JPS5911548A - Optical pickup device - Google Patents

Abstract

PURPOSE:To obtain a signal with high SN ratio, by inputting a laser light irradiated from one or both spots of a semiconductor laser being a light source to other spot different from the irradiated laser light spot, for decreasing the coherence of the semiconductor laser, and suppressing the optical noise of the semiconductor laser. CONSTITUTION:A laser light irradiated from a spot 8a of the semiconductor laser 1 is divided into a luminous flux going to a disc reflecting plane 6 and that going to a reflecting mirror 13c. The light going to the disc forms an optical spot 9 on a disc 6, the reflecting light passes through again a 1/4 wavelength plate and is directed in the direction of a main optical detector 7 with the direction of polarization of light changed by 90 deg. from the beginning, and the information of the disc is converted into an electric signal. On the other hand, the luminous flux reflected in the direction of the reflecting mirror 13c is focused with a lens 2b and given to the other irradiating spot 8b of the semiconductor laser. Further, the laser light irradiated from the spot 8b is separated into the luminous flux in the direction of a lens 2a and of a monitor optical detector 12 at a semi-transmitting mirror 11a, and the laser light irradiated in the lens 2a is stopped and given to the spot 8a.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical pickup device using a single-longitudinal mode semiconductor laser.

Conventionally, in single-longitudinal mode semiconductor lasers, when the light emitted from the element end face is reflected by an external reflective surface and returned to the emission end face, the optical output fluctuates even if the laser element is driven with a constant current. It is known that in the case of signal reproduction, etc., this becomes optical noise, making it difficult to detect a good signal.

For this reason, when semi-integrated lasers are used for optical information processing, such as in optical communication or optical disk playback devices, in order to prevent the generation of semiconductor laser noise due to feedback light, the light returns to the semiconductor laser element as much as possible by means such as polarization separation. I tried not to. However, it is actually impossible to reduce the amount of feedback light to completely zero, and if the amount of feedback light to the semiconductor laser becomes about 0.1% of the output light amount due to variations in the precision of optical components, assembly precision, etc. Optical noise increases rapidly. Therefore, the method of suppressing the optical noise of the semiconductor laser by minimizing the amount of feedback light to the semiconductor laser has the disadvantage that component precision and assembly precision are extremely critical.

FIG. 1(a) shows the basic configuration of a conventional pickup optical system for reproducing optical discs. The laser light emitted from the light emitting point 8 of the semiconductor laser 1 is converted into parallel light by the coupling 2, and after passing through the light-opening prism 3 and the quarter-wave plate 4, it is collimated by the focusing lens 5 onto the reflective surface 6 on the disk. Narrow it down to light and jft 9. The reflected light from this reflective surface is again narrowed down by a lens 5 and a 1/4e long plate 4.
The light passes through the polarizing prism 3, is kicked out laterally, reaches the photodetector 7, and is converted into an electrical signal.

Conventionally, this type of optical pickup device uses a 1/4 wavelength plate 4.
The combination of the polarizing prism 3 and the polarizing prism 3 prevents reflected light from the disk from returning to the semiconductor laser in principle. That is, as shown in Figure 1(b), the linearly polarized light emitted from the semiconductor laser is incident by making the plane of polarization at a 45° angle to the crystal axis of the quarter-wave plate. After passing through the quarter-wave plate, the laser light becomes circularly polarized light.

