JP3277120B2 - Optical demultiplexer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical demultiplexer used for ultra-high speed optical time division multiplex communication.

[0002]

2. Description of the Related Art In recent years, in the field of optical communication, the bit rate has been greatly increased, and data transmission at a bit rate of several tens of Gbps has become a reality. However, if it is attempted to transmit and receive a data signal of several tens of Gbps as it is, the bandwidth required for the electrical components in the transmitter and the receiver becomes very wide from 0 Hz to several tens of GHz (about the bit rate). It is very difficult to realize such components.

[0003] In order to realize such ultra-high-speed optical communication, the speed is increased by performing optical time division multiplex communication. That is, on the transmission side, a plurality of ultrashort optical pulse trains independently modulated at a lower bit rate are superimposed little by little in phase, and are substantially tens of Gb.
realize ps. Then, on the receiving side, at the time of the optical signal before being converted into an electric signal, the superposed pulse train is again separated into the original low bit rate pulse train, and then each pulse train is received.

An apparatus for separating a pulse train from an optical signal on the receiving side as described above is called an optical demultiplexer. As a conventional optical demultiplexer, for example, a configuration as shown in FIG. 12 is known. In FIG. 12, 102 is an optical distributor, 1
03 is an optical gate, 107 is a clock extraction unit, 109 is an optical receiver, and 124 is a fixed phase shifter (180 °).

In this optical demultiplexer, an incident optical signal is split into signals having the same number of sequences (two in FIG. 12 ) as the original low bit rate pulse train, and a modulator (optical device) provided in each branch is used. (Gate) 103 is turned on at the timing when the bit of the pulse train to be taken out in the branch comes, and the light is transmitted. A modulator that functions in this manner is called an optical gate. The example of FIG. 13 shows a state in which every other bit is taken out by the optical gate (one side for two multiplexing).

As a conventional optical demultiplexer, FIG.
In addition to the configuration shown in FIG. 2, there is a configuration shown in FIG. Here, four series of bit strings are extracted. In FIG. 14, reference numeral 125 denotes a 1/2 frequency divider.

Further, a configuration as shown in FIG. 15 using an optical switch 108 instead of the modulator 103 is also conceivable. For such a configuration, the phase of the drive signal for driving the optical gate 103 or the optical switch 108 of the optical demultiplexer is important. For example, in the optical demultiplexer shown in FIG. 12, the timing at which each optical gate 103 is turned on may be as shown in FIG. 13, but the phase is shifted, as shown in FIG. 16 (a) or (b). When it comes
Correct separation operation cannot be obtained.

[0008] Since the optical time division multiplexed signal is very fast, the delay difference between the drive signal of the optical gate or the optical switch and the optical signal is shifted by only a few psec to tens of psec (the allowable range of the shift is as follows). (Depending on the multiplexed bit rate), a correct separation operation is not possible. This degree of delay shift depends on the size of the system, but can sufficiently occur even if the ambient temperature changes, for example.

However, there is no measure against the fact that the phase difference between a signal for driving an optical gate or an optical switch used for pulse separation in an optical demultiplexer and an input optical signal varies depending on the surrounding environment. If the driving is continued, there is a possibility that a correct separation operation cannot be performed.

[0010]

As described above, conventionally,
If no countermeasures have been taken for the phase difference between the signal driving the optical gate or optical switch of the optical demultiplexer and the optical signal to change due to the surrounding environment, and the optical demultiplexer has been driven for a long time, There was a risk that correct separation operation could not be performed.

The present invention has been made in view of the above circumstances, and is an optical demultiplexer used for ultra-high-speed optical time division multiplexing communication, which can stably operate for a long time irrespective of a change in ambient temperature. An object of the present invention is to provide an optical demultiplexer capable of continuously performing a pulse separation operation.

[0012]

SUMMARY OF THE INVENTION The present invention relates to a pulse separating means for separating a desired pulse train from an input optical signal in which a plurality of pulse trains are time-division multiplexed and outputting an output optical signal, and the input optical signal or the pulse. A drive signal generating unit for generating a drive signal for the pulse separating unit based on a clock signal extracted from the output optical signal of the separating unit; and at least one of the input optical signal and the drive signal. a variable phase means for displacing the phase, the path
Clock of the output optical signal output from the pulse separation means.
Detecting means for detecting a frequency component, and the detecting means
So that the detected clock frequency component is maximized
And a phase shift amount control means for feedback- controlling the phase shift amount of the variable phase shift means.

