JPH02259702A - Polarization diffraction element - Google Patents

Abstract

PURPOSE:To separate incident light to two parallel luminous fluxes of different polarization components by providing two diffraction gratings which have the grating intervals equal to each other and are so formed as to have the mutual grating directions coincident with each other respectively on the different faces of a flat plate substrate. CONSTITUTION:The diffraction gratings 3, 4 are formed with rectangular projecting parts 6... within the equal grating intervals D and are so provided as to allow nearly 100% transmission of the P polarization component of the incident light 5 and to allow nearly 100% diffraction of the S polarization component. The P polarization component of the incident light 5 on the polarization diffraction element 1 at an incident angle thetai1 is transmitted to 0 order diffracted light 5a through the diffraction gratings 3, 4 and S polarization component is diffracted as 1st order diffracted light 5b by the diffraction grating 3 and is further diffracted by the diffraction grating 4. These light rays are respectively emitted from the element 1. Since the gratings 3, 4 are formed at the grating interval D equal to each other, the 0 order diffracted light 5a and the 1st order diffracted light 5b are emitted in parallel from the element 1.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a diffraction element used for optical pickup, etc., and specifically relates to a polarization diffraction element that separates incident light into light beams of different polarization components. .

[Conventional technology]

It has been known that a diffraction grating with a grating interval approximately equal to the wavelength of light has polarization properties (Yo et al.
komori, “Dielectric surfac
e-relief gratings with hig
h diffraction efficiency.

Applied Optics Vol, 23. No,
14. pp2303.1984).

As shown in FIG. 5, the polarization diffraction element 21 includes a diffraction grating 2 having polarization characteristics, which is formed on one surface of a transparent substrate 22 such as glass, with a grating interval approximately equal to a predetermined wavelength of incident light.
3 is formed by a method such as two-beam interferometry.

The diffraction grating 23 is made of, for example, photoresist, and its thickness and grating interval are set to 2 μm and 0.5 μm, respectively. Moreover, the diffraction grating 23 transmits almost 100% of the P polarized light component, and transmits approximately 100% of the S polarized light component.
It is made to cause 00% diffraction.

For example, the polarization diffraction element 21 has a wavelength of 0.8μ.
When the incident light 24 of m is incident at the Bragg angle, the incident light 2
The P-polarized light component among the four polarized light components is transmitted through the diffraction grating 23 as O-order diffracted light 24a, and is hardly diffracted as first-order diffracted light 24b. On the other hand, the S-polarized component of the incident light 24 is diffracted by the diffraction grating 23 as first-order diffracted light 24b,
It is almost never transmitted as the 0th order diffracted light 24a.

In addition, in order to detect the polarization components of the 0th-order diffracted light 24a and the 1st-order diffracted light 24b separated in this way, they are passed through condensing lenses 25 and 26 to photodetectors 27 and 28, respectively.
This is done by focusing the light on the

As described above, the diffraction grating 23 has the characteristic of separating light of different polarization, and therefore can be used as a polarizing beam splitter for optical pickup in a magneto-optical recording/reproducing device. If the polarization diffraction element 21 is used in a polarization beam splinter, the number of parts can be reduced and the optical pickup can be made smaller and lighter.

[Problem to be solved by the invention]

However, in the conventional polarization diffraction element 21, the diffraction angle of the diffracted light by the diffraction grating 23 depends on the wavelength of the incident light 24. For example, when a laser diode is used as the light source,
The wavelength of the light emitted from the laser diode changes due to changes in the surrounding temperature, and the above-mentioned diffraction angle changes.

