JPS59110048A - Reproducing device for semiconductor laser information - Google Patents

Abstract

PURPOSE:To make the influence of reflected light feedback hard to exert on the titled device and to suppress noise generation by decreasing the numerical aperture of a coupling lens or increasing a laser light output. CONSTITUTION:The noise generation originating the feedback of reflected light is suppressed by reducing the numerical aperture of the coupling lens 2 to <=0.15 and/or setting the light output of a laser 1 to >=3mW on a one-side projection end surface, obtaining a high S/N. The numerical aperture of the lens is equal to sintheta in a figure. There are two reasons that the influence of the reflected light feedback is made hard to exert on the device. Firstly, the NA of the coupling lens determines the efficiency of conversion from semiconductor laser light to a parallel beam. Secondly, the optical spot diameter of the reflected light when returning onto the laser projection end surface, the larger the NA, the smaller the rate of entrance in the active layer of the semiconductor laser.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an optical information reproducing device using a semiconductor laser as a light source, and particularly relates to an optical disk device such as an optical video disk that requires a high signal-to-noise ratio. .

[Prior art]

Optical video disks that use laser light to reproduce video information from a rotating disk are already commercially available as products that use a T-T e Ne laser. By using a semiconductor laser as a light source, it is expected that the device will be smaller, lighter, and less expensive, and therefore its development is active. However, due to optical output fluctuations of the semiconductor laser, that is, laser noise, there is a problem that the reproduced image quality is inferior to that when a HeNe laser is used.

[Purpose of the invention]

An object of the present invention is to provide a semiconductor laser information reproducing device that suppresses the laser noise of the semiconductor laser and can reproduce information with high SN.

[Summary of the invention]

Semiconductor laser noise occurs when laser light is reflected by an external optical system and the reflected light is fed back to the laser. In the present invention, by reducing the numerical aperture of the coupling lens or increasing the laser light output,
This suppresses noise generation due to the influence of reflected light feedback.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. Light emitted from the semiconductor laser 1 is collimated into a parallel beam by a coupling lens 2 and focused onto an information storage medium 4 by an objective lens 3.

The light intensity of the reflected light from the information storage medium 4 is modulated according to the information. A wave plate 6 and a polarizing prism 7 direct the reflected light beam to a photodetector 5 where it is converted into an electrical signal. If the laser light output fluctuates, it is detected as noise in the reproduced signal, which causes a deterioration in the reproduced image quality in the case of a video disc. Laser noise is caused by a portion of the reflected light from the information storage medium 4 being returned to the laser 1 without being guided to the photodetector 5.

The noise level is determined not only by the reflected light feedback rate.
Depends on temperature, laser light output, etc.

The present invention has the configuration shown in FIG. 1, and the numerical aperture of the coupling lens 2 is
By making the output power smaller than 0.15 and/or making the optical output of the laser 1 larger than 3 mW on one emission end face, noise generation due to reflected light feedback is suppressed and a high SN is achieved. Here, the numerical aperture of the lens (Nu
mericalAparture, NA) is the first
There is a relationship between θ and NA”!, linθ in the figure.

Next, the effect of noise suppression will be shown using actual measurement data.

Figure 2 shows the dependence of laser noise (frequency 5MHz) on laser light output using the NA of the coupling lens as a parameter.
P (Modified Channel 13 tripe
dT'1anar) type laser noise reflected light feedback rate is 1%, and the width is 27tl'. In the figure, the horizontal axis shows the laser light output on one end facet, and the vertical axis shows the laser noise RI N (R,el
active Intensity No.1se),
It is shown by . The reflected light feedback rate is shown in Figure 1.
The figure shows 100% when the wavelength plate 6 and polarizing prism 7 are removed and the ratio of light guided to the photodetector is O. Figure 2 shows that the smaller the NA of the coupling lens and the larger the laser light output, the less the effect of reflected light feedback.
< indicates that noise generation is small. In addition, in devices that require high SN playback such as optical video discs, RIN < 10-14 is required, and in the case of Figure 2, NA (0.15° optical output > 3 mW is satisfied). .

Laser noise also depends on temperature. FIG. 3 shows the temperature change of laser noise. Using NA as a parameter, the optical output is adjusted so that the DC output level of the photodetector is constant as shown in FIG.

In this example, under the condition of a reflected light feedback rate of 1%,
NA=0.10. Optical output = 5.8mW and always 'fLIN
<10-14, NA=0.15. 1 at optical output = 3mW
0"5 (RIN<10", NA=0, 19, optical output=2.5 mW, RIN>10-13 is always satisfied.

FIG. 4 shows the worst noise level in the temperature range of 20 to 50 tT for reflected light feedback.

