JPH0585970B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical pickup device using a semiconductor laser diode.

In an optical pickup device used to reproduce signals from optical discs, etc., the light beam from a laser diode, etc. is focused into a minute spot on the disc signal surface using an optical lens, etc., and the reflected light modulated by the disc signal is converted into an electrical signal by a photodetector. Convert and reproduce information.

The configuration of this type of optical pickup is shown in FIG.

The light beam from the laser diode 1 is made into a parallel light beam by a coupling lens 2, passes through a polarizing prism 3 and a quarter-wave plate 4, and is focused onto the signal surface of a disk 6 by a focusing lens 5. The reflected light passes through 5 and 4, is reflected by the polarizing prism 3, and is converted into an electrical signal by the photodetector 7. In this type of optical system, since the polarizing prism 3 and the quarter-wave plate 4 are used, reflected light does not return to the laser 1 in principle. In other words, by making the polarization plane incident at an angle of 45° to the crystal axis of the quarter-wave plate 4, the directly polarized light emitted from the semiconductor laser 1 becomes polarized after passing through the quarter-wave plate 4. The laser beam becomes circularly polarized. When this laser beam is reflected by the reflective surface of the disk 6 and passes through the quarter-wave plate 4 again, it becomes directly polarized light whose plane of polarization is rotated by 90 degrees from the initial incident polarization direction. Light with different polarization directions by 90° becomes linearly polarized light. Light having polarization directions different by 90° does not pass through the polarizing prism 3 and is reflected, so that it does not return to the semiconductor laser 1. However, due to parts precision, assembly precision, optical anisotropy (birefringence) of the disk, etc., the laser
It is impossible to return to zero and reduce the amount of returned light to zero. In this case, especially in a longitudinal single mode laser diode such as a refractive index guide structure, the longitudinal mode changes due to this small amount of feedback light, and the optical output changes. During signal reproduction, this output fluctuation becomes noise, making it difficult to detect a signal better than that from the disk.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical pickup device that reduces the drawbacks of optical output fluctuation noise and has excellent light utilization efficiency.

Laser diodes, which are light sources, can be roughly divided into so-called single mode lasers provided with the above-mentioned refractive index guide and multimode lasers using a gain guide. Single and multi are vertical modes. Single-mode lasers are susceptible to mode changes due to feedback light, but multi-mode lasers are less likely to change mode, so multi-mode lasers are generally superior in terms of noise caused by feedback light. However, since multimode lasers have astigmatism, using a cylindrical lens etc.
This has the drawback of having to be corrected. For this reason, although single mode lasers require fewer components, they have the disadvantage of producing the noise mentioned above. Conventionally, in order to reduce noise caused by the feedback light, we tried to prevent light from returning to the laser as much as possible. (References Nikkei Mechanical 12/21′
(1981, etc.) The applicant made detailed measurements of the relationship between the level of feedback noise and the amount of feedback using various semiconductor lasers, and found that the noise peaked at a certain amount of feedback, and that the noise level increased when a larger amount of feedback was returned. We discovered the fact that. FIG. 2 shows the relationship between the amount of reflected feedback light and the noise level of a single mode laser. This uses a CSP type laser with a numerical aperture of 0.15.
In this example, the amount of light passing through the lens system and reaching the disk surface is assumed to be 100%. In reality, the optical system does not use all of the amount of light emitted from the laser. This is because the emission pattern of the semiconductor laser is an anisotropic ellipse, and the aperture of the diaphragm lens needs to have uniform intensity in order to form a small spot. Therefore, it is normal for about 10 to 30% of the light to pass through the optical system, although it varies depending on the laser light emission pattern. The amount of feedback below assumes that the amount of light passing through this optical system is 100%. In FIG. 2, the horizontal axis indicates the feedback rate of laser light. The vertical axis indicates the relative noise intensity per 1 Hz bandwidth when the laser beam is observed with a photodetector and the DC output is set to 1. In other words, a relative noise intensity of 10 ^-13 corresponds to an S/N of 90 dB when measured at a 10 KHz (10 ^-4 ) bandwidth.
As can be seen from Figure 2, it was found that the noise decreased rapidly when the feedback was increased by about 10% or more. This is considered to be because the single mode laser becomes multi-like when the feedback exceeds a certain level.
If about 20% of the noise is returned to the laser, the noise will be reduced to a level where there is no problem.

