JPH04109687A - Semiconductor laser control device - Google Patents

Abstract

PURPOSE:To realize high accuracy, high speed operation and high resolution by adjusting the gain of a gain converter and setting the light output of a semiconductor laser. CONSTITUTION:A current amplifying part is structured by inputting a drain of a field effect transistor 7 to a base of a bipolar transistor 10, inputting a collector of the bipolar transistor 10 to a cathode of a semiconductor laser 9 and connecting an emitter of the bipolar transistor 10 to a gate of the field effect transistor 7 through a capacitor 8. A light receiving signal current from a light receiving part 12 is branched to a reference voltage 15 and a current input for feedback using a variable resistor 13, and a control signal for setting the reference voltage 15 and the current input point to the same voltage is then applied to a current input point. Thereby, high accuracy, high speed operation and high resolution can be realized only with a simple circuit structure.

Description

[Detailed description of the invention] Leather ratio! The present invention relates to a semiconductor laser control device, and more particularly, to a laser printer, an optical disk device, a digital copying machine,
The present invention relates to a semiconductor laser control device that controls the optical output of a semiconductor laser used as a light source in optical communication devices and the like.

Semiconductor lasers have been widely used in recent years as light sources for optical disk devices, laser printers, etc. because they are extremely small and can be directly modulated at high speed by a drive current.

However, the relationship between the driving current and the optical output of the semiconductor laser changes significantly depending on the temperature, which poses a problem when trying to set the optical intensity of the semiconductor laser to a desired value. In order to solve this problem and take advantage of the advantages of semiconductor lasers, various APC (Automatic Power Cable)
ontrol) circuit has been proposed.

This APC circuit can be broadly classified into the following three types.

■The light output of the semiconductor laser is monitored by a light receiving element,
A signal proportional to the light receiving current (proportional to the optical output of the semiconductor laser) generated in this light receiving element.

A method in which the optical output of the semiconductor laser is controlled to a desired value using an optical/electrical negative feedback loop that constantly controls the forward current of the semiconductor laser so that the light emission level command signal is equal.

■During the power setting period, the light output of the semiconductor laser is monitored by the light receiving element, and the light receiving current (
A signal proportional to (proportional to the optical output of the semiconductor laser) and 1
The optical output of the semiconductor laser is controlled by controlling the forward current of the semiconductor laser so that it is equal to the light emission level command signal, and by holding the value of the forward current of the semiconductor laser set in the power setting period outside the power setting period. A method that modulates the forward current of the semiconductor laser set during the power setting period based on the information, and adds information to the optical output of the semiconductor laser outside the power setting period.

■Measure the semiconductor laser temperature and use the measured temperature signal to control the forward current of the semiconductor laser or to keep the temperature of the semiconductor laser constant to adjust the optical output of the semiconductor laser to the desired level. A method of controlling by value.

Among the above classifications, the method (■) is preferable because it constantly controls the optical output of the semiconductor laser to the desired value, but the relationship between the emission level command signal and the optical output of the semiconductor laser When setting, there is a drawback that the open loop cross frequency in the optical/electrical negative feedback loop varies depending on the set value, making the control speed of the optical/electrical negative feedback loop unstable.

Purpose The present invention has been made in view of the above-mentioned problems, and has been made for the purpose of improving the above-mentioned drawbacks and providing a high-speed, high-precision, high-resolution, and stable semiconductor laser control device. be.

Structure In order to achieve the above object, the present invention provides a semiconductor laser control device that (1) detects the optical output of a driven semiconductor laser by a light receiving section, and detects a current proportional to a light reception signal current obtained from the light receiving section; In a semiconductor laser control device that controls the light emission level and the command signal current to be equal to each other,
A drain of a field effect transistor is connected to a base of a bipolar transistor, and a collector of the bipolar transistor is connected to a cathode of the semiconductor laser.

a current amplifying section configured by connecting the emitter of the bipolar transistor to the gate of the field effect transistor via a capacitor; and a variable resistor for feeding back the light receiving signal current from the light receiving section to a reference voltage. and a control circuit that applies a control signal to the current input point such that the reference voltage and the current input point are at the same potential;
In 1), the reference voltage is controlled by a second control circuit having the same characteristics as the first control circuit that performs control on the current input point so that the reference voltage and the voltage at the current input point are at the same potential. (3) In (2) above, the configurations of the first control circuit and the second control circuit are configured of bipolar transistors or field effect transistors having the same characteristics, and each of the configurations is the same. It is characterized by flowing a bias current of . Hereinafter, the present invention will be explained based on examples.

