

In this embodiment, the DFB is the same as in the second embodiment.
I explained the method of controlling the laser by modulating it at a low frequency.
You may control without modulating like 1st Example. still,
In FIG. 7, reference numeral 302 denotes a polarization beam splitter, 306
Is a low pass filter, 307 is a differential amplifier, and 309 is D
A current source corresponding to the center electrode of the FB laser 301, 31
Reference numerals 0 and 311 denote current sources corresponding to the electrodes on both sides of the DFB laser 301, respectively.

[0045]

Fourth Embodiment A fourth embodiment according to the present invention will be described with reference to FIGS. 9 and 10. FIG. 9 is a perspective view of the integrated semiconductor device, and FIG. 10 is a sectional view taken along the waveguide.

As shown in FIG. 9, the distributed feedback (D
An FB) laser 1101, a polarization mode splitter 1102 having a Y branch structure, and a photodetector 1103 that detects only one oscillation polarization mode are integrated. These waveguides are formed by a buried structure 1106 made of high resistance InP. In this embodiment, the polarization mode splitter 11
In No. 02, the SiN film is loaded and annealed only in the region 1104 to mix the superlattice forming the waveguide, thereby allowing only the TM polarized wave to be guided. Therefore, only the TM mode light can be received by the photodetector 1103, and the TE mode light is extracted from the emission unit 1105. At this time, the Y-branching polarization mode splitter 110
1 as the extinction ratio of TE / TM at each end of 2
0 dB is obtained. Of course, the light received by the photodetector 1103 may be set to the TE mode and the TM mode may be emitted. Further, the polarization mode splitter 1102 is manufactured by loading ZnO and annealing, and forming a mixed crystal by diffusion of Zn, or by loading a metal thin film, the loss of TM is larger, so that an asymmetric Y branch is formed and the metal thin film side is formed. The TE mode may be guided. The length of this element is 800 μm for the 1101 DFB laser section, about 2500 μm for the 1102 polarization mode splitter, and 20 for the 1103 photodetector section.
At 0 μm, the total length is about 3500 μm.

Next, an element structure and a manufacturing method will be described with reference to FIG. Reference numeral 1201 denotes n-InP serving as a substrate,
Reference numeral 1202 is an n-InP buffer layer in which a diffraction grating g having a depth of 0.05 μm is partially formed, and 1203 is a thickness of 0.2 μm.
m n-In _0.82 Ga _0.18 As _0.4 P _0.6
The lower light guide layer 1204 is i-In _0.28 Ga
_0.72 As well layer (thickness 10 nm), i-In
_0.82 Ga _0.18 As _0.4 P _0.6 Barrier layer (thickness 10 nm) Active layer having a strained superlattice structure consisting of 10 layers, 1
205 is a p-InP clad layer, 1206 is a p-In
_0.53 Ga _0.47 As contact layers, 1208 and 1208 ′ are electrode isolation regions where the contact layer 1206 is removed, and 1209 and 1209 ′ are p-side electrodes C
r / AuZnNi / Au layer, 1210 is a Cr / AuZnNi / Au layer, which is an electrode for passing a current in which signals are superimposed, 1
211 is an AuGeNi / Au layer which is a substrate side electrode, 12
20 is an AuGeNi / Au layer which is an electrode of the photodetector, 1
Reference numerals 212 and 1212 ′ are SiO films that serve as antireflection films.

In a part of the DFB laser 1101 and the Y branch waveguide 1102, after patterning, the contact layer, the clad layer, and the active layer are etched, and then the i-I
n _0.53 Ga _0.47 As well layer (thickness 3 nm), i
-InP barrier layer (thickness: 5 nm) 20 superlattice structure optical guide layer 1213, p-InP clad layer 12
14, p-In _0.59 Ga _0.41 As _0.9 P
_{A 0.1} contact layer 1215 (which is not formed in the Y branch waveguide portion) is formed by selective regrowth.
Then, after patterning into a Y shape and etching up to the substrate 1201, a high resistance InP burying layer is formed to form a burying structure 1106. The re-grown light guide layer 1213 has a wavelength corresponding to the energy band gap of about 1.3 μm and a laser oscillation wavelength of 1.55 μm.
The structure has a small loss for the light.

Here, in the DFB laser 1101, the active layer 1204 is a multiple quantum well layer having tensile strain, and corresponds to the transition energy between the ground levels of light holes and electrons (E _lh0 -E _e0 ). Wavelength is 1.56μ
m, the wavelength corresponding to the transition energy (E _hh0 −E _e0 ) between the ground level of the heavy hole and the electron is 1.56 μm, and the transition energy is designed to be the same.
The oscillation threshold for TM polarization is lower than that of the B laser,
It has a configuration that enables efficient polarization switching. Here, the distributed feedback wavelength by the diffraction grating g DFB portion _sets the pitch of the diffraction grating g so as to be near the wavelength corresponding to the E LH0 _{-E e0} to 240 nm, 1.562Myuemu in TE mode, TM mode Has a Bragg wavelength of 1.558 μm. These are the same as the laser of the first embodiment.

The DFB laser section 1101 has all other structures, for example, a structure without a diffraction grating g in a region without an active layer (a region with an electrode 1210), a structure with a shift region in the diffraction grating g. A structure in which the active layer 1204 is provided in the region may be used. Further, a DFB laser having a two-electrode configuration may be used for simplicity. That is, any semiconductor laser may be used as long as it is a polarization-modulating semiconductor laser.

Next, the driving method of the present device will be described with reference to FIG. 9 again. A DC bias 26 is applied to the electrodes 1209 and 1209 ′ by a direct current source 1110 and 1110 ′, respectively.
DC, 27.6 mA was applied to the electrode 1210 by the direct current source 111, and an amplitude of 2 mA was applied to the electrode 1210.
When the digital signal 1115 of is superposed, TE / TM
Switching occurs. This laser light is split into TE polarized light and TM polarized light by the polarization mode splitter 1102. The time waveform at this time is substantially the same as that shown in FIG. .

The TE light split by the polarization beam splitter 1102 is coupled to the optical fiber 1121 through the isolator 1120 and transmitted. Here, in the isolator, the reflected light from the transmission line or the like is reflected by the semiconductor laser 1101.
Not only is it prevented from disturbing the oscillation due to incidence on the beam, but also has the function of improving the extinction ratio of the TE polarization and the TM polarization by the Glan-Thompson prisms provided on both sides of the isolator 1120. At this time, the extinction ratio of TE and TM is 20 dB or more, and ASK transmission with this extinction ratio is possible.

