Patent Application
20090135701
The present invention relates to an optical pickup device by which information can be recording onto and reproducing from plural kinds of optical recording media, and to an optical disc device provided with the optical pickup device, especially relates to an optical pickup device that are configured in such a way that three kinds of light beams whose wavelengths are different from each other are received by a common photo-detector, and to an optical disc device provided with the optical pickup device.
In the conventional technology, in order to perform recording information onto, reproducing it from, or both recording onto and reproducing from DVD (digital versatile disc) and CD (compact disc), a two-light-source-type optical pickup device has been used that has a DVD laser output unit whose output wavelength is approximately 650 nm, and a CD laser output unit whose output wavelength is approximately 780 nm. Moreover, for the purpose of downsizing each of these light sources, a two-wavelength integrated laser output unit by which beams of two-kind wavelengths can be outputted using a single package has also been practically used. There are known as the two-wavelength integrated laser output unit, for example, a monolithic-type laser output unit that is obtained by forming two laser diodes on a monolithic-type semiconductor substrate, and a hybrid-type laser output unit that is obtained by bonding closely together two semiconductor substrates on each of which a laser diode is formed.
In the two-wavelength integrated laser output unit, beam-emitting portions of the two laser diodes (for DVD and CD) are slightly apart from each other, and the distance therebetween is generally approximately 110 μm. Therefore, when the optical axis of one of the laser diodes coincides with the system optical axis passing through the centers of the objective lens and the collimating lens of the optical pickup device, the optical axis of the laser beam emitted from the other laser diode is displaced from the system optical axis. In the as-is state, each of the return light beams having been outputted from the DVD laser diode and the CD laser diode and then reflected by the optical recording medium cannot be received by a common photo-detector. Accordingly, a method is proposed, in which both of the return light beams are guided to the common photo-detector by diffracting one of the return light beams or both of them, using a diffraction grating, etc., either or both of the return light beams (for example, refer to Patent Documents 1 and 2).
Moreover, in recent years, mass storage of optical recording media has been required, and an optical recording medium such as an optical recording disc, compatible with the blue-violet laser system, whose capacity is several times larger than that of DVD and CD has been practically used. Resultantly, from a viewpoint of device downsizing and cost reduction, a method has been required that information can be recorded onto and reproduced from, for example, DVD, CD, and an optical recording medium compatible with the blue-violet laser system by a single optical pickup device. Consequently, a three-light-source type pickup device having a blue-violet laser diode, in addition to a DVD laser diode and a CD laser diode, has been developed.
As an example of the three-light-source type optical pickup device, the following configuration has been proposed. That is, each of the optical axes of three-kind-wavelength optical beams whose output wavelengths are different from each other, using prisms corresponding to respective wavelengths, made coincident with the system optical axis of the optical pickup device; thereby, light beams of each wavelength are guided to an optical recording medium. Three-kind-wavelength return light beams of the light beams of each wavelength that are reflected by the optical recording medium pass through the respective prisms to be guided to a common photo-detector, and then detected by the photo-detector (for example, refer to Non-Patent Document 1).
[Patent Document 1] Japanese Laid-Open Publication No. 2001-143312
[Patent Document 2] Japanese Laid-Open Publication No. 2001-256670
[Non-Patent Document 1] “Philips, Holland, has developed optical head that enables recording onto and reproducing from CD, DVD, and Blu-ray Disc”, [ONLINE], Jul. 16, 2004, Nikkei Business Publications, Inc., [accessed on Feb. 20, 2005], Internet <http://TECHON.nikkeibp.co.jp/members/NEWS/20040716/104521/>
However, according to the first configuration (Non-Patent Document 1) described above, although the return light beams reflected by the optical recording medium can be received by the common photo-detector, many prisms etc. are needed for aligning with the system optical axis of the optical pickup device the optical axis of each laser output unit. Moreover, various parts are also needed for mounting onto the optical pickup device each laser output unit. As a result, the number of the constituent parts of the optical pickup device increases; therefore, a problem has occurred that it is difficult to downsize the device and reduce the cost.
