JPS6117103A - Polarizing beam splitter - Google Patents

Abstract

PURPOSE:To make a polarizing beam splitter thin and lightweight and to reduce the production cost by using a relief-type diffraction grating provided with a reflecting film having the dependence upon polarized light. CONSTITUTION:One of transparent members 21 and 22 having almost the same refractive index has plural slopes at least, and a reflecting film 23 having the dependence upon polarized light is provided on the boundary surface between them to form the relief-type diffraction grating. The reflecting film 23 is so formed that about 100% P-polarized light is transmitted through the relfecting film 23 and about 100% S-polarized light is reflected on the reflecting film 23. Consequently, this polarizing beam splitter functions as a mere paralle plane plate for a P-plarized incident light 26 and the light 26 is almost transmitted through the polarizing beam splitter, but an S-polarized incident light 27 is almost diffracted to become a diffracted light 27'. This diffraction grating is produced with a thin and lightweight constitution and a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polarizing beam splitter, and more particularly to a polarizing beam splitter suitable for use in an optical head device for recording or reproducing information by irradiating light onto an information recording carrier.

FIGS. 1(a) and 1(b) are diagrams explaining the functions of a conventional polarizing beam splitter used in an optical head device. (a) is used to eject a digital audio disc or optical video disc (7) a.
The polarizing beam splitter 1 is composed of right-angle prisms 2 and 3, and a reflective film 4 having polarization dependence formed on the joint surfaces thereof. This reflective film 4 has approximately 10
It shows a transmittance of 0% and a reflectance of almost 100% for S-polarized light. Therefore, for example, linearly polarized light (P
Most of the polarized light) 6 passes through the polarizing beam splitter 1, passes through the input quarter plate 5, becomes circularly polarized light, and is focused onto the recording surface of the information recording carrier by an objective lens (not shown). In addition, the reflected light 8 that is reflected by the recording surface and contains the information signal enters at /4
The light passes through the plate again, becomes S-polarized light, is almost reflected by the reflective film 4, becomes a light beam 9, and is guided to a photodetector and used for signal readout.

On the other hand, the polarizing beam splitter 11 shown in (b) is for reading out magnetically recorded information using the magneto-optic effect, and also consists of a right-angle prism 12, 13 and a reflective film 14. Here, the reflective film 14 is formed so as to reflect or transmit P-polarized light and S-polarized light at an appropriate ratio. For example, 30% of the energy of the P-polarized light 15 incident on the polarizing beam splitter 11 is reflected by the reflection jlI 14 to become a light beam 17, and the remaining 70% of the light beam 16 is transmitted through an objective lens (not shown). The light is focused on the recording surface of the magnetic recording medium. The plane of polarization rotates according to the information on the recording surface (
The reflected light 18 that has been subjected to Kerr rotation enters the polarization beam splinter 11 again. Here, the modulation component due to Kerr rotation is S
It is polarized light and is almost 100% reflected by the reflective film 14. On the other hand, 70% of the P-polarized light component in the reflected light 18 is transmitted through the reflective film 14, and only the remaining 30% is reflected and guided to the photodetector together with the S-polarized light component. In this way, the components of the incident light (
By relatively increasing the modulation component (S-polarized light) compared to P-polarized light, the Kerr rotation angle apparently increases, and the S/N
This makes it possible to read signals with a high ratio.

However, since the conventional polarizing beam splitter as described above is manufactured by joining two prisms at their facing angles,
It has the disadvantage that it requires complicated processing and alignment adjustment, making it difficult to reduce costs. Furthermore, since it is approximately cubic in shape, when used in optical head devices, etc., it becomes a factor that prevents devices from being made thinner.

An object of the present invention is to provide a polarizing beam splitter factory that is thin, lightweight, and can be manufactured at low cost.

The above-mentioned object of the present invention is to provide a polarization-dependent strong reflective film on the interface between first and second transparent members having substantially the same refractive index and a plurality of inclined surfaces on at least one side, and to provide relief-type diffraction. This is achieved by a polarizing beam splitter formed by a grating.

