JPH0613981A - Optical timing extraction circuit - Google Patents

Abstract

PURPOSE:To generate a short optical clock pulse by specifying a repetitive a frequency of an optical pulse stimulated from a passive mode clock laser when an optical signal with a specific bit rate is inputted from a transmission line. CONSTITUTION:An input optical signal 1 is a signal whose bit rate is M (b/s) and an optical clock output 2 is outputted synchronously with the input optical signal 1. An optical circulator 3 inputs the input optical signal 1 to a semiconductor laser 7 and outputs separately the optical clock output 2 outputted from the semiconductor laser 7. When an optical signal whose bit rate is M is inputted via the optical circulator 3 to the passive mode clock laser in which a high speed saturable absorbing body 6 of multiple quantum well structure is inserted in an optical resonator comprising the incident side end face of the semiconductor laser 7 and a full reflecting mirror 5, the frequency is M/m (Hz, m is a natural number). An output pulse synchronously with the input optical signal 1 has a pulse width of several 10ps-several 100fs and has a time band product close to a transform limit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical timing extraction circuit used in an optical communication device, and more particularly to an optical timing for generating an optical clock pulse synchronized with an optical pulse signal train having a very high bit rate. Regarding the extraction circuit.

[0002]

2. Description of the Related Art The bit rate of an optical communication system currently in practical use in Japan is 1.6 Gb / s, but in order to construct a higher speed (> several 10 Gb / s) optical communication system, It is necessary to develop an optical signal processing technology that does not suffer from the speed limitation of electronic circuits. Generating a clock synchronized with an input optical signal is one of the main techniques in an optical communication system.

As an attempt to optically realize the synchronous clock, there is a method described below conventionally.

(1) An optical tank circuit used for extracting a clock component of an input optical signal by utilizing the periodic transmission characteristic of an optical resonator (M. Jinno et al., “Optical tank circuits used for
all-optical timing recovery ”, IEEE J. Quantum Ele
ctron., vol. 28, no. 4, April 1992), (2) Optical PLL circuit using a semiconductor laser amplifier as an optical phase comparator (Kawanishi et al., “P using a 1.5 μm traveling wave LD amplifier”).
10GHz timing extraction by LL circuit ", Proceedings of the 1992 Spring Symposium National Congress B-997), (3) Optical injection locking circuit using injection locking to self-pulsating semiconductor laser (M. Jinno et. al., “All-optical timing e
xtraction using a 1.5 μm self pulsating multielec
trode DFB LD ”, Electron. Lett., vol.24, no. 23, p
p.1426-1427, November 1988).

The self-pulsation in the semiconductor laser means that defects in the semiconductor laser and a low current injection region formed by nonuniform current injection function as a saturable absorber.
When the life of absorption saturation is shorter than the gain saturation life of the gain region, pulsed self-excited oscillation occurs. A pulsed light is generated by repeating the cycle of gain increase, absorption saturation, laser oscillation, gain decrease, laser oscillation stop, and absorption increase. The repetition frequency is about the relaxation frequency of the semiconductor laser, which is at most 10's GHz.

[0006]

However, the above-mentioned conventional
The optical tank circuit of (1) uses optical interference, and therefore has a drawback that the requirement for coherence of an input optical signal is severe. Further, it is difficult to tune the center frequency of the input optical signal and the resonance frequency of the optical resonator.

Next, the conventional optical PLL circuit of (2) can obtain only an electric clock output, so that a process for producing an optical clock from the electric clock is further required. In addition, the operating speed is limited by the carrier life of the semiconductor laser amplifier.

Finally, the injection locking method for the self-pulsation semiconductor laser of (3) has a drawback that the operating speed is about the relaxation oscillation frequency of the semiconductor amplifier (up to 10 and several Gb / s).

Further, the pulse width and pulse waveform of the optical clock pulse generated by the above-described conventional configuration do not necessarily match the pulse width and pulse waveform required in the optical processing circuit of the next stage, and some kind of intermediate Processing is required.

