JP2001094199A - Optical clock sampling circuit and optical communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a superhigh-speed optical clock sampling circuit useful to a large-capacity optical communication. SOLUTION: An input signal optical pulse train 21 is injected in a first mode synchronous semiconductor laser 1 to sample the same frequency sampling clock optical pulse train 22, which has a frequency equal to the clock frequency of the pulse train 21 and a timing synchronized with that of the pulse train 21. This optical pulse train 22 is injected in a second mode synchronous semiconductor laser 2, having a repeat frequency of 1/integer of the clock frequency of the optical pulse train 22 via an optical gate 3 and a dividing sampling clock optical pulse train 23, which is a synchronous clock optical pulse train of a repetition frequency of 1/integer of the clock frequency of the injected clock optical pulse train 22, is generated. One part of this clock optical pulse train 23 is extracted by an optical coupler 11 and is incident in the gate 3 via an optical delay device 13, a light amplifier 4 and the coupler 11 to change the transmissivity of the gate 3 and the pulse train 22, which enters the laser 2 is modulated in intensity in a divided clock frequency.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system, and more particularly to a system for extracting a data clock frequency from a data optical signal train in ultra-high-speed optical communication.

[0002]

2. Description of the Related Art In a current optical communication system, a configuration is used in which an optical signal is temporarily converted to an electric signal after processing the optical signal. The problem with such a system, in addition to the complexity of the electronics components, is that the speed of the electrical processing is typically several gigabits per second (G).
b / s).

[0003] In order to solve the complexity of the device and the limitation of the processing speed, it is necessary to at least partially process the optical signal without using electricity. The importance of a technique for extracting an optical clock signal sequence synchronized with a signal is increasing. In this background, a method of directly outputting an optical clock by injecting an optical data stream into a mode-locked laser has recently attracted attention.

[0004] The optical clock extracting element and method are described in, for example, Japanese Patent Application No. 10-124852 by Hashimoto and Yokoyama.
In this publication, a method using a mode-locked semiconductor laser and its application in optical communication are described. The method using the mode-locked semiconductor laser can realize a high-speed clock extraction function with a simple configuration.However, in optical communication applications, the speed of the high-speed optical signal train is further reduced at the receiving end. In addition, a function for reducing the processing speed to a minimum is also required.

[0005] Therefore, a method of dividing the optical clock frequency by a factor of 1 at the receiving end and dividing the optical signal sequence in time using this frequency has been developed. What is important here is that when generating a frequency-divided clock light pulse that is an integer fraction of the clock frequency of the transmitted optical signal train, the phase relationship with the optical signal is determined, and the timing is adjusted. To prevent discrete deviation in the clock cycle.

For this purpose, a phase locked loop using electronics is often used, but this method inevitably complicates the system to some extent. Therefore, as a method of generating a frequency-divided clock all-optically, a method of thinning out an optical clock with an optical gate has been proposed. See IEEE Photonics Technolog by Lee and Kim on how to do this.
An example is described in the paper described in y Letters, Vol. 11, No. 4, pp. 469-471.

FIG. 6 shows the principle of the method. The clock light pulse train 25 is a semiconductor laser amplifier 35
Then, the light enters the light shaping / amplifying circuit 51 via the optical delay circuit 13, and a part of the output is returned to the semiconductor laser amplifier 35 after an appropriate time delay by the optical delay device 13.

In this configuration, when one light pulse enters the light shaping amplifier circuit 61, its output is returned and the gain of the semiconductor laser amplifier 35 is consumed, thereby increasing the transmittance of the next light pulse. The optical shaping amplifier circuit 6
1 can be eliminated. Then, for the next optical pulse, the gain of the semiconductor laser amplifier 35 does not decrease, and the optical pulse can reach the optical shaping amplifier 61 again.

In this way, the frequency is finally reduced to 1/2.
Is obtained as an output. FIG. 7 shows this state in a timing chart. That is, the gain 107 of the semiconductor laser amplifier
Greatly fluctuates at twice the period of the initial clock light pulse train 105, so that a divided clock light pulse 106 is obtained.

