JPH0634533B2 - Time division optical switch - Google Patents

Description

TECHNICAL FIELD The present invention relates to a time-division optical switch that controls exchange of time-division optical signals.

(Prior Art and Its Problems) Optical communication systems using optical fibers as transmission lines are capable of high-speed and large-capacity signal transmission, and various transmission systems have been put to practical use. Particularly, the time division transmission method of digital signals utilizing high speed is one of important methods. In an optical communication system currently put into practical use, an optical signal is simply transmitted through an optical fiber, and the signal is exchanged after it is exchanged with a supporting electric signal. As described above, in the method of converting an optical signal into an electric signal and exchanging the electric signal, it is necessary to convert the electric signal into the optical signal again when the exchanged signal is transmitted again. It has the drawback of being expensive. Further, there is a drawback that it is difficult to exchange a high-speed signal of 100 megabits / second or more with a conventional time division switch for electric signals. As a means for solving this drawback, an optical switch that exchanges a time-division optical signal as an optical signal is proposed. This optical switch is described in Japanese Patent Laid-Open No. 53-117311, and is constructed by connecting a plurality of matrix-like optical switches having N × M input / output terminals with optical fiber groups having mutually different lengths. That is, an optical fiber is used as a delay line. Basically, the time-division optical signal incident on the optical switch is distributed to different fibers for each time slot, delayed by different times, and then combined in the next-stage optical switch to change the time sequence of the signals. Is. However, in an optical switch using such an optical fiber delay line, the delay time is determined by the length of the optical fiber, and a long optical fiber is required to exchange a signal with a long frame period. However, there was a defect such as an increase in size. Also, because time division switching is performed by simply selecting the delay time of the optical transmission line, loss occurs at the connection between the optical switch and the optical fiber, etc., and the output optical signal level drops significantly compared to the input optical signal level. There was also the drawback of doing so.

To eliminate this drawback, a bistable operation is shown between a write optical switch with one input and multiple outputs and a read optical switch with multiple inputs and one output. The inventor of the present application has proposed a time division optical switch having a structure in which a semiconductor laser is inserted in Japanese Patent Laid-Open No. 59-107675.
The configuration is such that the input time-division optical signal should be written into different optical memories, that is, bistable semiconductor lasers in sequence with the writing optical switch for each time slot to hold the input signal and the reading optical switch should output each signal. It selectively cuts into time slots to perform time division optical switching. With this configuration, the length of the frame cycle to be exchanged can be arbitrarily set, the device can be easily downsized, and a high output optical signal level can be easily obtained. The write and read optical switches that constitute the time-division optical switch require high speed, so a waveguide type optical switch is used. However, the waveguide type optical switch is usually large in size as the number of channels is large, and requires a large number of manufacturing steps, which is very expensive. Therefore, this time-division optical switch using two optical switches has a drawback that it has a large shape and is expensive.

(Object of the Invention) An object of the present invention is to provide a small-sized and inexpensive time-division optical switch, which requires a small number of optical switches, excluding the conventional drawbacks.

(Structure of the Invention) A time-division optical switch according to the present invention includes an optical branch circuit having one input end and a plurality of output ends, and light emitted from the output end is incident from one end of a laser resonator. A plurality of semiconductor lasers installed corresponding to each output end and exhibiting bistable operation, and a plurality of input ends and one output end corresponding to each semiconductor laser so that light emitted from these semiconductor lasers respectively enter A read optical switch comprising: a current drive circuit for driving each of the semiconductor lasers;
An optical switch drive circuit that drives the readout optical switch, and a central control device that controls the timing of each of the current drive and optical switch drive circuits are configured.

