JPH10104427A - Wavelength plate, and optical pickup unit equipped with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a wavelength plate for actualizing an optical pickup unit which has a superior jitter characteristics by suppressing a variation in a photodetection quantity of return light by an optical detector and maintaining a photodetection quantity at a specific level. SOLUTION: The wavelength plate 1 has two kind of phase difference areas A and B formed in specific pattern on the light passing surface of its substrate 1. This pattern is so formed that when light travels in the wavelength plate 1 forward and backward, light passing through the same phase difference area and light passing through the different phase difference area become equal in the quantity of light. Therefore, when the wavelength plate 1 is used for the optical pickup unit, the level of the photodetection quantity of return light by the optical detector and the range of the variation in the photodetection quantity can be set by controlling the phase difference between the phase difference areas A and B. Consequently, the wavelength plate 1 can be provided which is not affected by the birefringence of an optical recording body and has superior jitter characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup device for reproducing / recording an optical recording medium or performing one of the operations, for separating outgoing light from a light source and returning light from an optical recording medium. It relates to a wave plate such as a quarter wave plate to be used.

[0002]

2. Description of the Related Art An optical pickup device for reproducing an optical recording medium such as a compact disk (CD) includes:
2. Description of the Related Art A polarization type optical pickup device in which a polarization beam splitter (PBS) and a 波長 wavelength plate are arranged on an optical path from a laser light source to a photodetector is known. In such an optical pickup device, light emitted from a laser light source is
After passing through the お よ び wavelength plate and the 波長 wavelength plate, it is irradiated as a light spot on the recording surface of the optical recording medium, and the return light from this recording surface passes through the １ ／ wavelength plate and the PBS again. . The return light from the recording surface is changed into a laser light having a polarization direction different from the polarization direction of the emitted light from the laser light source by 90 degrees after passing through the quarter wavelength plate, and is provided in a direction different from the direction of the laser light source. The light is guided to a photodetector. Such a polarization type optical pickup device is excellent in light use efficiency, and is often used in an optical pickup device for recording information on an optical recording medium.

[0003]

Here, the reflecting surface of the optical recording medium also has birefringence. For this reason, not only the phase difference due to the quarter-wave plate but also the phase difference due to the birefringence of the optical recording medium occurs in the return light from the optical recording medium. Therefore, the return light passing through the quarter-wave plate is not linearly polarized light but elliptically polarized light, and is not only a linearly polarized light component perpendicular to the polarization direction of light emitted from the laser light source, but is a parallel linearly polarized light component. Will be included. If the amount of birefringence due to the optical recording medium cannot be ignored, the following problems will occur.

FIG. 6A shows the relationship between the phase difference caused by the birefringence of an optical recording medium and the amount of return light received by a photodetector when a quarter wavelength plate is used. Note that the vertical axis indicates the ratio Pr / Po of the amount of light Pr received by the photodetector and the amount of light Po emitted from the laser light source. As shown in this figure, in an optical pickup device using a quarter-wave plate, when a phase difference due to birefringence of an optical recording medium is included in return light, a photodetector is used in accordance with the phase difference. The received light amount Pr of the return light decreases. Especially,
When the phase difference caused by the birefringence of the optical recording medium is π radian, the return light guided to the photodetector becomes zero,
An information signal from the optical recording medium cannot be obtained.

Further, since the birefringence of the optical recording medium differs depending on the location, the phase difference given to the return light changes with the rotation of the optical recording medium. For this reason, if the phase difference caused by the birefringence of the optical recording medium varies in the range of 0 to π radians, the amount Pr of light received by the photodetector of the return light varies in the range of 0 to 1. The fluctuation of the amount of received light Pr of the return light deteriorates the fluctuation (jitter) characteristic of the output signal from the photodetector.

Here, if the phase difference caused by the birefringence of the optical recording medium is within the range of 0 to π radians, it is 1/4.
By using a ８ wavelength plate instead of a wavelength plate, FIG.
As shown in (B), return light guided to the photodetector can be prevented from becoming zero. Further, the fluctuation of the amount of received light Pr of the return light by the photodetector can be kept in a range of approximately 0.5 to 1. However, the fluctuation width of the received light amount Pr is large, and the possibility that the return light guided to the photodetector becomes zero remains.

SUMMARY OF THE INVENTION An object of the present invention is to maintain the amount of return light received by a photodetector at a constant level, to eliminate the possibility that the amount of received light becomes zero, to suppress fluctuations in the amount of received light, and to reduce the amount of received light. It is an object of the present invention to propose a wavelength plate for realizing an optical pickup device having excellent jitter characteristics by maintaining the wavelength plate at a constant level.

