JPH11183946A - Pulse light generator - Google Patents

Abstract

(57) [Problem] To provide a pulse light generation device that generates a light pulse having a very narrow time width. SOLUTION: A branching array waveguide grating 202 selectively transmits and separates a spectral component constituting a supercontinuum light incident on an optical input port 201. The intensities of the separated spectral components are independently adjusted by the variable attenuators 203-1 to 203-k to be constant. The phase difference of each spectrum component is adjusted to zero by the variable phase adjusters 204-1 to 204-k. The adjusted spectral components are multiplexed by the multiplexing array waveguide grating 205 and output from the optical output port 206.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse light generator, and more particularly, to a pulse light generator suitable for use in an ultra-high-speed optical transmission system using optical pulses.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram showing an example of a conventional pulsed light generator. Hereinafter, the operation of the conventional pulsed light generator will be described with reference to FIG.

In FIG. 4, reference numeral 41 denotes a reference signal oscillator,
2 is an optical pulse generator and 43 is an optical amplifier. 44 is a low dispersion optical fiber, and 45 is an optical output port. The electric signal generated by the reference signal oscillator 41 is input to an optical pulse generator 42 and used to generate an optical pulse having a constant repetition frequency. Usually, a sine wave generated from a low noise oscillator is used as the reference signal.

[0004] The optical pulse generator 42 generates an optical pulse having a frequency equal to the frequency of the input reference signal, or a frequency that is an integral multiple of the frequency, or an integer fraction thereof. As the optical pulse generator 42, a mode-locked fiber ring laser, a mode-locked semiconductor laser, a gain-modulated semiconductor laser, a collision pulse mode-locked laser, or the like can be used.

Here, the mode-locked fiber ring laser will be described with reference to FIGS.

FIG. 5 is a diagram showing a configuration example of a mode-locked fiber ring laser. FIG. 5A shows a basic configuration,
FIG. 5B shows a typical spectrum characteristic obtained by mode locking, and FIG. 5C shows a time response characteristic of the output waveform.

In FIG. 5A, 51 is an oscillator, 52
Is a power amplifier, and 53 is an optical modulator. 54 is an optical fiber, 55 is an optical amplifier, 56 is an optical isolator, and 57 is an output optical fiber coupler.

In FIG. 5A, an output of an oscillator 51 having a predetermined frequency is amplified by a power amplifier 52 and applied to an optical modulator 53. The optical modulator 53 modulates the loss or phase of light at the frequency of the oscillation output by the electro-optic effect. The optical amplifier 55 amplifies the modulated optical pulse. The optical isolator 56 regulates the traveling direction of the light pulse and blocks the reflected return light. The output optical fiber coupler 57 is used to extract the amplified optical pulse to the outside.

The modulator 53, the optical amplifier 55, the optical isolator 56, and the output optical fiber coupler 57 are coupled in a ring shape via the optical fiber 54, thereby forming a ring resonator 58. As the optical amplifier 55, a rare earth-doped optical fiber amplifier to which a rare earth such as erbium (Er) or neodymium (Nd) is added,
A semiconductor laser amplifier is used.

FIG. 6 is a diagram showing a configuration example of a rare earth-doped optical fiber amplifier.

In FIG. 6, reference numeral 61 denotes an Er-doped fiber, 62 denotes a wavelength-multiplexed wave device, and 63 denotes an excitation light source. An optical pulse is input from one end (left end in the figure) of the rare earth doped optical fiber, and the amplified optical pulse is emitted from the other end (right end in the figure). The pumping light from the pumping light source 63 is incident on the rare-earth-doped optical fiber via the emission end, the incident-end, or the multi-wavelength filter 62 connected to both ends of the light pulse of the rare-earth-doped optical fiber. In the rare earth-doped optical fiber, the light pulse is amplified by the excitation light.

FIG. 6A shows a backward pumping configuration in which a wavelength-multiplexed optical device 62 is connected to the output end of a rare-earth-doped optical fiber, and FIG. The forward excitation configuration connected to the input end is shown in FIG.
(C) shows a bidirectional pump configuration in which the wavelength-multiplexed wave device 62 is connected between the input end and the output end of the rare earth doped optical fiber.

Next, a semiconductor laser amplifier which can constitute the optical amplifier 55 will be described with reference to FIG.

FIG. 7 is a diagram showing a configuration example of a semiconductor laser amplifier.

