JP2009147076A - External resonator type wavelength variable laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an external resonator type wavelength variable laser device having excellent stability in laser oscillation mode. <P>SOLUTION: The external resonator type wavelength variable laser device includes: an optical amplifier 2 for amplifying a laser beam; a wavelength controller 8 for controlling the wavelength of the laser beam by a liquid crystal driven by an alternating current voltage; and a phase adjustor 3 for adjusting the phase of the laser beam by changing a refractive index based on a modulation signal. The amplitude of the modulation signal changes based on the amplitude of the alternating current voltage. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an external resonator type wavelength tunable laser device.

  In recent years, with the rapid spread of the Internet, there has been a demand for further increase in communication traffic capacity. Therefore, improvement of transmission rate per channel and multi-channelization by wavelength division multiplexing (WDM) are being promoted. In WDM, a plurality of optical signals assigned to different carrier wavelengths (channels) can be transmitted simultaneously. That is, the communication capacity can be increased by increasing the number of channels. For example, sufficient modulation can be performed at 10 gigabits / second per channel, and 100 channels can be transmitted by one optical fiber. As a result, the communication capacity reaches 1 terabit / second.

  As a wavelength band used in medium-to-long distance optical communication in recent years, a C band (1530 to 1570 nm) that can be amplified by a rare earth element erbium-doped optical fiber amplifier (EDFA: Erbium Doped Fiber Amplifier) is widely used. Usually, a laser device corresponding to the wavelength of each standard channel used in optical communication is required. For example, for 100 channels, 100 types of laser devices are required. Therefore, there is a demand for practical use of a wavelength tunable laser device that can cover all the C band that can be amplified by the EDFA with a single laser device.

  On the other hand, there is also a demand for the construction of a flexible network that can dynamically set a path according to an increase or decrease in traffic or a failure. Therefore, there is a long-awaited development of a network infrastructure that can provide more diverse services. In order to construct such a large-capacity, high-function, high-reliability photonic network, a technology for freely controlling the wavelength is indispensable. Therefore, the wavelength tunable laser has become an extremely important system key device.

  Patent Document 1 discloses a wavelength tunable laser in which a plurality of distributed feedback (DFB) lasers are arranged in parallel. In this tunable laser, the oscillation wavelength of each DFB laser is set in advance and the wavelength is roughly adjusted by switching the laser. Further, fine adjustment is performed using the change in refractive index due to temperature.

  However, the wavelength tunable laser disclosed in Patent Document 1 requires an optical coupler that combines one output port of each DFB laser. Therefore, when the number of parallel DFB lasers is increased, the loss in the optical coupler increases. That is, there is a problem that the wavelength variable range and the light output are in a trade-off relationship.

  However, the wavelength tunable laser based on the DFB laser has an advantage that it can be combined with the wavelength locker described in Patent Document 2 because it can be finely adjusted by temperature. The wavelength locker is an etalon type filter having a periodic transmission amplitude on the frequency axis. Near the center of the amplitude, the transmitted light intensity of the etalon filter changes sensitively according to the laser frequency. Therefore, it is possible to tune to a desired laser frequency by detecting the transmitted light intensity with the monitor current of the photoelectric conversion element. Thus, the combination of the DFB laser and the wavelength locker is an effective means for locking the laser wavelength with high accuracy with respect to the standard channel wavelength.

  On the other hand, an external resonator type tunable laser has been proposed as a tunable laser having no trade-off relationship. In the external resonator type wavelength tunable laser, an external resonator is formed by using a semiconductor optical amplifier (SOA) and an external reflecting mirror. Then, wavelength selection characteristics are realized by inserting a wavelength controller such as a wavelength tunable mirror or a wavelength tunable filter in the external resonator. In this external cavity type wavelength tunable laser, a wavelength tunable width that covers the entire C band can be obtained relatively easily, and research and development are thriving.