In the present invention, the delay of the components and cables in the system changes depending on the temperature, and as a result, the phase for driving the pulse separating means deviates (relatively) from the phase at which the desired optical pulse can be correctly separated. In order to prevent this from happening, for example, how much the temperature changes and the phase of the drive signal deviates (relatively) are measured in advance, and based on the information, the variable phase shift means (for example, put into the optical path) is used. The variable optical phase shifter or the electrical variable phase shifter inserted in the drive signal path) changes the delay so as to substantially cancel the change in delay due to temperature. By doing so, a desired light pulse can be separated at a correct phase regardless of the temperature.

[0014]

According to the present invention, the most appropriate phase state can be obtained by detecting how much the pulse separation operation deviates from the most appropriate phase state and feeding that information back to the variable phase means. The most appropriate phase state is, for example, a state in which an optical pulse is gated substantially at the center of an optical gate waveform, or a state in which an optical pulse is approximately at the center of an adjacent switch time.

[0016]

According to the present invention, in order to perform delay compensation, at least one of the optical signal path and the drive signal path includes
A device having a delay characteristic that almost correctly cancels a change in delay due to a temperature of a component or a cable used in the optical demultiplexer is inserted, and the change in delay is canceled by each other, so that the temperature is reduced. , The phase for driving the pulse separation means can be prevented from deviating from a substantially correct state. As described above, according to the present invention, the pulse separation operation can be stably performed for a long time irrespective of the occurrence of a temperature change in the surrounding environment.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) First, a first embodiment of the present invention will be described.

The present embodiment extracts an optical input that is optical time-division multiplexing, the optical input itself (or a transformation into pulses separated <br/> was light output or the electric signal it from light input) In an optical demultiplexer that performs pulse separation by an optical gate or an optical switch driven by a drive signal generated based on the generated clock, the temperature within the optical demultiplexer system (for example, inside the housing containing the element or The relationship between the temperature measured in the vicinity and the phase shift amount between the optical input and the drive signal is checked beforehand, and the phase shift amount is calculated from the temperature measured by the temperature detecting element provided in or near the housing. By estimating and giving a phase shift to the optical input or the drive signal (or both the optical input and the drive signal) using a variable phase shifter to cancel the above-mentioned phase shift amount, It is obtained so as to eliminate.

FIG. 1 shows the configuration of the optical demultiplexer according to the present embodiment. In the present embodiment, an example of a 1: 2 optical demultiplexer using an optical gate as a pulse separation element is shown, and optical distributors 2 _a and 2 _b , optical gates 3 ₁ and 3 ₂ , and a variable phase shifter 4 are shown. ₁ , 4 ₂ , a temperature detection unit 5, a signal processing unit 6, and a clock extraction circuit 7.

As shown in FIG. 1, in the optical demultiplexer 1 according to the present embodiment, a part of an incident optical signal is branched by an optical distributor _2a , and then a clock component is extracted by a clock extracting circuit 7, and pulse separation is performed. A drive signal to be used is generated.

The remaining light branched by the optical distributor 2 _a is branched into two by the following optical distributor 2 _b for pulse separation, two light gates 3 _1, 3 ₂ are respectively incident on. Light gate 3
_1, 3 to _2, for example, it is used and an electro-absorption modulator and LiNO ₃ intensity modulator.

Two optical gates 3 from the clock extracting circuit 7
_1, 3 to the drive signal path for sending a ₂ to drive signals, branch unit b
Variable phase shifters 4 ₁ and 4 ₂ are arranged in the rear part of ₁ in such a manner as to be inserted respectively. These variable phase shifters 4 ₁ and 4 ₂
The phase of the drive signal is shifted by an amount corresponding to the signal from the signal processing unit 6.

