JPH01150121A - Optical circuit for self-timing extraction - Google Patents

Abstract

PURPOSE:To obtain an optical clock signal synchronized directly from an input optical signal and without having jitter by realizing an optical loop circuit consisting of an optical directional coupler and an optical delay element and furthermore, an optical band-pass filter with high Q value by using a light amplifier, and combining the filter with a light limiter. CONSTITUTION:The optical directional coupler 2 includes a second output port, and a branching means is included in the coupler 2, and branches the signal of a second input port to the second output port. The second output port is coupled with an optical limiter 5 via an optical waveguide 4 optically. The optical loop circuit 3 is constituted of the optical delay element, and length L is set at C/nT assuming the clock cycle of an input light signal as T. Where, C shows luminous flux, and (n) is the refractive index of the loop circuit 3. The optical BPF is formed by the coupler 2 and the loop 3. It is permitted to provide the light amplifier 6 on the loop 3. The amplifier 6 is a semiconductor laser whose injection current is suppressed less than an oscillation threshold value, and the optical limiter 5 is the semiconductor laser having an electrode bi-sected in the direction of a resonator. By constituting a circuit in such way, it is possible to extract the optical clock synchronized with input whose pulse strength is modulated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is applied to an optical communication device. In particular, the present invention relates to an optical circuit that extracts an optical clock signal synchronized with an input optical signal from the input optical signal itself in order to process the optical signal without converting it into an electrical signal.

[Conventional technology]

In current optical communication equipment, only the transmission section is where the optical signal actually flows; repeaters, multiplexing/demultiplexing equipment, and exchanges convert the optical signal into an electrical signal and then use various electronic circuits to convert it into an electrical signal. It is necessary to process the signal and then convert it back into an optical signal. Therefore, the device configuration becomes complicated.

In addition, in processing by electronic circuits, 10 to several 106 bits
There is a theoretical limit to the processing speed at a bit rate of about /s. In order to solve the problem of complicated equipment configuration and limited processing speed due to the intervention of electrical processing, it is possible to process optical signals as they are without converting them into electrical signals. is necessary.

In order to achieve relaying, multiplexing/demultiplexing, switching, and other functions using optical signals without converting them to electrical signals, it is necessary to extract an optical clock signal synchronized with the input optical signal from the input optical signal itself. Technology is essential. As a self-timing extraction technology using light, Japanese Patent Application Laid-Open No. 62-6
No. 3923 discloses an optical phase locked loop.

FIG. 11 shows a block diagram of the optical phase-locked loop.

The phase comparator 113 receives the input optical signal incident on the end face 111 of the optical waveguide 112 and the optically controlled negative resistance light emitting element 117.
It receives and receives an optical clock signal inputted into the leading waveguide 118 from the first output end of the waveguide 118 , and outputs a phase difference optical signal to the leading waveguide 114 . This phase difference optical signal is guided to an optical fiber 115 via a leading wavepath 114. The optically controlled negative resistance light emitting element 117 emits a pulse intensity modulated clock signal from a first output end to which the optical waveguide 118 is connected and a second output end to which the optical waveguide 119 is connected. As a result, an optical clock signal is obtained at the end face 120 of the optical waveguide 119.

[Problem that the invention seeks to solve]

However, optical phase-locked loops, like electrical phase-locked loops, ■ have large circuit scales, and ■ attempt to give steep low-pass characteristics to the transfer function of a phase-locked loop, resulting in a sharp gain increase in the transfer function. This has the drawback that jitter at a specific frequency is amplified by multi-stage relay.

The present invention solves the above problems, and can extract an optical clock signal synchronized with a pulse intensity modulated input optical signal without converting it into an electrical signal with a simple passive resonance type configuration. It is an object of the present invention to provide a self-timing extraction optical circuit that does not have a gain for specific jitter.

[Means for solving problems]

The self-timing extraction optical circuit of the present invention includes an optical directional coupler that couples optical signals input to two or more input ports to one or more output ports, and a first input port of the optical directional coupler. manual means for supplying a pulse intensity modulated input optical signal; and the optical directional coupling by delaying the optical signal output from the output port of the optical directional coupler by a time equal to the clock period of the input optical signal. an optical loop circuit that supplies a second input port of the device, a branching means for branching an optical signal from the optical loop circuit, and a branching means that shapes the waveform of the branched optical signal and synchronizes it with the input optical signal. It is characterized by comprising an optical limiter that outputs an optical clock signal.

