JP2001004841A - Optical element, optical head and optical recording / reproducing device - Google Patents

Abstract

(57) [Problem] To realize a small-sized and low-cost optical head that can use a polarizing optical system for each of a plurality of light sources having different wavelengths. SOLUTION: A plurality of light sources 1A and 1B having different wavelengths, condensing optical systems 4 and 6 for condensing light of the light sources on an optical recording medium 7, and an optical system arranged in an optical path from the light source to the optical recording medium. Element 5;
Separation means 2A, 2B for separating the reflected light from the optical recording medium from the optical path of the light emitted from the light source, and photodetectors 8A, 8B, 9A, 9B for receiving the reflected light from the separated optical recording medium. Is provided. The retardation of the optical element 5 is set so as to be substantially an odd multiple of 1/4 of the wavelength of any light source.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head and an optical recording / reproducing apparatus which are used for optical information processing or optical communication, and particularly include a plurality of light sources having different wavelengths.

[0002]

2. Description of the Related Art In recent years, digital versatile disks (DVDs) have been used to store digital information on compact disks (C).
Since recording can be performed at a recording density approximately six times that of D),
It has attracted attention as a large-capacity optical recording medium. In order to further increase the recording capacity by increasing the density, it is effective to shorten the wavelength of the light source used, and the red light source (wavelength 660 nm) currently used in DVD is changed to the blue light source (wavelength 400 nm). Is thought to be. Therefore,
An optical head equipped with a red light source for reproducing a current DVD and a blue light source for reproducing a higher density medium can be considered. In this case, if an independent optical system is used for different wavelengths, the optical head is required. However, there are problems such as that the number of constituent parts increases and it is not suitable for downsizing the optical head. In order to solve this problem, Japanese Patent Application Laid-Open No. 4-32413 has been proposed.
In Japanese Patent Publication No. 0, a special wavelength plate is used to integrate optical components, thereby enabling downsizing of an optical head. The characteristic of this wave plate is that for a certain wavelength, its retardation (the value obtained by multiplying the birefringence amount and the thickness will be described later) is an integral multiple of that wavelength, and for the other wavelength, The retardation is 1/4 of that wavelength.

Here, an example of the above-mentioned conventional optical head will be described with reference to the drawings. FIG. 3 is a configuration diagram of a conventional optical head (also referred to as an optical pickup). As shown in the figure, the optical head includes a first light source 30a, a second light source 30b, a first collimator lens 32a, a second collimator lens 32b, and a first beam shaping optical system 33.
a, a second beam shaping optical system 33b, a first wavelength filter 34, a rotating mirror 35, a diffraction grating 36, a polarization beam splitter 37, a wavelength plate 38, an objective lens 39, an optical disk 40, a second wavelength filter 42, First lens 4
3, 1/2 wavelength plate 44, Wollaston prism 45, first photodetector 46, second lens 47, beam splitter 48, second photodetector 49a, and third photodetector 4
9b.

The first and second light sources 30a and 30b are composed of, for example, semiconductor laser elements and output coherent light for recording and reproduction to the recording layer of the optical disk 40, and have different wavelengths. First and second
Are collimator lenses 32a and 32b that convert divergent light emitted from the first and second light sources 30a and 30b into parallel light, and the first and second beam shaping optical systems 33a and 33b are first and second beam shaping optical systems. Second light sources 30a and 3
Ob is an optical system for converting an elliptical beam emitted from Ob into a perfect circle. The first wavelength filter 34 transmits 100% with respect to the wavelength of the first light source, and transmits with respect to the wavelength of the second light source. The turning mirror 35 is for changing the optical path of the light emitted from the light source 30b, and the diffraction grating 36 is for changing the incident light to three. The polarizing beam splitter 37 has a transmittance of 90% or more for p-polarized light and a reflectance of 95% or more for s-polarized light with respect to the wavelength of the first light source, and has a reflectance of 95% or more with respect to the wavelength of the second light source. 70% transmittance for p-polarized light, 95% for s-polarized light
%, The wavelength plate 38 has a retardation of １ ／ wavelength with respect to the wavelength of the first light source, and the wavelength with respect to the wavelength of the second light source. The objective lens 39 is a lens for condensing light on the recording layer of the optical disk 40, and the second wavelength filter 42 is a lens for the wavelength of the first light source. Has a characteristic of reflecting 100% and transmitting 100% with respect to the wavelength of the second light source, and the first and second lenses 43 and 47 transmit the light reflected by the recording layer of the optical disc 40 to the first and second lenses. First to third photodetectors 46, 49a and 4
9b, the half-wave plate 44 rotates the polarization direction by 45 degrees, and the Wollaston prism 45 changes the optical path according to the polarization direction.
The beam splitter 48 has characteristics of a transmittance of 50% and a reflectance of 50%, and the first to third photodetectors 46 and 49.
Reference numerals a and 49b denote light received by the recording layer of the optical disk 40 and convert the light into an electric signal.

