JP2593215B2 - Semiconductor laser driving method - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for driving a semiconductor laser, and more particularly to noise reduction.

[Prior art] Conventionally, when a semiconductor laser is used as a light source in an information reproducing apparatus that optically reproduces information from an optical video disk or the like, return light from the optical disk returns to the output light of the semiconductor laser and light-induced noise is generated. It is known that there is an obstacle to the reproduction of information. In the field of optical communication, etc.,
The reflected light from the optical components such as the end of the optical fiber returns to the semiconductor laser, and the return light-induced noise is generated, which is a problem.

As a countermeasure, a method has been proposed in which a high-frequency current is superimposed on a DC current to reduce noise so that a semiconductor laser oscillates in multiple longitudinal modes, as shown in JP-B-59-9086.

In addition, in the field of optical communication, speckle pattern noise caused by a difference in propagation constant between modes at a light receiving end of a multimode optical fiber has become a problem in addition to return light induced noise. As a countermeasure, as shown in JP-B-61-24837, a method has been proposed in which the semiconductor laser electrode is divided into a plurality of parts, and a high-frequency voltage is applied to some of them to oscillate the semiconductor laser in multiple longitudinal modes. Have been.

[Problems to be Solved by the Invention] As shown in Japanese Patent Publication No. 59-9086, the effect of noise reduction is such that the high-frequency current amplitude is reduced so that the high-frequency current cuts the oscillation threshold of the semiconductor laser deeply. And when the semiconductor laser is pulsed, there is a sufficient noise reduction effect. Therefore, in the case of an apparatus that performs only reproduction, such as the above-mentioned Japanese Patent Publication No. 59-9086, there is no problem as long as the average light power is low and constant, but there is no problem. Necessary information recording / reproducing devices and optical communication devices have the following problems. In these devices, a high extinction ratio during modulation is required to obtain a good signal-to-noise ratio, and thus high optical power is required. However, in order to apply a high-frequency current that cuts the threshold value deeply to the laser even at a high output, there is a restriction that the peak light output by the high-frequency current does not exceed the maximum rating of the laser. Therefore, the level of the AC component (usually a rectangular wave) of the erasing, recording, and transmission signals modulated according to the information must be equal to or lower than the midpoint between the threshold current and the maximum rated current. There is a problem that a high optical output required for erasing and recording and an extinction ratio of a required modulation cannot be obtained.

Therefore, as shown in JP-A-62-119743 and JP-A-62-142721, at a high output level such as erasing or recording,
A method has been proposed in which superposition of a high-frequency current is stopped to ensure a high optical output. However, for this reason, return light-induced noise at a high output level such as erasing or recording is not sufficiently reduced, and there is a problem in reliability. In particular, when the recording medium is a medium in which the change in light energy or power is slow and the threshold is not clear, such as a dye medium, the quality of bits erased or recorded due to return light-induced noise is poor. become. Also, when the medium is in the form of an optical card or optical tape and the relative speed between the optical head and the medium is low, the servo band for auto focus and auto tracking and the information signal band are very close to or overlap with each other. At the time of modulation, the crosstalk of the information signal to the servo signal is large. Therefore,
The noise is not only the information signal but also the servo signal. In the case of an information signal, particularly in the case of digital modulation, by applying an error correction code or the like, it is possible to cope with the influence of noise to some extent at the expense of information capacity and transfer speed.
Servo such as auto focus and auto tracking cannot deal with noise, and these errors are serious errors for the system.

Also, in the method disclosed in the above-mentioned Japanese Patent Publication No. 61-24837 in optical communication, simply dividing the electrodes of the semiconductor laser does not prevent the average optical output in the information signal band from being reduced due to the high-frequency current, and reduces the extinction ratio. A decrease, a decrease in signal-to-noise ratio, and an increase in error cannot be avoided, and application to an optical information recording / reproducing apparatus is difficult.

[Means for Solving the Problems] In view of the problems of the conventional example described above, the present invention can solve the problem of a decrease in light output, perform good modulation such as erasing and recording of information, and have sufficient noise. It is an object of the present invention to provide a stable, reliable, and inexpensive laser driving method that can achieve a reduction effect and is a simple method.

According to the present invention, a semiconductor laser as a light source has a plurality of current injection regions, and each of at least two injection regions has:
It is characterized in that a current containing an AC component whose phase is substantially inverted at substantially the same frequency is injected, and the magnitude of at least one AC component of one phase is switched according to modulation.

Example Hereinafter, a specific example of the semiconductor laser driving method of the present invention will be described in detail with reference to the drawings.

