JPH05211364A - Light output control circuit of laser diode - Google Patents

Abstract

PURPOSE:To enable a laser diode to stably output a laser beam by a method wherein a pulse current feed circuit which feeds a modulation signal current and a bias current feed circuit which feeds a current close to a threshold current in intensity are connected together in parallel, which is connected to the laser diode in series. CONSTITUTION:A pulse current feed circuit 3, a fixed bias current feed circuit 4, and an automatic control bias current feed circuit 5 are connected together in parallel, and these circuits are connected to a laser diode 1 in series. When input pulse signal and its inverted signal are applied to the input terminals 3a and 3b of the pulse current feed circuit 3, drive signals having the same phase with the input signal ad the opposite phase are outputted from the current feed circuit 3, and a pulse current is applied to the laser diode 1. The fixed bias current feed circuit 4 feeds an irreducible minimum bias current at a low temperature to the laser diode 1. The automatic control bias current feed circuit 5 controls a bias current in intensity corresponding to the photocurrent of a photodetector 2 which monitors the laser diode 1 receiving its the optical output power of the laser diode 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode optical output control circuit used for a light source for digital optical communication.

[0002]

2. Description of the Related Art A laser diode, which is widely used as a light source for optical communication, is characterized by easy high-speed driving and high light output. However, the characteristics of the laser diode, especially the light output level, are It changes greatly due to changes over time. Various characteristics and operating principles of the laser diode described above are known. A light output stabilizing circuit is usually provided for such fluctuations. An example of a conventional laser diode drive circuit is shown in FIG. 4 (see, for example, “Topics in Applied”).
Physics Volume39 Semicond
uctor Device "p. 184 Figure 5.1
9).

In FIG. 4, an input signal VIN is applied to the gate terminal of a field effect transistor Q, and a modulation signal current flows between the drain and source of the transistor Q. That is, a transistor that bypasses the signal current is provided in parallel with the laser diode. Further, another conventional example is shown in FIG. 5 (see Japanese Patent Laid-Open No. 1-91484). In this example, a transistor that drives a current (modulation signal current and bias current) flowing in the laser diode is provided in series with the laser diode, and this transistor takes a mode of supplying a bias current and a modulation signal current. There is.

[0004]

However, in the conventional optical output control circuit of the laser diode used for the light source for digital optical communication, it is possible to increase or decrease the appropriate bias current according to the temperature variation of the laser diode. This is not possible, and the laser diode has a large change in the optical output due to the influence of the temperature change, and the optical output cannot be stabilized.
Since the current supply circuit does not have a sufficient current supply capacity, a reliable and stable function could not be expected.

The present invention has been made on the basis of the above background, and an object of the present invention is to stabilize the optical output of a laser diode by increasing / decreasing an appropriate bias current according to temperature fluctuations. Another object of the present invention is to provide a light output control circuit for a laser diode, which can provide a reliable current supply capability and can exhibit a highly reliable and stable function.

[0006]

As a result of various studies to solve the above-mentioned problems, the present inventors found that in order to stabilize the optical output of a laser diode By changing the appropriate bias current up or down,
It is necessary to supply a large bias current to the laser diode in order to obtain high optical output satisfactorily at high temperature, and in order to expect a reliable and stable function, the supply circuit must have a sufficient current supply. A semiconductor transistor that requires high capacity and a bias current flows must have a large maximum allowable collector loss. On the other hand,
It is preferable that the semiconductor transistors that make up the circuit have the same specifications in order to expect lower costs, productivity, or integration considering these, and to provide multiple bias current supply circuits to supply to the laser diode. As a result, it was found that the collector loss of the used transistor can be reduced (dispersed), and the present invention was completed.

That is, the laser diode optical output control circuit of the present invention uses a pulse current supply circuit for supplying a modulation signal current and a threshold for supplying the laser diode to the laser diode when the laser diode is used as a light source for digital optical communication or the like. A bias current supply circuit (consisting of a fixed bias current supply circuit and an automatic control bias current supply circuit) that supplies a current near the value current is connected in parallel, and these circuits are connected in series to the laser diode. It is characterized by. Therefore, a feature of the present invention is that the bias current supply circuit is composed of two blocks, a fixed bias current supply circuit and an automatic control bias current supply circuit, and the collector loss of the transistor used is greatly reduced. Further, the bias current supply circuit blocks only the modulated signal from the optical output monitor (light receiving element) of the laser diode, and therefore has only a DC operation function.