FIG. 1 is a circuit diagram for explaining one embodiment of the semiconductor laser control device of the present invention, in which 1 is a bias voltage source;
2 is a differential amplifier, 3 and 6 are current sources, 4.10 is a bipolar transistor, 5 and 11° 14 is a resistor, 7 is a field effect transistor, 8 is a capacitor, 9 is a semiconductor laser, 1
2 is a light receiving element, 13 is a variable resistor, 15 is a reference voltage, 16
is a constant voltage diode, and Vc is a DC power supply. In FIG. 1 with the above circuit elements, the light emission level command signal current I0
is input to the emitter of bipolar transistor 4. The collector of the bipolar transistor 4 is connected to the gate of the field effect transistor 7, and the collector of the bipolar transistor 4 is connected to the gate of the field effect transistor 7.
The drain of is connected to the base of bipolar transistor 1o,
The emitter of the bipolar transistor 10 is connected to a constant voltage diode 16, and the anode of the constant voltage diode 16 is connected to the gate of the field effect transistor 7 via the capacitor 8. Configures a negative feedback loop in the amplifier section.

Then, a driven semiconductor laser 9 is connected as a load to the collector of the bipolar transistor 10. Luminous level command electrician. When applied, this is amplified, a forward current flows through the semiconductor laser 9 serving as the load, and the semiconductor laser 9 emits light. This light output is detected by the light receiving element 12, and a light receiving signal current i proportional to the light output of the semiconductor laser 9 flows through the light receiving element 12. This light reception signal current i is divided into a variable resistor 13 and a resistor 14. The current shunted through the variable resistor 13 is input to the emitter of the bipolar transistor 4, and the current shunted through the resistor 14 flows toward the reference voltage 15. In this case, by changing the resistance value of the variable resistor 13, the magnitude of the shunt current of the light reception signal current i input to the emitter of the bipolar transistor 4 can be adjusted, and in this way, the optical/electrical negative feedback loop is configured.

In the above configuration, when one light emission level command signal current Io is given, the variable resistor 13 is adjusted so that the output of the semiconductor laser 9 outputs a desired value P, and the attenuation coefficient K of the current in the variable resistor 13 is set to a predetermined value. When set to a value, the light and
The cross frequency of the electrical negative feedback loop can be f0. We will prove this below.

In this example, the open loop gain A of the system is A=G・
It can be expressed as η·αS−K (1).

However, G: Gain of the current amplification section (consisting of bipolar transistor 10 and field effect transistor 7) η: Differential quantum efficiency of the semiconductor laser α: Coupling coefficient between the light receiving element and the semiconductor laser S: Light receiving radiation sensitivity of the light receiving element.

K: Attenuation coefficient of current in variable resistance.

Looking at equation (1), the open loop gain A of the system is determined by the differential quantum efficiency of the semiconductor laser 9, the light receiving element 12 and the semiconductor laser 9.
It can be seen that this is affected by variations in the coupling coefficient with the light receiving element 12, the light receiving radiation sensitivity of the light receiving element 12, etc.

Further, the gain G of the current amplification section of this circuit is approximately set to be G=17jωCR(2). Here, CR is a constant determined by the characteristics of the current amplification section, and ω (=2πf, f: frequency) is the angular velocity.

Further, the light reception signal current 1 is the optical output P of the semiconductor laser 9.

1=Po・αS (3
).

It also has a light emitting level command signal current engineer. Since I0=-1(4), the optical output P of the semiconductor laser 9 is P,=1. /kas,
(5) Also, the cross frequency f of the optical/electrical negative feedback loop.

is the frequency where A=1, so from (1) and (2) f, = η·αS −k/2πCR (6).