(Operation of the Invention) In the present invention, the input time-division optical signal is split by the optical branching circuit and is made incident on all the bistable semiconductor lasers at the same time, and the injection current of each bistable semiconductor laser is written as a signal. Only the desired time slot is increased to a current value that can be triggered by the input optical signal, the written state is maintained for the rest of the time, and the current value is kept low enough that the input optical signal does not trigger. By doing so, the write operation and the optical memory operation are performed.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of 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 and four output slots for outputting a second time division optical signal 190 each consisting of four time slots carrying different information. Between the waveguide 380,
An optical branch circuit 320 having an input terminal connected to the input optical waveguide 310 and an output terminal connected to the optical waveguides 341, 351, 361, 371 guided to the entrance ends of the semiconductor lasers 340, 350, 360, 370 exhibiting bistable operation, and the exit end of the bistable semiconductor laser. A read 4 × 1 optical switch 330 having input terminals connected to the optical waveguides 342, 352, 362, and 372, respectively, and output terminals connected to the output optical waveguide 380 is installed.
An optical switch drive circuit 331 is connected to the optical switch 330, and a current drive circuit 345 for injecting a bias current into each of the bistable semiconductor lasers 340, 350, 360, 370,
355, 365, 375 are connected. Optical switch drive circuit 331
And the current drive circuits 345, 355, 365, 375 are connected to a central control unit (not shown). The central control unit is composed of a timing extraction circuit, a memory circuit, etc., and generates a control signal for controlling the optical switch drive circuit and the current drive circuit.

Each current drive circuit is a device that generates a three-valued current in response to a command from the central controller, and a simple example thereof is shown in the circuit diagram of FIG.

This current drive circuit includes two transistors T _r1 , T
_It consists of _r2 and two resistors. The collectors of these transistors T _r1 and T _r2 are bistable semiconductor laser LDs.
Connected to ₁ . The setters are resistors R ₁ and R ₂ , respectively.
Is connected to a -V power source, and the base terminal serves as an input terminal for a control signal. In this figure, a base voltage of the transistor _{T r1, T r2} is V smaller than the current in the transistor _{T r1, T r2} bistable semiconductor laser LD _1, which is inserted in series with the collector side since both turned off flow Absent. On the other hand, when the voltages _V 1 to the base of the transistor _{T r1 (V} 1> _V) is applied transistor _{T r1}
Is turned on, and the current i of the bistable semiconductor laser LD ₁ is i =
i ₁ = (V ₁ −V) / R ₁ . Similarly, when the voltage V ₁ is applied to the base of _{Tr 2} , i = i ₂ = (V ₁ −V)
/ R ₂ . When the voltage V ₁ is applied to both T _r1 and T _r2 , i = i ₃ = i ₁ + is applied to the bistable semiconductor laser.
The current i ₂ flows. Thus, with the circuit of FIG.
A three-valued current can be passed through the bistable semiconductor laser LD ₁ by an external signal.

A specific example of the readout 4 × 1 optical switch and the optical branch circuit of this embodiment, and specific examples of the bistable semiconductor lasers 340, 350, 360, 370 will be described.

First, FIG. 3 (a) shows a plan view of an example of a directional coupling type optical switch that can be used as the 4 × 1 optical switch 330 for reading. It is composed of four directional coupling type optical switches 21, 22, 23 and 30 formed on a ferroelectric crystal or semiconductor crystal substrate 20. For example, the above directional coupling type optical switch can be obtained by diffusing titanium on a lithium niobate crystal to form an optical waveguide and installing electrodes on the optical waveguides close to each other. The directional coupling type switch 30 is a switch for blocking an optical signal passing through the optical waveguide 24. Optical waveguide 24 for output, optical waveguides 25, 26, 27, 28
Can be used as a 4 × 1 optical switch for reading.

The optical switch drive circuits 321 and 331 generate electric signals for controlling the optical switch 330 under the control of the central controller. For example, optical switch 330
In the case of the optical switch shown in Fig. 3 (a), directional coupling type optical switches 21, 22, 23, 30 are supplied with different voltages.
It has a function of supplying to. When the voltage applied to each of these directional coupling type optical switches 21, 22, 23, 30 is 0, the light wave incident on one of the optical waveguides passes through the incident optical waveguide as it is, and when the voltage is V ₁ , the incident light is close to each other. The optical switch drive circuit 331
In the simplest case, can be configured by a circuit using one transistor for each directional coupling type optical switch.

For example, the voltage V
₁ is connected to the power source, the emitter is grounded, the base is used as an input terminal for a control signal, and the output voltage terminal is output from the collector. In this circuit, the output voltage is V ₁ when the voltage of the control signal is 0, and the output voltage is 0 when the voltage of the control signal is positive.