Here, in the two-light-source-type optical pickup device, device downsizing and cost reduction have been realized by using the two-wavelength integrated laser output unit and a phase-differential-type diffraction grating (Patent Documents 1 and 2); similarly, in the three-light source type pickup device, device downsizing and cost reduction are required to be realized by using the two-wavelength integrated laser output unit and the phase-differential-type diffraction grating. In this case, a method can be considered that, two light beams, whose wavelengths are different from each other, outputted from a two-wavelength integrated laser output unit and single-wavelength light beam emitted from a single-wavelength type laser output unit are guided to the same optical path by using a prism, etc., and received by a common photo-detector. However, in the phase-differential-type diffraction grating, because a relationship of
n sin θ−nb sin θb=mλ/p
is established among the wavelength λ of the incident light beam, the incident angle θ of the incident light beam, the refractive index n of the light-incident-side medium, the exiting angle θb of the exiting light beam, the refractive index nb of the light-exiting-side medium, the order number m of the diffracted light beam, and the pitch p of the diffraction grating, diffraction angles of the first- or higher-order diffracted light beams of the three light beams, whose wavelengths are different from each other, incident in parallel on the diffraction grating, are different from each other; therefore, a problem occurs that it is difficult to guide to the common photo-detector the return light beams having the three wavelength components.
An objective of the present invention, which is made to solve the above problems, is to provide an optical pickup device, for recording information onto and reproducing it from plural kinds of optical recording media (for example, DVD, CD, and the optical disc compatible with the blue-violet laser system) in which the wavelengths of the light beams used are different from each other, by which three kinds of return light beams reflected by an optical recording medium can be detected by a common photo-detector.
Moreover, another objective of the present invention is to provide an optical disc device that is configured to include the optical pickup device as described above.
An optical pickup device according to the invention, is enable to record/reproduce information to/from an optical recording media, the optical pickup device comprising:
a first light emitting portion that emits a first light beam with a first wavelength;
a second light emitting portion that emits a second light beam with a second wavelength;
a third light emitting portion that emits a third light beam with a third wavelength;
an adjusting element for optical axes enable to control the optical axis of the return light beam that is output from said light emitting portion and is reflected by said optical recording media; and
a single photo detector that receives said return light beam passing through said adjusting element for optical axes; and
wherein said first light emitting portion and said second light emitting portion are arranged in such a way that the optical axis of said first light beam and the optical axis of said third light beam approximately coincide with each other;
said adjusting element for optical axes controls the axis of the return light beam of said second light beam; and
said single photo detector receives the return light beams of said first light beam, said second light beam and said third light beam.
In the optical pickup device according to the invention, by a simple configuration and an easy control method, three kinds of return light beams reflected by the optical recording medium can be detected by the common photo-detector; therefore, downsizing of the optical pickup device and of the optical disc device using the optical pickup device, and their cost reduction can be realized.
In the optical pickup device, a laser output unit 1 and a laser output unit 2 are provided as light sources, in which the laser output unit 1 includes a semiconductor substrate 18 on which a light emitting portion 15 that emits light beam whose wavelength is λ1 (approximately 405 nm) is formed, while the laser output unit 2 includes a semiconductor substrate 19 on which a light emitting portion 16 that emits light beam whose wavelength is λ2 (approximately 650 nm), and a semiconductor substrate 20 on which a light emitting portion 17 that emits light beam whose wavelength is λ3 (approximately 780 nm) is formed; thus, the emitting portions 15, 16, and 17, formed on the semiconductor, each are configured to emit the light beams with the wavelengths of λ1, λ2, and λ3, when a voltage is applied to each of the emitting portions. In
The laser output unit 1 is arranged in the optical pickup device in such a way that the optical axis of the light emitting portion 15 emitting the light beam with the wavelength of λ1 is made coincident with the optical axis passing through the center of a collimating lens 8 and an objective lens 10 (the system optical axis A of the optical pickup device).
