JPH0318079A - Semiconductor laser driver - Google Patents

Abstract

PURPOSE:To enable stable driving high in reliability with simple and low-cost constitution by connecting the high frequency signal source in a frequency band, which includes positive feedback signals being the modulated signals corresponding to current, with feedback means, for feedback means which positively feedback signals to a semiconductor laser through the injected current of the semiconductor laser. CONSTITUTION:For feedback means 10-14 which positively feedback signals through the injected current of a semiconductor laser 1, a high frequency signal source 15 in the frequency band, which includes positive feedback signals being the modulated signal corresponding to the modulation degree of current, is connected to feedback means 10-14. Accordingly, when changing the modulation degree of injected current between the time of erasion and the time of regeneration of external information, for example, record, the high frequency signal in the frequency band, which includes a positive feedback signal, is superimposed on a bias current, and it can be stabilized into a desired light output value, and noise reduction effect can be maintained. Hereby, stable driving high in reliability can be done with simple and low-cost constitution.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention detects the optical output of a semiconductor laser as a light source,
The present invention relates to a semiconductor laser driving device that extracts a signal in a desired frequency band from the detection signal and feeds it back to the injection current.

[Prior Art] Conventionally, in an information reproducing device that optically reproduces information from a video disk or the like, when a semiconductor laser is used as a light source, the return light from the optical disk is converted into the output light of the semiconductor laser. The noise induced by this noise was present, causing problems in information reproduction. Further, in technical fields such as optical communications, reflected light from the end of an optical component, such as an optical fiber, returns to the semiconductor laser, and the returned light induces noise, which is a problem.

As a countermeasure against such noise generation, as disclosed in Japanese Patent Publication No. 59-9086, a constant high-frequency current has been applied to the DC current bias injected so that the semiconductor laser oscillates in multiple longitudinal modes. It has been proposed to reduce the noise by overlapping the signals. However, in addition to the normal drive device, a separate high-frequency current source is required, and the output from this current source is large enough to obtain a large modulation degree that deeply cuts the bias current threshold. must be set. Therefore, the scale of the device becomes large and the cost also increases. Furthermore, since the output of the high-frequency current source is large, there is a problem in that noise resulting from unnecessary electromagnetic radiation is large.

Therefore, JP-A-59-129948 and JP-A-62-35
As disclosed in Japanese Patent No. 34, the optical output from a semiconductor laser is detected, and the high frequency component of the detection signal, preferably the resonance frequency component of the relaxation oscillation of the semiconductor laser, is applied to the injection current of the semiconductor laser. A positive feedback method has been proposed.

According to this method, the scale of the device can be reduced, and the power required for noise reduction can be reduced.

[Problems to be Solved by the Invention] However, in this method, "fluctuations" in the optical output of the semiconductor laser, that is, noise, are detected and this is positively fed back to the bias current. The strength controls the initiation of high-frequency oscillation due to positive feedback, and stable high-frequency oscillation is not always obtained. In addition, the characteristics of the semiconductor laser, such as the resonance frequency of relaxation oscillation, the laser capacity, and the load, are determined by the injection current, temperature,
Since it changes depending on the amount of light returned to the semiconductor laser, the delay time of the returned light, etc., the stability of high-frequency oscillation is poor, and oscillation and non-oscillation occur unstablely.

Furthermore, in this system as disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 59-129948,
As disclosed in Japanese Patent No. 9-9086, the noise reduction effect cannot be sufficiently exerted unless the modulation degree is large enough to cut the threshold of the semiconductor laser. Moreover, the optical output of a semiconductor laser is a pulse emission depending on the frequency of high-frequency oscillation, so even if the same bias current is injected, the average optical output of the semiconductor laser will be different between high-frequency oscillation and non-oscillation. It will make a big difference. That is, there is a problem in that the optical output of the semiconductor laser changes due to repeated high-frequency oscillation/non-oscillation. In addition,
As mentioned above, it is an unstable element even during high-frequency oscillation, and the possibility of optical output remains.