A temperature detecting section 5 is installed inside (or near) the housing of the optical demultiplexer 1, and the temperature is detected by the temperature detecting section 5, and the result is sent to the signal processing section 6 as temperature information. Have given. For a temperature measurement point, one-point measurement or multiple-point measurement may be used. When the temperature is measured at a plurality of points, all the data may be used as the temperature information, or the data obtained by performing processing such as averaging may be used as the temperature information.

The signal processing section 6 stores information on the relationship between the temperature inside (or near) the housing measured in advance and the amount of phase shift between the optical input and drive signals. The temperature measurement location for creating this information is preferably the same as the installation location of the temperature detector 5 or as close as possible.

The signal processing unit 6 is located inside (or near) the housing.
Optical gates 3 ₁ the temperature transitions, 3-position <br/> phase change for canceling a phase shift occurring between the input optical signal and the drive signal in the ₂ to give to the driving signal, the temperature from the temperature detecting section 5 The variable phase shifters 4 ₁ and 4 ₂ are controlled based on the above information. Needless to say, in order to perform two-sequence pulse separation, the phase difference between the two drive signals applied to the optical gates 3 ₁ and 3 ₂ is always set to be 180 degrees with respect to the phase of the optical input pulse. You.

As described above, even if the inside of the housing (or near
Even if the temperature of the ( side) temperature changes, the optical gates 3 ₁ , 3 ₂
Since the phase shift between the optical signal and the drive signal with respect to the light is eliminated, the light incident on the optical gates 3 ₁ and 3 ₂ is subjected to the pulse separation operation of the optical gates 3 ₁ and 3 ₂ by the temperature compensated drive signal. Thus, the pulse is accurately separated as a two-series pulse train.

Next, the contents of temperature measurement and control will be described in more detail. For example, pulses of light input at a predetermined reference temperature is the optimum state condition substantially at the center of the optical gate waveform, referenced to the timing of punching the optical gates 3 _1, 3 ₂ at this time. Then, in order to determine how much the phase of the variable phase shifters 4 ₁ and 4 ₂ should be changed with respect to the detected temperature, a temperature test of the optical demultiplexer system is performed. Light gates 3 ₁ and 3
Measure how much the timing of punching ₂ deviates from the reference (how much the phase deviates).

As a result of the temperature test, if the phase shift to be punched is linear with respect to the change in temperature (or if the shift is linear with respect to the change in the output value of the temperature detection unit 5), the temperature detection unit 5 outputs a signal such as an operational amplifier. through simple linear circuit, and converted into a control signal for changing the amount of the appropriate phase (phase amount to cancel the phase shift due to temperature variation), variable phase shifter 4 _1,
4 may be input _2.

Even if the variation of the punching phase with respect to the temperature is nonlinear, as long as the variation is relatively simple, a simple nonlinear element such as a diode can be used as in the case described above.
A control signal for canceling the phase shift due to temperature can be input to the variable phase shifters 4 ₁ and 4 ₂ .

When the temperature changes in a complicated manner, it is preferable that the information be stored in a memory and controlled using a device capable of higher control such as a microprocessor.

It is desirable that the variable phase shifter 4 changes the phase continuously with respect to the control signal. Further, even if the signal changes discretely, the signal does not break when the phase changes (or if the signal does
Anything that is much shorter than a bit and hardly affects operation) can be used.

In the example of FIG. 1, the fixed phase shifter is omitted.
The phase difference (180 ° in the case of the above two multiplexing) set between the drive signals applied to the respective optical gates is controlled only by the variable phase shifter. However, the fixed phase shifter 24 may be used together as shown in FIG. Absent.

In FIG. 1, the variable phase shifters 4 ₁ and 4 ₂ are inserted in the path from the clock extracting unit 7 to the optical gates 3 ₁ and 3 ₂ behind the branching unit b ₁ , respectively. If the delay characteristics with respect to the temperature of the two paths ahead of the signal branch part b1 are substantially the same, a variable phase shifter is provided in a part (part A in FIG. 1) ahead of the branch part b1 as shown in FIG. May be arranged only one. In this case, there is an advantage that the number of variable phase shifters can be reduced. In the figure, reference numeral 25 denotes a fixed phase shifter.

