JPH0439131B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus detection device, and more particularly to a focus detection device suitable for use in optical information recording and reproducing devices such as video disks, optical memories, and magneto-optical memories.

In the above-mentioned optical information recording and reproducing apparatus that records or reproduces information by projecting a light beam onto a recording medium,
to focus a light beam onto the recording medium;
The distance between the objective lens and the recording medium must be detected, and the optical head must be controlled according to the distance so that the beam always has the minimum diameter on the recording medium. Conventional methods for detecting the distance between the objective lens and the recording medium, or so-called focus detection, include a method in which a cylindrical lens is used to detect the in-focus position based on the shape of a point image, and a knife edge is used to move the position of the point image. A method of detecting the focal point position has been used. However, these methods have drawbacks such as a complicated structure, requiring many parts to be spatially arranged with high precision, troublesome assembly and adjustment, and the optical head becoming large and heavy.

SUMMARY OF THE INVENTION An object of the present invention is to provide a focus detection device that has a simple configuration and can reduce the size and weight of an optical head.

The present invention includes a means for focusing a light beam emitted from a light source onto an irradiated object, a means for focusing at least a part of a reflected light beam from the irradiation object, and a means for focusing the focused light beam on two different polarization directions. A means for dividing the light beam into light beams, two optical fibers placed before and after the convergence position of the divided light beams, and a light beam on the irradiated object based on the difference or ratio of the amount of light transmitted by these optical fibers. The above object is achieved by a focus detection device comprising means for detecting a focus state.

Embodiments of the present invention will be described below with reference to the drawings.

A first embodiment of the invention is shown in FIG. The light beam emitted from the laser light source 1 is focused onto the optical fiber 5 by the focusing lens 22. Optical fibers are polarization-maintaining optical fibers such as elliptical core fibers, elliptical cladding fibers, and side bit fibers, and linearly polarized light incident from one end is
It has the function of transmitting light to the other end without changing the polarization state. The linearly polarized light beam emitted from the other end of the optical fiber 5 passes through the polarizing beam splitter 7 and then passes through the λ/4 plate 8 to become circularly polarized light.
The light is condensed by a condenser lens 11, passes through a substrate 14, and is condensed onto an information carrier surface 15. Information carrier 1
The light beam reflected and modulated by lens 1
1, the light passes through the λ/4 plate 8 again, becomes linearly polarized light having a plane of vibration in a direction perpendicular to that at the time of incidence, and is reflected by the polarizing beam splitter 7. This reflected light beam is split by a beam splitter 12 and enters single mode optical fibers 29 and 30. When a light beam is incident on a single mode optical fiber, the amount of light that effectively enters the fiber is very sensitive to the sphericity of the light beam and the size of the spot. The most efficient case is when the size of the optical spot is the same as the size of the distribution of the light flux transmitted within the single mode optical fiber, and the light flux forms a beam waist at the end face of the optical fiber. The light beams transmitted by the optical fibers 29 and 30 are detected by the photodetectors 31 and 32, and the output difference between the two is calculated by the subtracter 42 to become an out-of-focus signal. Detection is performed.

FIG. 2 shows how the amount of defocus can be detected using single-mode optical fibers placed before and after the light-converging surface shown in FIG. 1. In the figure, the horizontal axis is the amount of defocus from the reference position, and the vertical axis is the amount of light. Here, the two curves shown by the solid line and the dashed line are
The amount of light extracted from each of the two optical fibers is shown. By taking the difference between the output values of the light amounts, information on front focus and rear focus and their amounts are detected. Note that, as is clear from the above explanation, detection of out-of-focus is possible in principle just by using a single fiber. Further, although the single mode optical fiber described above enables highly sensitive detection, the present invention is not limited thereto, and an optical fiber or optical waveguide having an appropriate core diameter can also be used. Automatic focus adjustment becomes possible by slightly moving the optical head in the optical axis direction at a sufficiently high frequency in accordance with the defocus signal obtained in this manner.

A second embodiment of the invention is shown in FIGS. 3a, b, and c. The figure shows only the part where light is detected using an optical fiber.

In FIG. 3a, a parallel plate 50 of crystal is placed in the linearly polarized beam converged by the focusing lens 11, making an angle of 45° with the plane of vibration of the beam. The light beam incident on the crystal is divided into an ordinary ray and an extraordinary ray, and the difference in refractive index between the two rays causes separation of the focal point in the optical axis direction. The optical fiber 20 is a polarization-maintaining optical fiber that transmits ordinary rays and extraordinary rays independently. Figure 3b is
This is a diagram showing the signal detection section, with the optical fiber 20
After the light beam emitted from the collimation lens 21 becomes a parallel light beam, the light beam exits from the polarizing beam splitter 2.
3 and detected by photodetectors 31 and 32. A focus signal is obtained from the difference or ratio of the amounts of light incident on both photodetectors.