While, in the laser output unit 2, the semiconductor substrate 19 that emits the wavelength of λ2, and the semiconductor substrate 20 that emits the wavelength of λ3, are arranged in parallel to each other in the interior thereof, here, due to space limitation, the emitting portion 16 that emits light beam with the wavelength of λ2, and the emitting portion 17 that emits light beam with the wavelength of λ3 are arranged slightly apart from each other. The distance between the emitting portion 16 and the emitting portion 17 is, for example, 110 μm. Here, in the optical pickup device, the laser output unit 2 is arranged in such a way that the emitting portion 17 emitting the light beam with the wavelength λ3 is made coincident with the system optical axis A of the optical pickup device, while the light beam of λ2 is emitted from the emitting portion 16 emitting light beam with the wavelength λ2 so as to be slightly apart from the system optical axis A, and to be in parallel to the system optical axis A.
The λ1 light beam emitted from the laser output unit 1 passes through the system optical axis A, and then through a grating 3. The grating 3 is used for forming a sub beam, generally performed in optical pickup devices, that is needed for detecting a tracking error signal (a three-beam method, a differential push-pull method, etc.). The λ3 light beam emitted from the laser output unit 2 passes through the system optical axis A, and then through a grating 4. The λ2 light beam travels in parallel along the system optical axis A at a position slightly apart from the system optical axis A of the optical pickup device, and then passes through the grating 4.
The λ1 light beam having passed through the grating 3 is incident onto a dichroic mirror 5. The dichroic mirror 5 switches between the reflection operation and the transmission operation according to the wavelength of the incident light. In Embodiment 1, the mirror face of the dichroic mirror 5 is set in such a manner that the light beam with the wavelength of λ1 is almost transmitted through the mirror face, while the light beams with the wavelengths of λ2 and λ3 are almost reflected there; here, the light beam with the wavelength of λ1 almost passes through the dichroic mirror 5. The λ3 light beam passing through the grating 4 is reflected by the mirror face of the dichroic mirror 5. The light beam with the wavelength of λ3 reflected by the dichroic mirror 5 passes through the system optical axis A of the optical pickup device, similarly to the case of the light beam with wavelength λ1. The λ2 light beam passing through the grating 4 is reflected by the mirror face of the dichroic mirror 5. The light beam with the wavelength of λ2 reflected by the dichroic mirror 5 travels in parallel along the system optical axis A at the position slightly apart from the system optical axis A of the optical pickup device.
The light beam with the wavelength of λ1 having passed through the dichroic mirror 5, or the light beam with the wavelength of λ2 or λ3 reflected by the dichroic mirror 5 is incident onto a polarization prism 6. The polarization prism 6 acts as a polarization beam splitter that switches between the reflection operation and the transmission operation corresponding to the polarization direction of the incident light. The crystal-axis direction (polarization direction) of the polarization prism 6 is set so as to pass linearly-polarized light beams with the wavelengths of λ1, λ2, and λ3 that have passed through the dichroic mirror 5.
The optical pickup device further includes a mirror 7 that reflects light beam having passed through the prism 6, the collimating lens 8 onto which the light beam reflected by the mirror 7 is incident, and a wavelength plate 9 onto which the light beam having passed through the collimating lens 8 is incident. The collimating lens 8 is used for changing the incident light into parallel light. The wavelength plate 9 is a so-called quarter-wavelength plate having a function for changing the linear polarization into circular polarization. The light beam having passed through the wavelength plate 9, which is changed to have the circular polarization, is incident onto the objective lens 10, and then, is focused onto a signal recording face of an optical disc 11 (DVD, CD, or an optical disc for a blue-violet laser).