[0010]

In the above-described method of generating the divided clock light pulse, the frequency of the input clock light pulse that can be processed is limited to about the response time of the carrier in the semiconductor laser amplifier, and the frequency is about 10 GHz or more. The frequency division operation cannot be performed on the clock light pulse train. Therefore, the purpose cannot be sufficiently achieved in terms of the ultra-high-speed optical signal processing that exceeds the electrical speed limitation by the all-optical method, which was the original purpose.

An object of the present invention is to provide an optical clock extracting circuit capable of generating a highly stable frequency-divided clock light pulse from a signal light pulse train of several tens Gb / s or more regardless of the electric speed limitation. .

Still another object of the present invention is to provide an optical communication system using such an optical clock extracting circuit as a part of a receiver.

[0013]

An optical clock extracting circuit according to the present invention generates a first extracted clock optical pulse train having the same frequency as the clock frequency in synchronization with a signal optical pulse train in response to the incidence of a signal optical pulse train. A first mode-locked semiconductor laser to be transmitted and a first extracted clock light pulse train output from the first mode-locked semiconductor laser,
A second mode-locked semiconductor laser that generates a second extracted clock light pulse train having a frequency that is a fraction of the clock frequency in synchronism with the first extracted clock light pulse train and outputs it as a frequency-divided extracted clock light pulse A light modulating means optically coupled between the first and second mode-locked semiconductor lasers for modulating a first extracted clock light pulse train output from the first synchronous semiconductor laser; And a modulation signal extracting means for extracting a modulation signal to be supplied to the means from a second extracted clock pulse train output from the second mode-locked semiconductor laser.

The light modulating means is constituted by an optical gate which operates to modulate the light transmittance by the incidence of light, and the modulating signal extracting means comprises an optical output from the second mode-locked semiconductor laser. A first optical coupler for extracting a part of the light to the outside, an optical delay unit for providing an optical delay to the optical output from the first optical coupler, an optical amplifier for amplifying the optical output from the optical delay unit, The optical amplifier is characterized by being constituted by a second optical coupler for inputting the optical output amplified by the optical amplifier to the optical gate.

Alternatively, the light modulating means is constituted by an optical modulator which operates to modulate the light transmittance by an electric signal, and the modulated signal extracting means comprises a light from a second mode-locked semiconductor laser. An optical coupler for extracting a part of the output to the outside, an optical delay unit for providing an optical delay to the optical output from the optical coupler, an optical amplifier for amplifying the optical output from the optical delay unit, and an optical amplifier from the optical amplifier. It is characterized by comprising a high light-tolerance photodetector for receiving an optical output, and means for applying an electric signal from the photodetector to the optical modulator.

Alternatively, the light modulating means is constituted by an optical modulator operable to modulate light transmittance by an electric signal, and the modulated signal extracting means is configured to modulate light from the second mode-locked semiconductor laser. An optical coupler for extracting a part of the output to the outside, an optical delay device for providing an optical delay to the optical output from the optical coupler, a photodetector for receiving the optical output from the optical delay device, It is characterized by comprising a high-frequency amplifier for amplifying an electric signal output from a detector, and means for applying an electric signal from the high-frequency amplifier to the optical modulator.

In the above-mentioned optical clock extracting circuit, the first mode-locked semiconductor laser and the second mode-locked semiconductor laser in the optical clock extracting circuit have a saturable absorption region and a gain region, respectively. The mode-locked semiconductor laser alone performs passive mode locking.

Further, by using the above-described optical clock extracting circuit as a part of an optical communication receiver, an optical communication system can be constructed.

In the optical clock extraction of the present invention, when a signal light pulse train having a clock frequency substantially equal to the light pulse repetition frequency or a frequency that is an integral multiple of the light pulse repetition frequency is injected into a semiconductor laser which is passively mode-locked by the saturable absorption effect. The operation of the mode-locked semiconductor laser is synchronized with the timing.

At the time of this timing synchronization, in the extraction of the optical clock having a frequency that is a fraction of an integer, the intensity of the light incident on the mode-locked semiconductor laser is modulated at the frequency of the optical clock finally obtained. The effect of suppressing the timing deviation is used.

