JPS60170041A - Optical pickup device - Google Patents

Abstract

PURPOSE:To reduce the level of noise caused by the feedback of the laser output light by applying a deep pulse modulation to a semiconductor laser having a reduced reflection factor on its output end face. CONSTITUTION:The output extraction factor of a semiconductor laser 1 is improved by setting the reflection factor at <=10% for a resonance surface for extraction of output and also at >=90% for a counter resonance surface respectively. a pulse generator 8 is connected to the laser 1 via a capacitor; while a light emitting diode 9 is connected with branching to the drive circuit of the laser 1 and then to an output circuit 11 via an LPF10. Therefore a DC current flows to the laser 1 from the circuit 11 in response to the signal given from the diode 9. Then a pulse current given from the generator 8 is superposed on the DC current of the laser 1. Thus the laser 1 is driven. This suppresses the fluctuation of the optical output of the laser 1 due to the feedback of reflected light and then reduces the noise level.

Description

[Detailed Description of the Invention] Technical Field> The present invention relates to a pink amplifier device that is effective for an optical disk information processing device that optically writes and reproduces information, and particularly relates to an optical pickup device that uses a semiconductor laser as a light source. It is related to the device.

<Prior Art> Optical pickup devices that use semiconductor lasers as means for writing or reproducing information on optical discs have been put into practical use.

An example of the basic configuration of an optical pickup device is shown in FIG. The light emitted from the semiconductor laser 1 is turned into parallel light by a collimator lens 2, and is sequentially irradiated onto a disk 7 through a beam 1/splitter 3, a polarizing prism 4, a 74-wave plate 5, and a condenser lens 6.

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 disk 7, the semiconductor laser 1 is driven with a direct current, and the detector 8 detects the amount of reflected light that is modulated depending on the presence or absence of pits on the disk 7. In the optical pickup as described above, the image of the f-conductor laser is transferred to the disk 7.
Since the structure is such that an image is formed upward, a part of the separated light from the disk 7 is returned to the semiconductor laser l. It is generally known that the noise level of a semiconductor laser increases when a part of its emitted light is fed back.For example, even if the amount of feedback light is about 0.1 inch of the emitted light, the relative The noise intensity increases by more than 1000 times, deteriorating the performance of the original disk information processing device. For this purpose, as shown in FIG. 1, a 174 wavelength plate 5 and a polarizing prism 4 are inserted to prevent the reflected light from the disk 7 from returning to the semiconductor laser l. However, even if such a feedback prevention mechanism is added, it is not possible to completely prevent the reflected light from returning. A polymer resin material is generally used for a disk substrate used in an optical disk device.

This polymer resin plate has a slight birefringence, and therefore the quarter wavelength plate 5 and the polarizing prism 4 for preventing feedback are used.
Even if disk 7 is attached, due to the birefringence of disk 7,
A part of the infrared light is fed back to the semiconductor laser l. Reflected light is also fed back due to optical adjustment deviations of the shunted quarter-wave plate 5 and polarizing prism 4. Therefore, due to these reasons, several beams of reflected light from the disk 7 are 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 disk device include, in addition to low noise, the need to read information at high speed.
It is essential that high output oscillation of about 0r+zW is possible. In order to obtain such high output, semiconductor lasers are designed to have a reflectance of 10% or less for the output extraction resonant surface that constitutes the Fugg-Reperot resonator, and a reflectance of 9.0% or more for the opposing resonant surface. Measures are in place to increase extraction efficiency. For this reason, the influence of the feedback light becomes even greater than in a normal semiconductor laser whose end face reflectance is 32 inches.

Next, the relationship between feedback light and noise generation will be explained in detail. Figure 2 shows the reflectance of the output end face of 10% or more when there is no feedback.
FIG. 3 is a characteristic diagram showing the optical output, relative noise intensity, and spectrum with respect to the current of the semiconductor laser. The semiconductor laser used is VSIS (V-channel 5ubstr
The output end surface reflectance is set to 2% and the back surface reflectance is set to 2% and 95%, respectively, using a laser element of the type (inner 5 tripe) type.

As shown in FIG. 2, when an AR coating (resonant end face coating with a dielectric layer) is applied to a refractive index guided VSIS laser with a controlled transverse mode, multiple longitudinal mode oscillation occurs up to an output of about smw.

Figures 3 and 4 show the relative feedback amount: (++i) Return nail ÷
Edge output Q (I light fij: ) F = 8%, 0.0O
The optical output, relative noise intensity, and spectrum are shown respectively when itib is set as Itib. When the amount of feedback light is relatively large, such as F=3, as shown in FIG. 3, the relative noise intensity has a large peak up to about 45 W. Moreover, the spectrum has multiple longitudinal modes in the entire output range. This increase in noise due to the feedback light is due to foster noise due to the feedback light, and increases as the output end face reflectance decreases. On the other hand, as shown in FIG. 4, when the amount of feedback light is relatively small, such as F=0.001, the output power up to 3 mW becomes a multiple longitudinal mode, and the increase in noise is not significant. However, when the distance between the optical disk and the semiconductor laser exceeds 3 fflW, the noise increases or decreases at a period of half the wavelength of the laser oscillation light (λ/2 to 0.4 μm). Accordingly, the spectrum repeats two states, as shown in the figure, a single-longitudinal mode (when the noise is small) and a 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 with normal λ/2 thickness end coating.
Kokichi has been proposed, which suppresses mode competition noise by superimposing a high-frequency sine wave on a 2% laser to generate multimode oscillation (Japanese Patent Application No. 55-11 'a 515-'6).
. However, simply causing multimode oscillation does not suppress noise due to competition between external resonator modes. Therefore, in reality, it is necessary to weight a signal with a sufficiently large frequency (~600 MH2) so that the coherence length is shorter than that of the external resonator.

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

As already mentioned, when a semiconductor laser is used as a light source for an optical pickup device, laser noise becomes a problem mainly when the signal M is small, but the laser output during reproduction is tJf.
It is desirable that it be as small as possible.

This is to prevent deterioration of the optical disc, and normally the laser output when low is set to 1 to 5 mWK. The dependence of the relative noise intensity on the amount of feedback light when the laser output is 3 mW and 5 ynW in DC driving is shown by solid lines in FIGS. 5 and 6, respectively. As is clear from the figure, the relative noise intensity gradually increases as the amount of feedback light increases. In a region where the amount of feedback light is small, mode competition occurs when the power is 5 mW, and noise increases.

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

<Object of the Invention> The object of the Kihatsu 8A is to provide a new and useful optical pink-up device m that reduces noise due to feedback of laser output light in a pick amplifier device using a semiconductor laser.

<Embodiment> FIG. 7 is a block diagram of an optical big amplifier device showing an embodiment of the invention J. In the figure, the same symbols "J (l to 7) as in FIG. 1 indicate the same contents.

This embodiment includes a pulse generator 8, a light receiving diode 9 that receives the output of the semiconductor laser 1, a reduction p-wave generator 10 that extracts the low frequency component of the output of the light-receiving diode 9, and an output of the low-frequency p-wave generator IO that is constant. It is composed of a constant output circuit 11 that supplies a direct current to the semiconductor laser 1 so that the following equation is obtained. When driving the semiconductor laser 1, by superimposing a pulse current, fluctuations in the optical output of the semiconductor laser due to feedback of reflected light can be suppressed. The pulse generator 8 is connected to the semiconductor laser l via a capacitor, and is also connected to the light receiving diode 9.
is branch-connected to the drive circuit of the semiconductor laser 1 and has a low frequency P
It is connected to a constant output circuit 11 via a wave generator 10. Therefore, a direct current is supplied to the semiconductor laser 1 from the constant output circuit 11 in accordance with the signal from the light receiving diode 9, and a pulse current from the pulse generator 8 is superimposed on this to drive the semiconductor laser 1.

FIG. 8 shows the average optical output and average noise intensity when the relative feedback source is set to 3% and when a DC current is applied and a pulse current is simultaneously superimposed. From Figure 8, the maximum value t of optical output
d 6 mW, but the average value is 3 mW. On the other hand, regarding the noise, when driven only with DC current shown by the solid line in the figure ((IXIQ-
11H2, but 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 relative noise intensity corresponding to the maximum optical output of 6 mW is 5XIO-13Hz.
-1 half, 2.5 x 10-+3 Hz-1, is the noise intensity with an average output of 3 mW. 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 I using pulses, etc., the FM
The modulation and broadening of the spectral 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 be shortened.