For example, when the wavelength of the incident light 24 is a predetermined wavelength, 1
The next diffraction light 24b is diffracted at a predetermined diffraction angle, and is correctly focused on the photodetector 28 by the condenser lens 26. At this time, when the ambient temperature decreases and the wavelength of the incident light 24 becomes smaller than the predetermined wavelength, the diffraction angle also becomes smaller accordingly, and the first-order diffracted light 24b diffracted by the diffraction grating 23 is As shown by the two-dot chain line, the light beam travels far away from the predetermined optical path. For this reason, the first-order diffracted light 24b is no longer focused on a predetermined position of the photodetector 28, resulting in the inconvenience that the S-polarized light component cannot be detected.

As mentioned above, in the polarization diffraction element 21, since the grating spacing of the diffraction grating 23 is set to about the wavelength, a slight change in the wavelength of the incident light 24 causes a large change in the diffraction angle.
The optical path of the first-order diffracted light 24b is moved. Therefore, it is conceivable to increase the size of the light receiving part of the photodetector 2 to receive the first-order diffracted light 24b that is out of the optical path. This also increases the size of the optical pickup, which is a negative factor when trying to make the optical pickup smaller and lighter. Furthermore, even if the photodetector 28 is made larger, the focal point of the first-order diffracted light 24b will no longer be formed at a fixed position on the photodetector 28 due to changes in the wavelength of the incident light 24, and the detection accuracy of the S-polarized light component will decrease. There also arises the problem of a decrease in the power.

Furthermore, the diffraction angle for diffracting the first-order diffracted light 24b is 100″
Since they are before and after, the 1st-order diffracted light 24b and the 0th-order diffracted light 24a
The photodetector 27 and
28 must be provided at positions spaced apart from each other. As a result, the optical pickup becomes large, making it difficult to reduce the size and weight of the optical pickup as in the case described above.

By the way, the diffraction grating 23 absorbs almost 100% of the P polarized light component.
However, in reality, when the incident light 24 enters only one diffraction grating 23, the P polarized light component becomes the O-order diffracted light 24.
Diffraction efficiency η when transmitted as a. 9, and the diffraction efficiency η when the S-polarized component is diffracted as the first-order diffracted light 24b.
1. are both about 0.99. Therefore, the O-order diffracted light 24a transmitted through the diffraction grating 23 has a diffraction efficiency η of about 0.01. , a slight amount of the S-polarized light component transmitted through the diffraction grating 23 is included. Furthermore, the first-order diffracted light 24b diffracted by the diffraction grating 23 has a diffraction efficiency η of about 0.01,
A small amount of the P-polarized light component diffracted by the diffraction grating 23 at p is included.

In this way, when the incident light 24 is separated into one diffraction grating 23, the ratio of the other polarized light components in the desired polarized light component, that is, the degree of polarization, is the diffraction efficiency η with respect to the diffraction efficiency η6p in the 0th-order diffracted light 24a. , expressed as the ratio of
In the first-order diffracted light 24b, it is expressed as the ratio of the diffraction efficiency η19 to the diffraction efficiency η. According to this, the degree of polarization of the 0th-order diffracted light and the 1st-order diffracted light is found to be approximately 0.
．． 01, and it cannot be said that the degree of separation between the P-polarized light component and the S-polarized light component is sufficiently satisfactory for practical use.

[Means to solve the problem]

In order to solve the above problems, the polarization diffraction element according to the present invention has a first diffraction grating and a second diffraction grating formed with a grating interval having a width approximately equal to a predetermined wavelength of incident light. The first diffraction grating and the second diffraction grating are provided on different surfaces of a shaped substrate, and the first diffraction grating and the second diffraction grating are formed so that they have equal grating intervals and their grating directions match. It is characterized by

[For production]

With the above configuration, when incident light of a predetermined wavelength enters the polarization diffraction element, the O-order diffracted light that has passed through the first diffraction grating is further transmitted through the second diffraction grating, while Of the incident light, the first-order diffracted light that is diffracted by the first diffraction grating enters the second diffraction grating, where it is further diffracted. The incident light is thus separated into beams of different polarization components.

