JP4296662B2 - Optical head device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device used for a recording device or a reproducing device for an optical recording medium such as an optical disk using two wavelength lights as light sources.
[0002]
[Prior art]
Various optical head devices for recording information on or reproducing information on an information recording surface of an optical recording medium such as an optical disk such as a CD or DVD and a magneto-optical disk have been proposed. In an optical head device mounted on such a recording apparatus or reproducing apparatus for an optical recording medium, light emitted from a semiconductor laser as a light source is reflected by the optical recording medium to become return light, and this return light is a beam splitter or the like. Is guided to a light receiving element which is a photodetector. In recent years, a beam splitter using a hologram element which is a kind of diffraction element has been put into practical use. With the hologram element, the light traveling direction can be bent by diffraction and guided to a light receiving element disposed near the semiconductor laser, so that the optical head device can be miniaturized.
[0003]
Also, CD / DVD compatible type optical head devices have been commercialized in order to record or reproduce information on optical disks of CD and DVD, which are optical recording media of different standards, with the same optical head device. In particular, assuming reproduction of a CD-R or the like that uses a medium with high wavelength dependence for the optical recording medium layer, a semiconductor laser having a 790 nm wavelength band is used for a CD, and a semiconductor laser having a 650 nm wavelength band is used for a DVD. It has been.
[0004]
FIG. 6 is a side view showing a configuration example of a CD / DVD compatible type optical head device. Light emitted from the semiconductor laser 51A that outputs laser light in the 650 nm wavelength band and the semiconductor laser 51B that outputs laser light in the 790 nm wavelength band is converted into parallel light by the collimating lenses 54A and 54B, and the optical axis is output by the color synthesis separation dichroic prism 58. Are transmitted through a diffraction element 57 such as a hologram beam splitter, which is a kind of light beam control element, and focused by an objective lens 53 on an optical disk 55 such as a CD or DVD as an optical recording medium.
[0005]
The reflected light from the optical disk 55 passes through the objective lens 53 again, is diffracted by the diffraction element 57, is separated into light of each wavelength band by the color synthesis / separation dichroic prism 58, and is condensed by the collimating lenses 54A and 54B. The light reaches the light receiving elements constituting the photodetectors 56A and 56B. The light receiving element converts the received reflected light into an electrical signal and outputs it. The output signal is amplified by an amplifier, and the gain is corrected by an automatic gain correction circuit to adjust the signal level within a certain range. . The diffraction element 57 composed of a hologram beam splitter or the like may be used integrally with the objective lens 53 as shown in FIG. 6 or may be used as an integrated unit disposed in the vicinity of the semiconductor laser and the photodetector.
[0006]
The example of the conventional optical head device shown in FIG. 6 has a configuration in which a light source and a photodetector are separated in a DVD system and a CD system, but recently, a semiconductor laser in a 790 nm wavelength band for CD and a 650 nm for DVD. Monolithic dual-wavelength lasers in which semiconductor lasers in the wavelength band are formed in one chip and dual-wavelength lasers in which laser chips in each wavelength band are arranged and fixed at intervals of 100 to 300 μm are being used. By using such a laser light source, stable optical head characteristics with high emission point position accuracy can be obtained, and optical components such as the color synthesis / separation dichroic prism 58 and the collimating lenses 54A and 54B shown in FIG. 6 can be omitted. Therefore, the reduction in the number of parts can be expected to reduce the size and weight of the optical head device and to simplify the design of the optical system.
[0007]
As a light beam control element such as the above-described diffraction element, a non-polarizing hologram beam splitter having a diffraction characteristic without polarization dependence, in which a diffraction grating made of an isotropic optical material is formed on the upper surface of a glass substrate, has been conventionally used. However, since the light beam is diffracted in the forward path and the return path, it is difficult to obtain a light utilization efficiency of 10% or more.
[0008]
Accordingly, in order to improve the light utilization efficiency, it has been proposed to use a polarizing hologram beam splitter and a quarter wavelength plate whose diffraction efficiency changes depending on the polarization direction of light as a light beam control element. In this case, since the polarization direction of the forward path and the polarization direction of the return path are orthogonal polarizations rotated by 90 ° by the ¼ wavelength plate, the polarization of the forward path does not act as a diffraction grating and is diffracted only by the polarization of the return path By having the polarization dependency acting as a grating, a light utilization efficiency higher than 10% which is the theoretical round-trip efficiency of the non-polarizing hologram beam splitter can be obtained as a result.
