JPS6161450B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for transmitting semiconductor laser light to a microscopic diameter (approximately φ
This relates to a device that optically records and reproduces information such as video and audio with high density, or a device that reproduces already recorded information by focusing the light (1 μm) on a recording medium coated with an optical recording material. be.

In general, semiconductor lasers have the following characteristics: their output light can be directly modulated by electrical signals without using an optical modulator, and they are small compared to other lasers. It is being used as a light source for various devices. For example, research is being conducted on how to use it as a light source for optical communications, taking advantage of the above characteristics. Practical use is also being considered as a light source for playback of known optical video discs.

When a semiconductor laser is used as a light source in the optical recording/reproducing device described above, it is necessary to generate light with a relatively high output power corresponding to the sensitivity of the optical recording material. For example, when using a silver halide film that requires development and fixing processing after light irradiation as an optical recording material, such output power is not required, but real-time recording materials that do not require development and fixing processing, such as metal thin film In the case of using a thin pigment or the like, recording energy of 100 to 1000 mJ/cm ² is required on the recording material, and the output power of the light source is also required to be several tens of mW or more.

Furthermore, in order to satisfy the above recording energy on the optical recording material, it is necessary to make the transmission efficiency of the light as high as possible in the optical system that narrows the output light of the semiconductor laser to a minute diameter. Since the output light of a semiconductor laser is essentially spread light,
Particular consideration must be given to this point.

Furthermore, the beam diameter of the light irradiated onto the optical recording material needs to be narrowed down to the smallest possible beam diameter in order to satisfy the above-mentioned recording energy conditions and to record and reproduce signals as densely as possible. Furthermore, it is necessary to always maintain a minute spot portion having the above-mentioned light beam diameter on the surface of the optical recording material.

The cross-sectional shape of the light beam at the light emitting surface of a high-power semiconductor laser is generally flat, 1 to 2 μm in the thickness direction of the bonded surface and approximately 10 μm in the direction parallel to the bonded surface, in relation to the bonded surface. It is taking shape. In addition, the light beam emitted from it has a fairly large spread angle, for example, ±15° to 25° in the direction perpendicular to the joint surface, and ±15° in the direction parallel to the joint surface.
It has a spread of 5° to 10°, and is similar to general HeNe.
Requires significantly different handling than gas lasers.

One of the conventional examples of an optical system used in an optical video disc is disclosed in Japanese Patent Application Laid-Open No. 129246/1983, and its outline is shown in FIG. 11 is a light source, and a semiconductor laser or a gas laser is used. A thin line I extending from the light source 11 indicates an optical path around the optical axis (indicated by a dashed line). Reference numerals 12 and 13 designate objective lenses. The objective lens 12 once forms an image of the light source at point L, and the aperture objective lens 13 narrows down the light beam that spreads from the image at point L and irradiates it onto the optical recording disk 14. The aperture objective lens 1
The change in the distance between the optical recording disk 14 and the optical recording disk 14 can be determined by the well-known astigmatism method, and an outline of the astigmatism method will be explained below. The reflected light beam from the optical recording disk 14 is again passed through the aperture objective lens 1.
The list will be narrowed down through 3. Reference numeral 15 denotes a beam splitter, and the reflected light beam is separated from the incident optical path by the beam splitter 15 (represented by optical path R) and is focused toward point M. 16 is a convex cylindrical lens whose axis (indicated by the cross in FIG. give. That is, the first
FIG. b shows the optical path R of the reflected light beam portion affected by the cylindrical lens 16. The reflected light beam is once narrowed down toward the N point, then expands again and is irradiated onto the photodetector 17. FIG. 1c (in this case, the axis of the convex cylindrical lens 16 is drawn parallel to the plane of the paper) shows the optical path R of the reflected light beam portion that is not affected by the cylindrical lens 16, and the reflected light beam is transmitted toward point M. However, it is not narrowed down, and in the process, the photodetector 17
is irradiated. Here, if the optical path R in FIG. 1b corresponds to the optical path of the reflected light beam to the upper and lower end portions of the optical path I of the incident light beam, the optical path R in FIG. 2c corresponds to the optical path to the left and right end portions of the optical path I. Equivalent to. When the incident light is accurately focused on the optical recording disk 14 by the aperture objective lens 13, the thickness of the reflected light beam is the same in the direction in which the convex cylindrical lens 16 affects and in the direction in which it does not. A photodetector 17 will be installed. In this case, consider how the shape of the light beam on the photodetector 17 changes as the distance between the aperture objective lens 13 and the optical recording disk 14 changes. When the distance between the aperture objective lens 13 and the optical recording disk 14 becomes closer, the M point moves away from the convex cylindrical lens 16 in the direction shown in FIG. 1c, and the beam on the photodetector 17 becomes thicker. On the other hand, in the direction shown in FIG. 1b, point N is further away from the convex cylindrical lens 16, and the thickness of the light beam on the photodetector 17 becomes thinner. Also, when the distance between the aperture objective lens 13 and the optical recording disk 14 becomes large, the M point also becomes N.
The point also approaches the convex cylindrical lens 16, and the shape of the light beam on the photodetector 17 becomes narrower in the direction of FIG. 1c, and conversely becomes thicker in the direction of FIG. 1b.