[0008]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention relates to a substrate and a light transmitting surface of the substrate, which provide different phase differences to light passing through the light transmitting surface. The following configuration is adopted in a wave plate having at least three types of retardation regions. That is, the first
The size of these phase difference regions is such that the amount of light passing through the same type of phase difference region and the amount of light passing through the different type of phase difference region are substantially equal. The structure formed so that it may become is employ | adopted.

In the wavelength plate of the present invention, when light passes through the phase difference region, it receives a predetermined phase difference, and the phase difference received at this time differs for each phase difference region passing therethrough. Of light having different phase differences. For this reason, in a polarization type optical pickup device that transmits or reflects only light having a polarization direction perpendicular to the polarization direction of light emitted from a light source in a predetermined direction,
Even if the return light from the optical recording medium includes a phase difference due to the birefringence of the optical recording medium, the return light passing through the wave plate of the present invention has a polarization direction perpendicular to the polarization direction of the output light. There is always light with the component Therefore, the amount of return light read by the photodetector does not become zero, so that an information signal from the optical recording medium can be reliably obtained.

Further, in the wave plate of the present invention, the amount of return light received by the photodetector can be made equal to or more than a certain level by controlling the phase difference of each phase difference region.
Fluctuations in the amount of received light can also be reduced. Therefore,
A wavelength plate suitable for an optical pickup device having excellent jitter characteristics can be provided.

In the case of a wavelength plate having first and second retardation regions, the difference between the phase difference due to the first retardation region and the phase difference due to the second retardation region is a multiple of π. It is desirable to set so that With this setting, even if a phase difference occurs due to the birefringence of the optical recording medium, the amount of light received by the photodetector can be maintained at a constant level. For this reason, by using the wave plate of the present invention, it is possible to realize a polarization type optical pickup device which is not affected by the birefringence of the optical recording medium.

Further, as a second configuration of the wave plate of the present invention, the size of each phase difference region is such that the amount of passing light reciprocating through the same type of phase difference region passes through the phase difference regions having different amounts. A configuration formed so as to be twice as much as the light amount of the passing light that reciprocates can be adopted. In such a configuration, the first
And the wavelength plate having the second retardation region, the difference between the phase difference due to the first retardation region and the phase difference due to the second retardation region is a multiple of 2π / 3. It is preferable to set.

Each of the retardation regions of the wave plate of the present invention can be formed of a birefringent film having the same crystal optic axis but different thicknesses. Further, each retardation region can be formed of at least one of a birefringent film, a dielectric film, and an air layer. Further, if an antireflection film is formed in the retardation region, light reflection can be prevented, so that a wavelength plate excellent in light use efficiency can be provided.

The substrate of the wave plate of the present invention is rectangular, and the direction of each side of the substrate is the direction of the crystal optical axis of the first retardation region or the second retardation region. Is preferably set to be a predetermined angle with respect to. In this way, when the wavelength plate is mounted on the optical pickup, each side serves as a mark in the direction of the crystal optical axis. For this reason, it is easy to mount the wave plate on the optical pickup.

An optical pickup device using a wave plate according to the present invention includes a light source for emitting light passing through the wave plate, an objective lens for condensing the light passing through the wave plate as a light spot on an optical recording medium. And a beam splitter for guiding return light from the recording medium to a photodetector. In such an optical pickup device, when the direction of the crystal optical axis of the optical recording medium is known, it is desirable to match the direction of the crystal optical axis of the wave plate of the present invention with the direction of the crystal optical axis of the optical recording medium. .

[0016]

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

[Embodiment 1] FIG. 1A is a perspective view of a wave plate to which the present invention is applied. As shown in this figure, a wave plate 1 of the present example has a rectangular substrate 2 and two types of retardation regions A formed on a surface (light passing surface) 201 of the substrate 2.
And B. The direction of each side of the substrate 2 is set to be a predetermined angle with respect to the direction of the crystal optical axis of the phase difference region A or the phase difference region B. The substrate 2 is formed of, for example, a glass substrate, a silicon substrate, a plastic substrate, or the like, and is transparent to the wavelength of light incident on the wave plate 1. The substrate 2 has a flat front surface 201 and a flat back surface 202. These surfaces 201 and 202 are used as light passing surfaces 201 and 20 through which light emitted from a light source and returned light from an optical recording medium pass.
It is 2. In FIG. 1A, a circular region R indicated by a broken line is a region irradiated with laser light. In the following description and drawings of the present specification, only the circular region R to which light is irradiated out of the wave plate is taken out and described and illustrated.

