JPH0397130A - Noise reducing circuit for semiconductor laser - Google Patents

Abstract

PURPOSE:To always obtain a proper degree of modulation to reduce the noise by obtaining a differential quantum efficiency from the DC current detected at the time of driving a semiconductor laser with each light emission power and varying the gain of a gain variable amplifier in accordance with it to control the extent of superposition of a high frequency current on the DC current. CONSTITUTION:A DC driving circuit 5 which flows the DC current to a semiconductor laser 1, an oscillating circuit 12 which flows the high frequency current to the semiconductor laser 1, a gain variable amplifier 13 connected to the oscillating circuit 12 and the semiconductor laser 1, a light emission power control means 10 which causes the semiconductor laser 1 to emit light with two or more stages of power, and a detecting circuit 9 which detects the DC current flowing to the semiconductor laser 1 are provided. The differential quantum efficiency is obtained from the DC current detected at the time of driving the semiconductor laser 1 with each light emission power, and the gain of the gain variable amplifier 13 is varied in accordance with this obtained differential quantum efficiency to control the extent of superpositon of the high frequency current on the DC current. Thus, a sufficient degree of modulation is always secured to reduce the noise even in the case of the variance in current-light characteristic of the semiconductor laser due to the temperature or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a semiconductor laser noise reduction circuit for an optical information processing device.

Conventional technology Generally, when a semiconductor laser is used in an optical disk drive device, etc., noise may increase significantly due to reflected light (return light) from an optical information recording medium (return light does not always increase noise. (For example, noise may not increase depending on conditions such as disk reflectance, ambient temperature, emission power, and semiconductor laser variation.)
. In any case, as the noise of the semiconductor laser increases,
This results in the inconvenience that the signal information recorded on the optical information recording medium cannot be read correctly.

For this reason, as a noise reduction method for semiconductor lasers, "Laser Noise Reduction in Video Disk Players Equipped with Semiconductor Lasers Using High-Frequency Current Superimposition Method" in the document "Optics (Vol. 14, No. 5, October 1985)" or,
There is one disclosed in Japanese Patent Laid-Open No. 60-192377. For example, in Japanese Patent Application Laid-Open No. 60-192377, the driving current of a semiconductor laser is composed of a direct current and a high frequency current, and the amount of superimposition of the high frequency current is always constant.
The output intensity of the semiconductor laser is controlled by controlling the direct current based on the detection of low frequency components of the output light.

Problems to be Solved by the Invention However, with such a method, it is not always possible to obtain a sufficient modulation degree (140% or more) due to fluctuations in the current-optical characteristics of the semiconductor laser. Now, suppose that the semiconductor laser has a current-light characteristic (IL characteristic) as shown by a solid line A in FIG. 4 under a 25 DEG C. condition. At this time, in order to obtain the average required power Po, it is assumed that a direct current IDC and a high frequency current INF are passed through the semiconductor laser, and a light emission amount waveform as shown by a solid line C in FIG. 4 is obtained. Also,
The modulation degree M at this time is M= (Po /poc) x, where PDC is the light emission power due to only the DC current ■oc
It is assumed that M is 100 [%] and M≧140%. However, if we consider a case where the I-L characteristic changes as shown by the broken line B in FIG. 4 due to an increase in the environmental temperature, the DC current is changed to I in order to obtain the average required power PO. If it is changed to C', the light emission power Pl)c' due only to this DC current IDC' becomes equal to the average required power Po, and a sufficient degree of modulation cannot be obtained.

Means for Solving the Problems A high-frequency current superimposition type semiconductor laser used in an optical information processing device that reproduces, records, or erases information by condensing and irradiating a laser beam emitted from a semiconductor laser onto an optical information recording medium using an objective lens. The noise reduction circuit comprises: a direct current drive circuit for passing a direct current through the semiconductor laser; an oscillation circuit for passing a high frequency current through the semiconductor laser; and a variable gain amplifier connected to the oscillation circuit and the semiconductor laser; A light emitting power control means for causing the semiconductor laser to emit light at two or more levels of power, and a detection circuit for detecting a direct current flowing through the semiconductor laser, are provided, and a detection circuit is provided to detect the direct current detected when the semiconductor laser is deceived at each light emitting power. The differential quantum efficiency is determined, and the gain of the variable gain amplifier is varied in accordance with the determined differential quantum efficiency to control the amount of high frequency current to be superimposed on the direct current.

