JPH02159780A - Drive circuit of laser - Google Patents

Abstract

PURPOSE:To enable a laser to keep an emission volume constant by a method wherein the emission volume of laser rays is controlled basing on an error voltage which is obtained from the information of voltage difference between a laser ray emission volume monitor voltage and a specified reference voltage and a laser drive power information. CONSTITUTION:A laser drive means Tr1 which controls an emission volume PL of laser rays 3 emitted from a laser 2 basing on an error voltage VLD is provided, where the error voltage VLD is obtained by computing a difference information VSD between a monitor voltage VM which is corresponding to the emission volume PL of the laser rays 3 and a reference voltage VR and a power information Vp2 which drives the laser 2. By this setup, a laser drive circuit 10 can be obtained, which can keep the emission volume PL of the laser rays 3 emitted from the LD 2 constant even if a power voltage VCC fluctuates.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a laser drive circuit, and in particular to an APC (auto
It is suitable for application to circuit configurations such as matic power control).

B. Summary of the Invention The present invention provides a laser drive circuit that calculates the amount of laser light emitted from a laser based on an error voltage obtained by calculating difference information between a monitor voltage and a reference voltage and information on a power source that drives the laser. By controlling the amount of light emitted from the laser, the amount of light emitted by the laser can be maintained constant even if the power supply voltage fluctuates.

C. Conventional technology Conventionally, in semiconductor lasers used for optical pickup in compact disc devices, for example, a light receiving element such as a photodiode is provided in the same package as a light emitting element made of the semiconductor laser, and this reduces the amount of light emitted by the semiconductor laser. The so-called APC (automatic) monitors and automatically controls the amount of light emitted to a certain level.
A laser drive circuit having a power control circuit configuration is used.

That is, as shown in FIG. 5, this laser drive circuit 1
, a predetermined error voltage VL is supplied to the base via the base resistor RE, and the power supply V The collector current of the connected transistor Tr+ having an emitter follower configuration is supplied as a laser drive current (■) to a semiconductor laser diode (T,,D)2 to which an electrostatic damage prevention capacitor C2 is connected in parallel. The LD 2 is driven to emit light and the laser beam 3 is emitted.

The laser beam 3 emitted from the LD 2 is received by a photodiode (PD) 4 placed, for example, on the back side within the same package as the LD 2, and as a result, the laser beam 3 is emitted from the PD 4 according to the light emission amount PL of the laser beam 3. A monitor current [M] consisting of the light receiving current is a variable resistor R for current-voltage conversion whose one end is grounded.
, and the resulting voltage is supplied as a monitor voltage VH to the non-inverting input terminal of a differential amplifier circuit 4 having an operational amplifier configuration.

This differential amplifier circuit 4 is negatively fed back by a resistor R1, and its inverting input terminal is grounded via a resistor R2 having a sufficiently small resistance value. In this way, the difference output (2) between the monitor voltage VM and the reference voltage VR is obtained from its output terminal.

This differential output 5 is supplied via input resistors R4 and R5, each having the same resistance value, to the inverting and non-inverting input terminals of a buffer circuit 6 which is configured as an operational amplifier and is negatively fed back by a resistor R6. At the output end of circuit 6,
Error voltage generated by amplifying differential output ■5 with a predetermined gain■
1 is output, and the transistor T is output via the base resistor RB.
Supplied to the base of r+.

In this way, this laser drive circuit 1 as a whole constitutes a feedback loop, and the light emission amount PL of the laser beam 3 emitted from the LD 2 is adjusted against external fluctuations such as temperature characteristics.
is automatically controlled to a constant amount.

D Problems to be Solved by the Invention By the way, in the laser drive circuit 1 having such a configuration, L
Since a collector drive method is adopted as the driving transistor Tr+ of D2, the bias voltage varies due to voltage variation of the power supply V((), and as a result, the light emission amount P of the laser beam 3 emitted from the LD2 varies. There is a problem with doing so.

That is, in this laser drive circuit 1, the gain of the non-inverting input terminal of the differential amplifier circuit 4 is 4, the gain of the inverting input terminal is 9, and the voltage-current conversion gain of the transistor Tr1 is K. , L
Assuming that the current-light power conversion efficiency of D2 is KL and the light-power current conversion efficiency of PD4 is , it can be represented by a block diagram BL1 as shown in FIG.

Here, the voltage-current conversion gain Ko of the transistor Tr1 can be expressed by the following formula.This block diagram BLI can be summarized as a block diagram BL2 as shown in FIG. It can be seen that the amount of light emission PL of the laser beam 3 fluctuates as expressed by the following formula %.

Note that the block diagram BL2 in this laser drive circuit 1
In the system, the power supply ■. , has no feedback loop, the block diagram BL3 as shown in FIG.
Therefore, the voltage fluctuation of the power supply VCc is expressed by the following equation. Actually, in the laser drive circuit 1, (
Compared to the case of equation 3), it has a power supply voltage suppression ability expressed by the following equation = 6 (4) 1 +KOKLKMRMKP.

