JPS6136298B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for optically reading information recorded on a reflective information recording carrier, and more specifically to an optical method for reading information using a semiconductor laser as a light source and utilizing its self-coupling effect. This relates to a method for reading digital information.

He-Ne is used as a light source for optical information reading devices.
A gas laser like a laser was used,
Since gas lasers are large and expensive, attempts have been made to improve the above drawbacks by using semiconductor lasers as light sources. Furthermore, in semiconductor lasers, it is known that when the emitted laser light is reflected from the outside and enters the active layer of the semiconductor laser again, the optical and electrical characteristics of the semiconductor laser change. This phenomenon is called the self-coupling effect of semiconductor lasers, and it has been proposed to apply this phenomenon to optical information reading. This is discussed in the magazine ``Optics Communications''.
``Self-Cut Pulled'' by Mitsuhashi et al.
Optical pickup (Self-coupled)
opticab pickup)

Conventionally, two methods have been proposed for reading information using the self-coupling effect of semiconductor lasers. The first method is to detect changes in the voltage or current of a semiconductor laser due to the intensity of reflected light. In this case, since the information is extracted as an electrical signal from the beginning, it has the advantage of not requiring a photodetector such as a photo diode at all, but the voltage and current changes due to the strength of the feedback light are very small. , there is a drawback that a sufficient S/N ratio cannot be obtained.

On the other hand, the second method uses a photodetector such as a photodiode to detect changes in the optical output of the semiconductor laser due to the intensity of reflected light.
In this case, the S/N ratio is improved compared to the first method, but it is still not sufficient. In addition, the magnitude of changes in the voltage, current, and optical output of the semiconductor laser due to the strength and weakness of the feedback light will not increase beyond a certain maximum value even if the driving current of the semiconductor laser is increased to increase the optical output. Both the first and second methods have the drawback that they cannot be said to effectively utilize the optical output of the laser.

The purpose of the present invention is to eliminate the above-mentioned drawbacks,
An object of the present invention is to provide an optical information reading method that has a good S/N ratio and can effectively utilize the optical output of a semiconductor laser.

The present invention detects a change in the light intensity of a specific mode after a change in the light intensity of the semiconductor laser, which is caused by irradiating an information recording carrier with emitted laser light from a semiconductor laser and returning this reflected light to the active layer of the semiconductor laser. It is something to do.

Hereinafter, the present invention will be explained in detail with reference to the drawings. FIG. 1 is a block diagram of an experiment conducted to observe the self-coupling effect of the semiconductor laser used in the present invention. A laser beam 3 emitted from a semiconductor laser 2 driven by a power source 1 is transmitted to a converging optical system 4.
As a result, the light is converged so as to be focused on a reflecting mirror 5 placed at a distant point in front of the semiconductor laser 2 and the converging optical system. The laser light reflected by the reflecting mirror 5 passes through the converging optical system 4 again and returns to the active layer of the semiconductor laser 2 as a feed back light 6. On the other hand, a part of the laser light is guided to a photodetector 8 by a beam splitter 7 placed between the reflecting mirror 5 and the converging optical system 4, and the optical output of the semiconductor laser 2 is determined depending on the presence or absence of the feedback light 6. changes are observed. Note that a change in the optical output from the other emission end facet of the semiconductor laser 2 may be detected. FIG. 2 shows changes in the relationship between the semiconductor laser light output L and the current I depending on the presence or absence of the feedback light, as measured by the system shown in FIG. According to this, it can be seen that the change in the semiconductor laser light output due to the presence or absence of the feedback light is greatest near the threshold, and does not become larger even if the semiconductor laser light output is increased. Furthermore, the magnitude of the change is not sufficient. On the other hand, FIG. 3 shows the change in the oscillation longitudinal mode depending on the presence or absence of the feedback light at point A in FIG. 2. At point A, while the change in the intensity of the total optical output of the semiconductor laser due to the feedback light is small, there is a large change in the light intensity of each oscillating longitudinal mode. (Figure 3b) In other words, when there is no feedback light, it oscillates in a state close to a single longitudinal mode (Figure 3a).
The number of oscillation longitudinal modes increases due to the feedback light, and the light intensity of each longitudinal mode is averaged. Such a phenomenon was always observed stably in the current range above the threshold. Therefore, a specific one of the oscillation longitudinal modes of the emitted laser light of the semiconductor laser
By observing the light intensity of one or several longitudinal modes,
Large changes in light intensity can be obtained by adjusting the strength of the feed back light. Furthermore, since this change in light amount is proportional to the optical output of the semiconductor laser, the optical output of the semiconductor laser can be used effectively. The present invention applies this phenomenon to an optical information reading method, whereby a very good S/N ratio can be obtained and the optical output of a semiconductor laser can be used effectively.

FIG. 4 is a diagram showing an embodiment of the optical information reading method according to the present invention, and is an example applied to reading a light reflective video disc. Emitted laser light 3 from a semiconductor laser 2 driven by a power source 1 is focused by a focusing optical system 4 onto a disk 10 on which information to be read is recorded. The laser light reflected by the disk 10 passes through the converging optical system 4 again, becomes feedback light 6, and returns to the active layer of the semiconductor laser 2. The emitted laser beam 11 from the other emission end facet of the semiconductor laser 2 is transmitted to a wavelength separator 12.
As a result, only a part of the longitudinal mode light 13 out of the many oscillation longitudinal modes is extracted, and a change in the light intensity is detected by the photodetector 14 and extracted as an electrical signal to the output terminal 15. The intensity of the feedback light 6 is modulated by the information pattern recorded on the disk 10, and as a result, the characteristics of the semiconductor laser 2 change, and this change is output as an electrical signal to the output terminal 15. Since changes in the light intensity of only a portion of the longitudinal mode light 13 of the longitudinal modes are detected, a very large change in light intensity can be extracted, a good S/N ratio can be obtained, and the semiconductor laser light output can be used effectively. It becomes possible. Here, as the wavelength separator 12, a device that spatially separates wavelengths such as a grating or a prism may be used, and a portion thereof may be taken out with a slit, or an optical filter or an etalon may be used. . In this example, application to a light-reflecting information recording carrier such as a light-reflecting video disk was shown.
In the case of a light transmission type information recording carrier, if a reflecting mirror is installed behind the information recording carrier, it can be equivalently regarded as a reflective type information recording carrier and can be read in the same manner as in this embodiment.

As explained in detail above, according to the present invention, in an optical information reading method that utilizes the self-coupling effect of a semiconductor laser, it is possible to greatly improve the S/N ratio and effectively utilize the semiconductor laser light output. .

[Brief explanation of the drawing]

1, 2, and 3 are diagrams for explaining the self-coupling effect of a semiconductor laser, and FIG. 4 is a diagram showing an embodiment of an optical information reading device according to the present invention. In the figure, 1 is a power supply, 2 is a semiconductor laser, 3 is a radiation laser beam, 4 is a converging optical system, 5 is a reflecting mirror, 6
is the feed back light, 7 is the beam splitter,
8 and 14 are photodetectors, 10 is a disk, 12 is a wavelength separator, 13 is a part of longitudinal mode light of the laser oscillation longitudinal mode, and 15 is an output terminal.

Claims (1)

[Claims] 1. Irradiating an information recording carrier with a laser beam emitted from a semiconductor laser, feeding back the laser beam reflected by the information recording carrier to an active layer of the semiconductor laser, and detecting a change in the characteristics of the semiconductor laser. In an optical information reading method for reading information recorded on the information recording carrier,
An optical information reading method comprising separating and extracting only a part of the longitudinal oscillation modes of the semiconductor laser and detecting changes in light intensity thereof.