Further, instead of inserting a variable phase shifter for an electric signal into the path of the drive signal, as shown in FIG.
Between each optical gate and the optical distributor (positions C and D in FIG. 1)
4B, or only one optical variable phase shifter 34 may be inserted before the optical distributor (position B in FIG. 1) with respect to the optical gate as shown in FIG. 4B. Phase shifter 34
May be added.

[0036] Although an optical gate or an optical switch can be used like a nonlinear loop mirror, in this case, there pass the light in the path of the drive signal from the clock extracting unit 7 to the optical gates 3 _1, 3 ₂ Therefore, a variable optical phase shifter may be inserted into the optical path portion of the drive signal path.
In this case as well, one may be inserted before the branch part, or two may be inserted after the branch part, as described above.

In the above example, the columns to be separated at each branch are distinguished by the phase difference of the drive signal applied to each optical gate. Instead, a fixed phase shifter is provided between the optical gate and the optical distributor. And pulse separation may be performed by giving a phase difference between the optical signals input to the respective optical gates. Also, in this case, when adopting the configuration in which the variable optical phase shifters are inserted between the respective optical gates and the optical distributor as described above, the function of the fixed phase shifter is the same as in FIG. A variable phase shifter may be used, or a fixed phase shifter and a variable phase shifter may be provided independently as in FIG .

FIG . 5 shows another example of the configuration of the optical demultiplexer of the present embodiment. The present embodiment is an example of a 1: 2 optical demultiplexer using an optical switch as a pulse separation element, and includes an optical distributor 2 _a , a variable phase shifter 4, a temperature detector 5, a signal processor 6, and a clock extractor. A unit 7 and an optical switch 8 are provided. Optical distributor 2 _a, variable phase shifter 4, the temperature detection unit 5, the signal processing unit 6, the clock extraction part 7 are those having the same configuration and function as FIG. The optical switch 8
A signal is alternately output to two output paths in synchronization with the frequency of the drive signal.
It is composed of NO ₃ or the like. The details of the temperature measurement and control are the same as described above.

In the first embodiment, as an example of the optical demultiplexer using the optical gate as the pulse separating element, the description has been made with the 1: 2 optical demultiplexer for easy understanding. In addition, the present invention is applicable to a 1: n optical demultiplexer that shifts out a pulse train of each system by 360 / n (n is an integer of 2 or more) degrees. Also in this case, either a variable phase shifter for electric signals or a variable phase shifter for light may be used, one of them may be provided commonly to each system, and one may be provided for each system. May be.

In each of the above examples, the clock is extracted from the input optical signal . However, the clock may be obtained from the optical demultiplexer output signal as shown in FIG. 6 , or may be obtained from the receiver as shown in FIG . In these cases, it is preferable to collect the system including the part up to the point where the clock is extracted as compact as possible, configure the system so that the internal temperature does not vary, measure the temperature, and control the phase in the same manner as described above. In addition,
In FIG. 6 , 1 is an optical demultiplexer, 2 is an optical distributor, 7 is a clock extracting unit . In FIG. 7 , 1 is an optical demultiplexer, 9 is an optical receiver, 10 is a clock signal or an optical gate or an optical switch. Is the driving signal of the first embodiment. As described above, various methods can be used as a path until an input to a clock extraction unit that outputs a drive signal of an optical gate or an optical switch is obtained in the following embodiments.

(Second Embodiment) Next, a second embodiment of the present invention will be described. As a second embodiment, an embodiment will be described in which a deviation from a phase state that provides an optimal separation operation is detected, and the detection result is fed back to a variable phase shifter to maintain an optimal phase state. In addition,
In the second embodiment, in order to make the description easy to understand,
A portion for extracting one series from an optical signal multiplexed by n (n is an integer of 2 or more) in a time division manner is shown as a basic configuration, and the description of the entire configuration is omitted.

FIG. 8 shows the basic configuration of an optical demultiplexer according to this embodiment. The optical demultiplexer 2 _c , optical gate 3, variable phase shifter 4, signal processor 6, clock extractor 7, photodetector 11, band A pass filter 12 is provided. In addition,
In the figure, a path until an optical input is given to the optical gate 13,
Although a path up to obtaining an input to the clock extracting unit 7 that outputs a drive signal of the optical gate 3 is omitted, various types can be considered as described in the first embodiment.

In FIG. 8, an optical signal branched by one or a plurality of optical distributors (not shown) provided on the input side is pulse-separated by an optical gate 3. A part of the output of the optical gate 3 is branched by the optical distributor _2c , received by the photodetector 11, and the frequency component of the carrier of the extracted data string is extracted by the bandpass filter 12.

This carrier frequency component (clock frequency)
The state in which component (2) is maximized is the optimum phase shift state. Therefore, the signal processing unit 6 to which the output of the band-pass filter 12 has been given is set so that the frequency component is maximized (for example, if the bit rate of the data stream is 10 Gbps,
Gbps component is maximized), and the clock extraction unit 7
The phase shift amount of the variable phase shifter 4 inserted in the path of the drive signal from the optical gate 3 to the optical gate 3 is controlled.

In the control for maximizing the carrier frequency component, the phase of the variable phase shifter 4 is always shifted back and forth by a small amount as in the hill-climbing method, and the phase is changed in such a manner that the carrier frequency component increases. In either case, the phase at which the carrier frequency component decreases is the optimum point. Alternatively, when the carrier component decreases below a certain threshold, the phase of the variable phase shifter 4 may be shifted back and forth by a small amount to change the phase in the direction in which the carrier frequency component increases.

At this time, the gate waveform of the optical gate is as shown in FIG.
As shown in (a), the waveform must have a maximum near the center and decrease as approaching both ends.
As shown in FIG. 9B, in the rectangular gate waveform, even if the phase of the gate waveform is slightly shifted, the optical power transmitted through the optical gate hardly changes until approaching the edge of the gate waveform. This is because no change is observed. The gate waveform changed as shown in FIG.
It can be easily obtained by simply driving the optical gate (or switch) by the O3 modulator with a sine wave.

As described above, according to the present embodiment, the phase difference between the optical pulse and the optical gate waveform is detected, and feedback control is performed so that the phase difference is an appropriate value. A stable optical demultiplexing operation can be performed.

Although a part of the light is split by the optical distributor 2 in FIG. 8 , an electric signal received by a photodetector of a receiver (not shown) may be split and used.

Also in this embodiment, instead of providing the variable delay unit 4 for each of a plurality of streams for performing pulse separation, only one variable delay unit 4 is provided on the path between the clock extraction unit 7 and the branch unit. May be provided (see FIG. 3), and a variable delay may be inserted into the optical path (see FIG. 4).
The same can be said for the points described in the first embodiment.

The advantage of performing the feedback control as in the present embodiment is that it can cope with a change in the delay variation characteristic of the system with respect to temperature due to aging or replacement of parts.

(Third Embodiment) Next, a third embodiment of the present invention will be described. In the present embodiment, the phase change of the waveform driving the optical gate or the optical switch is optimized even if the temperature changes, by canceling out the changes in the delay due to the temperature of the components and cables used in the optical demultiplexer. Is to be held.

FIG . 10 shows the basic configuration of the optical demultiplexer according to the present embodiment, which includes an optical distributor 2, an optical gate 3, a component 23 having a delay compensation function, and a clock extracting unit 7. Figure
After all necessary components and cables are mounted in a system such as 10 , a temperature test is performed to determine how much the phase difference between the optical pulse passing through the gate and the optical gate waveform varies with temperature. For example, if the phase difference shifts by 1 ps with respect to a temperature of 1 degree in the direction in which the optical gate waveform advances, a component having a delay compensation function such as a cable that delays at a rate of 1 ps / ° C. to the path of the signal for driving the optical gate. 23
Should be inserted.

FIG . 11 shows another example of the basic configuration of the optical demultiplexer according to the present embodiment, which comprises an optical gate 3, an optical receiver 9, and a component 23 having a delay compensation function. In the system where the clock is extracted by the receiver as shown in FIG. 11, the phase change due to the temperature of the loop that applies the drive signal from the optical gate to the receiver and the optical gate through the clock extraction system is almost equal to the combination of the components. What is necessary is just to make it zero. If the value does not become zero, a component 23 having a delay compensation function for adjusting the difference may be inserted in the same manner as described above.

