JPH0640595B2 - Injection current control circuit for bistable semiconductor laser - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an injection current control circuit for a bistable semiconductor laser that exhibits a hysteresis characteristic between an injection current and the amount of emitted light.

(Prior art and its problems) Optical communication using an optical fiber in a transmission line has an advantage that a large amount of information can be transmitted because the optical fiber has a wide band and the optical fiber does not receive induced noise. Therefore, it is expected to be widely used in the future. In this optical communication, the information to be sent is changed from an electric signal to an optical signal by a transmitting device, the information is transmitted by an optical fiber, and the information is converted again into an electric signal by a receiving device. In this case, the optical signal is merely a transmission means for transmitting the signal from one side to the other utilizing the fact that the transmission loss of the optical fiber of the transmission line is extremely small, and the optical signal is used for signal processing such as logical operation and storage of optical information. The signal has not reached an active role. If optical information can be stored and the storage result can be obtained as an optical signal, it is extremely effective for diversifying the functions of the optical communication system.

As this optical information storage circuit, as described in Japanese Patent Application No. 57-216708, "Time Division Optical Exchanger", a saturable absorption portion, for example, a portion where current is not injected is provided in a part of a resonator of a semiconductor laser. As a result, the injection current vs. optical output characteristic has a hysteresis characteristic, and the injection current is set within the hysteresis (hereinafter, the value of the injection current is referred to as a bias current), so that either one of the binary light quantities is stored. Also known is one that can be set by injecting an optical signal with it and reset by reducing the injection current.

In such an optical information storage circuit, the closer the bias current value is to the rise of the hysteresis characteristic, the smaller the amount of injected light can be set.

On the other hand, if the value of the bias current approaches the rise of the hysteresis characteristic, the optical information storage circuit is set by a slight change in the bias current, resulting in an error in the stored information.

Therefore, an extremely stable injection current control circuit is required in order to set a small injection light amount and to store optical information without error.

FIG. 1 (a) is a sectional view showing a specific example of a bistable semiconductor laser. The structure is almost the same as that of a current injection type semiconductor laser that is normally used. For example, GaAlAs / GaAs or InGaAsP
This is a laser with a double heterojunction structure made of / 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 the current acts as a saturable absorber, the bistable semiconductor laser of FIG. 1 (a) can have a hysteresis characteristic in the injection current-optical output characteristic. 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. Details of such a bistable semiconductor laser are described in the literature, Electronic Letter (Vol. 17, page 741) and Proc.

FIGS. 1 (b), (c) and (d) are diagrams for explaining the operation of the bistable semiconductor laser of FIG. 1 (a). FIG. 1 (b) is a diagram showing the relationship between the injection current i and the emitted light amount Pout when the incident light amount Pin = 0. That is, when the injection current i is increased from i ₀ , the emitted light amount Pout rapidly increases at i = i _a ,
Emission light intensity Po in the case of the injection current i in the opposite reduced from i _t
ut exhibits a hysteresis characteristic that decreases sharply at i = i _c , and has two stable points A and B of the emitted light amounts P ₀ and P ₁ at i = i _b . FIG. 1 (c) is a diagram showing the relationship between the incident light amount Pin and the outgoing light amount Pout when the injection current i = i _b in FIG. 1 (b). That is, the first of the emitted light quantity Pout = P ₀
When the incident light amount Pin is increased from 0 at the stable point A of, the output light amount Pout sharply increases at Pin = P _a , and when the incident light amount Pin = P _t decreases thereafter, the output light amount Pout is almost Without decreasing, the light amount Pout goes to the second stable point B where Pout = P ₁ . Points A and B in Fig. 1 (c) are respectively shown in Fig. 1 (b).
Represents the same points as points A and B in. FIG. 1 (d) 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 injection current i is i _b and the incident light Pin is 0, the bistable semiconductor laser is at one of the two stable points A and B in FIG. 1 (b) according to the previously written data. It is positioned and holds the emitted light quantity P ₀ or P ₁ . In FIG. 1 (b), when the bistable semiconductor laser holds one stable point B (emitted light amount P ₁ ), the injection current i is once set to i ₀ and returned to i _b again, the emitted light amount Po
ut changes in the order of B → D → A, and thereafter holds the other stable point A (emitted light amount P ₀ ). That is, the bistable semiconductor laser is reset. Further, in FIG. 1 (c), when the bistable semiconductor laser holds the stable point A (emitted light amount P ₀ ), the incident light amount is once set to P _t and returned to 0 again, the emitted light amount P out becomes A → E.
→ It changes in the order of B, and thereafter the stable point B (the amount of emitted light P ₁ ) is held. That is, the bistable semiconductor laser is set. Further, when the injection current i is i ₀ and the incident light quantity Pin is P _t , the outgoing light quantity Pout shows P ₂ according to the characteristics of the bistable semiconductor laser, but in the present invention, this light quantity is not used directly, so the explanation is omitted. To do.

