JPS60192377A - Driving method for semiconductor laser element - Google Patents

Abstract

PURPOSE:To reduce noise attributable to laser energy fed back to the emitting element by a method wherein the intensity of the energy emitted by a semiconductor laser element is detected and the direct current value is regulated according to the intensity of the detected value. CONSTITUTION:A direct current smaller than the threshold current of a semiconductor laser 1 is yielded by a constant output circuit 11. The yield is joined by a pulsating current outputted by a pulse generator 8 to pulse-drive the semiconductor laser 1. In this way, fluctuations caused by the feedback of reflected energy may be suppressed. The pulse generator 8 is connected to the semiconductor laser 1 with the intermediary of a capacitor. A light-receiving diode 9 is connected to the driving circuit for the semiconductor laser 1, and to the constant output circuit 11 through the intermediary of a low-pass filter 10. Accordingly, the direct current, supplied by the constant output circuit 11 through a coil to the semiconductor laser 1 on the basis of the signals transmitted by the light-receiving diode 9, is regulated. The regulated direct current is joined by a pulsating current, originating inth pulse generator 8, to drive the semiconductor laser 1 to yield constant-intensity pulses.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a method for driving a semiconductor laser element that is effective as a light source means for a pickup device for an optical information processing device that optically writes and reproduces information.

<Prior Art> An optical pickup device is used as a means for writing information on an optical disk or reproducing information stored on an optical disk, and a semiconductor laser has been put into practical use as its light source.

An example of the basic configuration of an optical pickup device using a semiconductor laser is shown in FIG. The light emitted from the semiconductor laser 1 is turned into parallel light by a collimating lens 2, and is irradiated onto a beam splitter 3, a polarizing prism 4, a quarter wavelength plate 5, and a condensing lens 6 in order. When writing information on the disk 7, the semiconductor laser 1 is pulse-driven at a frequency according to the signal. When reading information from the disc 7, the semiconductor laser 1 is driven with a direct current, and the detector 8 detects the amount of repulsive light that is modulated depending on the presence or absence of pits on the disc 7'. In the optical pickup as described above, since the image of the semiconductor laser is formed on the disk 7, a part of the repulsive light from the Dinuk 7 is fed back to the semiconductor laser 1. It is generally known that in a semiconductor laser, the noise level of the laser increases when a part of the emitted light is fed back.For example, even if the amount of returned JW light is about 0.1% of the amount of emitted light, The relative noise intensity of the laser increases by more than 1000 times,
Deteriorates the performance of the optical disk information processing device. For this purpose, as shown in FIG.
A mechanism is provided to allow the reflected light from the disk 7 to return to the semiconductor laser 1. However, even if such a feedback prevention mechanism is added, it is not possible to completely prevent the reflected light from returning. A DINUK substrate used in an optical disk device is generally made of a polymer resin material. This polymer resin plate has a slight birefringence, and even if a 1/4 wavelength plate 5 and a polarizing prism 4 are attached to prevent feedback, the reflected light from the Dinuk 7 will not be reflected due to this birefringence. A portion is fed back to the semiconductor laser 1. Further, the reflected light can also be returned due to optical adjustment deviation of the wavelength plate 5 or the polarizing prism 4. Therefore, due to these reasons, several percent of the reflected light from the disk 7 is fed back to the semiconductor laser 1, increasing the noise level of the laser output light. The increase in laser noise level poses a major problem during signal reproduction.

By the way, the specifications required for a semiconductor laser as a light source for a pickup device of an optical Dinuk device include low noise and the need to write information at high speed.
? It is essential that high output oscillation of about 7LW is possible. In order to obtain such high output, semiconductor lasers must have a resonant surface for output extraction that constitutes a Fabry-Perot resonator with a deflection rate of 10% or less, and a resonant surface that opposes this with a reflectance of 90% or more. A means for increasing the extraction efficiency is provided. Therefore, the influence of the feedback light becomes even greater than in a normal semiconductor laser whose end face reflectance is 32°.

Next, the relationship between feedback light and noise generation will be explained in detail. FIG. 2 is a characteristic diagram showing the optical output, relative noise intensity, and spectrum according to the current of a semiconductor laser in which the reflectance of the emission end face is 10% or less in the case of no feedback. The semiconductor laser used is a VS with an internal stripe structure with a V-groove cut into the substrate.
IS (V-channel 5ubstrate
The laser element is of the inner 5tripe type laser element, and the output end face reflectance is set to 2% and the back face reflection rate is set to 95%. Refractive index guided type VS with controlled transverse mode as shown in Figure 2
Even in IS lasers, if AR coating (resonant end face covering with a dielectric film) is applied, there will be a total loss of multiple longitudinal modes up to an output of about 3 to 7 LW. FIGS. 3 and 4 show the optical output, relative noise intensity, and spectrum when the relative feedback amount (feedback light amount/end face emitted light amount) is F-3 shadow and 0.001 shadow, respectively. In the case where the amount of feedback light is relatively large (F = 3% as shown in Figure 3), the relative noise intensity is 4 m at the output, W
It has a large peak in the middle.