The operation of the optical head thus configured will be described. First, light emitted from the first light source 30a will be described. The linearly polarized light emitted from the first light source 30a enters the first collimator lens 32a, is converted into parallel light, enters the first beam shaping optical system 33a, is converted into a substantially perfect circle, 1 wavelength filter 34
, Approximately 100% of the incident light is transmitted and incident on the polarization beam splitter 37, and 90% or more of the incident light is transmitted, converted into circularly polarized light by the wave plate 38, and converted by the objective lens 39. The light is focused on the optical disk 40. Next, the light reflected by the optical disk 40 is transmitted through the objective lens 39 and is incident on the wave plate 38. The incident circularly polarized light is converted by the wave plate 38 into the first light source 30a.
The emitted light is converted into linearly polarized light in a direction orthogonal to the polarization direction of the emitted light, and is incident on the polarization beam splitter 37.
The incident light is separated by the polarizing beam splitter 37 into
5% or more is reflected, and almost 1
00% is reflected, passes through the lens 47, and is split into two optical paths by the beam splitter 48, and the photodetectors 49a,
49b. The photodetectors 49a and 49b output a focus error signal indicating a focus state of light on the optical disk 40, and output a tracking error signal indicating a light irradiation position. The focus error signal is given to focus control means (not shown), and the focus control means moves the position of the objective lens 39 in the optical axis direction based on the focus error signal so that light is always focused on the optical disc 40 in a focused state. To control. Further, tracking control means (not shown) controls the position of the objective lens 39 based on the tracking error signal so that light is focused on a desired track on the optical disc 40. Further, the light detector 49
a and 49b reproduce the information recorded on the optical disk 40.

Next, light emitted from the second light source 30b will be described. In this case, it is assumed that the reproduced optical disk is a magneto-optical disk. The linearly polarized light emitted from the second light source 30b enters the second collimator lens 32b, is converted into parallel light, and is converted into a second beam shaping optical system 33.
b, is converted into a substantially perfect circle, the optical path is changed by the rotating mirror 35, the beam is divided into three beams by the diffraction grating 36, and the beam is incident on the first wavelength filter 34. 100% of the light is reflected and incident on the polarization beam splitter 37, and approximately 70% of the incident light is transmitted and incident on the wave plate 38. Here, since the retardation of the wave plate 38 is an integral multiple of the wavelength, the polarization direction is not changed, and the polarization direction of the transmitted light remains the same as the polarization direction of the light emitted from the second light source. The light transmitted through the wave plate 38 is collected on the optical disk 40 by the objective lens 39. Next, the light reflected by the optical disk 40 becomes linearly polarized light slightly different in direction from the linearly polarized light emitted from the light source 30b due to the Kerr effect of the optical disk. This light is transmitted through the objective lens 39 and is incident on the wave plate 38. As in the forward path, the polarization direction does not change and the polarization beam splitter 3
7 is incident. In the polarization beam splitter 37, 70% of the light in the polarization direction of the linearly polarized light emitted from the light source is transmitted, and 95% or more of the light in the direction orthogonal to the light is reflected. The reflected light is 1 by the wavelength filter 42.
The half-wave plate 44 is transmitted through the lens 43 and transmitted through the lens 43.
The polarization direction is rotated by 45 degrees, and the light is separated by the Wollaston prism 45 in accordance with the polarization direction.
6 is incident. The photodetector 46 outputs a focus error signal indicating a focused state of light on the optical disc 40, and outputs a tracking error signal indicating a light irradiation position. The focus error signal is given to focus control means (not shown), and the focus control means adjusts the position of the objective lens 39 in the optical axis direction based on the focus error signal so that light is always focused on the optical disc 40 in a focused state. To control. Further, a tracking control means (not shown) controls the position of the objective lens 39 based on the tracking error signal so that the light is focused on a desired track on the optical disc 40. Further, the photodetector 46 reproduces information recorded on the optical disk 40 (the method of reproducing the magneto-optical disk is a normal method, and will not be described here).