Generally, the diffraction angle of diffracted light is determined by the grating spacing of the diffraction grating and the wavelength of the incident light. Therefore, if the grating spacing of the first diffraction grating and the grating spacing of the second diffraction grating are made equal to each other, The diffraction angle by the second diffraction grating becomes equal to the diffraction angle by the second diffraction grating. Therefore, when the first-order diffracted light diffracted by the first diffraction grating enters the second diffraction grating and is diffracted, it is emitted from the polarization diffraction element in parallel with the zero-order diffraction light.

Here, for example, as the wavelength of the incident light becomes shorter, the diffraction angles of the first-order diffraction light by the first diffraction grating and the second diffraction grating become smaller. However, the grating intervals of each diffraction grating are equal to each other. Since the diffraction angles are equal to each other and the first-order diffraction light is emitted in parallel with the zero-order diffraction light, the first-order diffraction light is As the diffraction angle becomes smaller, it deviates from the predetermined optical path and travels inside the substrate, but since the substrate is thin, the deviation from the optical path is minimal, and the first-order diffracted light is emitted from the polarization diffraction element. Therefore, when incorporating such a polarization diffraction element into an optical pickup, the first-order diffracted light is focused on a photodetector via an optical system such as a condensing lens. Even if the wavelength of the incident light changes, the first-order diffracted light can be detected at a predetermined position of the photodetector.

In addition, since the 0th-order diffracted light and the 1st-order diffracted light separated by the polarization diffraction element travel in parallel, when detecting these polarized light components, a photodetector for detection must be installed close to the photodetector. The space required can be reduced. Therefore, when incorporating a polarization diffraction element into an optical pickup,
Optical pickups can be easily made smaller and lighter.

Furthermore, by passing the incident light through two diffraction gratings, it is possible to reduce the degree of polarization of the O-order diffracted light and the first-order diffraction light, increase the degree of separation of polarized light components, and improve the accuracy of polarization detection.

[Embodiment 1] An embodiment of the present invention will be described below based on FIGS. 1 to 3.

As shown in FIG. 1, the polarization diffraction element 1 includes diffraction gratings 3 and 4 provided on both sides of a transparent, flat substrate 2 made of glass or the like. The grating spacing of the diffraction gratings 3 and 4 is set to about 50 wavelengths of incident light, and the grating directions of both gratings coincide with the direction perpendicular to the plane of the drawing. Further, the diffraction gratings 3 and 4 are manufactured as relief type diffraction gratings by etching the substrate 2, for example.

As shown in FIG. 2, the diffraction gratings 3 and 4 have rectangular convex portions 6 formed within equal grating intervals. The diffraction gratings 3 and 4 transmit almost 100% of the P-polarized component of the incident light 5 whose electric field vibrates parallel to the plane of the paper in FIG. The S-polarized light component vibrating in the direction perpendicular to the plane of the paper is approximately 100
% diffraction. Therefore, for example, if the wavelength of the incident light 5 in FIG. 1 is 0.8 μm and the refractive index n of the substrate 2 is 1.5, the lattice spacing is approximately 0.5 μm.
m, the depth of the grating is also about 1 μm, and the ratio of the width of the convex portion 6 to the grating interval, that is, the duty D, /D is set to 0.5.

In the above configuration, the incident light 5 enters the polarization diffraction element 1 at an incident angle θ1. , the P polarized light component becomes the 0th order light 5a
The light passes through the diffraction gratings 3 and 4 as a beam and exits from the polarization diffraction element 1. On the other hand, the S-polarized component of the incident light 5 is the first-order diffracted light 5
As b, the light is diffracted by the diffraction grating 3 at a diffraction angle θ1, further diffracted by the diffraction grating 4 at a diffraction angle θ2, and output from the polarization diffraction element 1.