[0009]
[Problems to be solved by the invention]
However, when the light beam control element including the polarizing hologram beam splitter and the quarter wavelength plate as described above is applied to an optical head device in which a two-wavelength laser is used as a light source, there are the following problems. .
[0010]
That is, sin θ with respect to the diffraction angle θ of ± first-order diffracted light when light of wavelength λ is incident on a hologram beam splitter composed of a single diffraction grating with pitch P is proportional to wavelength λ and inversely proportional to grating pitch P. Therefore, the wavelength λ₁= 650 nm and wavelength λ₂When light from a two-wavelength laser of 790 nm is incident on the hologram beam splitter, the diffraction angle of the two wavelength lights is different, so the light receiving surface must be enlarged to receive the diffracted light with the same photodetector. . For this reason, there are problems that the noise component increases and a high S / N cannot be secured, and it is difficult to obtain a good high frequency characteristic. In addition, there is a drawback that it is easily affected by stray light.
[0011]
On the other hand, wavelength λ₁And wavelength λ₂When separate light receiving elements are formed for light reception, the number of light receiving elements is twice as large, and the scale of the apparatus increases, resulting in a problem that the signal processing circuit becomes complicated.
[0012]
An object of the present invention is to record or reproduce information on an information recording surface of an optical recording medium having different operating wavelengths by using two wavelength lights as a light source and receiving the two wavelength lights with the same photodetector. It is an object to provide a compact and lightweight optical head device that can be stably and efficiently produced with a small number of components.
[0013]
[Means for Solving the Problems]
  In the present invention, the wavelength λ₁And wavelength λ₂(Λ₁<Λ₂) A semiconductor laser that emits light of two wavelengths, an objective lens that condenses the emitted light of the semiconductor laser onto an optical recording medium, and a photodetector that detects reflected light reflected by the optical recording medium,A collimating lens for collimating the reflected light from the optical recording medium while making the emitted light from the semiconductor laser into parallel light;A hologram beam splitter for guiding the reflected light to the photodetector;The wavelength λ in the optical path between the objective lens and the hologram beam splitter ₁ A quarter wave plate forAn optical head device comprising at least the hologram beam splitter,Each cross-sectional shape is periodic unevenFirst diffraction gratingAnd secondFrom two diffraction gratingsBecome,The first diffraction grating is a polarization hologram whose diffraction efficiency changes depending on the polarization direction of light, and the phase difference of transmitted light between the concave portion and the convex portion is the wavelength λ. ₂ An integer multiple of 2π and a wavelength λ ₁ Is a non-integer multiple of 2π, and the wavelength λ ₂ And the wavelength λ from the semiconductor laser toward the optical recording medium ₁ Wavelength λ transmitted through the optical recording medium and reflected by the optical recording medium ₁ The second diffraction grating has a phase difference of transmitted light between the concave portion and the convex portion, the wavelength λ ₁ An integer multiple of 2π and a wavelength λ ₂ Is a non-integer multiple of 2π, where f and NA are the focal length and numerical aperture of the collimating lens, and P is the grating pitch of the second diffraction grating. The distance from the second diffraction grating is 2 × f × P × NA / λ ₂ Set to be longer,Further, it is diffracted by the hologram beam splitter.AboveWavelength λ₁andAboveWavelength λ₂Light of the sameAboveProvided is an optical head device which is guided to a photodetector.
[0014]
  Also,The wavelength λ ₁ Is a wavelength band for DVD of 650 nm, and the wavelength λ ₂ Is the 790nm wavelength band for CDAn optical head device is provided.