[0007]

However, the optical head having the above-described structure has a problem when it is desired to increase the reciprocal light utilization efficiency for both of the lights from the two light sources having different wavelengths. . For example, when a red light source and a blue light source are respectively mounted and both light sources are used for recording on an optical recording medium (for example, a phase change disk), or when a low reflectivity disk is reproduced using either light source, etc. It is necessary to increase the light use efficiency of the round trip for the light from the light source, and it is necessary to use a polarization optical system for both light sources. In consideration of such a case, in the wavelength plate described in the conventional example, since the retardation is １ ／ wavelength for one light source 30a, a polarizing optical system can be used. Since the retardation is an integral multiple of the wavelength for one light source 30b, a polarizing optical system cannot be used, and the light use efficiency in the round trip cannot be increased.

The present invention has been made in view of such conventional problems, and provides an optical element which can use a polarizing optical system for any of light from two light sources having different wavelengths. The first purpose is to do so. Also,
By using this optical element, it is possible to use a polarizing optical system for both light sources to enable recording and reproduction of information from an optical disk having a low reflectance, and to realize miniaturization and cost reduction of an optical head. This is the purpose of 2.

[0009]

In order to achieve the above object, the optical element of the present invention is made of a birefringent material, and has a value obtained by multiplying the birefringence amount of the birefringent material by the thickness. The retardation is substantially an odd multiple of (１ ／) × λ1 with respect to the wavelength λ1 of one light source, and (1/1/2) with respect to the wavelength λ2 of another light source different from the wavelength of the light source.
4) It is characterized by being substantially an odd multiple of × λ2. This makes it possible to convert linearly polarized light into circularly polarized light for two wavelengths of light. In addition, since one optical element has characteristics as a quarter-wave plate for both of two wavelengths, when applied to an optical system having a plurality of light sources having different wavelengths, the number of optical components can be reduced. Can be.

In the above optical element, the wavelength λ1 is 640.
Preferably, the wavelength λ2 is a constant value within a range of 390 nm to 430 nm. Thereby, the above configuration can be applied to the red light source and the blue light source.

It is preferable that the optical element has an antireflection film for preventing reflection of light of wavelength λ1 and light of wavelength λ2. Thereby, the light use efficiency can be further increased.

According to another aspect of the present invention, there is provided an optical head comprising: a plurality of light sources having different wavelengths; a condensing optical system for condensing light from the light source on an optical recording medium; The retardation, which is arranged in the optical path to the optical recording medium and is made of a birefringent material, is a value obtained by multiplying the birefringence amount and the thickness of the birefringent material with respect to the wavelength λ1 of one light source. (１ ／) × λ1 and a wavelength λ of another light source different from the wavelength of the light source.
An optical element that is substantially an odd multiple of (1/4) × λ2 with respect to 2; a separating unit that separates reflected light from the optical recording medium from an optical path of light emitted from the light source; And a photodetector for receiving reflected light from the separated optical recording medium. Accordingly, since an optical element having a characteristic of converting linearly polarized light into circularly polarized light with respect to light of two wavelengths is used, a polarization optical system can be used for both optical systems of two wavelengths. . Further, since one optical element has characteristics as a quarter-wave plate for both of two wavelengths, the number of optical components can be reduced, and a compact and low-cost optical head can be realized.

In the above optical head, the wavelength λ1 is 640.
Preferably, the wavelength λ2 is a constant value within a range of 390 nm to 430 nm. Thereby, the above-described effect is exhibited in an optical head equipped with a red light source and a blue light source.

Further, an optical recording / reproducing apparatus according to the present invention has the above-described optical head, controls the optical head based on a signal output from the optical head, and reproduces information from an optical recording medium. Alternatively, information is recorded on the optical recording medium. Thus, an optical recording / reproducing apparatus equipped with a plurality of small and low-cost light sources can be realized.

[0015]

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

(First Embodiment) A first embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a configuration diagram of an optical head according to the first embodiment of the present invention. In FIG. 1, 1A is the first
, 1B is a second light source, 2A is a first polarization hologram, 2B is a second polarization hologram, 3 is a wavelength filter, 4 is a collimator lens, 5 is an optical element of the present invention (details will be described later), 6 is an objective lens, 7 is an optical recording medium, 8A is a first photodetector, 8B is a second photodetector, 9
A is a third photodetector, and 9B is a fourth photodetector.
Here, the condensing optical system includes a collimator lens 4 and an objective lens 6, and the separating unit includes first and second polarization holograms 2A and 2B.

Here, the first and second light sources 1A and 1B
Is a light source composed of, for example, a semiconductor laser element and outputting coherent light for recording / reproducing to the recording layer of the optical recording medium 7, using the first light source 1A as a red light source and setting its wavelength (λ1) to 640 nm or more. The wavelength (λ2) is set to 660 nm, which is 680 nm or less, and the wavelength (λ2) is set to 400 nm, which is 390 nm to 430 nm, using the second light source 1B as a blue light source. The first and second polarization holograms 2A and 2B change the refractive index by proton-exchanging a desired portion of a lithium niobate substrate, which is a birefringent substrate, transmit 100% for a certain linearly polarized light, and the polarization direction thereof. Is an optical element that diffracts 100% with respect to linearly polarized light in a direction perpendicular to the direction (see JP-A-63-314502). The wavelength filter 3 has a characteristic of reflecting 100% of light having the wavelength of the first light source 1A and transmitting 100% of light having the wavelength of the second light source. The collimator lens 4 is a lens that converts divergent light emitted from the first and second light sources 1A and 1B into parallel light. The optical element 5 of the present invention converts both linearly polarized light emitted from the first and second light sources 1A and 1B into circularly polarized light,
This element converts circularly polarized light into linearly polarized light (the details will be described later). The objective lens 6 is a lens that focuses light on the recording layer of the optical recording medium 7. The first to fourth photodetectors 8A, 8B, 9A, 9B receive the light reflected on the recording layer of the optical recording medium 7 and convert the light into an electric signal.

The operation of the optical head thus configured will be described with reference to FIG. First, the light (λ1 = 660 nm) emitted from the first light source will be described. The linearly polarized light emitted from the first light source 1A is incident on the first polarization hologram 2A, is transmitted almost 100%, is incident on the wavelength filter 3, is reflected almost 100%, and is reflected on the collimator lens 4. The incident light is collimated by the collimator lens 4 to be incident on the optical element 5. The incident linearly polarized light is converted into circularly polarized light by the optical element 5, incident on the objective lens 6, and condensed on the optical recording medium 7. Next, the light reflected from the optical recording medium 7 passes through the objective lens 6 and enters the optical element 5. The circularly polarized light incident on the optical element 5 is converted into linearly polarized light having a polarization direction orthogonal to the polarization direction of the linearly polarized light emitted from the first light source 1A. The converted linearly polarized light passes through the collimator lens 4 and is reflected 100% by the wavelength filter 3.
The light is incident on the first polarization hologram 2A, is diffracted by almost 100%, the + 1st-order light of the diffraction is incident on the photodetector 8A, and the -1st-order light of the diffraction is incident on the photodetector 8B. Photodetector 8
A and / or 8B output a focus error signal indicating a focused state of light on the optical recording medium 7, and output a tracking error signal indicating a light irradiation position. The focus error signal is supplied to focus control means (not shown), and the focus control means adjusts the position of the objective lens 6 based on the focus error signal so that the light is always focused on the optical recording medium 7 in a focused state. Control in the axial direction. A tracking control unit (not shown) controls the position of the objective lens 6 based on the tracking error signal so that the light is focused on a desired track on the optical recording medium 7. Further, information recorded on the optical recording medium 7 is also obtained from the photodetectors 8A and / or 8B.

Next, the light emitted from the second light source (λ2 =
400 nm). The linearly polarized light emitted from the second light source 1B is incident on the second polarization hologram 2B, almost 100% is transmitted, is incident on the wavelength filter 3, almost 100% is transmitted, and is transmitted to the collimator lens 4. The incident light is collimated by the collimator lens 4 to be incident on the optical element 5. The incident linearly polarized light is converted into circularly polarized light by the optical element 5, incident on the objective lens 6, and condensed on the optical recording medium 7. Next, the optical recording medium 7
Is transmitted through the objective lens 6 and is incident on the optical element 5. The circularly polarized light incident on the optical element 5 is converted into linearly polarized light having a polarization direction orthogonal to the polarization direction of the linearly polarized light emitted from the second light source 1B. The converted linearly polarized light passes through the collimator lens 4, is transmitted by the wavelength filter 3 at 100%, is incident on the second polarization hologram 2B, is almost 100% diffracted, and has a +
The primary light is incident on the photodetector 9A, and the -1st order diffracted light is incident on the photodetector 9B. Photodetector 9A and / or 9B
Outputs a focus error signal indicating a focused state of light on the optical recording medium 7 and outputs a tracking error signal indicating a light irradiation position. The focus error signal is supplied to focus control means (not shown), and the focus control means adjusts the position of the objective lens 6 based on the focus error signal so that the light is always focused on the optical recording medium 7 in a focused state. Control in the axial direction. A tracking control unit (not shown) controls the position of the objective lens 6 based on the tracking error signal so that the light is focused on a desired track on the optical recording medium 7. Further, information recorded on the optical recording medium 7 is also obtained from the photodetectors 9A and / or 9B.

Next, the optical element 5 of the present invention will be described in detail. FIG. 2 is a sectional view of the optical element 5 according to the first embodiment of the present invention. In FIG. 2, 20 is the first glass, 21 is the UV curable resin layer, 22 is the polyimide film, 2
3 is a second glass. The optical element 5 is configured by bonding and laminating glass plates 20 and 23 on both surfaces of a polyimide film 5 with a UV curable resin layer 21.

Here, the definition of the amount of birefringence will be described.
The difference between the major axis and the minor axis of the ellipse formed when an ellipsoid (index ellipsoid) whose main axis is the refractive index of the crystal axis of the birefringent material is made perpendicular to the incident light is birefringence. Quantity. Now, the amount of birefringence of the polyimide film 22 can be freely changed depending on the manufacturing method.
(See IEICE Technical Report (1994), pp. 67-72). When the thickness of the polyimide film is 9.3 μm,
The retardation (the value obtained by multiplying the amount of birefringence and the thickness) of the optical element 5 is obtained by 9.3 × 0.053. This value is compared with the wavelengths of the light sources 1A and 1B at 660 nm and 400 n.
Calculating the ratio to m gives the following:

[0023]

(9.3 × 0.053) /0.66=0.75 (1) (9.3 × 0.053) /0.40=1.23 (2)

According to the equation (1), the optical element 5 is 660 nm.
Becomes a ／ wavelength plate for the wavelength of
It can be seen that for a wavelength of 00 nm, it becomes a 5/4 wavelength plate. Therefore, this optical element 5 has a wavelength of 660 nm and a wavelength of 4 nm.
When linearly polarized light of 00 nm is incident, linearly polarized light of either wavelength is converted to circularly polarized light.

As described above, the optical element of the present invention has a retardation that is substantially an odd multiple of (1/4) × λ1 for a certain wavelength (λ1), and has another wavelength (λ2). )
Has a retardation that is substantially an odd multiple of (1/4) × λ2, it is possible to realize an optical element that can convert linearly polarized light of two wavelengths into circularly polarized light.

Further, in an optical head having a plurality of light sources having different wavelengths, by using the above-described optical element capable of converting linearly polarized light into circularly polarized light with respect to light of two wavelengths, light of either wavelength can be obtained. Also, a polarizing optical system can be used, and the light use efficiency can be increased for both wavelengths. In addition, since the optical element has characteristics as a quarter-wave plate for any of the two wavelengths, it is not necessary to use a quarter-wave plate corresponding to each of the wavelengths. It is possible to reduce the size and cost of the optical head while mounting a plurality of light sources.

In an optical recording / reproducing apparatus for reproducing or recording information from an optical recording medium by controlling the optical head based on the control signal output from the optical head, even if the optical head itself has a plurality of light sources, Since the cost is low and it is suitable for miniaturization, a low-cost and compact optical recording / reproducing apparatus can be realized.

It is needless to say that providing a well-known antireflection film to the optical element of the present embodiment increases the light use efficiency and is useful.

Although the embodiments of the present invention have been described with reference to the examples, the present invention is not limited to the above-described embodiments, and may be applied to other embodiments based on the technical idea of the present invention. Can be.

For example, in the optical element of the above embodiment, the polyimide film 22 is used as the birefringent material, but there is no problem if another polymer film or liquid crystal is used. There is no problem even if an inorganic material such as quartz is used. In this case, it is not necessary to sandwich the transmitted wavefront with glass in order to improve the transmitted wavefront.

In the above embodiment, an optical element functioning as a ５ wavelength plate for a wavelength of 400 nm and a ／ wavelength plate for a wavelength of 660 nm is exemplified. There is no problem if the wavelength is set to be approximately an odd multiple of 1/4 wavelength.

In the above embodiment, the wavelength of the light source is 66
Although the case of 0 nm and 400 nm has been described, applicable wavelengths are not limited to these, and there is no problem even if one or both wavelengths are changed. Also, by setting the retardation of the optical element to be approximately an odd multiple of 波長 wavelength of each wavelength with respect to three or more different wavelengths, the above effect can be obtained for light of each wavelength. .

In the optical head of the above embodiment, the polarization hologram is used for each of a plurality of light sources, but may be used as a common component. In this case, the number of polarization holograms is reduced, and the size and cost of the optical head can be further reduced.

Further, in the above embodiment, the case where the optical element 5 of the present invention is arranged in the parallel system between the collimator lens 4 and the objective lens 6 is shown. It may be arranged at any place as long as it is on the recording medium side.

In the above embodiment, an infinite optical head is shown. However, a finite optical head that does not use the collimator lens 4 may be used. In this case, the cost is low since no collimator lens is used.

In the above embodiment, the reflected light from the optical recording medium is separated from the optical path from the light source by the polarization hologram and is incident on the photodetector. However, the structure of the separating means is not limited to this. For example, a separation optical element such as a half mirror may be used to separate the optical path from the light source and make the light incident on the photodetector.

In the above embodiment, an optical head having two light sources is described, but a plurality of light sources (three or more) are provided.
There is no problem even if there is.

In the above embodiment, the case where the optical recording medium is an optical disk has been described. However, a card-shaped optical recording medium or the like may be used. It can be widely applied to equipment.

[0039]

As described above, according to the present invention, the optical element of the present invention has a (1/4) × wavelength for a certain wavelength (λ1).
It has a retardation that is approximately an odd multiple of λ1 and also has a retardation that is approximately an odd multiple of (×) × λ2 for another wavelength (λ2). An optical element that can convert linearly polarized light into circularly polarized light can be realized.

In the optical head of the present invention, by using this optical element, a polarizing optical system can be used for any light source, and in that case, 1 / indispensable for each light source. Since the four-wavelength plate can be constituted by one optical element, a small and low-cost optical head equipped with a plurality of light sources can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of an optical head of the present invention.

FIG. 2 is a schematic sectional view showing an embodiment of an optical element used in the optical head of the present invention.

FIG. 3 is a schematic configuration diagram illustrating an example of a conventional optical head.

[Explanation of symbols]

 Reference Signs List 1A First light source 1B Second light source 2A First polarization hologram 2B Second polarization hologram 3 Wavelength filter 4 Collimator lens 5 Optical element 6 Objective lens 7 Optical recording medium 8A First photodetector 8B Second light Detector 9A Third light detector 9B Fourth light detector

Continued on the front page (72) Inventor Tetsuo Hosomi 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture F-term in Matsushita Electric Industrial Co., Ltd. CA09 DA01 DA05 EC47 FA08 JA12 JA14 JA25 JA26 JA31 JA32 JA65 KA02 LB07

Claims (6)

[Claims] 1. A retardation made of a birefringent material, which is a value obtained by multiplying a birefringence amount and a thickness of the birefringent material, by (1/1) with respect to a wavelength λ1 of one light source.
4) An optical element characterized by being substantially an odd multiple of × λ1, and being substantially an odd multiple of (１ ／) × λ2 with respect to a wavelength λ2 of another light source different from the wavelength of the light source. 2. The wavelength λ1 is 640 nm or more and 680 n.
m, and the wavelength λ2 is 390
The optical element according to claim 1, wherein the optical element has a constant value in a range from nm to 430 nm. 3. The optical element according to claim 1, further comprising an anti-reflection film for preventing reflection of light of wavelength λ1 and light of wavelength λ2. 4. A plurality of light sources having different wavelengths from each other; a condensing optical system for condensing light from the light source onto an optical recording medium; and a light source disposed in an optical path from the light source to the optical recording medium;
The retardation, which is a value obtained by multiplying the birefringence amount and the thickness of the birefringent material, is a substantially odd number of (1/4) × λ1 with respect to the wavelength λ1 of one light source. An optical element that is twice as large as the wavelength λ2 of another light source different from the wavelength of the light source, and is approximately an odd multiple of (1/4) × λ2; An optical head comprising: a separating unit that separates an emitted light from an optical path; and a photodetector that receives reflected light from the optical recording medium separated by the separating unit. 5. The wavelength λ1 is 640 nm or more and 680 n.
m, and the wavelength λ2 is 390
The optical head according to claim 4, wherein the optical head has a constant value within a range of not less than nm and not more than 430 nm. 6. An optical head according to claim 4, wherein the optical head is controlled based on a signal output from the optical head, and information is reproduced from an optical recording medium, or the optical recording is performed. An optical recording / reproducing apparatus for recording information on a medium.