[0008]

The laser diode optical output control circuit of the present invention having the above-described structure operates and functions as follows. This will be described with reference to FIG. 1, which is a block diagram of an example of a light output control circuit for a laser diode according to the present invention. The laser diode light output control circuit of the present invention receives the input pulse signals S1 and S-1 from the outside and controls the pulse current IP supplied to the laser diode 1 based on the input pulse signals S1 and S-1. A pulse current supply circuit 3, a fixed bias current supply circuit 4 for supplying a fixed bias current IB1 to the laser diode 1, and an automatic control bias current supply circuit 5 for supplying a control bias current IB2 to the laser diode 1 are connected in parallel, The pulse current supply circuit 3
The fixed bias current supply circuit 4 and the automatic control bias current supply circuit 5 are connected to the laser diode 1 in series.

In order to obtain a stable and stable optical output waveform of the laser diode, it is necessary to appropriately supply to the laser diode a bias current in the vicinity of the threshold current according to the fluctuation of the threshold current due to temperature change. .. The change in the threshold current due to the change in the ambient temperature of the laser diode generally has almost linearity with respect to the temperature. In general, it is also possible to obtain a linear photocurrent from a light receiving element (for example, a photodiode) built in to monitor and output the optical output level of the laser diode. For example, if a red visible light laser diode that is easily available is taken as an example and the temperature variation range is 0 to 50 ° C., the change in threshold current within that range is about several tens mA.
There is also a numerical value. When an output having a large absolute value of pulsed light output and a small extinction ratio is required, it is necessary to stably supply a bias current of about 100 mA or more to the laser diode depending on conditions such as ambient temperature.

The laser diode 1 has an input pulse S1
, And the modulation signal current Ip is supplied from the pulse current supply circuit 3 as a modulation signal corresponding to S-1. On the other hand, as for the bias current, the fixed bias current IB1 which is required to flow at least within the temperature range for maintaining the operation is supplied from the supply circuit 4,
In order to obtain a desired light output, an appropriate bias current IB2 that changes due to temperature fluctuations is detected from the monitor current IM of the light receiving element 2 and supplied from the automatic control bias current supply circuit 5. With the above configuration, it is possible to compensate for the characteristic change due to the temperature fluctuation of the laser diode and maintain a stable optical output operation.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The light output control circuit of the laser diode of the present invention will be specifically described with reference to embodiments. FIG. 1 is a block diagram of a light output control circuit of a laser diode showing an embodiment of the present invention. FIG. 3 is a schematic diagram showing changes in laser diode drive current due to temperature changes and changes in input / output waveforms due to temperature characteristics of the laser diode. As shown in FIG. 1, the optical output control circuit of the laser diode 1 of the embodiment has a pulse current supply circuit 3, a fixed bias current supply circuit 4, and a photocurrent of a light receiving element 2 for monitoring the optical output of the laser diode 1. The laser diode 1 is driven by receiving the input pulse signal S1 and its inverted signal S-1.

The pulse current supply circuit 3 is provided with, for example, a differential input terminal, and when the input pulse signal S1 and its inverted signal S-1 are applied to the input terminals 3a and 3b, the drive signal S2 of the same phase and an opposite phase thereto. , S3, and pulse current IP is supplied to the laser diode. As shown in FIG. 3, substantially the same pulse current IP can be supplied regardless of (I) low temperature and (II) high temperature. The fixed bias current supply circuit 4 and the automatic control bias current supply circuit 5 supply a bias current near the superimposed threshold current to the laser diode. As shown in FIG. 3, the fixed bias current supply circuit 4 supplies to the laser diode a bias current IB1 that should be supplied at a minimum at (I) low temperature which is the lower limit of the expected operating temperature range. The automatically controlled bias current supply circuit 5 changes the laser diode temperature fluctuation, that is, the threshold current changes in the range of (I) low temperature to (II) high temperature, so that a desired appropriate bias current is I
This is a circuit that provides within the fluctuation of B2L to IB2H, and a light receiving element 2 that receives and monitors the optical output power of the laser diode 1.
It has a function of controlling the amount of bias current according to the photocurrent IM of.

This will be described with reference to FIG. 3. It is assumed that the operating temperature range is from (I) low temperature to (II) high temperature. High level (high level) of the digital logic of the desired optical pulse signal
The optical output power level of (level) is Pb, and the optical output power level of low level (Low level) is Pa. For example (I)
At a low temperature, the current IB1 received from the fixed bias current supply circuit 4 and the monitor current IM of the light receiving element 2 from the automatic control bias current supply circuit 5 are appropriately controlled.
2L is added to obtain a bias current IB1 + IB2L (= Ic), which is supplied to the laser diode 1. On the other hand, 3a,
A modulation signal current IP corresponding to the input pulse signals S1 and S-1 applied to the 3b terminal is obtained. Therefore, the superimposed drive current of IB1 + IB2L + IP (= If) is supplied to the laser diode 1.