Furthermore, in the method described in Japanese Patent Application Laid-Open No. 62-3534, an example is shown in which the output of a semiconductor laser is modulated by information. Since the modulation degree of high frequency oscillation changes, considering the above problem,
The noise fluctuations and the optical output fluctuations are actually changed by the modulation using the above-mentioned information, and the S/N ratio becomes poor.

[Object of the Invention] The present invention has been made based on the above circumstances, and is capable of sufficiently reducing noise of a semiconductor laser when optically recording, reproducing, erasing, or transmitting information, and moreover, provides a simple and convenient method. It is an object of the present invention to provide a stable, highly reliable, and low-cost semiconductor laser driving device that can accomplish this by means of the present invention.

[Means for Solving the Problems] Therefore, in the present invention, the optical output of a semiconductor laser as a light source is detected, and a signal in at least a part of the frequency band of the detection signal is positively transmitted through an injected current to the semiconductor laser. In a semiconductor laser driving device for feedback, a high frequency signal source in a frequency band including the positive feedback signal with a modulation signal corresponding to the degree of modulation of the current is provided to the feedback means for positive feedback of the signal through the injection current of the semiconductor laser. is configured to be connected to the feedback means.

[Function 1] In such a configuration, when the modulation degree of the injected current is changed between external information, for example, recording/erasing and reproducing, a high frequency signal in a frequency band including the positive feedback signal is generated during reproducing. It is superimposed on the bias current, and can be stabilized at a desired optical output value, and can maintain a sufficient noise reduction effect. Furthermore, since the high frequency band is limited, the device can be realized on a small scale and at low cost.

[Example] Hereinafter, an example of the present invention will be specifically described with reference to FIGS. 1 and 2. In the figure, reference numeral 1 denotes a semiconductor laser as a light source, and a monitoring photodiode 2 is disposed behind it to monitor the rear output of the semiconductor laser 1. The processing circuit 4 has an input terminal 3
is connected to which a modulation command signal modulated at a level corresponding to the information is input. The output signal from the processing circuit 4 is sent to the amplifier 5, the comparator (■) 9, and the comparator (n).
14 and a high frequency signal source (e.g. high frequency voltage source) 15
is output to. The output of the amplifier 5 is applied to the semiconductor laser 1 via the voltage/current converter 6 (further via an inductance circuit (not shown)), and the output of the high frequency voltage source 15 is input to the amplifier 1O. 10
The output of is applied to the semiconductor laser 1 via the voltage/current converter 11 (further via a coupling capacitor (not shown)). The output current of the monitoring photodiode 2 passes through a low-pass filter 7 to block high frequency components, and then is supplied to a comparator (I) 9 via a current/voltage converter 8, where it is compared with the signal from the processing circuit 4. be done. The resulting output signal is given to the amplifier 5, and is passed through the monitor photodiode 2, comparator (I) 9, amplifier 5, etc. to the laser 1 through a negative feedback system.
Achieve auto power control.

On the other hand, the output current of the monitor photodiode 2 passes through a high-pass filter l2 to cut off low frequency components, and then
It is supplied to a comparator (II) 14 via a current/voltage converter 8 and compared with the signal from the processing circuit 4. The resulting output signal is given to the processing circuit 4, and is also sent from the monitor photodiode 2 given to the amplifier 10 to the comparator (I
I) High frequency oscillation is achieved by the positive feedback system passing through the amplifier 10 and the laser l, but at this time, depending on the command signal from the processing circuit 4, a predetermined high frequency signal is output from the high frequency voltage source l5. Anbu 10
superimposed on the signal.