The light beam focused on the signal recording face of each optical disc 11 is modulated corresponding to an information signal recorded on the signal recording face, and is reflected to be return light beam; then, the light beam becomes parallel light again after passing through the objective lens 10, and is incident on to the wavelength plate 9. The polarization of the light beam after passing through the wavelength plate 9 changes from circular to linear; however, the linear polarization direction in this state is approximately 90 degrees different from that when the light beam has traveled forward. The return light beam passing through the wavelength plate 9 passes through the collimating lens 8 to be focusing light flux; then, the light flux is incident onto the polarization prism 6 after being reflected by the mirror 7.
As illustrated in
The adjusting element 13 for optical axes has a function for varying the optical-axis direction of at least one of three return light beams having the respective different wavelengths of λ1, λ2, and λ3. Specifically, due to the diffraction function of a diffraction element provided in the adjusting element 13 for optical axes, the optical-axis direction of the return light beam with the wavelength λ2 is varied; thereby, the return light beams having the respective wavelengths of λ1, λ2, and λ3 are configured to be received by the common photo-detector.
The return light beams having the respective wavelengths of λ1 and λ3 travel so that each of the optical axes approximately coincides with the system optical axis A, of the optical pickup device, passing through the centers of the collimating lens 8 and the objective lens 10, and then are incident onto a photo-detector 14 after having passed through the adjusting element 13 for optical axes. While, because the light emitting portion 16 of the semiconductor substrate 19 that emits the light beam with the wavelength λ2 is arranged at the position slightly apart from the light emitting portion 17 that emits the light beam with the wavelength λ3, the return light beam with the wavelength λ2 is incident, in a state of the optical axis displaced from the system optical axis A, onto the adjusting element 13 for optical axes, and then incident onto the photo-detector 14 after having been diffracted by a binary-blazed diffraction grating provided in the adjusting element 13 for optical axes. That is, also regarding every return-light beam having each wavelength of λ1, λ2, and λ3, the photo-detector 14 can receive them, and each light signal can be detected.
Next, operations and configurations of the binary-blazed diffraction grating of the adjusting element for optical axes are explained.
As represented in
As represented in
On the other hand, the return light beam with the wavelength λ2 passes through an optical path displaced from the optical axis of the return light beams having the respective wavelengths of λ1 and λ3, is incident, at a constant incident angle, onto an incident face 21b of the diffraction grating 21, and then its first-order diffracted light beam exits from the grating face 21a of the diffraction grating 21. The first-order diffracted light beam of the return light beam with the wavelength λ2 is incident, at a constant incident angle (different from the incident angle onto the diffraction grating 21), onto the photo-detector 14.
Because the adjusting element 13 for optical axes, etc. are configured as the above, by moving the adjusting element 13 for optical axes or the photo-detector 14 along the optical-axis direction (optical-axis direction of the return light beams with the wavelengths of λ1 and λ3) of the incident light, the position at which the return light beam with the wavelength λ2 is received can be aligned in the detection face (in the face to which the optical axis of the incident light is perpendicular) of the photo-detector 14. Regarding the return light beams having the respective wavelengths of λ1 and λ3, because the zero-order diffraction ones are utilized, even if the adjusting element 13 for optical axes and the photo-detector 14 are moved along the optical axis, the light receiving position on the photo-detector 14 does not vary. As a result, the light receiving position of the return light beam with the wavelength λ2 can coincide therewith, on the photo-detector 14, of the return light beams having the respective wavelengths of λ1 and λ3.
Here, assuming that the refractive index, for the wavelength λ3, of the material forming the diffraction grating 21 is n3, and m is an integer number not smaller than 1, the step height d of the diffraction grating 21 represented in
d≈mλ
3/(n3−1) Eq. 1
Assuming that the wavelength λ3 is 780 nm, and the order m is 1; then, determining the refractive index of the diffraction grating 21 based on the refractive-index data of BK7 grade glass as a general glass material, the step height d can be obtained to be approximately 1.53 μm by Equation 1. Accordingly, the step height d of the diffraction grating 21 is set at 1.53 μm in this embodiment. In the condition given by Equation 1, the intensity of the zero-order diffracted light thereof with the wavelength λ3 is most strengthened.
