JPH0669603A - Passive mode synchronizing semiconductor laser device - Google Patents

Abstract

PURPOSE:To synchronize a repetitive frequency of a passive mode synchronizing semiconductor laser with a frequency of natural number (n)-times a reference frequency (f). CONSTITUTION:An electric signal of a reference frequency (f) is generated from a reference frequency generator 105, a referential light pulse 116 of a repetitive frequency of f+DELTAf is generated from a referential laser 104 based on the electrical signal, and a correlation electric signal of a frequency n.DELTAf is obtained by taking optical correlation between the referential light pulse 116 and an output signal light pulse 115 of a passive mode synchronizing semiconductor laser 101. Meanwhile, an electric signal of (n) times a frequency DELTAf is generated, phase difference of two electric signals of the frequency n.DELTAf is detected as an error signal and an oscillation repetitive frequency is synchronized with (n.f) by controlling a temperature of the passive mode synchronizing semiconductor laser 101 by the error signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is used for optical communication and optical information processing using ultrafast optical pulses. In particular, it relates to setting the oscillation repetition frequency of a passive mode-locked semiconductor laser used as an ultrafast optical pulse light source.

[0002]

2. Description of the Related Art Conventionally, a passive mode-locked semiconductor laser is known which oscillates by self-excited pulse by utilizing a saturable absorption effect in a laser resonator without being modulated by an electric signal. As a method of synchronizing the oscillation repetition frequency of such a passive mode-locked semiconductor laser with an external signal, an optical pulse whose repetition frequency is locked is injected into the passive mode-locked semiconductor laser from the outside and the passive mode-locked semiconductor laser is injected. There is a method of forcibly synchronizing the mode-locked frequency of 1 to the repetition frequency of the injected light pulse.

[0003]

However, in such a conventional method, there is a problem that jitter due to dynamic behavior in the laser resonator is likely to occur and stable synchronization is difficult. Further, when the repetition frequency of the passive mode-locking semiconductor laser is several tens Hz or higher, it is difficult to generate the injection light pulse which is repeated at such a high frequency.

The present invention solves such a problem, and
It is an object of the present invention to provide a passive mode-locking semiconductor laser device that can be stably synchronized even at a repetition frequency of 0 GHz or higher.

[0005]

A passive mode-locking semiconductor laser device of the present invention includes a reference frequency generator for generating an electric signal of a reference frequency f, a passive mode-locking semiconductor laser for periodically oscillating, and this semiconductor. The laser oscillation repetition frequency is a frequency n · f that is a natural number n times the reference frequency f.
In a device provided with a frequency synchronization means for synchronizing with
The frequency synchronization means electrically oscillates at a frequency Δf, a means for generating an electric signal of a frequency f ± Δf from the output of the comparison frequency generator and the output of the reference frequency generator, and A reference laser that repeatedly generates a reference light pulse at the frequency of an electric signal, a signal light pulse with a repetition frequency n · f output from a passive mode-locked semiconductor laser, and a reference light pulse with a repetition frequency f ± Δf from the reference laser are input. And an optical correlation signal generator for generating a correlation signal of frequency n · Δf, a frequency multiplier for multiplying the output frequency Δf of the comparison frequency generator by n, an output of this frequency multiplier and an output of the optical correlation signal generator. And a means for adjusting the oscillation repetition frequency of the passive mode-locked semiconductor laser by an error voltage proportional to the detected phase difference. It is characterized in.

It is desirable that the means for adjusting the oscillation repetition frequency includes a Peltier element that electrically controls the temperature of the passive mode-locked semiconductor laser.

As the optical correlation signal generator, it is preferable to use a sum frequency generator using a second-order optical nonlinear medium having a waveguide structure. That is, the oscillation wavelength λ ₂ of the local laser
The leave oscillation wavelength lambda ₁ and different values of the optical pulse laser, wavelength optical correlation signal generator is a sum frequency component and a wavelength lambda ₁ of the signal light pulse and the wavelength lambda ₂ of the reference optical pulse lambda A sum frequency generator that generates light of ₁ · λ ₂ / (λ ₁ + λ ₂ ), a detection unit that detects the sum frequency component as an electric signal, and a frequency n · Δf from the detected electric signal.
And an electric filter for extracting the component of

Alternatively, a third-order optical nonlinear medium having the optical Kerr effect can be used as the optical correlation signal generator. That is, the optical correlation signal generator detects an optical Kerr switch that transmits the signal light pulse when the reference light pulse is incident at the same time as the signal light pulse, and the signal light pulse that has passed through the optical Kerr switch as an electrical signal. The configuration may include a detection unit and an electric filter that extracts a component of frequency n · Δf from the detected electric signal.

