JPH11120598A - Optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve an optical characteristic and to miniaturize and lighten a device by constituting an opening control element provided in an optical path of plural light sources with different wavelengths getting to an information recording surface, of multi-layer volume hologram transmitting light through in its central part area containing an optical axis at a high linear transmission rate, diffracting the light in its peripheral part area and changing the optical path. SOLUTION: The volume hologram 1A, 1B with different diffraction wavelength bands are constituted of a diaphragm part having a wavelength selectivity showing the high linear transmission rate in the central part area (b) containing the optical axis for the light of two wavelengths 650 nm and 780 nm, and showing the high linear transmission rate for the light of the wavelength 650 nm and the low linear transmission rate for the light of 780 nm in the peripheral part area (a) not containing the optical axis. Substrates 2 hold the laminated volume hologram 1A, 1B into between them, and stick them, and assemble integrally them to constitute the opening control element. Thus, surface flatness is improved, and handleability and assembly in an optical head are simplified, and the optical head is miniaturized and thinned, and a cost is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head device which can be used for an information reproducing device or a recording device of an optical disk device.

[0002]

2. Description of the Related Art CD-R (write-once compact disc)
For recording and reproducing CD (compact disk) type discs including such as those described above, a semiconductor laser having a wavelength of 780 nm, an objective lens having an NA (numerical aperture) of 0.45, and a disc having a thickness of 1.2 mm are used as light sources. While DV
For recording / reproducing a D (Digital Video Disk) disk, a semiconductor laser having a wavelength of 650 nm band and an NA of 0.6 are used.
And a 0.6 mm thick disk.

A single optical head device records and reproduces both CD and DVD discs by mounting two semiconductor lasers of each wavelength used for CD and DVD and two objective lenses of each NA. This method can be realized by switching between them. However, this method has two drawbacks: the optical system has two systems, the shape is large, the weight is heavy, the number of parts is large, the assembly is complicated, the control power is large, and the cost is high. there were.

Therefore, recently, a more compact optical head device can be realized by combining and separating light emitted from semiconductor lasers having different wavelengths with a wavelength-selective combining / separating mirror and using the same objective lens. It is considered. However, the thickness of the disc differs between 1.2 mm and 0.6 mm between the CD system and the DVD system, the NA of the objective lens differs between 0.45 and 0.6, respectively, and the wavelength of the light source is 780 nm band and 650 nm. When recording and reproducing using the same objective lens for a CD and a DVD, it is necessary to change the NA of the objective lens according to the wavelength because of the difference between the bands.

As a method of changing the NA of the objective lens according to the wavelength, a central region including the optical axis (hereinafter, simply referred to as a central region) allows light of the two wavelengths to pass straight through and a peripheral region not including the optical axis. Area (hereinafter simply referred to as the peripheral area)
Transmits light having a wavelength of 650 nm in a straight line,
By disposing an aperture control element having a function of not transmitting light of the wavelength in the optical path to the light source and the information recording surface,
The NA is switched between CD and DVD.

Conventionally, the aperture control element is constituted by a single volume hologram manufactured by interfering light with a photosensitive film for a volume hologram. However, due to careless handling, the portions where the diffraction gratings are formed have flaws, and light leaked from these flaws becomes noise, causing malfunctions and changing the NA of the objective lens.

In the spectral waveform of the diffraction efficiency of a single volume hologram, the half-value width (hereinafter referred to as diffraction wavelength bandwidth) with respect to the maximum value differs depending on the material.
The reflection type hologram has a defect of usually 20 to 30 nm, and the aperture control element has a narrow diffraction wavelength bandwidth.

In order to widen this diffraction wavelength bandwidth, a diffuser plate is used instead of a reflecting mirror for irradiating the light irradiated from the light source to the photosensitive film for a volume hologram again to the photosensitive film for a volume hologram. An exposure method has been proposed in which the diffraction wavelength bandwidth is widened by exposing to light, but the volume hologram produced by the exposure method using scattered diffusion light has a drawback of lowering the diffraction efficiency.

Further, since the diffraction efficiency and the diffraction wavelength bandwidth largely depend on the ability of the material to be used and the laser power at the time of exposure, the cost of the material is increased. There is also a disadvantage that the yield is reduced because the variation in the bandwidth is large.