The present invention has been made in view of the above points, and it is possible to generate a short optical clock pulse having a simple structure and close to the transform limit synchronized with an input optical signal train of several tens Gb / s or more. An object is to provide an optical timing extraction circuit.

[0011]

According to the present invention, in an optical timing extraction circuit for generating an optical clock pulse train synchronized with an optical signal, a passive mode clock laser for generating an optical pulse and a bit rate M inputted from a transmission line are M. (B /
s) means for inputting an optical signal to the passive mode clock laser, and optical pulse output means for outputting an optical pulse generated from the passive mode clock laser. When no optical signal is input, a passive mode lock laser is provided. The repetition frequency of the optical pulse generated from is approximately M / m (Hz), where m is a natural number.

[0012]

The optical timing extraction circuit of the present invention is based on the optical injection locking method in the conventional optical injection locking circuit using the injection locking to the self-pulsating semiconductor laser. As a means, a passive mode clock laser is used instead of a self-pulsation semiconductor laser.

Passive mode-locked lasers generate an optical pulse train close to the transform limit by repeatedly undergoing intensity modulation by a saturable absorber or the like while light reciprocates in a laser resonator. Then, the repetition of the pulse generation is 2 times the laser cavity length.
It will be about the reciprocal of double. As a result, the optical timing extraction circuit of the present invention utilizes the injection locking to the passive mode-locked laser capable of generating extremely short optical pulses with extremely high repetition rate and uses the optical clock path synchronized with the input optical signal train. It is what gives rise to columns.

[0014]

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

The present invention uses a passive mode-locked laser as an optical pulse generator. A passive mode-locked laser generates a pulsed optical pulse near the transform limit by repeatedly undergoing intensity modulation by a saturable absorber while the light reciprocates in a laser resonator.

In a passive mode-locked laser based on a semiconductor laser, the cavity length can be easily reduced to 1 mm or less, and therefore the repetition frequency is several tens to several hundreds GHz.
Up to. In fact, it has been reported that a saturable absorber with a multiple quantum well structure is integrated in a semiconductor laser and pulsed at 350 GHz (YK Chen et al., “Subpicosec”).
ond monolithic colliding-pulse mode-locked multipl
e quantum well lasers ”, Appl. Phys. Lett. vol. 5
8 (12), pp. 1253-1255, 1991).

The present invention utilizes injection locking to a passive mode-locked laser capable of generating extremely short optical pulses with extremely high repetition rate in order to generate an optical clock pulse train synchronized with an input optical signal train. , To extract the optical timing.

The injection locking mechanism will be described below.
The injection locking phenomenon is a phenomenon that has been used in oscillators using a vacuum tube in the old days, and is recently observed in microwave oscillators and lasers. This is a phenomenon in which, when a signal near the self-excited oscillation frequency is externally input to an oscillator that is performing self-excited oscillation, the self-excited oscillation frequency is pulled in in synchronization with the input signal frequency. Injection locking is a phenomenon in which the input signal frequency is pulled in in synchronization with the input frequency. Injection locking also occurs when the input signal frequency is an integral multiple of the self-excited oscillation frequency or a fraction of an integer.

The optical timing extraction circuit of the present invention is a passive circuit for generating an optical signal having a bit rate of M (b / s) and an optical pulse having a repetition frequency of approximately M / m (Hz, m is a natural number). When inputting to the mode-locked laser and the input optical signal is sensitive to the saturable absorber, the passive mode-locked laser is synchronized with the input optical signal and has a bit rate of 1 / m0 of the input optical signal. The optical clock is generated repeatedly.