As shown in FIG. 1B, a circular region R of the light passing surface 201 of the substrate 2 to which the laser beam is irradiated is
Two types of retardation regions A and B are formed in a predetermined pattern with the same area (size). In the wavelength plate 1 of this example, the phase difference region A is formed in a range of 135 degrees with respect to the center O of the light passing surface 201, and the phase difference region A is clockwise from the phase difference region A to a range of 45 degrees in the drawing. Phase difference area B, 4
The phase difference region A is formed in the range of 5 degrees, and the phase difference region B is formed in the range of 135 degrees in this order.

The phase difference region A is formed from a birefringent film 3 having a predetermined thickness. The birefringent film 3 is made of, for example, an inorganic substance such as tantalum pentoxide, tungsten oxide, bismuth trioxide, titanium oxide, etc., in a direction making a predetermined angle with respect to the light passing surface 201 of the substrate 2 with respect to its normal direction. It has been deposited. The birefringent film 3 can be formed so as to have an optical action such as a １ ／ wavelength plate or a ８ wavelength plate by adjusting the film thickness. For this reason, the light that has passed through the phase difference region A receives a phase difference of a predetermined wavelength A ′.

The retardation region B is also formed of a birefringent film 4 having a predetermined thickness, and is formed by depositing the same inorganic substance as that of the retardation region A. Therefore, by adjusting the thickness of the birefringent film 4, the light that has passed through the phase difference region B also receives a phase difference of a predetermined wavelength B '.

The birefringent films 3 of the retardation regions A and B
And 4 need not be formed of the same inorganic substance, and may be formed of a birefringent material such as quartz or a polymer film. Instead of giving both of the retardation regions A and B birefringence, one of the retardation regions may be formed of a material transparent to an air layer or light. Further, one of the retardation regions A and B may be formed from a dielectric film. Furthermore, an anti-reflection film may be formed on the retardation regions A and B in order to prevent light reflection and increase light use efficiency.

The method for producing the birefringent films 3 and 4 is as follows.
Instead of vapor deposition, a sputtering method can be used. When the birefringent films 3 and 4 are formed by vapor deposition or sputtering, the use of a metal mask makes it easier to form the phase difference regions A and B in a predetermined pattern. Alternatively, the polymer film may be stretched in one direction to produce the birefringent films 3 and 4, and thereafter, the stretched polymer film may be attached to a substrate. Further, it is also possible to cut crystal having a birefringence, lithium niobate (LiNbO3), or the like into a predetermined shape by crystal growth, and attach the cut crystal or the like to a substrate.

As shown in FIG. 2, the incoherent light L
Is incident on the wave plate 1, passes through the phase difference region A, and A ′
The incoherent light L including the light La having received the phase difference of the wavelength and the light Lb having passed the phase difference region B and having received the phase difference of B ′ wavelength is emitted from the wavelength plate 1. afterwards,
When the incoherent light L is reflected by the optical recording medium or the like and is incident again on the wave plate 1, as shown in FIG. 3, the light is emitted from a position which is point-symmetric with the light La and Lb emitted from the wave plate 1. Light La and Lb enter.

As a result, when the incoherent light L passes through the wave plate 1 again, it passes through the phase difference region A twice and
Light Laa having received a phase difference of A 'wavelength, light Lab having passed a phase difference region A, and then having passed a phase difference region B and having received a phase difference of (A' + B ') wavelength, phase difference region B After passing through the phase difference region A, the light beam Lba that has received a phase difference of (A '+ B') wavelengths, and the light beam Lba that has passed through the phase difference region B twice and has received a phase difference of 2B 'wavelengths. Light Lbb will be present. Further, these four types of light Laa, La
Since b, Lba, and Lbb occupy the same proportion in the incoherent light L, the respective light amounts are all equal. In the case of coherent light, the light emitted from the wave plate 1 can be condensed on one point and can be regarded as being emitted from that point as a light source. Also in this case, there are four types of light Laa, Lab, Lba, and Lbb, and the light amounts of these lights Laa, Lab, Lba, and Lbb are all equal.

The patterns of the phase difference regions A and B formed on the light-irradiated portion of the light passing surface 201 of the wave plate 1 are not limited to the above-mentioned patterns, and the incoherent light reciprocates through the wave plate. But four kinds of light Laa, La
What is necessary is just to employ | adopt the pattern which makes b, Lba, and the light quantity of Lbb all equal. For example, FIG.
To (D). In the patterns shown in FIGS. 4A to 4D, the amounts of light applied to the phase difference regions A and B are equal, and the four patterns of light Laa, Lab, Lba, and
And Lbb are equal. In the case where only coherent light is used, a pattern in which concentric circles having substantially equal areas and the period of the diameter is such that a lens effect does not occur as shown in FIG. 4E may be used. Further, instead of forming two types of retardation regions A and B, three or more types of retardation regions can be formed. In this case, the size of these phase difference regions is set so that the amount of light passing through the same type of phase difference region and the amount of light passing through the different phase difference region are substantially equal. It may be formed.