As shown in FIG. 1 (c), when the value of the injection current i is increased from i _b to i _b ′ in FIG. 1 (b), the incoming / outgoing light amount characteristic is such that the outgoing light amount Pout rises from P _a This reduces to P _a ′, which allows the bistable semiconductor laser to be set with a smaller amount of incident light than P _t .

However, at this time, the bias current value i _b ′ _becomes | i _t −
A fluctuation of i _b ′ | or more occurs, and once it reaches i _t , the bistable semiconductor laser is set, which results in an error in stored information.

FIG. 2 is a circuit diagram showing an example of an injection current control circuit for a bistable semiconductor laser.

According to FIG. 2, the injection current control circuit of the semiconductor laser is such that one end is the injection electrode of the bistable semiconductor laser 200 and the other end is the power supply
A constant current source 201 connected to V ₊ ′, a resistor 202 having one end connected to a power source V ₊ and having a resistance value R, and an emitter at the other end of the resistor 202 are injected into a bistable semiconductor laser 200. The transistor 204 includes a collector connected to the electrode and a base connected to the control signal input terminal 203.

A voltage V is always applied to the control signal input terminal 203 shown in FIG. 2, and a constant current I ₂ determined by the following equation is obtained at the collector of the transistor 204. This constant current I ₂ is bistable. It is injected into the semiconductor laser 200.

I ₂ = (V ₊ −V−V _BE ) / R (1) Here, the voltage V _BE indicates the emitter-base voltage of the transistor 204. A constant current I ₂ is further injected into the bistable semiconductor laser 200 by a constant current source 201, and the sum I ₁ + I ₂ with the collector current I ₁ of the transistor 204 is _first calculated.
By setting i _b shown in FIG. _6B , _{one of the} binary output light amounts P ₁ and P ₀ is held.

In this state, the bistable laser diode 200 is shown in Fig. 1 (c).
When the incident light Pin having a light amount of P _t or more shown in is injected, the bistable semiconductor laser 200 is set, and thereafter the incident light P
Even if in is removed, the emitted light amount P ₁ is retained.

Here, a voltage V ₊ equal to the power supply voltage is applied to the control signal input terminal 203.
Is added, the collector current I ₂ of the transistor 204 becomes 0, and only the current I _{1 from the} constant current source 201 is injected into the bistable semiconductor laser 200.

Here, if i ₀ shown in FIG. 1 (b) is used as the value of this current I _1, the bistable semiconductor laser 200 is reset by this, and the voltage applied to the control input terminal 203 thereafter is V
Even after returning to, the emitted light quantity P ₀ is maintained.

In Fig. _{2, if} the fluctuations of currents I ₁ and I _{2 due} to temperature changes and changes over time are designated as △ I ₁ and △ I ₂ , respectively, the sum of these fluctuations △ I ₁ + △ I ₂ is shown in Fig. 1 (b). It is necessary to satisfy the following equation using i _b and i _t shown above.

ΔI ₁ + ΔI ₂ <i _t , i _b (2) That is, in the control circuit of the bistable semiconductor laser shown in FIG. 2, the current I ₁ of the constant current source 201 and the collector current I _{2 of the} transistor 204 are Since the sum of the two is used as an injection current for holding the optical information, the fluctuations ΔI _{1 and} ΔI _{2 of} these currents are also added, resulting in an error in the stored information.

(Object of the Invention) It is an object of the present invention to provide an injection current control circuit for a bistable semiconductor laser having high stability which can be set by a small amount of injected light and which does not cause an error in stored information.

(Structure of the Invention) According to the present invention, a bistable semiconductor laser having a hysteresis characteristic between an injection current and an emitted light amount, and an injection current having a first value in a hysteresis loop of the hysteresis characteristic of the bistable semiconductor laser. And a means for reducing the injected current to a second value outside the hysteresis loop of the hysteresis characteristic of the bistable semiconductor laser by the input control signal and smaller than the first value. An injection current control circuit for a bistable semiconductor laser is obtained.

(Embodiment) FIG. 3 is a circuit diagram showing a first embodiment of the present invention. In FIG. 3, the same reference numerals as those in FIG. 2 denote the same components as those in FIG.

