JPS6142182A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To directly control peak power and bias power at the time of recording by a method wherein the peak current and the bias current are controlled by sampling the output of a photo detector on the basis of code values changing into the peak power and the bias power in modulation signals. CONSTITUTION:By means of the reproduction mode, a terminal alpha led out of a semiconductor laser 1 is connected to a terminal beta on the basis of a mode signal (a), and a transistor 15 is controlled by comparing the monitor current from the photo detector 2 with the voltage value from a reproducing reference voltage source 13, thus controlling the photo output of the laser 1 to a constant reproduction power. By means of the recording mode, the terminal alpha is connected to a modulation circuit 20, which circuit is changed over on the basis of a modulation signal (b). When the signal (b) is of the bias power, a transistor 24 is controlled by comparing a monitor current Im with a bias reference voltage source 23, thus controlling the photo output of the laser 1 to the bias power. When the signal (b) is of the peak power, this output is controlled to the peak power by comparison with a reference voltage source 27.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a semiconductor laser drive circuit that modulates the optical output of a semiconductor laser, and particularly to a semiconductor laser drive circuit that performs power servo control during modulation with a digital signal.

2. Description of the Related Art Structures of Conventional Examples and Their Problems In recent years, semiconductor lasers have come to be widely used in fields such as optical communication and optical recording.

In the field of optical recording, optical disks are attracting attention as large-capacity memories because they enable high-density recording. A semiconductor laser drive circuit used in an optical disk device will be explained below.

FIG. 1 shows changes in optical output due to drive current of a semiconductor laser when recording and reproducing on an optical disk. To record a signal on an optical disc, a laser beam is irradiated onto the recording layer of the optical disc to make holes in the recording layer or change the reflectance of the recording layer to form recording bits. A signal is recorded depending on the presence or absence of this recording bit.

Hereinafter, the optical output of the semiconductor laser when forming recording bits will be referred to as peak power, and will be represented by Pp in FIG.

When recording a digital signal, a recording bit is made to correspond to "1" of a modulation signal, for example. When "o" is selected, laser light is irradiated to an extent that does not cause any change in the recording layer.

The optical output at this time is called bias power, and P
Represented by b. By applying bias power, the recording layer is preheated and recording characteristics can be improved. To reproduce signals recorded from an optical disc, weak laser light is irradiated and changes in the amount of reflected light depending on the presence or absence of recorded bits are detected. The optical output of the semiconductor laser at this time is called the reproduction power and is represented by Po in FIG.

Further, as the value of each power, for example, Po-1 mW.

Pb-2mW1Pp-8mW.

FIG. 2 shows an example of changes in the optical output of a semiconductor laser. In FIG. 2, the semiconductor laser initially emits light at the reproducing power P0 in the reproducing mode. The recording mode is then entered, and recording is performed by emitting light with peak power Pp or bias power Pb in accordance with the modulation signal shown at the top of the figure. After that, it is in playback mode again.

FIG. 3 shows the configuration of a conventional semiconductor laser drive circuit. In FIG. 3, 1 is a semiconductor laser.

2 is a photodetector, which receives the light toward the rear of the semiconductor laser 1 or a part of the laser light passing through the optical system;
The optical output of the semiconductor laser 1 is monitored. Reference numeral 3 denotes a current source that determines the reproduction power, and supplies a reproduction current I0. 4 is a power servo circuit that controls the magnitude of the reproduction current 10 according to the output from the photodetector 2. Reference numeral 6 denotes a current source that determines the bias power during recording, and supplies a bias current 11.

8 is a current source that determines the peak power during recording, and supplies a peak current I2. Reference numeral 5 denotes a mode switching circuit, which switches between recording and reproduction mode in response to a mode signal a. 7 is a switching circuit, which is turned on and off by a modulation signal.

First, the operation in playback mode will be explained.

A mode signal a is inputted from the input terminal 1o, and the mode switching circuit 6 is opened. At this time, the semiconductor laser 1 has a reproduction current I0
flows and emits light at the reproduction power P0.

Next, in the recording mode, the mode switching circuit 5 is closed by the mode signal a. A modulation signal S is inputted from the input terminal 9, and when the modulation signal is "1'", the switching circuit 7 is closed, and when it is "O", it is opened.

When the modulation signal is "P0", the sum of the reproducing current and the bias current: I.-1-71 flows through the semiconductor laser 1, and the semiconductor laser 1 emits light with the bias power Pb.Also, when the modulation signal is "1", Furthermore, peak current I2 is added, total work.+11
A current of +I2 flows, and the semiconductor laser 1 emits light with a peak power Pp.