In such a configuration, a signal (digital or analog signal) modulated according to information is input to the processing circuit 4 from the input terminal 3. Next, a case will be described in which the present invention is applied to recording and reproducing information. During reproduction, a signal for bringing the average optical output of the semiconductor laser I to the level of the optical powers P and I for reproduction is sent to the comparator (I) 9, the amplifier 5, and the comparator (II) 14 (comparator for high-frequency oscillation positive feedback). ) from the processing circuit 4. Specifically, a voltage is applied to the mode that is adjusted in advance so that the laser power is PR at room temperature, and the voltage output from the amplifier (■) 5 is converted into a corresponding DC current by the voltage/current converter 6. Inject into the semiconductor laser l. Further, from the high frequency voltage source 9, a high frequency voltage having an amplitude at an allowable noise level is inputted to the amplifier (II) 10, which is injected as a high frequency current into the semiconductor laser 1 via the voltage/current converter 1l, and then superimposed on the direct current component of When the output PR of the semiconductor laser 1 fluctuates at a frequency lower than the high frequency oscillation frequency due to the influence of ambient temperature, etc., the output current of the photodiode 2 also fluctuates in proportion to this, so only the low frequency component is low-passed. It is extracted by a filter 7 and input as a voltage by a current/voltage converter 8 (and amplified if necessary) to a comparator (I) 9.

The comparator 9 compares the reference voltage input from the processing circuit 4 with the output voltage from the photodiode 2, and calculates Send the inverted error signal to Amb5. In other words, for the semiconductor laser l, there are a photodiode 2, a low-pass finoretor 7, a current/voltage converter 8, and a comparison ffl(1
) 9, Negative feedback is applied in the low frequency range by the amplifier 5 and the voltage/current converter 6, and the low frequency range of the optical output is stabilized.
Auto power control is achieved. At this time, as described above, for the semiconductor laser 1, the photodiode 2, the high-pass finoretor l2, the current/voltage converter l3, the comparator (I1) l4, the amplifier IO, and the voltage/voltage converter l2,
Positive feedback in the high frequency range is applied by the RS flow converter l1, and oscillation can be made at a high frequency sufficiently higher than the information signal frequency, basically using the same principle as the conventional technology. At this time, the reference voltage from the processing circuit 4 applied to the comparator (II) 14 is set so as to be fed back positively to the bias applied to the semiconductor laser 1. In FIA terms, the allowable noise of the semiconductor laser l required for reproduction (for example, relative noise intensity I
When there is noise exceeding 0-"/+12), positive feedback is always applied, high-frequency oscillation with a large amplitude that is below the allowable noise occurs, and a high-frequency current with a large modulation factor is injected into the semiconductor laser 1. Specifically, to ensure that oscillation is kept below the allowable noise level, the allowable noise level is kept low (frequency band limited by low-pass and high-pass filters) and each element of the positive feedback circuit is made high. It is necessary to improve the performance (by taking a margin and making the frequency characteristics of noise level, gain, and phase fairly good).

Therefore, in the embodiment of the present invention, by providing a high frequency band signal source (voltage #t) 15 and inputting its output to the amplifier 10, it is possible to eliminate the need for higher performance of the positive feedback circuit and reduce costs. I made it. In other words, comparator (I1)
As an output of 14, there is a comparison output between the allowable noise and the detected noise, which is input to the processing circuit 4, and when this is small, that is, when the noise level is close to the allowable noise, the signal source is output from the processing circuit 4. 15 to output a signal in a high frequency band. By inputting this high frequency signal to the amplifier IO, stable positive feedback is provided. The output waveform of the signal source 15 can be a waveform including various frequencies and phases, but a pulse or a step-like waveform with a sharp rise (fall) is preferable because the circuit is simple. By adding such a simple circuit,
Once oscillation begins, a normal positive feedback circuit continues the oscillation and reduces laser noise.

FIG. 2 is a schematic diagram showing the current-optical output characteristics of the semiconductor laser 1, where the horizontal axis represents the DC current injected from the amplifier 5;
The vertical axis is the average optical output obtained by averaging the high frequency oscillation band.

