JPH0740369B2 - Polarization separation type hologram head - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head for detecting an information signal of an optical disc and a servo signal such as focus or tracking, and more particularly to a magneto-optical head for detecting a signal by using polarized light or It can be used for an optical head that records an information signal in a gray pattern or a bit pattern.

2. Description of the Related Art As a method for detecting an information signal recorded on an information carrier by using the polarization of a light beam, a method using a polarization beam splitter or a Wollaston prism has been conventionally known. A method of detecting a signal of, for example, a magneto-optical disk using these techniques is disclosed in Japanese Patent Laid-Open No. 62-78755, and therefore detailed description thereof will be omitted.

The technology to detect a servo signal using a polarization hologram is
For example, it is well known in JP-A-62-97141.

However, even if such a polarization hologram is used, the combination of functions cannot be sufficiently achieved. In particular, in order to detect polarized light, it was necessary to combine with a polarization beam splitter or a polarization separation element such as a Wollaston prism.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention As is clear from the above-mentioned conventional example, the conventional element for polarization separation is an element that only performs polarization separation, and it is usually difficult to combine it with other functions. It was In the one-function / one-element type optical head, the number of parts is large and the adjustment is complicated. Therefore, in order to downsize and simplify the optical head, it is necessary to combine the functions.

The present invention has been made in view of the above points, and an object thereof is to provide a polarization hologram head capable of achieving a composite function.

Another object of the present invention is to utilize polarization separation by a polarization hologram to simplify the functions of wavelength plate, Wollaston prism, polarization beam splitter, etc., which have been conventionally required, by integrating them into one element, thus reducing the cost. The purpose is to simplify the assembly process.

In order to achieve the above object, the present invention provides a radiation light source, an optical means for receiving a light beam emitted from the radiation light source and converging the light beam on an information recording carrier, and the optical means. The beam splitter located between the radiation sources, the polarizing hologram that reflects the information carrier and transmits or reflects the light beam that is split by this beam splitter, and the polarizing plane that emitted this polarizing hologram alternate with each other. 0th order light +
The configuration includes a plurality of photodetectors that receive the first-order diffracted light and the −1st-order diffracted light.

Operation The present invention has the above-described configuration, and can utilize the polarization separation by the polarization hologram to simplify the configuration of the optical head.

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

FIG. 1 is a basic configuration example of the first embodiment of the present invention. An example using the semiconductor laser 1 as a radiation light source is shown. The light beam emitted from the semiconductor laser 1 passes through the beam splitter 4 and enters the objective lens 2. The light beam converged by the objective lens 2 is incident on the optical disc 3 which is an information carrier. The light beam reflected by the disk 3 is transmitted again through the objective lens 2 and the beam splitter 4
Is reflected by. In FIG. 1, the light emitted from the semiconductor laser 1 is transmitted through the beam splitter 4 first, but the light emitted from the semiconductor laser 1 is first reflected by the eame splitter 4 depending on the optical system design. You may do it.

The light beam transmitted through the objective lens 2 and reflected by the beam splitter 4 is incident on the polarization hologram 5.

Generally, in a polarization hologram, P-polarized light polarized in a direction perpendicular to the grating direction of the diffractive element forming the hologram becomes 0-order light, and S-polarized light polarized in a direction horizontal to the grating direction of the diffractive element is +1 next time. It is split into a folded light and a -1st order diffracted light.

In this embodiment, a part of the zone plate as shown in FIG. 2 or FIG. 3 is used as the polarization hologram 5. Therefore, in the polarization hologram 5, S polarization is present in the + 1st order diffracted light and the −1st order diffracted light. , P-polarized light is split into 0th-order light. These light beams are respectively converted to optical signal detectors 7,8,
Receive at 6.

In the configuration of FIG. 1 (a), when the semiconductor laser 1 has insufficient power, the single lenses 51 and 52 are provided between the semiconductor laser 1 and the beam splitter 4, respectively, so that parallel light is incident on the beam splitter 4. It is also possible to arrange it between the polarization hologram 5 and the polarization hologram 5 to form the structure shown in FIG. As a method of forming a polarization polarization grating, for example, a method of forming a relatively shallow groove-shaped grating in an anisotropic material as shown in plan views and sectional views in FIGS. 2 (a) and 2 (b), and FIG. A method of forming a relatively deep groove-shaped grating by etching or pressing is well known as shown in plan views and sectional views in FIGS.