On the other hand, when the aperture control element constituted by the single volume hologram is incorporated in an optical head, the diffraction wavelength shifts due to fluctuations in ambient temperature because of the narrow diffraction wavelength bandwidth, and the diffraction wavelength shifts from the required wavelength range. There were drawbacks.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide an optical head device which has excellent optical characteristics and is suitable for reduction in size and weight.

[0012]

The present invention comprises a plurality of light sources having different wavelengths, and an aperture control element disposed in an optical path from the light source to an information recording surface. In the central region including, the light of the plurality of light sources is transmitted at a high linear transmittance, and in the peripheral region not including the optical axis, light of one or more wavelengths out of the light of the plurality of wavelengths is diffracted. An optical head device for performing aperture control by an aperture control element having a function of changing the aperture is characterized in that the aperture control element is a multilayer volume hologram in which a plurality of volume holograms are stacked.

[0013] The present invention also provides the optical head device, wherein the diffraction wavelength bands of the plurality of stacked volume holograms are distributed so as to partially overlap the adjacent diffraction wavelength bands.

In the multilayer volume hologram, the diffraction wavelength bandwidth of the n-th adjacent diffraction wavelength band is Y _n (nm), and the diffraction wavelength bandwidth of the (n-1) -th bandwidth is Y. _n-1 (nm), and the n-th and (n-
1) The difference between the center wavelengths of the diffraction wavelength bands of the
_{When n−1} (nm), 10 ≦ X _n−1 ≦ (Y _n−1 + Y
_n ) The optical head device satisfying the relationship of (2) / 2 is provided.

The multi-layer volume hologram also provides the optical head device, wherein the diffraction efficiency at the point where the spectral waveforms of adjacent diffraction wavelength bands intersect is 55% or more.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention has two types of light sources, a semiconductor laser for DVD and a semiconductor laser for CD, and has a central portion including an optical axis near an objective lens between the objective lens and the light source. An aperture control element having the property of transmitting the light of the two wavelengths at a high linear transmittance in the region and diffracting the light of the one wavelength in the peripheral region not including the optical axis to change the optical path,
By arranging and arranging a multilayer volume hologram formed by laminating n volume holograms, light of different wavelengths emitted from the two types of light sources, the semiconductor laser for DVD and the semiconductor laser for CD, is emitted. Because the NA of the objective lens is controlled with respect to the wavelength of and the light is focused on the disk, a single optical system achieves a high light-gathering ability even for disks having different thicknesses.

This aperture control element is formed by laminating a plurality of volume holograms having holograms having the same or slightly different diffraction wavelength peaks due to light interference in a peripheral region not including the optical axis. Thus, an aperture control element having a wide diffraction wavelength bandwidth that prevents malfunctions and changes in NA due to the scratches is realized.

The hologram photosensitive film used for the volume hologram is a so-called integrally formed film made of the same material irrespective of the central region and the peripheral region. In the peripheral region, the thickness and the average refractive index are substantially the same, and the optical length can be substantially the same.

Materials suitable for these volume holograms include photopolymers and dichromated gelatin, and holograms usually have an area of several hundred μm to several mm square and several μm to several tens μm. It is thick and has a light diffraction function. As this hologram, a volume hologram such as a Lippmann type can be widely used. As the hologram material, various photosensitive materials such as polyvinyl carbazole, gelatin dichromate, photo resist, photopolymer, and silver salt can be used.

As the volume hologram, there are a transmission type in which a diffraction grating is formed in the hologram medium at an angle close to the hologram surface and a reflection type in which a diffraction grating is formed in the hologram medium at an angle close to the hologram surface. Can be used.

In the case of using this transmission type, the wavelength 7 incident on the peripheral portion of the hologram (peripheral region not including the optical axis) is used.
By diffracting the 80 nm light to change the traveling direction of the light, it prevents the light of the wavelength of 780 nm incident on the peripheral region from being effectively focused on the disk, and has substantially the same effect as having a diaphragm. Is obtained. In addition, in the case of using the reflection type, the light having a wavelength of 780 nm incident on the periphery of the hologram is reflected and diffracted, so that the light can be prevented from reaching the disk. Therefore, unnecessary reflected light from the disk does not become stray light. preferable.

The multilayer volume hologram has a smooth surface.
When a plurality of photosensitive films for holograms are sandwiched between sheets, the smoothness of the surface can be easily improved, and the adverse effect of light having a desired wavelength on the surface of the aperture control element can be prevented. Preferably, the reliability can be improved.