[0021]

FIG. 1 is a configuration diagram of an optical clock extraction circuit showing a first embodiment of the present invention. In the first mode-locked semiconductor laser 1 shown in FIG. 1, an input signal light pulse train 21 is first injected to extract a same-frequency-extracted clock light pulse train 22 whose clock frequency is equal to and synchronized in timing.

Next, the optical pulse train is injected via the optical gate 3 into the second mode-locked semiconductor laser 2 having a repetition frequency that is approximately one-integral of the clock frequency. A frequency-divided extracted clock light pulse train 23 is generated, which is a synchronous clock light pulse having a repetition frequency divided by a fraction of the clock frequency of the frequency-extracted clock light pulse train 22.

An optical output of a part of the clock light pulse train is divided and taken out by the first optical coupler 11, passed through an optical delay device 13 for giving an appropriate time delay to the taken out light pulse, and furthermore by an optical amplifier 4. After being amplified to an appropriate intensity, the light enters the optical gate 3 via the second optical coupler 12. The delay time of the optical delay unit 13 can be adjusted so that the frequency-divided extracted clock optical pulse train 23 is output most stably. Generally, it is set to be about the clock cycle of the injected same-frequency extracted clock light pulse train 22 or slightly shorter.

As a result, the transmittance of the optical gate 3 is modulated by the divided clock frequency of the frequency-divided extracted clock optical pulse train 23. Thus, the same frequency extracted clock light pulse train 22 from the first mode-locked semiconductor laser 1 is intensity-modulated at the divided clock frequency.

Therefore, the second mode-locked semiconductor laser 2
, The same frequency extracted clock light pulse train 22 from the first mode-locked semiconductor laser 1 that has been intensity-modulated at the divided clock frequency is incident. By this feedback process, when the second mode-locked semiconductor laser 2 synchronizes with the same frequency-extracted clock light pulse train 22 from the first mode-locked semiconductor laser 1 at the divided frequency, the second mode-locked semiconductor laser 2 changes to a high-intensity light pulse. Selective synchronization occurs.

Therefore, in the frequency division frequency synchronization process, it is possible to prevent the timing of the synchronization from being discretely and accidentally shifted in the cycle unit of the clock light pulse train from the first mode-locked semiconductor laser 1.

FIG. 2 is a diagram showing the mutual timing of the time waveforms of the input signal light pulse train and each extracted clock light pulse train in FIG. Input signal light pulse train waveform 1
01 and the same frequency extraction clock light pulse train waveform 102 are synchronized, but after the optical gate 3, the same frequency extraction clock light pulse train 22 is intensity-modulated like the same frequency extraction clock light pulse train modulation waveform 103. Reflecting this, the frequency-divided extracted clock light pulse train 23 finally obtains the frequency-divided extracted clock light pulse train waveform 104 in synchronization with the high-intensity injected light pulse.

In the present embodiment, the first mode-locked semiconductor laser 1 and the second mode-locked semiconductor laser 2 each have an InGaAsP-based bulk active layer structure and have a saturable absorption region and a gain region, and a distributed feedback type. A semiconductor laser device having a reflection structure having a module structure in which optical components such as an optical isolator and a lens are incorporated to facilitate optical coupling was used.

In each of the passive mode-locked states, the first mode-locked semiconductor laser 1 performs a mode-locked operation at 80 GHz and the second mode-locked semiconductor laser 2 performs a mode-locked operation at 10 GHz. To generate a coherent light pulse. Therefore, the speed of the input signal light pulse train 21 is 80 Gb here.
/ S.

As the optical gate 3, a waveguide chip having a semiconductor layer structure similar to that of a semiconductor laser, also having a module structure, was used by applying a reverse bias electrically. An erbium fiber amplifier was used for the optical amplifier 4. All optical clock extraction circuits including these components were optically coupled by optical fibers.

With the above configuration, finally, the frequency-divided clock pulse train 23 becomes 10G without any timing shift.
Hz optical pulse train was obtained.

