JPS637950Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention irradiates concentric circles or spiral tracks on a rotating disk with light from a light source as minute light spots, records information as a series of pits, and records information in the same manner. The present invention relates to a focus detection device for detecting out-of-focus between a light spot and a medium in an optical information recording and reproducing device for reproducing recorded information.

In recent years, the technology of optically reproducing information recorded as a series of minute pits in a concentric or spiral pattern on a disk-shaped recording medium (hereinafter referred to as the medium) has progressed, and this has been put to practical use as video disks, audio disks, etc. In addition, the development of optical disk memory devices that use the same technology to record as well as reproduce information and are used as file memories, etc. is also progressing. In these devices, the convergent beam diameter of the laser light used for recording and reproduction and the size of the recording pits are made very small, at 1 to 2 μm or less, making it possible to record and reproduce information at high density. For this reason, a servo mechanism is indispensable and very important for constantly irradiating the medium with a light spot in focus.

Conventionally, the following three methods have been considered as typical methods for detecting this defocus.

1. Astigmatism method 2. Asymmetry method 3. Critical angle method 1) inserts a lens and a cylindrical lens in succession into the optical path of the reflected light from the medium to give astigmatism to the reflected light. A four-part detector is placed to detect changes in the pattern on the detector due to focus shift. Reference numeral 21 converges the reflected light from the medium using a lens, asymmetrically masks the middle of the converged beam with a knife edge, etc., and detects a focus shift using a two-split detector placed near the convergence point.
Although these methods are sufficiently practical and have already been applied to various devices, they essentially require the use of a lens to converge the reflected light, resulting in a large number of parts and the need for lenses. It is not suitable for miniaturization because it requires an optical path length equal to the focal length of .

On the other hand, in order to eliminate the above drawback, 3) detects the focus shift without converging the reflected light.
This is the critical angle method. This critical angle method will be explained with reference to the drawings. FIG. 1 is a diagram for explaining one embodiment of a focus detection method using the critical angle method. Here, for convenience of explanation, an example using a semiconductor laser is shown. A linearly polarized laser 2 from a semiconductor laser 1 is collimated by a collimator lens 3, passes through a polarizing beam splitter 4, a λ/4 wavelength plate 5, becomes circularly polarized, and is focused onto a disk-shaped medium 7 by an objective lens 6. be done. The reflected light from the disk-shaped medium 7 is again passed through the objective lens 6 and the λ/4 wavelength plate 5.
The reflected light 8 is converted into a polarization state orthogonal to the incident state by the polarization beam splitter 4.
is extracted as. Here, when the disk-shaped medium 7 is in a focused state, a prism 9 is inserted into the optical path so that its reflecting surface 9a forms a substantially critical angle with the reflected light 8. When the disk-shaped medium 7 is in a focused state, the reflected light 8 is almost uniformly incident on the reflective surface 9a at a critical angle, so that the light intensity distribution of the totally reflected light 10 by the reflective surface 9a is almost uniform. When the disk-shaped medium 7 enters a defocus state in this state, the reflected light 8 enters a convergence state according to the amount of defocus and enters the reflection surface 9a. Therefore, in the light rays above the central axis 8a of the reflected light 8 in FIG. On the other hand, of the reflected light 8, the light rays below the central axis 8a are totally reflected because the angle of incidence on the reflecting surface 9a is even larger than the critical angle. Therefore, the light intensity distribution of the totally reflected light 10 in FIG. 1 is bright on the right side and dark on the left side. Conversely, when the disk-shaped medium 7 approaches the objective lens 6 side from the focused state, the light intensity distribution of the totally reflected light 10 will be dark on the right side and bright on the left side, contrary to the above case.

Therefore, if a change in the light intensity distribution of the totally reflected light 10 is detected using, for example, a two-divided detector divided into left and right halves, a defocus can be detected from this. In this way, the critical angle method has the great advantage of being able to detect defocus using parallel light without requiring any converging lenses, but in order to improve detection sensitivity, it is necessary to increase the number of reflections. However, this has the drawback that increasing the number or length of the prisms results in an increase in the overall size.

An object of the present invention is to eliminate the above-mentioned drawbacks and to provide a defocus detection device that does not require a converging lens and allows the entire system to be made compact while maintaining high sensitivity.