Regarding the fluctuations in the current-optical characteristics of the semiconductor laser, as shown in Figure 3, the fluctuations in the laser oscillation starting current Ith are unrelated to the fluctuations in the degree of modulation when the average emission power of the semiconductor laser is controlled by DC current. , and only the differential quantum efficiency η defined by ΔL/Δ■ affects the modulation degree. That is, as the differential quantum efficiency η changes, the power emitted by the high-frequency current also changes. Here, in this solution, the differential quantum efficiency is determined from the DC current flowing through the semiconductor laser, and the amount of superimposition of the high-frequency current is varied according to the variation, so that the ■-L characteristics of the semiconductor laser due to temperature etc. Even if there are fluctuations, a sufficient degree of modulation can always be ensured.

Embodiment An embodiment of the present invention will be described with reference to FIGS. 1 to 3. First, the emitted laser beam is passed through an objective lens (not shown).
A semiconductor laser 1 is provided which is used for reproducing (or recording or erasing) information by condensing and irradiating an optical information recording medium (not shown) through an optical information recording medium (not shown). A photoelectric converter 2 is provided that receives a portion of the laser beam emitted from the semiconductor laser l and obtains a voltage proportional to the amount of emitted light. The power converter 2 is connected to a control circuit 4 via an amplifier 3. This control circuit 4 uses a voltage input from the amplifier 3 as a reference voltage Vref.
The constant current circuit 5 connected to the semiconductor laser 1 is controlled so that the current is equal to .

Here, different reference voltage generators 6 and 7 are prepared, and the reference voltage V ref applied to the control circuit 4 via the switch circuit 8 is switched between two stages: Vref and Vref. The constant current circuit 5 serves as a DC drive circuit that causes a DC current IDC to flow through the semiconductor laser 1. An A/D converter (detection means) that detects the direct current ■Dc flowing through the constant current circuit 5 to the semiconductor laser l
9 is provided, and input to a controller (light emission power control means) 10 having a CPU configuration that controls the entire system. This controller 10 also takes charge of switching control of the switch circuit 8.

On the other hand, a high frequency current superimposing circuit 1l for superimposing a high frequency current on the semiconductor laser l is provided in parallel with the constant current circuit 5. The high-frequency current superimposition circuit 11 includes an oscillation circuit 12 for generating a high-frequency current and a variable gain amplifier 13 connected to the oscillation circuit 12 and whose gain can be varied. The driving of the oscillation circuit 12 is controlled by the controller IO, and the gain of the variable gain amplifier l3 is controlled by the controller IO.
It is controlled via a converter 14.

In such a configuration, first, the controller 10 turns off the oscillation circuit l2 and sets the high frequency current IHF to 20, and then controls the output of the amplifier 3 to be equal to the reference voltage Vref of the reference voltage generator 6. The constant current circuit 5 is controlled by the circuit 4 to maintain the emission power of the semiconductor laser 1 at, for example, the reproduction emission power P1. At this time, the DC current I DCI flowing through the semiconductor laser l is taken into the controller 10 by the A/D converter 9. Next, high frequency current ■
. = 0, the switch circuit 8 is switched to the reference voltage generator 7 side by the controller 10, and a different reference voltage V is set.
ref, is input to the control circuit 4. As a result, the output of the amplifier 3 becomes the reference voltage Vref. The constant current circuit 5 is controlled so that the emission power of the semiconductor laser l becomes equal to the emission power P. is maintained. At this time, a direct current ■ flows through the semiconductor laser l. . ．． is also input to the controller 10 by the A/D converter 9.

The controller 10 has a predetermined luminous power P,,
P, and the DC current I Dct when driving at each power
From e I ocs, the differential quantum efficiency η. of the semiconductor laser 1 at that time is determined. Calculate. That is, '7-= l P- '1l/ l Ioc- T
DC1, and the calculated differential quantum efficiency η. According to the value of , the controller 10 variably controls the gain of the variable gain amplifier l3 via the D/A converter l4. Therefore, when the oscillation circuit 12 is turned on during actual modulation driving, the value (superimposition amount) of the high frequency current IHF superimposed on the DC current IDC changes.

Thereafter, the controller 10 switches and controls the switch circuit 8, inputs the reference voltage Vref, to the control circuit 4, turns on the oscillation circuit 12, and superimposes the high-frequency current rop on the direct current IDC from the constant current circuit 5 in the semiconductor laser l. do. Therefore, while keeping the average emission power at P1,
Differential quantum efficiency η. A high frequency current corresponding to the current is superimposed, and an appropriate degree of modulation is obtained.