In FIG. 3, a voltage 0 is constantly applied to the control input terminal 203, so that the transistor 300 is kept off. Thus, the bistable semiconductor laser 200 and only the current I ₁ of the constant current source 201 is injected, the bistable semiconductor laser 200 by using the i _b shown in FIG. 1 (b) to the value P ₀ Alternatively, the output light amount of either _one of P ₁ is held.

Here, the set of this bistable semiconductor laser 200 is shown in FIG.
This can be performed by applying the incident light Pin having a light amount _{equal to} or _larger than P _t shown in (c), and the emitted light amount P ₁ is retained even if the incident light Pin is subsequently removed.

Further, in FIG. 3, a positive voltage V is applied to the control signal input terminal 203.
Is added, the transistor 300 turns on and the constant current source 2
Most of the current I ₁ injected into the bistable semiconductor laser 200 by 01 flows into the collector of the transistor 300. As a result, the bistable semiconductor laser 200 is reset and holds the emitted light amount P ₀ even if the voltage applied to the control input terminal becomes 0 thereafter.

Unlike the circuit shown in FIG. 2, the injection current control circuit of the bistable semiconductor laser shown in FIG. 3 is different from the current I of the constant current source 201.
For use only _one as an injection current for storing optical information bistable semiconductor laser 200, cause the stored information erroneous Mari becomes only variation △ I ₁ of the current I ₁ of the constant current source 201, FIG. 2 More stable operation can be performed than the circuit shown in FIG.

FIG. 4 is a circuit diagram showing a second embodiment of the present invention.

In FIG. 4, the same reference numerals as those in FIG. 2 denote the same components as those in FIG.

In FIG. 4, the voltage V ₋ is constantly applied to the control input terminal 203, which keeps the transistor 400 off. Thus the bistable semiconductor laser 200 are injected only current I ₁ of the constant current source 201, the bistable semiconductor laser 200 by using the i _b shown in FIG. 1 (b) to the value P ₀ or It is possible to hold the emitted light amount of either one of P ₁ .

This setting of the bistable semiconductor laser 200 can be performed by giving the incident light Pin having a light quantity of P _t or more shown in FIG. 1 (c).

On the other hand, the bistable semiconductor laser 200 is reset by applying a voltage V to the control input terminal 203, whereby a current I ₂ given by the resistance value R of the resistor 401 is given to the collector of the transistor 400 as follows. It

I ₂ = (V−V ₀ −V _BE ) / R (3) Here, the voltage V _BE represents the base-emitter voltage of the transistor 401.

Therefore, at this time, the current injected into the bistable semiconductor laser 200 is I ₁ -I ₂ , and this value is i ₀ shown in FIG. 1 (b).
By using, the bistable semiconductor laser 200 can be reset.

The injection current control circuit shown in FIG. 4 uses the current I of the constant current source 201.
For use only _one as an injection current for storing optical information bistable semiconductor laser 200, cause erroneous Mari stored information is only variation △ I ₁ of the current I ₁ of the constant current source 201, a more stable Can perform actions. In FIG. 4, the diode 402 functions to prevent reverse bias from being applied to the bistable semiconductor laser 200.

(Effects of the Invention) As described above, according to the present invention, it is possible to obtain an injection current control circuit for a bistable semiconductor laser having a high degree of stability in which stored information is not erroneously generated.

[Brief description of drawings]

FIG. 1 (a) is a sectional view of a bistable semiconductor laser used in the present invention, and FIG. 1 (b) is a relationship between the injection current i and the emitted light amount Pout of the bistable semiconductor laser shown in FIG. 1 (a). Fig. 1, Fig.
(c) is the incident light intensity of the bistable semiconductor laser shown in Fig. 1 (a).
FIG. 1 is a diagram showing the relationship between Pin and the emitted light amount Pout, FIG. 1 (d) is a table showing the operation of the bistable semiconductor laser, and FIG. 2 is a circuit showing an example of the injection current control circuit of the bistable semiconductor laser. FIGS. 3 and 4 are circuit diagrams showing a first embodiment of the present invention, and FIG.
The drawing is a circuit diagram showing a second embodiment of the present invention. In the figure, 200 is a bistable semiconductor laser, and 201 is a constant current source.

Claims (1)

[Claims] 1. A bistable semiconductor laser having a hysteresis characteristic between an injection current and an emitted light quantity, and a constant current source for supplying an injection current having a first value in a hysteresis loop of the hysteresis characteristic of the bistable semiconductor laser. And, the injection current is outside the hysteresis loop of the hysteresis characteristic of the bistable semiconductor laser by an input control signal, and
An injection current control circuit for a bistable semiconductor laser, comprising: a means for reducing the value to a second value smaller than the first value.