JPS6256581B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical information reproducing device using a semiconductor laser as a light source, and in particular uses a semiconductor laser with a flat emitted light beam to condense a light spot on the micron order. be.

A conventional device of this type is shown in FIG. In the figure, 1 is a semiconductor laser, 2 is an emitted light beam from the semiconductor laser with a flat cross-sectional shape, 3 is a collimating lens that transforms the diffused flat emitted light beam into parallel light 2a with a flat cross-sectional shape as shown, and 4 is a polarized light beam. Beam splitter, 5 is a quarter wavelength phase plate,
Reference numeral 6 denotes a tracking actuator, which is composed of a rotating shaft 7 that rotates in the direction of arrow A in response to a tracking signal, and a mirror 8 fixed to this shaft, and changes the angle of incidence of the light beam 2b entering the condenser lens 9, thereby changing the angle of incidence of the light beam 2b. Tracking is corrected by displacing the position of the spot 10 in the direction of arrow B. 11 is an information recording medium containing a video signal, PCM audio signal, etc.; 12 is a pit constituting an information unit of the information recording medium; and 13 is an information track consisting of a row of pits, for example 1.7μ.
It is recorded in m pitches. 14 is a motor that rotates the information recording medium 11; 15 is an autofocusing actuator that moves the condensing lens 9 in the optical axis direction C; 2c is a reflected light beam containing the information signal read from the information recording medium; 17 is a reflected light beam; This is a photodetector that converts the luminous flux 2c into an electrical signal. Figure b is a diagram showing the relationship between the effective diameter of the condenser lens 9 and the size of the light beam 2b incident thereon. It is composed of Note that the size of the luminous flux 2b is defined as the size of the point where the maximum intensity at the center is 1/e ² .

Next, the operation will be explained. An emitted light beam 2 with a flat cross section emitted from the semiconductor laser 1 is converted into flat parallel light 2a by a collimating lens 3, transmitted through a flat beam splitter 4 and a quarter wavelength phase plate 5, and reflected by a mirror 8. and condensing lens 9
The light passes through the light beam, becomes a flat light spot 10, is irradiated onto an information track 13, and is reflected. The intensity of this reflected light, which passes through the condenser lens 9 again, differs depending on whether it is irradiated onto the pit 12 on the information track 13 or when it is irradiated onto a location other than the pit.
In the light spot 10 of the information recording medium, the spot diameter in the direction parallel to the major axis direction of the polarized light beam 2b incident on the condenser lens 9 is smaller than the spot diameter parallel to the minor axis direction. For example, at a wavelength of 0.78 μm,
When using a semiconductor laser with a ratio of major axis to minor axis of 4 and a condensing lens with a numerical aperture of 0.5, and making the size of the flat beam in the major axis direction equal to the effective diameter of the condensing lens, the Kirchhoff diffraction integral is When the spot diameter is calculated using this method, the spot diameter in the direction parallel to the major axis direction of the flat light beam is 1.29 μm, and the spot diameter in the direction parallel to the short axis direction of the flat light beam is 4.0 μm.

The light 2c reflected by the information recording medium 11 is focused by a condenser lens 9, passes through a mirror 8 and a quarter wavelength phase plate 5, is separated by a polarizing beam splitter 4, and enters a photodetector 17. The video signal is converted into an electrical signal and read out from this signal.
PCM audio signals etc. are extracted. This device is equipped with an auto-focusing sensor (not shown, omitted) that detects the positional deviation of the focal point (minimum focus position) of the condenser lens from the information recording medium surface. Output is auto focusing actuator 1
5, and controls the distance between the condenser lens 9 and the information recording medium 11 so that the focal point of the condenser lens is always located on the surface of the information recording medium. This device also has a light spot 10 from above the information track 13.
The output of the auto-tracking sensor is applied to the auto-tracking actuator 6 so that the light spot 10 is constantly positioned on the information track 13. controlled by.