Finally, the power servo circuit will be explained. Generally, the optical output of a semiconductor laser changes depending on temperature changes. This is shown in FIG. When the temperature increases from t to t', the optical output decreases (P-+P') even if the same current I flows. In order to keep the optical output constant, the current flowing through the semiconductor laser is
must be increased from I' to I'. A power servo circuit performs this control. The configuration of the power servo circuit is shown in FIG. The optical output of the semiconductor laser 1 is received by a photodetector 2, and changes in the amount of received light are monitored by a current flowing through the photodetector 2. It can be expressed as a change in For example, a PIN photodiode or the like can be used as a photodetector.If the resistor 12 is Rm and the reference voltage source 13 is vR, the comparison voltage vfn==VR-minR1n due to a change in the monitor current - Output. The switching circuit 14 switches the connection of the base of the transistor 160 based on the mode signal a. In play mode, base terminal α
is connected to the terminal β from the operational amplifier 11. As a result, the comparison voltage V is applied to the base of the transistor 16, and the collector current flowing through the transistor 16, that is, the driving current of the semiconductor laser is controlled.

In this servo circuit, when the optical output of the semiconductor laser 1 decreases, the current of the photodetector 2 decreases, the comparison voltage vm increases, the drive current flowing through the transistor 15 increases, and the optical output of the semiconductor laser 1 increases. . In addition, the semiconductor laser 1
When the optical output increases, control in the opposite direction to the above is applied. As described above, the power servo circuit performs control so that the reproduction power is always kept constant.

Further, the sample switch 16 is controlled by the mode signal a, and is closed in the playback mode and opened in the record mode. In the playback mode, the comparison voltage v! from the operational amplifier 11 is output! n to the hold capacitor 17.

In the recording mode, the sample switch 16 is turned on, and the switching circuit 14 connects the terminal α to the terminal β. The comparison voltage held by the hold capacitor 17 is vm, which is applied to the operational amplifier 18. passes through the switching circuit 14 and is added to the base of the transistor 15;
The drive current of the semiconductor laser 1 is maintained. Note that the operational amplifier 18 performs impedance conversion in order to hold the comparison voltage vm with the hold capacitor 17.

As described above, in the recording mode, the drive current (corresponding to the reproduction current I0) in the reproduction mode is held, and the bias current 11. The configuration is such that the peak current I2 is superimposed. As described above, since servo control is not performed in the recording mode, the optical output changes due to temperature changes during recording, making it impossible to perform good recording. Therefore, in the recording mode, it is necessary to shorten the continuous recording time, switch to the reproduction mode at regular intervals, and perform power control.

For example, since a modulation signal used when recording a digital signal generally has many low-frequency components, it is necessary to control the peak power and bias power separately.

OBJECTS OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks, and it is an object of the present invention to provide a semiconductor drive circuit that can perform power control on peak power and bias power during recording.

Structure of the Invention The present invention comprises a peak sample and hold circuit that samples the output of a photodetector using a code value that is the peak power in a modulated signal, and a bias sample and hold circuit that samples the output of the photodetector using a code value that is the bias power. This is a semiconductor laser drive circuit equipped with a circuit that controls the peak current using the hold value of the peak sample and hold circuit, and controls the bias current using the hold value of the bias sample and hold circuit. Power and peak power can be directly controlled.

DESCRIPTION OF EMBODIMENTS First, the basic configuration of the present invention will be explained with reference to the drawings. FIG. 6 shows the basic configuration of a semiconductor laser drive circuit in the first embodiment of the present invention. In FIG. 6, 1 is a semiconductor laser, 2 is a photodetector, and 11 is an operational amplifier.

12 is a resistor, 13 is a reproduction reference voltage source, and 16 is a transistor, which is the same as the configuration shown in FIG. 5. 1
Reference numeral 9 denotes a mode switching circuit which switches the connection of the semiconductor laser 1 according to the mode signal a. 2° is a modulation circuit that switches the connection of the semiconductor laser 1 according to a modulation signal. 21 is an operational amplifier, 22 is a resistor, 23 is a bias reference voltage source, 24
are transistors, and these control the magnitude of the bias current. 26 is an operational amplifier, 26 is a resistor, 27 is a peak reference voltage source, and 28 is a transistor, which controls the magnitude of the peak current.

First, the operation in playback mode will be explained. The operation in the regeneration mode is similar to that of the power servo circuit shown in FIG. 6 in the regeneration mode. In the reproduction mode, mode signal a causes the mode switching circuit 19 to switch the terminal α from the semiconductor laser 1.
Connect to terminal β.