In the binary-blazed diffraction grating 21, if the step height d is an integral multiple of λ/(n−1), because the optical-path-length differences, due to the step height d, each become an integral multiple of the wavelength λ, the maximum value of the zero-order diffraction efficiency can be obtained. Assuming that the wavelength λ1 is 405 nm, while λ3 is 780 nm, the ratio between the wavelengths is approximately 1.92; thus, the ratio is close to 2. Therefore, if the step height d is set in such a way that the optical-path-length differences each become an integral multiple of the wavelength λ3, when assuming that n1=n3, a value that is also approximately an integral multiple of the wavelength λ1 is obtained; consequently, a high zero-order-diffraction efficiency can be obtained for both of the wavelength λ1 and the wavelength λ3.
Generally, the refractive index of material such as glass and plastic slightly increases with shortening the wavelength. For example, in a case of BK7 grade glass being a general glass material, n=1.53 for the wavelength of 405 nm, while n=1.51 for the wavelength of 780 nm. In a case in which calculation is performed using, as material for the diffraction grating 21, the refractive-index data of the BK7 grade glass being a general glass material, the ratio between λ3/(n3−1) and λ1/(n1−1) becomes 1.99; thus, the ratio appears to be closer to an integral multiple than when assuming that n1=n3. Accordingly, if the step height d of the diffraction grating 21 is set to an integral multiple of λ3/(n3−1) so that the maximum value of the zero-order diffraction efficiency of the wavelength λ3 is obtained, the value of the step height where the maximum value of the zero-order diffraction efficiency of the wavelength λ1 is obtained, that is, λ1/(n1−1) also approaches an integral multiple. As a result, high zero-order-diffraction efficiencies can be obtained for both of the wavelength λ1 and the wavelength λ3.
Using the refractive-index data of the BK7 grade glass being a general glass material, in a case of the level number P=2, where the grating structure is simplest, assuming that the step height d of the diffraction grating 21 is a variable parameter (d=h, when the level number is 2), the diffraction efficiency of each return light beam is calculated; thereby, regarding the diffraction efficiency of each return light beam, almost maximum values of the zero-order diffraction efficiency at d=1.53 μm can be obtained for both of the wavelengths of λ1 and λ3.
As described above, the level number P of the diffraction grating 21 is the step number (including the grating bottom) of the diffraction grating 21; thus, P=5 in the example represented in
Moreover, as illustrated in
In
As represented in
As represented in
As represented in
As represented in
As represented in
As represented in
As represented in
Generally, as the light amount received by the photo-detector 14 increases, the signal detection becomes easier. In this embodiment, by setting the level number P to 5, a large value of the first-order diffraction light efficiency for the wavelength λ2, has been obtained at a value of the groove depth h, when the efficiencies of the zero-order diffraction light beams with the wavelengths of λ1 and λ3 each reach approximately the maximum value. Accordingly, intensity of not only the return light beam whose wavelengths are λ1 and λ3 but also the return light beam whose wavelength is λ2 increases; consequently, the signal detection can be favorably performed.
As explained above, in this embodiment, by controlling, using the adjusting element 13 for optical axes, the optical axis of at least one of the return light beams with the wavelengths of λ1, λ2, and λ3 having been reflected on the optical recording medium, the return light beams with the respective wavelengths can be detected by the common photo-detector 14. Accordingly, downsizing and low-cost manufacturing of an optical pickup device and an optical disc device using it can be realized.
In this embodiment, because the zero-order diffracted light of the return light beams with the wavelengths of λ1, and λ3 is configured to be guided to the photo-detector 14, without varying the light receiving position, on the photo-detector 14, of the return light beams with the wavelengths of λ1, and λ3, by controlling the position of the diffraction grating 13 or the photo-detector 14 along the optical axis, the light receiving position of the return light beam with the wavelength λ2 can be made coincident with the light receiving position, on the photo-detector 14, of the return light beams with the wavelengths of λ1 and λ3. Accordingly, the optical axis can be controlled by a simple control method to guide the return light beams with the wavelengths of λ1, λ2, and λ3 into the common photo-detector 14.