FIG. 2 is a flowchart showing such operation control. Here, for simplicity, the calculated differential quantum efficiency η
．． An example will be shown in which the variable gain amplifier 13 is divided into three regions and the gain of the variable gain amplifier 13 is variably controlled in three stages. For example, η. kuηr
When ef, the differential quantum efficiency η. is small, so a sufficient degree of modulation cannot be obtained unless the superimposed high-frequency current is increased. Therefore, data D0 is given to the D/A converter l4, and the gain of the variable gain amplifier l3 is maximized. ηref, (
When 77 $ < 77 ref, D/A converter 14
is given data D, and the gain of the variable gain amplifier l3 is set to an intermediate level. η. 〉ηref, the D/A converter 14
Data D. is given, and the gain of variable gain amplifier l3 is minimized. In this way, the differential quantum efficiency η. Depending on the value of , the amount of superimposition of the high frequency current INF becomes variable, resulting in an appropriate degree of modulation (for example, 140-180%).

By measuring the differential quantum efficiency in this way and performing a control operation to vary the amount of superimposition of high-frequency current when the power is turned on, or at fixed time intervals or during idle time,
An appropriate degree of modulation can be obtained. Furthermore, a temperature sensor may be connected to the controller to control the high frequency current that is superimposed when a predetermined temperature change is detected.

Furthermore, although the above explanation is an example of a semiconductor laser drive circuit that outputs only reproduction light emission power, it is also possible to use a drive circuit that has the function of outputting recording light emission power (or erasing light emission power) as well as reproduction light emission power. Can be applied. For example, reference voltages Vref, , Vre are included in the reproduction power setting circuit shown in FIG.
The differential quantum efficiency can be similarly detected by generating f, and detecting the DC current value by connecting an A/D converter to the first drive circuit. In this case, the first switching circuit in Figure 1 of the publication is connected between f and h, the second switching circuit is connected between b and c, and the third switching circuit is connected between d and h.
Leave space between e open.

Furthermore, in FIG. 1, the output voltage of the amplifier 3 is A/D converted by a second A/D converter (not shown), and a direct current I is generated. C. Luminous power P1' and DC current I when measuring. c. is inputted into the controller 10, and the differential quantum efficiency η. The calculation of 77-=I P-'P,'I/II
oc. It is also possible to obtain a more accurate degree of modulation by calculating a more accurate differential quantum efficiency using Ioc1.

In addition, in this example, the degree of modulation is changed by varying the amount of superimposition of high-frequency current by determining the differential quantum efficiency η, but the obtained differential quantum efficiency η is used to stably output recording or erasing power. It can also be used to detect deterioration of the semiconductor laser l.

Effects of the Invention The present invention is configured as described above by focusing on the differential quantum efficiency in the current-optical characteristics of a semiconductor laser, and calculates the differential quantum efficiency from the DC current detected when driving the semiconductor laser at each emission power, The gain of the variable gain amplifier is varied according to the obtained differential quantum efficiency to control the amount of high-frequency current superimposed on the DC current, so even if there are changes in the environmental temperature, etc., it will always be appropriate. A semiconductor laser light source with a high degree of modulation and low noise can be used, and reproduction operations with low errors can be performed.

[Brief explanation of drawings]

1 to 3 show an embodiment of the present invention,
Fig. 1 is a block circuit diagram, Fig. 2 is a flowchart, Fig. 3 is a current-light characteristic diagram showing calculation of differential quantum efficiency, and Fig. 4 is a block circuit diagram.
The figure is a current-light characteristic diagram showing a conventional example. ■... Semiconductor laser, 5... DC drive circuit, 9...
・Detection means, 10... Light emission power control means, l2...
・Oscillation circuit, l3...variable gain amplifier Applicant Lico Co., Ltd. 1 3 It 3 LI-Ba

Claims (1)

[Claims] In a semiconductor laser noise reduction circuit using a high frequency current superimposition method in an optical information processing device that performs reproduction, recording, or erasing of information by condensing and irradiating a laser beam emitted from a semiconductor laser onto an optical information recording medium using an objective lens, a direct current drive circuit for passing a direct current through the semiconductor laser; an oscillation circuit for passing a high frequency current through the semiconductor laser; a variable gain amplifier connected to the oscillation circuit and the semiconductor laser; A light emitting power control means for emitting light with a power of A noise reduction circuit for a semiconductor laser, characterized in that the gain of the variable gain amplifier is varied according to the differential quantum efficiency obtained to control the amount of high frequency current superimposed on the direct current.