Since the conventional device is configured as described above, compared to the case where the output light beam 2 is an isotropic circular output beam condensing the output light from a light source (for example, a circular beam with a wavelength of 0.78 μm, When condensing light with a condensing lens with a numerical aperture of 0.5, assuming that the effective diameter of the condensing lens is equal to the diameter of the circular beam, the calculated value of the spot diameter based on Kirchhoff's diffraction integral is 1.42 μm.) Condensing lens The spot diameters in the direction parallel to the major axis direction of the incident light beam are approximately equal as calculated above, but the spot diameters in the direction parallel to the minor axis direction are much larger. Therefore, as shown in FIG.
When the light is focused so that the longitudinal direction of the track is aligned, the ratio of simultaneous reproduction of adjacent tracks becomes higher than when reproduction is performed by isotropic light concentration. In other words, the crosstalk characteristics deteriorate. Furthermore, if the light is focused so that the longitudinal direction of the light spot is parallel to the information track, the spatial frequency characteristics will deteriorate compared to the case where reproduction is performed using isotropic light. There were other drawbacks.

This invention was made in order to eliminate the drawbacks of the conventional ones as described above, and the size of the flat light beam 2b incident on the condenser lens in the long axis direction is made larger than the effective diameter of the condenser lens. By reducing the size of the light spot and oriented the long axis direction parallel to the information track 13, crosstalk characteristics and spatial frequency characteristics are improved.

An embodiment of the present invention will be described below with reference to FIG. In this example, the size of the collimating lens 3a in the major axis direction of the incident light beam 2b is such that the size of the collimating lens 3a in the long axis direction is set to the condensing lens 9 as shown in FIG.
is larger than the effective diameter of the optical spot 10, and the major axis direction of the incident light beam 2b is parallel to the information track 13, and as a result, the major axis direction of the optical spot 10 is perpendicular to the information track 13, as shown in FIG. 2c. Configure. For example, when using a semiconductor laser with a wavelength of 0.78 μm and a ratio of major axis to minor axis of 4 and a condenser lens with a numerical aperture of 0.5, the size of the incident light beam 2b in the minor axis direction is made equal to the effective diameter of the condenser lens. , Kirchhoff's diffraction integral is used to calculate the spot diameter, the spot diameter in the direction parallel to the major axis direction of the incident light beam 2b is 1.24 μm, and the spot diameter in the direction parallel to the minor axis direction is 1.48 μm.

Note that when the optical system is configured as described above, a large amount of light escapes to the outside of the condenser lens. For example, if a semiconductor laser with a ratio of major axis to minor axis of 4 is arranged so that the effective diameter of the condenser lens is equal to the minor axis, the effective power condensed by the condenser lens is calculated as follows: At 33%, the remaining 67% is invalid power. However, many recent semiconductor lasers can produce a power of 3 mw or more, and as a result, in this example, an effective power of 1 mw or more can be obtained. For such an information reproducing device, if the power is 1mW,
Usually sufficient. Therefore, even if some power escapes to the outside of the condenser lens, the normally required laser power can be used. Note that the larger the laser power is, the more light can escape to the outside of the condenser lens, and the size of the incident light beam 2b can be made larger than the effective diameter of the condenser lens. As a result, a small spot diameter can be achieved.

Note that the other configurations are the same as the conventional device.

As described above, the present invention is configured so that the size of the flat light beam incident on the condenser lens in the major axis direction, or in the major axis direction and the minor axis direction, is made larger than the effective diameter of the condenser lens. Using a condensing lens with the same numerical aperture, the present invention can condense light into a smaller light spot than the conventional device, and has the effect of significantly improving spatial frequency characteristics and crosstalk characteristics.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a conventional optical information reproducing device, and FIG. 2 is a diagram showing an embodiment of the present invention. In the figure, 1 is a semiconductor laser, 2 is an emitted light beam, 2b is an incident light beam, 2c is a reflected light beam, 3 is a collimating lens, 4 is a polarizing beam splitter, and 5 is a polarizing beam splitter.
1/4 wavelength plate, 8 is tracking actuator, 9
10 is a condensing lens, 10 is a light spot, 11 is an information recording carrier, 13 is a recording track, and 17 is a photodetector. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A semiconductor laser with a flat emitted light beam, an information recording medium, a condensing lens for condensing the emitted light beam from the semiconductor laser onto an information track on the information recording medium, and a condensing lens for condensing the emitted light beam from the semiconductor laser onto an information track on the information recording medium. and an optical system that guides the light to the condenser lens, the flat light beam incident on the condenser lens is arranged so that the major axis direction of the flat light beam incident on the condenser lens is parallel to the information track of the information recording medium. An optical information reproducing device in which the optical system is configured such that the size of the light beam in the short axis direction is larger than the effective diameter of the condensing lens.