Here, 16 is the characteristic when high frequency oscillation is not occurring, and l7 is the characteristic when high frequency oscillation is occurring, and indeed,
This is the characteristic when l8 is high-frequency oscillating, but the amplitude of the high-frequency oscillation is small due to any of the reasons mentioned above, and the characteristics 19 and 20 are when the average optical output is the reproduction level PR.
．． It corresponds to 21. That is, amplifier 5 and lO
This is an example of the current waveform that is finally injected into the semiconductor laser. As is clear from the figure, when the high frequency oscillation is unstable, the pulsed light emission is unstable and the average optical output fluctuates, and fluctuates between characteristics 16 and 17. For example, in the worst case, the output P, l is obtained with the current waveform l7, but if the bias current is constant and the high-frequency oscillation stops as shown in the current waveform 21, which is different from the bias current value, the laser oscillation will stop from the characteristic l6. It will be understood that this will not be done. However, according to the present invention, such inconveniences are unlikely to occur because, as described above, fluctuations in the average output at frequencies lower than the high frequency oscillation frequency are removed by the above-mentioned automatic power control. In other words, in Fig. 2, when there is a fluctuation in high frequency oscillation and a change in the average optical output, the bias current value is controlled by auto power control, and the control is performed as shown in waveforms 19, 20, and 21, for example. Since it operates to maintain the average optical output at the reproduction power PR, stable and low-noise optical output can be obtained in the low frequency range.

Further, according to the present invention, when a high frequency signal is appropriately applied as described above from the high frequency signal source 15, more stable high frequency oscillation can be obtained, and the amount of returned light, the returned light delay time (the phase of the returned light), etc. This reduces the fluctuations shown in FIG. 2.

Next, during recording, a modulation signal corresponding to the information to be recorded is applied from the input terminal 3 to the processing circuit 4, where waveform shaping is performed and a signal for modulating the semiconductor laser 1 is applied to the amplifier 5.

For example, in the case of binary recording, the optical power for reproduction is usually P7
and high output half power P. If we modulate the corresponding binary values, then the reference voltage applied to the comparator Nl)9 is also
Its PR, P. is modulated into binary values corresponding to . As a result, auto power control
The respective optical outputs PR and Pll are stabilized and good optical modulation can be performed.

The processing circuit 4 also includes a comparator (I) for positive feedback of high frequency oscillation.
I) Group 1 of 14! It is applied as a voltage to turn positive feedback on and off in response to modulation of the laser's optical output. This is done by cutting off the positive feedback circuit so that high frequency oscillation does not occur when the optical output is large Pw, and by changing the polarity of the reference voltage to create negative feedback. When it is l, the same voltage as during reproduction is applied, enabling high frequency oscillation and reducing noise. Similarly, the control signal given from the processing circuit 4 to the signal source 15 is such that when the optical output is Pw, pulses and step waveforms are not output to prevent high frequency oscillation, and the optical output PII is During playback, the system is controlled to generate high-frequency oscillation in the same way as during playback, thereby reducing noise.

In this way, PR, P. At any optical output level, or even if a factor that fluctuates the high frequency oscillation occurs, the power control and the high frequency band signal source l5
As a result, the desired low-noise Pll and Pw are output from the semiconductor laser 1, and stable recording operation is performed.

In the above description, the reason why positive feedback is stopped and high frequency oscillation is not performed when the optical output is Pw is as follows.

That is, as shown in FIG. 2, when the bias current becomes high, even if the current amplitude of high-frequency oscillation becomes large as shown in characteristic 17, the peak on the valley side of the corresponding current waveform becomes
This is because the threshold value is no longer exceeded, and characteristic 17 (in FIG. 2) and characteristic 16 match at the higher bias current. In other words, at high output, the degree of modulation of high frequencies decreases, and no noise reduction effect can be expected. Not only that, the peak on the mountain side of the current waveform may exceed the maximum rating of the laser, potentially shortening the laser's lifespan, so high-frequency modulation is interrupted on the high output side.

In addition, in the above embodiment, the output terminal of the signal source 15 is
Preferably, a bandpass filter that passes only high frequency bands is provided.