  In the external cavity type tunable laser, most of the basic characteristics are determined by the tunable mirror and the tunable filter. As the wavelength tunable mirror, for example, there is an electrically controlled wavelength tunable mirror disclosed in Patent Document 3 in which the external mirror itself has a wavelength tunable characteristic. Examples of the wavelength tunable filter include a filter that rotates an etalon disclosed in Patent Document 4, a filter that rotates a diffraction grating disclosed in Patent Document 5, an acoustic engineering filter and a dielectric filter disclosed in Patent Document 6, and the like. There is.

  Various external cavity type wavelength tunable lasers using such a wavelength tunable filter and wavelength tunable mirror are known. For example, Patent Document 7 discloses an external resonator type wavelength tunable laser including a semiconductor optical amplifier, an etalon, and a wavelength tunable filter. The wavelength tunable filter has a relatively wide transmission bandwidth, and the laser mode is not stable even if the laser resonator is configured by itself. Therefore, an etalon having a transmission bandwidth narrower than that of the wavelength tunable filter is inserted into the laser resonator to stabilize the laser mode. Furthermore, since the transmission bandwidth of the wavelength tunable filter is wide, the accuracy of the transmission peak wavelength is relatively insensitive. Therefore, the wavelength variable filter can be controlled by open loop. That is, feedback control is not required.

  Further, the etalon in the laser resonator of Patent Document 7 operates as a wavelength selection filter (hereinafter, abbreviated as a wavelength selection filter) having periodic transmission characteristics on the frequency axis. At this time, since the transmission peak wavelength is fixed, if the laser oscillation mode can be adjusted to the transmission peak wavelength, the transmittance of the wavelength selection filter can be maximized. That is, the optical loss in the laser resonator can be minimized. At the same time, since the submode transmittance can be minimized, the mode stability can also be improved.

  In order to adjust the laser oscillation mode to the peak wavelength of the wavelength selection filter, the phase may be adjusted in the laser resonator. The phase adjustment is to effectively change the optical length (refractive index n × actual length L) of the laser resonator. Specifically, a method for changing the refractive index n by arranging a substance that changes the refractive index, such as a semiconductor, in the laser resonator, or a method for changing the actual optical length L by a mechanical method. Can be considered.

  A configuration to which a phase adjustment mechanism for changing the refractive index of a semiconductor is added is disclosed in Patent Document 6 and Non-Patent Document 1, for example. In these configurations, as in Patent Document 7, an etalon is used as the wavelength selection filter. However, the configuration of the wavelength tunable filter is different from Patent Document 7. In patent document 6, it is the structure which combined the acousto-optic filter and the reflective mirror as a wavelength variable filter. On the other hand, in Non-Patent Document 1, an electrically controlled wavelength tunable mirror using a change in the refractive index of liquid crystal is used.

  The principle of the wavelength selection operation by such an external resonator type wavelength tunable laser will be briefly described with reference to FIGS. 8, 9A, 9B, 9C, and 9D. FIG. 8 is a side view showing a configuration of a related art external resonator type tunable laser device, and FIGS. 9A, 9B, 9C, and 9D illustrate laser oscillation modes of the external resonator type tunable laser device of FIG. It is a figure for doing. 8 includes a semiconductor element 51, a semiconductor optical amplifier 52, a low reflection coating surface 53, a non-reflection coating surface 54, a collimator lens 55, an etalon 56, a wavelength tunable filter 57, and a total reflection mirror 58. A subcarrier 59 and a temperature controller 101. The low-reflection coating surface 53, the semiconductor optical amplifier 52, the non-reflection coating surface 54, the collimating lens 55, the etalon 56, the wavelength variable filter 57, and the total reflection mirror 58 constitute an external resonator. 9A shows the transmission characteristics of the tunable filter 57, FIG. 9B shows the transmission characteristics of the etalon 56, FIG. 9C shows the Fabry-Perot mode of the external resonator, and FIG. 9D shows the laser oscillation mode of the external resonator.