Actually, in a VSIS type laser device with a threshold current of 40fflA, the modulation frequency was 50MH21 and the modulation current was 16m.
A. When driven with a DC current of 35 mA, the coherence length is 1? ? It became below IFF+.

The mode competition noise that occurs when the relative feedback light intensity 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. do not.

There are two reasons why the coherence length is sufficiently short even when the modulation frequency is 50 MHz, which is one order of magnitude smaller than the above-mentioned 600 MH2. (1) The semiconductor laser used in the present invention 811 has a small output surface reflectance of, for example, about 2%, and therefore has a short photon lifetime and a coherence length of about one digit compared to a normal laser element with a reflectance of 32%. (2) Since it is pulse driven, even if the pulse repetition frequency is small, there is modulation due to the harmonic components of the pulse;
The line width broadening due to FM modulation is greater than that due to single frequency sine wave modulation.

The repetition frequency of the pulse is reproduced from the central limit theorem (r
The frequency must be twice or more the upper limit frequency of the frequency component of j, and practically it is required to be about 5 times ll-i. However, this value is sufficiently small compared to 600MH2 mentioned in l3fj, and when the present invention is applied to an optical disk device for recording and reproducing, there is an advantage that the pulse drive circuit for recording can be used as is.

In FIGS. 5 and 6, the dependence of the relative noise intensity on the amount of feedback light of a vsts 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 direct current drive shown by the solid line in the figure, noise reduction has been achieved over the entire range.

<Effects of the Invention> As described in detail above, according to the present invention, by providing a pulse applying means that applies deep pulse modulation below the threshold current to a semiconductor laser whose output end face has a low reflectance, it is possible to Both the problematic quantum noise and the mode competition noise that appears when the amount of feedback light is small can be reduced, and an optical pink amplifier device for an optical disk information processing device with high characteristics and reliability can be obtained.

[Brief explanation of drawings]

Figure 1 shows an optical pickup using a semiconductor laser.
FIG. 2 is a configuration diagram showing the basic configuration. Figure 2 shows the reflectance of the laser output end face when there is no optical feedback.
FIG. 3 is a characteristic diagram showing optical output, relative noise intensity, and spectrum with respect to drive current of a semiconductor laser set to 0% or less. Figures 3 and 4 show relative feedback amounts of 3% and 0.00, respectively.
FIG. 3 is a characteristic diagram showing the drive current dependence of optical output, relative noise intensity, and spectrum when the ratio is 1%. Figures 5 and 6 show direct current 'ton current drive IIv' (solid line) and pulse drive (
FIG. 4 is a characteristic diagram showing the dependence of the relative intensity noise on the amount of feedback light in the case of (broken line). FIG. 7 is a block diagram of an optical pickup device showing one embodiment of the present invention. FIG. 8 is an explanatory diagram illustrating the operating principle of the present invention. I...Half-Ichiku Laser, 7...Disc, 8...
Pulse generator, 9... Light receiving diode, 10... Low frequency wave generator, 11... Constant output circuit. Agent Patent attorney Aihiko Fuku (and 2 others) Fig. 1 Nishi Yoku wo Tora (mA) Single acting (circle (mA) Fig. 7 Fig. 8

Claims (1)

[Scope of Claims] 1. The reflectance of the output end face for irradiating the laser beam onto the irradiated surface is set to 10% or more, and the reflectance of the back surface facing the output end face is set to 90% or higher. one? a conductive laser element,
a self-current driving means for driving the semiconductor laser element; a means for superimposing a high-frequency current having a frequency that is twice or more than the upper limit frequency in the information value to the semiconductor laser element;
An optical big amplifier device comprising: 2. A means for applying a high-frequency current in a superimposed manner to the semiconductor laser element includes: a DC power supply that drives the semiconductor laser element; a means for generating a high-frequency current to be superimposed on the DC power supply; and a photodetector to which a portion of the laser light is irradiated. a low-frequency p-wave generator that converts the low-frequency component of the output of the photodetector into a p-wave; and a low-frequency p-wave generator that feeds back the output of the low-frequency p-wave generator to the DC power supply to generate the low-frequency component of the output of the semiconductor laser element. 2. An optical pickup device according to claim 1, comprising means for making the optical pickup constant.