At this time, the above diffraction angles θ1 and θ2 are the diffraction gratings 3 and 4.
Since it is determined by the grating spacing of
a and the first-order diffraction light 5b are emitted from the polarization diffraction element 1 in parallel. Furthermore, if the incident angle θi1 of the incident light 5 is set to the so-called Bragg angle that satisfies the formula θt+=sin −' (λ/2D), where the wavelength of the incident light is λ, then the above incident angle θif can be made equal to the incident angle θiz at which the first-order diffracted light 5b diffracted by the diffraction grating 3 enters the diffraction grating 4, making it easier to match the characteristics of the diffraction gratings 3 and 4.

Next, the 0th order light beams 5a and 1 separated as described above are
Detection of the next diffracted light 5b will be explained.

As shown in FIG. 3, a condenser lens 7 is provided on the optical path of the 0th-order index light 5a at a predetermined distance from the polarization diffraction element 1, and a photodetector 8 is provided at a predetermined condensing position. It is provided. On the other hand, a condenser lens 9 is provided on the optical path of the first-order diffracted light 5b at a predetermined distance from the polarization diffraction element 1, and a photodetector 10 is provided at a predetermined condensing position. The photodetectors 8 and 10 are arranged in packages 11 and 12, respectively.

In such a configuration, if the wavelength of the incident light 5 becomes shorter than a predetermined wavelength for some reason, both the diffraction angles θ and .theta.2 become smaller and the diffraction angle θ becomes smaller.

θ2'. Therefore, the first-order diffracted light 5b diffracted by the diffraction grating 3 travels away from the predetermined optical path as shown by the two-dot chain line in the figure, and the position where it enters the diffraction grating 4 is also shifted. However, since the substrate 2 is thin, the deviation of the optical path is very small, and 1
The position where the next diffraction light 5b exits from the polarization diffraction element 1 is also slightly shifted. Furthermore, as described above, since the grating intervals of the diffraction gratings 3 and 4 are equal to each other, the diffraction angle θ.

- θ2' is also made equal, and the first-order diffracted light 5b is emitted from the polarization diffraction element 1 in parallel with the zero-order index light 5a. Therefore, even if the wavelength of the incident light 5 is small (even if the wavelength of the incident light 5 becomes small), the output position of the first-order diffracted light 5b in the polarization diffraction element 1 does not shift significantly, and the first-order diffracted light 5b is directed to the photodetector 10 by the condenser lens 9. The light can be focused at a predetermined position.

Furthermore, the degree of polarization by the diffraction gratings 3 and 4 will be explained.

As shown in FIG. 1, the O-order pointing light 5a of the incident light 5 transmitted through the diffraction grating 3 actually has a diffraction efficiency η. In addition to the P-polarized component transmitted by 2, the diffraction efficiency η. .

It contains a small amount of the S-polarized light component that was transmitted through the camera.

The degree of polarization at this time is the diffraction efficiency η. 2・η0. Assuming that they are 0.99 and 0.01, respectively, it becomes approximately 0.01.

The diffraction efficiency η is the same as that of the diffraction grating 3. 2・η. When the zero-order light 5a is incident on the diffraction grating 4 having a diffraction efficiency of η. ,・η. 3 are each squared, and the P polarization component is the diffraction efficiency η. p′, and the S polarized component has a diffraction efficiency η
. Transmits at S'. Therefore, as above, the 0th order instruction 5
The polarization degree of a is approximately 0°0001, for example,
The value is 1/100 compared to the case where only the diffraction grating 3 is transmitted.

On the other hand, the incident light 5 is diffracted by the diffraction grating 3 as first-order diffracted light 5b with a diffraction efficiency ηIp of the P-polarized light component and a diffraction efficiency η,5 of the S-polarized light component. And this first-order diffracted light 5
b is incident on the diffraction grating 4, and the P polarized light component has a diffraction efficiency ηtp
t, and the S-polarized component is diffracted with a diffraction efficiency η,3'. Therefore, the diffraction efficiency η and p・η are each 0.0.
1 and 0.99, the degree of polarization of the first-order diffracted light 5b is approximately 0.01 at the diffraction grating 3, and approximately 0.01 at the diffraction grating 4.
OOO1, which is S included in the 0th order instruction 5a
It is so small that it can be considered that there is almost no polarization component,
This means that the P-polarized light component included in the first-order diffracted light 5b also becomes smaller.