As is clear from the above equations (5) and (6), the light receiving element 12
Even if the coupling coefficient α of the semiconductor laser 9 and the light-receiving radiation sensitivity S of the light-receiving element 12 change, a predetermined light emission level command current can be maintained.

variable resistor 1 so as to obtain the desired optical output P,
When the damping coefficient k of 3 is adjusted, α5=const (7), so it is clear from equation (5) that the crossover frequency f0 can be automatically set to a desired value.

Further, in this configuration, the light reception signal current i is divided by the resistance ratio of the variable resistor 13 and the resistor 14 without flowing unnecessary bias current.

In other words, if the resistance value of the variable resistor 13 is Rv, and the resistance value of the resistor 14 is R, then the current attenuation coefficient is = R/ (R+
Rv) (8).

Additionally, in this configuration, the luminous level command signal works. The emitter of the bipolar transistor 4 is the summation point of a part of the light receiving signal current i to which negative feedback is applied, but when the current flows into the emitter, the emitter potential fluctuates.In order to eliminate this fluctuation, in the present invention, the emitter potential is set to the reference voltage. Control is applied so that the potential is the same as that of 15. That is, by inputting the bias voltage source 1 at the same voltage as the reference voltage 15 to the differential amplifier 2 together with the emitter of the bipolar transistor 4, and inputting its output to the base of the bipolar transistor 4, a negative feedback loop is constructed.

Next, the operation of the negative feedback loop will be explained. Assume that the emitter potential of the bipolar transistor 4 has decreased.

In this case, the difference between the emitter potential and the potential of the bias voltage source 1 (the potential of the reference voltage 15) is amplified by the differential amplifier 2 to try to raise the base potential of the bipolar transistor 4. Then, the potential between the base and emitter of the bipolar transistor 4 increases, and the bipolar transistor 4 attempts to allow a large amount of current to flow. Then, a large amount of current flows through the resistor 5, raising the emitter potential. When the emitter potential is stabilized in this way, the light reception signal current i is divided by the resistance ratio of the variable resistor 13 and the resistor 14 without flowing an unnecessary bias current. In other words, the attenuation coefficient of the current is calculated by equation (8), and the current is shunted with high accuracy, so that a more accurate optical/electrical negative feedback loop is constructed.

Also, in this configuration, bias voltage source 1 and reference voltage 15 are provided, but this is because the circuit is driven only with a positive power supply, and if a negative power supply can also be used, the bias voltage source 1
It is also possible to consider a configuration in which the and reference voltage 15 are respectively grounded, and the resistor 5 is connected not to the ground but to the negative power supply.
It is clear that the effects of the present invention can also be obtained with this configuration.

Here, it is clear that the effects of the present invention can be obtained even if the resistor 5 is a current source.

Further, in this configuration, the light emission level command signal current 1 is input to the emitter of the bipolar transistor 4, but the current of the current source 3 is input to the light emission level command signal circuit. It is clear that the effects of the present invention can be obtained even if controlled by

Further, in this configuration, the bipolar transistor 4 is used as a means for impedance conversion, but the effects of the present invention can be obtained even if a field effect transistor is used in this part or a circuit configured with a plurality of transistors. is clear.

As described above, according to the present invention, when the optical output of the semiconductor laser is simply set to a desired value, the crossover frequency of the system can be automatically set to be constant.

Moreover, a semiconductor laser control device with high precision, high speed, and high resolution can be realized.

FIG. 2 is a circuit diagram for explaining another embodiment of the semiconductor laser control device of the present invention, in which circuit elements having the same functions as those in FIG. 1 are given the same reference numerals. 18,19
．． 20 is a field effect transistor, 21 is a constant voltage diode, and 22 is a current amplifier.

Here, the current amplifier 22 is a simplified version of the same circuit as the current amplification section in FIG. It is assumed that the operations are the same.

In FIG. 2, the basic operation is the same as that in FIG. 1, so a description thereof will be omitted here. The difference between FIG. 2 and FIG. 1 is that the bipolar transistor 4 is replaced by a field effect transistor 20, and the anode of the constant voltage diode 21 is connected to the light emission level command signal current I0 and the light receiving signal current i attenuated by the variable resistor 13.
This is the current addition point. In addition, the field effect transistor 18 and the field effect transistor 19 are each composed of elements having the same characteristics, and the drain of the field effect transistor 18 is connected to the current mirror circuit 17, the gate is connected to the bias voltage source 1, and the source is connected to the ground. , the drain of the field effect transistor 19 is connected to the current mirror circuit 17, the gate is connected to the anode of the constant voltage diode 21, the source is connected to the ground, and the gate of the field effect transistor 20 is connected to the drain of the field effect transistor 19. It constitutes a circuit. The operation of this control circuit will be briefly explained. Assuming that the anode potential of the constant voltage diode 21 decreases, the drain current of the field effect transistor 19 decreases, and as a result, the drain current of the field effect transistor 20 decreases.
The gate potential of the field effect transistor 20 increases, the gate-source voltage of the field effect transistor 20 increases, and a large amount of current tends to flow. Therefore, the anode potential of the constant voltage diode 21 is equal to the voltage of the bias voltage source 1 (that is, the voltage of the reference voltage 15).
Control is applied at high speed so that it is equal to .

The field effect transistors 18 and 19°2o are used here because if the light reception signal current attenuated by the variable resistor is small, it will be affected by the gate leakage current (base current for bipolar transistors) of these transistors. However, since the gate leakage current of a field effect transistor is generally on the order of several tens of nanoamperes, this is done in consideration of the fact that it is less susceptible to this effect than a bipolar transistor. Therefore, this part may be configured using a bipolar transistor if the light reception signal current attenuated by the variable resistor is relatively large, or a bipolar transistor may be used for the structure of the field effect transistor 20 that performs impedance conversion even if the current is small. It is clear that the effects of the present invention can be obtained even if the configuration is a Darlington connection or an emitter follower (source follower).

FIG. 3 is a circuit diagram for explaining still another embodiment of the present invention, in which circuit elements having the same functions as those in FIGS. 1 and 2 are given the same reference numerals.

23 is a current source, 24, 28, 29 are field effect transistors, 25 is a constant voltage diode, 26 is a resistor, 27 is a current mirror circuit, and 30 is a bias voltage source.

In FIG. 3, the reference voltage is constructed from a circuit having the same characteristics as the control circuit described above. That is, current source 3
and current source 23, field effect transistor 20 and field effect transistor 24, constant voltage diode 21 and constant voltage diode 25, resistor 5 and resistor 26, field effect transistor 1
8.19 and field effect transistors 28, 29. It is assumed that the bias voltage source 1 and the bias voltage source 30 are each composed of elements having the same characteristics, and that the current mirror circuit 17 and the current mirror circuit 27 have the same characteristics.

If you have this kind of circuit configuration, the constant voltage diode 2
Rather than controlling the anode potential of the constant voltage diode 21 and the anode potential of the constant voltage diode 25 to be the same voltage as the reference voltage, which is a fixed potential, the anode potential of the constant voltage diode 21 and the anode potential of the constant voltage diode 25 are kept relatively at the same potential even if they fluctuate. Control is applied so that Then, since each control system has the same characteristics, the relative potential becomes O, and a system can be constructed in which the light reception signal current i is accurately divided by the resistance ratio of the variable resistor 13 and the resistor 14.

As described above, according to the present invention, when the optical output of the semiconductor laser is easily set to a desired value, the crossover frequency of the system can be automatically set to be constant, and the system can be set with high precision, high speed, and high resolution. A semiconductor laser control device can be realized.

FIG. 4 is a circuit diagram for explaining still another embodiment of the present invention, in which circuit elements having the same functions as those in FIGS. 1 and 2 are given the same reference numerals.

31, 34 are bipolar transistors, 32° and 35 are resistors, and 33 is a current mirror circuit.