In the case of (II) high temperature, the optical output of the laser diode 1 is attenuated as compared with (I) low temperature.
As a result, the photocurrent IM generated in the light receiving element 2 which receives the optical output of the laser diode 1 is also reduced. Here, the automatic control bias current supply circuit 5 functions to recognize the reduced IM and supply an appropriate current of IB2H. Therefore, the bias current IB1 + IB2H (= Id) is obtained, and I
A drive current of B1L + IB2H + IP (= Ie) is supplied. Therefore, when the temperature variation range is (I) low temperature to (II) high temperature, Pb is output as the high level of the digital logic of the optical output pulse as the optical output power from the laser diode 1, and Pa is output as the low level.

FIG. 2 is a circuit diagram showing a specific example of the circuit shown in FIG. As shown in FIG. 2, the pulse current supply circuit 3
Is composed of transistors 10 and 11 which are differentially connected, a resistor 19 which is connected to the collector of the transistor 10 and a constant current source 30, and the laser diode 1 is provided between the collector of the transistor 11 and the voltage source Vcc. It is connected. The fixed bias current supply circuit 4 includes a transistor 1
2 and 13, resistors 20 and 21, and a variable resistor 26. The collector of the transistor 12 is connected to the cathode of the laser diode 1. The automatic control bias current supply circuit 5 includes transistors 14, 15, 16, 17,
18, a resistor 22, 23, 24, 25, 27, a variable resistor 28, a capacitor 29, and a light receiving element 2 for monitoring the light output of the laser diode 1 between the collector of the transistor 14 and the voltage source Vcc. It is connected.

The operation of the circuit of this aspect will be described below.
The non-inverted and inverted input pulse signals S1 and S-1 are input to the differential input terminals 30 and 31. When the non-inverted input pulse signal S1 is at a high level and the inverted input pulse signal S-1 is at a low level, the transistor 11 is turned on, the transistor 10 is turned off, and the optical output of the laser diode 1 is emitted at a high level (Pb). Move to operation. The pulse current value IP as the modulation signal current is calculated by the transistor 1
It is set by the constant current circuit 30 between the emitters of 0 and 11 and GND. Next, the differential input terminal 30 has a low level 31
In the case of a high level, the transistor 11 is turned off, the pulse current IP is cut off in the laser diode 1, the transistor 10 is turned on, and the pulse current IP flows through the resistor 19 connected to the collector. A current switching type logic circuit composed of transistors 10 and 11 is a non-saturation type E if it is a bipolar transistor.
High speed drive is possible by the CL operation.

Next, the bias current which is superimposed on the pulse current IP as the modulation signal and is supplied to the laser diode 1 will be described. The fixed bias current supply circuit 4 is
It supplies a lower limit of the operating temperature range of the laser diode 1, that is, a current value IB1 in the vicinity of the threshold current peculiar to the laser diode required at the lowest temperature of the expected operation. Supply to diode 1. The fixed bias current IB1 is applied to the fixed resistor 20,
21 depends on the resistance value of the variable resistor 26, and if the resistance value is set to an appropriate resistance value, by changing the variable resistor 26, I
B1 can be changed within a certain range. Transistor 12,
Since 13 has a constant current configuration, a stable bias current IB1 is generated from the laser diode 1 anode → cathode → transistor 12 collector → emitter → resistor 20.
→ About IB1 flows through the route of GND.

The automatic control bias current supply circuit 5 is a circuit for supplying a bias current superposed on the fixed bias current IB1 and providing an optimum bias current according to the threshold current which changes with the temperature change of the laser diode 1. is there. The mode of operation will be described with reference to FIG. When the light receiving element 2 receives the optical output of the laser diode 1 (optical output corresponding to the bias current and the amount of the modulation signal), the photocurrent IM flows into the transistors 14 and 15 according to the amount of light. Here, in the constant current type circuit mainly composed of the transistors 14 and 15, by setting the capacitor 29 connected between the base terminals of both the transistors and GND to an appropriate capacitance value, It has a function of bypassing only the modulated signal component superimposed on the photocurrent IM to the GND. LPF
(Low Pass Filter) or HCF (H
It has a function of high Cut Filter). As a result, the monitor output of the light output fluctuation from the light receiving element 2 is biased from the collector of the transistor 15 due to the appropriately set resistance values of 22, 23, 27 and 28 and the capacitance value of the capacitor of 29. The direct current optical output corresponding to the current component, that is, Pa can be extracted in the form of being converted into a voltage. Therefore, the collector potential of the transistor 15 (base potential of the transistor 16) operates in such a direction that it increases when the optical output Pa level of the laser diode 1 decreases, and conversely decreases when the optical output Pa level of the laser diode 1 increases (Pa level).