FIG. 3c shows a modification of the second embodiment of the invention shown in FIG. 3a. The light beam reflected from the information carrier surface is condensed by a condenser lens 13 having different refractive indexes for ordinary rays and extraordinary rays. One end of the polarization-maintaining optical fiber 20 is placed between the focusing positions for the ordinary ray and the extraordinary ray produced during focusing. As a result, exactly the same effect as in FIG. 3a is obtained, and the in-focus position, the amount of out-of-focus, and its direction are detected.

A third embodiment of the present invention, in which the present invention is applied to a recording/reproducing head of an optical disk using magneto-optical recording, is shown in FIGS. 4a and 4b.

In the figures, parts that are the same as those in the first embodiment are given the same reference numerals. The light beam emitted from the laser light source 1 is focused by a focusing lens 22 and is incident on the polarization maintaining optical fiber 5. The light beam emitted from the optical fiber 5 is turned into a parallel light beam by a collimation lens 16, and is transmitted through a beam splitter 17 having polarization characteristics. The polarizing beam splitter 17 used here has optimal characteristics for magneto-optical readout using Kerr rotation, for example, it has a transmittance of 70% and a reflectance of 30% for linearly polarized incident light. It has a reflectance of 100% for linearly polarized light having a plane of vibration perpendicular to this. The linearly polarized light beam that has passed through the polarization beam splitter 17 passes through the prism 9 and is focused by the hologram lens 10 onto the magneto-optical recording material 45 through the substrate 44 . The light beam reflected by this magneto-optical recording material and subjected to Kerr rotation is diffracted by the hologram lens 10, becomes a parallel light beam, passes through the prism 9, and is transmitted through the beam splitter 1.
After being reflected by the light beam 7, the light is focused by the focusing lens 11. Beam splitter 12 near the focal point
is placed, and the light beam is split into two and enters the polarization maintaining optical fibers 20 and 24. These optical fibers are arranged before and after the focal point of the reflected light when it is in focus on the information carrier 15, so the total amount of light transmitted through each optical fiber and detected is A signal corresponding to the amount of defocus can be obtained. Further, each fiber is attached in a direction necessary for differential amplification of the light beams separated according to polarization so that changes in polarization state due to Kerr rotation in magneto-optical recording can be detected with the highest signal-to-noise ratio. Collimation lenses 21, 25 are placed at the exit end of the optical fiber, followed by polarizing beam splitters 23, 24, and photodetectors 31, 32, 33 are placed at the exit end of each beam splitter. , 34 are placed.
A focusing signal is detected by taking the difference or ratio between the amounts of light emitted from each optical fiber,
On the other hand, depending on the polarization state, the beam splitter 23,
The magneto-optical recording signal is read by taking the difference in the amount of light separated and detected at 24.

Although the explanation has been made using various embodiments up to this point, the scope of application of the present invention is not limited to the optical head of the information recording/reproducing device shown here. It will be obvious that the present invention is applicable to a variety of optical devices.

As explained above, the present invention has the following effects on conventional focus detection devices: (1) It reduces the number of parts and simplifies assembly and adjustment; and (2) It enables the optical head to be made smaller and lighter. It is something.

[Brief explanation of drawings]

FIG. 1 is a diagram illustrating a first embodiment of the present invention,
FIG. 2 is a diagram explaining the principle of focus detection in the first embodiment, and FIGS.
FIGS. 4a and 4b are partial views illustrating a third embodiment of the present invention, respectively. In the figure, 1 is a laser light source, 5 is a polarization maintaining optical fiber, 7 is a polarizing beam splitter, 8 is a λ/4 plate, 11 and 22 are condensing lenses, 14 is a substrate, 15 is an information carrier, and 12 is a beam splitter. Pritzter, 29 and 30 are single mode optical fibers,
31 and 32 are photodetectors, 41 is an adder, and 42 is a subtracter.

Claims (1)

[Claims] 1. A means for focusing a light beam emitted from a light source onto an irradiated object, a means for focusing at least a part of a reflected light beam from the irradiated object, and a means for dividing the focused reflected light beam into two light beams. a first optical fiber having one end disposed in front of the convergence position of one of the divided light beams, and a second optical fiber having one end disposed behind the convergence position of the other divided light beam. , two photodetectors that respectively detect the amount of light incident on one end of these optical fibers and transmitted to the other end, and an arithmetic element that calculates the difference or ratio of the detection outputs of these photodetectors. one end of each of the first and second optical fibers is arranged so that equal amounts of light are incident on the arithmetic element when the light beam irradiated onto the object is in focus; A focus detection device characterized in that it is configured to detect a focus state on an irradiated object from the output of the irradiation object.