[0009]

The reference light pulse having the repetition frequency f + Δf or f−Δf is generated with respect to the repetition frequency n · f of the output signal light pulse of the passive mode-locked semiconductor laser.
Then, an optical correlation between the reference light pulse and the signal light pulse is obtained to obtain a correlated electrical signal of frequency n · Δf. On the other hand, n · Δ obtained by electrically multiplying the frequency Δf
The signal of f is generated in advance, the phase difference between the two electric signals of frequency n · Δf is detected as an error signal, and the temperature of the passive mode-locking semiconductor laser is controlled by this error signal.
As a result, the oscillation repetition frequency of the passive mode-locked semiconductor laser can be locked at n · f.

In the present invention, since the repetition frequency of the reference light pulse is set to a low value of about 1 / n of the signal light pulse,
The reference light pulse can be easily generated by using a normal active mode locking or other electrical modulation technique.

In this case, the upper limit of the repetition frequency of the passive mode-locked semiconductor laser which is phase-locked is determined by the response speed of the optical correlation signal generator used and the pulse width of the reference optical pulse. When a sum frequency generator using the second-order optical nonlinear effect or an optical Kerr switch using the third-order optical nonlinear effect is used as the optical correlation signal generator, the response speed can be set to about several picoseconds. Also, the width of the reference light pulse can be set to 1 to 2 picoseconds by applying active mode locking to the semiconductor laser which is the reference laser. Therefore, an operation of 10 picoseconds or less is possible as a whole, and it is possible to synchronize with a repetition frequency of at least about 100 GHz.

[0012]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention.

In this embodiment, a reference frequency generator 105 for generating an electric signal having a reference frequency f, a passive mode-locking semiconductor laser 101 for periodically oscillating a pulse, and an oscillation repetition frequency of the semiconductor laser 101 are used as reference frequencies. and a frequency synchronizing means for synchronizing with a frequency n · f that is a natural number n times f. Here, the feature of the present embodiment is that, as a frequency synchronizing means, a comparison frequency generator that electrically oscillates at a frequency Δf, and a frequency f + Δf is obtained from the output of this comparison frequency generator and the output of the reference frequency generator. A mixer 107, a bandpass filter 117 and a modulation driver 108 as means for generating an electric signal, a reference laser 104 for repeatedly generating a reference light pulse 116 at the frequency of the electric signal, and a passive mode-locked semiconductor laser 101.
Signal light pulse 115 of repetition frequency n · f output by
And the reference light pulse 116 of the repetition frequency f + Δf from the reference laser 104 are input, and the output frequency Δf of the optical correlation signal generator 111 and the comparison frequency generator 106 for generating a correlation signal of frequency n · Δf is multiplied by n. Frequency multiplier 112 and phase comparator 1 for detecting the phase difference between the output of the frequency multiplier 112 and the output of the optical correlation signal generator 111.
13 and a low pass filter 114 as a means for adjusting the oscillation repetition frequency of the passive mode-locking semiconductor laser 101 by an error voltage proportional to the detected phase difference, a Peltier element control circuit 103, and a Peltier element 102. At the input of the optical correlation signal generator 111, Er-doped optical fiber amplifiers 109 and 110 are provided for amplifying the signal light pulse 115 and the reference light pulse 116, respectively.

The passive mode-locked semiconductor laser is InGaA.
The sP semiconductor laser has a structure in which a saturable region is provided, and by applying passive mode locking, a wavelength of 1.5
At 5 μm, a signal light pulse having a repetition frequency of about 80 GHz and a pulse width of 1.1 picoseconds is output. The Peltier element 102 is the passive mode-locking semiconductor laser 101.
The temperature is controlled by the Peltier element control circuit 103.

The reference laser 104 is an InGaAsP-based semiconductor laser and has a wavelength of 1.53 μm when modulated by an electric signal.
m, a reference light pulse having a repetition frequency of about 20 GHz is generated.

The reference frequency generator 105 has a frequency f = 20.
A reference frequency signal of GHz is generated, and a comparison frequency generator 1
06 generates a comparison frequency of frequency Δf = 1 MHz.
The Er-doped optical fiber amplifiers 107 and 108 have a maximum saturated average output of about 100 mW, and signal optical pulse,
Amplify the reference light pulse.

The operation of this embodiment will be described.

The output of the reference frequency generator 105 and the output of the comparison frequency generator 106 are mixed by the mixer 107, and one of the sum frequency signal and the difference frequency signal, in this embodiment, the sum frequency is output by the band pass filter 117. The reference laser 104 is extracted and modulated by the modulation driver 108. Due to this modulation, the reference laser 104 is subjected to active mode locking, and the reference laser 104 outputs the reference light pulse 116. The repetition frequency of the reference light pulse 116 matches the sum frequency of the oscillation frequency f = 20 GHz of the reference frequency generator 105 and the oscillation frequency Δf = 1 MkHz of the comparison frequency generator 106. The pulse width of the reference light pulse 116 was measured to be about 1.2 picoseconds. The signal light pulse 115 and the reference light pulse 116 are amplified by the Er-doped optical fiber amplifiers 109 and 110, respectively, and input to the optical correlation signal generator 111. The optical correlation signal generator 111 calculates the correlation between two optical pulses to obtain a frequency n · Δ.
Generate a correlation signal of f = 4 MHz.

The output of the comparison frequency generator 106 is also branched and input to the frequency multiplier 112. The frequency multiplier 112 multiplies the frequency of the input signal by n = 4,
Generate a reference signal of Δf = 4 MHz.

The correlation signal output from the optical correlation signal generator 111 and the reference signal output from the frequency multiplier 112 are input to the phase comparator 113. The phase comparator 113 detects the phase difference (instantaneous frequency difference) between the two. This phase difference is between the phase difference between the signal light pulse 115 and the reference light pulse 116, that is, the synchronization frequency of the passive mode-locking semiconductor laser 101 and the value n = 4 times the reference frequency from the reference frequency generator 105. It corresponds to the difference.

The output of the phase comparator 113 is input to the low pass filter 114. The low pass filter 114 extracts an error voltage proportional to the phase difference between the correlation signal and the reference signal from the output of the phase comparator 113, and supplies this to the Peltier element control circuit 103. Peltier element control circuit 10
3 finely changes the temperature of the Peltier element 102 according to this error voltage. Due to this temperature change, the effective length of the laser resonator of the passive mode-locking semiconductor laser 101 is minutely changed, and the synchronization frequency is minutely changed.

Here, the operation of the Peltier element control circuit 103 is set so that the change of the synchronization frequency of the passive mode-locking semiconductor laser 101 due to the application of the error voltage acts in the direction of reducing the phase difference detected by the phase comparator 113. That is, the loop composed of the passive mode-locking semiconductor laser 101, the optical correlation signal generator 111, the phase comparator 113, and the passive mode-locking semiconductor laser 101 works as a phase-locked loop. This phase-locked loop is stable in that the synchronization frequency of the passive mode-locking semiconductor laser 101 becomes equal to four times the reference frequency generated by the reference frequency generator 105. Therefore, when the loop becomes stable, the synchronization frequency of the passive mode-locking semiconductor laser 101 is synchronized with a value that is four times the reference frequency 20 GHz output from the reference frequency generator 105, that is, 80 GHz.

FIG. 2 is a block diagram showing an example of the optical correlation signal generator. Here, an example using a sum frequency generator will be described.

This optical correlation signal generator uses the signal light pulse 2
01 (corresponding to 115 in FIG. 1) and the reference light pulse 2
02 (corresponding to 116 in FIG. 1) and λ /
Two-phase plates 203 and 204, half mirror 205, coupling lens 206, sum frequency generator 207, collimating lens 2
08, a wavelength filter 209 that transmits only light having a wavelength of 0.77 μm, a photodetector 210, and a cutoff frequency of 5 MH
an electrical low pass filter 211 of z
Output the correlation signal 212 of Δf = 4 MHz. As the photodetector 210, for example, a Si avalanche photodiode (APD) is used.

