JP2001281432A - Diffraction grating for two wavelengths and optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a diffraction grating for two wavelengths capable of keeping the O-order diffraction efficiency of one out of two kinds of wavelengths of incident light maximum and constant and keeping the O-order diffraction efficiency of the other variable. SOLUTION: The diffraction grating for the two wavelengths comprises a diffraction grating 11B with a projecting and recessing sectional shape formed on a transmissive substrate 11A, of which the ratio of the width to the period of the projecting parts of the grating has a value other than 0.5, the phase difference of the transmitted light between the projecting part and the recessing part is 2π with respect to the light with one out of the two wavelengths and the 0-order diffraction efficiency is adjusted to a specified value with respect to the light with the other wavelength.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a two-wavelength diffraction grating and an optical head device.

[0002]

2. Description of the Related Art In recent years, optical disks such as CDs and DVDs,
Alternatively, various optical head devices for recording and reproducing information on an information recording surface of an optical recording medium such as a magneto-optical disk (hereinafter collectively referred to as an optical disk) have been used. Normally, in this optical head device, information is optically recorded / reproduced on an information recording surface using a laser beam, but the laser beam is focused on a track on the information recording surface. In order to trace on a track while following the rotation of an optical disk, tracking methods such as a three-beam method and a differential push-pull method have been developed.

Here, a conventional optical head device in which a semiconductor laser of a 790 nm wavelength band and a semiconductor laser of a 650 nm wavelength band are arranged in a separated state will be described with reference to a configuration example of FIG.

In this optical head device, a semiconductor laser 3A
Light emitted from the (650 nm wavelength band) and 3B (790 nm wavelength band) is synthesized on the same optical axis by the wavelength synthesizing prism 9, passes through the beam splitter 4, is converted into parallel light by the collimating lens 5, and is converted into an objective lens. 6 is incident.
The beam transmitted through the objective lens 6 and condensed on the information recording surface of the optical disk 7 is reflected on the information recording surface, and the reflected light (hereinafter, referred to as signal light) travels in the same optical path as the original outward path. I will do it.

That is, this signal light is again converted into parallel light by the objective lens 6 and is condensed on the light receiving surface of the photodetector 8 via the collimator lens 5 and the beam splitter 4. Then, the light is converted into an electric signal by the photodetector 8.
Reference numeral 10 denotes a diffraction grating for generating three beams.

As a semiconductor laser that emits light of two wavelengths, for example, a monolithic two-wavelength semiconductor laser in which a semiconductor laser having a wavelength band of 790 nm and a semiconductor laser having a wavelength band of 650 nm are formed in one chip, A two-wavelength semiconductor laser comprising a plurality of chips in which laser chips in a band are arranged so that the distance between light emitting points is about 100 to 300 μm has also been proposed. When these two-wavelength semiconductor lasers are used, the number of parts is reduced, and the size and cost are reduced as compared with the conventional optical head device in which the two semiconductor lasers shown in FIG. I can do it.

[0007]

However, in the above-described optical head device, when a diffraction grating used for generating three beams by a three-beam method or a differential push-pull method is used in combination with a two-wavelength semiconductor laser, 79 for CD playback
Even if light in either the 0 nm wavelength band or the 650 nm wavelength band for DVD reproduction is incident on the diffraction grating, the diffracted light is formed. Therefore, extra diffracted light may become stray light and enter the photodetector. There is a problem that information cannot be recorded / reproduced.

When the three-beam method or the differential push-pull method is used only for reproducing a CD or a DVD, the diffracted light generated by the diffraction grating emits light of the other wavelength. There is a problem that loss occurs and the signal light decreases. Further, when the diffraction grating used for the three-beam method or the differential push-pull method and the phase plate for reducing the return light to the semiconductor laser are separately arranged, the wavefront aberration value of each optical element is reduced. Since the addition is performed, a problem arises that the total wavefront aberration value increases.