[0015]
  Further, the first diffraction grating and the second diffraction grating are installed in an optical path between the semiconductor laser and the objective lens, and in an optical path between the photodetector and the objective lens,AboveFirst diffraction caseThe child is the quarter wave plateIntegrated with the objective lensAboveAn optical head device is provided.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0017]
[First Embodiment]
FIG. 1 is a side view showing a schematic configuration of an optical head device according to a first embodiment of the present invention. The optical head device of the present embodiment has a first wavelength λ₁And the second wavelength λ₂A two-wavelength semiconductor laser (monolithic two-wavelength semiconductor laser, etc.) 1 that is a semiconductor laser light source capable of emitting light of two wavelengths, and a wavelength λ₂Of light and wavelength λ₁The first diffraction grating 21 that diffracts the light and the wavelength λ₁Of light and wavelength λ₂From a wavelength-selective hologram beam splitter 2 comprising a second diffraction grating 22 that diffracts the light, an objective lens 3 that focuses the light on an optical disk 5 that is an optical recording medium, and a two-wavelength semiconductor laser 1 A collimating lens (collimator lens) 4 that collimates the emitted light and collects the reflected light from the optical disc 5, a photodetector 6 that receives the reflected light from the optical disc 5 and detects the amount of light, and a first A quarter wavelength plate 7 disposed between the diffraction grating 21 and the objective lens 3 is provided. The two-wavelength semiconductor laser 1 has a wavelength λ for DVD-based optical recording media.₁= 650 nm light for a CD type optical recording medium₂= 790 nm light is switched and emitted according to the optical recording medium.
[0018]
Wavelength λ emitted from the two-wavelength semiconductor laser 1₁Light passes through the second diffraction grating 22 without being diffracted, becomes parallel light by the collimating lens 4, passes through the first diffraction grating 21, and passes through the information recording surface of the optical disk 5 by the objective lens 3. Focused. The light reflected by the information recording surface is again converted into parallel light by the objective lens 3 and enters the first diffraction grating 21. At this time, of the diffracted light diffracted by the first diffraction grating 21, the + 1st order diffracted light is condensed on the light receiving surface of the photodetector 6 by the collimating lens 4.
[0019]
The first diffraction grating 21 used here is a polarization hologram, which will be described later, and does not diffract with respect to the polarization direction of the laser light in the forward path, but the objective lens 3 and the polarization hologram in the return path. In this structure, diffraction is generated with respect to the polarized light whose polarization direction is rotated by 90 ° by passing through the quarter-wave plate 7 disposed between them twice. Even in the return optical path, the wavelength λ₁The laser light passes through the second diffraction grating 22 without being diffracted and reaches the photodetector 6.
[0020]
On the other hand, the wavelength λ emitted from the two-wavelength semiconductor laser 1₂The 0th-order diffracted light component that passes without being diffracted by the second diffraction grating 22 is converted into parallel light by the collimating lens 4 and is transmitted through the first diffraction grating 21 without being diffracted. 3 is focused on the information recording surface of the optical disk 5. The light reflected by the information recording surface again becomes parallel light by the objective lens 3 and passes through without being diffracted by the first diffraction grating 21, and becomes parallel light by the parallelizing lens 4 and becomes the second diffraction grating. 22 is incident. At this time, the second diffraction grating 22 is fabricated so that the + 1st order diffracted light out of the diffracted light diffracted by the second diffraction grating 22 is condensed on the light receiving surface of the photodetector 6.
[0021]
Wavelength λ used here₁Of light and wavelength λ₂The second diffraction grating 22 that diffracts the light is a non-polarizing hologram that does not depend on the polarization direction of incident light, which will be described later.
[0022]
The first diffraction grating 21 which is a polarization hologram is provided integrally with the objective lens 3 together with the quarter-wave plate 7 and is integrally driven by a known technology actuator (not shown). By this actuator, focusing control for condensing the focused beam at a predetermined focal point (spot diameter) on the information recording surface and tracking control for tracking the focused beam to a predetermined information recording track are performed. The second diffraction grating 22 which is a non-polarizing hologram is fixedly provided on a light incident / exit surface of a unit in which the semiconductor laser 1 and the photodetector 6 are integrated.
[0023]
Here, the use conditions and characteristics of the hologram beam splitter 2 which is a diffraction grating for two wavelengths in the configuration of FIG. 1 will be described.