FIG. 3 is a block diagram showing a second embodiment of the present invention. In this embodiment, instead of the optical gate 3 in the optical clock extraction circuit in FIG. 1, an optical modulator 31 whose light transmittance is modulated by an electric signal is used. Instead, the light detector 32
And a high-frequency amplifier 33 for amplifying the electric signal output are arranged in the optical clock extracting circuit.

The basic operation principle of generating the frequency-divided extracted clock light pulse train is the same as that of FIG. However, the same frequency extracted clock optical pulse train 22 generated from the first mode-locked semiconductor laser 1 is obtained by photoelectrically converting the frequency-divided extracted clock optical pulse train 23 by the photodetector 32 and driving the optical modulator 31 by this electric signal. Is intensity modulated. Also in this configuration, it is possible to finally obtain a 10 GHz optical pulse train without any timing shift as the divided clock optical pulse train 23.

FIG. 4 is a configuration diagram showing a third embodiment of the present invention. In this embodiment, in the optical clock extraction circuit of FIG. 1, an optical modulator 31 is provided in place of the optical gate 3 and operates so that the light transmittance is modulated by an electric signal. In addition, a high light tolerance photodetector 34 for receiving the optical output from the second mode-locked semiconductor laser 2 is used.

In this example, as in the configuration of FIG. 3, the same frequency extracted clock light pulse train 22 generated from the first mode-locked semiconductor laser 1 is obtained by dividing the frequency-divided extracted clock light pulse train 23 by the high light tolerance photodetector 34. Driving the optical modulator 31 with the photoelectrically converted electric signal performs intensity modulation.

FIG. 5 is a block diagram showing an embodiment of an optical communication system to which the optical clock extracting circuit according to the present invention is applied. The optical clock extracting circuit according to the present invention is used as a clock timing extracting circuit in a data demodulating unit on the receiving side. A circuit is used.

As shown in FIG. 5A, from the transmitting side,
Optical data of, for example, 100 Gb / s, in which the optical output from the short pulse light source 51 is signal-multiplexed by the data modulator 52, is transmitted. This optical data is passed through an optical fiber 53, an optical amplifier 54, a regenerative repeater 55, etc., and received by a data demodulator 56 on the receiving side, where the multiplexed optical signals are separated as an output signal group of, for example, 10 Gb / s, and output. Is done.

Further, as shown in FIG. 5B, the data demodulator 56 receives the 10 Gb / s output signal group from the received 100 Gb / s optical data by the optical gate 57 in order to separate the group. An optical clock extraction circuit 58 according to the present invention that extracts a 10 GHz optical clock from 100 Gb / s optical data is used.

[0039]

According to the present invention, by using two mode-locked semiconductor lasers, several tens G
It is possible to provide an optical clock extracting circuit capable of generating a stable frequency-divided clock light pulse having no timing shift from a signal light pulse train of b / s or more.

Further, the optical clock extracting circuit of the present invention does not limit the electric speed at the receiving end.
An optical communication system capable of transmitting an optical signal at a speed of several tens Gb / s or more per wavelength can be constructed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of an optical clock extraction circuit according to the present invention.

FIG. 2 is a diagram showing mutual timings of time waveforms of an input signal light pulse train and each extracted clock light pulse train in the optical clock extraction circuit according to the present invention.

FIG. 3 is a configuration diagram showing a second embodiment of the optical clock extraction circuit according to the present invention.

FIG. 4 is a configuration diagram showing a third embodiment of the optical clock extraction circuit according to the present invention.

FIG. 5 is a block diagram showing an optical communication system to which the optical clock extraction circuit according to the present invention is applied.

FIG. 6 is a configuration diagram of a frequency-divided optical clock generation circuit in a conventional example.

FIG. 7 is a diagram showing a relationship between a time change of a gain of a semiconductor laser amplifier and generation of a divided clock optical pulse in a divided optical clock generation circuit of a conventional example.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 first mode-locked semiconductor laser 2 second mode-locked semiconductor laser 3 optical gate 4 optical amplifier 11 first optical coupler 12 second optical coupler 13 optical delay unit 21 input signal light pulse train 22 same frequency extraction clock light pulse train 23 Frequency-divided extracted clock optical pulse train 25 Clock optical pulse train 26 Frequency-divided clock optical pulse train 31 Optical modulator 32 Photodetector 33 High-frequency amplifier 34 High light tolerance photodetector 51 Short pulse light source 52 Data modulator 53 Optical fiber 54 Optical amplifier 55 Reproduction Repeater 56 data demodulator 57 optical gate 58 optical clock extraction circuit 61 optical shaping amplifier 101 input signal optical pulse train waveform 102 identical frequency extracted clock optical pulse train waveform 103 identical frequency extracted clock optical pulse train modulated waveform 104 frequency-divided extracted clock optical pulse train waveform 105 Clock Optical pulse train waveform 106 frequency-divided clock pulse train waveform 107 semiconductor laser amplifier gain time change 108 the semiconductor laser amplifier transmitting optical clock pulse train waveform

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5F073 AB25 AB28 AB30 BA01 CA12 EA14 GA38 5K002 BA02 BA04 BA13 CA13 CA17 DA07 FA01

Claims (6)

[Claims] A first mode-locked semiconductor laser that generates a first extracted clock light pulse train having the same frequency as the clock frequency thereof in synchronization with the signal light pulse train when the signal light pulse train is incident; 1st mode-locked semiconductor laser
In response to the input of the extracted clock light pulse train, a second extracted clock light pulse train having a frequency that is a fraction of an integer of the clock frequency is generated in synchronization with the first extracted clock light pulse train, and the frequency-divided extracted clock light train is generated. A second mode-locked semiconductor laser outputting as a pulse; and a first extracted clock light optically coupled between the first and second mode-locked semiconductor lasers and output from the first synchronous semiconductor laser. Light modulation means for modulating a pulse train; and modulation signal extraction means for extracting a modulation signal to be supplied to the light modulation means from a second extracted clock light pulse train output from the second mode-locked semiconductor laser. An optical clock extraction circuit, comprising: 2. The light modulating means is constituted by an optical gate which operates so that light transmittance is modulated by incidence of light, and the modulation signal extracting means operates on a light output of the second mode-locked semiconductor laser. A first optical coupler for extracting a part to the outside, an optical delay unit for providing an optical delay to the optical output from the first optical coupler, an optical amplifier for amplifying the optical output from the optical delay unit, 2. The optical clock extracting circuit according to claim 1, further comprising a second optical coupler for inputting the optical output amplified by the optical amplifier to the optical gate. 3. The optical modulator according to claim 1, wherein the optical modulator comprises an optical modulator that operates to modulate light transmittance by an electric signal, and the modulated signal extractor comprises an optical output from the second mode-locked semiconductor laser. An optical coupler for extracting a part of the optical signal to the outside, an optical delay unit for giving an optical delay to the optical output from the optical coupler, an optical amplifier for amplifying the optical output from the optical delay unit, and a light from the optical amplifier. 2. The optical clock extracting circuit according to claim 1, further comprising a high light-tolerance photodetector for receiving an output, and means for applying an electric signal from the photodetector to the optical modulator. 4. The optical modulation means comprises an optical modulator which operates to modulate light transmittance by an electric signal, and wherein the modulation signal extracting means comprises an optical output from the second mode-locked semiconductor laser. An optical coupler for extracting a part of the optical output to the outside, an optical delay unit for providing an optical delay to the optical output from the optical coupler, a photodetector for receiving the optical output from the optical delay unit, and the photodetector 2. The optical clock extracting circuit according to claim 1, further comprising a high-frequency amplifier for amplifying an electric signal output from the optical amplifier, and a unit for applying an electric signal from the high-frequency amplifier to the optical modulator. 5. The first mode-locked semiconductor laser and the second mode-locked semiconductor laser have a saturable absorption region and a gain region, and each mode-locked semiconductor laser alone performs passive mode locking. Claim 1.
The optical clock extraction circuit according to any one of claims 1 to 4. 6. An optical clock using the optical clock extraction circuit according to claim 1 as means for extracting a frequency-divided reference clock pulse light from an incident signal light pulse train. Communications system.