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

FIG. 2 is a diagram explaining the polarizing beam splitter according to the present invention, in which (a) schematically represents and analyzes the configuration, and (b) shows an enlarged view of part A of the diffraction grating in (a). It is. Here, at least one of the transparent members 21 and 22 having substantially the same refractive index has a plurality of sloped surfaces, and a polarization-dependent reflective film 23 is provided on the boundary surface of these to form a relief shape. It forms a diffraction grating. Further, the transparent members 21 and 22 are supported so as to be sandwiched between the parallel plates 24 and 25.
25 is not necessarily necessary.

The reflective film 23 is configured to exhibit approximately 100% transmittance for P-polarized light and approximately 100% reflectance for S-polarized light. Therefore, the incident light 26 of P polarization is deflected into this polarized beam and becomes diffracted light 27'.

In the analysis example, a one-leaf circular lattice is used as shown in Fig. 2(b).
Shown below. Pinch d=207im, depth Δ=10ILm,
It was analyzed as a triangular shape with an inclination angle α=400. In this example, the energy of the diffracted light is determined by the characteristics of the reflective film 23.

Therefore, by forming the reflective film 23 in the same manner as the conventional reflective film 14 shown in FIG. For relief type photosensitive materials such as
It can be easily produced by exposing and developing interference fringes obtained by superimposing light beams split from the same laser light source, or by directly mechanically cutting a substrate made of a transparent member. In addition, by processing the matrix by cutting and transferring the grid pattern to a transparent material such as plastic by a method such as injection, compression, or thin layer copying, it can be manufactured with good productivity and at low cost. In this case, for example, the plurality of inclined surfaces are formed on one transparent member, provided with a reflective film, and then offset by the other transparent member. In addition, by forming a plurality of inclined surfaces corresponding to the unevenness on both transparent members, and then joining these after forming a reflective film, it can be used as a polarizing beam splitter.
.

Got by me. Such a dielectric multilayer film is described in the literature (Hiroshi Kubota, "Wave Optics JP 236, February 2, 1971").
For example, by alternately depositing layers of a material with a low refractive index and a layer of a material with a high refractive index, it exhibits a high reflectance for S-polarized light.
Shows low reflectance for polarized light. At this time, the thickness d of each layer is determined from nd cos θ=incident/4, where n is the refractive index of the material constituting the layer, θ is the incident angle of light, and is the wavelength used.

1.38) and high refractive index ZnS (refractive index nH=2,
30) were alternately stacked in four and five layers, respectively, to form a reflective film consisting of a total of nine layers. In FIG. 3, a broken line 31 indicates a transmittance ItP+2 for P-polarized light, and a solid line 32 indicates a transmittance 1ts12 for S-polarized light. Therefore, near 100% transmittance for P-polarized light can be obtained in the long wavelength range where the wavelength used is present. Although not particularly shown, the reflectance 1r512 for S-polarized light has a value close to 100% in the wavelength range used. - When using the polarizing beam splitter invented by Rikiki for reading magneto-optical recording, it is necessary to set the reflectance 1rs12 for S-polarized light to approximately 100% and the reflectance 1rP12 for P-polarized light to an appropriate value of about 30%. However, such a polarizing beam nullifier can be similarly realized by changing the design of the reflective film.

FIG. 4 shows an example of an optical head device constructed using the polarizing beam splitter of the present invention. The light (P-polarized light) emitted from the semiconductor laser 41 becomes a parallel beam of light by the collimator lens 42, and is converted into a parallel beam by the polarization beam sputterer 43 of the present invention.
incident on .

This polarizing beam splitter 43 consists of two parallel flat plates 44
．． It consists of transparent members 45 and 46 sandwiched by 47. A plurality of inclined surfaces are formed at a part of the boundary between the transparent members 45 and 46, and approximately 100% of the P-polarized light is transmitted through this boundary, and the light is S-polarized. Most of the incident light passes through the polarizing beam splitter 43, becomes circularly polarized by the input quarter plate 48, and forms a spot on the recording surface 51 of the information recording carrier (via the plate 50) by the objective lens 49.

Information is recorded on the recording surface 51 by changes in reflectance, etc., and one reflected light beam is modulated in light amount according to the information.