The final stage of the automatic control bias current supply circuit 5 includes transistors 16, 17, 18 and resistors 24, 2.
It is composed of 5. The emitter potential of the transistor 16 acts in the same direction as the collector potential of the transistor 15 described above. Transistors 17, 18 and resistor 2
Reference numerals 4 and 25 are constant current type circuit configurations, and the current supplied from the transistor 16 is
Flows into the base of. Resistors 24 and 25 connected between the emitters of the transistors 17 and 18 and GND have the same coefficient of change in threshold current value (temperature coefficient of threshold current) with respect to the ambient temperature of the laser diode 1. That is, by appropriately setting the resistance values 24 and 25, it becomes possible for the laser diode 1 to supply the appropriate IB2 required by the temperature change to the laser diode 1 via the collector and emitter of the transistor 25.

The present invention can be variously modified without departing from the gist of the present invention, and is not limited to the above embodiments. For example, in the above embodiment, the transistor is the NPN type, but the PNP type may be used, or two or more fixed bias current supply circuits may be provided.

[0021]

The present invention has the following effects. (1) A stable light output operation can be performed by compensating for the characteristic change of the laser diode caused by the temperature change. (2) Since the DC bias current value and the modulation signal current value can be independently set according to the light emitting state of the laser diode, it can be widely applied to laser diodes having arbitrary temperature characteristics. (3) By shunting the bias current (it is also possible to provide two or more blocks), it is possible to reduce the collector loss of the transistor used, and it is possible to employ a small-signal transistor or the like, especially when the power supply voltage is high. It is effective, and it is effective in the case of integration. Also,
By reducing the types of parts used, productivity can be improved and prices can be reduced.

(4) The bias current supply circuit has only a DC operation function by blocking the modulation signal from the light output monitoring light receiving element of the laser diode, and requires a complicated peak hold circuit or the like. It can be easily realized with a simple circuit configuration and a single power supply. (5) High-speed driving is possible by adopting the ECL configuration for the first-stage configuration that drives the modulation signal current.

[Brief description of drawings]

FIG. 1 is a block diagram of a light output control circuit of a laser diode according to an embodiment of the present invention.

FIG. 2 is a specific circuit diagram of the embodiment shown in FIG.

FIG. 3 is a schematic diagram showing a change in laser diode drive current due to a temperature change and a change in input / output waveform due to a temperature characteristic of the laser diode.

FIG. 4 is a circuit diagram of a conventional optical output control circuit of a typical laser diode.

FIG. 5 is a circuit diagram of a light output control circuit of another conventional laser diode.

[Explanation of symbols]

 1 ... Laser diode 2 ... Photodetector 3 ... Pulse current supply circuit 4 ... Fixed bias current supply circuit 5 ... Automatic bias current supply circuit

Claims (4)

[Claims] 1. A fixed bias current supply circuit for supplying a fixed bias current to a laser diode, and an automatic control bias current supply circuit for supplying a control bias current to the laser diode in response to a signal detecting the optical output of the laser diode. A pulse current supply circuit that receives an input pulse signal from the outside and supplies a modulation signal current according to the input pulse signal is connected in parallel, and the fixed bias current supply circuit, the automatic control bias current supply circuit, and the pulse current supply circuit An optical output control circuit for a laser diode, wherein and are connected in series to the laser diode. 2. A light output control circuit for a laser diode according to claim 1, wherein the fixed bias current supply circuit comprises a resistance element and a transistor. 3. An automatic control bias current supply circuit, which comprises a capacitor, a resistance element and a transistor, functions according to the photocurrent of a light receiving element for detecting the optical output power of a laser diode and stabilizes the optical output. Claim 1 having
Alternatively, the optical output control circuit of the laser diode according to the item 2. 4. The laser diode optical output control circuit according to claim 1, 2 or 3, wherein the pulse current supply circuit comprises a resistance element and a transistor.