Polarizing holograms can be formed using the same process.

If the optical disc is one that records and reproduces a magneto-optical signal, the P that is branched by the polarization hologram 5
The magneto-optical signal can be reproduced by detecting the difference signal between the polarized wave and the S polarized wave.

Further, since a part of the zone plate is used as the polarization hologram 5, as shown in FIG.
The focal positions of the secondary diffracted light in the optical axis direction are different from each other, and the focus error signal and the tracking error signal can be obtained from these diffracted lights.

Next, the situation will be described with reference to FIG. Only the polarization hologram 5 and the optical signal detectors 7, 8 and 6 are shown in FIG. 4 to simplify the drawing.

4A shows the case where the position of the disc 3 in FIG. 1 is away from the objective lens 2, and FIG. 4B shows the case where the disc 3 is at the focal position of the objective lens 2. (C) is a case where the disc 3 is in the direction of approaching the objective lens 2. The detectors 7, 8, 6 are (a), (b),
Assuming that the shape is as shown at the right end of (c), the focus error signal is (7-2)-(8-2) ... equation (1) or {(7-1) + (7-3) + (8-2)}-{(7-2) + (8-1) + (8-3)} ...... can be obtained by calculating the detector output of the formula (2) (for example, JP JP-A-61-258339, JP-A-61-33
30, JP-A-60-59545, etc.).

The information signal recorded by magneto-optical recording is {(7-1) + (7-2) + (7-3)} + {(8-1) + (8-2) + (8-3) }-(6) --- Obtained from the calculation of the detector output of equation (3). A normal bit-type recorded information signal or a grayscale pattern recorded information signal is {(7-1) + (7-2) + (7-3)} + {(8-1) + (8 -2) + (8-3)} + (6) ... It can be obtained from the calculation of the detector output of equation (4).

Further, when the three-thick line of the detectors 7 and 8 is parallel to the track recorded on the disk 3, a far field tracking error signal for tracking can be obtained. The method of obtaining the far-field tracking signal has been described in detail in, for example, US Pat. No. 4,491,940, and the description thereof will be omitted. In Fig. 4, the far-fielded tracking signal is obtained from, for example, {(7-1) + (8-3)}-{(7-3) + (8-1)} ... equation (5) However, it is also possible to divide the polarization hologram and extract the far-field signal separately.

FIG. 5 shows such an example, and FIG.
The light beam 10 is projected onto the polarization holograms 12 to 16 which are multi-divided as shown in FIG. Further, in the light beam 10, the tracking signal 11 corresponding to the position of the track recorded on the disk 3 partially forms a bright portion or a dark portion (whether the portion is a bright portion or a dark portion, the track is concave or convex). It depends on what).

With this polarization hologram, a polarization signal such as a magneto-optical signal and a focus error and field error tracking error signal can be obtained at the same time. That is, a polarization hologram
12, 14 and 16 are the same polarization holograms as shown in FIG. 5 (b).
The first-order diffracted lights are incident on detectors 7 and 8 to form light beams 21 and 22, respectively. However, the shaded portions of the light beams 21 and 22 are polarization holograms 13 and 1 as described below.
Since it is diffracted by 5, it is incident on another detector.

The light beam passing through the polarization hologram 13 has + 1st order and -1st order.
Next-order diffracted light enters detectors 17 and 18, respectively.
The light beam passing through the polarization hologram 15 has + 1st order and -1st order.
Next-order diffracted light enters detectors 19 and 20, respectively. Therefore, the focus error signal is calculated based on the equations (1) and (2) as described with reference to FIG.
And the output of each detector of 8 are obtained. The tracking error signal can be obtained by calculating the detector output of {(17) + (18)}-{(19) + (20)} (6), and the magneto-optical signal can be obtained by It can be obtained by adding all the detector outputs that receive the + 1st order diffracted light and the −1st order diffracted light based on (3) and taking the difference from the output of the detector 6 that receives the 0th order light.

In this way, the focus error signal is obtained by calculating the outputs of the detectors 7 and 8, so that the light beams 21, 2
In 2 there is no light in the part corresponding to the polarization holograms 13 and 15 (hatched part), but this part has almost no effect on the focus error signal, and rather the interference of the tracking error signal with the focus error signal is reduced. This gives good results.

