JPH01253843A - Hologram head - Google Patents

Abstract

PURPOSE:To obtain information of a high S/N by projecting semiconductor laser beams on a disk through an objective lens, permitting reflected light beams to be made incident on a first detector through a first hologram and a first detection lens and permitting light beams branched in the first hologram to be made incident on a second detector through a second hologram and a second detection lens. CONSTITUTION:Laser beams from the semiconductor laser 1 is made to be parallel light beams in a collimator lens 2 and projected on a half mirror 3. The reflected light beams are made incident on the disk 5 through the objective lens 4. Next, the reflected light beams are transmitted through the mirror 3 and made incident on the first detector 8 through the first hologram 6 and the first detection lens 7. At the same time, the light beams branched in the first hologram 6 are made incident on the second hologram 9, and the outputs from here are made incident on the second detector 11 through the second detection lens 10. Thus, the polarization characteristic of the high S/N, which is optimum for reproducing information of an optical magnetic signal or the like, can be obtained, and light and small optical head constitution is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a hologram head that extracts information using polarized light.

A typical conventional technology in which information is extracted in the form of polarized light is a magneto-optical optical disc.

A method of detecting a magneto-optical signal prior to the present invention will be explained below with reference to FIG.

The light beam emitted from the semiconductor laser 61 is made into substantially parallel light by a collimator lens 62, and is incident on a magneto-optical disk 66 via a half mirror 63 and an objective lens 64.

During reproduction, when reflected by the disk 66, the information signal portion is subjected to Kerr rotation, resulting in a light beam containing the information signal as polarized light. The reflected light beam passes through the objective lens 64° half mirror 63 again and enters the detection lens 6e. The light beam incident on the Volaston prism 67 is divided into P waves and S waves, which are incident on a detector Ass and a detector B 69, respectively. A magneto-optical signal can be obtained from each of the detectors by separating the polarized light, and a signal with a good S/N ratio can be obtained by differential signals.

How is the signal obtained by polarization separation and the SIN
What you need to do to get a good result is
Since it is generally well known, detailed explanation will be omitted here. Normally, the better the degree of separation of polarized light, the better the signal with a good S/N ratio can be obtained, but generally a degree of separation of about 100:1 or more is required.

In this regard, the Guorastone prism has a separation degree of 1000:1.
The above is sufficient.

In the optical system described above, it is also possible to extract a focus error signal from one of the detectors using the astigmatism method, the Knifezi method, or the like. It is also possible to extract the rank error signal (by Bushpull method, sample format tracking method, etc.). 1. Problems to be Solved by the Invention The device described above uses a Volaston prism or a Volaston prism for polarization separation. Ronochon prism.

It is necessary to use crystal or calcite such as Senarmont Prism or Savard 7” IJism.

However, these prisms require the use of optically high-quality crystal or calcite, which makes them expensive, and they also require length, resulting in a heavy and bulky head.

It is conceivable to use a polarizing beam splitter in place of these prisms, but if a polarizing beam splitter is used, it is necessary to bend the reflected beam by about 900 degrees, making it difficult to use an in-line configuration.

Also, although it is cheaper than a prism using crystal, it is still a fairly high-cost optical component.

Furthermore, when trying to obtain a focus signal or the like using the above-mentioned prism, an additional optical element for generating the focus signal is required. For example, to obtain an astigmatism focusing signal, a parallel plate or cylindrical lens is required, and to obtain a knife focusing signal, a knife or wedge prism is required. That is, in the current optical system, since an optical system combined with the detection system is required in this way, adjustment becomes complicated.

Thus, there has been a need for an inexpensive optical element that integrates these functions.

On the other hand, polarizing hologram technology has been known for a long time, and if this hologram can be used, the structure can be simplified.

However, even when these holograms are used, the degree of polarization separation is extremely poor at 6 to 8 to 1, making it difficult to use them practically, and the large dispersion of the holograms makes it difficult to detect servo signals. there were.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide a hologram head that uses a hologram, has a simple configuration, and can detect information with good S/N characteristics.

Means for Addressing the Problem The present invention includes a first optical system that converts a light beam emitted from a radiation source into substantially parallel light, and a second optical system that receives this substantially parallel light and focuses it on an information carrier. a third optical system that splits the light beam reflected or transmitted by this information carrier, a first hologram provided in the optical path of this split light beam, and light that passed through this first hologram. a first photodetector that receives the beam; a second hologram that receives the n-th order diffracted light diffracted by the first hologram;
and a second photodetector that receives −3rd order diffracted light diffracted by the hologram.

Effect With this configuration, by using two holograms, it is possible to improve the polarization separation characteristics, and to remove or reduce optical axis fluctuations of the light beam by using the phase correction of the holograms, thereby making it possible to transmit optical information. Good SIN can be detected. .

Example An embodiment of the present invention is shown in FIG.

The light beam emitted from the semiconductor laser 1, which is the irradiation light source, is
The collimating lens 2 converts the light into substantially parallel light, which passes through the half mirror 3 and the objective lens 4 and is focused onto the magneto-optical disk 6 . The light beam reflected by the disk 6 enters the detection optical system and is split. The zero-order light transmitted through the hologram A6 provided in the optical path is condensed by the detection lens A7, and is incident on the detefter A8. The light beam diffracted by hologram A6 is incident on hologram B9.