The wavelength plate 1 configured as described above can be used as one of the optical components constituting the same in a polarization type optical pickup device for an optical recording medium.
With reference to FIG. 5, a description will be given of a polarization type optical pickup device using the wavelength plate 1 to which the present invention is applied. As shown in FIG. 5, a schematic configuration of the optical pickup device 20, the optical system of the optical pickup device 20 focuses laser light Lo emitted from a laser diode (LD) 21 as a laser light source on an optical recording medium 25. Outgoing path and optical recording medium 2
And a return path for guiding the return light Lr from 5 to the photodetector 29. On the outward path, the optical recording medium 25 is
The grading lens 22, the polarizing beam splitter (PBS) 23, the wave plate 1, and the objective lens 24 are arranged in this order. The grading lens 22 is configured to split the laser light Lo from the LD 21 into five laser lights.

In the optical pickup device 20, the LD2
The laser light Lo emitted from 1 is divided into five laser lights by the grading lens 22. The five divided lights are changed in traveling direction by approximately 90 degrees by the PBS 23 and guided to the wave plate 1.

In the wavelength plate 1, the directions of the crystal optical axes of the birefringent films 3 and 4 constituting the phase difference regions A and B are aligned with the direction of the crystal optical axis of the optical recording medium 25. As described above, in the wavelength plate 1, the direction of each side of the substrate 2 and the direction of the crystal optical axis of the phase difference region A or the phase difference region B are set at a predetermined angle. Therefore, by arranging the wave plate 1 with each side of the substrate 2 as a mark, the retardation regions A and B
Can be easily matched with the direction of the crystal optical axis of the birefringent films 3 and 4 and the direction of the crystal optical axis of the optical recording medium 25.

The wave plate 1 has a phase difference region A of 3/8.
The thicknesses of the birefringent films 3 and 4 are determined so that the wave plate and the retardation region B have an optical function as a １ ／ wave plate. The film thickness, for example, film thickness of 1/8 wavelength plate d1 is, λ / 8 = d1 (n e -n o) where, n _e: refractive index for extraordinary light n _o: a refractive index for ordinary light lambda: laser light Lo is the wavelength of 3/8 thickness d3 of the wave plate is a _{3λ / 8 = d3 (n e} -n o). As for the thickness of the birefringent films 3 and 4 in the retardation regions A and B, the birefringent films constituting the retardation regions A and B are provided if the retardation between the retardation regions A and B is satisfied. It is also possible to make the thicknesses of the retardation regions A and B the same by making the materials 3 and 4 different. Regarding the selection of the material of the birefringent films 3 and 4,
It is preferable to select a material that reduces the wavefront aberration.

The split light that has passed through the wavelength plate 1 has the light La that has passed through the phase difference area A and has received a phase difference of ／ wavelength as described above, and the light La that has passed through the phase difference area B has 1 Light Lb having a phase difference of / 8 wavelength is generated. The divided lights including these lights La and Lb are respectively condensed as light spots on the recording surface 251 of the optical recording medium 25 via the objective lens 24. The condensed split light is applied to the recording surface 251.
Are reflected while undergoing intensity modulation based on the data recorded in the optical path, and are respectively returned to the return path as return light Lr.

On the return path, the optical detector 2
The objective lens 24, the wave plate 1, and the PBS 23 are arranged in this order toward 9. The return light Lr from the optical recording medium 25 is guided to the wave plate 1 via the objective lens 25.
When the return light Lr passes through the wave plate 1, as described above,
The light Laa received a phase difference of 、 ３ wavelength and the light L received a phase difference of 波長 wavelength by the phase difference areas A and B.
ab, light Lba also having a phase difference of 波長 wavelength,
And a light Lbb that has received a phase difference of 1/4 wavelength. These lights Laa, Lab, Lba, and Lbb
Are all equal. These four types of light Laa, La
Return light Lr containing b, Lba, and Lbb is PBS
Only the polarization direction component perpendicular to the polarization direction of the light Lo emitted from the LD 21 is guided to the photodetector 29 through the PBS 23. The photodetector 29 includes five light receiving elements, and the return light Lr is focused on these light receiving elements. A focusing error signal (FE signal), a tracking error signal (TE signal), and a pit signal (RF signal) can be detected based on the light spots received by these five light receiving elements.