The defocus detection device according to the present invention includes a light source, a lens system that converges the light emitted from the light source onto an irradiated object, and a splitter that is inserted in the middle of the lens system and extracts reflected light from the irradiated object. and at least one pair of lenses installed at symmetrical positions with respect to the central optical axis within a plane including the central optical axis of the reflected light from the irradiated object extracted by the splitter, with their incident surfaces tilted in the same direction by a predetermined angle. The illumination target of the lens system is composed of a waveguide element having a light receiving angle limit of This device is characterized by being configured to detect out-of-focus on an object.

The present invention will be explained in detail below with reference to the drawings. Figure 2 shows one of the defocus detection devices according to the present invention.
It is a figure which shows an example. Again, for convenience of explanation, a disk-shaped medium recording/reproducing device using a semiconductor laser is taken as an example.

Linearly polarized laser light 2 from semiconductor laser 1
is collimated by a collimator lens 3, passes through a polarizing beam splitter 4 and a λ/4 wavelength plate 5, becomes circularly polarized, and is irradiated onto a disk-shaped medium 7 as a minute spot by an objective lens 6. The laser beam reflected by the disk-shaped medium 7 passes through the objective lens 6 and the λ/4 wavelength plate 5 again, and is converted into a linearly polarized state perpendicular to that at the time of incidence, so that it is extracted as reflected light 8 by the polarizing beam splitter 4. Here, a pair of waveguide elements 20a and 20b are installed at symmetrical positions with respect to the central axis within a plane including the central axis 8a of the reflected light 8, with their incident end faces tilted in the same direction. Here, a step-index type multimode fiber was used as the waveguide element. Here, the optical waveguide mechanism of such a waveguide element will be explained.

FIG. 3 schematically shows the optical waveguide mechanism of the waveguide element in one dimension. The waveguide element has a core 31 surrounded by a cladding 32 made of a medium with a refractive index lower than that of the core 31, and light 33 incident from the end face is guided by total reflection at the core-cladding interface.

If the refractive index of the core 31 is n ₁ and the refractive index of the cladding 32 is n ₂ , then light with an incident angle θ _M that satisfies sinθ _M = √ ² ₁ − ² ₂ will be approximately at the critical angle of incidence at the interface between the core and the cladding, and θ> Light at θ _M is not guided. Therefore, by setting θ=θ _M , the waveguide element becomes equivalent to a series of many reflecting surfaces with critical angles set. Therefore, defocus can be detected using the same principle as in the critical angle method described above. That is, if the outputs of the optical waveguide elements 20a and 20b are received by photodetectors (not shown) and their differential outputs are obtained, a signal corresponding to the defocus can be obtained. Moreover, according to the present device, since the shape of the waveguide elements 20a and 20b is small, the number of total reflections can be greatly increased while the shape remains very small, so that a small device with high detection sensitivity can be obtained. Furthermore, when optical fibers are used, the flexibility and light weight of the optical fibers allow for flexibility in the arrangement of the system and light weight.

In this example, an optical fiber was used as the optical waveguide element, but it is also possible to use an ordinary planar guide or the like, or an array bundle of these may be used.In this case, a photodetector The amount of incident light can be increased.

Furthermore, although this embodiment has shown a configuration in which the light from the light source is once parallelized, it goes without saying that the application of the present invention is not limited to this configuration.

As described above in detail, the defocus detection device according to the present invention does not require convergence of reflected light, and can provide a defocus detection device with high detection sensitivity and miniaturization of the entire system.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a conventional defocus detection device, FIG. 2 is a diagram showing an embodiment of the defocus detection device according to the present invention, and FIG. 3 is a diagram for explaining a waveguide mechanism of a waveguide element. It is. In the figure, 1 is a semiconductor laser, 2, 8, 10,
33 is a laser beam, 3 and 6 are lenses, 4 is a polarizing beam splitter, 5 is a λ/4 wavelength plate, 7 is a disk-shaped medium, 8a is an optical axis, 9 is a prism, 9a is a reflective surface, 31 is a core, 32 is Clad, 20a, 2
0b is a waveguide element.

Claims (1)

[Scope of utility model registration request] a light source, a lens system that focuses the light emitted from the light source onto the irradiated object, a light splitting element inserted in the middle of the lens system to extract the reflected light from the irradiated object, and the light splitting element. At least one pair of light receiving angles installed at symmetrical positions with respect to the central optical axis within a plane including the central optical axis of the reflected light from the irradiated object extracted by the element, with the incident surfaces tilted in the same direction by a predetermined angle. a waveguide element having a limit; and at least one pair of photodetectors that receive light emitted from the waveguide element;
A defocus detection device characterized in that the defocus detection device is configured to detect a defocus of the lens system toward the irradiated object based on differential outputs of a pair of photodetectors.