FIG. 1 shows the configuration of the first embodiment of the optical timing extraction circuit of the present invention. The input optical signal 1 is a signal having a bit rate of M (b / s), and the optical clock output 2 is output in synchronization with the input optical signal 1. The optical circulator 3 is
The input optical signal 1 is input to the semiconductor laser 7, and the optical clock output 2 output from the semiconductor laser 7 is separated and output.
The antireflection film 8 is provided on one surface of the semiconductor laser 7. The saturable absorber 6 has a small absorption coefficient for strong incident light intensity. Further, the lens ₄₁ to ₄ and the total reflection mirror 5 is provided.

In this structure, the end face of the semiconductor laser 7 on the incident side and the total reflection mirror 5 constitute an optical resonator. The saturable absorber 6 has a multi-quantum well structure (MQW structure) in which a material having a large electron affinity and a material having a small electron affinity are alternately laminated, and the thickness of these layers is set to be equal to or less than the quantum mechanical wavelength of electrons. What was done can be used. MQ like this
In the W structure, steep exciton absorption exists even at room temperature, and this absorption changes extremely rapidly by the incident light electric field. When such a high-speed saturable absorber is inserted into the optical resonator, the saturable absorber 6 modulates the light intensity in the optical resonator to generate a pulse without applying an electric signal from the outside. This is called passive mode lock. Here, the repetition frequency R of the optical pulse generated from the passive mode-locked laser is R = c / (where L is the length of the optical resonator, n is the effective refractive index, and c is the speed of light in vacuum. 2 nL).

To such a passive mode-locked laser, an optical signal having a bit rate M of about mR (b / s, m is a natural number) and having sensitivity to the saturable absorber 6 is supplied to the optical circulator 3 and the lens 4. To enter via. Then, the repetition frequency generated by the passive mode-locked laser in synchronization with the input optical signal changes from R (Hz) to M / m (Hz), and the optical clock output 2 synchronized with the input optical signal 1 is generated. This is separated from the input optical signal 1 by the optical circulator 3 and becomes an optical clock output. At the same time, the optical circulator 3 functions to prevent the influence of the optical clock on the input side, the return light of the optical clock, and the influence of light from the next stage. Output pulse width of passive mode-locked laser is 10ps
Since this has a time band product of several hundred fs and close to the transform limit, it is convenient for optical processing such as optical demultiplexing and optical identification reproduction using this pulse.

The MQW semiconductor laser in which the MQW structure described above is introduced into a semiconductor laser is superior in high frequency characteristics and differential gain to a semiconductor laser having a normal structure.
If the electrode of such an MQW semiconductor laser is divided into two, one of which is the saturable absorption region and the other is the gain region, a passive mode-locked laser in which the saturable absorption region and the gain region are integrated can be constructed.

FIG. 2 shows the configuration of an optical timing extractor according to the second embodiment of the present invention. In the figure, the same components as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted. By using the passive mode-locked laser described above, an optical Taming extractor as shown in the figure can be realized. The saturable absorption region 10 and the gain region 11 in the figure are realized by electrode decomposition of an MQW semiconductor laser. These are electrically independent and optically coupled. In the configuration shown in FIG.
The antireflective film 8 is applied and the total reflection mirror 5 is used to form an external resonator.

FIG. 3 shows the configuration of an optical timing extractor according to the third embodiment of the present invention. In the figure, the same components as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be omitted.

When the required repetition of the optical clock output is fast, the optical resonator length of the passive mode-locked laser can be short. For this reason, the passive mode-locked laser may be configured by a single MQW semiconductor laser without using an external mirror. In this case, the optical clock output 2 is taken out from the end face opposite to the incident side. The optical isolator 9 functions to prevent the influence of the optical clock on the input side, the return light of the optical clock, and the influence of light from the next stage.

In the structure of the passive mode-locked laser shown in FIGS. 1 and 2, if the total reflection mirror 5 is replaced with a partial transmission mirror, the structure as shown in FIG. 3 can be obtained without using the optical circulator 3. It is also possible to take out the optical clock output on the side opposite to the incident side.