This reflected light passes through the objective lens 49 and the input/quarter plate 48 again, becomes S-polarized light, enters the photodetector 8 via the sensor lens 52, and the information is read.

It should be noted that tracking control, which controls the track on the recording surface so that the spot always scans correctly, and focusing control, which aligns the focus position of the objective lens with the recording surface, are essential for optical head devices. The embodiment can perform such control by combining with a conventionally known control method. For example, if the sensor lens 52 is an anamorphic optical system and the photodetector 53 is a 4-split photodetector, the light intensity distribution of the light incident on the photodetector will depend on the focused state of the spot on the recording surface 51. changes, and by detecting this change on the divided light-receiving surfaces, a focus error signal can be obtained. This method is generally well known as the astigmatism method.

FIG. 5 shows other configuration examples in which the polarizing beam splitter of the present invention is used in an optical head device for magneto-optical recording. The P-polarized light emitted from the semiconductor laser 61 is turned into a parallel beam by the collimator lens 62, and is incident on the polarizing beam splitter 63 constructed based on the present invention. The transparent member 65 and 66 are sandwiched between the transparent members 65 and 66, and a polarization-dependent reflective film 68 is provided on a part of their boundary surface to form a relief-shaped circular grating.

This reflective film 68 has a reflectance of approximately 100 for S-polarized light.
%, and the reflectance for P-polarized light is 30%. Therefore, 70% of the incident light (P-polarized light) passes through the polarizing beam splitter 63, passes through the objective lens 69, becomes a convergent light beam, and passes through the substrate 70 onto the recording surface 71 on which information is magnetically recorded. A spot of around 1 gm is formed. The reflected light reflected by the recording surface 71 is modulated into light whose polarization plane is rotated in the opposite direction according to the recorded information (that is, due to a change in the magnetization direction). As described in the objective description again, this reflected light has a Kerr rotation component (S-polarized light).The incident light is guided while repeating total reflection on the surface of the parallel plate 64 or 67, and is guided at the end face of the polarizing beam splitter 63. The light enters a four-split photodetector 75. An analyzer 74 is provided immediately in front of the photodetector 75, and converts the magneto-optical signal into a change in the amount of light.

The polarizing beam splitter 63 shown in FIG.
5, four light-receiving surfaces are arranged in series in the direction of the paper. The light intensity distribution on the photodetector 75 changes depending on the focused state of the spot on the recording surface. For example, when the focal position of the objective lens 69 coincides with the recording surface 71, the reflected light 72 becomes parallel light, and the diffracted light 73 becomes a solid line in FIG.
It is incident in the shape shown in 73b. In addition, the objective lens 69
If the light 72 is too close to or too far from the recording surface, the reflected light 72 becomes a diverging light or a convergent light, and the rotating light 73
In FIG. 6, they are shown as dashed lines or broken lines, respectively, and take the shape shown as 73c or 73a, respectively, on the photodetector 75. The principle of detecting a focus error signal using such a change in the shape of the light beam will be explained in detail below.

FIGS. 7(a), (b), and (c) are views of the 4-split photodetector 75 viewed from the light incident side, and (b) shows the focused state fm, (a),
(c) shows an out-of-focus state. Here, 75a, '75
b, 75c, and 75d indicate the divided light-receiving surfaces, and the shape of the incident light beam is as described above at 73a, 73b, and 73c.
and changes. Light receiving surfaces 75a, 75b, 75c.

Letting the outputs from 75d be Ia, Ib, Ic, and Ici, respectively, the electrical system as shown in Figure 8(a) is (Ib+Ic).
By performing the calculation -(Ia+Id), a focus error signal as shown in FIG. 8(b) is obtained at the output terminal 78 of the differential amplifier 77. In FIG. 8(b), the horizontal axis shows the distance (focus error) between the objective lens and the recording surface when the in-focus position is zero, and the vertical axis shows the signal output. Autofocus is made possible by moving the objective lens 69 or the entire optical head relative to the disk along the optical axis of the incident light via an actuator (not shown) in accordance with the obtained focus error signal.

Next, the principle of auto-tracking in the embodiment shown in FIG. 5 will be explained. Figure 9(a).

