JPH0439837B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical switch that performs exchange control of time-division optical signals.

Optical communication systems using optical fibers as transmission paths are capable of high-speed, large-capacity signal transmission, and various transmission methods have been put into practical use. In particular, time-division transmission of digital signals that takes advantage of high speed is one of the most important methods.
It is one. In optical communication systems currently in practical use, optical signals are simply transmitted through optical fibers, and the signals are exchanged after being converted into electrical signals. As mentioned above, in the method of converting optical signals into electrical signals and exchanging them, when transmitting the exchanged signals again, it is necessary to convert the electrical signals back into optical signals, which makes the equipment complicated and costs high. The disadvantage is that it is expensive. Another drawback is that it is difficult to exchange high-speed signals of 100 megabits per second or more using conventional time-division switching equipment for electrical signals.

As a means to solve the above-mentioned drawbacks, an optical exchanger that exchanges time-division optical signals as optical signals has been proposed. This optical switch was published in the patent application published in 1983.
117311, and is constructed by connecting a plurality of matrix optical switches each having N×M input/output terminals with a group of optical fibers having mutually different lengths. That is, an optical fiber is used as a delay line. Basically, time-division optical signals input to an optical switch are distributed to different fibers for each time slot, each delayed by a different time, and then combined in the next optical switch to change the time order of the signals. It is. However, in optical switching equipment that uses such optical fiber delay lines, the delay time is determined by the length of the optical fiber, and long optical fibers are required to exchange signals with long frame periods. Therefore, there were drawbacks such as an increase in the size of the device. In addition, since time division switching is performed simply by selecting the delay time of the optical transmission line, loss occurs at the connection between the optical switch and the optical fiber, and the output optical signal level is lower than the input optical signal level. It also had the disadvantage of a significant drop.

The purpose of the present invention is to eliminate the above-mentioned drawbacks of the conventional time-division optical signal exchanger, to allow the length of the frame period to be exchanged to be arbitrarily set, to easily miniaturize the device, and to easily achieve a high output optical signal level. The purpose of the present invention is to provide a time-division optical switching system that can be obtained in a timely manner.

The time-division optical switch of the present invention includes a writing optical switch having one input end and a plurality of output ends, and a light emitted from the output end entering each output end from a laser resonator end face. Semiconductor lasers exhibiting bistable operation that are installed in correspondence with each other, and a plurality of input terminals and one output that correspond to each of the semiconductor lasers so that the emitted light from the semiconductor lasers enters the semiconductor lasers. The optical switch comprises an optical readout switch having an end, an optical switch drive circuit for each of the optical switches, a current drive circuit for the semiconductor laser, and a central control unit that controls the timing of each of the drive circuits.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of a time division optical switch according to the present invention. In FIG. 1, an input optical waveguide 310 for inputting a first time-division optical signal 100 consisting of four time slots carrying different information, and an output optical waveguide for outputting a second time-division optical signal 190. 380, the input terminal is connected to the input optical waveguide 310, and the output terminal is connected to the input optical waveguide 310.
Semiconductor lasers 340, 350, exhibiting bistable operation
Optical waveguide 34 guided to the input ends of 360 and 370
1,351, 361, and 371, respectively, and an optical waveguide 34 guided to the output end of the bistable semiconductor laser.
A readout 4×1 optical switch 330 is installed whose input terminals are connected to 2, 352, 362, and 372, respectively, and whose output terminals are connected to the output optical waveguide 380. Optical switch drive circuits 321 and 331 are connected to the optical switches 320 and 330, respectively, and bistable semiconductor lasers 340, 350,
Current drive circuits 345, 355, and 36 for injecting bias currents are provided at 360 and 370, respectively.
5,375 are connected. Optical switch drive circuits 321, 331 and current drive circuits 345, 35
5, 365, 375 are connected to a central control unit (not shown). The central control unit includes a timing extraction circuit, a memory circuit, etc., and generates control signals for controlling the optical switch drive circuit and current drive circuit.

The current drive circuit is a device that generates a binary current according to a command from the central controller, and a simple example thereof is shown in FIG. 2a.

This current drive circuit consists of a transistor Tr and two resistors. The collector of the transistor Tr is connected to the bistable semiconductor laser via a resistor _R1 . The controller and collector are connected through a resistor _R2 , and the base terminal serves as the input terminal for the control signal.