[0024]
Since DVD-based optical recording media require focusing and tracking with higher accuracy than CD-based recording media, the hologram beam splitter is preferably driven integrally with the objective lens. However, when using a non-polarizing hologram beam splitter that is driven integrally with the objective lens, the wavelength λ₁The component of the first-order diffracted light diffracted in the forward path in a straight line (0th-order diffracted light) or ± second-order diffracted in the forward path is condensed on the same light receiving surface as the signal light in the photodetector 6. Therefore, it becomes a noise component and the signal cannot be reproduced.
[0025]
On the other hand, when a polarizing hologram is used for the first diffraction grating 21 as described above, such a problem does not occur because the diffracted light generated in the forward path is very small. The second diffraction grating 22 has a wavelength λ₁The light is transmitted almost without being diffracted, so that the light utilization efficiency is not lowered and the generation of diffracted light that becomes stray light is small.
[0026]
CD system wavelength λ₂In contrast, the first diffraction grating 21 has a wavelength λ.₂Light is transmitted without being diffracted, so that a decrease in light utilization efficiency is suppressed, and the generation of diffracted light that becomes stray light is small. On the other hand, the second diffraction grating 22 is a non-polarizing hologram, and therefore has a wavelength λ in the forward path and the return path.₂Diffract light. As a result, the problem of stray light due to multiple diffraction in the forward and return paths described above occurs.
[0027]
As a workaround, an arrangement configuration and a hologram structure may be used so that the diffracted light from the second diffraction grating 22 does not reach the effective numerical aperture NA of the collimating lens 4 having the focal length f in the forward path. Specifically, when the grating pitch of the second diffraction grating 22 is P, the distance between the collimating lens 4 and the second diffraction grating 22 is 2 × f × P × NA / λ.₂Longer and P is 0.5 × λ₂What is necessary is just to set so that it may become smaller than / NA.
[0028]
The second diffraction grating 22 is also a polarizing hologram, so that the wavelength λ₁As in the case of, the diffracted light in the forward path disappears and the problem of stray light is solved. In particular, when it can be applied to an optical recording medium such as a CD-R that can be written to, it requires a high-intensity writing light.
[0029]
Further, since the CD-type optical disc has a disc thickness twice that of the DVD-type optical disc, the spatial birefringence fluctuation of the disc, which is a resin molded body, tends to remain. As a result, in the case of a polarization hologram that diffracts only one polarization component, the signal light intensity is likely to fluctuate according to the spatial birefringence fluctuation of the disk, and thus stable reproduction may not be possible.
[0030]
As a measure for such an optical disk having birefringence, a CD wavelength λ₂The second diffraction grating 22 that diffracts is preferably a non-polarizing hologram or a partially polarizing hologram that diffracts both polarization-dependent but orthogonal polarization components.
[0031]
In the configuration of the present embodiment, separate wavelength-selective hologram beam splitters are used for the respective wavelength lights of the two-wavelength semiconductor laser, so that the emission point position of the two-wavelength semiconductor laser and the arrangement relationship of the hologram beam splitter are limited. By reducing the grating pitch of each wavelength selective hologram beam splitter, it is easy to condense light of a plurality of wavelengths on the same light receiving element.
[0032]
Next, an example of a polarization hologram used in the optical head device of the present invention will be described with reference to the cross-sectional configuration diagram shown in FIG.
[0033]
In the polarization hologram 31 of this example, an isotropic filler 31C is filled in a birefringent diffraction grating 31B having a rectangular wave cross section formed on a translucent substrate 31A, and a translucent substrate 31D is disposed thereon. Thus, the birefringent diffraction grating 31B is sandwiched between the translucent substrates 31A and 31D. Here, for example, the ordinary refractive index n is used as the birefringent diffraction grating 31B._o= 1.52, extraordinary refractive index n_e= 1.67 polymer liquid crystal, refractive index n as isotropic filler 31C_s= 1.52 isotropic medium is used.
[0034]
This polarizing hologram 31 is applied to the first diffraction grating 21 in FIG. In this case, the wavelength λ of the DVD system emitted from the two-wavelength semiconductor laser 1₁= 650 nm and CD system wavelength λ₂= 790 nm emission light is incident on the polarization hologram 31. Here, the polarization direction of each wavelength is the ordinary refractive index n of the birefringent diffraction grating 31B._oThe orientation of the polarizing hologram 31 is set so as to be S-polarized light corresponding to.
