JPH0637377A - Phase-locked optical pulse generator - Google Patents

Abstract

PURPOSE:To provide a phase-locked optical pulse generator which can stably generate a demultiplied optical pulse accurately phase-synchronized with a repetition frequency of 10GHz or more. CONSTITUTION:A signal optical pulse and an output light of a semiconductor laser 1 are incident to optical intensity modulators 3a, 3b in which a modulation signal of a frequency deviated by an infinitesimal frequency DELTAf with respect to a repetition frequency f of the signal optical pulse is inputted, and intensity- modulated to generate a correlation component of the frequency DELTAf, which is converted to an electric signal through photodiodes 7a, 7b, low pass filters 8a, 8b, etc., and an error voltage proportional to a phase difference of both is generated by a phase comparator 10. After it is regulated to a voltage responsive to a demultiplying ratio by a variable attenuator 11, it is supplied to a voltage-controlled oscillator 2 to control its oscillation frequency thereby to control a driver 13 of the laser 1, thereby synchronizing the phase of the signal optical pulse with that of a demultiplied optical pulse.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase-locked optical pulse generator used for timing extraction for optical gate switches in optical communication and optical information processing using ultrafast optical pulses.

[0002]

2. Description of the Related Art In a conventional phase-locked optical pulse generator of this type, the phase is synchronized with a signal light pulse having a wavelength of λ ₁ and a repetition frequency of f, and the repetition frequency is f / n at a wavelength of λ _2.
(However, n is a natural number)
The signal light pulse is converted into an electric signal by using a photodetector, the component of the repetition frequency f contained therein is divided into 1 / n by an electric frequency dividing circuit, and the divided division is repeated. A phase-divided divided optical pulse is generated by modulating a laser oscillating at a wavelength λ ₂ with an electric signal of frequency f / n.

[0003]

However, in such a conventional phase-locked optical pulse generator, when the repetition frequency exceeds 10 GHz, the response speed of the electric circuit is not so fast, so that stable frequency division is difficult and the accuracy is high. There was a problem that it was difficult to achieve phase synchronization.

In view of the above-mentioned conventional problems, the present invention provides a 10G
It is an object of the present invention to provide a phase-locked optical pulse generator capable of stably generating a frequency-divided optical pulse that is accurately phase-locked even at a repetition frequency of Hz or higher.

[0005]

Since the present invention SUMMARY OF THE INVENTION To achieve the above object, a wavelength repetition frequency lambda ₁ is phase synchronized with the signal light pulse f, wavelength repetition frequency lambda ₂ f
In a phase-locked optical pulse generator that generates a divided optical pulse of / n (where n is a natural number), oscillates at a wavelength λ ₂ ,
A phase-locked optical pulse laser that generates an optical pulse that repeats at the frequency of an electrical modulation signal applied from the outside, and a free-running oscillation frequency near f / n, and the oscillation frequency is proportional to the control voltage applied from the outside. A variable voltage controlled oscillator, a drive circuit that modulates a phase-locked optical pulse laser with the output of the voltage controlled oscillator, and first and second intensity-modulating incident light at a frequency that is deviated from the frequency f by Δf. Optical intensity modulating means, first and second optical coupling means for inputting part of the outputs of the signal light pulse and the phase locked optical pulse laser to the first and second light intensity modulating means, respectively, and First and second light detecting means for respectively detecting the light output of the second light intensity modulating means and converting the light output into an electric signal, and the first light detecting means.
And first and second frequency selecting means for selectively extracting only the frequency .DELTA.f component from the outputs of the second and second light detecting means, and two frequencies .DELTA.f respectively extracted by the first and second frequency selecting means. A phase comparison unit that detects a phase difference between the components and generates an error voltage proportional to the detected phase difference, and a voltage adjustment unit that adjusts the error voltage of the phase comparison unit according to a division ratio to create a control voltage We propose a phase locked optical pulse generator.