When this laser beam is reflected by the reflecting surface and passes through the quarter-wave plate again, the polarization direction is changed to the inclination angle 1V with the polarization plane rotated by 90 degrees from the initial polarization direction. Light with polarization directions different by 90° is reflected without passing through the polarizing prism, and therefore does not return to the semiconductor laser. However, it is unavoidable that the laser beam returns to the light emitting point 8, albeit slightly, due to the accuracy of optical components, assembly accuracy, optical anisotropy (birefringence), etc. For example, when the polarizing prism 3 is adjusted so that the reflected light from the reflective surface 6 of the disk is guided to the photodetector 7 to the maximum in FIG. 200:1, semiconductor laser 1 and coupling lens 2
The coupling efficiency η, that is, the ratio of the amount of light after passing through the coupling lens to the total amount of light emitted in front of the semiconductor laser is 25%, the reflectance of the reflective surface is 80%, and the ratio of the amount of light that passes through the coupling lens to the total amount of light emitted in front of the semiconductor laser is 25%, the reflectance of the reflective surface is 80%, and the ratio of the amount of light that passes through the coupling lens is 80%. If the light efficiency is 80%,
Approximately 0.08 to 0.1% of the forward emitted light from the semiconductor laser returns to the light emitting point 8a. This degree of return υ
Depending on the amount of light, a single-longitudinal mode semiconductor laser generates optical noise that interferes with signal reproduction.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical pickup device that suppresses the generation of optical noise in a single-longitudinal mode semiconductor laser and has a good S/N ratio.

As mentioned above, in the index-guided semiconductor laser, the amount of light returned to the element is closely related to the optical noise level.

When the applicant made detailed measurements of the relationship between the amount of feedback light and optical noise in various semiconductor lasers, he discovered that the noise level does not increase monotonically with respect to the amount of feedback light, but has a peak at a certain amount of feedback light. An example of this is shown in FIG. The horizontal axis indicates the feedback light amount rate, that is, the ratio of the amount of feedback light to the device light emitting point to the total amount of light emitted per end face of the semiconductor laser.

The vertical axis represents the relative noise intensity per bandwidth IHz when the DC output is set to 1 when the laser beam is observed with a photodetector. In other words, relative noise intensity 1o-1m is 10KH
This corresponds to an S/N of 90 dB when measured with two bandwidths.

Measurements were made in the 500 KJ4Z band using a refractive index guided single-longitudinal mode semiconductor laser with a constant optical output of 3 mW while changing the element temperature from 20 C to 60 C. Figure 2 plots the maximum noise level at that time. This is what I did.

According to this result, the peak of the noise is at a feedback light rate of 0.1.
%, and if the feedback light amount rate is increased beyond this level, the noise will decrease. Therefore, the generation of noise can be reduced by increasing the feedback light amount rate to 1% or more, rather than decreasing it a little, and by using a semiconductor laser on the right side of the peak.

FIGS. 3(a) and 3(b) show how the relative noise intensity changes when the horizontal axis represents temperature at 0.1% feedback and 2% feedback. It can be seen that with optical feedback of about 2%, the fluctuation in noise intensity is reduced by more than 10 dB compared to when feedback is 0.1%, which is effective in reducing noise.

This noise reduction effect is thought to be due to a decrease in the coherency of the laser beam due to the optical feedback to the laser.

In order to reduce the optical noise of semiconductor lasers through optical feedback as described above, the applicant previously proposed an optical pickup device having a structure that returns laser light to the same light emitting point from which it was emitted. .

In contrast, the present invention is characterized by injecting heavy laser light from another light emitting point different from the light emitting point emitted in a semiconductor laser which normally has two light emitting points. Note that the amount of injected light is suitably equivalent to 1% or more of the total amount of emitted light per single light emitting point.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 shows an embodiment in which laser light is injected into a light emitting point using a semi-transmissive mirror 11a and reflecting mirrors 13a, 13b, and 13c. The laser light emitted from one light emitting point 8a of the semiconductor laser 1 is collimated by the collimating lens 2a, and enters the semi-transparent mirror 11a, where it is divided into a beam of light directed toward the reflective surface 6 of the disk and a beam directed toward the reflecting mirror 13c. It can be divided into The light directed toward the disk is the first
Similar to the conventional pickup shown in the figure, after passing through a polarizing prism 3 and a quarter-wave plate 4, a light spot 9 is formed on a disk 6 by an aperture indentation lens 5.