As the material of the surface substrate, glass or plastic is preferable because of its excellent flatness. As the plastic, polycarbonate, polyolefin, acryl and the like can be used. Alternatively, the substrate and the hologram photosensitive film may be attached to each other with an adhesive layer, and the protective film may be formed on the substrate or the hologram photosensitive film. It is preferable to use a material having a refractive index as close as possible to the hologram and the substrate.

Further, it is preferable to form a non-reflection coating layer on the surface of the substrate, because the transmission efficiency can be increased and return light due to surface reflection can be reduced.

When manufacturing this multilayer volume hologram,
By forming a plurality of volume hologram elements each having a size of several mm on a large substrate and manufacturing the volume hologram element by cutting, it is preferable because productivity can be increased, mass production can be performed, and low cost can be realized.

The method of manufacturing the multilayer volume hologram, particularly the reflection type hologram, will be described. First, in the case of the first layer volume hologram (hereinafter referred to as the first volume hologram), the photosensitive film is sandwiched between substrates. After irradiating, for example, ultraviolet light (UV light) to a region to be transmitted with both wavelengths and then exposing the region to change the optical path by diffracting light of one wavelength, irradiate laser light of a wavelength to be reflected and opposite to the light source. A reflection hologram can be formed by placing a reflection mirror on the side and causing the irradiated laser light to interfere with the light reflected by the reflection mirror. At this time, it is easy to use the same wavelength as the wavelength to be selectively reflected as the laser wavelength used for exposure.However, when using a different wavelength, it is necessary to change the exposure incident angle so as to satisfy the Bragg condition. I just need.

Similarly, the volume hologram of the second layer (hereinafter referred to as second
In the case of a volume hologram), the photosensitive film is sandwiched between the substrates, and a region where both wavelengths are to be transmitted is irradiated with ultraviolet light (UV light) in the same manner, and then light of one wavelength is diffracted. The area where the optical path is changed is irradiated with a laser beam having a wavelength different from the wavelength of the laser beam irradiated at the time of the first volume hologram, for example, by 10 nm, and a reflection mirror is placed on the side opposite to the light source. By interfering the light reflected by the hologram, a reflection hologram having a wavelength different in diffraction efficiency from that of the first volume hologram can be formed.

Here, instead of making the wavelengths of the laser beams for exposing the first and second volume holograms different from each other, the laser beams having the same wavelength are irradiated at different incident angles so that the diffraction wavelength bands differ by about 10 nm. Good.

Since the first and second volume holograms have different center wavelengths of the diffraction wavelength bands, when these volume holograms are stacked so as to partially overlap the two diffraction wavelength bands, the diffraction wavelength bandwidth becomes 2 It becomes equal to the value calculated by the logical sum of the diffraction wavelength bandwidths of the two volume holograms, and a wide diffraction wavelength bandwidth characteristic can be obtained. That is, by stacking a plurality of volume holograms, a volume hologram having a wide diffraction wavelength band can be obtained. FIG. 5 shows diffraction wavelength band characteristics showing the concept of broadening the diffraction wavelength band. The vertical axis indicates the diffraction efficiency, and the horizontal axis indicates the wavelength.

In FIG. 5, reference numeral 15 denotes a spectral waveform of the diffraction efficiency of the first volume hologram, 16 denotes a spectral waveform of the diffraction efficiency of the second volume hologram, and 17 denotes a diffraction waveform of the first and second volume holograms. 5 shows a spectral waveform of a combined diffraction efficiency (hereinafter, referred to as a combined diffraction efficiency) in a region where wavelength bands overlap. When the diffraction wavelength bandwidths of the first and second volume holograms are Y ₁ (nm) and Y ₂ (nm), respectively, and the difference between the center wavelengths of the diffraction wavelength bands is X ₁ (nm), the diffraction wavelength bandwidth is In order to widen X, it is better that X ₁ is large, but the diffraction wavelength bandwidth of one reflection hologram is usually 20 to 30.
Therefore, X ₁ ≧ 10 is preferable.