A groove 70 is formed in the substrate 70 of the information carrier as shown in (b,) and (C).
If a is formed, the objective lens 69
The incident light beam is focused near this groove 70a. here(
b) is a state where a spot is generated on the target groove, (
In a) and (C), if the spot occurs to the right or left of the groove, tracking information is included due to scattering. When the reflected light is received by the 4-split photodetector 75 shown in FIG. 5, the amount of light received by the light receiving surfaces 75a, 75b, 75c, and 75d is as shown in FIGS. 9(a) and 9(b).

FIG. 1O(a), respectively, depending on the state of (c).

It changes as shown in (b) and (c). Therefore, No. 111g
In the electrical system shown in (a), (Ia + Ib) (
By performing the calculation Ic+Ic1), a tracking error signal as shown in FIG. 11(b) is obtained at the output terminal 80 of the differential amplifier 79. Figure 11(b)
, the horizontal axis shows the tracking error and the vertical axis shows the signal output. Auto) racking is possible by driving a tracking actuator (not shown) in accordance with the obtained tracking error signal and moving the objective lens perpendicularly to the optical axis. Here, we have explained the case where grooves as guide tracks are formed in advance on the substrate 70, but even if there is no such groove, the part (recording track) on the recording surface 71 where information is recorded, In other parts, the amount of light passing through the analyzer 74 differs due to the above-mentioned magneto-optic effect, and an imbalance occurs in the light amount distribution on the photodetector 75 depending on the positional relationship between the recording track and the spot. Therefore, even in such a case, a tracking error signal can be similarly obtained by calculating the output of each light receiving surface of the four-split photodetector 75 as shown in FIG. 11(a).

For example, when a transparent member is used for the folded lattice as shown in FIG. 12, a focusing effect can be provided by an optical system as shown in FIG. 13. In FIG. 13, parallel light beams 81 and 82 emitted from the same laser light source and split by an optical system (not shown) enter parallel to the rotation axis 85 into conical mirrors 83 and 84 that share the rotation axis 85, respectively. The two light beams reflected by each conical mirror become a conical wavefront having a focal line on the rotation axis 85 and are incident on the hologram sensitive material 87 on the substrate 86 . At this time, the area 88 on the photosensitive material surface
The interference fringes generated in this case have a three-dimensional conical shape with the rotation axis 85 as the center of rotation. Therefore, it is exposed in this way.

As explained above, the present invention utilizes a relief-type diffraction grating provided with a polarization-dependent fold-reflection film, thereby making a polarizing beam splitter thinner and lighter, and achieving effects such as reducing manufacturing costs. It's something you get.

[Brief explanation of the drawing]

FIGS. 1(a) and (b) are diagrams each illustrating the functions of a conventional polarizing beam splitter. FIGS. 2(a) and (b) are diagrams respectively illustrating the configuration of the polarizing beam splitter of the present invention. 4 and 5 are schematic diagrams showing an example of a configuration in which the polarizing beam splitter of the present invention is used in an optical receiver, respectively. Figure 6 is a diagram of the polarizing beam splitter in Figure 5 viewed from the semiconductor laser side, and Figures 7 (a), (b), and (c) are diagrams showing changes in the light amount distribution on the photodetector due to focus errors, respectively. FIGS. 8(a) and 8(b) are diagrams showing an electrical system for focus error detection and a focus error signal, respectively. FIGS. 9(a), (b), and (c) are diagrams showing the positional fluctuations of the light spot on the recording surface, and FIGS. 10(a) and (b), respectively.
, (c) are diagrams showing changes in the amount of light on the photodetector, respectively. Figures 11 (a) and (b) are perspective views showing the electrical system for tracking error detection and the shape of one side of the tracking error signal member, respectively. FIG. 13 is a diagram illustrating an example of a method for manufacturing the inclined surface shown in FIG. 12.

Claims (1)

[Claims] (1) A polarization-dependent reflective film is provided on the interface between first and second transparent members having substantially the same refractive index and having a plurality of inclined surfaces on at least one side to form a relief-type diffraction grating. A polarizing beam splitter consisting of: