JP3092757B2 - Optical pulse laser frequency division synchronization signal generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for optical communication and optical information processing using ultrafast optical pulses. In particular, it relates to generation of a frequency-divided signal used for synchronization such as multiplexing and demultiplexing of optical signals.

[0002]

2. Description of the Related Art In recent years, research has been made on using output light of a laser that repeatedly oscillates pulses for optical communication and optical information processing. Such repetitive pulse oscillation can be self-excitedly generated by utilizing the saturable absorption effect in the laser resonator and the like without using modulation by an electric signal. Such a laser that oscillates by self-pulsation is referred to as an optical pulse laser in this specification.

When the output light of such an optical pulse laser is used for optical communication or optical signal processing, it is necessary to generate a frequency-divided electric signal having a frequency f / n in synchronization with the pulse oscillation repetition frequency f. . n is a natural number. In a conventional frequency synchronization signal generating apparatus for an optical pulse laser, an optical pulse is directly converted into an electric signal using a photodetector, and a repetition frequency component contained therein is converted to 1 / n using an electric frequency dividing circuit.
To generate the divided frequency f / n.

[0004]

However, in such a conventional frequency-divided synchronizing signal generating device, when the optical pulse repetition frequency exceeds 10 GHz, the response speed of the electric circuit is not so fast, so that a stable electric There was a drawback that the circumference was difficult and accurate phase synchronization was difficult.

[0005] The present invention solves such a problem and solves the problem.
It is an object of the present invention to provide an optical pulse laser frequency-division synchronous signal generating device that stably generates a frequency-divided electric signal phase-locked even with a repetition frequency of GHz or more.

[0006]

According to the present invention, a frequency division synchronization signal generating apparatus for an optical pulse laser receives a signal light pulse from an optical pulse laser that oscillates periodically and receives a repetition frequency f of the signal light pulse. A frequency division synchronous signal generator for an optical pulse laser that generates an electric signal of a frequency f / n synchronized with a local laser that periodically generates a reference light pulse at the frequency of an externally applied electric modulation signal; Is set to approximately the frequency f / n, and the oscillation frequency is changed by an externally applied voltage, a comparison signal oscillator electrically oscillates at the frequency Δf, an output of the comparison signal oscillator and the voltage control oscillator Means for generating an electric modulation signal having a frequency of f / n ± Δf from the output of the optical pulse and applying the signal to a local laser; An optical correlation signal generator that receives a light pulse and a reference light pulse having a repetition frequency f / n ± Δf from a local laser and generates a correlation signal of a frequency n · Δf, and multiplies the output frequency Δf of a comparison signal oscillator by n times Frequency multiplier, a phase comparator for detecting the phase difference between the output of the frequency multiplier and the output of the optical correlation signal generator, and the oscillation frequency of the voltage controlled oscillator based on an error voltage proportional to the detected phase difference. Feedback means for controlling the oscillation frequency and synchronizing the oscillation frequency with the repetition frequency f of the signal light pulse.

As the optical correlation signal generator, a sum frequency generator using a secondary optical nonlinear medium having a waveguide structure is preferably used. That is, the oscillation light 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 Sum frequency generator for generating light of ₁ · λ ₂ / (λ ₁ + λ ₂ ), detecting means for detecting the sum frequency component as an electric signal, and frequency n · Δf from the detected electric signal
And an electric filter that extracts the components of

Alternatively, a third-order optical nonlinear medium having an 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 simultaneously with the signal light pulse, and detects the signal light pulse transmitted through the optical Kerr switch as an electric signal. A configuration including a detecting unit and an electric filter for extracting a component of the frequency n · Δf from the detected electric signal may be employed.

[0009]

A voltage-controlled oscillator having an oscillation frequency near f / n and a comparison signal oscillator oscillating at a low frequency Δf are provided, and the local laser is modulated with a mixed wave (electric signal) of the two, so that the repetition frequency is f / n. A reference light pulse of n ± Δf is generated. Then, an optical correlation between the reference light pulse and the signal light pulse is obtained to obtain a correlated electric signal having a frequency n · Δf. On the other hand, the output of the comparison signal oscillator is electrically multiplied to obtain an electric signal of the same frequency n · Δf. The phase difference between the two electric signals having the frequency n · Δf is detected as an error signal, and the error signal is fed back to the voltage controlled oscillator. As a result, a frequency-divided electric signal having a frequency f / n synchronized with the repetition frequency f of the signal light pulse is generated from the voltage controlled oscillator.

In the present invention, since the phase of the signal light pulse and the phase of the reference light pulse are compared with each other with a low frequency component (frequency Δf), the electric circuit sufficiently follows the low frequency component with its response time. it can. In this case, the upper limit of the repetition frequency at which phase synchronization is performed is determined by the response speed of the optical correlation signal generator used and the pulse width of the reference light 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 reduced to 1 to 2 picoseconds by applying active mode locking to the semiconductor laser which is a local laser. Therefore, an operation of 10 picoseconds or less is possible as a whole, and it is possible to apply phase synchronization to a repetition frequency of at least about 100 GHz.

[0011]

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

The apparatus of this embodiment receives a signal light pulse 113 from an optical pulse laser 101 which periodically oscillates pulses and receives an electric signal having a frequency f / n synchronized with a repetition frequency f of the signal light pulse 113, that is, an electric signal. A local laser 102 that periodically generates a reference light pulse 114 at the frequency of an externally applied electric modulation signal, and a oscillating frequency that is substantially set to f / n. A voltage-controlled oscillator 103 whose oscillation frequency changes according to the voltage applied from the
and a means for generating an electric modulation signal having a frequency f / n ± Δf from the output of the comparison signal oscillator 104 and the output of the voltage controlled oscillator 103 and adding the signal to the local laser 102. Mixer 10
5. Bandpass filter 116 and modulation driver 106
And a signal light pulse 113 having a repetition frequency f from the optical pulse racer 101 and a reference light pulse 114 having a repetition frequency f / n ± Δf from the local laser 102 to generate a correlation signal having a frequency n · Δf. The output frequency Δf of the signal generator 109 and the comparison signal oscillator 104
A frequency multiplier 110 that multiplies the frequency multiplier by n
And a phase comparator 111 for detecting a phase difference between the output of the optical control signal generator 109 and the output of the optical correlation signal generator 109. The oscillation frequency of the voltage controlled oscillator 103 is controlled by an error voltage proportional to the detected phase difference. As feedback means for synchronizing the oscillation frequency with the repetition frequency f of the signal light pulse, the output of the phase comparator 111 is connected to the voltage controlled oscillator 103 via the low-pass filter 112. Optical correlation signal generator 10
The input 9 is provided with Er-doped optical fiber amplifiers 107 and 108 for amplifying the signal light pulse 113 and the reference light pulse 114, respectively.

The optical pulse laser 101 has a structure in which a saturable region is provided in an InGaAsP-based semiconductor laser.
By applying passive colliding pulse mode locking, a signal light pulse having a repetition frequency of about 40 GHz and a pulse width of 1.1 picoseconds at a wavelength of 1.55 μm is output.

The local laser 102 is an InGaAsP-based semiconductor laser. Active mode locking is performed by modulation with an electric signal of about 10 GHz, and the local laser 102 has a wavelength of 1.5 GHz.
A reference light pulse of 3 μm and a repetition frequency of about 10 GHz is generated.

The free-running oscillation frequency of the voltage controlled oscillator 103 is 10 GHz, the frequency Δf of the signal generated by the comparison signal oscillator 104 is 100 kHz, and the Er-doped optical fiber amplifier 1
The maximum saturation average power of the power amplifiers 07 and 108 is about 100 mW.

The operation of this embodiment will be described.

The output of the voltage control generator 103 and the output of the comparison signal oscillator 104 are mixed by the mixer 105, and one of the sum frequency signal and the difference frequency signal, in this embodiment, the sum frequency signal is band-passed. The signal is extracted by the filter 116 and modulated by the modulation driver 106 to the local laser 102. Due to this modulation, active mode locking is applied, and the local laser 102 receives the reference light pulse 11
4 is output. The repetition frequency of the reference light pulse 114 is equal to the oscillation frequency f / n (10 G
Hz) and the oscillation frequency Δf (1
00 kHz). Reference light pulse 1
The pulse width of 14 was about 1.2 picoseconds as measured. The signal light pulse 113 and the reference light pulse 114 are
The signals are amplified by the Er-doped optical fiber amplifiers 107 and 108 and input to the optical correlation signal generator 109. The optical correlation signal generator 109 correlates the two optical pulses and generates a correlation signal having a frequency n · Δf = 400 kHz.

The output of the comparison signal oscillator 104 is also branched and input to the frequency multiplier 110. The frequency multiplier 110 multiplies the frequency of the input signal by n = 4, and
A reference signal of Δf = 400 kHz is output.

The correlation signal output from the optical correlation signal generator 109 and the reference signal output from the frequency multiplier 110 are input to a phase comparator 111. The phase comparator 111 detects a phase difference between them (an instantaneous frequency difference). This phase difference corresponds to the phase difference between the signal light pulse 113 and the reference light pulse 114, that is, the difference between the repetition frequency of the signal light pulse and n = 4 times the oscillation frequency of the voltage controlled oscillator 103. .

The output of the phase comparator 111 is input to a low-pass filter 112. The low-pass filter 112 extracts an error voltage proportional to the phase difference between the correlation signal and the reference signal from the output of the phase comparator 111, and supplies this to the control voltage terminal of the voltage controlled oscillator 103.

Due to the error voltage input to the control voltage terminal, the oscillation frequency of the voltage controlled oscillator 103 fluctuates in the direction in which the phase difference between the correlation signal and the reference signal decreases. That is, the voltage controlled oscillator 103 → the local laser 102 →
A loop composed of the optical correlation signal generator 109 → the phase comparator 111 → the low pass filter 112 → the voltage controlled oscillator 103 functions as a phase locked loop. This phase locked loop is stable in that the quadrupled value of the oscillation frequency of the voltage controlled oscillator 103 becomes equal to the repetition frequency of the signal light pulse 113. Therefore, when the loop is stabilized, the repetition frequency is synchronized with the signal light pulse of 40 GHz, and 1
The 10 GHz frequency-divided synchronization signal 115 divided by / 4 is output from the voltage controlled oscillator 103.

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 generates a signal light pulse 2
01 (corresponding to 113 in FIG. 1) and reference light pulse 2
02 (corresponding to 114 in FIG. 1) as input, and λ /
2-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 700
It has an electrical low-pass filter 211 of kHz and outputs a correlation signal 212 having a frequency of 4 · Δf (400 kHz). 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 the a-axis of the crystal in the light propagation direction, the b-axis parallel to the substrate surface, and the −c axis perpendicular to the substrate surface. Thus, an optical waveguide is formed. On the substrate surface, c
Periodic domain inversion is formed on the substrate by periodically irradiating an electron beam at a pitch d parallel to the b-axis from the axial direction, and the quasi-phase matching condition for sum frequency generation is established. I have. The pitch d at which the quasi-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 equation, λ ₁ is the wavelength of the signal light pulse, and λ ₂ is the wavelength of the reference light pulse. The wavelength λ ₃ represents the wavelength of the sum frequency of the two lights, and N ₁ , N ₂ , and N ₃ represent the wavelengths λ ₁ , λ ₂ , and λ ₃ , respectively.
Represents the effective refractive index for the c-axis polarized wave.

Here, λ ₁ = 1.55 μm, λ ₂ = 1.
If it is 53 μm, the sum frequency wavelength λ ₃ is 1 / λ ₁ + 1 / λ ₂ = 1 / λ ₃ because the reciprocal of the wavelength is proportional to the frequency.
Λ ₃ = 0.77 μm. At this time, if d is set to 20 μm, a quasi-phase matching condition that a c-axis polarized sum frequency component is generated by the c-axis polarized signal light pulse 201 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 polarization of 02 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 enters 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 a sum frequency component of the signal light pulse 201 and the reference light pulse 202.

When the sum frequency generation process is viewed in terms of time, the greater the degree of temporal overlap between the signal light pulse 201 and the reference light pulse 202, the greater the intensity of the generated sum frequency component. In the present embodiment, since the repetition frequency is different between the signal light pulse 201 and the reference light pulse 202, the degree of overlap between the two pulses fluctuates periodically, and the beat component of the light intensity corresponding to the change in the overlap (correlation) Signal) is superimposed on the sum frequency. Since the time width of the reference light pulse 202 is sufficiently narrow as much as the time width of the signal light pulse 201, the generated beat frequencies are the repetition frequency of the signal light pulse 201 and the harmonic frequency (repetition frequency) of the reference light pulse 202. N times). Therefore, in the setting in this example, the beat frequency is 4 · Δf (400 kHz).

The output light of the sum frequency generator 207 is collimated by a collimating lens 208 and
After only the sum frequency component is extracted in step 09, it is converted into an electric signal by the photodetector 210. The correlation signal (frequency 4.Δf) in the detected electric signal is converted to a low-pass filter 211.
Is taken out by

As described above, the sum frequency generator shown in FIG.
, The correlation signal between the signal light pulse 113 and the reference light pulse 114 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 utilizing a third-order nonlinear effect.

This optical correlation signal generator has a wavelength of 1.55 μm.
An m signal light pulse 301 and a 1.53 μm wavelength reference light pulse 302 are input, and λ / 2 phase plates 303 and 30 are input.
4, polarizer 305, half mirror 306, coupling lens 3
07, optical nonlinear optical medium 308, collimating lens 30
9, a wavelength filter 31 that transmits only 1.55 μm light
0, an analyzer 311, a photodetector 312, and a low-pass filter 313 having a cutoff frequency of 700 kHz, and output a correlation signal 315 having a frequency of 4 · Δf (400 kHz). As the optical nonlinear optical medium 308, for example, a length of 1 m
As ₂ S ₃ 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 set at 45 degrees from the x-axis of the optical nonlinear optical medium 308.
The polarization direction of the analyzer 311 is shifted by 90 degrees from the polarization direction of the polarizer 305 in accordance with the inclination direction.
The signal light pulse 301 propagates through the polarizer 305 to the optical nonlinear optical medium 308 by adjusting the λ / 2 phase plate 303 so that the polarization direction of the signal light pulse 301 matches the polarization transmission direction of the polarizer 305. However, the signal light pulse 301 is emitted from the nonlinear optical medium 308, is collimated by the collimating lens 309, and passes through the wavelength filter 310, but is blocked by the analyzer 311 whose polarization transmission direction is different 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 adjusted to the y-axis polarization. Reference light pulse 3
02, the reference light pulse 30
2 causes an optical Kerr effect, and the optical nonlinear optical medium 30
Birefringence occurs in 8. Due to the birefringence, the polarization of the signal light pulse 301 propagating with the reference light pulse 302 is made elliptical, and a polarization component different in the 90-degree direction, that is, an orthogonal polarization component is generated. The 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 state where there is no reference light pulse 302, the signal light pulse 301 incident in the polarization direction 316a inclined at 45 degrees from the x-axis of the optical nonlinear optical medium 308 changes the nonlinear optical medium 308 while maintaining its polarization direction. And is cut off by the analyzer 311. On the other hand, when the reference light pulse 302 is incident in the polarization direction 316b coincident 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 overlaps with the reference optical pulse 302 at the output end. Polarization direction 316b to pulse 310
A component in the polarization direction 317a orthogonal to the above occurs. this is,
This means that the signal light pulse 301 has been switched by the reference light pulse 302. Only the switched light pulse 314 enters 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, and the intensity of the orthogonal polarization component increases as the overlap increases. . Since the repetition frequency is different between the signal light pulse 301 and the reference light pulse 302, the degree of overlap between the two pulses is determined by the difference between the two light's repetition frequencies (including the difference component from the harmonic frequency). Change. This periodic change appears as a change in the intensity of the envelope of the orthogonal polarization component. Therefore, the orthogonal polarization component is converted into an electric signal by the photodetector 312, and the low-frequency component is extracted through the low-pass filter 313, whereby the correlation signal can be detected.

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.

Here, A is used as the optical nonlinear optical medium.
Although an s ₂ S ₃ -based fiber has been described as an example, the present invention can be similarly implemented using a silica-based fiber or an organic material waveguide as long as it has a third-order optical nonlinear effect.

[0040]

As described above, the frequency-dividing synchronization signal generating apparatus for an optical pulse laser according to the present invention does not convert a signal light pulse into an electric signal and then electrically divides the signal. The frequency is divided by combining the processing. Therefore, the frequency-divided signal can be generated in synchronization with the repetition frequency of the optical pulse repeatedly output from the passive mode-locked laser without imposing a load on the electric circuit.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a frequency-divided synchronization signal generator according to an embodiment of the present invention.

FIG. 2 is a configuration diagram illustrating an example of an optical correlation signal generator;
The figure which shows the example using 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]

 Reference Signs List 101 optical pulse laser 102 local laser 103 voltage controlled oscillator 104 comparison signal oscillator 105 mixer 106 modulation driver 107, 108 Er-doped optical fiber amplifier 109 optical correlation signal generator 110 frequency doubler 111 phase comparator 112 low-pass filter 113 signal light Pulse 114 reference light pulse 115 frequency division synchronization signal 116 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 collimating lens 209 wavelength filter 210 light detection Device 211 low-pass filter 212 correlation signal 301 signal light pulse 302 reference light pulse 303, 304 λ / 2 phase plate 305 polarizer 306 half mirror 307 coupling lens 308 Form optical medium 309 collimating lens 310 wavelength filter 311 analyzer 312 photodetector 313 low-pass filter 314 switched optical pulse 315 correlation signal

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI H04B 10/152 (72) Inventor Shigeo Ishibashi 1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) Reference Document JP-A-52-8879 (JP, A) JP-A-5-327493 (JP, A) JP-A-63-10218 (JP, A) JP-A-4-106989 (JP, A) 13981 (JP, A) Patent 3055735 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 3/00-4/00 H01S 5/00-5/50 H03L 7/08 H04B 10/04 H04B 10/06 H04B 10/142 H04B 10/152 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. A signal light pulse from an optical pulse laser that periodically oscillates is input, and an electric signal having a frequency f / n (n is a natural number) synchronized with a repetition frequency f of the signal light pulse is generated. In a frequency-divided synchronization signal generating apparatus for an optical pulse laser, a local laser (102) that periodically generates a reference optical pulse at the frequency of an externally applied electric modulation signal, and an oscillation frequency set to approximately the frequency f / n A voltage-controlled oscillator (103) whose oscillation frequency changes according to an externally applied voltage; and a comparison signal oscillator (10) electrically oscillating at a frequency Δf.
4) and the frequency f / n + Δf or f / n−Δf (hereinafter f / n−Δf) from the output of the comparison signal oscillator and the output of the voltage-controlled oscillator.
/ N ± Δf) (105, 116, 10)
6), a signal light pulse having a repetition frequency f from the optical pulse racer and a repetition frequency f / n from the local laser.
An optical correlation signal generator (109) that receives a reference light pulse of ± Δf and generates a correlation signal of a frequency n · Δf; and a frequency multiplier (110) that multiplies the output frequency Δf of the comparison signal oscillator by n. A phase comparator (111) for detecting a phase difference between the output of the frequency multiplier and the output of the optical correlation signal generator; and an error voltage proportional to the detected phase difference. And a feedback means for controlling the oscillation frequency to be synchronized with the repetition frequency f of the signal light pulse. 2. The oscillation light wavelength λ ₂ of the local laser is set to a value different from the oscillation light wavelength λ ₁ of the optical pulse laser, and the optical correlation signal generator generates a signal light pulse having a wavelength λ ₁ and a wavelength λ _{1 The} wavelength λ ₁ · λ ₂ / (λ ₁ + λ) which is the sum frequency component of the _two reference light pulses
_2. A sum frequency generator for generating the light of ₂ ), detecting means for detecting the sum frequency component as an electric signal, and an electric filter for extracting a component of a frequency n · Δf from the detected electric signal. Frequency pulse synchronization signal generator for optical pulse laser. 3. The optical correlation signal generator, comprising: an optical Kerr switch that transmits a signal light pulse when a reference light pulse is incident simultaneously with a signal light pulse; and an optical Kerr switch that transmits the signal light pulse that has passed through the optical Kerr switch. 2. The optical pulse laser frequency division synchronizing signal generating apparatus according to claim 1, further comprising: detecting means for detecting the signal as a signal; and an electric filter for extracting a component of a frequency n · Δf from the detected electric signal.