Therefore, in the laser drive circuit 1, the voltage of the transistor Tr+ is
Depending on the time constant determined by tn RB and capacitor C1, there is a shedding effect equal to the reciprocal of the open loop gain expressed by equation (4).

However, since the open loop gain itself, which is the denominator of equation (4), is set to a finite value based on the cutoff band, this disturbance factor could not be completely eliminated.

The present invention has been made in consideration of the above points, and aims to propose a laser drive circuit that can automatically control the amount of laser light emitted to a constant level even if the power supply voltage fluctuates.

E Means for solving the problem In order to solve this problem, in the present invention, the laser beam 3 emitted from the laser 2 is received, and the monitor voltage (4) is set according to the amount of light emission Pt of the laser beam 3. Error detection that generates an error voltage VLD by calculating the difference information V3tl between the generated laser light monitoring means 4, R, the monitor voltage VM and the predetermined reference voltage ■2, and the power supply information ■2□ for driving the laser 2 Means 11, 12, 13, 14, and laser driving means T, □ for controlling the light emission amount PL of the laser beam 3 emitted from the laser 2 based on the error voltage VLD are provided.

Error voltage obtained by calculating the difference information sD between the F action monitor voltage 4 and the reference voltage VR and the power supply information VP2 for driving the laser 2 1. By controlling the light emission amount PL of the laser beam 3 emitted from the laser 2 based on
can be maintained constant.

G example An embodiment of the present invention will be described in detail below with reference to the drawings.

In FIG. 1, in which parts corresponding to those in FIG.
A monitor current I is a light receiving current corresponding to the light emission amount PL of the laser light 3 received by the monitor current I. is supplied to the other end of the current-voltage converting variable resistor RM whose one end is grounded, and the resulting voltage is the monitor voltage (2). The signal is then supplied to the inverting input terminal of the first differential amplifier circuit 11 configured as an operational amplifier via the input resistor R11.

The first differential amplifier circuit 11 is negatively fed back by a resistor R12 having a resistance value equal to that of the input resistor R11, and its non-inverting input terminal is grounded via a resistor R13, and the reference voltage obtained from the reference power supply 5 is (2) 8 is inputted via an input resistor R14 having a resistance value equal to that of the resistor R13.

As a result, the non-inverting input terminal of the first differential amplifier circuit 11 is provided with a non-inverting input voltage obtained by dividing the reference voltage 8 by 1/2 by the first resistive voltage divider circuit 12 consisting of resistors R13 and R14. Vpl is input, and thus the monitor voltage V is output from its output terminal. and the first difference output v between the non-inverting input voltage VF6
sn.

The first differential output vsn is inputted via an input resistor R15 to an inverting input terminal of a second differential amplifier circuit 13 configured as an operational amplifier subjected to negative feedback with a resistor R16.

A reference voltage vR obtained from the reference power supply 5 is inputted to the non-inverting input terminal of the second differential amplifier circuit 13 via an input resistor R17, and a voltage 1/2 obtained by dividing the power supply voltage VCC by 1/2 is input. A voltage obtained by further dividing 2Vcc with resistors R18 and R19 is input via an input resistor R20.

As a result, the reference voltage 7 and the 1/2 power supply voltage 1/2vcc are applied to the non-inverting input terminal of the second differential amplifier circuit 13.
The non-inverting input voltage vrz divided by the second resistance voltage divider circuit 14 made up of resistors R17, R18, R19, and R20 is inputted, and the output terminal thereof is inputted from the first differential amplifier circuit 11. An error voltage V1. D is output, and this error voltage VLD is supplied to the base of the transistor Tr+ via the base resistor R8.

In this way, this laser drive circuit 10 as a whole constitutes a feedback loop, and the first differential amplifier circuit 1
1), L I) against disturbance fluctuations such as temperature characteristics.
The light emission amount Pt of the laser beam 3 emitted from the LD 2 is automatically controlled to a constant amount, and in the second differential amplifier circuit 13, the laser beam 3 emitted from the LD 2 is The amount of light emitted PL is automatically controlled to a constant amount.

Note that in the first resistive voltage divider circuit 12, the resistors R13 and R14 have the same resistance value, so the non-inverting input voltage V p 1 input to the non-inverting input terminal of the first differential amplifier circuit 11 is calculated by the following formula: % Formula % On the other hand, in the second resistance voltage divider circuit 13, for example, the resistance values of the resistors R17, R18, R19, and R20 are:
15.5.11.2.84.3 [KΩ], respectively, so that the non-inverting input voltage VP2 input to the non-inverting input terminal of the second differential amplifier circuit 13 is calculated by the following formula: It is set to the value expressed by the % expression %.

Here, equations (5) and (6) are simplified and expressed as the following equation (%). If the gain on the non-inverting input side of the second differential amplifier circuit 13 is KPZ, and the gain on the inverting input side is KN□, then this laser drive circuit 10 is as shown in FIG. It can be represented by a block diagram BLIO as shown in FIG. 2 in which corresponding parts are given the same reference numerals.