Moreover, the spectrum 1-)V becomes multiple longitudinal modes in the entire output region. This increase in noise due to the feedback light is due to quantum noise due to the feedback light, and increases as the output end face reflectance decreases. On the other hand, as shown in FIG. 4, F = 0. OO
], % and when the amount of feedback light is relatively small, the output is 3 m.
In the W mode, there are multiple longitudinal modes, and the increase in noise is significant. However, at 3 mW or more, the distance between the optical disk and the semiconductor laser is half the laser oscillation light wavelength (λ/2 to 04 μm,
) The noise increases or decreases in cycles.

Accordingly, the spectrum also repeats two states, as shown in the figure: single-longitudinal mode (@ when the noise is small) and mode competition (when the noise is large). This mode competition includes not only competition between laser longitudinal modes but also competition between longitudinal modes of an external resonator constituted by a laser and a disk.

By the way, the reflectance is 3 when the end face is coated with a normal λ thickness.
It has been proposed to suppress mode competition noise by superimposing a high frequency sine wave on a 2% laser to generate multimode oscillation (Japanese Patent Application No. 113515/1982). but,
Noise caused by competition between external resonator modes cannot be suppressed simply by causing multimode oscillation. Therefore, in reality, it is necessary to superimpose a signal with a sufficiently large frequency (~600 MHz) so that the coherence length is shorter than that of the external resonator.

In addition, quantum noise that appears when the amount of feedback light is large, which is noticeable in lasers with low reflectance coating on the end face, cannot be reduced simply by shortening the coherence length. Therefore,
In a laser with low end face reflectance due to high output, it is impossible to reduce optical feedback noise simply by superimposing a high frequency sine wave.

When a semiconductor laser is used as a light source for an optical pickup device, it has already been mentioned that laser noise is a problem mainly during signal processing, but it is desirable that the laser output during reproduction be as small as possible.

This is to prevent deterioration of the optical disc, and the laser output during reproduction is usually set at 1 to 5 m, W. The dependence of the relative noise intensity on the amount of feedback light when the laser output is 3rn, W and 57rLW in DC driving is shown by solid lines in FIGS. 5 and 6, respectively. As is clear from the figure, as the amount of feedback light increases, the relative noise intensity gradually increases. In the region where the amount of reflected light is small, mode competition occurs in the case of 5771 W, and noise increases.

Such an increase in noise degrades the functionality of the system, so it is essential to reduce it.

<Objective of the Invention> The present invention provides a novel and useful method for driving a semiconductor laser probe that reduces noise caused by feedback of laser output light, which is extremely effective when a semiconductor laser is used as a signal light source for a pickup device or the like. The purpose is to provide.

<Embodiment> FIG. 7 is a configuration diagram of an optical pickup device using a semiconductor laser to explain an embodiment of the present invention. In the figure, the same symbols (1 to 7) as in FIG. 1 indicate the same contents.

The drive circuit for the semiconductor laser 1 includes a pulse generator 8 and a pulse generator 1''.
A light receiving diode 9 receives a part of the output light of the conductive laser 1, a reducing OIS device 10 extracts the low frequency component of the output of the light receiving diode 9, and a semiconductor laser 1 so that the output of the low frequency OIS device 10 is constant. It is composed of a constant output circuit 11 that supplies a direct current to. When driving the semiconductor laser 1, a DC current with a current value below the threshold current of the semiconductor laser 1 is output from the constant output circuit 11, and by superimposing the pulsed electric current fMk output from the pulse generator 8 on this, the semiconductor laser 1 is driven. Pulsed. Such pulse driving can suppress fluctuations in the optical output of the semiconductor laser due to feedback of reflected light.

The pulse generator 8 is connected to the semiconductor laser 1 via an equalizer, and the light receiving diode 9 is branch-connected to the drive circuit of the semiconductor laser 1 and is connected to a constant output circuit 11 via a low frequency transducer 10. . Therefore, the DC current supplied from the constant output circuit 11 to the semiconductor laser 1 via the coil is controlled in accordance with the signal from the light receiving diode 9, and the pulse current from the pulse generator 8 is superimposed on this to control the DC current supplied to the semiconductor laser 1 via the coil. is driven with a constant pulse output intensity.