[0035]
Therefore, the polarization hologram 31 has a wavelength λ₁And wavelength λ₂2 is incident, as shown in FIG. 2A, the refractive index n of the isotropic filler 31C, which is a uniform refractive index material, is obtained._sAnd ordinary refractive index n of the birefringent diffraction grating 31B which is a birefringent material_oAre approximately equal, so the wavelength λ₁And wavelength λ₂No diffraction grating is generated for any light, and almost no diffracted light is generated.
[0036]
In addition, a wavelength λ between the optical disk 5 and the polarization hologram 31 is used.₁= Wavelength λ by placing a quarter wave plate 7 for 650 nm₁Is reciprocated through the quarter-wave plate 7, the reflected light reflected by the information recording surface of the optical disk 5 becomes P-polarized light whose polarization plane is rotated by 90 °, and is incident on the polarizing hologram 31 again.
[0037]
When P-polarized light is incident on the polarization hologram 31, as shown in FIG. 2B, the refractive index n of the isotropic filler 31C that is a uniform refractive index material._sAnd the extraordinary refractive index n of the birefringent diffraction grating 31B which is a birefringent material_eIs different, a diffraction grating is formed. The grating depth of the birefringent diffraction grating 31B processed into the diffraction grating shape is d₁Then, the phase difference φ generated with respect to the incident light having the wavelength λ is 2π (n_e-N_s) Xd₁/ Λ.
[0038]
Here, the grating depth d of the birefringent diffraction grating 31B₁Is the wavelength λ with the plane of polarization rotated by 90 °₂= (N so that no diffraction occurs for P-polarized light of 790 nm_e-N_s) Xd₁Is the wavelength λ₂To be an integer multiple of.
[0039]
Thus the lattice depth d₁To set the wavelength λ₂Of the optical disk reflected light, the S-polarized component whose polarization plane does not change is transmitted in a straight line as in the forward path, and the orthogonal P-polarized component whose polarization plane is rotated by 90 ° is also an integer multiple of 2π in phase change by the polarization hologram 31. Therefore, it passes straight without being diffracted.
[0040]
Therefore, the wavelength λ₂Phase difference with respect to φ₂Becomes an integral multiple of 2π and the wavelength λ₁Phase difference with respect to φ₁Lattice depth d such that is a non-integer multiple of 2π.₁By setting the wavelength λ₁Becomes a diffraction grating for P-polarized light of the wavelength λ₂A wavelength-selective polarization hologram that does not act as a diffraction grating for S-polarized light and P-polarized light is obtained.
[0041]
Specifically, (n_e-N_s) Xd₁Is λ₂That is, for example, the lattice depth d₁Is set to 10.53 μm, the wavelength λ₁= Polarizing hologram 31 having wavelength selectivity for diffracting light of only 650 nm with high ± first-order diffraction efficiency of 35% or more is realized. Since the diffracted light loss in the forward path is small, a high light utilization efficiency is achieved as the round-trip efficiency.
[0042]
In this way, a two-wavelength polarization hologram used in the optical head device of the present embodiment having the configuration described with reference to FIG. 1 is obtained. In addition, about the birefringent material and a processing process, since the well-known technique reported variously may be used, description is abbreviate | omitted here.
[0043]
Next, an example of a non-polarizing hologram used in the optical head device of the present invention will be described with reference to the cross-sectional configuration diagram shown in FIG.
[0044]
The non-polarizing hologram 32 in this example has a configuration in which a uniform refractive material 32B having a refractive index n is processed into a concavo-convex diffraction grating shape and formed on a translucent substrate 32A, and forms an interface with air.
[0045]
The step of the diffraction grating shape is d₂Then, the phase difference φ generated with respect to the incident light having the wavelength λ is 2π (n−1) × d.₂/ Λ, so the wavelength λ₁Phase difference with respect to φ₁Becomes an integral multiple of 2π and the wavelength λ₂Phase difference with respect to φ₂Step d so that is a non-integer multiple of 2π₂Set. From this, the wavelength λ₂Becomes a diffraction grating and wavelength λ₁In contrast, a wavelength-selective non-polarizing hologram that does not act as a diffraction grating can be obtained.
[0046]
Specifically, (n−1) × d₂Is λ₁That is, for example, the lattice depth d₂To 1.3 μm, the wavelength λ₂= For the 790 nm only, the 0th-order transmittance of the forward path is approximately 70%, the ± 1st-order diffraction efficiency of the return path is approximately 11%, and the signal light of approximately 8% is guided to the photodetector as the total efficiency of the forward path and the return path. Thus, a non-polarizing hologram 32 having a wavelength selectivity that can be realized is realized.
[0047]
In this manner, a two-wavelength non-polarizing hologram used in the optical head device of the present embodiment having the configuration described with reference to FIG. 1 is obtained. In addition, about the processing process of a diffraction grating, since the well-known technique reported variously may be used, description is abbreviate | omitted here.
[0048]
Further, when a collimating lens having a numerical aperture NA = 0.07 and a focal length f = 22 mm is used as the collimating lens 4, θ is half of the incident light angle incident on the collimating lens._c= 4.0 °. At this time, the wavelength λ other than the signal light in the forward and return paths₂In order to prevent the multiple diffracted light beams from entering the photodetector, a grating is formed only in the light passing region of the non-polarizing hologram 32 corresponding to NA = 0.07, and ± 1st order diffracted light beams in the forward path The diffraction angle of θ_cIt is sufficient to make it twice or more. Specifically, the grating pitch P of the non-polarizing hologram 32₂0.5 × λ₂/ NA or less, that is, 5.6 μm or less.
[0049]
Lattice pitch P₂A non-polarizing hologram 32 having a wavelength of about 4.5 μm₂+ 1st order diffracted light from the non-polarizing hologram 32 is collected on a photodetector about 0.71 mm away from the light emitting point. At this time, since the incidence of diffracted light, which is stray light other than signal light, is slight, the noise component is reduced, and recording / reproduction with a high S / N becomes possible.
[0050]
Here, the wavelength λ by the polarization hologram 31₁= Platform pitch P of the polarizing hologram 31 so that + 1st order diffracted light of 650 nm is condensed on the same photodetector.₁Is set. Specifically, f × λ₁/ P₁Is about 0.71 mm. That is, P₁= 20 μm is sufficient.
[0051]
The optical head device is configured using the hologram beam splitter 2 which is the diffraction grating for two wavelengths obtained in this way. In this optical head device, a stable recording and reproduction of a DVD-type optical disk having a high light utilization efficiency with a small configuration with a small number of parts using a two-wavelength semiconductor laser 1 emitting two wavelengths and a single photodetector 6. In addition, it is possible to perform stable recording and reproduction on an optical disc in which birefringence remains in a CD system.
[0052]
In FIGS. 2 and 3, a simple two-step stepped diffraction grating-shaped hologram is illustrated, but instead, a multi-step grating shape having a stepped section or a sawtooth wave-shaped blazed grating shape is used. Thus, since the diffraction efficiency of a specific diffraction order can be improved, various designs and manufactures are possible depending on the application.
[0053]
[Second Embodiment]
FIG. 4 is a side sectional view showing a configuration of a polarization hologram according to the second embodiment of the present invention. The second embodiment shows a modification of the polarization hologram in the first embodiment.
[0054]
The polarizing hologram 41 of the second embodiment is a birefringent diffraction comprising a four-step step grating shown in FIG. 4 instead of the birefringent diffraction grating 31B having a rectangular wave cross section shown in the first embodiment. This is a configuration using a lattice 41B. The birefringent diffraction grating 41B formed on the translucent substrate 41A is filled with an isotropic filler 41C, and the translucent substrate 41D is disposed thereon. Each step width in the birefringent diffraction grating 41B is equally distributed in pitch.
[0055]
Here, the grating depth d of each step of the birefringent diffraction grating 41B₁(N_e-N_s) Xd₁Is the wavelength λ₂As in the case of the first embodiment, the wavelength λ₂It is assumed that no diffraction occurs for P-polarized light.
[0056]
Specifically, (n_e-N_s) Xd₁Is λ₂That is, for example, by setting the grating depth of each step to 0, 5.27 μm, 10.53 μm, and 15.80 μm in a four-stage structure, the wavelength λ₁= Polarizing hologram 41 having wavelength selectivity for diffracting only light of 650 nm with high + first order diffraction efficiency of 70% or more is realized.