In this embodiment, the zero-order diffracted light beams with the wavelengths of λ1 and λ3 are used; however, as represented in
Moreover, in the binary-blazed diffraction grating, when the step height d is an integral multiple of λ/(n−1), the difference of the optical path lengths depending on the step height d becomes an integral multiple of the wavelength λ; consequently, the maximum value of the zero-order diffraction efficiency can be obtained. In this embodiment, the step height d of the diffraction grating is set to d≈mλ3/(n3−1), in which the maximum value of the zero-order diffraction efficiency can be obtained at the wavelength of λ3. Assuming that the wavelength λ1 is 405 nm, and the wavelength λ3 is 780 nm, the ratio of the wavelengths approximately equals to 1.92, which is close to 2. Therefore, if the step height d is set so that the difference of the optical path lengths becomes an integral multiple of the wavelength λ3, the difference also becomes almost an integral multiple of the wavelength λ1; consequently, high efficiencies of the zero-order diffraction can also be obtained for both the wavelengths of λ1 and λ3. As a result, signal detection can be favorably performed for the return light beams with the wavelengths of λ1 and λ3.
As the configurations of the laser output units according to this embodiment, the semiconductor substrate 20, on which the light emitting portion 17 that emits light beam whose wavelength is λ3 (approximately 780 nm) is formed, is included in a laser output unit 1a, and the semiconductor substrates 18 and 19, on which the light emitting portions 15 and 16 that emit light beams whose wavelengths are λ1 (approximately 405 nm) and λ2 (approximately 650 nm) are formed, respectively, are included in a laser output unit 2a; by applying voltage to each of the light emitting portions 15, 16, and 17 that are formed on the semiconductor substrates, the light beams having the respective wavelengths of λ1, λ2, and λ3 are configured to be emitted. In
According to this embodiment, the light emitting portion 15 for the wavelength λ1 is arranged in such a way that its optical axis coincides with the system optical axis A of the optical pickup device. The light emitting portion 16 for the wavelength λ2 is arranged, due to spatial limitation, at a position slightly apart from the light emitting portion 15 for the wavelength λ1. The light emitting portion 17 for the wavelength λ3 is arranged in such a way that its optical axis coincides with the system optical axis A of the optical pickup device. Here, the wavelength λ1 and the wavelength λ2 are used after having been reflected by the mirror surface of a dichroic mirror 5a, while the wavelength λ3 is used after having passed through it.
As the configurations of the laser output units according to this embodiment, the semiconductor substrates 18 and 19, on which the light emitting portions 15 and 16 that emit light beams whose wavelengths are λ1 (approximately 405 nm) and λ2 (approximately 650 nm) are formed, respectively, are included in a laser output unit 1b, and the semiconductor substrate 20, on which the light emitting portion 17 that emits light beam whose wavelength is λ3 (approximately 780 nm) is formed, is included in a laser output unit 2b; by applying a voltage to each of the light emitting portions 15, 16, and 17 that are formed on the semiconductor substrates, the light beams having the respective wavelengths of λ1, λ2, and λ3 are configured to be emitted. In
According to this embodiment, the light emitting portion 15 for the wavelength λ1 is arranged in such a way that its optical axis coincides with the system optical axis A of the optical pickup device. The light emitting portion 16 for the wavelength λ2 is arranged, due to spatial limitation, at a position slightly apart from the light emitting portion 15 for the wavelength λ3. The light emitting portion 17 for the wavelength λ3 is arranged in such a way that its optical axis coincides with the system optical axis A of the optical pickup device. Here, the wavelength λ1 and the wavelength λ2 are used after having passed through the mirror surface of a dichroic mirror 5b, while the wavelength λ3 is used after having been reflected thereby.