In addition, when we measure the number of frame errors of the PCM signal recorded on a compact disk with a diameter of 12 cm in response to the amount of laser feedback, we find that the number of frame errors in the PCM signal recorded on the disk is as shown in Figure 3. It was also found that the number decreased rapidly.

Conventionally, it is possible to simplify the optical system by using a half prism or a half mirror, but in this case, the efficiency of light utilization is greatly degraded. Figure 5 shows the relationship between the amount of laser feedback and the signal level (light utilization efficiency) in the case of an optical system using a polarizing prism and a λ/4 plate (a).
The case of a half mirror is shown in (b). For the same amount of feedback of 20%, a polarizing prism is nearly four times more efficient than a half mirror.

Therefore, it is better to return some light to the laser while using a polarizing prism. For this purpose, the quarter-wave plate should be configured so that the phase does not shift by exactly 90° with respect to the wavelength of the regular laser. For example, a feedback amount of 20% can be created by setting the optical axis of the quarter-wave plate at about 30° with respect to the incident polarization direction, instead of the normal 45° on the axis. This can also be easily achieved by tilting the quarter-wave plate with respect to the incident optical axis.

Next, an embodiment of the present invention is shown in FIG. This embodiment is an example in which the optical axis of the quarter-wave plate is shifted from its original position with respect to the incident polarization direction described above.

(a) is a conventional example. 1/ with respect to the incident polarization direction
The four-wavelength plate 4a forms an angle of 45°. (b) is an example of the present invention, in which the optical axis of the quarter-wave plate 4b is set at 60° with respect to the direction of incident polarization. This deviates from the 90° phase change of the incident light, making it possible to reduce the amount of feedback to the laser to 20%.

Furthermore, instead of using a 1/4 wavelength plate so that about 20% of the 1/4 wavelength plate itself returns to the laser, a plate whose phase shift amount itself is shifted from the 1/4 wavelength may be used.

As described above, the present invention makes it possible to reduce noise caused by changes in the longitudinal mode of the laser by returning part of the light from the laser diode, making it possible to detect signals of higher quality than from the disk. Furthermore, it is possible to provide an optical pickup with excellent light utilization efficiency.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of an optical pickup device, FIG. 2 is a characteristic diagram of the relationship between the amount of laser feedback light and noise according to the present invention, and FIG. 3 is a characteristic diagram of the relationship between the amount of laser feedback light and the number of frame errors according to the present invention. Figure 4 is a diagram showing the relationship between the λ/4 wavelength plate and the optical axis, a is the conventional one, b is the embodiment of the present invention, and Figure 5 is a diagram showing the relationship between the amount of laser feedback light and the signal level. In addition, Fig. 4a shows the case of the embodiment of Fig. 4b, and Fig. 5
Figure b shows the case where a half mirror is used. 1...Laser diode, 2...Coupling lens, 3...Polarizing prism, 4...1/4 wavelength plate,
5... Aperture lens, 6... Disc.

Claims (1)

[Claims] 1. The output light beam of a semiconductor laser with a single longitudinal mode is focused and irradiated onto a disk through a polarizing prism and a quarter-wave plate, and the reflected light from the disk is passed through the quarter-wave plate. Further, in an optical pickup device in which light is reflected by the polarizing prism and picked up by a photodetector, a predetermined amount of light from the reflected light from the disk is transmitted through the polarizing prism and returned to the semiconductor laser. In order to change the longitudinal mode of the semiconductor laser from a single mode to a multi-like mode, the crystal axis of the quarter-wave plate is set at 45° with respect to the direction of incident polarization.
An optical pick-up device characterized by being shifted from a reference angle determined in .