On the other hand, TM light is detected and monitored by the photodiode 1103. When this signal is converted into a signal having a band of 10 kHz or less by the low-pass filter 114,
The component modulated by the high-speed signal of 100 Mbps or more is averaged, and the output voltage changes according to the deviation amount of the bias point as shown in FIG. 4 of the first embodiment. After comparison with the current source 1111 that causes a DC bias to flow through the electrode 1210 via the control circuit 1112.
By feeding back to, it is possible to automatically stabilize the bias point. Note that feedback is applied to the electrode that does not modulate.
It may be a current source 1110 or 1110 ′ that flows a C bias. This automatic stabilization is performed by the PI by the control circuit 1112.
(Proportional, integral) control was performed, but by optimizing the gain and the integration time, it was possible to keep the change in extinction ratio within 1% over a long period of time. This makes the first
Similar to the embodiment, the long-term reduction in error rate, which was a problem in the conventional polarization modulation transmission, could be improved.

Further, the DFB laser 1101 has the electrode 12
The lasing wavelength can be changed by changing the ratio of the currents flowing through 09 and 1209 ′. When the same current was applied as above, the oscillation wavelength was 1.562 μm, but if the ratio was changed from 1: 1 to 1: 9 while keeping the total current constant at 52 mA, a wavelength of about 3 nm was reached up to 1.565 μm. I was able to change.

As described above, the integrated semiconductor device according to the present embodiment becomes a light source suitable for wavelength division multiplexing optical communication by the driving method described above.

[0056]

Fifth Embodiment A fifth embodiment according to the present invention will be described with reference to FIG. FIG. 11 shows LiNb having a shape as shown.
A Y-branch polarization mode splitter 1 on the O ₃ substrate 1301.
The apparatus 304 is a device in which a three-electrode type DFB laser 1302 and a photodiode 1303 are hybridly mounted with an adhesive. The polarization mode splitter 1304 is
A metal thin film 1305 is loaded on one waveguide of the branch so that the TE mode propagates to this side. The TM mode propagates to the other waveguide and the photodiode 1
Detected by 303. The DFB laser has the same structure as the DFB laser section of the fourth embodiment, and polarization modulation can be performed by applying a modulation signal with an amplitude of 2 mA to the center electrode. Hybrid mounting optical coupling parts a and b, and TE mode emission end c
Has a non-reflective coating to suppress reflection.
The driving method and the like are the same as in the fourth embodiment.

[0057]

Sixth Embodiment A sixth embodiment according to the present invention is one in which a vertical type forward coupler as shown in FIG. 12 is used as a polarization mode splitter and is monolithically integrated on a semiconductor substrate.

The structure and manufacturing method of this device will be described. n-I
On the nP substrate 1401, the n-InP buffer layer 140
2. n-InGaAs with bandgap wavelength of 1.1 μm
P lower waveguide layer 1403, n-InP clad layer 1404
Are sequentially stacked in the first growth. Clad layer 14
In No. 04, a fine diffraction grating 1406 with a pitch of 240 nm is formed in the DFB laser section, and a coarse diffraction grating 1407 with a pitch of 14.5 μm is formed in the forward coupler section. No diffraction grating is formed in the photodetector. A bandgap wavelength of 1.1 μm is formed on the clad layer 1404.
N-InGaAsP optical guide layer 1405, n-InGaAsP upper waveguide layer 1 having a bandgap wavelength of 1.3 μm
408, well layer i-In _0.28 Ga _0.72 As (thickness 10 n
m), the barrier layer i-In _0.82 Ga _0.18 As _0.4 P _0.6 (thickness 10 nm), and the active layer 14 having a strained superlattice structure composed of 10 layers.
09, p-InP clad layer 1410, In _0.53 Ga
_{A 0.47} As contact layer 1411 is deposited in the second growth.

Subsequently, the upper waveguide layer 1408 is etched to remove the active layer 1409 in the center portion of the DFB laser portion and the forward coupler portion, and the p-InP cladding layer 1412 and the In _0.53 Ga _0.47 As contact layer are formed. 14
13 is laminated by the third selective growth. The waveguide is 2.5μ
An embedded structure made of high resistance InP (not shown) having a width of m was adopted. Next, a p-side electrode 1414 and an n-side electrode 1415 are formed, and the p-side electrode 1414 is formed by removing three electrodes in the DFB laser section, and removing the electrode and contact layer in the forward coupler section. After the element was cut out, antireflection coatings 1416 and 1416 ′ were applied to both end surfaces. The length of each part is 800 μm in the DFB laser part, 1000 μm in the forward coupler part, and 200 μm in the photodetector part.

The operation of this apparatus will be described. In the DFB laser section, an amplitude of 2 mA is applied to the center electrode as in the fourth embodiment.
Polarization modulation can be performed by adding the modulation signal of. In the forward coupler, part of the light A propagating through the upper waveguide layer 1408 is coupled (B) to the lower waveguide layer 1403, propagates, and can be extracted to the outside. FIG. 13 shows the coupling characteristic from the upper waveguide 1408 to the lower waveguide 1403 in this forward coupler that functions as a wavelength discriminator. As shown in this figure, for TE mode light, a filter characteristic having a laser oscillation wavelength of 1.55 μm as a center wavelength is shown. Its full width at half maximum is about 5 nm
Is. On the other hand, for TM light, about 30 nm
It has a characteristic of having a central wavelength on the short wavelength side. Therefore, the light with the laser oscillation wavelength of 1.55 μm propagates only through the TE light through the lower waveguide 1403 and can be extracted to the outside. TM
The light propagates through the upper waveguide 1408 as it is and is detected by the photodetector.

The light emitted from the lower waveguide 1403 is
Since the polarization-modulated light has only the TE mode, it is an ASK signal and is coupled to the optical fiber 1417. At this time, in order to cut the TM light propagating through the upper waveguide 1408 and leaking from the rear part of the photodetector, only the rear part of the photodetector may be damaged by etching or the like to scatter the leaked light. Also, an isolator may be inserted when coupling to the optical fiber 1417.

The method of driving the device is the same as that of the fourth embodiment. The DFB laser is about 3 as in the fourth embodiment.
It has a wavelength tunable characteristic of nm, but in that wavelength range, TE
The coupling efficiency of light to the lower waveguide 1403 is about 25% down.