The angle between the + 1st and -1st order diffraction directions of the polarization hologram to obtain the tracking error signal and the + 1st and -1st order diffraction directions of the polarization hologram to obtain the focus error signal should be designed to be as small as possible. Is desirable. This is because the angle of analysis when obtaining a magneto-optical signal is a polarization hologram as shown in FIG. 5 (b).
This is because 12, 14, 16 and 13 and 15 are different. As a method of making this angle as small as possible, a polarization hologram 13,1
There is a method to change the carrier frequency of 4. By changing the carrier frequency of the polarization hologram, the diffraction angle of the light beam diffracted by the polarization hologram can be changed.

In FIG. 5 (b), the carrier frequencies of the holograms having polarizability are almost the same for all the polarisable holograms 12 to 16, although they are exaggerated in angle.

FIG. 6 shows an example in which the carrier frequency of the polarization hologram is changed. Detectors 25 and 26 corresponding to the polarization hologram 23 and a detector 2 corresponding to the polarization hologram 24
There are 7,28. As shown in FIG. 6 (b), both holograms 2
The carrier frequencies of 3, 24 are higher than those of the polarization holograms 12, 14, 16 for the focus error signal, and by arranging them as shown in the figure, the difference in the angle of analysis can be reduced.

Furthermore, if the carrier frequency of one of the polarization holograms is increased, it is possible to completely match the analyzing angles. FIG. 7 shows detectors 29 and 30 corresponding to such a polarization hologram. By doing so, the magneto-optical signal can be detected without changing the analyzing angle by the multi-divided polarization hologram.

Although FIGS. 1 to 7 show examples of the same focusing error detection method, astigmatism is eliminated by using polarization holograms 5 ′ and 5 ″ as shown in FIGS. 8 (a) and 8 (b). It is also possible to use the detector 31 of the generating method.

The polarizing hologram has a function of a cylindrical lens that causes astigmatism. ± 1 at this time
Astigmatism is generated in the second-order diffracted light, and a detector is inserted in the vicinity of the minimum confusion circle to perform astigmatic focusing operation.

In addition, the polarization hologram 5 as shown in FIG.
It is also possible to use a double knife edge type detector 32 (see JP-A-61-120352).

For example, as disclosed in Japanese Patent Laid-Open No. 61-120352, a semi-circular beam having a different focus state can be positioned on a detector. In this case, the polarization hologram must be a polarization hologram that gives different focal points in the upper half and the lower half on the optical axis, and must be designed to be blazed for + 1st order light. At this time, the diffraction efficiency of the 0th-order light and the + 1st-order light is slightly changed, but this can be balanced by adjusting the gain of the amplifier of the buffer connected to each detector.

FIG. 9 shows a circuit configuration used for realizing the present invention. The numbers in parentheses at the left end are the numbers of the detectors in the drawings shown in FIGS. 2 and 3, and here indicate that the outputs are input from the respective detectors. Reference numerals 34 to 38 are buffer amplifiers, which mainly convert the impedance of the input signal. The outputs of the buffer amplifiers 34 and 35 are subtracted by the differential amplifier 39 to obtain the focus error signal 43. The output of the buffer amplifiers 36 and 37 is the differential amplifier 40.
Is subtracted to obtain a far-field tracking error signal. The outputs of the buffer amplifiers 34 to 37 are summing amplifiers.
When the information carrier that is added in 41 and subtracted from the output of the buffer amplifier 38 by the differential amplifier 42 to obtain the magneto-optical signal 45 is a normal phase pattern or a light and shade pattern,
The output of the amplifier 41 and the output of the amplifier 38 can be added to obtain an information signal as in the conventional case.

EFFECTS OF THE INVENTION As described above, according to the polarization hologram of the present invention, it is possible to easily detect the magneto-optical signal and to combine the functions that have been difficult with the conventional methods.