Also, assuming that the diffraction efficiency (hereinafter referred to as intersection diffraction efficiency) at the intersection of the spectral waveforms in the diffraction wavelength band of the first and second volume holograms is N (%) and the combined diffraction efficiency is T (%). The combined diffraction efficiency T is approximately T = N +
Since it is expressed by (100−N) N / 100, when the intersection diffraction efficiency is 50% or more of the maximum value of the diffraction efficiency in each band (hereinafter, peak diffraction efficiency), the combined diffraction efficiency of each band is It is preferable because the peak diffraction efficiency is 75% or more. In order to satisfy the condition that the intersection diffraction efficiency is 50% or more of the peak diffraction efficiency of each band, the relationship between the diffraction wavelength bandwidths Y ₁ and Y ₂ and the difference X ₁ between the center wavelengths of the diffraction wavelength bands is required. Can be achieved if X ₁ ≦ (Y ₁ + Y ₂ ) / 2.

[0032] Thus, when the diffraction wavelength band width Y ₁ of the two volume holograms, Y ₂ and relationship of the difference between X ₁ of the center wavelength of the diffraction wavelength band _{10 ≦ X 1 ≦ (Y 1} + Y 2) is / 2, The intersection diffraction efficiency of the two volume holograms is 50% or more of the peak diffraction efficiency of each, so that a high combined diffraction efficiency and a wide diffraction wavelength bandwidth can be achieved, and the intersection diffraction efficiency is 55% or more. X ₁ , Y ₁ , Y
By adjusting ₂ , a high combined diffraction efficiency of about 80% or more can be achieved, so that it is more preferable as an aperture control element in which volume holograms are stacked.

On the other hand, the difference X ₁ between the diffraction wavelength bandwidths Y ₁ , Y _{2 of the} two volume holograms and the center wavelength of the diffraction wavelength bandwidth.
Is X ₁ ≧ (Y ₁ + Y ₂ ) / 2, the overlapping portion of the diffraction bands by the two volume holograms decreases,
As the difference between X ₁ and (Y ₁ + Y ₂ ) / 2 is larger, the combined diffraction band characteristic has a larger drop in diffraction efficiency near the center of the center wavelength of the two diffraction wavelength bands, and the combined diffraction efficiency is lower. It is not preferable because it lowers.

In the above description, the case where two volume holograms are stacked (n = 2) is described. However, the same applies to the case where three or more volume holograms are stacked (n ≧ 3). The diffraction wavelength bandwidth of the n-th band of the band (hereinafter, adjacent band) is represented by Y _n (nm), (n−
1) The diffraction wavelength bandwidth of the _first band is Y _n-1 (nm), and the difference between the center wavelengths of the adjacent bands is X _n-1.
(Nm), the volume hologram is configured such that the relationship of 10 ≦ X _n−1 ≦ (Y _n−1 + Y _n ) / 2 is established between two adjacent diffraction wavelength bands, and further, An aperture control element having a wide diffraction wavelength bandwidth can be realized.

In the case where one of the two stacked volume holograms has a flaw or the like, the diffraction efficiency of the flawed portion is P (%), and the diffraction of the portion corresponding to the flawed portion of the flawless volume hologram is Assuming that the efficiency is Q (%), the combined diffraction efficiency (hereinafter referred to as “scratch portion combined diffraction efficiency”) corresponding to the flaw portion of the stacked volume hologram at this time is S (%), and the flaw portion combined diffraction efficiency S Is approximately represented by S = Q + (100−Q) * P / 100. For example, when Q = 55% and P = 20%, S is 6
4 (%), which is three times or more the diffraction efficiency as compared with the diffraction efficiency of 20% when one scratched volume hologram is used. The same applies to the case where n volume holograms are stacked. Even if there are a plurality of volume holograms having a flaw or the like, a high flaw portion synthetic diffraction efficiency can be realized if the flaws do not overlap.

The hologram exposed by the method described here is a specular reflection type. However, when a specular reflection type aperture control element is used, light emitted from a semiconductor laser as a light source is diffracted and reflected by the hologram, and is reflected on the semiconductor laser. It can return and cause noise. As a countermeasure against this noise, by using a non-specular reflection hologram with a different exposure method or a non-specular reflection hologram with a lens effect, the diffracted reflected light from the aperture limiting part (the hologram peripheral part) is applied to the semiconductor laser. It is preferable because it is not possible to return.

Further, as the mirror used at the time of the exposure,
One of the laser beams during exposure can be exposed as diffused light having a distorted wavefront by forming minute irregularities on the surface. Since it is scattered and diffracted in the hologram region produced by such an exposure method, it depends on the diffusion state recorded in the hologram, but it returns to the semiconductor laser as compared with the hologram produced by the exposure method using a specular reflection mirror. Light is preferred because it can be less than half.