[0057]
This second embodiment has a wavelength λ as compared with the first embodiment.₁Since the light utilization efficiency at = 650 nm is improved approximately twice, the signal intensity in the light detection system is increased, and the S / N can be further improved.
[0058]
[Third Embodiment]
FIG. 5 is a side view showing a schematic configuration of an optical head device according to the third embodiment of the present invention.
[0059]
In the optical head device, the third embodiment is a DVD-system wavelength λ.₁This is different from the first embodiment in that an integrated hologram beam splitter 12 is configured by arranging a first diffraction grating 21 that diffracts the light in the vicinity of the second diffraction grating 22. The hologram beam splitter 12 has a configuration in which a first diffraction grating 21 and a second diffraction grating 22 are formed on the front and back surfaces of a single light-transmitting substrate. The two-wavelength semiconductor laser 1 and the photodetector 6 It is fixedly placed on the integrated unit side. In FIG. 5, the same components as those of the first embodiment shown in FIG.
[0060]
In the example of FIG. 5, since the case where the first diffraction grating 21 is a polarization hologram as in the first embodiment is shown, the quarter wavelength plate 7 is provided, but the first diffraction grating 21 is formed of a non-polarization hologram. In this case, a quarter wave plate may not be used.
[0061]
In the third embodiment, the grating pitch of the first diffraction grating 21 is set to the wavelength λ.₁The reflected light from the optical disk 5 has a wavelength λ₂The pitch is narrower than the grating pitch of the first embodiment so that the light is condensed on the same light receiving element.
[0062]
Wavelength λ₂The second diffraction grating 22, which is a non-polarization hologram that diffracts the light beam, is replaced with a non-polarization hologram comprising a three-step step grating instead of the diffraction grating having a rectangular cross section in the first embodiment shown in FIG. It forms using. Each step width in the diffraction grating is equally distributed in pitch.
[0063]
Here, the grating depth d of each staircase having a refractive index n = 1.5₂(N−1) × d₂Is the wavelength λ₁By making it an integral multiple of₁Diffracted light does not occur in the forward and return paths and passes straight through.
[0064]
Specifically, (n-1) d₂Is λ₁That is, for example, by forming a three-stage structure with a grating depth of 0, 1.3 μm, 2.6 μm, the wavelength λ₂= For the 790 nm only, the 0th-order transmittance of the forward path is about 39%, the ± 1st-order diffraction efficiency of the return path is about 34%, and the signal light of about 13% is guided to the photodetector as the total efficiency of the forward path and the return path A possible wavelength-selective non-polarizing hologram 22 is realized. In this case, the wavelength λ is compared with the configuration of the first embodiment.₂Since the light utilization efficiency at the time is improved by about 1.6 times, the signal intensity in the light detection system is increased, and the S / N can be further improved.
[0065]
If the first diffraction grating 21 is also a non-polarizing hologram, the diffracted light in the forward path is prevented from becoming stray light in the same way as in the first embodiment so that the multiple diffracted light in the forward path and the return path does not become stray light. The configuration specification is such that it does not enter the aperture of the collimating lens 4. At this time, although the light utilization efficiency is lowered, it is possible to perform more stable recording / reproduction with respect to the residual birefringence of the DVD type optical disk.
[0066]
As described above, according to each of the embodiments described above, in the optical head device corresponding to two wavelengths, the light source and the photodetector can be configured by a single two-wavelength semiconductor laser and a photodetector, and the number of parts can be reduced. Stable recording and reproduction with high light utilization efficiency can be realized with a small and lightweight configuration. In this case, stable recording and reproduction can be performed on both a DVD optical disk having a short operating wavelength and a CD optical disk having a thick disk thickness and residual birefringence.
[0067]
Although not described in the above embodiment, in order to stably record and reproduce a plurality of types of information recording media having different specifications such as DVD-type and CD-type optical discs, any optical system has optical aberrations. Various conventional techniques can be applied to use of reduced compatible objective lenses and aperture control elements.
[0068]
Further, when a tracking method such as a three-beam method or a differential push-pull method is employed in the optical head device, a diffraction grating for forming a sub beam is separately used, but the description and illustration are omitted.