FIG. 3B shows an optical fiber branch circuit as a specific example of the optical branch circuit 320. Reference numeral 40 is an input optical fiber, and 42, 43, 44 and 45 are output optical fibers. This branch circuit can be manufactured by bundling four optical fibers and heating and fusing the bundled portion 41. Further, a waveguide type optical branch circuit can be constructed by using an optical waveguide provided on a substrate similar to that shown in FIG. 3 (a).

FIG. 4 is a sectional view showing a specific example of the bistable semiconductor lasers 340, 350, 360, 370. The structure is almost the same as that of a commonly used current injection type semiconductor laser, for example, GaAlAs / GaA.
It is a laser with a double heterojunction structure made of s or InGaAsP / InP. However, this is different from a normal semiconductor laser in that the electrodes are not uniform and there is a portion where current is not injected. Since the non-injection region of this current acts as a saturable absorber, this bistable semiconductor laser can have bistable characteristics in the injection current vs. optical output. In addition, instead of making the electrodes non-uniform, a similar bistable characteristic can be provided by providing non-uniformity in a part of the active layer which is a light emitting region and disposing 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 injected light from the outside. For details of such a bistable semiconductor laser, refer to
It is described in the 17th volume, page 741 of the Electronics Letter and in 272nd volume of the conference of the Institute of Electronics, Communications and Communications, National Division of Light and Radio, 1987 (Volume 2).

5 (a), (b) and (c) are characteristic diagrams for explaining the operation of the bistable semiconductor laser of FIG. Fig. 5 (a) shows the amount of incident light Pin =
The relationship between the injection current i and the emitted light amount Pout when 0 is shown. That is, the injection current i rapidly emitted light amount Pout increases with i = i _a in when increasing from i _0, the emission light intensity Pout in the case of the injection current i in the opposite reduced from i _t is i
= _{I c} in indicates hysteresis characteristic as rapidly decreases, two stable points _{A 1} and _{B 1} of the emitted light amount _{P 01} and _{P 11} in the current value i _{= i b1} slightly larger than _{i c}
Have. Further, also at a current value i = i _b2 that is larger than i _{b1 and} slightly smaller than i _a , similarly, there are two stable points A ₂ and B ₂ of the emitted light amounts P ₀₂ and P ₁₂ .

FIG. 5 (b) is a diagram showing the relationship between the incident light amount Pin and the emitted light amount Pout when the injection current i = i _b1 in FIG. 5 (a). That is, the first stable point A ₁ of the emitted light amount Pout = P ₀₁
When the incident light amount Pin is increased from 0 in the case of, the output light amount Pout sharply increases at Pin = P _a1 and thereafter the incident light amount Pin increases.
= P _t1 and the second stable point B ₁ of the emitted light amount Pout = P ₁₁ with almost no decrease in the emitted light amount Pout
Move on to. Points A ₁ and B ₁ in FIG. 5 (b) represent the same points as points A ₁ and B ₁ in FIG. 5 (a), respectively. Similarly, FIG. 5 (c) shows the relationship between the incident light amount Pin and the outgoing light amount Pout when i = i _b2 in FIG. 5 (a). In this case, the point P _{t2 at} which the emitted light amount sharply increases is P
_{It is} smaller than _t1 .

The following Table 1 shows the operation of the bistable semiconductor laser for the injection current i and the incident light Pin.

That is, when the injection current i is i _b1 , the bistable semiconductor laser is written before. It is located at either one of the two stable points A ₁ and B ₁ in FIG. 5 (a) according to the collected data, and the emitted light quantity P ₀₁ or P ₁₁ is maintained. That is, data is written in the bistable semiconductor laser, and even if an optical signal is incident in this state, the amount of light is P _a1 in FIG. 5 (b).
In the following cases, this state is preserved while the current keeps i _b1 . If the output of the bistable semiconductor laser is taken out during this state, the data can be read. Fig. 5 (a)
The injection current i when the bistable semiconductor laser is holding one of the stable points B _{1 (emission} light intensity P ₁₁₎ at a time in i ₀
Then, when returning to i _b1 again, the output light amount Pout is B ₁ → D → A ₁
, And then the other stable point A ₁ (emitted light amount P ₀₁ )
Hold. Further, even if the same change of the injection current as that described above is given while A ₁ is held, A ₁ → D → A _{1 is obtained and the} stable point A ₁ is restored. That is, the operation of the bistable semiconductor laser as an optical memory is reset. On the other hand,
= I _b2, and in FIG. 5 (c), when the bistable laser holds the stable point A ₂ (outgoing light amount P ₀₂ ), the optical signal “1” is incident, that is, the incident light amount once becomes P _t2 and again. When it returns to 0, the emitted light amount Pout changes in the order of A ₂ → E → B ₂ and moves to the stable point B ₂ (emitted light amount P ₁₂ ). That is, the bistable semiconductor laser is set to the "1" state. When the optical signal is “0”, that is, the incident light amount Pin = 0, the state of A ₂ is held. When the current is set to i = i _{b1 in} this state, the stable points are held at B ₁ and A ₁ , respectively. As described above, in the state of i = i _b1 , the signal of “1”, that is, Pin = P
_{Even if t2} is input once, the stable point does not move.

FIG. 6 shows a bistable semiconductor laser 34 of the embodiment shown in FIG.
6 is a time chart for explaining write and read operations of 0. Referring to FIGS. 5 and 6, the time-division optical signal 100 is guided to the optical waveguide 310 shown in FIG. In the time division optical signal 100, the information of A, B, C and D is sequentially written in one time slot in that order. The optical signal 400 of FIG. 6 is the information A of the time division optical signal 100,
A specific example in which B, C, and D are 1-bit NRZ signals will be described. Here, the optical signal is a bistable semiconductor laser 340,
In the amount of light incident signal "0" to 350, 360, 370 0, is set to be the amount of light P _t2 shown in FIG. 5 the signal "1". Each bistable semiconductor laser has a current drive circuit 345, 355, 365, 375 except when resetting and writing.
Therefore, the current value i _b1 shown in FIG. 5 is always injected. When using the current drive circuit shown in FIG. 2 in this state it is turned on only the transistor T _r1. The optical waveguide 380 is constantly guided by the optical switch 330.
Separated from 362,372. Optical switch 330 is shown in Fig. 3.
In the case of the configuration shown in (a), at this time, the voltage V ₁ is applied to the directional coupling type optical switch 30 by the optical switch drive circuit 331, and the directional coupling type optical switch 30 is in the cutoff state.

In FIG. 1, the writing of the information A of the time division optical signal 100 to the bistable semiconductor laser 340 is performed as follows. That is, first, in the first period within the first time slot of the time division optical signal 100, the bistable semiconductor laser 340
To i _b2 injection current i to the after subtraction once i ₀ as shown in Figure 6 410. This is achieved, for example, by turning off the transistor T _r1 in FIG. 2 and then turning back on T _r1 and T _r2 simultaneously. As a result, even if the emitted light quantity Pout of the bistable semiconductor laser 340 has previously held the light quantity P ₁₁ as shown in FIG. 6A, it is reset to P ₀₁ at the trailing edge 411 of the injection current pulse 410. . Next, in the second period following the first period in the first time slot of the time division optical signal 100, the current is i _b2.
Kept in. As a result, the emitted light amount 440 of the bistable semiconductor laser 340 is set to P ₁₂ at the leading edge 431 of the current pulse 410 when the incident light amount is P _t2 , and P when the incident light amount 430 is 0. It becomes ₀₂ . After that, since the injection current pulse is held at i _b1 , the bistable semiconductor laser 340 holds the information P ₁₁ of 1 or the information P _{01 of} 0.

In this way, the information A of the time division optical signal 100 is written in the bistable semiconductor laser 340. Similarly, the current drive circuits 355, 365, 375 are bistable semiconductor lasers 350, 360, 3 respectively.
The injection current to 70 is i ₀ only when writing as in the above.
→ i _b2 and varied, information B of the split optical signal 100 when by controlling so as to i _b1 in the other time, C, respectively D bistable semiconductor lasers 350, 360, and 370
Write in. In this way, the bistable semiconductor laser is
The information A written in 340 is read out to the time division optical signal 190 as follows. That is, the optical switch 330 connects the optical waveguide 342 to the optical waveguide 380 in the fourth time slot of the time division optical signal 190, for example. FIG. 6D shows the control voltage supplied to the optical switch 330 by the optical switch drive circuit 331 to cause the optical switch 330 to perform the connecting operation. Here, the optical switch 330 connects the optical waveguide 342 and the optical waveguide 380 only when the control voltage 450 is V ₁ . For example, when the optical switch shown in FIG. 3 (a) is used as the optical switch 330, the control voltage 450 indicates the voltage applied to the directional coupling type optical switches 21 and 22. As a result, an optical signal obtained by extracting the emitted light 440 of the bistable semiconductor laser 340 by the control voltage 450 is obtained in the optical waveguide 380 as shown in FIG. Similarly, the information D, C, and B held in the bistable semiconductor lasers 370, 360, and 350 in the fourth, fifth, and sixth time slots of the time division optical signal 190 are read out to the optical waveguide 380 in the same manner. In the time division optical signal 190 thus obtained in the optical waveguide 380, information is written in the order of A, D, C and B, and the information A and D and the information B and C of the time division optical signal 100 are exchanged. Is performed.

(Effect of the Invention) As described above, in the present invention, the length of the frame period of information to be exchanged can be arbitrarily set by the control of the write and read optical switches, and a long optical fiber is not used. Therefore, it is easy to downsize the device. Further, by using a bistable semiconductor laser having a high emission light level, the light quantity of the second time division optical signal 100 can be made larger than that of the first time division optical signal 190. Furthermore, since only one optical switch is used, it is possible to obtain a time-division optical switch that is smaller and cheaper than conventional ones.

The present invention is not limited to the above embodiment. For example, in the embodiment shown in FIG. 1, an example is shown in which writing to the bistable semiconductor laser is performed periodically and reading is performed according to the information to be exchanged, but the writing to the bistable semiconductor laser is exchanged. The same exchange operation can be obtained by periodically performing reading according to the information to be read. When one time slot is composed of n bits (n ≧ 2), n bistable semiconductor lasers are provided instead of one bistable semiconductor laser shown in the present embodiment. By sequentially performing writing and reading of n bistable semiconductor lasers, the exchange can be performed between time slots. Further, as the read optical switch, an optical switch having a structure in which a gate unit for blocking light is provided during each branch of the optical branch circuit can be used.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of a time division optical switch according to the present invention, FIG. 2 is a circuit diagram of an example of a current drive circuit of the semiconductor laser of FIG. 1, and FIGS. 3 (a) and 3 (b) are respectively. FIG. 4 is a sectional view of an example of a read optical switch and an optical branch circuit that can be used in the present invention, FIG. 4 is a sectional view of an example of a bistable semiconductor laser, and FIGS. FIG. 6 is a characteristic diagram showing the operation of the bistable semiconductor laser, and FIG. 6 is a time chart explaining the operation of this embodiment. In the figure, 320 ... Optical branch circuit, 3
30 ... Readout optical switch, 340, 350, 360, 370 ... Bistable semiconductor laser.

Claims (1)

[Claims] 1. An optical branching circuit having one input end and a plurality of output ends to which a time-division multiplexed optical signal enters, and light emitted from the output end enters from one end of a laser resonator. And a plurality of semiconductor lasers whose oscillation states exhibit bistable operation with respect to the injected current and the amount of injected light, which are installed corresponding to the respective output terminals, and the semiconductor lasers so that the light emitted from these semiconductor lasers respectively enter. A read optical switch having a plurality of input terminals and one output terminal corresponding to, and injection into the semiconductor laser in synchronization with the optical signals simultaneously injected into the semiconductor lasers by the optical branch circuit. A current drive circuit for driving each semiconductor laser so as to sequentially obtain an oscillation state corresponding to the information of the optical signal by increasing / decreasing the current, and a semiconductor drive circuit in an oscillation state corresponding to the information. Time division, comprising an optical switch drive circuit for driving the emitted light of the body laser so as to be sequentially read by an optical switch, and a central control device for controlling the timing of the current drive circuit and the optical switch drive circuit. Optical switch.