In addition, in the above embodiment, the noise level judgment output is sent to the comparator (
TI) was taken from 14 and led to processing circuit 4, but
The output amplified by 0 may also be used. Although the output of the signal source 15 is input to the amplifier 10, it may be input anywhere between the filter l2 and the terminal of the laser 1 as long as it is in the positive feedback circuit. In this case, the human input signal is optimized to a signal suitable for the input location. In this way, with the above-described configuration of the present invention, the noise of the semiconductor laser 1 can be sufficiently reduced, and moreover, stable and highly reliable laser driving can be performed regardless of the presence or absence of modulation of the output of the semiconductor laser 1. Become.

Next, another embodiment of the present invention will be described with reference to FIG. Here, positive feedback for high frequency oscillation and negative feedback for stabilizing optical output are configured in one feedback circuit system.
The device has been simplified. That is, from the input terminal 3, a modulated signal corresponding to the information is manually input to the processing circuit 4. Here, as in the previous embodiment, the present invention will be explained by applying it to an apparatus for recording and reproducing information.

At the time of reproduction, the output of the semiconductor laser l is changed to the optical power for reproduction P.
R is kept constant and high frequency oscillation is performed.

As in the previous embodiment, the power P. A voltage corresponding to the voltage is applied to the amplifier 22.

Then, the output voltage of the amplifier 22 is converted into a current by the voltage/current converter 23, and the current is injected into the semiconductor laser l to obtain the reproduction power PR. The output current of the monitor photodiode 2 is converted into a voltage corresponding to the output light of the semiconductor laser 1 by a current/voltage converter 24 whose frequency characteristics extend to the high frequency oscillation band. Here current/voltage converter 2
4, its frequency characteristics can be switched in two stages, and the frequency characteristics of the high frequency oscillation band are controlled by a signal from the processing circuit 4. Specifically, the filter characteristics are switched using a high-speed analog switch or a digital filter. The monitor voltage thus obtained is input to the comparator 25 and compared with the reference voltage input from the processing circuit 4, as in the previous embodiment.
The inverted error signal is input to amplifier 22.

Therefore, based on the reference voltage given to the comparator 25 from the processing circuit 4, the photodiode 2, the current/voltage converter 24, the comparator 25, the amplifier 22, and the voltage/current converter 23 are connected to the semiconductor laser l. Return is applied. The frequency characteristics of the gain and phase of the feedback are determined by the frequency characteristics of the current/voltage converter 24, and is basically a negative feedback configuration using an inverting amplifier. At low frequencies, the phase is shifted by 180 degrees, normal negative feedback is applied, and auto power control is performed as in the previous embodiment, and at high frequencies, the phase is shifted by 360 degrees.
Positive feedback is required due to degree deviation. The gain in that band is set to 1 or more, and oscillation is started. During playback, feedback with frequency characteristics that enable high-frequency oscillation is selected. Specifically, the frequency characteristics of the current/voltage converter 24 are switched so that the feedback gain is 1 or more in the high frequency oscillation band and the phase rotates by 360'' (or an integral multiple thereof).

Further, as in the above embodiment, stabilization of high frequency oscillation is realized by the high frequency band signal source 26 whose output is controlled by the processing circuit 4. A comparison output between the allowable noise and the detected noise is obtained from the comparator 25, and based on the output, the processing circuit 4
controls the signal source 26. When the signal source 26 receives an output command from the processing circuit 4, it outputs a pulse or step waveform including a high frequency band. This is input to the comparator 25, and stable oscillation in the high frequency band begins due to forced positive feedback, as in the previous embodiment. As described above, according to the present invention, the number of parts can be reduced, and output stability and noise reduction can be realized.

During recording, the modulation signal applied from the input terminal 3 is waveform-shaped by the processing circuit 4, and the bias of the laser I is modulated via the amplifier 22 and voltage/current converter 23, as in the previous embodiment. Further, a reference voltage modulated in accordance with the modulation is applied to the comparator 25, thereby realizing intelligent power control in accordance with the modulation. Indeed, in this embodiment as well, high frequency oscillation is stopped when the output level is high during recording, and oscillation is performed at a low output level. To do this, the frequency characteristics of the high frequency oscillation band of the current/voltage converter 24 are switched using a signal corresponding to the modulation from the processing circuit 4. Specifically, this can be achieved by making the gain in the high frequency oscillation band of the feedback less than 1 or by shifting the phase from 360 degrees (or an integral multiple thereof), or by achieving both at the same time. be. Further, when the output is high, the output of the signal source 26 is prohibited, so that high-frequency oscillation does not occur, and when the output is low, the appropriate output from the signal source 26 is used to stably cause high-frequency oscillation, as in the case of reproduction.