The photodetector 17 is shown in Fig. 2, and the photodetector 1 is shown in Fig. 3.
7 shows the shape of the light beam on top. The photodetector 17 is divided into four parts, and each photodetection part is designated as A, B, C, and D as shown in FIG. taken in a direction parallel to the optical axis of In this case, if the distance between the aperture objective lens 13 and the optical recording disk 14 is equal to the distance at which the incident light beam is precisely focused by the aperture objective lens 13, the light beam will take the form shown in FIG. If the aperture 14 approaches the objective lens 13, it will take the shape shown in FIG. 3b, and if it moves away from it, it will take the shape shown in FIG. 3c. If S is the difference between the sum of the amounts of light irradiated to photodetectors A and B and the sum of the amounts of light irradiated to C and D, then S can be positive or negative depending on the distance between the optical recording disk 14 and the objective lens 13. Takes a value.

FIG. 4 shows S relative to the position of the optical recording disk 14.
This shows the value of . The zero point of the optical recording disk 14 is set at a position where the incident light beam is accurately focused by the aperture objective lens 13. The positive direction is defined as the direction in which the optical recording disk 14 and the aperture objective lens 13 move away from each other. A more detailed explanation of FIG. 4 will be given later.

If information is only to be reproduced from an optical recording disk on which information has already been recorded, a low-power semiconductor laser may be used. Some low-power semiconductor lasers have a nearly circular light-emitting surface, and by using the optical system shown in Figure 1, it is possible to create a roughly circular light spot on an optical recording disk and obtain a high-quality reproduction signal. . However, in order to record and reproduce information in real time, high optical power is required as described above, and high-power semiconductor lasers generally have an oval light emitting surface. In order to focus the optical system shown in Figure 1 into a minute, approximately circular light spot (for example, 1 μm in diameter) on the surface of the optical recording disk, the geometrical optical oval image inevitably becomes approximately circular due to light diffraction. It requires extremely high magnification. The geometrical optical image in the optical system of FIG. 1 is analogously (fh/eg) times the size of the emission surface. In order to achieve good aperture in terms of wave optics, it is necessary to increase the numerical aperture of the aperture objective lens 13, and in terms of geometric optics, h cannot be increased to obtain a small image.
The aperture radius of the aperture objective lens 13 is limited.
Therefore, in order to narrow down the aperture, it is necessary to increase g, which increases the vignetting of light at the aperture objective lens 13 and reduces the light transmission efficiency. In order to improve the transmission ratio, f can be increased to reduce the spread of the light beam at point L, but in this case, the geometrical optic aperture image becomes large.

As described above, the conventional optical system cannot concentrate high optical power onto a minute portion on the surface of an optical recording disk, and is therefore unsuitable for real-time recording and reproduction of information.

The present invention is a compact optical device that can focus a high-power light beam to a minute spot on the surface of an optical recording disk, and that can always maintain the minute spot portion on the surface of the optical recording disk. The present invention provides a recording/playback device.

An embodiment of the present invention will be described above with reference to FIG. Components that are the same as those in FIG. 1 are given the same numbers. In FIG. 5, reference numeral 11 denotes a high-output semiconductor laser, and a thin line I indicates an optical path around the front axis (indicated by a chain line). FIG. 5a is a plan view showing a direction perpendicular to the bonding surface of the active layer of the semiconductor laser 11, while FIG. 5b is a side view showing a direction parallel to the bonding surface. 12 is a condensing lens, which collects the light spreading from the light source 11 and
It is assumed that the parallel light is thick in the direction shown in Figure 5a and thin parallel light in the direction shown in Figure 5B. 22 and 23 are concave and convex cylindrical lenses, respectively, and by aligning the focal points of the two cylindrical lenses 22 and 23 (represented by point F), the light of the semiconductor laser, which generally has a rectangular light emission form, is The parallel light beam is expanded in the direction shown in FIG.

In this way, two cylindrical lenses 22, 2
3, it is possible to create an approximately circular or approximately square image and concentrate high optical power in a minute area, and furthermore, the optical beam narrowed by the aperture objective lens 13 can record and reproduce information in real time. The light is irradiated onto the optical recording medium 21 that can be used. The optical recording medium 21 is formed by directly modulating the optical output of the semiconductor laser 11 according to the information to be recorded, and is irradiated with a further narrowed light beam, so that its optical properties can be locally changed according to the information to be recorded. , information is recorded as minute bits. To reproduce information, a light beam with a power weaker than that used during recording is irradiated onto the optical recording medium 21 on which information has already been recorded, and the reflected light from the optical recording medium 21 is caused by the difference in optical properties in the form of minute pits. This is done by sensing changes.

During recording, as mentioned above, it is necessary to concentrate high optical power into a minute spot, and for this purpose, the most narrowed part of the light beam is irradiated onto the optical recording medium 21 by the aperture objective lens 13. In other words, it is necessary to keep the distance between the optical recording medium 21 and the objective lens 13 constant so that the light is focused on the optical recording medium 21. For this purpose, a method using astigmatism caused by the cylindrical lens described above is used.