The sum frequency generator 207 diffuses Ti into a MgO: LiNbO ₃ crystal substrate having a crystal a-axis in the light propagation direction, a b-axis parallel to the substrate surface, and a −c-axis perpendicular to the substrate surface. Then, an optical waveguide is formed. C on the substrate surface
Periodic domain inversion is formed on the substrate by periodically irradiating the electron beam at a pitch d parallel to the b-axis from the axial direction, and the quasi-phase matching condition for generating the sum frequency is established. There is. The pitch d at which the pseudo phase matching is established is expressed by the following equation.

D = | N ₃ / λ ₃ −N ₂ / λ ₂ −N ₁ / λ ₁ | ⁻¹ (1) In this formula, λ ₁ is the wavelength of the signal light pulse, and λ ₂ is the reference light pulse. The wavelength, λ ₃ , represents the wavelength of the sum frequency of the two lights, and N ₁ , N ₂ , N ₃ are the wavelengths λ ₁ , λ ₂ , λ ₃ , respectively.
Represents the effective refractive index for the c-axis polarized wave at.

Here, λ ₁ = 1.55 μm and λ ₂ = 1.
Assuming that the sum frequency wavelength λ ₃ is 53 μm, the reciprocal of the wavelength is proportional to the frequency, and therefore 1 / λ ₁ + 1 / λ ₂ = 1 / λ ₃
And λ ₃ = 0.77 μm. At this time, if d = 20 μm is set, a quasi phase matching condition that a sum frequency component polarized on the c-axis is generated by the signal light pulse 201 polarized on the c-axis and the reference light pulse 202 is obtained.

The generation of the optical correlation signal by the sum frequency generator 207 will be described in more detail.

Signal light pulse 201 and reference light pulse 2
The 02 polarized light is made to coincide with the c-axis direction by the λ / 2 phase plates 203 and 204, combined by the half mirror 205, and then incident on the sum frequency generator 207 via the coupling lens 206. Since the quasi-phase matching condition is satisfied, the sum frequency generator 207 outputs light having a wavelength of 0.77 μm, which is the sum frequency component of the signal light pulse 201 and the reference light pulse 202.

In terms of the process of generating the sum frequency, the intensity of the sum frequency component generated increases as the degree of temporal overlap between the signal light pulse 201 and the reference light pulse 202 increases. In the present embodiment, since the signal light pulse 201 and the reference light pulse 202 have different repetition frequencies, the degree of overlap of the two pulses periodically fluctuates, and the beat component (correlation of the light intensity) corresponding to the fluctuation of the overlap is correlated. Signal) is superimposed on the sum frequency. Since the time width of the reference light pulse 202 is sufficiently narrow as the time width of the signal light pulse 201, the beat frequency generated is the repetition frequency of the signal light pulse 201 and the harmonic frequency of the reference pulse 202 (of the repetition frequency n times). Therefore, in the setting of this example, the beat frequency is 4 · Δf = 4 MHz.

The output light of the sum frequency generator 207 is collimated by the collimator lens 208, and the wavelength filter 2
After only the sum frequency component is extracted by 09, it is converted into an electric signal by the photodetector 210. The correlation signal (frequency 4 · Δf) in the detected electric signal is the low-pass filter 21.
Taken out by 1.

As described above, the sum frequency generator is used in FIG.
By configuring the optical correlation signal generator 111 in, the correlation signal between the signal light pulse 115 and the reference light pulse 116 can be detected.

FIG. 3 is a block diagram showing another example of the optical correlation signal generator. This example uses an optical Kerr switch that utilizes the third-order nonlinear effect.