Therefore, as described above, by passing the incident light 5 through the diffraction gratings 3 and 4, the P polarization component and the S
The degree of separation from polarized light components can be made high enough to be practically satisfactory.

[Embodiment 2] Next, another embodiment of the present invention will be described based on FIG. 4. It should be noted that members having the same functions as those in the first embodiment will be denoted by the same numbers and their explanations will be omitted.

As shown in FIG. 4, a condenser lens 13 for condensing both the 0th-order index light 5a and the 1st-order diffracted light 5b is provided on the optical path of each diffracted light, and a predetermined condensing lens 13 is provided on the optical path of each diffracted light. The photodetectors 8 and 10 are placed in the same package 14 at the same location.

In such a configuration, the O-order light 5a and the 1st-order diffracted light 5b are condensed by one condenser lens 13 to a position approximately midway between the two light components, and the photodetectors 8 and 10 are housed in the same package 14. They can be arranged side by side on the inner substrate part, or formed on a single semiconductor substrate such as St, and arranged close to each other.

In the polarization diffraction element 1 of this example and the above-mentioned example 1, the diffraction gratings 3 and 4 are formed on both sides of the substrate 2 by etching, but in addition to this, they may be formed of, for example, photoresist. Of course, it is also good. Also, the diffraction grating 3
- The shape of the lattice 4 is not limited to the rectangular shape shown in FIGS. 1 and 2.

〔Effect of the invention〕

As described above, the polarization diffraction element according to the present invention has a first grating formed with a grating interval having a width approximately equal to a predetermined wavelength of incident light.
a diffraction grating and a second diffraction grating are provided on different surfaces of a flat substrate, and the first diffraction grating and the second diffraction grating have equal lattice spacing, and , the lattice directions are formed to match each other.

According to this, the incident light is separated into light beams of different polarization components by the first diffraction grating and the second diffraction grating, and is always emitted in parallel from the polarization diffraction element. therefore,
Even if the wavelength of the incident light changes, the parallel relationship between the light beams of the polarized light components can be maintained.

Therefore, when a polarization diffraction element is incorporated into an optical pickup, the optical pickup can be made smaller by providing photodetectors for detecting different polarization components in close positions. Further, by guiding each polarized light component to a predetermined position of the photodetector via an optical system such as a condensing lens, it is possible to improve the detection accuracy of the polarized light component.

[Brief Description of the Drawings] Figures 1 to 3 show an embodiment of the present invention, and Figure 1 shows the configuration of a polarization diffraction element and the state of diffraction of incident light by the polarization diffraction element. 2 is a partially enlarged view of FIG. 1 showing the configuration of the diffraction grating, FIG. 3 is a vertical sectional view showing the configuration for detecting polarized light components separated by a polarization diffraction element, and FIG. 4 is a partial enlarged view of FIG. FIG. 7 is a vertical cross-sectional view showing another embodiment of the invention and showing a configuration for detecting polarized light components separated by a polarization diffraction element. FIG. 5 is a vertical cross-sectional view showing the configuration of a conventional polarization diffraction element and the state of diffraction of incident light by the polarization diffraction element. 2 is a substrate, and 3 and 4 are diffraction gratings. Patent Applicant: SHARP Co., Ltd. F1

Claims (1)

[Claims] 1. A first diffraction grating and a second diffraction grating formed with a grating interval having a width approximately equal to a predetermined wavelength of incident light are provided on different surfaces of a flat substrate, and A polarization diffraction element characterized in that the diffraction grating and the second diffraction grating have equal grating intervals and are formed so that their grating directions coincide with each other.