As the configurations of the laser output units according to this embodiment, the semiconductor substrates 19 and 20, on which the light emitting portions 16 and 17 that emit light beams whose wavelengths are λ2 (approximately 650 nm) and λ3 (approximately 780 nm) are formed, respectively, are included in a laser output unit 1c, and the semiconductor substrate 18, on which the light emitting portion 15 that emits light beam whose wavelength is λ1 (approximately 405 nm) is formed, is included in a laser output unit 2c; by applying a voltage to each of the light emitting portions 15, 16, and 17 that are formed on the semiconductor substrates, the light beams having the respective wavelengths of λ1, λ2, and λ3 are configured to be emitted. In
According to this embodiment, the light emitting portion 15 for the wavelength λ1 is arranged in such a way that its optical axis coincides with the system optical axis A of the optical pickup device. The light emitting portion 17 for the wavelength λ3 is arranged in such a way that its optical axis coincides with the system optical axis A of the optical pickup device, and the light emitting portion 16 for the wavelength λ2 is arranged, due to spatial limitation, at a position slightly apart from the light emitting portion 17 for the wavelength λ3. Here, the wavelength λ2 and the wavelength λ3 are used after having passed through the mirror surface of a dichroic mirror 5c, while the wavelength λ1 is used after having been reflected thereby.
According to this embodiment (as in
Although, in Embodiment 1 described above, the level number P of the diffraction grating 21 included in the adjusting element for optical axes has been set to 5, the level number P of the diffraction grating 21 in this embodiment is set within the range from 4 to 6. The other configurations of an optical pickup device according to this embodiment are similar to those in Embodiment 1 described above.
The configuration of the diffraction grating 21 represented in
As represented in
As explained above, in this embodiment, by using the binary-blazed diffraction grating whose level number P is from 4 to 6, the relatively high diffraction efficiency can also be obtained for the return light beam having the wavelength of λ2, in addition to that for the return light beams having the respective wavelengths of λ1 and λ3; thereby, the favorable signal detection can be performed by the photo-detector 14.
Especially, when the level number P is set to 4, the step number is smaller and the structure is simpler than those in a case of the level number P being 5 or 6; therefore, an advantage is also be obtained that the diffraction grating 21 can easily produced.
In Embodiment 1 described above, the refractive index of the diffraction grating 21 included in the adjusting element 13 for optical axes is assumed to be equivalent to that of BK7 as general glass material; however, in this embodiment, material having the refractive index that satisfies the following condition is selected as that of the diffraction grating 21. The other configurations of an optical pickup device according to this embodiment are similar to those in Embodiment 1 described above.
According to this embodiment, a material of the diffraction grating 21 included in the adjusting element 13 for optical axes is selected among materials in which a relationship
1.0≦(n1−1)/(n3−1)≦1.08 Eq.2
is satisfied, where the refractive indexes of the material for the wavelengths λ1 and λ3 are n1 and n3, respectively.
In Embodiment 1 described above, it has been described that the wavelengths of λ1 and λ3 are approximately 405 nm and 780 nm, respectively; however, generally the wavelengths outputted from the blue-violet semiconductor laser output unit and the CD laser output unit vary with certain margins, for example, λ1=405±8 nm, and λ3=780±15 nm; therefore, it is not necessary that λ1=405 nm, and λ3=780 nm.
As explained also in Embodiment 1, when the step height d of the diffraction grating 21 is an integral multiple of λ/(n−1), the maximum zero-order diffraction efficiency can be obtained; therefore, the most suitable step height d for the wavelength λ1 is an integral multiple of λ1/(n1−1), and the most suitable step height d for the wavelength λ3 is an integral multiple of λ3/(n3−1). The value of λ3/λ1 is approximately “2”; however, considering the difference between the refractive indexes n1 and n3, when a relationship
2λ1/(n1−1)=λ3/(n3−1) Eq. 3
is established, the maximum zero-order diffraction efficiencies for the wavelengths λ1 and λ3 can be simultaneously obtained. By modifying Equation 3, the following Equation 4
(n1−1)/(n3−1)=2λ1/λ3 Eq. 4
can be obtained.