These methods have the advantage that they are very simple and inexpensive. The present invention is not limited to the above-described embodiment, and can be implemented with various modifications within the technical scope.

[0055]

According to the present invention, the temperature in the optical demultiplexer is detected, and the fluctuation of the phase difference between the optical signal and the drive signal due to the temperature is controlled based on the detected temperature. A long-term stable optical demultiplexing operation can be performed.

[0056] Further, according to the present invention, detects the phase difference of the light signal and the drive signal, whereby is possible to perform feedback control with such a so that an appropriate value, long-term stability that does not depend on changes in the surrounding environment A simple optical demultiplexing operation becomes possible.

Furthermore, according to the present invention, the components and temperature delay characteristics of cables used in the optical demultiplexer adjusted, configured to delay variation with temperature change occurs is absorbed cancel each other By
An optical demultiplexing operation that is stable for a long period of time regardless of changes in the surrounding environment can be performed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an optical demultiplexer according to a first embodiment.

FIG. 2 is a diagram showing a modification of the embodiment.

FIG. 3 is a diagram showing another modification of the embodiment.

FIG. 4 is a view showing still another modification of the embodiment.

FIG. 5 is a diagram showing another configuration of the optical demultiplexer according to the embodiment;

FIG. 6 is a diagram showing a modification of the embodiment.

FIG. 7 is a view showing another modification of the embodiment.

FIG. 8 is a diagram illustrating a configuration of an optical demultiplexer according to a second embodiment.

FIG. 9 is a diagram for explaining a relationship between a gate waveform of an optical gate and an optical pulse.

FIG. 10 is a diagram illustrating a configuration of an optical demultiplexer according to a third embodiment.

FIG. 11 is a diagram showing another configuration of the optical demultiplexer according to the embodiment;

FIG. 12 is a diagram showing an example of a conventional optical demultiplexer.

FIG. 13 is a diagram for explaining the operation of pulse separation of the optical demultiplexer.

FIG. 14 is a diagram showing another example of a conventional optical demultiplexer.

FIG. 15 is a diagram showing still another example of the conventional optical demultiplexer.

FIG. 16 is a diagram for explaining a phase shift in a conventional optical demultiplexer.

[Explanation of symbols]

1 ... optical demultiplexer 2, 2 _a, 2 _b, 2 _c ... light distributor 3,3 _1, 3 ₂ ... optical gate 4,4', 4 _1, 4 ₂ ... variable phase shifter 5 ... temperature detector 6 ... Signal processing unit 7 ... Clock extraction unit 8 ... Optical switch 9 ... Optical receiver 11 ... Photo detector 12 ... Band pass filter 13 ... Electro-absorption modulator 23 ... Parts with delay compensation function 24 ... Fixed phase shifter 34 ... Variable Phase shifter

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Morita Itsuro 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation (72) Inventor Shu Yamamoto 2-3-Nishi-Shinjuku, Shinjuku-ku, Tokyo 2.International Telegraph and Telephone Co., Ltd. No. 1, Toshiba Town Inside R & D Center, Toshiba Corporation (56) References JP-A-7-58777 (JP, A) JP-A-5-153108 (JP, A) JP-A-9-214470 (JP, A) ( 58) Fields surveyed (Int. Cl. ⁷ , DB name) H04J 3/00-3/26 H04L 5/18-5/26 H04B 10/00 JICST file (JOIS)

Claims (1)

(57) [Claims] A pulse separation means for separating a desired pulse train from an input optical signal in which a plurality of pulse trains are time-division multiplexed and outputting an output light signal, and a pulse separation means for the input light signal or the pulse separation means. A drive signal generating means for generating a drive signal for the pulse separation means based on a clock signal extracted from the output optical signal, wherein the phase of at least one of the input optical signal and the drive signal is shifted. Variable phase shifting means for detecting, a detecting means for detecting a clock frequency component of the output optical signal output from the pulse separating means, and the variable shifting means for maximizing the clock frequency component detected by the detecting means. An optical demultiplexer comprising: a phase shift amount control means for feedback-controlling the phase shift amount of the phase means.