An object of the present invention is to record / reproduce information on / from different optical recording media such as a CD optical disk and a DVD optical disk by using light of two wavelength bands by using a semiconductor laser for two wavelengths as a light source. Another object of the present invention is to provide a two-wavelength diffraction grating and an optical head device that can perform stable signal detection.

[0010]

According to the present invention, there is provided a two-wavelength diffraction grating in which light of at least one of the wavelengths λ ₁ and λ ₂ (λ ₁ ≠ λ ₂ ) is incident. A diffraction grating whose cross-sectional shape is composed of periodic irregularities,
The ratio w / P of the width w of the convex portion and the period P has a value other than 0.5, and transmits the incident light of _one wavelength λ ₁ and diffracts the incident light of the other wavelength λ ₂ , The _two- wavelength diffraction device, wherein the phase difference of the transmitted light between the portion and the concave portion is 2π with respect to the transmitted light having the wavelength λ ₁ , and the diffraction efficiency for the light having the wavelength λ ₂ is adjusted to a predetermined value. Provide a grid.

[0011] comprising a light source for emitting light having a wavelength lambda ₁ and wavelength lambda _2, and an objective lens for condensing the light of the wavelength lambda ₁ and wavelength lambda ₂ to the optical recording medium, the recording of information on an optical recording medium・
An optical head device for performing reproduction, wherein the two-wavelength diffraction grating is provided in an optical path between the light source and the objective lens.

[0012]

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

"First Embodiment of Two-Wavelength Diffraction Grating" FIG. 1
The two-wavelength diffraction grating 1 is composed of a light-transmitting substrate 11A that is optically isotropic, and has a diffraction grating 11B having a uniform refractive index formed of periodic irregularities on one surface. The grating depth (thickness) d ₁ of the concavo-convex portion of the diffraction grating 11B and the refractive index n ₁ of the convex portion satisfy the following formulas 1 and 2 with respect to the incident light having the wavelengths λ ₁ and λ _2. It is formed as follows.

The phase difference formed by the difference in the refractive index between the incident light of wavelength λ ₁ and air is expressed by the following equation (1). Similarly, the phase difference formed by the difference in the refractive index between the incident light having the wavelength λ ₂ and the air is expressed by Expression 2. According to the scalar theory, the efficiency of light diffracted by the diffraction grating 11B thus formed satisfies the following Expressions 3 and 4. The zero-order light diffraction efficiency η ₁ (0) and the ± first-order light diffraction efficiency η ₁ (± 1) with respect to the incident light having the wavelength λ ₁ are expressed by Expressions 3 and 4, respectively.

On the other hand, the 0th-order light diffraction efficiency η ₂ (0) and ± 1st-order light diffraction efficiency η ₂ (± 1) for incident light of wavelength λ ₂
Is represented by Equations 5 and 6, respectively.

[0016]

(Equation 1)

Here, a is a ratio between the pitch P of the diffraction grating and the width w of the convex portion of the diffraction grating, and is defined by Expression 7 (hereinafter, referred to as “a”).
Duty). Equations 5 and 6 suggest that by adjusting the duty a, a desired diffraction efficiency can be obtained for incident light of wavelength λ ₂ . The desired diffraction efficiency with respect to the wavelength λ ₂ is, for example, a value of the zero-order diffraction efficiency of about 70 to 80%, and may be changed according to the power of the light traveling straight.

For incident light of wavelength λ ₁ , η ₁ (0) = 1, that is, 100
From Equation 4, η ₁ (± 1) = 0, that is, 0% regardless of the duty a (see FIG. 2).

In the conventional single-wavelength three-beam diffraction grating, the duty a is fixed to a = 0.5, and the zero-order light diffraction efficiency η ₂
(0) and ± 1st order light diffraction efficiency η ₂ (± 1) were adjusted by changing the grating depth. However, in the case of a two-wavelength diffraction grating, the depth of the grating is uniquely determined by the constraint of transmitting one wavelength light, and the 0th-order light diffraction efficiency η ₂ (0) and ± 1st-order light diffraction of the other wavelength light. The efficiency η ₂ (± 1) is given by
And in equation 6, fixed at a value for a = 0.5. According to the configuration of the present invention, by adjusting the value of the duty a, the zero-order light diffraction efficiency η ₂ (0) becomes a large value,
The ± first-order diffraction efficiency η ₂ (± 1) can be adjusted to a small value.

As shown in FIG. 2, the duty a is 0.5
By setting the value to a value other than the above, the zero-order diffraction efficiency of the 790 nm wavelength light can be further increased. The 0th-order diffraction efficiency increases as the value is larger than 0.5 or smaller than 0.5.
That is, when incident thus formed 2-wavelength diffraction grating wavelength lambda ₁ and wavelength lambda ₂ different two wavelengths, as shown in FIG. 1 (A), one of the incident wavelength lambda ₁ Light is not diffracted as shown in Equations 3 and 4,
1B, the incident light of the other wavelength λ ₂ is diffracted at a desired efficiency given by Equations 5 and 6, and passes through the two-wavelength diffraction grating. I do.
In other words, a wavelength-selective diffraction grating that acts as a diffraction grating adjusted to obtain a desired diffraction efficiency for light of one wavelength, but does not act as a diffraction grating for light of the other wavelength. realizable.

"Second embodiment of two-wavelength diffraction grating" FIG. 3
The second embodiment is a modification of the first embodiment described above.
is there. In the two-wavelength diffraction grating 1 of the present embodiment, the light-transmitting group
Just forming diffraction grating 11B on one surface of plate 11A
However, as shown in FIG.
Form diffraction grating 11C with periodic unevenness and uniform refractive index
I do. In this case, the grating depth of the uneven portion of the diffraction grating 11C
(Thickness) d_TwoAnd the refractive index n of the convex part_TwoIs the wavelength λ ₁and
Wavelength λ_TwoSatisfies the following relational expression for the incident light of
It is formed as follows.

The phase difference formed by the difference in the refractive index between the incident light having the wavelength λ ₁ and air is expressed by the following equation (8). Similarly, the phase difference formed by the difference in refractive index between the incident light having the wavelength λ ₂ and air is represented by Expression 9.

[0023]

(Equation 2)

In this diffraction grating 11C, similarly to the diffraction grating 11B, a desired diffraction efficiency can be obtained with respect to the incident light having the wavelength λ ₁ by adjusting the duty a. Here, the lattice depth d ₁ is the same as that in FIG. FIG. 4 shows that when the duty a is set to a value other than 0.5,
This shows that the 0th-order diffraction efficiency of light having a wavelength of 50 nm can be further increased.

As described above, it is possible to realize a wavelength-selective diffraction grating adjusted so as to obtain desired diffraction efficiencies for lights of different wavelengths. That is, as shown in FIG. 3 (A), to the wavelength lambda ₁ of the incident light, the diffraction grating 11C
Exerts a diffractive action, and can generate zero-order light and ± first-order light. On the other hand, as shown in FIG. 3B, the diffraction grating B exerts a diffractive action on the incident light of the wavelength λ ₂ , and can generate the zero-order light and the ± first-order light.

The two-wavelength diffraction grating 1 according to the present embodiment has a periodic grating functioning as a diffraction grating on only the light of wavelength λ _{1 and} the light of wavelength λ _{2 on} both surfaces of the light incidence surface and the light emission surface. The uneven shape may be formed to generate three differently used beams corresponding to CD-type and DVD-type optical disks.

"Third Embodiment of Two-Wavelength Diffraction Grating" FIG. 5
In the two-wavelength diffraction grating 2 of the third embodiment, a phase plate 21C is provided between a light-transmitting substrate 21A on which a diffraction grating adjusted to obtain a desired diffraction efficiency is formed and a light-transmitting substrate 21D. It is configured to be inserted and fixed with the adhesive 21E.
As described above, the integration is preferable because the element can be downsized.

The phase plate 21C is made of an organic thin film. For example, a polycarbonate film is stretched to form a birefringent film whose optical axes are aligned in the stretching direction to generate a phase difference. In this case, when the linearly polarized light of the incident light of wavelength lambda ₁ is transmitted through the organic thin film, the retardation value of the phase plate 21C such that the phase difference becomes substantially circularly polarized light is generated and the fast axis (birefringence axis) direction incident The direction of linear polarization of light is adjusted. The diffraction grating 21B is the same as that in FIG.

As the phase plate 21C, an organic thin film having a phase difference generating function may be formed directly on the translucent substrate 21A or 21D. For example, specifically, after an alignment film is applied on a transparent substrate and subjected to an alignment treatment, a solution of a liquid crystal monomer which is a birefringent material is applied so that the optical axis of the liquid crystal molecules is aligned in the alignment direction of the alignment film. Align Furthermore, a photopolymerization curing agent is previously contained in the liquid crystal monomer solution, and the monomer is polymerized by irradiating a light source for photopolymerization to form a polymer liquid crystal layer. A phase plate can be formed.

Next, the second embodiment of the first to third embodiments will be described.
An optical head device equipped with a wavelength diffraction grating will be described. Here, for example, a wavelength λ ₁ for a DVD-based optical disc is used.
Is 650 nm, and the wavelength λ ₂ for CD optical discs is 790.
nm.

In the optical head device shown in FIG. 6, the light of wavelength λ ₁ emitted from the two-wavelength semiconductor laser 3 is transmitted straight on the optical axis without being diffracted by the two-wavelength diffraction grating 1 or 2, and The light passes through the beam splitter 4 and is converted into parallel light by a collimating lens 5. Thereafter, the parallel light is converged by the objective lens 6 on an information recording track on an information recording surface of the optical disk 7 (DVD system).

The light reflected on the information recording surface again passes through the objective lens 6 and the collimator lens 5, is reflected by the beam splitter 4, and is condensed on the light receiving surface of the photodetector 8. Here, the two-wavelength diffraction grating 1 represents the diffraction grating itself shown in FIG. 1 or FIG. 3, and the two-wavelength diffraction grating 1 means an integrated phase plate and diffraction grating.

On the other hand, the light of wavelength λ ₂ emitted from the two-wavelength semiconductor laser 3 has a desired diffraction efficiency (for example, 10% to 40%) at the two-wavelength diffraction grating 1 or 2.
), The light is diffracted as ± first-order light, further passes through the beam splitter 4, and is made parallel by the collimator lens 5. Thereafter, the parallel light is converged by the objective lens 6 on the information recording track of the information recording surface of the optical disk 7 (CD system) as three beams of zero-order light and ± first-order light. The light reflected on the information recording surface again passes through the objective lens 6 and the collimating lens 5, is reflected by the beam splitter 4, and is collected on the light receiving surface of the photodetector 8.

Thus, the two-wavelength diffraction grating 1 or 2
In the case of an optical head device equipped with a λ ₁ , the light of wavelength λ ₁ is transmitted straight without being diffracted by the two-wavelength diffraction grating 1 or 2, so that there is no reduction in efficiency and no stray light. Therefore, as an optical detection method for a DVD-based optical disk, a tracking error signal is detected by a heterodyne detection method or a phase difference method, and an optical disk information is detected by an astigmatism method using a general photodetector composed of four divided light receiving surfaces. Detection of a focus signal on a recording surface and detection of a pit signal as recording information can be stably performed.

On the other hand, in the case of the optical disk of the CD system, the detection of the focus signal and the detection of the pit signal on the information recording surface of the optical disk by the astigmatism method are performed by using the photodetector having the same four-divided light receiving surface as that of the DVD system. In addition, by detecting ± primary light on two light receiving surfaces that are further divided in addition to the four divisions of the photodetector, a tracking error signal is detected by a three-beam method. The grating pitch of the two-wavelength diffraction grating 1 or 2 is appropriately determined according to the optical system of the optical head device on which it is mounted and the tracking method of the optical recording medium.

In the example of the optical head device shown in FIG. 6, the beam splitter 4 is used, and the unit of the two-wavelength semiconductor laser 3 and the photodetector 8 are separated from each other. Instead of using a hologram beam splitter, the light reflected on the information recording surface is diffracted and separated, and focused on a photodetector located near the semiconductor laser in the two-wavelength semiconductor laser unit. You may comprise. In this case, since the semiconductor laser and the photodetector are arranged in the same unit, the size of the optical head device can be reduced.

The technique of forming a two-wavelength diffraction grating of which diffraction efficiency is adjusted by changing the ratio of the width and the period of the convex portion of the diffraction grating in the two-wavelength diffraction grating of the present invention is as follows.
The two different wavelengths are not limited to the combination of the 650 nm wavelength band and the 790 nm wavelength band, and can be applied to the combination of the 405 nm wavelength band and the 650 nm wavelength band, or the combination of the 405 nm wavelength band and the 790 nm wavelength band.

[0038]

EXAMPLE 1 Example 1 is a modification of the first embodiment shown in FIG.
This is a specific example. The first light-transmitting substrate 11A has a refractive index n₁But
Constructed with 1.5 uniform refractive index material, processed into uneven shape
A diffraction grating 11B that forms an interface with air was formed. And
Lattice depth d of this uneven part₁To (n₁-1) · d₁Is λ₁
So that d₁= 1.3 μm. This
With such a configuration, the wave used for DVD
Long λ₁= 650 nm incident light has a phase difference of 2π
On the other hand, the wavelength λ used for a CD optical disk _Two
= 790nm incident light, if the resulting phase difference is 2π
Absent. As a result, as shown in FIG._Two
1A acts as a diffraction grating for the light of FIG.
The wavelength λ₁Acts as a diffraction grating for light
An unselective wavelength-selective diffraction grating was obtained.

Here, the pitch of the diffraction grating is 12 μm.
m, the duty a was set to 0.2, and the diffraction efficiency of the zero-order light with respect to the incident light of wavelength λ ₂ was set to approximately 80% and the diffraction efficiency of the ± first-order light was set to approximately 5%. The translucent substrate 1
1A, an anti-reflection film is formed to suppress the reflection loss at the interface with air to 1% or less.

By mounting the two-wavelength diffraction grating having such a structure on an optical head device, a desired diffraction efficiency can be obtained with respect to light having a wavelength used for a CD-type optical disk. Could be used.

Example 2 Example 2 is a specific example of the third embodiment shown in FIG. A first light-transmitting substrate 21A (refractive index 1.5) on which a diffraction grating 21B having a duty a of 0.2 and a grating depth of 1.3 μm is formed, and a second light-transmitting substrate 21
D was fixed with an adhesive 21E with the phase plate 21C interposed therebetween.

The phase plate 21C is obtained by stretching a polycarbonate film to induce birefringence. Here, by adjusting the stretching conditions, it gave a retardation value equivalent to a quarter-wave plate for the wavelength lambda _1. Specifically, by a fast axis inclined by 45 ° disposed with respect to the linear polarization direction of the wavelength lambda ₁ of the phase plate 21C, when the incident linearly polarized light passes through the phase plate 21C, becomes circularly polarized light emitted did.

Therefore, the wavelength λ ₁ and the wavelength λ ₂ whose linear polarization directions are inclined by + 45 ° or −45 ° with respect to the optical axis of the phase plate 21C are added to the two-wavelength diffraction grating 1 having such a configuration.
When the linearly polarized light of different wavelengths are incident, one wavelength lambda ₁
Is linearly polarized without being diffracted and transmitted straight through, while the other linearly polarized incident light of wavelength λ ₂ is
Approximately 80% was diffracted as 0th-order light and approximately 5% was diffracted as ± 1st-order light, and emitted as elliptically polarized light. In other words, it acted as a diffraction grating for light of one wavelength, but did not act as a diffraction grating for light of the other wavelength.

[0044]

As described above, according to the two-wavelength diffraction grating of the present invention, the zero-order diffraction efficiency is kept constant at the maximum with respect to one of the incident two wavelengths of light while the other is incident on the other. On the other hand, the zero-order diffraction efficiency can be changed.

By mounting the two-wavelength diffraction grating on an optical head device having a two-wavelength semiconductor laser,
When recording / reproducing information on / from different types of optical recording media such as a D-type optical disk and a DVD-type optical disk, stable signal detection can be performed.

[Brief description of the drawings]

1A and 1B are configuration diagrams of a first embodiment of a two-wavelength diffraction grating according to the present invention, in which FIG. 1A shows the progress of the optical path of one wavelength light, and FIG. 1B shows the optical path of the other wavelength light. FIG.

FIG. 2 shows 790 nm by the two-wavelength diffraction grating of the present invention.
(A) is a graph of the 0th-order diffraction efficiency, and (B) is a graph showing an example of the duty dependence of the diffraction efficiency with respect to the light having the wavelength of (a).
Is a graph of ± 1st-order diffraction efficiency.

3A and 3B are configuration diagrams of a second embodiment of a two-wavelength diffraction grating according to the present invention, in which FIG. 3A shows the progress of the optical path of one wavelength light, and FIG. 3B shows the optical path of the other wavelength light. FIG.

FIG. 4 shows 650 nm by the two-wavelength diffraction grating of the present invention.
(A) is a graph of the 0th-order diffraction efficiency, and (B) is a graph showing an example of the duty dependence of the diffraction efficiency with respect to the light having the wavelength of (a).
Is a graph of ± 1st-order diffraction efficiency.

5A and 5B are configuration diagrams of a three-wavelength diffraction grating according to a third embodiment of the present invention, in which FIG. 5A shows the progress of the optical path of one wavelength light, and FIG. 5B shows the optical path of the other wavelength light. FIG.

FIG. 6 is a schematic diagram illustrating an example of an optical head device according to the invention.

FIG. 7 is a schematic configuration diagram showing a configuration example of a conventional optical head device.

[Description of Symbols] 1: Diffraction grating for two wavelengths 11A: Transparent substrate 11B, 11C: Diffraction grating 2: Diffraction grating for two wavelengths 21A, 21D: Transparent substrate 21B: Diffraction grating 21C: Phase plate 21E: Adhesion Agent 3: 2 wavelength semiconductor laser 3A, 3B: semiconductor laser 4: beam splitter 5: collimating lens 6: objective lens 7: optical disk 8: photodetector 9: wavelength combining prism

Claims (3)

[Claims] 1. A two-wavelength diffraction grating on which light of at least one of the wavelengths λ ₁ and λ ₂ (λ ₁ ≠ λ ₂ ) is incident. And a ratio w / P of the width w of the protrusion to the period P.
Has a value other than 0.5, transmits the incident light of _one wavelength λ ₁ , diffracts the incident light of the other wavelength λ ₂ , and the phase difference of the transmitted light between the convex portion and the concave portion is the wavelength λ. ₁ is 2π to transmitted light, the two-wavelength diffraction grating, wherein the diffraction efficiency for the wavelength lambda ₂ of light is adjusted to a predetermined value. 2. The two-wavelength diffraction grating is integrated with a phase plate having an organic thin film for changing a polarization state of transmitted light having at least one of the wavelengths λ ₁ and λ _2. 2. The two-wavelength diffraction grating according to item 1. Comprising a light source for emitting wherein the wavelength lambda ₁ and wavelength lambda ₂ light, and an objective lens for condensing the light of the wavelength lambda ₁ and wavelength lambda ₂ to the optical recording medium, the recording of information on an optical recording medium An optical head device for performing reproduction, wherein the two-wavelength diffraction grating according to claim 1 or 2 is provided in an optical path between the light source and the objective lens.