In FIG. 7, reference numeral 71 denotes a driving power source (current source).
, 72 are semiconductor laser amplifiers. The semiconductor laser amplifier 72 is configured to amplify an input optical pulse according to a current supplied from the driving power supply 71 and output the amplified optical pulse.

Here, returning to FIG. 5, the operation principle of the mode-locked fiber ring laser will be described.

Assuming that the optical path length of the ring resonator 58 is L, the physical length of each component (53, 55, 56, 57) of the ring resonator 58 is h, and the refractive index of each component is n, the optical path length L
Each physical length h _i a value obtained by multiplying the respective refractive index n _i (i.e., each of the optical path length) is the sum of, is calculated by the following equation (1).

[0018]

[Number 1] _{L = Σh i n i (1} ) By the way, the ring resonator 58 of FIG. 5, fr = c / L
There are a number of longitudinal modes with a frequency interval given by (where c is the speed of light).

Here, the light modulator 5 in the ring resonator 58
When the light modulation of the repetition frequency fm = N × fr (where N is an integer of 1 or more) is performed by using 3, the phases of all the longitudinal modes at the frequency interval N × fr are aligned as shown in FIG. A mode-locked oscillation state is set. At this time, FIG.
As shown in (c), the repetition frequency is 1 / (N × f
An optical pulse train of r) is obtained. Note that the pulse width of each light pulse corresponds to the reciprocal of the oscillation spectrum width δν determined by the envelope of a number of longitudinal mode spectra, and the center of the envelope of this spectrum is the center wavelength (center frequency νo) (FIG. 5). (See (b)).

Next, a mode-locked semiconductor laser and a gain-modulated semiconductor laser which can constitute the optical pulse generator 42 will be described with reference to FIGS.

FIG. 8 is a diagram showing a configuration example of a mode-locked semiconductor laser.

In FIG. 8, reference numeral 81 denotes a semiconductor laser;
And 84 are lenses, and 83 is a light intensity modulator. 85 is a reflecting mirror, 86 is an oscillator, and 87 is a half mirror for extracting output light. The light intensity modulator 83 is located at the center of the optical path between the reflecting mirror 85 where the light pulse reciprocates and the output light extracting half mirror 87. When the optical pulse is driven at a frequency that is twice the frequency of reciprocating this optical path, or at a frequency equal to an integer multiple thereof, mode-locking operation is realized according to the same principle as that of a mode-locked fiber ring laser. Generate.

The semiconductor laser 81 and the light intensity modulator 8
When the mirror 3 and the reflecting mirror 85 are monolithically integrated on the same semiconductor substrate, the lenses 82 and 84 in FIG.
Is unnecessary.

As shown in FIG. 8B, FIG.
By arranging the configuration of (a) symmetrically and arranging the saturable absorbing medium 88 at the center, it is possible to configure a collision pulse mode-locked laser capable of generating a short light pulse.

FIG. 9 is a diagram showing a configuration example of a gain modulation semiconductor laser.

In FIG. 9, 91 is an oscillator, 92 is an amplifier, 93 is a DC power supply, 94 is a direct bi-bias circuit, 95
Is a semiconductor laser, 96 is a normal dispersion fiber, and 97 is an output optical terminal.

The output of the oscillator 91 is amplified by the amplifier 92. This amplified output is superimposed on the DC voltage from the DC power supply 93 by the bias circuit 94 and input to the semiconductor laser 95. The oscillation frequency of the oscillator 91 is 1 GHz
In this case, the semiconductor laser 95 generates a short optical pulse of about 30 ps by relaxation oscillation. The optical pulse generated by the semiconductor laser 95 is output from the output optical terminal 97.

Here, the optical pulse generated by the semiconductor laser 95 has a frequency chirping whose wavelength changes in the long wavelength direction with the passage of time of the pulse. In order to eliminate the frequency chirping, the generated optical pulse may be output through the normal dispersion fiber 96. Pulse width is about 10p by normal dispersion fiber 96
s, and output from the output optical terminal 97.

Returning to FIG. 4, the light pulse generated from the light pulse generator 42 has the following two features.

1) The pulse width is 10 ps or less.

2) The time bandwidth product of the pulse is within twice the value of the transform limit (TL) light pulse.

In particular, a mode-locked fiber ring laser,
In a mode-locked semiconductor laser or the like, an almost complete TL light pulse can be generated.