The reflected light from the disk passes through the 1/4 wavelength plate again, so that the polarization direction of the light changes by 90' from the initial direction and is emitted toward the main photodetector 7. The information on the sidisk is converted into electrical signals. On the other hand, the semi-transparent mirror 11
The light beam reflected by a toward the reflecting mirror 13c is reflected by the reflecting mirrors 13c and 13b.

13a, and is focused by the lens 2b and injected into the other light emitting point 8b of the semiconductor laser. On the other hand, the laser light emitted from the light emitting point 8b becomes parallel light by the lens 2b, and the reflecting mirrors 13a, 13b, 1
3c, the semi-transmissive mirror 11a separates the light beam into light beams directed toward the lens 2a and toward the monitoring photodetector 12.

The laser light incident on the lens 2a is focused and injected into the light emitting point 8a. By configuring the optical system as described above, the laser beam from the light emitting point 8a is directed to the light emitting point 8b.
The laser beams from b can be injected into each 83, and the effect of reducing laser noise can be obtained. For example, lens 2a
and the coupling efficiency between 2b and semiconductor laser 1 is both 2
5%, and when the reflectance and transmittance of the semi-transmissive mirror are both 50%, approximately 12% of the amount of light emitted from each light emitting point is injected into the light emitting points 8a and 8b. become. A semi-transparent mirror 11a and a reflecting mirror 13a.

By changing the reflectance of 13b and 13c, it is possible to change the amount of injected light and increase the amount of signal light to the disk, but it is necessary that the amount of injected light be 1% or more.

The monitoring photodetector 12 is an automatic output control (ADC) that monitors the amount of light emitted from the laser 1 and keeps the optical output constant.
It is used for various purposes.

This example can be realized with a relatively small number of parts.
It also has the advantage of relatively high light utilization efficiency.

FIG. 5 shows an optical system consisting of two reflecting mirrors 13a and 13b.

This is an embodiment in which a semi-transmissive mirror 11b, a half-wave plate 10, and the like are used.

The laser beam emitted from the light emitting point 8a is reflected by the disk 6 in exactly the same way as in the optical system shown in FIG. This light flux enters the semi-transmissive @llb, and a part of it is transmitted as it is and guided to the main photodetector 7, where the information on the disk is converted into an electrical signal. On the other hand, the polarization direction of the light beam reflected by the semi-transmissive mirror 11b is changed by 90 degrees by the half-wave plate 10 and returned to the same direction as the polarization direction of the semiconductor laser 1. The light beam after passing through the 1/2 wavelength plate 10 is relayed by reflecting mirrors 13b and 13a, and then sent to lens 2b.
is injected into the light emitting point 8b. On the other hand, the light from the light emitting point 8b follows the completely opposite path, that is, the light emitting point 8b→
Lens 2b → Reflector 13 a, 13b - = 1/2 length plate 10 → Semi-transmissive mirror → Polarizing prism → 1/4 wavelength plate 4 → Aperture lens 5 → Reflective surface 6 → Aperture recess lens 5 → Aperture Lens 4 → Polarizing prism 3 → Light emitting point 8 via lens 2a
injected into a.

Here, the coupling efficiency of laser 1 and lenses 2a and 2b is 2
5%, and the transmittance and reflectance of the semi-transmissive mirror 11b are each 50%.
Assuming that the reflectance of the disk is 80% and the efficiency with which the light beam travels back and forth through the aperture lens 5 is 80%, about 8% of the light can be completely injected into the light emitting points 8a and 8b. Note that by changing the ratio between the transmittance and the reflectance of the semi-transmissive mirror 11b, it is also possible to change the ratio between the amount of injected light and the amount of signal light. In addition, by shifting the angle between the crystal axis of the quarter-wave plate 4 and the polarization plane of the incident light from the normal 45°, the light emitting point 8a
It is also possible to separately inject the light emitted from the light emitting point 8a and 8b into the light emitting points 8a and 8b, and the light emitted from the light emitting point 8b into the light emitting points 8b and 8a, respectively. However, in any case, it is desirable that the amount of light injected into each light emitting point be 1% or more of the amount of light emitted from the light emitting point.