Reflected optical feedback of ~1% is unavoidable in devices such as optical video discs due to the birefringence of the disc. Therefore, as shown in Figure 4, NA=0.10
．． If the optical output is 5.8 mW, IITN<1o-' under the conditions of temperature 20~50C1 reflected light feedback 41%
4 is fully satisfied. However, NA=0.19. When the optical output is 2.5'mW, the reflection feedback becomes 0.2% or more and RIN>10-14. NA=0.15.
At an optical output of 3 mW, R,IN<10-14 is satisfied when the reflection feedback is 0.7% or less, but RIN>10-14 is satisfied when the reflection feedback is 1% or more.

Therefore, 1% for reflected light feedback.

RI N<10-1 within the temperature range of 20C to 50tZ'
In order to always satisfy 4, NA<0.15. light output)
It is necessary to set it to 3mW.

The reason why it becomes less susceptible to the influence of reflected light feedback by reducing the NA of the coupling lens or increasing the laser light output will be explained.

First, the NA of the coupling lens primarily determines the conversion efficiency from the semiconductor laser to a parallel beam. That is, a semiconductor laser emits diverging light with a spread angle of θ=15 to 30°, but the smaller the NA, the more likely only a part of the light is used. This utilization efficiency is proportional to (NA)2. Furthermore, the optical spot diameter when the reflected light beam from the storage medium returns onto the laser emitting end face via the coupling lens is N.
The smaller A is, the larger it is, and the smaller the proportion of it entering the active layer of the semiconductor laser. The proportion of the returned light spot that enters the laser resonator is proportional to (NA)2. For the above two reasons, it is considered that the substantial feedback rate that governs laser noise generation is proportional to the fourth power of the coupling lens NA. Thus, for example, NA=0.15 and NA=0.1
0, the influence of reflected feedback is different by (0, 1510, 10)' = 5 times. By reducing the NA in this way, it is possible to suppress noise generation due to the influence of reflected light feedback, but as mentioned above, the smaller the NA, the lower the light utilization efficiency. However, if the NA is made too small, there will be a shortage of one laser beam, and it will take only millimeter to read out the signal from the disk. In order to avoid this insufficient amount of light, it is necessary to increase the optical output of the semiconductor laser, but there is also a limit to the optical output of the semiconductor laser, and the oscillation transverse mode must be kept stable as the fundamental mode and the lifetime must be guaranteed. The limit for the optical output is about 10 mW. 1/-The NA of the coupling lens is required to be 0.08 or more in order to obtain a view necessary for stably reading signals from the disk when the optical output is 10 mW. Therefore, the NA of the coupling lens is 0.08≦NA
The range is preferably 5, more preferably 1.0≦NA≦1.3.

Next, regarding the laser light output, if there is no reflected light feedback, the larger the light output, the smaller the noise. This is because the spectral linewidth of single mode oscillation becomes narrower and the phase fluctuation becomes smaller. In other words, it is thought that because the single mode property is strengthened, multimode oscillation is less likely to occur due to reflected light feedback, and therefore noise is less likely to occur. It should be noted that the higher the optical output of the semiconductor laser, the better, but as mentioned above, the optical output that keeps the oscillation transverse mode stable as the fundamental mode is currently about 10 m.
Since W is the limit, it is preferable to use it within this range from the viewpoint of reliability and life of the element, and it is more preferable to use it within the range of 3.5 mW or more and 6 mW or less.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to suppress the generation of noise due to feedback of reflected light from a semiconductor laser, thereby making it possible to reproduce optical information with high SN.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, and FIG. 2 is a diagram showing an embodiment of the present invention. FIG. 3 and FIG. 4 are diagrams for explaining the present invention. (9) 1... Semiconductor laser, 2... Coupling lens, 3...
Objective lens, 4... Information storage medium, 5... Photodetector, 6... Wave plate, 7... Polarizing prism. Agent Patent Attorney Toshiyuki Usuda (10) r 1 Ward 2nd Loop “Hikari” (8w) 'fIt Figure

Claims (1)

[Claims] 1. A semiconductor laser, an information storage medium, an optical system having a coupling lens and an objective lens, and guiding a light beam from the semiconductor laser to the information storage medium, and reflection from the information storage medium. 1. A semiconductor laser information reproducing device comprising a photodetector that receives light, wherein the coupling lens has a numerical aperture of 0.08 or more and less than 0.15. 2. The optical output of the semiconductor laser is 3 on one side of the output end face.
2. The semiconductor laser information reproducing device according to claim 1, wherein the semiconductor laser information reproducing device is larger than mW and is 10 mW or less.