As shown in FIG. 3, this block diagram BLIO is a block diagram B1 in a form corresponding to the block diagram BLI in FIG. , 11, where the terms a KPI KHz-b KPZ and K s
If the terms of + K N The block diagram B in FIG. 6 except that the voltage VCc is input through the I KrzC term.
It can be seen that it is equivalent to LI.

Therefore, in this laser drive circuit 10, KPZ, that is, the second differential amplifier circuit 13
By setting the gain on the non-inverting input side of the power supply voltage ■,. can be controlled independently of the overall system, thus the power supply ■. This is designed to prevent fluctuations in the amount of light emitted P due to voltage fluctuations in .

In addition, when the resistance values of the resistors R17, R18, R19, and R20 are as described above, the left side of the equation (11) is calculated as shown in the following equation, and thus the equation (11) is made to hold. There is.

In addition, in the case of this embodiment, the gain of the non-inverting input terminal of the first differential amplifier circuit 11 is equal to the gain KNI of the inverting input terminal and the gain KN2 of the inverting input terminal of the second differential amplifier circuit 13. is determined so as to satisfy the servo characteristic conditions of the entire feedback loop, such as the gain on the low frequency side and the cutoff band.

In the above configuration, for example, when the power supply voltage VCC fluctuates and the voltage attempts to rise, the error voltage VtO sent out from the second differential amplifier circuit 13 increases by that amount, so that the he-to-emitter voltage of the transistor Trl increases. V!
IE is held constant, so that even if the power supply voltage VCC fluctuates, the driving current (2) of the LD2 can be held constant, and thus the power supply voltage (2). It is possible to prevent variations in the amount of light emission P1 of the laser beam 3 emitted from the LD 2 due to variations in .

According to the above configuration, the laser beam 3 emitted from the LD 2
An error voltage VLD is generated based on the monitor voltage V obtained by receiving the light with the PD4, the differential output VSD of the reference voltage 6, and the power supply information Vpz, and based on this error voltage VLD. Laser light 3 emitted from LD2
By controlling the amount of light emitted P, even if the power supply voltage ■cc changes, the laser beam 3 emitted from the LD2
It is possible to realize a laser drive circuit 10 that can maintain a constant light emission amount PL.

Note that in the above embodiment, the second differential amplifier circuit 13
To generate the non-inverting input voltage VP2 of , the power supply voltage V
The case has been described in which the voltage 1/2Vcc obtained by dividing CC by 1/2 is used, but instead of this, the power supply voltage V((() may be used as it is. In this case, the second resistance voltage divider By arbitrarily selecting the resistance division ratio of , it is possible to achieve the same effect as in the above embodiment.

Furthermore, in the above embodiment, two operational amplifiers constitute the first and second differential amplifier circuits, and in the first differential amplifier circuit, the emission from the laser is The amount of laser light emitted from the laser is automatically controlled to a constant level, and the second differential amplifier circuit automatically controls the amount of laser light emitted from the laser in response to disturbance fluctuations in the power supply voltage. Although the circuit configuration is controlled to a constant value, the circuit configuration is not limited to this.In short, if the error voltage for driving the laser to emit light is obtained based on the difference information between the monitor voltage and the reference voltage, and the power supply information, the above implementation can be performed. The same effect as in the example can be achieved.

Further, in the above-described embodiments, a case was described in which a laser and a photodiode were sealed in the same package, but the present invention is not limited to this, but can also be applied to a case in which a laser beam is monitored outside the laser package. It is also widely applicable and suitable.

Effects of the Invention As described above, according to the present invention, the emission of laser light emitted from the laser is determined based on the error voltage obtained by calculating the difference information between the monitor voltage and the reference voltage and the power supply information for driving the laser. By controlling the amount, it is possible to realize a laser drive circuit that can maintain a constant amount of laser light emitted from the laser even if the power supply voltage fluctuates.

[Brief explanation of the drawing]

Figure 1 is a connection diagram showing one embodiment of the present invention, Figures 2 to 4
The figure is a block diagram to explain the loop characteristics.
The figure is a connection diagram showing a conventional laser drive circuit, Figures 6 to 8.
The figure is a block diagram for explaining the loop characteristics. ■, 10... Laser drive circuit, 2...
Laser diode, 3... Laser light, 4...
...Photodiode, 5...Reference power supply, 1
1.13...Differential amplifier circuit, 12.14...
...Resistive voltage divider circuit.

Claims (1)

[Scope of Claims] Laser light monitoring means for receiving laser light emitted from a laser and generating a monitor voltage according to the amount of light emitted by the laser light, and information on the difference between the monitor voltage and a predetermined reference voltage; comprising: an error detection means that calculates power supply information for driving the laser to generate an error voltage; and a laser drive means that controls the amount of laser light emitted from the laser based on the error voltage. Features a laser drive circuit.