In this embodiment, compared to the embodiment shown in FIG. 4, the optical path length through which the injected light passes can be made longer, so even if the amount of injected light is relatively small, the coherency of the laser light decreases quickly and noise is reduced. This has the advantage that the reduction effect is more significant.

Figure 6 shows two semi-transparent prisms l without using a polarizing element.
la, llb and two reflecting mirrors 13a.

13b and the like is shown.

Light from the light emitting point 8a is transmitted through the light emitting point 8a → lens 2a → semi-transparent fJ11a → focusing lens 5 → disk 6 → focusing lens 5 → semi-transmitting mirror 11a → cow transmitting mirror 11b → reflecting mirrors 13b, 13a → lens 2b is injected into the light emitting point 8b. At the same time, the light from the light emitting point 8a passes through the semi-transparent mirror 11a.
and the semi-transparent mirror 1 that is returned to the light emitting point 8a by
Some are guided to the main photodetector 7 by 1b. on the other hand,
A part of the laser light from the light emitting point 8b is injected into the light emitting point 8a through the reverse path, and the other part is guided to the main photodetector 7.
The other part is fed back to the light emitting point 8bK.

Here, assuming that the transmittance and reflectance of the semi-transmissive mirrors 11a and llb are equal to 50% each, and that the light efficiency of the other optical systems is the same as in the embodiment shown in FIG. About 2% of the light is completely injected into the light emitting point 8b, and about 4% is returned to the light emitting point 8a. On the other hand, about 2% of the light emitted from the light emitting point 8b is injected into the light emitting point 8a, and about 1% is injected into the light emitting point 8b.
will be returned to. Therefore, assuming that the amount of light emitted from both light emitting points is equal, and adding the amount of light injected to each light emitting point, the amount of light equivalent to about 6% is injected into the light emitting point 8a, and about 3% is injected into the light emitting point 8b. Ru.

Since this embodiment does not use a polarizing element, it has the advantage of being less susceptible to fluctuations in the amount of light injected into the laser due to the birefringence of the disk and fluctuations in the signal light cover to the photodetector.

As explained above, in an optical pickup device, the present invention provides a trap for injecting laser light emitted from one or both light emitting points of a semiconductor laser of a light source into another light emitting point different from the emitted light emitting point. Since the coherency of the semiconductor laser can be lowered, the optical noise of the semiconductor laser can be suppressed, and as a result, a signal of higher quality than the disk can be detected.

[Brief explanation of the drawing]

Figures 1 (a) and (b) are diagrams explaining the configuration of a conventional optical pickup, Figure 2 is a diagram showing the relationship between the amount of feedback light to the semiconductor laser and the laser noise level, and Figure 3 (a) and (b). Figures 4, 5, and 6 show the noise reduction effect of optical feedback.
The figure shows an embodiment of the present invention.

Claims (1)

[Claims] 1. A single-longitudinal mode semiconductor laser having two light emitting points that emit laser light in opposite directions, and at least one light beam from the semiconductor laser being directed through a light distribution element. An optical pickup device comprising an optical means focusing the light onto the reflective surface of the semiconductor laser, and a photodetector that receives the reflected light from the reflective surface via the light distribution element, in which light is emitted from at least one light emitting point of the semiconductor laser. 1. An optical pickup device comprising a light injection means for injecting a predetermined amount of laser light into a light emitting point different from the light emitting point from which the laser light was emitted. 2. The light pickup device according to claim 1, wherein the light injection means injects into the light emitting point an amount of light equal to or more than 1% of the total amount of light emitted from a single light emitting point.