  First, the light emitted from the semiconductor optical amplifier 52, which is a gain medium, includes a number of Fabry-Perot modes 63 depending on the total length of the external resonator as shown in FIG. 9C. However, among these modes, only a plurality of modes that match the period of the periodic transmission band 62 (FIG. 9B) of the etalon 56 that is the wavelength selection filter are selected and pass through the wavelength selection filter. Here, the Fabry-Perot mode that cannot pass through the wavelength selection filter is suppressed. Therefore, even in a configuration in which the frequency interval of the Fabry-Perot mode is relatively narrow, that is, a configuration in which the overall length of the external resonator is relatively long, submodes other than the channel can be easily suppressed.

  Next, only one of a plurality of modes transmitted through the wavelength selection filter is selected by the wavelength tunable filter 57 having the transmission characteristic 61 as shown in FIG. Reference numeral 64 in FIG. 9D denotes a mode that transmits through the wavelength tunable filter 57. The light transmitted through the wavelength tunable filter 57 is reflected by the total reflection mirror 58 and finally returns to the semiconductor optical amplifier 52. Thus, a feedback loop is configured. According to the configuration of FIG. 8, a wavelength tunable laser with high mode stability can be realized relatively easily. In addition, wavelength selection characteristics can be realized with relatively simple control.

  In the configuration of FIG. 8, the periodic wavelength of the wavelength selection filter is fixed, and the wavelength of the transmission peak coincides with the standard channel for optical communication. In the configuration of FIG. 8, a wavelength selection filter is arranged inside the external resonator. Therefore, the wavelength accuracy within the channel accuracy of the wavelength selection filter can be obtained without using a wavelength locker necessary for the wavelength tunable DFB laser.

  In this type of laser, the transmission peak wavelength of the etalon inside the resonator matches the standard channel in advance. Therefore, it is necessary to control the laser oscillation wavelength to match the etalon transmission wavelength by phase adjustment. This etalon is the least prone to degradation among the on-board components. Therefore, if the phase adjustment is performed so that the laser wavelength always matches the peak, the oscillation wavelength can be kept constant even if the semiconductor deteriorates.

This control is usually called dither control. While superimposing a low-frequency modulation signal (dither) on the DC component (bias) of the phase adjustment current and monitoring the laser light output, the DC component (bias) of the phase current is adjusted so that the amplitude of the modulation signal of the optical output is minimized. Feedback control. By such automatic control, correct phase adjustment is always performed even if the semiconductor element deteriorates. Also, feedback control in which dither is superimposed at a frequency different from the frequency of the AC drive voltage is also effective for aging of the wavelength tunable mirror or wavelength tunable filter.
JP 2003-023208 A JP 2001-257419 A US Patent No. US6215928B1 Japanese Patent Laid-Open No. 4-69987 Japanese Patent Laid-Open No. 5-48220 Japanese Patent Laid-Open No. 2000-261086 US Patent No. US6526071B1 Japanese Patent Application No. 2007-084642 J. De Merlier et al., “Full C-band external cavity wavelength tunable laser using a liquid-crystal-based tunable mirror”, IEEE Photonic Technology Letters, 2005, Vol. 17, p. 681

  In the external resonator type wavelength tunable laser disclosed in Non-Patent Document 1 and Patent Document 8, the liquid crystal wavelength tunable mirror or wavelength tunable filter is an optical bandpass filter. By changing the refractive index of the liquid crystal, the maximum transmission wavelength is changed to realize a wavelength tunable laser. However, since the change in the refractive index of the liquid crystal is used, there is a problem that the laser oscillation mode becomes unstable under certain conditions. The details will be described below.

  In the liquid crystal wavelength tunable mirror, when the amplitude of the AC drive voltage applied to the liquid crystal increases, the refractive index of the liquid crystal is modulated at the frequency of the AC drive voltage, and the laser oscillation mode becomes unstable. This phenomenon is hereinafter referred to as a liquid crystal self-phase modulation effect. This self-phase modulation effect always exists as long as the liquid crystal is driven with an AC voltage.