In the reproduction power comparison circuit A, an operational amplifier 11 and a resistor 12
The magnitude of the monitor current Irn from the photodetector 2 is compared with the value v0 of the reproduction reference voltage source 13, and the comparison voltage ■
. . occurs. Add this to the base of transistor 16. The transistor 15 is a reproduction current source, and controls the optical output of the semiconductor laser 1 to a constant reproduction power.

Next, the operation in recording mode will be explained. In the recording mode, the mode switching circuit 19 connects the terminal α from the semiconductor laser 1 to the terminal γ to the modulation circuit 20 in response to the mode signal a. The modulation circuit 2o connects the terminal δ to the terminal η to the collector of the transistor 24 of the bias current source in response to the modulation signal.
Alternatively, it is connected to the terminal ε to the collector of the transistor 28 of the peak current source, and the optical output of the semiconductor laser 1 is modulated to correspond to the peak power. In bias comparison circuit B, monitor current circuit. and bias reference voltage source 23 (vb
) as operational amplifier 21. The comparison voltage vfn is compared with the resistor 22.
b is applied to the transistor 24, and the optical output of the semiconductor laser 1 is controlled to be bias power. Similarly, in the peak comparison circuit C, the monitor current - and the peak reference voltage source 27 (vp) are connected to the operational amplifier 26. The comparison voltage V is compared with the resistor 26. adding p to transistor 28;
The optical output of the semiconductor laser 1 is controlled to reach the peak power.

As described above, in this embodiment, the reproduced power 2 bias power and peak power are individually detected by the monitor current of the photodetector 2, and compared with the respective reference voltage sources, the monitor current is set to a predetermined value. By controlling the magnitude so that it has a magnitude of , it is possible to control the optical output of the semiconductor laser 1 to have a predetermined power. In particular, since the peak power and bias power are determined by directly monitoring the optical output of the semiconductor laser according to the modulation signal, power servo is possible for long-term continuous recording.

In addition, in this embodiment, fine control is possible because power servo is performed for all cases of each code "0" or "1'" of the modulation signal, but on the other hand, when the data rate of the modulation signal becomes high, , when the same sign continues for a short period of time, the operation of the power servo circuit cannot keep up.

Next, an embodiment that can be used even when the data rate of the modulated signal is high will be described. FIG. 7 shows the configuration of a semiconductor laser drive circuit in a second embodiment of the present invention. In Fig. 7, 1 is a semiconductor laser, 2 is a photodetector, 11, 21°26 is an operational amplifier, 12, 22.2
6 is resistance.

13, 23, and 27 are for playback and bias, respectively.

The peak reference voltage sources 15, 24, and 28 are driving transistors, 19 is a mode switching circuit, and 2o is a modulation circuit, which is the same as the configuration shown in FIG. 6. A bias sample switch 29 samples the comparative voltage JE vm b using a bias sample signal C. 3Q is a bias hold capacitor, and 35 is an operational amplifier, which holds the sampled comparison voltage ■mb. Reference numeral 32 denotes a peak sample switch which samples the comparison voltage V using the peak sample signal d.

33 is a peak hold capacitor, and 34 is an operational amplifier, which holds the sampled comparison voltage vm.

Since the operation in the reproduction mode is the same as that in the first embodiment shown in FIG. 6, the explanation will be omitted, and the operation in the recording mode will be explained. Mote voltage in recording mode vmb, vmp
The operation of generating is also the same as in the first embodiment. In the first embodiment, the comparison voltage is directly connected to the current source transistor 24.
．． However, in the second embodiment, the current source is driven by a sampled and held value of this comparison voltage. In FIG. 7, the sample switch 29. The bias hold capacitor 0° pair amplifier 31 constitutes a sample and hold circuit, and samples and holds the comparison voltage vmb using the bias sample signal C. This hold voltage is applied to the transistor 24
In addition, the optical output of the semiconductor laser 1 is controlled so as to have a bias power.

The peak sample switch 32, peak hold capacitor 33, and operational amplifier 34 also constitute a sample and hold circuit, and the comparison voltage v is determined by the peak sample signal d.
Sample and hold mp.

Applying this hold voltage to the base of transistor 28,
The optical output of the semiconductor laser 1 is controlled to reach the peak power.