The optical directional coupler includes a second output port, branching means is included in the optical directional coupler, and the branching means utilizes the optical directional coupler to output the signal of the second input port. Alternatively, a second optical directional coupler provided on the optical loop circuit separately from the optical directional coupler may be used.

The optical loop circuit can include an optical amplifier.

[For production]

In the self-timing extraction optical circuit of the present invention, an optical directional coupler and an optical loop circuit constitute an optical band-pass filter having steep band-pass characteristics, and the output optical signal of this optical band-pass filter is shaped by an optical limiter. and output.

Therefore, an optical clock signal synchronized with the pulse intensity modulated input optical signal can be extracted from the pulse intensity modulated input optical signal without the need for optical-to-electrical conversion and electric-to-optical conversion. Furthermore, since an optical bandpass filter is used, jitter can be averaged, and specific jitter is not amplified.

Further, the optical circuit of the present invention does not require complicated elements such as an optical phase comparator and an optically controlled negative resistance light emitting element, which are required in a conventional optical phase-locked loop.

〔Example〕

FIG. 1 is a block diagram of a self-timing extraction optical circuit according to a first embodiment of the present invention.

This self-timing extraction optical circuit includes an optical directional coupler 2 that couples optical signals input into two human power ports to two output ports, and a pulse intensity input to the first input port of this optical directional coupler 2. As a manual means for supplying a modulated input optical signal, the optical signal outputted from the leading waveguide 1 and the first output port of the optical directional coupler 2 is delayed by a time equal to the clock period of the input optical signal. An optical loop circuit 3 for supplying to the second input port of the optical directional coupler 2, a branching means for branching an optical signal from the optical loop circuit 3, and a branching means for shaping the waveform of the branched optical signal. and an optical limiter 5 that outputs an optical clock signal synchronized with the input optical signal.

In this embodiment, the optical directional coupler 2 includes a second output port, and the branching means is included in the optical directional coupler 2 to route the signal of the second human-powered boat to the second output port. It has a branching configuration. This second output port is connected to the leading waveguide 4.
It is optically coupled to the optical limiter 5 via.

The optical loop circuit 3 is composed of an optical delay element, and its length L is c/n, where T is the clock period of the input optical signal.
It is set to T. Here, C is the speed of light in vacuum, n
is the refractive index of the optical loop circuit 3.

In this self-timing extraction circuit, the optical directional coupler 2 and the optical loop circuit 3 constitute an optical bandpass filter 10.

FIG. 2 is a block diagram of a self-timing extraction optical circuit according to a second embodiment of the present invention. This embodiment differs from the first embodiment in that an optical amplifier 6 is provided on the optical loop circuit 3.

FIG. 3 is a diagram showing the operating principle of the first embodiment and the second embodiment, in which (a) shows a pulse intensity modulated input optical signal;
(b) shows an optical signal output from the second output port of the optical directional coupler 2, and (C) shows an optical clock signal output from the optical limiter 5.

As shown in FIG. 3(a), the input optical signal has added jitter due to various noises and code patterns. Therefore, the phase of the optical pulse is advanced or delayed.

For such an input optical signal, an optical bandpass filter 10 is applied.
allows only optical signals whose intensity is modulated at a frequency that is an integral multiple of 1/T to pass through. Therefore, the optical signal shown in FIG. 3 is obtained at the second output port of the optical directional coupler 2. This optical signal has its jitter averaged and becomes an optical pulse train having a period equal to the clock period of the input optical signal.

The optical limiter 5 shapes the waveform of this optical pulse train and
As shown in Figure (C), a rectangular pulse train with jitter absorbed is output.

Next, the characteristics of the optical bandpass filter 10 will be explained.

If the first input port, second manual port, first output port, and second output port of the optical directional coupler 2 have optical intensities P3, P2, P3, and P4, respectively, the optical directional coupler 20 input/output characteristics can be expressed as: (1). Here, k represents the branching ratio of the optical directional coupler 2, and Q<k<1. Furthermore, the loss of the optical directional coupler 2 has been ignored for the sake of simplicity. When the amplification factor of the optical amplifier 6 is a, the impulse response h (t) of the optical bandpass filter 10 is expressed as 1 and (2) using a delta function δ(1). The first embodiment can be considered as a case where a=l. When formula (2) is subjected to Fourier transformation, the absolute value H(fT) of the transfer function becomes l H(fT) 1 -...- (3). Here, f is the intensity modulation frequency.

Furthermore, the light intensity when going around the optical loop circuit 3 is a(1
-k). This value is called "loop gain" and Gt
, I will write it as .

FIG. 4 shows calculated values of the frequency characteristics of the optical bandpass filter 100. In this calculation, the loop gain GL was set to 0.9, and the branching ratio k was used as a parameter.

As shown in this figure, the optical bandpass filter 10 exhibits bandpass characteristics for frequencies that are integral multiples of the clock frequency 1/T of the input optical signal. An index Q indicating the sharpness of the resonance of the bandpass filter 10 is defined as the reciprocal of the full width at half maximum of the resonance peak. The larger the Q value, the more the optical bandpass filter 10
jitter absorption is large. When the branching ratio k is 0.5, the Q value is expressed as...-(4).

FIG. 5 shows the change in Q value with respect to (1/(1-k))-a. However, the branching ratio is 0.5.

As shown in this figure, the amplification factor a of the optical amplifier 6 is 1/(
1-k), that is, the loop gain GL becomes "
1", the larger Q becomes, and the amplification factor a = 1.99.
When , a large value of Q=360 is obtained.

In the case of the first embodiment, a=l, but since the branching ratio k is 0.5, this is due to the horizontal axis (1/
(1-k)) This corresponds to when -a is "1".

At this time as well, the Q value is greater than "1" and bandpass characteristics are obtained.

As the optical amplifier 6, a semiconductor laser element whose injection current is biased below the oscillation threshold is used.

When an optical signal is incident on the active layer from one end face of this semiconductor laser element, the optical signal amplified by the stimulated emission light transmitted through the active layer can be extracted from the other end face. In order to weaken the dependence of the optical amplifier on the incident wavelength, it is effective to apply a non-reflection coating to both end faces of the semiconductor laser element. Furthermore, in order to suppress stimulated emission light, it is effective to install a wavelength filter on the output side of the optical amplifier.

Regarding optical amplifiers using such semiconductor laser elements, please refer to IREε Journal of Curontum Electronics Vol. 23, p. 1010, rl, 5 Fake Ga1n.
AsP) Labeling Wave Semiconductor Laser Amplifier J (TEEB Journal
of Quantum Electronics, V
ol, 23. pplolo, '1.5μm G
alnAsPtraveling-wave semi
conductor 1aser amplifier
') is described in detail.

FIG. 6 is a perspective view showing an example of the optical limiter 5. FIG. The limiter 5 has an electrode 62. which is divided into two in the direction of the resonator.
The semiconductor laser device 61 includes a semiconductor laser device 63.

To operate this element as a light limiter, the electrode 62
．． 63, the region 64 to which current is supplied from the electrode 62 is brought into a saturable absorption state with a low injection current state, and the region 65 to which current is supplied from the electrode 63 is brought into a gain state with a high injection current state. Furthermore, the gain over the entire region of the resonator is set to be less than the loss, so that laser oscillation does not occur.

In this state, an optical signal 66 is incident on the active layer from one end face of the semiconductor laser element 61. When the optical signal 660 intensity is low, the region 64 acts as an absorber, the resonator loss is large, and no laser oscillation occurs. Therefore, the optical output 67 of the semiconductor laser element 61 is only due to spontaneous emission light, and its intensity is suppressed to a small value. However, as the intensity of the optical signal 66 increases, the resulting optical excitation increases and the absorption of the region 64 becomes saturated and becomes transparent. Therefore, the resonator loss sharply decreases. The gain at this time is sufficiently larger than the laser oscillation threshold, so
A rapid rise in laser oscillation occurs, and the optical output 67 suddenly increases.

In this way, the electrode 62 is divided into two parts in the direction of the resonator.
A differential gain characteristic appears between the input light intensity and the output light intensity of the semiconductor laser element 61 including the semiconductor laser element 63.

This characteristic is shown in FIG.

Regarding the nonlinearity of such semiconductor laser devices, see Optical and Quantum Electronics Volume 19, "Absorbing and Dispersive Pistability in Semiconductor Injection Lasers".
Quantum Blectronics, Vol.
, 19゜” Absorptive and disp
ersive stability insem
iconductor 1injection 1ase
rs”).

FIG. 8 is a block diagram of a self-timing extraction optical circuit according to a third embodiment of the present invention.

This embodiment differs from the first embodiment in that a second optical directional coupler 7 is provided on the optical loop circuit 3 in addition to the optical directional coupler 2 as a branching means.

That is, an optical directional coupler 2 that couples optical signals input to two input ports to one or more output ports, and a pulse intensity modulated input light to a first manual boat of this optical directional coupler 2. An optical directional coupler is used as a manual means for supplying a signal by delaying the optical signal outputted from the first output port of the optical waveguide 1 and the optical directional coupler 2 by a time equal to the clock period of the input optical signal. 2, a second optical directional coupler 7 as branching means for branching an optical signal from this optical loop circuit 3, and this optical directional coupler 7 and an optical limiter 5 that shapes the waveform of the input optical signal and outputs an optical clock signal synchronized with the input optical signal.

In this embodiment, the optical directional coupler 2.7 and the optical loop circuit 3 constitute an optical bandpass filter 10'.

The impulse response of this optical bandpass filter 10' is expressed as follows (5), where both the branching ratios of the optical directional coupler 2.7 are k. When this is Fourier transformed, the absolute value of the transfer function I H(fT) l is 1 H(fT) l =□ 1+ (1-h) '-2(1-11) 'C092π
fT.

FIG. 9 shows the calculation results of the frequency characteristics of the optical bandpass filter 10' using the branching ratio k as a parameter.

In addition, the index Q representing the sharpness of resonance is is given by

FIG. 10 shows the relationship between the branching ratio and the Q value. The smaller the branching ratio k, the larger the Q value becomes, and the branching ratio k becomes 0.01.
When this happens, a Q value of 90 is obtained.

In this embodiment, as in the second embodiment, an optical amplifier can be inserted into the optical loop circuit 3 to increase the Q value.

〔Effect of the invention〕

As explained above, the self-timing extraction optical circuit of the present invention realizes an optical bandpass filter with a high Q value by using an optical loop circuit composed of an optical coupler and an optical delay element, and an optical amplifier. However, by combining this with an optical limiter, a jitter-free optical clock signal synchronized with the input optical signal can be obtained directly from the input optical signal without performing photoelectric conversion.

Since the present invention does not require electrical signal processing, it can be used in optical repeaters, optical exchanges, optical multiplexing/demultiplexing devices, etc., and has the effect of ultra-high-speed processing and simplified configuration. .

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a self-timing extraction optical circuit according to a first embodiment of the present invention. FIG. 2 is a block diagram of a self-timing extraction optical circuit according to a second embodiment of the present invention. FIG. 3 is a diagram showing the operating principle of the first embodiment and the second embodiment. FIG. 4 is a diagram showing calculated values of frequency characteristics of an optical bandpass filter. FIG. 5 is a diagram showing an example of a change in the Q value of an optical bandpass filter. FIG. 6 is a perspective view of the optical limiter. FIG. 7 is a diagram showing the characteristics of the optical limiter. FIG. 8 is a block diagram of a self-timing extraction optical circuit according to a third embodiment of the present invention. FIG. 9 is a diagram showing calculation results of frequency characteristics of an optical bandpass filter. FIG. 10 is a diagram showing the relationship between the branching ratio and the Q value. FIG. 11 is a block diagram of an optical phase-locked loop. 1.4... Leading waveguide, 2.7... Optical directional coupler,
3... Optical loop circuit, 5... Optical limiter, 6...
Optical amplifier, 10.10'...optical bandpass filter. Tail-condominium example;! Pi 1 口 2nd 宷 1st example 2nd figure 111 0911"11 10° 1 principle reflection country 3 fig. :J7 Figure Atsushi three major cases tail 8 Figure Iris 1o times Conventional example fan Figure 11

Claims (4)

[Claims] (1) An optical directional coupler that couples optical signals input to two or more input ports to one or more output ports, and a pulse intensity modulated input light to the first input port of this optical directional coupler. input means for supplying a signal; and a second input port of the optical directional coupler that delays the optical signal output from the output port of the optical directional coupler by a time equal to the clock period of the input optical signal. an optical loop circuit for supplying an optical signal to the input optical signal; a branching means for branching an optical signal from the optical loop circuit; and an optical branching means for shaping the waveform of the optical signal branched by the branching means and outputting an optical clock signal synchronized with the input optical signal. Self-timing extraction optical circuit with limiter. (2) The optical directional coupler includes a second output port, and the branching means is included in the optical directional coupler and is configured to branch the signal of the second input port to the second output port. A self-timing extraction optical circuit according to claim (1). (3) The self-timing extraction optical circuit according to claim 1, wherein the branching means includes a second optical directional coupler provided on the optical loop circuit separately from the optical directional coupler. . (4) The self-timing extraction optical circuit according to any one of claims (1) to (3), wherein the optical loop circuit includes an optical amplifier.