In the optical pickup device 20 provided with the wave plate 1 having the phase difference regions A and B having the optical function as the と し て wavelength plate and the ８ wavelength plate, the return light Lr Even if a phase difference due to the birefringence of the optical recording medium 25 is included, the four types of light Laa, Lab, Lba, and Lbb having different phase differences and the same amount of light are P
It is incident on BS23. For this reason, L
Even when only the light in the polarization direction perpendicular to the polarization direction of the laser light Lo from D21 is guided to the photodetector 29, the amount Pr of the return light Lr received by the photodetector 29 can be obtained based on the equation (1). As shown in FIG. 6C, the level is almost constant. Therefore, it is possible to solve the problem that the information signal from the optical recording medium 25 cannot be obtained. Further, a 波長 wavelength plate or a 1/4 wavelength plate as shown in FIGS.
The range in which the light receiving amount Pr of the return light Lr by the photodetector 29 fluctuates as compared with the case where the eight-wavelength plate is employed in the optical pickup device. Therefore, by using the wavelength plate 1, the optical pickup device 2 having excellent jitter characteristics can be obtained.
0 can be realized.

Equation (1) determines the ratio (Pr / Po) of the amount of light Po of the laser light Lo from the LD 21 and the amount of light Pr of the perpendicular component to the laser light Lo in the return light Lr from the optical recording medium 25. It is a calculation formula. In the formula (1), a is a phase difference due to the birefringence of the optical recording medium 25, delta1 is a phase difference A 'received when passing through the phase difference region A, and delta2 is received when passing through the phase difference region B. This is the phase difference B ′. In the wavelength plate 1 arranged in the optical pickup device 20, delta1 is 3λ.
/ 8 and delta2 correspond to λ / 8.

[0034]

(Equation 1)

In the wave plate 1, a 3/8 wave plate and 1
Instead of forming the phase difference regions A and B so as to have an optical function as a / 8 wavelength plate, the phase difference region B is used as an air layer so that no phase difference is generated, and the phase difference region A is reduced to 1/4. The birefringent film 3 having such a thickness as to have an optical function as a wavelength plate may be formed. In this case, when the phase difference caused by the birefringence of the optical recording medium 25 is in the range of 0 to π radian as shown in FIG. An amount of light equal to or more than half of Lo is guided to the photodetector 29. Moreover, in this case, the fluctuation of the light receiving amount Pr of the return light Lr by the photodetector 29 falls within a range of about 0.5 to 0.75. Further, even if the fluctuation width of the phase difference caused by the birefringence of the optical recording medium 25 increases, the amount Pr of the return light Lr received by the photodetector 29 does not become zero. For this reason, the optical pickup device 20 having more excellent light use efficiency and jitter characteristics
Can be realized. In addition, there is an advantage that the wave plate 1 can be easily manufactured because the phase difference region B is an air layer.

The phase difference A ′ (delta1) received by passing through the phase difference area A and the phase difference area B
Phase difference B ′ (delta
The birefringent films 3 and 4 may be formed so that the difference from 2) is a multiple of π. In this case, as shown in FIG. 6E obtained based on the equation (1), the light amount of the laser light Lo from the LD 21 is almost half of that of the laser light Lo without being affected by the phase difference due to the birefringence of the optical recording medium 25. Po is guided to the photodetector 29. Therefore, the optical pickup device 20 which is not affected by the phase difference due to the birefringence of the optical recording medium 25 can be realized.

In the optical pickup device 20 having the wavelength plate 1 as described above, the thickness of the birefringent films 3 and 4 constituting the retardation regions A and B of the wavelength plate 1 is controlled, so that the optical detector Receiving amount Pr of return light Lr due to 29
Alternatively, a range in which the amount of received light Pr fluctuates is set and the optical recording medium 2 is set.
Optical pickup device 2 less affected by birefringence of 5
0 can be realized. For example, when the phase difference due to the optical recording medium 25 is large, the level of the light reception amount Pr of the return light Lr by the photodetector 29 can be reduced to reduce the fluctuation of the light reception amount Pr. Further, when the phase difference of the optical recording medium 25 is small, the light receiving amount Pr of the return light Lr by the photodetector 29 can be designed to a high level.

The polarization type optical pickup device 20 including the wavelength plate 1 maintains the ability to condense the laser light Lo from the LD 21 onto the recording surface 251 of the optical recording medium 25 with almost 100% energy efficiency. However, the amount Pr of the return light Lr received by the detector 29 can be designed at a high level. Therefore, the configuration is optimal for an optical pickup device for recording on the optical recording medium 25.