After the optical pulse having these characteristics is amplified by the optical amplifier 43, it is incident on the low dispersion optical fiber 44.
The low dispersion optical fiber 44 has a dispersion value in the vicinity of the wavelength of the optical pulse generated from the optical pulse generator 42 (about 20 n).
m).

When an optical pulse obtained by amplifying the optical pulse generated by the optical pulse generator 42 enters the low dispersion optical fiber 44 satisfying the above conditions, the optical spectrum is expanded in the low dispersion optical fiber 44 by an optical nonlinear effect. The phenomenon (Super Continuum; SC) is seen. SC
Then, the spread width of the spectrum reaches 200 nm or more. The supercontinuum phenomenon is characterized by a light coherence of at least 10n in the spread spectrum.
m is maintained.

Therefore, when the spectrum broadened by the SC is cut by an optical bandpass filter having a bandwidth wider than the spectrum width of the incident light pulse, an optical pulse having a pulse width smaller than the pulse width of the incident light pulse is generated. be able to. It has been confirmed that an extremely short pulse having a pulse width of several ps to 170 fs can be generated by utilizing this phenomenon.

Detailed experimental results of generation of ultrashort pulses are as follows.
_{"T.Morioka, S.Kawan i sh i,} K.
Mori, and M.M. Saruwatari "Tra
nsform-limited, femtosecond
d WDM pulse generation by
spectral filtering of gi
gahertz supercontinuum ", E
electron. Lett "vol., 30, pp. 1
166-1168, 1994. "It is described in.

[0037]

However, the above-mentioned prior art has the following problems. That is, in order to reduce the pulse width of the output optical pulse to 100 fs or less, it is necessary to widen the band of the spectrum to be cut out to 10 nm or more, but when the spectrum band exceeds 10 nm, the longer wavelength side of the spectrum of the generated SC light Since power fluctuations and phase differences occur on the short wavelength side, it is difficult to reduce the pulse width in the time response of the optical pulse even if the cutout band is further extended.

In view of the above, the present invention has been made in view of the above-mentioned problems of the prior art, and generates generated SC light by using a bright line having a frequency interval corresponding to the repetition frequency of an incident light pulse generated therein. By taking out each spectral component separately, adjusting the light intensity and optical phase independently for each component, and combining the components to minimize the pulse width of the output optical pulse, the time width is extremely short It is an object of the present invention to provide a pulsed light generator capable of generating a light pulse having a narrow width.

[0039]

According to a first aspect of the present invention, there is provided an optical pulse generating means for generating an optical pulse which repeats at a predetermined frequency; Optical means having zero dispersion in the vicinity of the optical pulse, and optical means for generating supercontinuum light having spectral components at frequency intervals corresponding to the repetition frequency by incident light based on the light pulse. In the apparatus, separating means for selectively transmitting and separating the spectral components constituting the supercontinuum light, and independently adjusting the intensity and phase of each of the separated spectral components, Adjusting means for keeping the intensity of the spectral component constant and minimizing the phase difference between the spectral components; Characterized in that it comprises an optical multiplexing means for multiplexing the respective spectral components.

Here, in the apparatus according to the second aspect of the present invention, the optical unit, the separating unit, the adjusting unit, and the optical multiplexing unit can each have a polarization maintaining characteristic. .

[0041]

In the apparatus according to the first aspect of the present invention, the condition for generating a short light pulse can be satisfied for a wider spectral range of the supercontinuum light generated by the optical means.

In the apparatus according to the second aspect of the present invention, the polarization state in the process after the optical means can be kept constant.

[0043]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a diagram showing a first embodiment of an optical pulse generator to which the present invention is applied.

In FIG. 1, reference numeral 101 denotes a reference signal oscillator,
102 is an optical pulse generator, 103 is an optical amplifier, 104
Is a low dispersion optical fiber, 105 is a periodic optical filter, 106
Is an optical output port.

The electric signal generated by the reference signal oscillator 101 is input to an optical pulse generator 102 and used to generate an optical pulse having a constant repetition frequency.
Usually, a sine wave generated from a low noise oscillator is used as the reference signal.

The optical pulse generator 102 generates an optical pulse having a frequency equal to the frequency of the input reference signal, or a frequency that is an integral multiple of the frequency, or a fraction thereof. As the optical pulse generator 102, similarly to the optical pulse generator 42 shown in FIG. 4, a mode-locked fiber ring laser, a mode-locked semiconductor laser, a gain-modulated semiconductor laser,
A collision pulse mode-locked laser or the like can be used.

Therefore, the light pulse generated from the light pulse generator 102 also has the two characteristics described above (the pulse width is 10 ps or less, and the time bandwidth product of the pulse is the transform limit (TL) light pulse). Within 2 times the value of the above).

After the optical pulse having these characteristics is amplified by the optical amplifier 103, it is incident on the low dispersion optical fiber 104. The low dispersion optical fiber 104 is selected so that its dispersion value has zero dispersion near the wavelength of the light pulse generated from the light pulse generator 102 (within about 20 nm), as described in the related art. In the low dispersion optical fiber 104, a supercontinuum phenomenon occurs,
Generate C light.

In order to generate a short optical pulse narrower than the method using the prior art, the present embodiment is designed to satisfy the short optical pulse generation condition for SC light having a wider spectral range. The internal structure of the SC light spectrum is controlled by the periodic light filter 105. This periodic optical filter 105 can be constituted by an arrayed waveguide grating filter.

FIG. 2 is a diagram showing a configuration of an arrayed waveguide grating filter.

In FIG. 2, reference numeral 201 denotes an optical input port;
Numeral 02 denotes a branching array waveguide grating. 203-1 to 2
03-k (k is a natural number) is a variable attenuator, 204
Reference numerals -1 to 204-k denote variable phase adjusters. Reference numeral 205 denotes a multiplexing array waveguide grating, and reference numeral 206 denotes an optical output port.

Hereinafter, the operation of the arrayed waveguide grating filter of FIG. 2 will be described. The light incident from the optical input port 201 is divided into different wavelength components of the incident light by the demultiplexing arrayed waveguide grating 202 and output from the k output terminals. That is, the bright line spectral components constituting the SC light generated in the low dispersion optical fiber 104 are cut out for each component.

FIG. 3 shows one of the arrayed waveguide gratings 202 for demultiplexing.
FIG. 14 is a characteristic diagram illustrating frequency (wavelength) transmission characteristics of the kth to kth output terminals.

Assuming that fo is the oscillation frequency of the reference signal oscillator 101, the transmission center wavelength (frequency) of each output from the output terminals 1 to k of the demultiplexing arrayed waveguide grating 202 is as shown in FIG. And the transmitted light frequency of the shortest wavelength port is f
A bright line spectral component shifted by fo as c is transmitted. As described above, the input SC light is selectively transmitted at a good extinction ratio for each bright line spectral component having a frequency interval corresponding to the repetition frequency of the incident light pulse generated therein, and separated into each bright line component. Is done. Furthermore, this k
The bright line spectral components are independently controlled by the variable attenuators 203-1 to 203-k and the variable phase adjuster 20.
After the intensity and phase are adjusted by 4-1 to 204-k, they are multiplexed again by the multiplexing array waveguide grating 206.

Here, the variable attenuators 203-1 to 203-
By adjusting the intensity and the phase with k and the variable phase adjusters 204-1 to 204-k, an optimal state is set for the generation condition of the ultrashort optical pulse.

At this time, if the amplitudes of the k emission line spectral components are adjusted to be equal and there is no phase difference, the optical pulse S (t) recombined by the multiplexing array waveguide grating 206 ) Is represented by the following equation (2).

[0058]

(Equation 2)

The above equation is equivalent to the expansion of a periodic impulse waveform into a Fourier series. That is, the pulse width of the recombined light pulse S (t) is extremely narrow.

As an example, the pulse width in the time response of the re-combined light pulse is calculated assuming that the envelope of the combined bright-line spectrum sequence is Gaussian and its half-value width is Δλ.

Here, assuming, for example, that Δλ = 100 nm and the generated optical pulse is a transform-limited optical pulse, the time-bandwidth product of the optical pulse becomes a constant value (0.44). Since the half-width wavelength band of 100 nm is converted to a frequency band of 12 THz, the obtained time width is 3.6 × 10 ⁻¹⁴ [s], that is, 36 [f].
s]. This time pulse width can be set to a value of about 1/5 of the value achieved by the conventional technique.