Next, the bias sample signal C and the peak sample signal d will be explained. These sample signals are signals obtained by detecting a code pattern in which the same code continues for a predetermined period or longer necessary to charge the whole-root capacitor of the sample-and-hold circuit from the modulated signal. FIG. 8 shows an example of the configuration of the code pattern detection circuit. In FIG. 8, 35 is a shift register, 36 is an AND gate,
37 is Noah Gate. FIG. 8 shows a case where three bits of the modulation signal are used as the period required for sample and hold. The modulated signals are sequentially input to the shift register 35. A shift register 36 shifts the modulation signal one bit at a time.

The AND gate 36 outputs all 1” from the shift register.
When , it outputs 1°゛. In other words, when the modulation signal has three or more consecutive "1" bits, the peak sample signal d
becomes “1”. Further, the NOR gate 37 outputs °°1'' when all outputs of the shift register are °0''.

That is, when three or more bits of the modulation signal are consecutively "0", the bias sample signal C becomes "1".

As described above, in this embodiment, a pattern in which the same code continues for a predetermined period is detected from the modulation signal, the comparison voltage of each power comparison circuit is sampled during this period, and is held during other periods. By adopting a configuration that controls the driving current of the semiconductor laser, stable control can be performed even when the data rate of the modulation signal is high.

In addition, in the two embodiments, the semiconductor laser has a reproduction power,
In order to emit light with bias power and peak power, a configuration was adopted in which a single current source was used for each, and these were switched using a mode signal and a modulation signal, but as shown in the conventional example, the current of the three current sources It is also possible to have a configuration in which optical outputs of each power are obtained by adding .

Further, although the bias power -Pb is made to correspond to 0'' of the modulation signal, if the preheating effect of the bias power is not utilized, Pb may be set to o.

Effects of the Invention As described above, the present invention includes a photodetector that detects the optical output of a semiconductor laser, and compares the output of the photodetector with a reference value at a code value that corresponds to the peak power in a modulated signal to determine a predetermined value. During recording, the bias power and The peak power can be directly controlled, and therefore power can be controlled even during long-term continuous recording, which has great practical effects.

[Brief explanation of the drawing]

Fig. 1 is a characteristic diagram showing the optical output of the semiconductor laser, Fig. 2 is a schematic diagram showing the optical output of the semiconductor laser during recording and reproduction, and Fig. 3 is a block diagram showing the configuration of a conventional semiconductor laser drive circuit. , FIG. 4 is a temperature characteristic diagram of the optical output of the semiconductor laser, FIG. 6 is a circuit diagram showing the configuration of the power servo circuit, and FIG. 6 is a circuit diagram showing the semiconductor laser drive circuit in the first embodiment of the present invention. , FIG. 7 is a circuit diagram showing a semiconductor laser drive circuit in a second embodiment of the invention, and FIG. 8 is a circuit diagram showing a code pattern detection circuit in the second embodiment of the invention. 1...Semiconductor laser, 2...Photodetector, 16°24.28...Current source, A, B, C.
... Comparison circuit, D, E ... Sample hold circuit 0 Name of agent Patent attorney Toshi Nakao
1 other person Figure 1 Light output Figure 2 Figure 5 Figure 6 Figure 8? Ku-41'

Claims (3)

[Claims] (1) A semiconductor laser is configured to modulate its optical output according to a modulation signal with two values: a peak power with a large optical output and a bias power with a small optical output, and has a photodetector that detects the optical output of the semiconductor laser. a peak comparison circuit that compares the output of the photodetector with a peak reference value during the period of the code value that is the peak power in the modulation signal; a peak current source that controls the drive current of the semiconductor laser using this output; a bias comparison circuit that compares the output of the photodetector with a bias reference value during the period of the code value that is the bias power in the signal;
1. A semiconductor laser drive circuit comprising a bias current source that controls the drive current of a semiconductor laser using this output, and maintains peak power and bias power at predetermined levels. (2) A peak sample and hold circuit that samples the output of the peak comparison circuit during the period of the code value that is the peak power in the modulation signal, a peak current source that controls the drive current of the semiconductor laser by the hold output of this circuit, and a modulation It is characterized by comprising a bias sample and hold circuit that samples the output of the bias comparator circuit during the period of the code value that becomes the bias power in the signal, and a bias current source that controls the drive current of the semiconductor laser by the hold output of this circuit. A semiconductor laser drive circuit according to claim 1. (3) It has a code pattern detection circuit that detects a code pattern in which the same code continues for a predetermined period in the modulated signal, and performs sampling with a peak sample hold circuit using a peak sample signal that detects the successive codes that result in peak power; 3. The semiconductor laser drive circuit according to claim 2, wherein sampling is performed by a bias sample hold circuit using a bias sample signal that detects a succession of codes serving as bias power.