Further, in the present invention, the functions of the wave plate, the Wollaston prism, the polarization beam splitter, etc., which have been conventionally required, can be integrated into one element by utilizing the polarization separation by the hologram, and the cost can be reduced and the assembling process can be simplified. Can be realized.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of a basic optical system of a polarization separation type hologram head according to an embodiment of the present invention, FIG.
FIG. 3 is a plan view and a cross-sectional view of the polarization hologram, FIG. 4 is a schematic diagram showing the basic detector configuration and operation for obtaining the focus error signal and the magneto-optical signal, and FIG. 5 is the multi-division polarization hologram. FIG. 6 is a schematic diagram showing a configuration example of a polarizing hologram and a detector for obtaining a focus error signal, a tracking error signal, and a magneto-optical signal from FIG. 6, and FIG. FIG. 7 is a schematic diagram showing a configuration example, FIG. 7 is a schematic diagram showing an improved detector configuration example, FIG. 8 is a schematic diagram showing an embodiment of a polarizing hologram and a detector configuration for obtaining the focus error signal, and FIG. It is a circuit diagram which shows the circuit structural example. 1 ... Semiconductor laser, 2 ... Objective laser, 3 ... Disk, 4 ... Beam splitter, 5,12,13,14,15,16,23,24
...... Hologram, 6,7,8,17,18,19,20,25,26,27,28,29,3
0,31,32 …… Detector, 34〜38 …… Amplifier, 39,40,42
...... Differential amplifier, 41,47 …… Adder.

Claims (7)

[Claims] 1. A radiation source, an optical means for receiving the light beam emitted from the radiation source and converging the light beam on an information recording carrier, a beam splitter located between the optical means and the radiation source, The light beam reflected by the information carrier and split by the beam splitter is transmitted or reflected, and the polarization direction of the 0th-order light transmitted or reflected is +1.
A polarization separation type equipped with a polarization hologram having a function in which the polarization directions of the first-order diffracted light and the −1st-order diffracted light are orthogonal to each other, and a plurality of photodetectors for receiving the 0th-order light, the + 1st-order diffracted light, and the −1st-order diffracted light. Hologram head. 2. A polarization hologram having different sizes of + 1st-order diffracted light and -1st-order diffracted light on a photodetector when the distance between an information carrier and an optical means for converging a light beam is improper. When the distance between the information carrier and the optical means for converging the light beam is appropriate, the + 1st-order diffracted light and the -1st-order diffracted light are approximately the same size on the photodetector. The polarization split hologram head according to claim 1, wherein the polarization split hologram head is configured to be incident as a light beam of. 3. A polarization hologram wherein a + 1st-order diffracted light and a -1st-order diffracted light on a photodetector have different shapes when the distance between the information carrier and the optical means for converging the light beam is inappropriate. When the distance between the information carrier and the optical means for converging the light beam is appropriate, a + 1st-order diffracted light and a -1st-order diffracted light are formed on the photodetector and have substantially the same shape. The polarized light separation type hologram head according to claim 1, wherein the polarized light separation type hologram head is configured to be incident as. 4. A polarization hologram comprising at least three parts having different patterns, and corresponding + 1s, respectively.
And a photodetector for injecting the first-order diffracted light, the first-order diffracted light, and the zero-order diffracted light, and two parts of the different patterns of the polarization hologram receive a light beam mainly containing a diffracted signal by a track on an information carrier. If the distance between the information carrier and the optical means for converging the light beam in the remaining portion of the polarization hologram is improper, the + 1st-order diffracted light and the -1st-order diffracted light on the photodetector have different sizes. Light beam is incident on the photodetector when the distance between the information carrier and the optical means for converging the light beam is appropriate. The polarization split hologram head according to claim 1, wherein the polarization split hologram head is configured to be incident as a light beam. 5. A polarization hologram consisting of portions having at least three different patterns and corresponding + 1s, respectively.
And a photodetector for injecting the first-order diffracted light, the first-order diffracted light, and the zero-order diffracted light, and two parts of the different patterns of the polarization hologram receive a light beam mainly containing a diffracted signal by a track on an information carrier. A different shape between the + 1st-order diffracted light and the -1st-order diffracted light on the photodetector when the distance between the information carrier and the optical means for converging the light beam is improper in the remaining portion of the polarization hologram. Light beam having a substantially same shape as the + 1st-order diffracted light and the -1st-order diffracted light on the photodetector when the distance between the information carrier and the optical means for converging the light beam is appropriate. The polarization separation hologram head according to claim 1, wherein the polarization separation hologram head is configured to be incident as a beam. 6. A circuit for generating a differential signal between the output of a photodetector on which 0th-order diffracted light is incident and the sum of the output of a photodetector on which + 1st-order diffracted light is incident. The polarized light separation type hologram head according to any one of claims. 7. A polarization separating hologram head according to claim 4, comprising a polarizing hologram having at least three different patterns having at least two different carrier frequencies and a photodetector.