[Second Embodiment] The wavelength plate of the second embodiment is set so that (light amount of light Laa) = (light amount of light Lbb) = {(light amount of light Lab) + (light amount of light Lba)}. The phase difference areas A and B are formed in a predetermined pattern and size. In the wavelength plate 1a of the present example, portions having the same functions as those of the wavelength plate 1 of the first embodiment are denoted by the same reference numerals, and description thereof is omitted. Further, also in the second embodiment, a portion of the wave plate 1a to be irradiated with light will be described. As shown in FIG. 7, in the wave plate 1 a of the present example, the center O of the light passing surface 201 is
The phase difference region A is formed in the range of degrees, and the phase difference region A
The phase difference region B is formed in this order in the range of 60 degrees, the phase difference region A is formed in the range of 60 degrees, and the phase difference region B is formed in the range of 120 degrees in the clockwise direction.

As shown in FIG. 8, the incoherent light L
Is incident on the wave plate 1a, light La passing through the phase difference region A and receiving a phase difference of A ′ wavelength, and light Lb passing through the phase difference region B and receiving a phase difference of B ′ wavelength Is emitted from the wave plate 1a. Thereafter, when the incoherent light L is reflected by the optical recording medium or the like and is incident again on the wave plate 1a, as shown in FIG. 9, the light La and the light La emitted from the wave plate 1a are point-symmetric. Light La and Lb enter from the position.

As a result, when the incoherent light L passes through the wave plate 1 again, it passes through the phase difference region A twice and
Light Laa having received a phase difference of A 'wavelength, light Lab having passed a phase difference region A, and then having passed a phase difference region B and having received a phase difference of (A' + B ') wavelength, phase difference region B After passing through the phase difference region A, the light beam Lba that has received a phase difference of (A '+ B') wavelengths, and the light beam Lba that has passed through the phase difference region B twice and has received a phase difference of 2B 'wavelengths. Light Lbb will be present. These four types of light Laa, Lab, Lb
The relationship between the light amounts of a and Lbb is as follows.
According to the ratio of a, Lab, Lba, and Lbb to the incoherent light L, (light amount of light Laa) = (light Lb
b) = {(light amount of light Lab) + (light amount of light Lba)}.

The pattern of the phase difference areas A and B formed on the light-irradiated portion of the light passing surface 201 of the wave plate 1a has four patterns even if the incoherent light reciprocates through the wave plate. Light Laa, Lab, Lba, and L
The relationship between the light amounts of bb is (light amount of light Laa) = (light amount of light Lbb) = {(light amount of light Lab) + (light amount of light Lba)}
What is necessary is just to make the pattern which satisfies. For example, FIG.
If patterns such as those shown in FIGS. 0 (A) to (D) are adopted,
There are four types of light Laa, Lab, Lba, and Lbb, and the relationship between the light amounts of these lights is (light amount of light Laa) = (light amount of light Lbb) = ｛(light amount of light Lab) +
(Light amount of light Lba). Also, in the wavelength plate 1a, instead of forming two types of retardation regions A and B, it is also possible to form three or more types of retardation regions. In this case, a phase difference region having a size such that the light amount of the passing light reciprocating through the same type of phase difference region is twice the light amount of the passing light reciprocating through the different phase difference region is formed. I just need.

The wavelength plate 1a configured as described above can be used as one of the optical components constituting the optical pickup device 20 for the optical recording medium 25, similarly to the wavelength plate 1 of the first embodiment.

In the wavelength plate 1 a used in the optical pickup device 20, the directions of the crystal optical axes of the birefringent films 3 and 4 constituting the phase difference regions A and B coincide with the directions of the crystal optical axes of the optical recording medium 25. Let me do it. Also in the wavelength plate 1a, by arranging each side of the substrate 2 as a mark, the directions of the crystal optical axes of the birefringent films 3 and 4 constituting the phase difference regions A and B and the crystal optical axis of the optical recording medium 25 are determined. Can be easily adjusted. Further, when the phase difference area A is 1
The thicknesses of the birefringent films 3 and 4 are determined and formed so that the 波長 wavelength plate and the retardation region B have an optical function as a ８ wavelength plate.

In the polarization type optical pickup device 20 having such a wavelength plate 1a, the divided light having passed through the wavelength plate 1a receives a phase difference of 1/4 wavelength through the phase difference area A. The light La and the light Lb that has passed through the phase difference area B and received a phase difference of ８ wavelength are generated. The split light is applied to the recording surface 251 of the optical recording medium 25 via the objective lens 24 by a light spot. As light.