FIG. 2 is an explanatory diagram relating to the configuration of an optical head unit of the optical information recording / reproducing apparatus, and is an embodiment of an optical card. Although FIG. 2 will be described below, those skilled in the art will only give a brief description of known parts.

The divergent light from the semiconductor laser 2 having a plurality of current injection regions according to the present invention is converted into parallel light by the collimator lens 3 and has a light amount distribution substantially symmetric with respect to the optical axis by the beam shaping prism 4. It is shaped into a perfect circular parallel light. It is divided into three parallel lights by the diffraction grating 51,
The light transmitted through the non-polarizing beam splitter 6 is bent substantially orthogonally by the bending mirror 7, and is spotted by the objective lens 8 on the tracking track 9 on the optical card 1 and the recording area sandwiched between the tracks S <b> 1, S <b> 2, Image S3. The optical card 1 is reciprocated at a constant linear velocity in the direction of arrow R by a driving means (not shown), and the spot is scanned on the tracking track. The reflected light of each spot S1, S2, S3 passes through the objective lens 8 again, and the component reflected by the non-polarizing beam splitter 6 is converted by the astigmatic focusing lens system 10 including, for example, a cylindrical surface. Photodetector 11,1
Light is focused on 2,13 respectively. The auto focus controls the objective lens 8 in the F direction by a known astigmatism method to obtain a minute spot, and the auto tracking controls the objective lens 8 in the T direction by a known three-beam method, thereby ensuring reliable operation. Control of the spot is possible. The recording surface of the optical card 1 is protected by a protective layer (not shown), and light from the laser 2 obtains information from the recording surface through the protective layer.
Polycarbonate or the like is used as the protective layer to reduce the cost of the optical card. However, birefringence occurs due to stress, residual stress, and the like during production, use, recording and reproduction of the card. Therefore,
In a combination of a polarizing beam splitter and a quarter-wave plate, which has both effective use of light and an isolator, the amount of light reflected to the photodetector and the amount of light transmitted to the laser 2 depend on the degree of birefringence. When the light quality fluctuates and the light on the photodetector side is small and the signal-to-noise ratio is poor, the light returns most to the laser 2 side, and return light-induced noise is generated, further deteriorating the signal-to-noise ratio. There is such a problem.

Therefore, in the present embodiment, a non-polarized beam splitter 6 that does not depend on polarization in the wavelength range of the laser 2 is used to stabilize the light amount on the photodetector and the light amount returned to the laser 2. Specifically, a transmittance of 60 to 70% and a reflectance of 30 to 40% are appropriate ranges. However, since the amount of light returning to the laser 2 reaches several pulses, the laser 2 needs AC signal is superimposed. At this time, since the non-polarizing beam splitter 6 is used, a high-output laser must be used because the transmittance from the laser 2 to the card 1 is reduced. However, as described in the problem of the conventional example, it is practically impossible to apply normal high-frequency superposition at the time of recording, which requires a high level. Noise can be reduced.

FIG. 3 is a schematic enlarged view of the chip structure of the laser 2 according to the present embodiment.

The laser 2 is a so-called buried type semiconductor laser, and a plurality of current injection regions 23, 24 and 25 are formed in pairs with an n-type electrode 22 by a plurality of p-type electrodes with respect to a stripe 21 in an active layer which performs laser oscillation. doing. 24, 25 for implanted region 23
Since the region 2 is small, the laser 2 can oscillate only in the injection region 23 if highly excited, but the oscillation is controlled by the polarity of the current (carrier) injected into the regions 24 and 25. A photodiode 26 is monolithically mounted behind the laser 2, and can apply a reverse bias to extract a current corresponding to the output of the laser 2.
It should be noted that this type of semiconductor laser can be manufactured by a technique known to those skilled in the art, and can be manufactured by, for example, a molecular beam crystal growth method and a reactive ion etching method, but the description of the manufacture is omitted here. I do. Further, in order to increase the output of the laser 2, the reflectance of the front end face of the laser 2 is set to 5 to 5.
It is easy to lower to 30% and raise the rear end face reflectance to 70-90%.

FIG. 1 is a schematic diagram of a drive circuit for driving the laser 2 according to the present embodiment. FIG. 4 is a schematic diagram for explaining a signal waveform at each point.

The information to be recorded or the signal indicating the reproduction is recognized by the microprocessor MP.U35 via the interface 34, and the recording / reproduction switching signal shown in FIG. At the time of recording, information to be recorded is modulated by a modulation circuit 33 according to a predetermined modulation method, and a preamble, a track number, an end of track, an error correction code and the like are added as necessary, and the information is recorded on an optical card. Converted to format. The modulated signal injects a signal current (FIG. 4 (b)) into the injection region 23 via the drive circuit 31. During reproduction, a DC bias current is selected and injected.

On the other hand, a high-frequency current for reducing the return light-induced noise is obtained by the oscillation circuit 38, and the high-frequency current is superimposed on the injection region 25 for both recording and reproduction, as shown in FIG. I have. The DC bias of the current injected into the injection region 25 is provided by the bias circuit 36 via the coil 40, and the high-frequency current superimposed via the capacitor 41 cuts the effective threshold I _th ′ in the region 25. Is given as The current applied to one injection region 24 is
The phase of the high-frequency current obtained by the oscillation circuit 38 is shifted by 180 ° C. by the inversion circuit 39 to the DC bias current given from the bias circuit 37 through the coil 40, and the capacitor 41
The current shown in FIG.

The current applied to the injection region 24 is equal to the effective threshold value of the injection region 24 during reproduction, when the output of the laser 2 may be low.
Only the DC component slightly exceeds I _th ′, and the high-frequency current whose phase has been inverted is used as the effective threshold only during high output during recording.
Superimpose so as to cut I _th '. This is the recording / playback switching signal from the MPU 35, FIG. 4 (a) and the modulation signal FIG. 4 (b)
This is achieved by switching the inversion circuit 39 and the bias circuit 36.

As described above, by switching the amplitude and the bias of the high-frequency current having one phase inverted, a desired modulation can be given to the output of the laser 2. In addition, since a pair of currents whose phases are inverted is added, high output is possible, and high output at high output does not include high frequency components in principle. That is, with the same effect as that disclosed in Japanese Patent Publication No. 59-9086, noise is reduced, and at the same time, a plurality of problems of the conventional example are eliminated, and stable modulation is easily performed.
Furthermore, according to the method of the present invention, conventionally, it was necessary to modulate the value of the current injected into the entire injection region of the laser 2. However, the present invention provides that the current injected into the minute injection region 24 as the control region is controlled. Since modulation is sufficient, not only simplification of the device, but also reduction in unnecessary power consumption and unnecessary radiation noise, and reduction in load, the higher frequency driving becomes possible. Although not described, 32 is an auto power control circuit, which is a circuit for detecting the output of the semiconductor laser by the monitor photodiode 26 and compensating for output fluctuation of low frequency such as temperature fluctuation, which is well known to those skilled in the art. Things.

In addition, the present invention is not only effective in reducing general return-light-induced noise speckle and pattern noise, but is also effective in reducing mode hopping noise caused by changes in temperature and injection current. Since the modes are multiplied, there is an effect of reducing noise. Furthermore, it is effective not only for reducing the intensity noise of the laser, but also for reducing the noise generated due to the change in the diffraction pattern and interference fringe pattern on the detector due to the wavelength change. In other words, this kind of noise appears as unwanted undulations or discontinuities of erroneous signals in the servo band, which causes problems. The effect of reducing noise and reducing noise is known.

Further, in FIG. 1, a plurality of output levels during recording,
Corresponding to multiple output levels such as the reproduction level, the frequency, phase, amplitude,
It is easy to optimally switch the DC bias level and the like by the MPU, and a more stable information recording / reproducing operation can be performed.

For example, although not shown, in order to compensate for changes in the frequency and amplitude of the oscillation circuit due to temperature and the like, a deviation from the reference value is detected using a detection circuit known to those skilled in the art, and the oscillation circuit 36 and the bias The present embodiment can be modified so as to stabilize the current waveform by feeding back to the circuits 36 and 37. Although not shown, laser 2
The output of the photodiode 26 monitoring the output of
Alternatively, the levels of noise are detected from the outputs of the photodetectors 11, 12, and 13, and the current waveforms are controlled so that the noise is reduced to a desired level in accordance with the detected levels (FIGS. 4C and 4D). It is also possible. For example, if the amount of noise is sufficiently acceptable under certain conditions, the oscillation circuit 38 may be stopped and only the DC bias 37 may be injected into the injection regions 24 and 25, or nothing may be injected.

 Hereinafter, a plurality of other embodiments of the present invention will be described.

FIG. 5 is a cross-sectional view taken along the stripe direction of the semiconductor laser, showing an example in which a plurality of frequencies are used. In the figure, the injection region is nine, but the inner five usually erase is implanted region 44 for performing recording and reproduction, the implanted region 45 and 48, 180 ° out of phase with the frequency f ₁ current injected containing an AC component, the implanted regions 46 and 47 for injecting a current including an alternating current component shifted in phase by 180 ° at the frequency f _2. By adopting such a configuration, optimal current waveforms, that is, frequencies f ₁ and f ₂ and a DC bias are set for two bands, a signal band and a servo band, respectively.
By switching or combining them as necessary, a stable operation with lower noise is realized as the device.

FIG. 6 is a view showing another embodiment of the semiconductor laser, in which injection regions 49a and 49b for performing normal erasure, recording and reproduction are shown.
And the AC component injection region divided in the width direction of the stripe 21
50,51 were provided. In this configuration, since the AC component having a phase shift is injected in the stripe width direction, it also affects the transverse mode of the laser, so that a larger multiple longitudinal mode oscillation effect appears, and On the other hand, the injection region in the active layer becomes smaller, so that it can be driven with lower power and can be driven up to a higher frequency because of the smaller capacitance.

FIG. 7 is a modification of the embodiment of FIG.
On both sides of the injection region 52 with the upper normal electrode, the injection region 5
3,54 added. In the present embodiment, in order to further facilitate multiple longitudinal mode oscillation, the width of the buried stripe is widened by a portion 55 corresponding to the injection regions 53 and 54, so that the influence on the transverse mode is further increased.

FIG. 8 shows another embodiment in which alternating current component injection regions 57 and 58 having a TJS (Transveuse Junctior Stripe) structure are provided for the injection region 56 of a normal electrode. The TJS structure is a structure known to those skilled in the art.
It is possible to inject a current (carrier) into the active layer from substantially the lateral direction by selectively diffusing zinc. With such a configuration, the crossing force between the injection region 56 for erasure, recording, and reproduction and the AC component injection regions 57 and 58 can be reduced, so that a more reliable noise reduction effect can be realized.

As an optical information recording / reproducing apparatus, an optical card system has been described as an example, but the present invention is not limited to this, and it is needless to say that the present invention is also applicable to an optical disk, an optical tape, etc. which performs any of erasing, recording, and reproducing. Applicable.

Furthermore, in the embodiments, superposition of a rectangular wave AC signal has been described. However, the present invention is not limited to this, and may be a sine wave or a triangular wave, and the frequency, phase, and amplitude may have some modulation. You may have received it. In short, it is only necessary to inject a current into a plurality of injection regions so that the light output at the time of high output becomes substantially constant, so that it is not necessary to pair the two injection regions as described in the embodiment. You may comprise three or more. In addition, the present invention can be implemented even if the sizes, gains, losses, and the like of the injection regions are not uniform due to manufacturing accuracy, if the amplitude of the alternating current whose phase is inverted is independently controlled. Furthermore, it is also possible to keep the optical output substantially constant by a combination of a plurality of frequencies, a plurality of phase shifts, or the like, or a combination of random signals.

Further, in the above embodiments, the setting of the implantation region is performed only on one side and one electrode is always common. However, it is obvious that setting the implantation region from both sides can reduce the crosstalk between the implantation regions. Not even.

[Effects of the Invention] As described above, according to the present invention, a semiconductor laser having a plurality of current injection regions is used as a light source, and at least two of the injection regions have substantially the same frequency and substantially inverted phases. Injection and driving of current containing AC component enables noise reduction at the time of high output, and modulates the amplitude and bias of one AC component whose phase is inverted, so that erasing and recording can be performed. By achieving the optical output modulation described above, noise can be reduced in any of erasing, recording, reproducing, and transmitting, and a stable and reliable semiconductor laser driving method can be provided.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram relating to a configuration of a driving device of a semiconductor laser according to the present invention. FIG. 2 is an explanatory diagram relating to the configuration of the optical head section of the optical information recording / reproducing apparatus according to the present invention. FIG. 3 is an explanatory diagram relating to the configuration of the semiconductor laser used in the present invention. FIG. 4 is a diagram for explaining a current waveform injected into the semiconductor laser. FIGS. 5 to 8 are explanatory diagrams each showing the configuration of another embodiment of the semiconductor laser. 1 ... optical card, 2 ... semiconductor laser, 23,24,25 ... current injection area,

Claims (1)

(57) [Claims] 1. A semiconductor laser driving method for using a semiconductor laser as a light source and modulating the semiconductor laser according to information, wherein the semiconductor laser has a plurality of current injection regions, and each of at least two injection regions includes: A current including an AC component having a phase substantially inverted at substantially the same frequency higher than the maximum frequency of modulation according to the information is injected, and at least one amplitude and a DC component of the AC component having the inverted phase are converted to information. A method for driving a semiconductor laser, comprising modulating the optical output of the semiconductor laser by modulating the light in response to the light.