This optical correlation signal generator has a wavelength of 1.55 μm.
m signal light pulse 301 and a reference light pulse 302 having a wavelength of 1.53 μm as input, and λ / 2 phase plates 303, 30
4, polarizer 305, half mirror 306, coupling lens 3
07, optical nonlinear optical medium 308, collimator lens 30
Wavelength filter 31 that transmits only 9 and 1.55 μm light
0, an analyzer 311, a photodetector 312, and a low-pass filter 313 having a cutoff frequency of 5 MHz, and a frequency of 4
Output the correlation signal 315 of Δf = 4 MHz. The optical nonlinear optical medium 308 is, for example, As ₂ S having a length of 1 m.
₃ system fiber is used. As the photodetector 312, for example, Ge: APD is used.

In such a configuration, the transmission polarization direction of the polarizer 305 is 45 from the x-axis of the optical nonlinear optical medium 308.
The polarization direction of the analyzer 311 is shifted by 90 degrees with respect to the polarization direction of the polarizer 305 while being aligned with the tilted direction.
By adjusting the λ / 2 phase plate 303 to match the polarization direction of the signal light pulse 301 with the polarization transmission direction of the polarizer 305, the signal light pulse 301 propagates through the polarizer 305 to the optical nonlinear optical medium 308. However, this signal light pulse 301 is emitted from the non-linear optical medium 308, collimated by the collimator lens 309 and transmitted through the wavelength filter 310, but is blocked by the analyzer 311 whose polarization transmission directions differ by 90 degrees.

In this state, the λ / 2 phase plate 304 is adjusted,
The reference light pulse 302 is incident on the optical nonlinear optical medium 308 with its polarization direction aligned with the y-axis polarization. Reference light pulse 3
If the intensity of 02 is somewhat high, the reference light pulse 30
The optical Kerr effect is generated by 2 and the optical nonlinear optical medium 30
Birefringence occurs within 8. Due to this birefringence, the polarization of the signal light pulse 301 propagating in an overlapping manner with the reference light pulse 302 becomes elliptical, and polarization components different in 90 ° direction, that is, orthogonal polarization components are generated. This orthogonal polarization component is emitted from the optical nonlinear optical medium 308 and passes through the analyzer 311.
The reference light pulse 302 is blocked by the wavelength filter 310.

That is, in the absence of the reference light pulse 302, the signal light pulse 301 incident in the polarization direction 316a inclined by 45 degrees from the x-axis of the optical nonlinear optical medium 308 maintains the polarization direction and the nonlinear optical medium 308 is maintained. And is blocked by the analyzer 311. On the other hand, when the reference light pulse 302 is incident in the polarization direction 316b that coincides with the y-axis of the optical nonlinear optical medium 308, at the output end of the optical nonlinear optical medium 308, the signal light is overlapped with the reference optical pulse 302. Polarization direction 316b for pulse 301
A component of a polarization direction 317a that is orthogonal to is generated. this is,
This means that the signal light pulse 301 is switched by the reference light pulse 302. Only this switched light pulse 314 is incident on the photodetector 312.

The intensity of the orthogonal polarization component obtained by such switching depends on the degree of temporal overlap between the signal light pulse 301 and the reference light pulse 302. The stronger the overlap, the stronger the intensity of the orthogonal polarization component. . Since the signal light pulse 301 and the reference light pulse 302 have different repetition frequencies, the difference in the repetition frequencies of these two lights (including the component of the difference with the harmonic frequency) causes the degree of overlap of the two pulses to vary. Change. This periodic change appears as a change in the intensity of the envelope of orthogonal polarization components. Therefore, the orthogonal polarization component can be converted into an electric signal by the photodetector 312, and the low frequency component can be extracted through the low pass filter 313 to detect the correlation signal.

As described above, the correlation signal between the signal light pulse and the reference light pulse can be detected by using the optical Kerr switch.

Further, here, as an optical nonlinear optical medium, A
Although the s ₂ S ₃ system fiber has been described as an example, a silica system fiber, an organic material waveguide, or the like can be similarly used as long as it has a third-order optical nonlinear effect.

[0042]

As described above, in the passive mode-locking semiconductor laser device of the present invention, the optical processing for detecting the correlation signal between the optical pulses is combined with the electrically generated reference frequency signal. This makes it possible to synchronize the repetition frequency of the passive mode-locking semiconductor laser with an external electric signal.