Further, since the wavelength of the semiconductor laser used in the optical head has individual variations (for example, ± 20 nm), it is necessary to diffract all of the wavelength in the wavelength range.

The wavelength of the semiconductor laser shifts to the longer wavelength side when the temperature is increased. However, the wavelength temperature shift amount and the diffraction wavelength temperature shift amount due to thermal expansion and contraction of the volume hologram are adjusted to cover a wide temperature range. This is preferable because the semiconductor laser light can be diffracted. The wavelength shift amount is preferably from 0.05 to 0.6 nm / ° C, and particularly preferably from 0.2 to 0.3 nm / ° C. This temperature characteristic of the wavelength shift amount can be realized by selecting, for example, an acrylic photopolymer or the like as the material of the volume hologram.

Since the multilayer volume hologram has a characteristic that the diffraction wavelength bandwidth is wide, it is very effective as a measure against the individual variation of the wavelength of the semiconductor laser and a measure against the wavelength shift due to the temperature.

[0041]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of an aperture control element according to an embodiment of the present invention. FIG. 1A is a cross-sectional view of a wavelength selective stop viewed from a direction perpendicular to an optical axis, and FIG. It is the top view seen from the direction.

In FIG. 1, reference numerals 1A and 1B denote volume holograms having different diffraction wavelength bands, and show a high linear transmittance for light having two wavelengths of 650 nm and 780 nm in the central region b including the optical axis. In the peripheral region a not including the optical axis, a diaphragm portion having a wavelength selectivity exhibiting a high linear transmittance for light having a wavelength of 650 nm and a low linear transmittance for light having a wavelength of 780 nm. Have been. 2 is a substrate, and the volume holograms 1A, 1
By sandwiching and adhering the stacked layers of B and assembling them together to form an aperture control element, the surface flatness is increased and handling and assembly of the optical head are simplified.

FIG. 3 is a cross-sectional view showing steps in a method of manufacturing the aperture control element including the volume hologram. In FIG. 3A, the volume hologram 1A is a UV exposure mask 1 in which a photosensitive film 14 for a volume hologram is sandwiched between substrates 2 and is opened only in a region b where both wavelengths are to be transmitted.
Then, the photosensitive film 14 for volume hologram is irradiated with UV light 12 from the direction of the mask 10 for UV exposure. Thereby, the region b irradiated with the UV light is uniformly exposed, and the light is transmitted straight.

Next, in FIG. 3B, the mask is removed, and the laser beam 13 having the same wavelength as the wavelength desired to be reflected in the peripheral region of the photosensitive film for volume hologram 14 is obtained.
Is irradiated. At this time, a reflection mirror 11 for reflecting the laser light 13 is placed on the opposite side of the volume hologram photosensitive film 14 with respect to the laser light 13, and the irradiated laser light 13 and the light reflected by the reflection mirror 11 interfere with each other. The volume hologram 1A in which the reflection hologram is formed in the region a having wavelength selectivity of the volume hologram photosensitive film 14 can be manufactured. In addition, when a wavelength different from the wavelength to be selectively reflected is used as the wavelength of the laser beam used for the exposure, the same reflection hologram is formed by changing the exposure incident angle so that the Bragg condition is satisfied. it can.

Similarly, the volume hologram 1B is shown in FIG.
(1) In FIG. 3 (2), for example, by making the wavelength of the laser beam to be exposed differ by about 10 nm from the wavelength of exposing the volume hologram 1A, a reflection hologram having a center wavelength of the diffraction wavelength band different by about 10 nm can be formed. . Here, instead of exposing with a laser beam having a different wavelength, a laser beam having the same wavelength as the laser beam exposing the volume hologram 1A may be irradiated at a different incident angle so that the center of the diffraction wavelength band differs by about 10 nm.

Further, in FIG. 3 (2), for example, when a mirror glass whose surface is finished with a polishing material of JIS # 2000 and a metal coating is used as the reflection mirror 11, a laser beam 13 is used. Is a reflection mirror 1
The peripheral area a of the volume hologram photosensitive film 14 is diffusely reflected by the surface of the photosensitive film 1 and is exposed to diffused light whose wavefront is disturbed. As described above, the incident light is scattered and diffracted in the reflection type hologram region (the peripheral region a) formed by using the diffused light whose wavefront is disturbed as one of the exposure light. When the reflection type hologram prepared by the method is used as the aperture control element 7 of the optical head device having the configuration shown in FIG. 2, it is compared with the case where a reflection type hologram manufactured using a regular reflection mirror as the reflection mirror 11 is used. Thus, the return light to the semiconductor laser could be reduced to 50% or less.