As described above, according to the present embodiment, adjustment is made so that the pulse width of the output light pulse is minimized by satisfying the condition for generating the short light pulse for the SC light having a wider spectral range. An optical pulse having an extremely narrow time width can be generated, and an optical pulse having a pulse width narrower than that of the related art can be generated. Therefore, when the device of the present invention is used as a wavelength-variable ultrashort optical pulse light source in ultrahigh-speed optical transmission or optical signal processing, a remarkable effect can be obtained.

The order in which the separated bright line spectral components pass through the variable attenuators 203-1 to 203-k and the variable phase adjusters 204-1 to 204-k is opposite to that in FIG. May be.

In this embodiment, since the amplitude and the phase of each of the separated emission line spectra can be controlled independently, it is also possible to freely generate light pulses other than the Gaussian light pulse described above. Can be.

(Other Embodiments) Among the components shown in FIGS. 1 and 2, the low dispersion optical fiber 104, the array waveguide grating 202 for demultiplexing, and the variable attenuators 203-1 to 203 are used.
−k, the variable phase adjusters 204-1 to 204-k, and the multiplexing arrayed waveguide grating 205 may have a polarization maintaining configuration.

According to the above configuration, the low dispersion optical fiber 1
Since all the components in the process from the generation of the supercontinuum light by the optical waveguide 04 to the final combined output by the periodic optical filter 105 are of the polarization maintaining configuration, the polarization state in this process is changed. It can be kept constant. For this reason, a short light pulse can always be generated stably.

[0067]

As described above, according to the first aspect of the present invention, a spectrum having a frequency interval corresponding to a repetition frequency generated by inputting an optical pulse that generates an optical pulse repeated at a predetermined frequency. Selectively transmit and separate the spectral components of supercontinuum light that has components, and adjust the intensity and phase of each separated spectral component independently to satisfy the ultrashort pulse generation condition. By combining the spectral components obtained by the above, it is possible to generate an optical pulse having an extremely short time width,
It is possible to provide an optimal device as a wavelength-tunable ultrashort optical pulse light source in ultra-high-speed optical transmission and optical signal processing.

According to the second aspect of the present invention, since the polarization maintaining structure is provided after the optical means for generating the supercontinuum light, the polarization state in the process after the optical means is kept constant. Therefore, there is an effect that an optical pulse having a very short time width can always be generated stably.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of an optical pulse generator to which the present invention is applied.

FIG. 2 is a diagram showing a configuration of an arrayed waveguide grating filter which is a component of the device according to the first embodiment.

FIG. 3 is a diagram showing frequency (wavelength) transmission characteristics of each output terminal in the arrayed waveguide grating filter.

FIG. 4 is a configuration diagram illustrating an example of a conventional pulse light generation device.

FIG. 5 is a diagram illustrating a configuration example of a mode-locked fiber ring laser.

FIG. 6 is a diagram illustrating a configuration example of a rare earth-doped optical fiber amplifier.

FIG. 7 is a diagram illustrating a configuration example of a semiconductor laser amplifier.

FIG. 8 is a diagram showing each configuration example of a mode-locked semiconductor laser.

FIG. 9 is a diagram illustrating a configuration example of a gain modulation semiconductor laser.

[Explanation of symbols]

 Reference Signs List 101 Reference signal oscillator 102 Optical pulse generator 103 Optical amplifier 104 Low dispersion optical fiber 105 Periodic optical filter 106, 206 Optical output port 201 Optical input port 202 Array waveguide grating for demultiplexing 203-1 to 203-k Variable attenuator 204 -1 to 204-k Variable phase adjuster 205 Array waveguide grating for multiplexing

Claims (2)

[Claims] 1. An optical pulse generating means for generating an optical pulse repeated at a predetermined frequency, and an optical means having a zero dispersion near a wavelength of the optical pulse, wherein said optical pulse has a repetition frequency based on incident light based on said optical pulse. Optical means for generating supercontinuum light having a spectral component at a frequency interval corresponding to the pulse light generation device, wherein the spectral components constituting the supercontinuum light are selectively transmitted and separated. Separating means, and adjusting means for independently adjusting the intensity and phase of each of the separated spectral components so as to keep the intensity of each spectral component constant and to minimize the phase difference between each spectral component, An optical multiplexing means for multiplexing each spectral component obtained by adjusting the adjusting means. Light generator. 2. The pulse light generating device according to claim 1, wherein the optical unit, the separating unit, the adjusting unit, and the optical multiplexing unit have polarization maintaining characteristics, respectively.