When the return light Lr from the recording surface 251 of the optical recording medium 25 passes through the wavelength plate 1a, the light Laa having received a phase difference of 波長 wavelength by the phase difference areas A and B as described above. , Light La having received a phase difference of ／ wavelength
b, light Lba also having a phase difference of ３ wavelength and light Lbb having a phase difference of １ ／ wavelength are generated.
The relationship between the light amounts of these lights Laa, Lab, Lba, and Lbb is as follows: (light amount of light Laa) = (light amount of light Lbb)
= {(Light amount of light Lab) + (light amount of light Lba)}.

For this reason, even if a phase difference due to the birefringence of the optical recording medium 25 is given to the return light Lr, the phase difference is different and (light amount of light Laa) = (light amount of light Lbb) = ｛(light Lab) (The amount of light) + (the amount of light Lba)｝, the four kinds of light Laa, Lab, Lba, and Lbb incident on the PBS 23. Therefore, PBS
Even when only light having a polarization direction perpendicular to the polarization direction of the laser light Lo from the LD 21 is guided to the photodetector 29 by the light-receiving device 23, the amount Pr of the return light Lr received by the light detector 29
Is, as shown in FIG. 11A obtained based on the equation (2),
Almost constant level. In particular, when the phase difference due to the birefringence of the optical recording medium 25 is 0 to π / 2 radians, a very large amount of return light Lr of approximately 0.79 to 0.90 is guided to the photodetector 29. I will Further, FIG.
As compared with the optical pickup device 20 using the １ ／ wavelength plate or the ８ wavelength plate as shown in (A) and (B), the fluctuation of the light receiving amount Pr of the return light Lr by the photodetector 29 is reduced. Therefore, the optical pickup device 20 having excellent jitter characteristics can be realized by using the wavelength plate 1a.

The expression (2) shows that the amount of light Po of the laser light Lo from the LD 21 and the component of the return light Lr from the optical recording medium 25 in the direction perpendicular to the laser light Lo in the optical pickup device 20 using the wave plate 1a. Is a calculation formula for calculating the ratio (Pr / Po) of the light quantity Pr of the light source. In the equation (2), a is a phase difference due to birefringence of the optical recording medium 25, delta1 is a phase difference A 'received when passing through the phase difference area A, and delta2 is received when passing through the phase difference area B. This is the phase difference B ′. Optical pickup device 2
In the wavelength plate 1a arranged at 0, delta1 is λ /
4, delta2 corresponds to λ / 8.

[0049]

(Equation 2)

Also in the wavelength plate 1a, by controlling the film thickness of the birefringent films 3 and 4 constituting the phase difference regions A and B according to the phase difference due to the birefringence of the optical recording medium 25, the light The amount Pr of the return light Lr received by the photodetector 29 of the pickup device 20 and the range in which the amount Pr of received light fluctuates can be set. For example, when the phase difference due to the birefringence of the optical recording medium 25 varies in the range of 0 to π radian, the phase difference region B is set as an air layer so that no phase difference is generated, and the phase difference region A is set to 1 What is necessary is just to form the birefringent film 3 having such a thickness as to have an optical function as a / 4 wavelength plate. As a result, FIG. 1 obtained based on equation (2)
As shown in FIG. 1B, return light Lr having a light amount of approximately 0.50 to 0.67 of the laser light Lo from the LD 21 is guided to the photodetector 29.

Further, the return light Lr by the photodetector 29 is not affected by the phase difference due to the birefringence of the optical recording medium 25.
When it is desired to always maintain the received light amount Pr at a constant level, the phase difference area A is a １ ／ wavelength plate, and the phase difference area B is 2 /
A phase difference A ′ (delta1) received by passing through the phase difference region A like a three-wave plate and a phase difference B ′ (delta) received by passing through the phase difference region B
The birefringent films 3 and 4 may be formed so that the difference from 2) is a multiple of 2π / 3. As a result, as shown in FIG. 11C obtained based on the expression (2), the amount Pr of the return light Lr received by the photodetector 29 is not affected by the phase difference due to the birefringence of the optical recording medium 25. The amount of light is almost half of the laser light Lo from the LD 21 at all times.

As described above, also in the optical pickup device 20 having the wavelength plate 1a, by controlling the film thickness of the birefringent films 3 and 4 constituting the phase difference regions A and B of the wavelength plate 1a, the photodetector is controlled. Light receiving amount P of the return light Lr due to 29
By setting the range in which r and the amount of received light Pr fluctuate, the optical pickup device 20 that is less affected by the birefringence of the optical recording medium 25 can be realized.