In Fig. 2a, when a sufficiently large negative voltage is applied to the base of the transistor Tr, the transistor _Tr is turned off, so that a current i=i ₀
=V/(R ₁ +R ₂ +r ₁ ) flows. However, here r ₁ is the resistance of the bistable semiconductor laser LD ₁ .

On the other hand, when the base potential of the transistor Tr is 0, the transistor Tr is turned on and the current i of the bistable semiconductor laser LD ₁ becomes i=i ₆ =V/(R ₁ +r ₁ +r ₂ ). However, here r ₂ is the resistance of the transistor Tr,
Let R ₂ >> r ₂ . As described above, by using the circuit shown in Figure 2a, the bistable semiconductor laser LD ₁ and 2 are connected to each other by an external signal.
A value of current can flow.

Here, specific examples of the 1×4 optical switch 320 for writing and the 4×1 optical switch for reading of this embodiment, bistable semiconductor lasers 340, 350, 360, 37
A specific example of 0 will be explained.

First, FIG. 2b shows the 1×4 optical switch 32 for writing.
An example of a directional coupling type optical switch that can be used as a 0 or read 4x1 optical switch 330 is shown. Four directional coupling type optical switches 21 and 2 formed on a ferroelectric crystal or semiconductor crystal substrate 20
2, 23, and 30. For example, the above-mentioned directional coupling type optical switch can be obtained by creating an optical waveguide by diffusing titanium on a lithium niobate crystal and placing electrodes on the optical waveguide close to each other. The directional coupling type switch 30 is a switch for cutting off the optical signal passing through the optical waveguide 24. Optical waveguide 24 for input, optical waveguide 2
When 5, 26, 27, and 28 are used for output, they can be used as 1×4 optical switches for writing, and when the input and output are reversed, they can be used as 4×1 optical switches for reading.

The optical switch drive circuits 321 and 331 generate electrical signals for controlling the optical switches 320 and 330 under the control of the central control unit. For example, the light switch 32
In the case of the optical switches 0 and 330 shown in FIG. 2b, they have the function of supplying two different voltages to the directional coupling type optical switches 21, 22, 23, and 30, respectively. Each of the above directional coupling type optical switches 21, 22, 23, 30 has an applied voltage of 0.
When it is assumed that the light wave incident on one optical waveguide passes through the incident optical waveguide as it is, and when the voltage is set to _V1 , the incident light moves to the other adjacent optical waveguide. The simplest configuration can be a circuit using one transistor for each of the above-mentioned directional coupling type optical switches.

For example, connect the collector of a transistor to a power supply with a voltage of V ₁ through a resistor R, and ground the emitter.
It is constructed by using the base as an input terminal for a control signal and outputting an output voltage terminal from the collector. In this circuit, when the voltage of the control signal is 0, the output voltage
V ₁ and the output voltage becomes 0 when the control signal voltage is positive.

Figure 3a shows bistable semiconductor lasers 340, 35.
It is a sectional view showing a specific example of 0,360,370. The structure is almost the same as a commonly used current injection type semiconductor laser, for example, GaAlAs/
This is a laser with a double heterojunction structure made of GaAs or InGaAsP/InP. However, it differs from a normal semiconductor laser in that the electrodes are not uniform and there are some parts into which current is not injected. Since the current non-injection region acts as a saturable absorber, the bistable semiconductor laser shown in FIG. 3a can have bistable characteristics in the injection current vs. optical output characteristic. Note that instead of making the electrode non-uniform, the active layer, which is the light emitting region, is made non-uniform.
A similar bistable characteristic can be provided by providing a saturable absorption region in a part. In these bistable semiconductor lasers, by appropriately selecting the injection current i, bistable characteristics can be obtained in the characteristics of the emitted light with respect to the externally injected light. Details of such bistable semiconductor lasers are described in the literature Electronics Letter, Volume 17, page 741, and in the 1981 IEICE National Conference on Optical and Radio Division Proceedings (Vol. 2), No. 272. .