On the contrary, in the configuration of FIG. 3, if a total reflection film is provided on the end face of the saturable absorption region 10 on the output side of the semiconductor laser, the optical clock output can be taken out from the input side using the optical circulator. .

FIG. 4 shows the configuration of an optical timing extractor according to the fourth embodiment of the present invention. The configuration shown in the figure is a configuration in which the optical clock pulse width generated can be narrowed by using a technique called collision pulse mode locking (CPM) for a passive mode-locked laser. At this time, the MQW semiconductor laser is divided into three, and the saturable absorber 11 is placed at the center. In this configuration, two light pulses reflected by the two end faces of the MQW semiconductor laser and traveling in opposite directions arrive at the saturable absorber 11 at the same time and meet with each other, resulting in twice the light intensity as in the case of a normal mode-locked laser structure. Since absorption saturation occurs at, more rapid nonlinear absorption occurs, resulting in a narrow optical pulse width.

FIG. 5 shows the configuration of an optical timing extractor according to the fifth embodiment of the present invention. The configuration shown in the figure is an MQW semiconductor laser configured in a ring shape. Similar to the CPM laser described above, light pulses traveling in opposite directions in the ring 20 collide with the saturable absorber 11 to sharpen the pulse. Even when such a passive mode-locked laser having a CPM structure is used, the principle of timing extraction by injection locking is the same as that described with reference to FIG. However, repeatedly, the relationship between the frequency R and the cavity length L is slightly different. If the length of the MQW semiconductor laser is L in FIG. 4 and the length of the ring 20 of the MQW semiconductor laser is L in FIG. Are respectively given by R = c / (nL).

The optical timing extraction circuit of the present invention is a passive circuit for generating an optical signal having a bit rate of M (b / s) and an optical pulse having a repetition frequency of about M / m (Hz, m is a natural number). When inputting to the mode-locked laser and the input optical signal is sensitive to the saturable absorber, the passive mode-locked laser synchronizes with the input optical signal and is 1 / m of the bit rate of the input optical signal. The optical clock is generated repeatedly.

[0032]

As described above, by using the timing extraction circuit of the present invention, it is possible to generate an optical clock pulse synchronized with an input optical signal train of several tens Gb / s or more with a simple structure. The output pulse of the passive mode-locked laser has a pulse width of several tens of ps to several hundred fs and has a time-band product close to the transform limit, which is convenient for optical demultiplexing using this pulse and optical processing such as optical identification and reproduction. Is good.

Therefore, the structure is simple and several tens of Gb / s.
It is possible to generate a short optical clock pulse that is close to the transform limit and that is synchronized with the input optical signal train.

[Brief description of drawings]

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

FIG. 2 is a configuration diagram of an optical timing extraction circuit according to a second embodiment of the present invention.

FIG. 3 is a configuration diagram of an optical timing extraction circuit according to a third embodiment of the present invention.

FIG. 4 is a configuration diagram of an optical timing extraction circuit according to a fourth embodiment of the present invention.

FIG. 5 is a configuration diagram of an optical timing extraction circuit according to a fifth embodiment of the present invention.

[Explanation of symbols]

1 input destination signal 2 optical clock output 3 optical circulator 4 ₁ to 4 ₄ lens 5 total reflection mirror 6 saturable absorber 7 semiconductor laser 8 non-reflective film 9 optical isolator 10 saturable absorption region 11 gain region 20 ring

Claims (1)

[Claims] 1. An optical timing extraction circuit for generating an optical clock pulse train synchronized with an optical signal, wherein a passive mode clock laser for generating an optical pulse and an optical signal having a bit rate of M (b / s) input from a transmission line. Means for inputting to the passive mode clock laser and an optical pulse output means for outputting an optical pulse generated from the passive mode clock laser, and when the optical signal is not input, the optical signal is generated from the passive mode locked laser. An optical timing extraction circuit, wherein the repetition frequency of the optical pulse is approximately M / m (Hz), where m is a natural number.