[0006]

According to the present invention, a part of the output of the phase-locked optical pulse laser modulated by the output of the signal light pulse and the voltage controlled oscillator is passed through the first and second optical coupling means, respectively, to obtain the first and the second optical coupling means, respectively. 2 is inputted to the intensity modulating means, and at this time, the frequency Δf components generated respectively are inputted to the phase comparing means via the first and second photodetecting means and the first and second frequency selecting means, and the output thereof. The error voltage is applied to the voltage controlled oscillator via the voltage adjusting means, whereby the repetition frequency f / n is phase-locked with the signal light pulse having the repetition frequency f.
The frequency-divided optical pulse is output from the phase-locked optical pulse laser.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of a phase-locked optical pulse generator according to the present invention. In the figure, 1 is an InGaAsP semiconductor laser, 2 is a voltage controlled oscillator having a free-running oscillation frequency of about 2 GHz, and 3a. , 3b are 1.5 μm band InGaAs / In
AlAs multiple quantum well optical intensity modulator, 4 oscillator, 5
a, 5b, 6a and 6b are optical coupling lenses, and 7a and 7b
Is a Ge photodiode, and 8a and 8b have a cutoff frequency of 15
Low-pass filter of 0 kHz, 9a and 9b are low-frequency AC
An amplifier, 10 is a phase comparator, 11 is a variable attenuator, and 12 is a drive circuit of the semiconductor laser 1 including a DC power supply.

The semiconductor laser 1 emits light with a wavelength of 1.54 μm and is mode-locked by a modulated high frequency current applied from the outside. The oscillator 4 is one for the light intensity modulators 3a and 3b.
A modulation signal of 0 GHz +100 kHz is generated.

Reference numeral 13 is a signal light pulse having a wavelength of 1.55 μm, a repetition frequency of 10 GHz and a pulse width of 20 picoseconds, which is generated by gain switching an external signal light semiconductor laser, and 14 is repeated for the signal light pulse 13. Frequency is 1
It is a divided optical pulse having a wavelength of 1.54 μm, a repetition frequency of 2 GHz, and a pulse width of 20 picoseconds, which is divided into / 5 and phase-locked.

Next, the operation of this embodiment will be described.

First, in a state where no phase synchronization is applied, that is, in a state where no control voltage is applied to the voltage controlled oscillator 2, the voltage controlled oscillator 2 oscillates free-running at about 2 GHz. When the semiconductor laser 1 is modulated by this oscillation signal via the drive circuit 12, mode locking is applied to the semiconductor laser 1, and the semiconductor laser 1 outputs an optical pulse having a repetition frequency of about 2 GHz and a pulse width of 20 picoseconds. To be done. The output of the signal light pulse 13 and the output of the semiconductor laser 1 are respectively reflected by the lens 5a.
And 5b to enter the light intensity modulators 3a and 3b,
Intensity modulation is applied to both incident lights at the frequency of the oscillator 4.

At this time, since the repetition frequency of the signal light pulse 13 and the frequency of the oscillator 4 differ by Δf (= 100 kHz), the correlation component of the frequency Δf is present in the envelope of the intensity-modulated signal light pulse 13. Occurs. Further, since the output pulse width of the semiconductor laser 1 is about the same as the signal light pulse 13 (in other words, the spectrum of the output of the semiconductor laser 1 is about the same as the spectrum of the signal light pulse 13),
The envelope of the output light of the intensity-modulated semiconductor laser 1 also contains the correlation component of the frequency Δf.

These intensity-modulated lights are detected by the photodiodes 7a and 7b via the lenses 6a and 6b, respectively, and the Δf components contained in the detected electric signals are taken out by the low pass filters 8a and 8b. The two extracted Δf components are amplified by the low frequency AC amplifiers 9a and 9b, and then input to the phase comparator 10. Here, each Δf component includes a relative phase difference with respect to the modulation signal which is the output of the oscillator 4, and the phase difference between the two detected by the phase comparator 10 results in the signal light pulse 13 and the semiconductor laser. It is equal to the phase difference with the output pulse of 1.

When the detected phase difference is sufficiently small, the phase comparator 10 outputs the error voltage v (t) represented by the following equation (1) corresponding to the phase difference. v (t) = A · sin (2πf d t + θ d) A · (2πf d t + θ d) ...... (1) However, where _{f d = f s -5f r θ} d = θ s -5θ r, A is The coefficients f _s and _fr are the instantaneous repetition frequency of the signal light pulse 13 and the output pulse of the semiconductor laser 1,
θ _s and θ _d are the instantaneous phases of the signal light pulse 13 and the output pulse of the semiconductor laser 1. It should be noted that this error voltage indicates a phase difference corresponding to the signal light pulse 13, and in order to correspond to the relative phase difference of the output pulse of the semiconductor laser 1, it is necessary to divide by a division ratio of 5. The adjustment of this error voltage can be performed by the variable attenuator 11.

The error voltage v (t) is input to the voltage controlled oscillator 2 through the variable attenuator 11, and the voltage controlled oscillator 2 changes in oscillation frequency according to the error voltage. A change in the oscillation frequency of the voltage controlled oscillator 2 appears as a change in the repetition frequency of the output pulse of the semiconductor laser 1, and is finally fed back to the voltage controlled oscillator 2 as a change in the error voltage output from the phase comparator 10.

In this feedback process, the repetition frequency (10 GHz) of the signal light pulse 13 is equal to the oscillation frequency of the voltage controlled oscillator 2, that is, the division ratio (5) times the repetition frequency (2 GHz) of the output pulse of the semiconductor laser 1. 2GHz which is the output of the semiconductor laser 1
The divided optical pulse 14 of is synchronized in phase with the signal optical pulse 13.

According to this embodiment, since the signal light pulse and the divided light pulse are dropped to the Δf component, that is, the frequency component which the response speed of the electric circuit can sufficiently follow, the phase comparison is performed. The phase of the light pulse can be stably synchronized with the signal light pulse. The upper limit of the repetition frequency that can be phase-locked is determined by the modulation band of the optical intensity modulator.
In the case of nAlAs multiple quantum well optical intensity modulator, since the 3 dB modulation band is about 40 GHz, at least 40 GHz
It is possible to apply phase synchronization to a repetition frequency of about z.

[0018]

As described above, according to the present invention, the phase difference between the signal light pulse and the frequency-divided light pulse can be converted into a low-frequency signal for comparison, so that 10 GHz can be achieved without burdening the electric circuit. There is an advantage that a frequency-divided optical pulse that is accurately phase-synchronized can be stably generated even at the above repetition frequencies.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a phase-locked optical pulse generator of the present invention.

[Explanation of Codes] 1 ... InGaAsP semiconductor laser, 2 ... Voltage controlled oscillator, 3a, 3b ... 1.5 μm band InGaAs / InAlA
s multiple quantum well light intensity modulator, 4 ... Oscillator, 5a, 5
b, 6a, 6b ... Lenses for optical coupling, 7a, 7b ... Ge
Photodiodes, 8a, 8b ... Low-pass filter, 9
a, 9b ... Low-frequency AC amplifier, 10 ... Phase comparator, 11
... variable attenuator, 12 ... drive circuit, 13 ... signal light pulse,
14 ... Divided light pulse.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Shigeo Ishibashi 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (1)

[Claims] _{1. A} frequency-divided optical pulse having a wavelength of λ ₁ and a repetition frequency of f is phase-synchronized with a signal light pulse having a wavelength of λ ₂ and a repetition frequency of f / n (where n is a natural number). In a phase-locked optical pulse generator, a phase-locked optical pulse laser that oscillates at a wavelength of λ ₂ and generates an optical pulse that repeats at the frequency of an externally applied electrical modulation signal, and a free-running oscillation frequency near f / n Yes, a voltage-controlled oscillator whose oscillation frequency changes in proportion to a control voltage applied from the outside, a drive circuit for modulating the phase-locked optical pulse laser with the output of the voltage-controlled oscillator, and a deviation of Δf from the frequency f First and second light intensity modulating means for performing intensity modulation on the incident light at different frequencies, and part of the outputs of the signal light pulse and the phase-locked light pulse laser are inputted to the first and second light intensity modulating means, respectively. First And second light coupling means, first and second light detection means for detecting the light outputs of the first and second light intensity modulation means, respectively, and converting them into an electric signal, and the first and second light detection means. First and second frequency selecting means for selectively extracting only the frequency Δf component from the output of the light detecting means, and a phase difference between the two frequency Δf components respectively extracted by the first and second frequency selecting means. And a voltage adjusting means for adjusting the error voltage of the phase comparing means according to the frequency division ratio to create a control voltage. Phase-locked optical pulse generator.