3b, c, and d are diagrams for explaining the operation of the bistable semiconductor laser shown in FIG. 3a. Third
FIG. b is a diagram showing the relationship between the injection current i and the output light amount Pout when the incident light amount Pin=0. That is, when the injection current i is increased from i ₀ , the output light amount Pout increases rapidly at i=i _a , and conversely, the injection current i
When is decreased from i _t , the output light amount Pout is i=
It exhibits a hysteresis characteristic that rapidly decreases at i _c and has two stable points A and B of the output light amount P ₀ and P ₁ at i = i _b . FIG. 3c is a diagram showing the relationship between the amount of incident light Pin and the amount of output light Pout when the injection current i=i _b in FIG. 3b. In other words, if the input light amount Pin is increased from 0 when the output light amount Pout= _P0 is at the first stable point A, the output light amount
Pout increases rapidly at Pin=P _a , and after that, the incident light amount Pin
When the output light amount Pout is decreased from = _Pt , the output light amount Pout hardly decreases and moves to the second stable point B of the output light amount Pout= _P1 . Points A and B in FIG. 3c represent the same points as points A and B in FIG. 3b, respectively. FIG. 3d is a table showing the operation of the bistable semiconductor laser with respect to the injection current i and the incident light Pin. When the injected current i is i _b and the incident light Pin is 0, the bistable semiconductor laser is located at one of the two stable points A and B in Figure 3b according to the previously written data. , output light amount P ₀
Or maintain P ₁ . In Fig. 3b, the bistable semiconductor laser is at one stable point B (output light amount P ₁ ).
When holding the injected current i, once i ₀ , then
When returning to i _b , the output light amount Pout changes in the order of B→D→A, and thereafter maintains the other stable point A (output light amount P ₀ ). That is, the bistable semiconductor laser is reset. In addition, in Fig. 3c, when the bistable semiconductor laser maintains the stable point A (output light amount P ₀ ), the input light amount is changed to P _t once and then returned to 0 again.The output light amount Pout changes in the order of A→E→B. From then on, stable point B (output light amount
P ₁ ) is retained. That is, the bistable semiconductor laser is set. Furthermore, when the injection current i is i ₀ and the incident light amount Pin is P _t , the output light amount Pout shows a value P ₂ depending on the characteristics of the bistable semiconductor laser, but this light amount is not directly used in the present invention, so it will not be explained. Omitted.

FIG. 4 is a time chart specifically for explaining the writing and reading operations of the bistable semiconductor laser 340 among the operations of the embodiment shown in FIG.
Referring to FIGS. 3 and 4, the time-division optical signal 100 is guided to the optical waveguide 310 shown in FIG. The optical signal 400 in FIG. 4 is a time-division optical signal 100.
information A, B, C, D of 1 bit each
A specific example in the case of an NRZ signal will be shown. Here, the light intensity is 0 and P _t shown in Fig. 3 as signals 0 and 1, respectively.
Requires. In FIG. 1, a current having a current value i _b shown in FIG. 3 is constantly injected into bistable semiconductor lasers 340, 350, 360, and 370 by current drive circuits 345, 355, 365, and 375, respectively. When using the current drive circuit shown in FIG. 2a, the transistor Tr is on in this state. In addition, optical waveguides 341, 351, 36
1,371 is always separated from the optical waveguide 310 by the optical switch 320. Similarly, the optical waveguide 380 is always separated from the optical waveguides 342, 352, 362, and 372 by the optical switch 330. The light switches 320 and 330 are shown in FIG.
In the case of the configuration shown in FIG.
1 or 331, voltage _V1 is applied, and the directional coupling type optical switch 30 is in a cut-off state.

In FIG. 1, information A of the time-division optical signal 100 is written into the bistable semiconductor laser 340 as follows. That is, first, in the first period in the first time slot of the time-division optical signal 100, the current i injected into the bistable semiconductor laser 340 is
is once reduced to i ₀ and then returned to i _b as shown in FIG. 4 410. This is achieved, for example, by turning off the transistor Tr of FIG. 2a and turning it back on again. As a result, bistable semiconductor laser 3
The output light amount Pout of 40 is reset to _P0 at the leading edge 411 of the injection current pulse 410, even though it previously held the light amount _P1 , as shown in FIG. 4440. Next, in a second period following the first period in the first time slot of the time-division optical signal 100, the optical waveguide 3 is
10 is connected to the optical waveguide 341. In the case of the configuration shown in FIG. 2b, the voltages 0, 21, and 22 of the directional coupling type switch 30 are set to the voltage _V1 . Figure 4 42
0 indicates a control voltage supplied from the optical switch drive circuit 321 to the optical switch 320 in order to cause the optical switch 320 to perform the above connection operation. For example, in the case of the optical switch of FIG. 2b, the supply voltages to the directional coupling type optical switches 21 and 22 are shown. The optical switch 320 connects the optical waveguide 310 and the optical waveguide 341 only when the control voltage 420 is _V1 .