[0069]
The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the present invention.
[0070]
【The invention's effect】
As described above, according to the present invention, the two wavelength lights are used as light sources, and the two wavelength lights are received by the same photodetector on the information recording surface of the optical recording medium having different use wavelengths. The information can be recorded or reproduced efficiently and stably, and a small and lightweight optical head device with a small number of parts can be realized.
[Brief description of the drawings]
FIG. 1 is a side view showing a schematic configuration of an optical head device according to a first embodiment of the present invention.
FIG. 2 is a side sectional view of a wavelength-selective polarization hologram that is a component of the optical head device according to the first embodiment of the present invention.
FIG. 3 is a side cross-sectional view of a wavelength-selective non-polarizing hologram that is a component of the optical head device according to the first embodiment of the present invention.
FIG. 4 is a side sectional view of a wavelength-selective polarization hologram that is a component of an optical head device according to a second embodiment of the present invention.
FIG. 5 is a side view showing a schematic configuration of an optical head device according to a third embodiment of the present invention.
FIG. 6 is a side view showing a schematic configuration example of a conventional optical head device.
[Explanation of symbols]
1: Two-wavelength semiconductor laser
2, 12: Hologram beam splitter
3: Objective lens
4: Parallelizing lens
5: Optical disc
6: Photodetector
7: 1/4 wave plate
21: First diffraction grating
22: Second diffraction grating
31, 41: Polarizing hologram
31A, 41A: Translucent substrate
31B, 41B: birefringent diffraction grating
31C, 41C: Isotropic filler
31D, 41D: Translucent substrate
32: Non-polarizing hologram
32A: Translucent substrate
32B: Uniform refractive material

Claims (3)

A semiconductor laser that emits light of two wavelengths of wavelength λ ₁ and wavelength λ ₂ (λ ₁ <λ ₂ );
An objective lens that focuses the emitted light of the semiconductor laser onto an optical recording medium;
A photodetector for detecting reflected light reflected by the optical recording medium;
A collimating lens for collimating the reflected light from the optical recording medium while making the emitted light from the semiconductor laser into parallel light;
A hologram beam splitter for guiding the reflected light to the photodetector ;
An optical head device comprising at least a quarter-wave plate for the wavelength λ _{1 in} an optical path between the objective lens and the hologram beam splitter ,
The hologram beam splitter comprises a first diffraction grating and second diffraction grating cross-sectional shape are each periodic uneven,
The first diffraction grating is a polarization hologram whose diffraction efficiency changes depending on the polarization direction of light, and the phase difference of transmitted light between the concave portion and the convex portion is an integral multiple of 2π with respect to the light of the wavelength λ ₂ and a non-integral multiple of 2π to the wavelength lambda ₁ of the light transmitted through the wavelength lambda ₁ of the light from the wavelength lambda ₂ of the light and the semiconductor laser toward the optical recording medium, and, in the optical recording medium Diffracts the reflected light of wavelength λ ₁ ,
In the second diffraction grating, the phase difference of the transmitted light between the concave portion and the convex portion is an integer multiple of 2π with respect to the light with the wavelength λ ₁ and a non-integer multiple with 2π with respect to the light with the wavelength λ ₂ .
When the focal length and numerical aperture of the collimating lens are f and NA, respectively, and the grating pitch of the second diffraction grating is P, the distance between the collimating lens and the second diffraction grating is 2 × f ×. Set to be longer than P × NA / λ ₂ ,
Further, the hologram beam light of said wavelength lambda ₁ and the wavelength lambda ₂ that has been diffracted by the splitter optical head device, characterized in that guided in the same said photodetector. The wavelength λ ₁ Is a wavelength band for DVD of 650 nm, and the wavelength λ ₂ 2. The optical head device according to claim 1, wherein is a wavelength band of 790 nm for CD. The first diffraction grating and the second diffraction grating are installed in an optical path between the semiconductor laser and the objective lens, and in an optical path between the photodetector and the objective lens,
The first diffraction grating child optical head device according to 請 Motomeko 1 or 2, wherein the quarter-wave plate and the that is integrated with the objective lens.

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