In the above embodiments, the present invention has been explained with respect to an optical information recording/reproducing device, but of course, when erasing information,
It can also be applied to fields such as optical communication and optical measurement. Also,
When the optical output of the laser is not modulated and only a low output is used, for example, when only for reproduction, there is no need to switch the reference voltage of the processing circuit 4 or the comparator, or to switch the interwave number band, and the state is always high-frequency oscillation. In the case where it is fixed to , the device is further simplified.

In addition, in the above embodiment, in the low output and high output states of modulation,
High-frequency oscillation and non-oscillation are switched, but for example, in optical disk memory devices, semiconductor laser noise becomes a problem only in the information signal band during reproduction (for example, 100K to 20MHz), and in the servo band (0 to 50kHz). Ya, records,
If laser noise is not a problem during erasing (when the recording and erasing thresholds of the recording medium are clear and the so-called gamma is high), high-frequency oscillation may be performed only during reproduction.

In addition, in order to stabilize high-frequency oscillation, a part of the feedback circuit (for example, an amplifier) is saturated by increasing the amplitude of oscillation, and the actual gain (including loss) is set to 1. You can also take

Further, in this embodiment, the positive feedback circuit includes a filter, a comparator, an amplifier, a current/voltage converter, a voltage/current converter, a signal source, etc., but the configuration is not limited to this. Any configuration may be sufficient as long as positive feedback is applied in the high frequency oscillation band and negative feedback is applied in the lower frequency band. Further, the laser beam detector may be provided separately for high frequency and low frequency, and may also be used as another signal detector. Furthermore, all or part of these circuits may be integrated with a semiconductor laser.

In addition, in the above embodiment, the frequency band of smart power control using negative feedback is set close to the high frequency band, but if the frequency band of optical output fluctuation due to instability of high frequency oscillation is slightly lower, then Just match it. In addition, below the information signal band to be used or below the servo band,
It is also possible to make it below the temperature fluctuation band. It is also possible to divide these into multiple bands and perform automatic power control independently.

In addition, a photodiode inside the semiconductor laser package is used to monitor the output light of the semiconductor laser, but in order to extract the high output necessary for recording and erasing from the semiconductor laser, a low Reflectance: If a member with high reflectance is provided on the rear end face and there is return light, the similarity between the front output and the rear output may be lost, so it is preferable to provide a part of the front output separately. It is best to monitor with a photodetector.

Further, the high frequency band signal source may be one that can reliably perform this feedback, and is not limited to a pulse waveform or a step waveform.

[Effects of the Invention] As described in detail above, the present invention is capable of reducing laser noise and stabilizing high-frequency oscillation by performing positive feedback in a high-frequency band by high-frequency oscillation within an allowable noise level range. The semiconductor laser can be driven stably and reliably using a relatively simple and low-cost configuration.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing one practical example of the present invention, Fig. 2 is a graph explaining the output characteristics of the laser when driven by high frequency oscillation, and Fig. 3 is a block diagram showing another embodiment. be. ■...Semiconductor laser 2...Photodiode 9, l4...Comparator 8.13...Current/voltage converter 6, 1l...Voltage/current converter 15...Signal source

Claims (1)

[Claims] A semiconductor laser driving device that detects the optical output of a semiconductor laser as a light source, and positively returns a signal in at least a part of the frequency band of the detection signal to the optical output of the semiconductor laser through an injected current, A high frequency signal source in a frequency band including the positive feedback signal is connected to the feedback means for positive feedback of a signal to the semiconductor laser through the injection current of the semiconductor laser, with a modulation signal corresponding to the current. A semiconductor laser drive device characterized by being configured as follows.