The light beam reflected from the optical recording medium 21 passes through the aperture objective lens 13 and the convex cylindrical lens 2.
3 and is separated from the incident optical path by a beam splitter 15. The separated reflected light beams are focused by a convex lens 24 and irradiated onto a photodetector 17. In the direction of FIG. 5a, the reflected light beam (represented by optical path R) is reflected by the convex lens 2 as shown in FIG. 5c.
4, but in the direction of FIG. 5b, the reflected light beam is previously focused by both the convex cylindrical lens 23 and the convex lens 24,
The aperture focal length differs in the directions a and b in FIG.
Therefore, the shape of the light beam on the photodetector 17 changes in the same way as in the optical system shown in FIG.
3 and the optical recording medium 21 is exactly the focal length of the aperture objective lens 13, a circular light spot is irradiated onto the photodetector 17 as shown in FIG. 3a. If the directions A and B of the photodetector 17 in FIG. 2 (indicated by the dashed-dotted line) are installed parallel to the optical axis in FIG. 5b,
If the aperture objective lens 13 and the optical recording medium 21 are brought closer together, the shape of the light beam on the photodetector 17 will be as shown in FIG.
If S is the difference between the sum of the amounts of light irradiated on A and B and the sum of the amounts of light irradiated on C and D, then the optical recording medium 21
The change in S depending on the distance between the aperture and the objective lens 13 is as shown in FIG. As described above, the photodetector 1
The signal S obtained in step 7 is led to the terminal J, and the objective lens 13 is adjusted by the objective lens driving device 25 so that the value of S becomes zero. When the value of S is positive, the aperture objective lens 13 is brought closer to the optical recording medium 21, and when the value of S is negative, it is moved away. As can be seen from FIG. 4, when the distance between the aperture objective lens 13 and the optical recording medium 21 exceeds point T, the distance increases, and focus control becomes impossible beyond point T. Further, the sum of all the light amounts of the photodetectors A, B, C, and D can be used as an information reproduction signal.

In FIG. 1, the axis of the cylindrical lens 16 (perpendicular to the optical axis and indicated by the x mark as described above) is aligned with the direction of A and B of the photodetector 17 (indicated by the dashed line).
It is necessary to install the lens perpendicularly or parallel to the lens (in FIG. 1, the perpendicular case is shown as an example).In order to control the focus using the astigmatism method, at least one cylindrical lens is required. Since the optical system of the present invention uses two cylindrical lenses to improve the aperture performance of the semiconductor laser, the beam splitter 15 inserted between the two cylindrical lenses separates the reflected light from the incident optical path. Focus can be controlled by using a convex lens in the optical path. Unlike a cylindrical lens, a convex lens is symmetrical with respect to the optical axis, so it can be installed without paying attention to the directions A and B of the photodetector 17.

As described above, the present invention uses a high-output semiconductor laser to provide an optical system that can convert high optical power into a minute optical spot and focus it on an optical recording medium, and in addition, in the reflected optical path, There is no need to use a cylindrical lens with magnification in one direction, and the distance between the aperture objective lens and the optical recording medium can be kept constant by simply installing a convex lens without paying attention to the rotation direction with respect to the photodetector. has.

[Brief explanation of the drawing]

Figures 1 a to c show the configuration of a conventional example of an optical system that controls focus using an astigmatism method, where a is a plan view, b is a side view, c is a view taken along arrow A-A in b, and Figure 2 is a 4 A plan view of the divided photodetector, FIG. 3 is an explanatory diagram showing the shape of the light beam on the photodetector, and FIG. 4 is an aperture objective lens and an optical recording medium for the signal obtained from the photodetector. FIGS. 5a to 5c are block diagrams of the optical system of the present invention, in which a is a plan view, b is a side view, and c is a view taken along the line B-B of b. 11...Semiconductor laser, 22, 23...Concave and convex cylindrical lenses, 13...Aperture objective lens, 15...Beam splitter, 17...Photodetector, 21...Optical recording medium, 24...Convex lens.

Claims (1)

[Claims] 1. In an apparatus for recording and reproducing information by focusing a minute light beam onto a recording medium, a first optical means includes a semiconductor laser as a light source and condenses light from the semiconductor laser to generate parallel light; a second optical means for converting the parallel light into substantially circular or square parallel light having a width wider than the incident parallel light using at least two cylindrical lenses; and a recording medium for converting the substantially circular or square parallel light to a recording medium. a third optical means that apertures the recording medium as a minute light spot; a beam splitter installed between the two cylindrical lenses; and a beam splitter that directs the reflected light from the recording medium to the beam splitter. a photodetector disposed in a reflected optical path separated from an incident optical path by a convex lens between the beam splitter and the photodetector; and a distance between the recording medium and the third optical means. An optical recording/reproducing apparatus characterized in that the distance is controlled to be constant by detecting the change in shape of light on the photodetector.