As a result, the input end of the bistable semiconductor laser 340 receives a time-division optical signal 400 as shown in FIG.
An optical signal is obtained in which only the first time slot of the signal is extracted by the control voltage 420. As a result, the amount of light 440 emitted from the bistable semiconductor laser 340 is
When the incident light amount 430 is P _t , it is set to P ₁ at the leading edge 431 of the incident light pulse 430;
When the amount of incident light 430 is 0, P ₀ is held. In this way, information A of the time-division optical signal 100 is written into the bistable semiconductor laser 340.
Similarly, the optical switch drive circuit 321 and the current drive circuits 355, 365, 375 are connected to the optical switch 320 and the bistable semiconductor lasers 350, 36, respectively.
By controlling the currents injected into the bistable semiconductor lasers 350, 360, and 370, information B, C, and D of the time-division optical signal 100 are written into the bistable semiconductor lasers 350, 360, and 370, respectively. Time-division optical signal 1 of information A written in the bistable semiconductor laser 340 in this way
Reading to 90 is performed as follows. That is, the optical waveguide 342 is connected to the optical waveguide 380 by the optical switch 330 at, for example, the fourth time slot of the time-division optical signal 190.
FIG. 4 450 shows an optical switch drive circuit 331 for causing the optical switch 330 to perform the above connection operation.
shows the control voltage supplied to optical switch 330 by. Here, the optical switch 330 has a control voltage of 45
Optical waveguide 342 and optical waveguide 38 only when 0 is V ₁
0 shall be connected. For example, when the optical switch shown in FIG. 2b is used as the optical switch 330,
The control voltage 450 is connected to the directional coupling type optical switch 21,
The voltage applied to 22 is shown. As a result, the optical waveguide 38
0, an optical signal is obtained by extracting the emitted light 440 of the bistable semiconductor laser 340 using the control voltage 450, as shown in FIG. 4 460. Similarly, the optical waveguide 380 is further provided with the first signal of the time-division optical signal 190;
In the second and third time slots, information D, C, and B held in bistable semiconductor lasers 370, 360, and 350, respectively, are read out. In the time-division optical signal 190 obtained in the optical waveguide 380 in this way, the information A of the time-division optical signal 100 is
and D and B and C are exchanged.

In the embodiment shown in FIG. 1, the length of the frame cycle of exchanged information can be set arbitrarily by controlling the write and read optical switches, and the apparatus is compact because it does not use a long optical fiber as in the conventional case. It is easy to convert. Further, by using a bistable semiconductor laser with a high output light level, it is possible to make the light amount of the second time-division optical signal 100 larger than that of the first time-division optical signal 190.