  The degree of this self-phase modulation effect can be explained by the frequency response of the liquid crystal. The frequency response of the liquid crystal is described in detail in Patent Document 8. In FIG. 2 of Patent Document 8, the frequency response of the liquid crystal has a resonance peak from 100 Hz to 1000 Hz (= 1 kHz), depending on the type of liquid crystal. Usually, the part away from this resonance peak is used. In applications such as a display, even if there is some self-phase modulation, the level is not visible so that there is no problem.

However, slight self-phase modulation can be a problem in lasers for optical communication. When the refractive index of the liquid crystal is modulated, the phase of the oscillation mode of the external resonator laser using the liquid crystal is also modulated. Here, assuming that the liquid crystal of the liquid crystal wavelength tunable mirror is driven at a frequency ω [kHz] and an AC drive voltage amplitude α [V], the refractive index n (t) at time t is the initial refractive index n _0. The self-phase modulation degree Δn is expressed by the following equation (1). Further, as shown in Expression (2), both the initial refractive index n ₀ and the self-phase modulation degree Δn are proportional to α.
n (t) = n ₀ + Δn · EXP (ωt) (1)
Here, n ₀ ∝α and Δn∝α (2)
Since the reflection wavelength of the liquid crystal wavelength tunable mirror is determined by the initial refractive index n ₀ , the reflection wavelength can be controlled by the AC drive voltage amplitude α. On the other hand, a self-phase modulation degree Δn proportional to the AC drive voltage amplitude α is generated at the same time. This unintended self-phase modulation degree Δn becomes prominent when the amplitude α is large, and the following problems occur when this is applied to the oscillation mode.

  First, the tolerance for dithering the AC drive voltage and phase adjustment current of the wavelength tunable mirror is reduced. At the time of dither control, it is necessary to change the AC drive voltage amplitude and the phase adjustment current of the tunable mirror within a range in which the laser oscillation mode does not hop. However, the range of the stable region where the laser oscillation mode does not hop can be reduced.

  Second, when a dither is applied to suppress stimulated Brillouin scattering (SBS), the dither amplitude is limited. In ultra-long-distance optical fiber transmission, high-power laser light that causes nonlinear effects in the optical fiber may be used. In that case, it is known that SBS occurs as a nonlinear effect in the optical fiber. Due to the influence of the SBS, there arises a problem that desired optical power is not input into the optical fiber. As a method for suppressing SBS, a method of increasing the line width by intentionally applying FM modulation to laser light is generally known. This SBS suppression is more effective as the FM modulation degree of the laser is higher. Therefore, it is desired to apply FM modulation as much as possible within a stable range of the laser mode, but there is a problem that the stable range is reduced.

  The present invention has been made to solve the above-described problems, and an object thereof is to provide an external resonator type wavelength tunable laser excellent in stability of a laser oscillation mode.

  An external resonator type wavelength tunable laser device according to the present invention includes an optical amplifier that amplifies laser light, a liquid crystal that is driven by an AC voltage, a wavelength controller that controls the wavelength of the laser light, and a modulation signal. And a phase adjuster for adjusting the phase of the laser light by changing the refractive index, and the amplitude of the modulation signal changes based on the amplitude of the AC voltage.

  ADVANTAGE OF THE INVENTION According to this invention, the external resonator type | mold wavelength-variable laser excellent in the stability of the oscillation mode of a laser can be provided.

  Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings. However, the present invention is not limited to the following embodiment. In addition, for clarity of explanation, the following description and drawings are simplified as appropriate.

[First embodiment]
FIG. 1 is a block diagram showing the configuration of an external resonator type wavelength tunable laser device according to a first embodiment of the present invention. In this embodiment, the variable wavelength mirror is a voltage application type that utilizes a change in the refractive index of the liquid crystal, and has a reflection characteristic that is not periodic within the wavelength band to be used. As shown in FIG. 1, the external resonator type wavelength tunable laser device of this embodiment includes a semiconductor element 1 including a semiconductor optical amplifier 2, a collimator lens 6, an etalon 7, and a liquid crystal wavelength tunable mirror 8. ing. The semiconductor element 1 is obtained by integrating a phase adjuster 3 as a passive element in a semiconductor optical amplifier 2 as an active element. An AC drive voltage source 11 is connected to the liquid crystal wavelength variable mirror 8, and a bias DC current source 12 and a modulation signal source 13 are connected to the phase adjuster 3. The details are described below.

  In this embodiment, laser light is output from the left end face of the semiconductor optical amplifier 2. A low-reflection coating 4 having a reflectance of 1 to 10% is applied to the left end surface of the semiconductor optical amplifier 2. On the other hand, a non-reflective coating 5 having a reflectance of 1% or less is applied to the right end surface of the phase adjustment region 3. As a result, the low-reflection coating 4, the semiconductor optical amplifier 2, the phase adjustment region 3, the non-reflection coating 5, the collimator lens 6, the etalon 7, and the liquid crystal wavelength tunable mirror 8 constitute an external resonator 10. In this embodiment, the end face of the semiconductor optical amplifier 2 opposite to the phase adjuster 3 is used as the light output side. However, the end face of the phase adjustment region 3 opposite to the semiconductor optical amplifier 2 may be used as the light output side. .

  The semiconductor optical amplifier 2, which is an active element, is composed of multiple quantum wells (MQW), and generates and amplifies light in response to current injection. The phase adjuster 3, which is a passive element, is composed of a bulk composition or multiple quantum wells, and has a wide band gap that does not absorb laser oscillation light. The refractive index of the region changes in response to current injection or voltage application. The semiconductor optical amplifier 2 and the phase adjuster 3 may be formed using a known butt joint technique or selective growth technique. The semiconductor optical amplifier 2 and the phase adjuster 3 are electrically insulated by a sufficient separation resistance of 1 kΩ or more.

  A collimating lens 6 is disposed on the side opposite to the light output of the semiconductor element 1. The collimating lens 6 converts the light beam from the semiconductor element 1 into parallel light 14. The beam collimated by the collimating lens 6 is then reflected by the liquid crystal wavelength variable mirror 8 and fed back to the original semiconductor element 1. The liquid crystal wavelength tunable mirror 8 uses a mirror that can vary the reflection peak wavelength by applying a voltage to change the refractive index of the liquid crystal. Such a type of tunable mirror is described in Patent Document 3, for example.

  An etalon 7 is disposed between the collimating lens 6 and the wavelength variable mirror 8. The etalon 7 has periodic transmission characteristics with respect to the wavelength in the wavelength range to be used. In this embodiment, the free spectral range (FSR) of the etalon is 50 GHz, that is, the transmission peak wavelength interval is 50 GHz.

  Although not shown in FIG. 1, the elements constituting the external cavity laser 10 are arranged on a common subcarrier so that the light beam travels linearly. In addition, a thermistor for temperature monitoring is arranged at an appropriate position. Further, the subcarrier is mounted on a temperature controller (TEC: Thermo-Electric Cooler) and is controlled to a constant temperature by monitoring the thermistor temperature.

  The details of the operating principle of this external resonator type wavelength tunable laser are as described above, and the liquid crystal wavelength tunable mirror 8 is a light band-pass filter, and by changing the refractive index of the liquid crystal. The tunable laser is realized by changing the maximum transmission wavelength.

Here, when an AC driving voltage having a frequency ω [kHz] and an amplitude α [V] is applied to the liquid crystal wavelength tunable mirror 8, the reflection peak wavelength of the liquid crystal wavelength tunable mirror 8 changes according to the value of α. At the same time, self-phase modulation caused by the change in the refractive index of the liquid crystal occurs in proportion to the amplitude α. The relationship between the self-phase modulation and α is shown in FIG. In the present invention, as shown in FIG. 1, a bias current for phase control is supplied from the DC current source 12 to the phase adjuster 3, and in addition, an AC component having an amplitude β is supplied from the modulation signal source 13. Superimpose. By selecting the value of β so as to satisfy the following expression (3) using the coefficient γ with respect to the value of α, the self-phase modulation of the liquid crystal wavelength tunable mirror can be canceled.
β = γ × α (3)

  FIG. 3A shows a diagram representing β when Expression (3) is satisfied, and FIG. 3B shows the magnitude of the phase modulation that the laser oscillation mode receives as a result. Furthermore, in order to explain the phase relationship between α and β, FIG. 4 shows on the time axis. FIG. 4 shows a state of self-phase modulation of the liquid crystal modulated at ω [kHz] on the time axis and a state of phase modulation generated in the laser by injecting an AC signal into the phase adjuster 3. As shown in FIG. 5, it is possible to effectively cancel the phase modulation that the laser oscillation mode receives.

  However, since the opposite phase relationship is satisfied as the phase, the phase of the AC drive voltage of the liquid crystal wavelength variable mirror 8 and the phase of the modulation current of the phase adjuster 3 are preferably in phase. This is because the phase value received by the laser oscillation mode increases as the AC drive voltage of the liquid crystal wavelength tunable mirror 8 increases, but the phase received by the laser oscillation mode decreases as the phase current of the phase adjuster 3 increases. It is. The relationship between this phase change, voltage, and current value is shown on the right axis of FIG. Therefore, in appearance, the phase of the AC drive power supply 11 and the phase of the modulation signal source 13 are in phase.

  In the above example, the semiconductor element 1 in which the phase adjuster 3 is integrated is used. However, according to the present invention, any other component that can be modulated at the frequency ω [kHz] and can control the phase can be used. By introducing such a component into the resonator, the self-phase modulation of the liquid crystal wavelength tunable mirror 8 can be canceled.

  Further, as described in Non-Patent Document 1, also in this embodiment, in order to dither control the voltage of the liquid crystal wavelength tunable mirror 8, an alternating current power source driven at a frequency different from ω or a bias value of the phase current is dithered. To control, the wavelength of the laser can be controlled by adding an alternating current source having a frequency different from ω. In this case, the output laser light 9 can be received by a photodetector for optical output monitoring by splitting a part of the optical power including the dither signal for optical output monitoring by a beam splitter or the like. Thereby, the optical power of the output laser beam 9 can be known from the branching ratio of the beam splitter, and dither control becomes possible.

  Further, in order to suppress SBS, phase modulation (FM modulation) can be applied to the laser oscillation mode. For this purpose, a modulation signal source having a frequency different from ω can be added to the phase adjuster 3.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. FIG. 5 is a block diagram showing a configuration of an external cavity type wavelength tunable laser device and a control device according to a second embodiment of the present invention, and the same components as those in FIG. 1 are denoted by the same reference numerals. . The external resonator type wavelength tunable laser device of this embodiment includes a semiconductor element 1 including a semiconductor optical amplifier 2, a collimator lens 6, an etalon 7, and a wavelength tunable mirror 8. The semiconductor device 1 has a gain region 2 and a phase adjuster 3 integrated therein, and the phase adjuster 3 is normally adjusted by applying a direct current. In this embodiment, the phase adjuster is used. 3 and the liquid crystal wavelength tunable mirror 8 are connected to the same AC drive voltage source 16, an AC voltage is supplied to the liquid crystal wavelength tunable mirror 8, and a current is supplied to the phase adjuster 3 by a variable resistor 15 on the way. After conversion, AC current is supplied.

  In the embodiment of FIG. 1, the liquid crystal wavelength variable mirror 8 and the phase adjuster 3 use separate ω [kHz] AC power supplies, and the frequencies ω may be slightly different from each other. If it is slightly different, the phase of the signal will shift after a long period of use, making it impossible to cancel the self-phase modulation of the liquid crystal. Therefore, in this embodiment, control is performed by branching from one AC drive power supply 16 in order to set the same frequency. In the present embodiment, the variable resistor 15 is disposed between the AC drive voltage source 16 and the phase adjuster 3, and the relationship of Expression (3) is satisfied by adjusting the variable resistor 15. Can do.

  Since the same AC drive voltage source is branched, the AC drive voltage supplied to the wavelength variable mirror 8 and the AC modulation signal current supplied to the phase adjuster 3 satisfy the in-phase relationship. As described above, the self-phase modulation effect of the liquid crystal is canceled, and the phase can be canceled with higher accuracy than in the embodiment while simplifying the control circuit as compared with the embodiment.

  Further, as described in Non-Patent Document 1, also in this embodiment, in order to dither control the voltage of the liquid crystal wavelength tunable mirror 8, an alternating current power source driven at a frequency different from ω or a bias value of the phase current is dithered. To control, the wavelength of the laser can be controlled by adding an alternating current source having a frequency different from ω.

  Further, in order to suppress SBS, phase modulation (FM modulation) can be applied to the laser oscillation mode. For this purpose, a modulation signal source having a frequency different from ω can be added to the phase adjuster 3.

[Third embodiment]
Next, a third embodiment of the present invention will be described. The configuration of the third embodiment of the present invention can be the same as that of either the first embodiment or the second embodiment, but the difference from the above embodiment is supplied to the phase adjuster 3. This is the value of the amplitude β of the modulation current. In the above embodiment, as shown in the equation (3), the amplitude β of the phase current is proportional to the AC voltage amplitude α to the wavelength variable mirror 8. In the present embodiment, the following equation (4) is established using the coefficient σ.
β = σ ÷ α (4)
This eliminates the need to separately apply phase modulation for SBS suppression. The details are described below.

  In this embodiment, SBS suppression can be realized at the same time by utilizing the self-phase modulation effect in the liquid crystal wavelength tunable mirror 8. Due to the self-phase modulation effect of the liquid crystal, this laser is phase-modulated at the frequency α of the AC drive voltage of the wavelength variable mirror 8. This is the same as FM modulation applied to the laser for SBS suppression.

  However, since the self-phase modulation effect of the liquid crystal is proportional to the AC drive voltage α, the effect appears only when α is large. Therefore, as shown in Expression (4), the modulation current amplitude β to the phase adjuster 3 is set to be inversely proportional to α. As shown in FIG. 7A, when α is small, β increases, and when α is large, β decreases. As a result, a constant phase modulation is applied to the laser regardless of the value of α. The effect is shown in FIG. 7B. This is possible because the frequency of the phase modulation current is also the same as α. As described above, the self-phase modulation of the liquid crystal is not canceled but can be actively used.

  According to the present embodiment, it is possible to substantially apply phase modulation for SBS suppression to the laser without adding a separate circuit for SBS suppression, and the control circuit of the laser light source can be simplified. . In the first to third embodiments, a configuration in which a wavelength tunable filter and a total reflection mirror are combined instead of the wavelength tunable mirror may be employed. The configuration of the AC drive power supply in that case is the same as the configuration in FIG. 1 or FIG. 6, and even in that case, the modulation signal is input to the phase adjuster 3 as in the first to third embodiments. Thus, the same effect as described above can be obtained.

  According to the present invention, when the wavelength tunable mirror or wavelength tunable filter using the change in the refractive index of the liquid crystal is driven, the self-phase modulation of the liquid crystal generated can be reduced to a practically negligible level by the control circuit. . That is, a practically sufficient wavelength control stable range can be provided, and the following effects can be obtained.

  First, the oscillation mode of the laser is stabilized against changes in the surrounding environment such as temperature changes. Second, the laser oscillation mode is stabilized even with respect to the aging of the constituent elements. The reason for the first and second effects is that the stable range of the laser oscillation mode is expanded with respect to changes in the liquid crystal driving voltage and the phase adjustment current, which are wavelength control parameters. Third, ultra-long distance optical transmission is possible. This is because the FM modulation degree for suppressing SBS in the optical fiber can be increased while maintaining the laser oscillation mode in a stable state. The present invention achieves an external resonator type wavelength tunable laser device capable of performing ultra-long distance communication with a stable laser oscillation mode with respect to environmental changes and secular changes by the first to third effects described above. Can do.

It is a block diagram which shows the external resonator type | mold wavelength-tunable laser apparatus which concerns on 1st Example. It is a figure which shows the self phase modulation amplitude with respect to AC drive voltage amplitude in the wavelength variable mirror or wavelength variable filter using the refractive index change of a liquid crystal. It is a figure which shows the phase modulation amplitude by a phase adjuster with respect to the AC drive voltage amplitude to a liquid crystal in 1st Example. It is a figure which shows the phase modulation amplitude which a laser oscillation mode receives with respect to the AC drive voltage amplitude to a liquid crystal in 1st Example. It is a figure which shows the relationship between the self phase modulation of the liquid crystal on the time axis, and the phase modulation by the phase adjuster. At the same time, it is a diagram showing the relationship between the liquid crystal driving voltage amplitude and the phase modulation current that are caused by the respective modulations. It is a figure which shows all the phase modulations which a laser oscillation mode receives by the phase modulation added to the phase adjuster on a time axis. It is a block diagram which shows the external resonator type | mold wavelength-tunable laser apparatus which concerns on 2nd Example. It is a figure which shows the phase modulation amplitude by a phase adjuster with respect to the AC drive voltage amplitude to a liquid crystal in 3rd Example. It is a figure which shows the phase modulation amplitude which a laser oscillation mode receives with respect to the AC drive voltage amplitude to a liquid crystal in 3rd Example. It is a side view which shows the external resonator type | mold wavelength-tunable laser apparatus of related technology. It is a figure which shows the transmission characteristic of the wavelength variable filter of the wavelength variable laser apparatus of FIG. It is a figure which shows the transmission characteristic of the etalon of the wavelength tunable laser apparatus of FIG. It is a figure which shows the Fabry-Perot mode of the external resonator of the wavelength tunable laser apparatus of FIG. It is a figure which shows the laser oscillation mode of the external resonator of the wavelength tunable laser apparatus of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor element 2 Semiconductor optical amplifier 3 Phase adjuster 4 Low reflection coating 5 Nonreflection coating 6 Collimating lens 7 Etalon 8 Wavelength variable mirror 9 Output laser beam 10 External resonator type lasers 11 and 16 AC drive power supply 12 DC current source 13 Modulation Signal source 14 Parallel light 15 Variable resistance

Claims (6)

An optical amplifier for amplifying laser light;
A wavelength controller having a liquid crystal driven by an AC voltage and controlling the wavelength of the laser beam;
A phase adjuster that adjusts the phase of the laser beam by changing the refractive index based on a modulation signal;
An external resonator type wavelength tunable laser device in which the amplitude of the modulation signal changes based on the amplitude of the AC voltage.   2. The external resonator type wavelength tunable laser device according to claim 1, wherein the frequency of the modulation signal and the frequency of the AC voltage are substantially the same.   3. The external resonator type wavelength tunable laser device according to claim 2, wherein a change in the refractive index of the phase adjuster due to the modulation signal is opposite in phase to a change in the refractive index of the liquid crystal due to the AC voltage.   4. The external resonator type wavelength tunable laser device according to claim 2, wherein the amplitude of the modulation signal changes in proportion to the amplitude of the AC voltage.   4. The external resonator type wavelength tunable laser device according to claim 2, wherein the amplitude of the modulation signal changes in inverse proportion to the amplitude of the AC voltage.   6. The external resonator type wavelength tunable laser device according to claim 2, wherein a power source that generates the modulation signal and a power source that generates the AC voltage are the same.

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