In Figure 4, the light emission level command signal current function is shown. and variable resistor 1
The current addition point of the light-receiving signal current i attenuated in step 3 is set as the emitter of the bipolar transistor 4. The difference between FIG. 4 and FIG. 3 is the configuration of the potential control section. In this configuration, bipolar transistors 4, 31, 34, resistor 5
, 32.35 are elements with the same characteristics,
A bias current is passed through the bipolar transistor 4 and the bipolar transistor 34 using a current mirror circuit 33. In this case, since the base potentials of bipolar transistor 4 and bipolar transistor 31 are common, the same bias current flows, and the emitter potentials of each become the same potential. A current that inputs the previous light reception signal current i to the emitter of the bipolar transistor 4 via the variable resistor 13;
By dividing the current into the current input to the emitter of the bipolar transistor 31, the same effect as in claim 2 can be obtained with a simpler circuit configuration.

Further, in this circuit configuration, bipolar transistors 4, 31, and 34 are used, but it is clear that similar effects can be obtained even if the circuit is configured using field effect transistors.

As described above, according to the present invention, when the optical output of the semiconductor laser is simply set to a desired value, the crossover frequency of the system can be automatically set to be constant.

Moreover, a high-precision, high-speed, and high-resolution semiconductor laser control device can be realized with a simple circuit configuration.

Note that the basic operation in FIG. 5 is the same as that described in FIG. 3.

Effects As is clear from the above explanation, the present invention has the following effects.

(1) Effect corresponding to claim 1: In the optical/electrical negative feedback loop that electrically converts and amplifies the output light of the driven semiconductor laser and the output light, by adjusting the gain of the gain converter, By simply setting the optical output to the desired value, the open-loop crossover frequency of the optical/electrical negative feedback loop is automatically set to the desired value. By configuring a control circuit that controls the current input point so that the current input point and the current input point have the same potential, a highly accurate, high-speed, high-resolution semiconductor laser control device can be realized.

(2) Effect corresponding to claim 2: A second control circuit having the same characteristics as a first control circuit that controls the current input point so that the reference voltage and the current input point are at the same potential. By configuring the reference voltage, a highly accurate semiconductor laser control device can be realized.

(3) Effect corresponding to claim 3: By configuring the first control circuit and the second control circuit using bipolar transistors or field effect transistors having the same characteristics and flowing the same bias current respectively. , a highly accurate semiconductor laser control device can be realized with a simple circuit configuration.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram for explaining one embodiment of the semiconductor laser control device of the present invention, Figure 2 is a circuit diagram for explaining another embodiment of the present invention, Figures 3 and 4 are The figure is a circuit diagram for explaining still another embodiment of the present invention. 1.30...Bias voltage source, 2...Differential amplifier,
3゜6.23...Current source, 4,10,31,34...
・Bipolar transistor, 7, 18, 19, 20°2
4.28.29...Field effect transistor, 5゜1.
1, 14, 32, 35... Resistor, 8... Capacitor, 16.21, 25... Constant voltage diode, 9...
Semiconductor laser diode, 12... Light receiving element, 13.
- Variable resistor, 15... Reference voltage, 22... Current amplifier, 17.27.33... Current mirror circuit. Figure cc Figure cc Figure cc Figure cc

Claims (1)

[Claims] 1. The optical output of the driven semiconductor laser is detected by a light receiving section, and control is performed so that the current proportional to the light reception signal current obtained from the light receiving section is made equal to the light emission level command signal current. In a semiconductor laser control device, the drain of a field effect transistor is input to the base of a bipolar transistor, the collector of the bipolar transistor is input to a cathode of the semiconductor laser, and the emitter of the bipolar transistor is connected to the base of the field effect transistor through a capacitor. a current amplification section configured by connecting to the gate; and a variable resistor to divide the light reception signal current from the light reception section into a reference voltage and a current input point for feedback;
A semiconductor laser control device comprising: a control circuit that applies a control signal to the current input point such that the reference voltage and the current input point have the same potential. 2. The reference voltage is configured by a second control circuit having the same characteristics as the first control circuit that performs control on the current input point so that the reference voltage and the voltage at the current input point are at the same potential. The semiconductor laser control device according to claim 1, characterized in that: 3. The first control circuit and the second control circuit are constructed of bipolar transistors or field effect transistors having the same characteristics, and the same bias current flows through each of the first control circuit and the second control circuit, respectively. Semiconductor laser control device.