FIG. 4 shows a wavelength 780 n in the peripheral region.
Three volume holograms each having a m-band diffraction wavelength bandwidth of 30 nm and a difference in the center wavelength of each adjacent diffraction wavelength band of 25 nm are laminated, sandwiched between the substrates 2 and bonded with an adhesive. The linear transmittance of the aperture control element is shown.

FIG. 4A shows the linear transmittance of the central region b. In the central region b, the wavelength is approximately 500 to
It shows high linear transmittance in the range of 900 nm. FIG.
(2) shows the linear transmittance of the peripheral region a. In the peripheral region a, a low linear transmittance is shown in the wavelength range of about 740 to 820 nm, and in other wavelength regions, a high linear transmittance is shown.

By laminating a plurality of volume holograms in this manner, 780 nm ± 40 nm was realized as a wavelength region in which the transmittance of the peripheral region was 10% or less. At this time, the linear transmittance of the central region is approximately 96% at both the wavelengths of 780 nm and 650 nm, and the linear transmittance of the peripheral region is approximately 5% at the wavelength of 780 nm.
It was approximately 96% for a wavelength of m.

Also, since the peripheral region and the central region of the aperture control element are made of the same material, the peripheral portion and the central region have substantially the same average refractive index as well as the thickness. (Refractive index × thickness) was almost the same, and there was no adverse effect on the light-collecting characteristics on the disk. At this time, the difference in optical length (transmitted wavefront aberration) is such that the RMS (root mean square) in the effective plane is 0.01λ or less,
The PV (Peak to Valley) value was also 0.05λ or less (λ = 633nm).

The wavelength of the semiconductor laser shifts to the longer wavelength side when the temperature rises, and its temperature coefficient is 0.3 nm.
/ ° C. In order to compensate for the temperature characteristics of the semiconductor laser, for example, an acrylic photopolymer is selected as the material of the volume hologram, so that the temperature coefficient of the diffraction wavelength of the volume hologram can be set to approximately 0.3 nm / ° C. The temperature shift of the diffraction wavelength could be matched, and the semiconductor laser light could be diffracted over a wide temperature range.

Next, FIG. 2 is a schematic diagram showing the structure of an optical system of an optical head device using the aperture control element according to the present invention. The optical system includes a semiconductor laser 3 having a wavelength of 650 nm and a wavelength 78.
Collimator lenses 5a and 5b having two light sources of a semiconductor laser 4 of 0 nm, condensing light emitted from the respective light sources and converting them into parallel rays, and a wavelength-selective combining / separating mirror 6 for combining and separating two wavelengths. , An aperture control element 7 using a volume hologram, and an objective lens 8. In addition,
For simplification of the drawing, a light receiving portion of light including a signal from the disk 9 and the like are omitted.

The aperture control element 7 has an NA of 0.45 for light having a wavelength of 780 nm and an NA of 0.6 for light having a wavelength of 650 nm. The aperture control element 7 is arranged at a position where it can be moved integrally with the objective lens 8 in order to facilitate its function and control of the optical head.

When the optical head configured as described above is used for a CD, the wavelength 78 emitted from the semiconductor laser 4 is used.
The laser light of 0 nm is converted into substantially parallel light by the collimator lens 5b, reflected by the wavelength selective mirror 6 so as to change the direction by 90 degrees, and enters the aperture control element 7. Since the aperture control element 7 has an NA of 0.45 for light having a wavelength of 780 nm as described above, the aperture control element 7 has a thickness of 1.2 m.
The information recorded on the CD disk of m was successfully reproduced with little influence of aberration.

Similarly, when the optical head having this configuration is used for a DVD, the wavelength 650 nm emitted from the semiconductor laser 3 is used.
m is converted into substantially parallel light by a collimator lens 5a, passes through a wavelength-selective mirror 6, and passes through an aperture control element 7
Incident on. The aperture control element 7 has a NA of 0.6 for light having a wavelength of 650 nm, so that the D
The VD disc was also successfully reproduced.

The present invention is not limited to the above embodiment,
For example, the aperture control element 7 may be arranged away from the objective lens, and the arrangement positions of the plurality of light sources can be changed in combination with the wavelength-selective combining / separating mirror 6. The plurality of volume holograms may have different characteristics, and an aperture control element having an arbitrary bandwidth can be realized by combining various volume holograms. Furthermore, various modifications can be made without departing from the gist of the present invention, in combination with a method for detecting information recorded on a disc, a method for processing a detection signal, a method for controlling an optical head, and the like.

[0057]

According to the present invention, a plurality of light sources having different wavelengths are arranged in an optical path from the light source to an information recording surface, and the plurality of light sources are arranged in a central region including an optical axis. Light is transmitted at a high linear transmittance, and aperture control is performed by an aperture control element having a function of diffracting light of one or more wavelengths among the lights of the plurality of light sources and changing an optical path in a peripheral region not including the optical axis. In the optical head device, the aperture control element is composed of a plurality of stacked volume holograms, so that even if there is a defect such as a flaw in a portion where the diffraction grating of one volume hologram is formed, the other layer may have a defect. Since the diffraction function is supplemented by the volume hologram, the yield is improved, and the diffraction wavelength bands of the plurality of volume holograms are configured to be distributed so as to partially overlap with adjacent wavelength bands. The width is widened, the temperature change, can absorb variations in characteristics due to individual variations between the components can at the same objective lens is stably converged to different light having different thicknesses on the disc wavelengths.

Therefore, information recording and reproduction of both CD and DVD can be stably performed by an optical head having one optical system (one objective lens is mounted), realizing miniaturization and thinning. As well as low cost.

[Brief description of the drawings]

FIG. 1 shows a configuration of a volume hologram aperture control element according to the present invention, wherein (1) is a cross-sectional view and (2) is a plan view.

FIG. 2 is a schematic diagram illustrating a configuration of an optical head device according to the present invention.

FIGS. 3A and 3B are cross-sectional views showing two steps of a volume hologram manufacturing method according to the present invention.

FIG. 4 is a graph showing an example of a wavelength characteristic of a linear transmittance of an aperture control element according to the present invention.

FIG. 5 is a schematic diagram showing the concept of broadening the diffraction wavelength band of a volume hologram in the present invention.

[Explanation of symbols]

1A to 1B: Volume holograms having different diffraction wavelengths 2: Substrate 3: Semiconductor laser (λ = 650 nm) 4: Semiconductor laser (λ = 780 nm) 5a: Collimator lens 5b: Collimator lens 6: Wavelength-selective synthetic separation mirror 7: Aperture Control element 8: Objective lens 9: Disk 10: UV exposure mask 11: Reflection mirror 12: UV light 13: Laser light 14: Photosensitive film for volume hologram 15: Spectral waveform of diffraction efficiency of first volume hologram 16: Spectral waveform of diffraction efficiency of second volume hologram 17: Spectral waveform of combined diffraction efficiency in a region where the diffraction wavelength bands of the first volume hologram and the second volume hologram overlap

Claims (4)

[Claims] A plurality of light sources having different wavelengths; and an aperture control element disposed in an optical path from the light source to an information recording surface, wherein the plurality of light sources are arranged in a central region including an optical axis of the aperture control element. The light from the light source is transmitted at a high linear transmittance, and in the peripheral region not including the optical axis, an aperture control element having a function of diffracting light of one or more wavelengths among the lights of the plurality of light sources and changing the optical path is provided. An optical head device for controlling an aperture, wherein the aperture control element is a multilayer volume hologram formed by stacking n (n is a natural number of 2 or more) volume holograms. 2. The optical head device according to claim 1, wherein the diffraction wavelength bands of the plurality of stacked volume holograms are distributed so as to partially overlap adjacent diffraction wavelength bands. 3. The multi-layer volume hologram according to claim 1, wherein the diffraction wavelength bandwidth of the n-th adjacent diffraction wavelength band is Y.
_n (nm), the diffraction wavelength bandwidth of the (n-1) th band is Y _n-1 (nm), and the difference between the respective center wavelengths of the nth and (n-1) th diffraction wavelength bands. To X _n-1
(Nm), 10 ≦ X _n−1 ≦ (Y _n−1 + Y _n )
3. The optical head device according to claim 1, wherein a relationship of / 2 is satisfied. 4. The optical head device according to claim 1, wherein the multilayer volume hologram has a diffraction efficiency of 55% or more at a wavelength at which spectral waveforms of adjacent diffraction wavelength bands intersect.