[Brief description of drawings]

FIG. 1 is a block diagram showing a passive mode-locking semiconductor laser device according to an embodiment of the present invention.

FIG. 2 is a configuration diagram showing an example of an optical correlation signal generator,
The figure which shows the example which used the sum frequency generator.

FIG. 3 is a configuration diagram showing another example of the optical correlation signal generator, showing an example using an optical Kerr switch.

[Explanation of symbols]

 101 Passive Mode-Locked Semiconductor Laser 102 Peltier Element 103 Peltier Element Control Circuit 104 Reference Laser 105 Reference Frequency Generator 106 Comparative Frequency Generator 107 Mixer 108 Modulation Driver 109, 110 Er-Doped Optical Fiber Amplifier 111 Optical Correlation Signal Generator 112 Frequency Multiplier 113 phase comparator 114 low-pass filter 115 signal light pulse 116 reference light pulse 117 band pass filter 201 signal light pulse 202 reference light pulse 203, 204 λ / 2 phase plate 205 half mirror 206 coupling lens 207 sum frequency generator 208 collimate Lens 209 Wavelength filter 210 Photodetector 211 Low pass filter 212 Correlation signal 301 Signal light pulse 302 Reference light pulse 303, 304 λ / 2 phase plate 305 Polarizer 306 Mirror 307 Coupling lens 308 Non-linear optical medium 309 Collimating lens 310 Wavelength filter 311 Analyzer 312 Photodetector 313 Low-pass filter 314 Switched optical pulse 315 Correlation signal

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Shigeo Ishibashi 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (4)

[Claims] 1. A reference frequency generator (105) for generating an electric signal of a reference frequency f, a passive mode-locking semiconductor laser (101) which periodically oscillates a pulse, and an oscillation repetition frequency of the semiconductor laser is the reference frequency. In a passive mode-locking semiconductor laser device comprising a frequency synchronization means for synchronizing with a frequency (n · f) which is a natural number n times f, the frequency synchronization means is a comparison frequency generator (10) that electrically oscillates at a frequency Δf.
6) and the output of the comparison frequency generator and the output of the reference frequency generator from the frequency f + Δf or f−Δf (hereinafter f ± Δf).
means (107, 1) for generating an electric signal of
17, 108), a reference laser (104) that repeatedly generates a reference light pulse at the frequency of the electric signal, a signal light pulse having a repetition frequency n · f output from the passive mode-locking semiconductor laser, and the reference laser from the reference laser. An optical correlation signal generator (1 that receives a reference light pulse having a repetition frequency f ± Δf as an input and generates a correlation signal having a frequency n · Δf
11), a frequency multiplier (112) for multiplying the output frequency Δf of the comparison frequency generator by n, and a phase comparator for detecting a phase difference between the output of the frequency multiplier and the output of the optical correlation signal generator. (113) and means (114, 103, 102) for adjusting the oscillation repetition frequency of the passive mode-locking semiconductor laser by an error voltage proportional to the detected phase difference. apparatus. 2. The passive mode-locking semiconductor laser device according to claim 1, wherein the adjusting means includes a Peltier element (102) for electrically controlling the temperature of the passive mode-locking semiconductor laser. 3. The oscillation light wavelength λ ₂ of the reference laser is set to a value different from the oscillation light wavelength λ ₁ of the optical passive mode-locking semiconductor laser, and the optical correlation signal generator outputs a signal light pulse of wavelength λ _1. And the reference light pulse of wavelength λ ₂ and the sum frequency component of wavelength λ ₁ · λ ₂ / (λ ₁ + λ
_2. A sum frequency generator for generating the light of ₂ ), a detection means for detecting the sum frequency component as an electric signal, and an electric filter for extracting a component of frequency n · Δf from the detected electric signal. 2. The passive mode-locked semiconductor laser device described in 2. 4. The optical correlation signal generator comprises: an optical Kerr switch that transmits the signal light pulse when a reference light pulse is incident at the same time as the signal light pulse; and an optical Kerr switch that transmits the signal light pulse that passes through the optical Kerr switch. 3. The passive mode-locked semiconductor laser device according to claim 1, further comprising: a detection unit that detects the signal, and an electric filter that extracts a component of frequency n · Δf from the detected electric signal.