When the outputted-wavelength ranges described above are applied to Equation 4, Equation 2 described above can be obtained. If the diffraction grating 21 is formed using material that satisfies Equation 2, the maximum zero-order diffraction efficiencies for both the wavelengths λ1 and λ3 can be obtained at the same step height d. Therefore, by selecting suitable material for the diffraction grating 21 corresponding to the wavelengths outputted from the laser output units used, relatively high zero-order diffraction efficiencies can be simultaneously obtained for the wavelengths λ1 and λ3.
As explained above, in this embodiment, even in a case where the laser output units are used by which the light beams having the wavelengths with certain margins are outputted, by selecting, from material by which the relationship of 1.0≦(n1−1)/(n3−1)≦1.08 is satisfied in the binary-blazed diffraction grating 21, specified material adapted to the wavelengths outputted from the laser output unit, relatively high zero-order diffraction efficiencies can be obtained for both of the wavelengths λ1 and λ3; therefore, signals can be favorably detected by the photo-detector 14.
The optical disc device according to this embodiment is provided with a rotationally drive mechanism 102 for supporting and rotationally driving a DVD, a CD, or a blue-violet laser optical disc having capacity several times larger than that of them. The rotationally drive mechanism 102 positions the optical disc 11 with a chucking hole 11a provided at the center thereof being used as a reference, and rotationally drives.
The optical pickup device 100 is arranged in a state in which an objective lens is faced to the signal recording face of the optical disc 11 rotationally driven by the rotationally drive mechanism 102, and moved in the radial direction of the optical disc by a carrying mechanism 103. The optical pickup device 100, the rotationally drive mechanism 102, and the carrying mechanism 103 are controlled by a control circuit 101. By the optical pickup device 100, information is at least either recorded onto or reproduced from the optical disc 11, using a light beam having a wavelength, selected corresponding to the kind of an optical disc, among three kinds of wavelengths λ1, λ2, and λ3 that the laser output unit can emit. Signals read out from the optical disc by the optical pickup device 100 are demodulated by a demodulation circuit 104.
According to this embodiment, by using the optical pickup devices explained in Embodiments 1 through 4, downsizing of optical disc devices and their cost reduction can be realized.
Here, in each embodiment described above, the wavelengths λ1, λ2, and λ3 have been set to approximately 405 nm, 650 nm, and 780 nm, respectively; however, a combination of other wavelengths may be used corresponding to the kind of the used optical recording medium. For example, if one of the wavelengths among three wavelengths λ1, λ2, and λ3 approximately equals to an integral multiple of one of the two other wavelengths, similar effect can be obtained by an optical-system configuration similar to those in Embodiments 1 through 4.
Moreover, in an optical pickup device by which four light beams having wavelengths λ1, λ2, λ3, and λ4, respectively, can be switched, for example, when both of (λ1/λ2) and (λ1/λ3) are approximately natural numbers, if all optical axes of the light beams of λ1, λ2, and λ3 can be made coincident with the system optical axes A, similar effect can be obtained by an optical-system configuration similar to that in each embodiment of the invention. All optical axes of the light beams of λ1, λ2, and λ3 can be made coincident with the system optical axes A, for example, by a method using two dichroic mirrors.
Moreover, in each embodiment described above, the binary-blazed diffraction grating 21 has been used; however, not limited to the binary-blazed diffraction grating, a adjusting element for optical axes may be used by which the optical axis of at least one of the return light beams can be controlled so that the return light beams having the wavelengths λ1, λ2, and λ3 can be received by the common photo-detector 14.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-318176 | Nov 2005 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2006/318511 | 9/19/2006 | WO | 00 | 8/29/2008 |