Since this forward coupler can change the central wavelength of filtering by applying an electric field, if the central wavelength is shifted according to the change of the oscillation wavelength of the laser, the coupling efficiency becomes constant. it can.

[0064]

[Embodiment 7] In the seventh embodiment according to the present invention, as shown in FIG. 14, Y of the fourth embodiment is provided at both emission ends of the DFB laser 1101.
Polarization mode splitters 1102 and 1102 ′ having a branch structure
It is a collection of two. Photodetector 1103 ', 11
03 ″ is provided in one polarization mode splitter 1102 ′ so that light of any polarization can be detected. In the other, a photodetector so that only TM mode can be detected. Only one 1103 is provided.

The photodetector output of each light is used for stabilizing the extinction ratio as in the fourth embodiment, or an APC (Auto power c) for stabilizing the optical output.
control). Control of extinction ratio is T
This embodiment is effective when M light alone is not enough and TE light is also required.

The light extracted to the outside is the TE light output of one polarization mode splitter 1102, which is coupled to the optical fiber 1121 and transmitted.

As described above, the two polarization mode splitters 1102 and 1102 ′ are the hybrid mounting type as in the fifth embodiment, the vertical type forward coupler as in the sixth embodiment, or a combination thereof. But it's okay.

In the integrated optical semiconductor devices of the fourth to the present embodiments, a semiconductor optical amplifier may be integrated at the emitting end in order to compensate the power of the light extracted from the polarization mode splitter to the outside. .

[0069]

Eighth Embodiment An eighth embodiment according to the present invention will be described. In FIG. 15, reference numeral 2101 denotes a semiconductor distributed feedback (DFB) laser that serves as a light source used in this system, 2102 denotes a Fabry-Perot etalon that serves as a wavelength filter, and 2103 denotes an optical fiber that transmits an optical signal.

The structure of the semiconductor DFB laser 2101 which is the light source of this embodiment is substantially the same as that described with reference to FIG.

In the DFB laser 2101 having the above structure, when the bias current flowing through the end-side electrode 109 without AR coating is fixed at a certain value I ₂ and the current I ₁ flowing through the AR-side electrode 110 is changed, As shown in FIG. 16, the oscillation polarization mode switches from the TE mode to the TM mode with a certain value. Therefore, when a minute digital signal ΔI ₁ is superimposed with an amplitude such that the TE mode output and the TM mode output after switching become the same as shown in FIG. As shown in FIG. 17, T of the electric field component E _{y is in} phase with the superimposed current waveform.
The M mode oscillates, and the TE mode of the electric field component E _x oscillates in the opposite phase. The oscillation wavelength of each oscillation light is 1.562 μ in the TE mode in the structure of this embodiment.
The lasing wavelength of the m and TM modes has a resonance peak of 1.558 μm, and the lasing wavelength difference between the two modes is significantly different from 4 nm.

DFB which is a light source having the characteristics shown above
The emitted light of the laser 2101 is made incident on the Fabry-Perot etalon 2102 which serves as a wavelength filter as shown in FIG. Now, the transmission characteristic of the Fabry-Perot etalon 2102 has a peak at the oscillation wavelength of the TE mode as shown in FIG.
1 is sufficiently smaller than the wavelength difference between the two oscillation modes and does not have a transmission peak in the vicinity of the oscillation wavelength of the TM mode, by modulating with a current waveform as shown in FIG. Since only the oscillating light is transmitted,
As shown in FIG. 9 (b), an output waveform ASK-modulated in the opposite phase to the signal current is obtained. Similarly, as shown in FIG. 18B, the oscillation wavelength of the TM mode has a peak, and 10 dB
If the entire transmission width is sufficiently smaller than the wavelength difference between the two oscillation modes of the DFB laser 2101, and if it is configured so as not to have a transmission peak in the vicinity of the oscillation wavelength of the TE mode, modulation with a current waveform as shown in FIG. As a result, since only the TM mode oscillation light is transmitted, an output waveform ASK-modulated in the same phase as the signal current is obtained as shown in FIG.

Since this modulated light has little fluctuation in carrier density and photon density in the laser 2101, the dynamic wavelength fluctuation can be made extremely small at 0.01 nm or less at the modulation frequency of 5 GHz. Further, since the difference between the oscillation wavelengths of the two modes is as large as several nm, 10 dB of the optical filter 2102
Since it can be much larger than the full width of transmission, a large extinction ratio can be obtained, and reception with a low error rate can be performed with a simple configuration.

In this embodiment, the wavelength filter 210 is used.
Although a Fabry-Perot etalon was used as 2, a wavelength filter (DF having a transmission characteristic that does not cause crosstalk between the two oscillation modes of the DFB laser 2101).
Similar effects can be expected by using B filters, dielectric filters, etc.).

[0075]

[Ninth Embodiment] FIG. 20 shows a ninth embodiment of the present invention. Second
The embodiment is an application of the optical communication system of the first embodiment to a wavelength division multiplexing communication system.

In FIG. 20, reference numeral 2301 denotes D which is a light source.
FB laser, 2302 is a wavelength tunable DFB filter, 23
Reference numeral 03 is an optical fiber for transmitting an optical signal, 2304 is a wavelength tunable DFB filter on the receiving side, and 2305 is a light receiver.

A DFB laser 2301 serving as a light source in FIG.
Are substantially the same as those described with reference to FIG.

Three-electrode DFB laser 2 in this embodiment
The wavelength tunable characteristic in the TE mode of 301 is the same as that shown in FIG. That is, the currents injected into the electrodes on both sides are controlled independently of I ₂ and I ′ _2, and the value of I ₂ + I ′ ₂ is set to 6
The ratio is kept constant while keeping it constant at 0 mA. However, I ₁ is kept constant at 20 mA, and single oscillation occurs in the TE mode. In this way, by changing the current ratio from 0.1 to 0.4, the variable width of about 3.0 nm can be obtained while maintaining the single mode. The wavelength can be tuned in the TM mode by the same method. Further, by superimposing on I ₁ a modulation current having an amplitude of 2 mA, polarization modulation can be performed as in the above embodiment.

The light wave thus modulated is incident on the optical filter 2302. At this time, if the transmission center wavelength of the optical filter is set so that only the TE mode (or TM mode) of the DFB laser 2301 is transmitted, ASK-modulated signal light can be obtained as in the above embodiment. This signal light is transmitted by being coupled to the optical fiber 2303, and is selectively demultiplexed by the optical filter 2304 that constitutes the optical receiver, and then converted into an electric signal by the photodetector 2305.