At this time, most of the light beam diffracted by the hologram 80 becomes S-polarized light. Most of the light beam diffracted again by the hologram B9 can be diffracted because most of the incident light is S-polarized. The light beam diffracted again is incident on the detection lens B10 and then on the detector B11.

If the detector B11 is made into a 4-split detector and the hologram 80 and/or the hologram B9 is made to have astigmatism in addition to the polarization separation function, then the focus error signal obtained by the astigmatism method can be obtained from the output of the detector B11. can be taken out.

Normally, when a single hologram is used, it is impossible to obtain a stable servo signal because the spatial frequency is extremely large in the case of a hologram that also has polarization properties. this is,
This is because a hologram with a high spatial frequency has a very strong wavelength dependence, so even a wavelength shift of a semiconductor laser due to, for example, a slight temperature difference will affect servo signal detection.

FIG. 2 schematically shows a typical example of a polarizing hologram used in this device. In the light beam 22 incident on the hologram 21, mainly the S-polarized light component is diffracted on the hologram surface, and mainly the P-wave component is transmitted as the zero-order light beam 23.

The hologram has an aspect ratio of 1.6 to 2.6, and a hologram pitch of 0.8 to 1.2 μm, which is approximately the same length as the wavelength.

The degree of polarization separation between the zero-order light and the first-order diffracted light is approximately 8 to 1 to 1, which is insufficient for information detection by magneto-optics, but by configuring it as shown in Figure 1, polarization separation can be achieved. By doing this twice, the degree of polarization separation can be improved to 64 to 100:1, and information can be detected with good accuracy.

Furthermore, it will be explained with reference to FIG. 3 that by using two holograms, the wavelength dependence of the holograms can be canceled.

The light beam incident on the hologram A31 in the initial state is
The beam is incident on the hologram B32 with a diffraction angle of ψ. The light beam diffracted again by the hologram B32 becomes parallel to the original optical axis when the incident angles of the light beams to the two holograms are equal. On the other hand, when there is a wavelength fluctuation and the light beam indicated by the broken line in FIG. Therefore, it ends up being parallel to the original optical beam axis.

FIG. 4 is an embodiment of another form of the present invention. This uses hologram separation instead of the usual prism membrane separation. In this embodiment, a method similar to the SSD method (detailed explanation will be omitted since it is publicly known) using this hologram prism is proposed. The light beam that has not been diffracted by the hologram A43 is accurately projected onto a three-part detector B11, which is a slit-shaped detector at the center, before the light beam is most focused. - direction,
The first-order diffracted light diffracted by the hologram A43 is diffracted again by the hologram B44, and is incident on the detector B11. Since the path of this light beam is a little long, the light beam is focused most at the imaging position of the disk, and then enters the three-part detector B46, which is similar to the detector A, in a diverging state. In the figure, the detector A and the detector B are shown separated, but in reality they can be placed sufficiently close to each other, and a single-chip type detector is also possible.

FIG. 5 schematically shows a hologram used in the second embodiment, which is composed of a hologram prism and an intermediary medium such as an adhesive. The refractive index of this mediating medium must be sufficiently different from that of other prisms, and it is desirable that it has no anisotropy.

In this example, to simplify the explanation, we have shown an example in which two single lenses (detector lens 10 in Figure 1) are used, but these are equivalent to an integrated two-optical axis single lens. It may be advantageous to use it. Alternatively, the detection lens B10 can be set on the optical axis to allow two light beams to pass through one single lens.

Effects of the Invention As described above, according to the present invention, by using two holograms, the optimum S/N for reproducing information such as magneto-optical signals can be achieved.
It has become possible to improve polarization characteristics and construct an optical head that is lightweight, compact, and has a simple configuration.

Furthermore, by using the complementarity of two holograms, it has become possible to form a photodetection system without wavelength dependence.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a hologram head according to the first embodiment of the present invention, FIG. 2 is a schematic diagram showing an example of the hologram, FIG. 3 is a schematic diagram showing external use of the hologram, and FIG. A block diagram of a hologram head according to a second embodiment of the invention, FIG. 5 is a schematic diagram showing an example of the hologram, and FIG. 6 is a block diagram of a conventional optical head. 1... Semiconductor laser, 2... Collimating lens, 3... Half mirror 14... Objective lens, 5... Magneto-optical lens, 6... ...Hologram A, 7...Detection lens A18...
...Detector A, 9...Hologram B110
...Detection lens B111 ...Detector B
0 Name of agent Patent attorney Toshio Nakao and 1 other person31
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Claims (1)

[Claims] a radiation light source, a first optical system that converts a light beam emitted from the radiation light source into substantially parallel light, a second optical system that receives the substantially parallel light and focuses it on an information carrier, and a light beam reflected by the information carrier. Or a third optical system that branches the transmitted light beam,
a first hologram in the optical path of the branched light beam; a first photodetector that receives the light beam transmitted through the first hologram; and an n-th order diffracted beam diffracted by the first hologram. a second hologram that receives light;
and a second photodetector that receives −n-order diffracted light diffracted by the hologram.