FIG. 8 shows the average optical output and average noise intensity when the relative amount of feedback light is 3% and when a DC current is applied and a pulse current is simultaneously superimposed. From FIG. 8, the maximum value of the optical output is 6 rr+, W, but the average value is 3 mW. On the other hand, regarding noise, as shown by the solid line in the figure, when driven only by DC current, the output is 3 mW and the frequency is 1X10 Hz.
However, in the case of pulse drive shown by the broken line, the maximum value of the optical output is twice the average value, and the noise is generated only while the laser is oscillating due to the application of pulses. Therefore, when driving with a pulse with a duty ratio of 50%, the noise power is halved, so the maximum optical output is 6m.
The relative noise intensity corresponding to W is 2.5 x 10 Hz, which is half of 1 Hz, and the average output is 3 m,
The noise intensity is W. That is, in this case, pulse driving achieves a 16 dB noise improvement.

By the way, when deep modulation below the threshold current is applied to the semiconductor laser 1 using pulses or the like, the FM
The modulation and expansion of the vector line width is the same as in the case of superimposing a sine wave as described above, and it is expected that the coherence length will become shorter.

Actually, in a VSIS type laser device with a threshold current of 40 mA, the modulation frequency is 50 MI-Iz and the modulation current is 167
When driven with 1 LA and a DC current of 3571 LA, the coherence length was less than 1 LA.

The mode competition noise that occurs when the relative feedback light weight is small as shown in Figure 4 is generated by coherent interaction between the emitted light and the feedback light, so it does not occur at all in a semiconductor laser whose coherence length is shortened by pulse driving. .

There are two reasons why the coherence ratio is sufficiently short when the modulation frequency is 50 MHz, which is one order of magnitude smaller than the above-mentioned 600 MHz. (1) The semiconductor laser used in the present invention has a low emission surface deflection rate of, for example, about 2%, and therefore has a short photon lifetime and a coherence length of about one order of magnitude compared to a normal laser beam with a reflectance of 32. to become smaller,
(Since it is a 2+ pulse drive, even if the pulse repetition frequency is small, there is modulation due to the harmonic components of the pulse, and the line width broadening due to FM modulation is larger than with single frequency IF string wave modulation.) .

From the central limit theorem, the repetition frequency of the pulse must be twice or more the localized wave number in the frequency component of the Hio Hakugo, and practically, it needs to be about five times the frequency.

However, this value is sufficiently smaller than the above-mentioned 600 MHz, and there is an advantage that when the present invention is applied to an optical disk device for recording and reproducing, the pulse drive circuit for recording can be used as is.

In FIGS. 5 and 6, the dependence of the relative incoming sound intensity on the amount of feedback light of a vsrs type laser element modulated with a deep pulse below the threshold current is shown by a broken line. Compared to the characteristics in the case of curved current drive shown by the solid line in the figure, noise reduction has been achieved over the entire range.

<Effects of the Invention> As detailed above, according to the present invention, by providing a pulse application means for applying deep pulse modulation below the threshold current to a semiconductor laser whose output facet has a low reflectance, it is possible to improve the effect when the amount of feedback light is large. It is possible to reduce both quantum noise, which is a problem when the feedback light intensity is small, and moat competitive noise, which appears when the amount of feedback light is small. A high quality light source device can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the basics of an optical pickup using a semiconductor laser. Figure 2 shows the laser emission end face fouling rate when there is no optical feedback.
FIG. 3 is a characteristic diagram showing optical output, relative noise intensity, and nuvector with respect to drive current of a semiconductor laser set to 0Φ or less. Figure 3 and No. 41￥1 indicate the relative feedback amount of type 3, 0, and 0, respectively.
FIG. 2 is a characteristic diagram showing the dependence of optical output, relative noise intensity, and spectrum on drive current when set to 001%. In Figures 5 and 6, the optical output is 3 mW. 5?71. FIG. 7 is a characteristic diagram showing the dependence of relative intensity noise on the amount of feedback light in DC current driving (solid line) and pulse driving (broken line) when W is used. FIG. 7 is a configuration diagram of an optical pickup device for explaining one embodiment of the present invention. FIG. 8 is an explanatory diagram illustrating the operating principle of the present invention. 1...1 semiconductor laser, 7...de, f nuku, 8...
...Pulse generator, 9...Photodetector diode, 1o
...Low frequency f1 wave device, 11... Constant output circuit. Agent Patent Attorney Aihiko Fukushi (and 2 others) No. 1[2]
] 7r7I & ψ power (mA) Fig. 2 ThtoQ i (mA) Fig. 3

Claims (1)

[Claims] 1. The driving current of the semiconductor laser element is composed of a direct current below a threshold current value and a high frequency pulse current that is applied to the gate,
A semiconductor laser device characterized in that the pulsed light intensity of the semiconductor laser device is kept constant by detecting the output light intensity of the semiconductor laser device and controlling the current value of the direct current in response to a detection signal. Driving method.