FIG. 5 is another time chart for explaining especially the write and read operations of the bistable semiconductor laser 340 among the operations of the embodiment shown in FIG. Referring to FIGS. 3 and 5, the time-division optical signal 100 is guided to the optical waveguide 310 shown in FIG. The optical signal 500 in FIG. 5 is the time-division optical signal 1
00 information A, B, C, D of 1 bit each
A specific example in the case of an NRZ signal will be shown. Here, the light amount 0 and P _t shown in Fig. 3 are respectively set as signals 0 and 1.
Requires. In FIG. 1, a current having a current value i _b shown in FIG. 3 is constantly injected into bistable semiconductor lasers 340, 350, 360, and 370 by current drive circuits 345, 355, 365, and 375, respectively. In addition, optical waveguides 341, 351, 36
1,371 is separated from the optical waveguide 310 by an optical switch 320. Similarly, optical waveguide 3
80 is connected to the optical waveguide 3 by the optical switch 330.
42, 352, 362, 372. In FIG. 1, a bistable semiconductor laser 340
Information A of the time-division optical signal 100 is written in the following manner. That is, in the first time slot of the time-division optical signal 100, the optical switch 320 switches the optical waveguide 310 from the optical waveguide 34.
Connect to 1. 510 is a light switch 320
shows the voltage supplied from the optical switch drive circuit 321 to the optical switch 320 to perform the above connection operation. Here, it is assumed that the optical switch 320 connects the optical waveguide 310 and the optical waveguide 341 only when the control voltage 510 is _V1 . As a result, at the input end of the bistable semiconductor laser 340, as shown in FIG.
As shown in FIG. 3, an optical signal is obtained by extracting only the first time slot of the time-division optical signal 500 using the control voltage 510. At the same time, the current drive circuit 3
45, the current i injected into the bistable semiconductor laser 340 is once reduced to i ₀ with a pulse width narrower than the incident light pulse 520, as shown in FIG. 5, 530, and then returned to i _b again. As a result, the amount of emitted light 540 of the bistable semiconductor laser 340 is 0 compared to the amount of incident light 520.
If the leading edge 53 of the injection current pulse 530
1 resets it to _P0 . On the other hand, the amount of incident light is 520
is P _t , the amount of light emitted from the bistable semiconductor laser 340 during the period 532 when the injection current 530 is 0 is P ₂ shown in FIG. Set to P ₁ . In this way, the bistable semiconductor laser 3
Information A of the time-division optical signal 100 is written in 40. Similarly, the optical switch drive circuit 321 and current drive circuits 355, 365, and 375 drive the optical switch 320 and the bistable semiconductor laser 35, respectively.
By controlling the injection currents to the bistable semiconductor lasers 350, 360, and 370, information B, C, and D of the time-division optical signal 100 are transmitted to the bistable semiconductor lasers 350, 360, and 3, respectively.
I will write it to 70. The information A written in the bistable semiconductor laser 340 in this manner is read out into the time-division optical signal 190 in the following manner. That is, in the fourth time slot of the time-division optical signal 190, the optical switch 33
0 connects the optical waveguide 342 to the optical waveguide 380. FIG. 5 550 shows the control voltages supplied to optical switch 330 by optical switch drive circuit 331 to cause optical switch 330 to perform the above connection operation. Here, it is assumed that the optical switch 330 connects the optical waveguide 342 and the optical waveguide 380 only when the control voltage 550 is _V1 . As a result, an optical signal obtained by extracting the emitted light 540 of the bistable semiconductor laser 340 by the control voltage 550 is obtained in the optical waveguide 380, as shown in FIG. 5 560. Similarly, the optical waveguide 380 further includes a time-division optical signal 1.
In the first, second, and third time slots of 90, the bistable semiconductor lasers 370, 360,
Information D, C, and B held in 350 are read out. In the time-division optical signal 190 obtained in the optical waveguide 380 in this way, the time-division optical signal 1
00 information A and D and A and C are exchanged.

As described above, according to the present invention, it is possible to arbitrarily set the length of the frame period to be exchanged, and it is possible to easily miniaturize the device and also to easily obtain a high output optical signal level. An optical switch is obtained.

Note that the present invention is not limited to the above-described embodiments.

For example, in the embodiment shown in FIG. 1, writing to the bistable semiconductor laser is performed periodically and reading is performed according to the information to be exchanged. Exactly the same exchange operation can be obtained by periodically performing readout according to the timing.

In addition, when one time slot is composed of n bits (n≧2), n bistable semiconductor lasers are provided in place of the one bistable semiconductor laser shown in this embodiment, and each time slot has n bits (n≧2). By sequentially writing and reading n bistable semiconductor lasers, exchange between time slots can be performed.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of a time-division optical switch according to the present invention, FIG. 2a is a diagram showing an example of a current drive circuit for a semiconductor laser, and FIG. FIG. 3a is a diagram showing an example of a bistable semiconductor laser; FIGS. 3b, c, and d are diagrams showing the operation of the bistable semiconductor laser; FIGS. 4 and 5 The figure is a time chart for explaining the operation of the optical switch. In the figure, 320 is an optical switch for writing;
330 is an optical switch for reading, 340, 35
0,360,370 is a bistable semiconductor laser.

Claims (1)

[Claims] 1. A write optical switch equipped with one input end and a plurality of output ends, and one installed in correspondence with each output end so that the light emitted from the output end enters from the end face of the laser resonator. a semiconductor laser exhibiting bistable operation, and a readout light comprising a plurality of input terminals and one output terminal corresponding to each semiconductor laser so that light emitted from the semiconductor laser is incident thereon. 1. A time-division optical switching system comprising a switch, an optical switch drive circuit for each of the optical switches, a current drive circuit for the semiconductor laser, and a central control unit that controls the timing of each of the drive circuits.