[0053]

As described above, the wave plate of the present invention provides a predetermined phase difference when light passes through the phase difference region,
It is formed so as to be different for each phase difference region passing through the phase difference received at this time. Therefore, the light that has passed through the wave plate includes light having several different phase differences. For this reason, only light having a polarization direction perpendicular to the polarization direction of light emitted from the light source is transmitted in a predetermined direction,
In an optical pickup device that reflects or diffracts,
Even if the return light includes a phase difference due to the birefringence of the optical recording medium, an information signal from the optical recording medium can be reliably obtained.

Further, the wavelength plate of the present invention can set the level of the amount of light received by the photodetector and the range in which the amount of received light varies by controlling the phase difference in each phase difference region. Therefore, it is possible to realize an optical pickup device excellent in jitter characteristics which is not affected by the birefringence of the optical recording medium.

[Brief description of the drawings]

FIG. 1A is a perspective view of a wave plate to which the present invention is applied,
FIG. 3B is a perspective view showing only a circular region of the wave plate irradiated with laser light.

FIG. 2 is an explanatory diagram showing a state of incoherent light passing through a wavelength plate shown in FIG.

FIG. 3 is an explanatory diagram showing a state in which the incoherent light shown in FIG. 2 passes through the wave plate shown in FIG. 1 again.

FIG. 4 is a plan view showing a wave plate formed in a different pattern from the wave plate shown in FIG.

FIG. 5 is a schematic configuration diagram of an optical pickup device including the wavelength plate shown in FIG.

FIG. 6 is a diagram illustrating the amount of return light received by a photodetector when various wavelength plates are employed in an optical pickup device.

FIG. 7 is a perspective view showing a wave plate to which the present invention is applied.

FIG. 8 is an explanatory diagram showing a state of incoherent light passing through the wavelength plate shown in FIG. 7;

9 is an explanatory diagram showing a state in which the incoherent light shown in FIG. 8 passes through the wave plate shown in FIG. 7 again.

FIG. 10 is a plan view showing a wave plate formed in a different pattern from the wave plate shown in FIG. 7;

FIG. 11 is a diagram illustrating the amount of return light received by a photodetector when various wavelength plates are employed in an optical pickup device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 1a Wave plate 2 Substrate 201, 202 Light transmission surface 3, 4 Birefringent film 20 Optical pickup device 21 Laser diode (LD) 23 Polarized beam spiriter (PBS) 24 Objective lens 25 Optical recording medium 29 Photodetector A, B Phase difference area

Claims (10)

[Claims] 1. A semiconductor device comprising: a substrate; and two or more types of phase difference regions formed on a light passing surface of the substrate and providing different phase differences to light passing through the light passing surface. Is formed such that the light amount of the passing light reciprocating through the same type of the phase difference region is substantially equal to the light amount of the passing light reciprocating through the different type of the phase difference region. A wave plate characterized by the above-mentioned. 2. The phase difference region according to claim 1, further comprising a first phase difference region and a second phase difference region, wherein a phase difference between the first phase difference region and a phase difference due to the second phase difference region is provided. Is set to be a multiple of π. 3. A substrate comprising: a substrate; and two or more types of phase difference regions formed on a light passing surface of the substrate and providing different phase differences to light passing through the light passing surface. Is formed such that the amount of light passing through the same type of phase difference region and reciprocating through the phase difference region is twice the amount of light passing through the different type of phase difference region. A wave plate characterized by the above-mentioned. 4. A phase difference according to claim 3, further comprising a first phase difference region and a second phase difference region as the phase difference region, wherein a difference between the phase difference caused by the first phase difference region and the phase difference caused by the second phase difference region. Is set to be a multiple of 2π / 3. 5. The method according to claim 1, wherein
A wave plate, wherein each of the retardation regions is formed of a birefringent film having a different film thickness although the direction of the crystal optical axis is aligned. 6. The method according to claim 1, wherein
The wave plate, wherein the retardation region is formed of at least one of a birefringent film, a dielectric film, and an air layer. 7. The method according to claim 1, wherein
The wave plate, wherein the retardation region includes an anti-reflection film. 8. The method according to claim 1, wherein
The substrate is rectangular, and the direction of each side of the substrate is set at a predetermined angle with respect to the direction of the crystal optical axis of the first retardation region or the second retardation region. Wave plate. 9. A wave plate as defined in claim 1, a light source that emits light passing through the wave plate, and optical recording of light emitted from the light source that passes through the wave plate. An objective lens for condensing a light spot on the medium,
An optical pickup device, comprising: a beam splitter for guiding return light from the optical recording medium to a photodetector. 10. The optical pickup device according to claim 9, wherein the direction of the crystal optical axis of the wavelength plate coincides with the direction of the crystal optical axis of the optical recording medium.