The wavelength multiplexing number in the configuration of the wavelength multiplexing communication system as in this embodiment is the wavelength tunable DFB filter 23.
The wavelength tunable range of 02, 2304 (about 3 nm) and the transmission bandwidth (about 10 dB transmission bandwidth of about 0.03 nm). However, in general, the wavelength tunable DFB filter has polarization dependence, and even in the same driving state, TE mode and TM
Since the transmission center wavelengths of the modes have different values, it is necessary to set these values so that crosstalk does not occur. Now, when the transmission wavelengths of the wavelength tunable DFB filters 2302 and 2304 are adjusted to the TE mode, the TM mode oscillation wavelength is configured not to be transmitted even if it is changed in the variable range (about 3 nm) of the wavelength tunable DFB filters 2302 and 2304. If so, the TM mode oscillation light does not cause crosstalk. Therefore, the DFB filters 2302, 2304
Assuming that a wavelength variable range of 3 nm and a 10 dB transmission bandwidth of 0.03 nm are used, multiplex transmission of about 100 ch is possible as a wavelength multiplexing optical communication system within the wavelength variable range.

[0081]

Tenth Embodiment FIG. 21 is a diagram showing a tenth embodiment of the present invention. The tenth embodiment relates to a method for stabilizing the optical communication system of the present invention. In FIG. 21, reference numeral 2501 denotes a DFB filter serving as a light source, 2502 denotes a polarization independent AO wavelength filter that spatially performs wavelength division, and the input light 25
For 10 only the light wave of a specific wavelength is guided to the waveguide 2511.
In addition, light waves having other wavelengths are branched to the waveguide 2512. The transmission bandwidth of the wavelength filter 2502 is FW
HM (full-width at half max
Imm) = 2.6 nm. Further, 2503 is an optical fiber for transmitting an optical signal, 2504 is a photodiode for receiving a light wave (reference light) branched by the wavelength filter 2502, 2505 is a low-pass filter, 2506.
Is a power supply for driving the DFB laser, and 2507 is a differential amplifier.

In the DFB laser 2501 used in this embodiment, it is known that if the bias current I ₁ flowing through the electrode 2509 changes, a sufficient extinction ratio cannot be obtained even if the modulation current amplitude ΔI ₁ is increased. Therefore, a polarization beam splitter (PBS) is used to transmit light waves that are not transmitted as optical signals.
A method of performing the separation / detection by using, for example, and performing the feedback control using the value is described in the above embodiment. Therefore, in the present embodiment for obtaining a higher extinction ratio,
The transmission center wavelength of the wavelength filter 2502 becomes the signal light D
By setting the TE mode (or TM mode) of the FB laser 2501, two modes can be set in the waveguide 25.
It branches to 11, 2512. Of the branched light waves, the signal light guided to the waveguide 2511, that is, the TE mode (or TM mode) is coupled to the optical fiber 2503 and transmitted. The light wave guided to the waveguide 2512, that is, the TM mode (or TE mode) is received by the photodiode 2504. A low-frequency filter 2505 extracts a low-frequency component from the electric signal obtained here and feeds it back to the laser driving power source 2506 via the differential amplifier 2507, whereby driving with a stable extinction ratio can be realized.

[0083]

[Embodiment 11] An embodiment in which optical transmission is performed by using the light source device or integrated optical semiconductor device according to the present invention as a light source for optical communication will be described with reference to FIG. Reference numeral 3801 denotes a light source device or an integrated optical semiconductor device in which the extinction ratio is stably controlled and polarization modulation is performed according to the present invention. With this light source 3801, the wavelength can be changed within the range of 3 nm as described in the above embodiment. In addition, in polarization modulation, dynamic wavelength fluctuation called chirping, which is a problem in normal direct intensity modulation, is very small at 2 GHz or less (about 0.01 nm in wavelength), so that 10 GHz is required for wavelength multiplexing. (About 0.0
Even if they are arranged at wavelength intervals of about 5 nm) to about 0.03 nm, crosstalk is not given to the adjacent channel. Therefore, when this light source device 3801 is used, 3 /
Wavelength multiplexing of about 0.05 = 60 or 3 / 0.03 = 100 channels is possible.

The light emitted from the light source 3801 is coupled to the single mode fiber 3802 and transmitted. In the receiving device, the signal light transmitted through the optical fiber 3802 is selectively demultiplexed into light having a desired wavelength by the optical filter 3803, and the signal is detected by the photodetector 3804. Here, as the optical filter 3803, the one having the same structure as the DFB laser of the above-mentioned embodiment is used, and the current is biased below the threshold to be used. By changing the current ratio of the two electrodes, the transmission gain can be kept constant at 20 dB and the transmission wavelength can be changed within the range of 3 nm. This is shown in FIG.
This graph shows the characteristics of the optical filter 3803 with respect to the TE mode, and when coupled in the TM mode, there is a transmission peak on the short wavelength side of about 4 nm from this. Also, the transmission width of this filter 3803 at 10 dB down is 0.03.
nm, and as described above 0.05 nm to 0.03
Waiting for sufficient properties to wavelength multiplex at nm intervals.

As the optical filter, other ones, for example, the Mach-Zehnder type, the fiber Fabry-Perot type and the like mentioned in the conventional example may be used. In addition, here, the light source 38
Although only 01 and one receiving device are described, it is needless to say that some light sources or receiving devices may be connected by an optical coupler or the like for transmission.

[0086]

[Embodiment 12] FIG. 24 shows a method of driving a light source for optical communication according to the present invention, an optical communication system using the same, and the like.
Optical connected to each terminal when applied to AN system-
An example of the configuration of the electrical conversion unit (node) is shown, and FIG. 25 shows an example of the configuration of the optical LAN system using the node.

An optical signal is taken into the node 4000 by using the optical fiber 3901 connected to the outside as a medium, and a part of the optical signal is incident on the receiving device 3903 having the wavelength tunable optical filter and the like as in the above embodiment. To do. The receiving device 3903 extracts only an optical signal of a desired wavelength and performs signal detection. On the other hand, in the case of transmitting an optical signal from the node 4000, the semiconductor laser device 3904 is driven by the method of the above-mentioned embodiment, polarization-modulated, and output converted into intensity modulation by a polarization beam splitter, an isolator 3905, or the like. Light is transmitted through the branching unit 3906 to the optical transmission line 3
It is made incident on 901. At that time, a desired wavelength may be oscillated and stabilized by the driving method as in the above embodiment.

Further, the wavelength tunable range can be widened by providing two or more semiconductor laser devices and tunable optical filters.

As a network of the optical LAN system,
The bus type is shown in FIG. 25, and a large number of networked terminals 4001 and centers can be installed by connecting the nodes 4000 in the A and B directions. However, in order to connect a large number of nodes 4000, it is necessary to arrange optical amplifiers in series on the transmission line 3901 in order to compensate for the attenuation of light. Also, by connecting two nodes 4000 to each terminal 4001 and using two transmission lines 3901, bidirectional transmission by the DQDB system becomes possible.

In such an optical network system, if the device driving method, the optical transmission system and the like according to the present invention are used, the multiplicity of 60 to 1 as described in the above embodiment.
No. 00 high-density WDM optical transmission network can be constructed. In addition, as a network system, A and B in FIG.
It may be a loop type or a star type in which the above are connected, or a combination thereof.

[0091]

[Embodiment 13] A wavelength division multiplexing optical CATV having a topology as shown in FIG. 26 can be constructed by a device driving method, an optical communication system and the like according to the present invention. CATV Center 4501
In the above, the semiconductor laser device or the wavelength tunable laser is polarization-modulated by the driving method of the above-mentioned embodiment to form a wavelength multiplex light source.
On the side of the subscriber 4502, which is the receiver, the receiver equipped with the wavelength tunable filter as in the above embodiment is used. Conventionally, due to the influence of the dynamic wavelength fluctuation of the DFB laser, the DF
Although it was difficult to use a B filter in such a system, the present invention made it possible.

Further, the subscriber 4502 is provided with an external modulator, and the signal from the subscriber 4502 is received by the reflected light from the modulator (one form of the simplified type bidirectional optical CATV, for example, Ishikawa, Furuta “Hikari”). LN external modulator for bidirectional transmission in CATV subscriber system ", OCS 91-82, p.
1) By constructing a star-type network as shown in FIG. 24, bidirectional optical CATV becomes possible and the service can be made highly functional.

In the present invention, the DFB laser I
The structure of the nP / InGaAsP system has been described.
The same effect can be expected for a short wavelength system structure such as As / AlGaAs.

[0094]

According to the present invention, a high-density wavelength multiplexing optical communication system such as a coherent optical communication, an advanced wavelength control technique and an electronic circuit technique can be applied to a high-density wavelength-division multiplexed optical communication system by using a direct polarization modulation system having an extremely small dynamic wavelength fluctuation. It can be built without the need for etc. Further, it is possible to construct a compact and highly productive light source for optical communication for a high-density wavelength division multiplexing optical communication system using a direct polarization modulation system with extremely small dynamic wavelength fluctuation.

Furthermore, in the optical communication system according to the present invention, the light source used in the communication system modulates the current flowing through a part of the optical waveguide, so that the polarization wavefront and the wavelength are different.
One of two oscillation modes is selected by a means for discriminating the wavelength of the output light modulated by the semiconductor laser by using a structure in which two polarization modes are switched and a semiconductor laser that selectively emits oscillation of one oscillation mode is used. It is characterized in that a light wave is coupled to an optical transmission line, and the light wave is ASK-modulated to be used as a signal for optical communication. Therefore, it is possible to drive a laser by a direct modulation method without a dynamic wavelength fluctuation, and to achieve a high-density wavelength. It is possible to construct a multiplex communication system and the like by a simple method.

Further, in addition to the semiconductor DFB laser device, an optical device such as a polarization beam splitter and a photodiode is required. Therefore, when it is used as a light source for optical communication, it is difficult to make it into a small module, and the module The present invention solves the problem that the optical adjustment is required when the film is turned into a thin film and the productivity is poor. .

[Brief description of drawings]

FIG. 1 is a first method of driving a semiconductor laser according to the present invention.
The figure explaining an Example.

FIG. 2 is a diagram showing the element configuration and characteristics of the first embodiment of the present invention.

FIG. 3 is a diagram for explaining the driving principle of the semiconductor laser used in the first embodiment of the present invention.

FIG. 4 is a diagram showing a relationship between a bias point shift and the intensity of TM polarized light.

FIG. 5 is a second method of driving a semiconductor laser according to the present invention.
The figure explaining an Example.

FIG. 6 is a diagram for explaining the output of the balanced modulator according to the second embodiment.

FIG. 7 is a third method of driving a semiconductor laser according to the present invention.
The figure explaining an Example.

FIG. 8 is a diagram showing wavelength tunable characteristics of the wavelength tunable laser according to the third embodiment.

FIG. 9 is a diagram illustrating an integrated optical semiconductor device and a driving method thereof according to a fourth embodiment of the present invention.

FIG. 10 is a sectional view of the integrated optical semiconductor device of the fourth embodiment in the waveguide direction.

FIG. 11 is a diagram illustrating a hybrid mounting integrated optical semiconductor device according to a fifth embodiment of the present invention.

FIG. 12 is a sectional view of an integrated optical semiconductor device according to a sixth embodiment of the present invention.

FIG. 13 is a diagram for explaining the characteristics of the forward coupler in the integrated optical semiconductor device according to the sixth embodiment of the present invention.

FIG. 14 is a perspective view of an integrated optical semiconductor device according to a seventh embodiment of the present invention.

FIG. 15 is a diagram for explaining an eighth embodiment according to the present invention.

FIG. 16 is a diagram showing current-light output characteristics of a semiconductor laser.

FIG. 17 is a diagram showing a modulation waveform of a semiconductor laser.

FIG. 18 is a diagram showing an oscillation spectrum of a light source and a transmission characteristic of an optical filter according to an eighth embodiment of the present invention.

FIG. 19 is a diagram showing a modulation waveform according to an eighth embodiment of the present invention.

FIG. 20 is a diagram for explaining a ninth embodiment according to the present invention.

FIG. 21 is a diagram illustrating a tenth embodiment according to the present invention.

FIG. 22 is a diagram for explaining the optical communication system of the eleventh embodiment according to the present invention.

FIG. 23 is a diagram showing the wavelength tunable characteristic of the wavelength tunable filter of the eleventh embodiment.

FIG. 24 is a diagram showing a configuration example of an optical node which is a twelfth embodiment according to the present invention.

FIG. 25 is a diagram illustrating an optical LAN network.

FIG. 26 is an optical CA that is a thirteenth embodiment according to the present invention.
The figure explaining a TV system.

FIG. 27 is a diagram showing a relationship between an extinction ratio of polarization modulation and a modulation current amplitude.

[Explanation of symbols]

1, 201, 301, 1101, 1302, 210
1, 2301, 2501 Semiconductor DFB laser 2, 220, 302 Polarizing beam splitter 3, 221, 304, 1103, 1103 ', 110
3 ″, 1303, 2504 Photodiodes 4, 223, 306, 1114, 2505 Low-pass filters 5, 6, 216, 217, 309, 310, 311, 1
110, 1110 ′, 1111, 2506 DC current source 7, 1115 modulator 8, 224, 307, 1113, 2507 differential amplifier 9, 402, 1121, 1417, 2103, 230
3, 2503, 3802, 3901 Optical fiber 10, 219, 1120, 2503, 3905 Optical isolator 101, 201, 1201, 1301, 1401 Substrate 102, 202, 1202, 1402 Buffer layer 103, 203, 213, 1203, 1213, 140
5 Light guide layer 104, 204, 1204, 1409 Active layer 105, 205, 214, 1205, 1214, 140
4, 1410, 1412 Clad layer 106, 206, 206 ', 215, 1206, 121
5, 1411, 1413 Contact layers 107, 207, 1106 Buried layers 108, 208, 208 ', 1208, 1208'
Electrode separation part 109, 110, 111, 209, 209 ', 210,
211, 1209, 1209 ', 1210, 1220,
1211, 1414, 1415, 2509 electrodes 112, 212, 212 ', 1212, 1212', 1
416, 1416 'Anti-reflection coating film 218, 308 Sinusoidal oscillator 222, 305 Balanced modulator 303 Fabry-Perot etalon 1102, 1102', 1304 Polarization mode splitter 1104 Mixed crystal region 1105 Emission part 1112 Control circuit 1305 Metal thin film 1403, 1408 Waveguide layer g Diffraction grating 1406 Fine pitch diffraction grating 1407 Coarse pitch diffraction grating 2102, 2502, 3803 Wavelength filter 2302, 2304 Semiconductor DFB filter 2305, 3804 Photoreceiver 2511, 2512 Waveguide 3801 Light source for optical communication 3902, 3906 Light Merging / branching device 3903 Optical receiving unit 3904 Optical transmitting unit 4000 node 4001 terminal 4501 center 4502 subscriber

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location H04B 10/04 10/06 H04J 14/00 14/02

Claims (53)

[Claims] 1. A driving method when a semiconductor laser is used as a light source for optical communication, wherein the semiconductor laser modulates a current flowing through a part of an optical waveguide (modulation signal) to generate two oscillations having different polarization planes. And a means for branching two polarization modes having different polarization planes of the modulated output light of the semiconductor laser into different propagation directions, and one of the two polarization modes. A means for coupling light to the optical transmission line and a means for converting the remaining one light into an electric signal are provided, and the electric signal is coupled so that the modulation state of the light coupled to the optical transmission line is stabilized. A method for driving a light source for optical communication, characterized in that a current flowing through the semiconductor laser is controlled based on the above. 2. The driving method according to claim 1, wherein the current controlled in the semiconductor laser is a current which flows in the same portion as an optical waveguide portion through which a modulation signal flows. 3. The driving method according to claim 1, wherein the current controlled in the semiconductor laser is a current passed through a portion different from the portion of the optical waveguide through which the modulation signal is passed. 4. The light hole in which the semiconductor laser is a distributed feedback semiconductor laser having a diffraction grating adjacent to an optical waveguide including a light emitting layer, wherein the light emitting layer is composed of quantum wells and the order of holes is a light hole. The pitch of the diffraction grating is set so that the Bragg wavelength is in the vicinity of the wavelength corresponding to the energy band gap between the order and the ground order of the electrons, and the threshold gain at the Bragg wavelength is almost the same in the two polarization modes. 4. The driving method according to claim 1, wherein the semiconductor lasers configured to be equal to each other are used as light sources. 5. The light emitting layer of the distributed feedback semiconductor laser is composed of a multiple quantum well in which tensile strain is introduced,
5. The driving method according to claim 4, wherein the light source is a semiconductor laser configured such that the heavy hole rank, which is the hole rank, is equal to the light hole rank, or the light hole rank is closer to the base rank of the electrons. 6. An output obtained by comparing the electric signal with a reference voltage by a differential amplifier through a low pass filter is fed back to a direct current source for driving the semiconductor laser at an appropriate feedback rate by a proportional and integrating circuit. 2. The current flowing through the semiconductor laser is controlled by the.
4. The driving method according to any one of 3 to 3. 7. A means for modulating a current flowing through the semiconductor laser with a low-frequency sine wave, and a means for multiplying the electric signal by the low-frequency sine wave, wherein the output of the multiplying means is in a low range. A current is supplied to the semiconductor laser by comparing the output with a reference voltage by a differential amplifier through a pass filter and feeding back the output to a direct current source for driving the semiconductor laser with an appropriate feedback ratio by a proportional and integrating circuit. The driving method according to any one of claims 1 to 3, wherein: 8. The oscillation wavelength of the semiconductor laser is variable by changing the ratio of the currents flowing in at least two parts of the waveguide, and the fluctuation of the wavelength of the transmitted light is changed immediately before the means for converting into an electric signal. The driving method according to claim 1, further comprising a wavelength discriminator that converts the intensity variation. 9. An output obtained by comparing an electric signal of light input to the means for converting the electric signal through the wavelength discriminator with a reference voltage by a differential amplifier through a low-pass filter, by a proportional and integrating circuit. 9. The driving method according to claim 8, wherein the oscillation wavelength is stabilized by a control method for controlling a current flowing through the semiconductor laser by feeding back to a direct current source for driving the semiconductor laser at a high feedback rate. . 10. A low-frequency sine signal and an electric signal of light input to a means for modulating a current flowing through the semiconductor laser with a low-frequency sine wave, and a means for converting the current into the electric signal through the wavelength discriminator. Means for multiplying the wave, comparing the output of the multiplying means with a reference voltage by a differential amplifier through a low-pass filter, and driving the semiconductor laser at a proper feedback rate by a proportional and integrating circuit. 9. The driving method according to claim 8, wherein the oscillation wavelength is stabilized by a control method of controlling a current flowing through the semiconductor laser by feeding back the direct current source to the semiconductor laser. 11. The semiconductor laser has a variable oscillation wavelength by changing a ratio of currents flowing through at least two portions of a waveguide, and a variation of the wavelength of transmitted light is changed immediately before a means for converting into an electric signal. A wavelength discriminator for converting into a change in intensity is provided, and an output obtained by comparing an electric signal of light input to the means for converting into the electric signal via the wavelength discriminator with a reference voltage through a low-pass filter is output. , A proportional and integrator circuit feeds back to a direct current source for driving the semiconductor laser at an appropriate feedback rate to control the current flowing through the semiconductor laser to stabilize the oscillation wavelength at the same time. The driving method according to claim 6 or 7. 12. The semiconductor laser has a variable oscillation wavelength by changing a ratio of currents flowing through at least two portions of a waveguide, and a variation in wavelength of transmitted light is changed immediately before a means for converting into an electric signal. A wavelength discriminator for converting into intensity fluctuation, a means for modulating a current flowing through the semiconductor laser with a low frequency sine wave, and an electric signal of light input to the means for converting into an electric signal through the wavelength discriminator. And a means for multiplying the low frequency sine wave, the output of the multiplying means is compared with a reference voltage by a differential amplifier through a low-pass filter, and the output is proportional and integrated by an integrating circuit at an appropriate feedback ratio. The oscillation wavelength is also stabilized at the same time by a control method of controlling a current flowing through the semiconductor laser by feeding back to a direct current source for driving the semiconductor laser. The driving method according to 6 or 7. 13. An optical communication system characterized in that a light source for optical communication is directly modulated by the driving method according to any one of claims 1 to 12, transmitted through an optical fiber, and directly detected by an optical receiver. . 14. An optical fiber according to any one of claims 8 to 1.
2. A plurality of light sources for optical communication that are directly modulated by the driving method according to any one of 2 are connected, light of a plurality of wavelengths is respectively modulated and transmitted, and an optical receiver equipped with an optical filter converts the light into a desired wavelength. An optical communication system characterized by performing wavelength division multiplexing transmission so that only the carried signals can be taken out. 15. An optical-electrical device characterized in that an optical communication light source and an optical receiver used in the optical communication system according to claim 14 are combined into one to perform optical transmission and reception by the optical communication system according to claim 14. Converter. 16. An optical-electrical conversion device for performing optical transmission / reception according to the optical communication system according to claim 14, wherein the optical communication light source and the optical receiver used for the optical communication system according to claim 14 are combined into one. A wavelength division multiplexing optical transmission system characterized by the above. 17. A broadcasting center is provided with a light source for optical communication that is directly modulated by the driving method according to claim 8, and an optical receiver is provided with an optical filter for an optical subscriber. Item 14. An optical CATV system characterized by performing the optical communication system according to Item 14. 18. A structure having a semiconductor laser, wherein the semiconductor laser has a structure in which two oscillation polarization modes having different polarization planes are switched by modulating (modulating signal) a current flowing through a part of an optical waveguide, Means for splitting two polarization modes having different polarization planes of the modulated output light of the semiconductor laser into different propagation directions; means for coupling one light of the two polarization modes to an optical transmission line; And a means for converting the remaining one light into an electric signal, and controlling a current flowing through the semiconductor laser based on the electric signal so that the modulation state of the light coupled to the optical transmission line is stabilized. A device for driving a light source, which is characterized by: 19. The light source driving device according to claim 18, wherein the branching means is a polarization beam splitter. 20. The means for coupling one light into an optical transmission line includes an isolator.
A driving device for the light source described. 21. A semiconductor laser having a structure in which two oscillating polarization modes having different polarization planes are switched by modulating (modulating signal) a current flowing through a part of an optical waveguide including a light emitting layer,
At least one of a waveguide-type polarization mode splitter for branching two polarization modes of the semiconductor laser, which have different polarization planes, into different propagation directions, and at least one of the two polarization modes. An integrated optical semiconductor device, characterized in that a photodetector for converting one light into an electric signal is integrated on the same substrate. 22. The integrated optical semiconductor device according to claim 21, wherein all are integrated monolithically on the same semiconductor crystal substrate. 23. The integrated optical semiconductor device according to claim 21, wherein all are integrated in a hybrid on the same substrate. 24. The semiconductor laser is a distributed feedback semiconductor laser provided with a diffraction grating in the vicinity of an optical waveguide including a light emitting layer, the light emitting layer being a quantum well, and a light hole in the order of holes. The pitch of the diffraction grating is set so that the Bragg wavelength is in the vicinity of the wavelength corresponding to the energy band gap between the order and the ground order of the electrons,
22. The integrated optical semiconductor device according to claim 21, wherein a semiconductor laser configured so that the threshold gain at a Bragg wavelength is substantially equal in the two polarization modes is used as a light source. 25. The light emitting layer of the distributed feedback semiconductor laser is composed of a multiple quantum well in which tensile strain is introduced,
22. The integrated optical semiconductor device according to claim 21, further comprising a semiconductor laser configured such that a heavy hole rank, which is a hole rank, is equal to a light hole rank, or a light hole rank is closer to a ground rank of electrons. . 26. The polarization mode splitter is formed of a Y-branch waveguide layer on the same plane, the waveguide layer includes a semiconductor superlattice, and only one of the Y-branch waveguide layers is the semiconductor layer. 22. The integrated optical semiconductor device according to claim 21, wherein the superlattice is mixed crystal. 27. The polarization mode splitter is formed of a Y-branch waveguide layer on the same plane, and a metal is loaded only on one of the Y-branch waveguide layers. Item 22. An integrated optical semiconductor device according to item 21. 28. The polarization mode splitter is formed by at least two waveguide layers in a vertical direction, and a diffraction grating is provided near any one of the waveguide layers. Integrated optical semiconductor device. 29. The integrated optical semiconductor device according to claim 21, wherein two polarization mode splitters are provided on both sides of the resonator of the semiconductor laser. 30. The integrated optical semiconductor device according to claim 21, wherein only one photodetector is provided to detect only light in one waveguide layer of the polarization mode splitter. . 31. The integrated optical semiconductor device according to claim 21, wherein two photodetectors are provided to detect light in both waveguide layers of the polarization mode splitter. 32. A current flowing through the semiconductor laser is controlled based on an electric signal obtained by the photodetector so that a modulation state of light emitted to the outside of the integrated optical semiconductor device is stabilized. 32. The integrated optical semiconductor device according to claim 21, wherein the integrated optical semiconductor device is a semiconductor device. 33. The integrated optical semiconductor device according to claim 32, wherein the current controlled in the semiconductor laser is a current which flows in the same portion as the portion of the optical waveguide through which the modulation signal flows. 34. The integrated optical semiconductor device according to claim 32, wherein the current controlled by the semiconductor laser is a current passed through a portion different from the portion of the optical waveguide through which the modulation signal is passed. 35. An output obtained by comparing the electric signal with a reference voltage by a differential amplifier through a low pass filter is fed back to a direct current source for driving the semiconductor laser at an appropriate feedback rate by a proportional and integrating circuit. 35. The integrated optical semiconductor device according to claim 32, wherein a current flowing through the semiconductor laser is controlled by. 36. The integrated optical semiconductor device according to claim 21, wherein an output of an electric signal compared with a reference voltage by a differential amplifier through a low-pass filter is proportional,
The modulation signal is used as an optical communication light source for directly modulating by a driving method for controlling a current flowing in the semiconductor laser by feeding back to a direct current source for driving the semiconductor laser with an appropriate feedback rate by an integrating circuit An optical communication system characterized in that it is transmitted by the optical receiver and directly detected by an optical receiver. 37. The integrated optical semiconductor device according to any one of claims 21 to 31, wherein an output obtained by comparing an electric signal with a reference voltage by a differential amplifier through a low-pass filter is proportional,
One optical fiber is used as a light source for optical communication for directly modulating by a driving method for controlling a current flowing in the semiconductor laser by feeding back to a direct current source for driving the semiconductor laser with an appropriate feedback rate by an integrating circuit. It is characterized by performing wavelength division multiplex transmission so that a plurality of connections are made, light of a plurality of wavelengths is modulated and transmitted, and an optical receiver equipped with an optical filter extracts only the signal on the desired wavelength of light. Optical communication system. 38. An optical-electricity characterized in that an optical communication light source and an optical receiver used in the optical communication system according to claim 37 are combined into one to perform optical transmission and reception by the optical communication system according to claim 37. Converter. 39. An optical-electrical conversion device for performing optical transmission / reception according to the optical communication system according to claim 37 is used, in which an optical communication light source and an optical receiver used for the optical communication system according to claim 37 are combined. A wavelength division multiplexing optical transmission system characterized by the above. 40. A broadcasting center is provided with the integrated optical semiconductor device according to claim 21 as a light source for optical communication, and an optical receiver is provided with an optical filter for optical subscribers. 37. An optical CATV system characterized by performing the optical communication system described in 37. 41. A semiconductor laser having a structure in which two oscillating polarization modes having different polarization planes are switched by modulating (modulating signal) a current passed through a part of an optical waveguide including a light emitting layer,
A means for branching two polarization modes having different polarization planes of the modulated output light of the semiconductor laser into different propagation directions, and at least one light of the two polarization modes divided into an electric signal. The semiconductor is based on an electric signal obtained by the photodetection means so that at least one of the modulation state and the oscillation wavelength of the light emitted to the outside of the device having the photodetection means for conversion is stabilized. A method for driving a semiconductor laser, which comprises controlling a current passed through the laser. 42. In an optical communication method, a structure in which a light source used in the communication system switches two oscillation modes having different polarization planes and wavelengths by modulating (modulating signal) a current flowing through a part of an optical waveguide. Of the modulated output light of the semiconductor laser, the one of the two oscillation modes is selected by the wavelength discriminating means and is coupled to the optical transmission line. Is an ASK-modulated signal and is used for optical communication signals. 43. A distributed feedback semiconductor laser in which the semiconductor laser is provided with a diffraction grating in the vicinity of an optical waveguide including a light emitting layer, the light emitting layer being a quantum well, and a light hole which is a hole order. The pitch of the diffraction grating is set so that the Bragg wavelength is in the vicinity of the wavelength corresponding to the energy band gap between the order and the ground order of the electrons,
43. The optical communication system according to claim 42, wherein a semiconductor laser configured so that the threshold gain at the Bragg wavelength is substantially equal in the two polarization modes is used as the light source. 44. The light emitting layer of the distributed feedback semiconductor laser is composed of a multiple quantum well in which tensile strain is introduced,
43. The optical communication system according to claim 42, wherein the light source is a semiconductor laser configured such that the heavy hole rank, which is the hole rank, is equal to the light hole rank, or the light hole rank is closer to the base rank of electrons. . 45. As the wavelength discriminating means, a fixed filter for coupling any one of two oscillation modes emitted from the semiconductor laser to an optical transmission line is used, and the signal light is ASK-modulated. 43. The optical communication system according to claim 42, wherein: 46. The optical communication system according to claim 45, wherein the fixed optical filter is a DFB filter. 47. The optical communication system according to claim 42, wherein the ASK-modulated signal light is transmitted through an optical fiber and directly detected by an optical receiver. 48. The wavelength discriminating means comprises a Fabry-Perot etalon.
2. The optical communication method described in 2. 49. The oscillation wavelength of the two oscillation modes of the semiconductor laser is variable by changing the ratio of currents flowing in at least two or more portions of the optical waveguide,
As the means for discriminating the wavelength, a wavelength tunable filter that tunes to the oscillation wavelength of the semiconductor laser and couples one of two oscillation modes to the optical transmission line is used.
43. The optical communication system according to claim 42, wherein the signal light is ASK-modulated. 50. The optical communication system according to claim 49, wherein the wavelength tunable filter is a DFB filter. 51. A plurality of light sources for optical communication are connected to one optical fiber, lights of a plurality of wavelengths are respectively modulated and transmitted, and the light having a desired wavelength is put on the light by an optical receiver equipped with an optical filter. 43. The optical communication system according to claim 42, wherein wavelength division multiplexing transmission is performed so that only signals are extracted. 52. As the means for discriminating wavelengths, an element for spatially separating a light wave of a specific wavelength and a light wave other than that is used, and one of two oscillation modes is coupled to an optical transmission line, and the remaining 1 And a modulation state of the light wave coupled to the optical transmission line and oscillation by setting a driving condition of the semiconductor laser serving as the light source based on the electric signal. 42. The optical communication system according to claim 41, wherein at least one of the wavelengths is stabilized. 53. A semiconductor laser having a structure in which a light source switches two oscillation modes having different polarization planes and wavelengths by modulating a current flowing through a part of an optical waveguide, and the modulated output of the semiconductor laser. Among the lights, the output light is ASK by selecting and outputting one of the two oscillation modes by means of wavelength discrimination.
An optical communication device characterized by modulating.