JP3970254B2 - Optical pickup device - Google Patents

Description

  The present invention relates to an optical pickup device, and more particularly to an optical pickup device that can handle a plurality of types of optical information recording media.

  CDs (Compact Discs) and DVDs (Digital Versatile Discs) are widely used as optical information recording media on which information is optically read or written using a light beam such as a laser beam. In recent years, as represented by “HD DVD (trademark)” and “Blu-ray Disc (trademark)”, development of optical information recording media having a larger storage capacity than CD and DVD has been progressing. However, various types of optical information recording media are expected to be developed.

  When reading or writing recorded information on such an optical information recording medium, it is necessary to use a light beam having an optimum wavelength according to the type of the optical information recording medium. For this reason, in an optical pickup apparatus that can handle a plurality of types of optical information recording media, a plurality of light sources may be provided, and a plurality of light beams having different wavelengths may be emitted from each light source.

  When light beams are emitted from a plurality of light sources, each light beam is usually emitted from the light source in a state where the optical axes do not coincide. When reading information recorded on various optical information recording media using a plurality of light beams whose optical axes do not coincide with each other, an information reading error may occur.

For this reason, when light beams are emitted from a plurality of light sources, a device for improving the reading accuracy of information recorded on an optical information recording medium has been conventionally used. For example, in order to make the optical axes of light beams emitted from a plurality of light sources coincide with each other, two laser beams emitted as parallel light beams are placed on the same optical axis using a first diffraction grating and a second diffraction grating. A technique leading to the above is known (see Patent Document 1).
JP 2002-323677 A

  In a diffractive optical element such as a diffraction grating, a light beam is diffracted based on the diffraction efficiency according to the characteristics of the diffractive optical element itself and the characteristics of the incident light beam. It is difficult to maintain the diffraction efficiency at 100% with respect to all the incident light beams, and the light beam diffracted by the diffractive optical element causes energy loss. Therefore, the fact that the light beam emitted from the light source is diffracted by the diffractive optical element before being irradiated on the optical information recording medium is a viewpoint of irradiating the optical information recording medium with a small energy loss. Is not preferable. In particular, in an optical pickup device capable of writing new information as well as reading information recorded on an optical information recording medium, it is necessary to use a high intensity light beam when writing information. Therefore, it is necessary to emit a light beam having a very large intensity.

  Further, in order to improve the reading accuracy of the information recorded on the optical information recording medium, the light beam reflected on the information recording surface of the optical information recording medium is applied to the optical information detecting means such as a photodiode. It is important to irradiate properly. In particular, preventing the position of the light beam applied to the optical information detection means from deviating from the normal position is very important for improving the reading accuracy.

  In addition, as in the above-described prior art, an optical pickup device that can handle light beams emitted from two light sources can be realized with a relatively simple structure. However, the light pickup device can emit light beams emitted from three or more light sources. The structure of an optical pickup device that can be used is complicated. Therefore, it is desired to propose a method for realizing an optical pickup device that can appropriately cope with light beams emitted from three or more light sources with a simple structure. In particular, in addition to CDs and DVDs, there is a growing need for optical pickup devices that are compatible with optical information recording media that will be developed in the future, such as HD DVDs and Blu-ray Discs described above. It is expected to come.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to efficiently detect information recorded on an optical information recording medium by efficiently using light beams emitted from a plurality of light sources. An object of the present invention is to provide an optical pickup device that can be used.

  The present invention relates to an optical pickup device. The optical pickup device includes a light emitting unit including two or more light sources capable of emitting a light beam, a light beam irradiating unit that irradiates an optical information recording medium with the light beam emitted from the light emitting unit, and the optical information. Optical axis correction means for correcting the optical axis of the light beam reflected by the recording medium, and information recorded on the optical information recording medium irradiated with the light beam whose optical axis is corrected by the optical axis correction means Optical information detecting means for detecting light based on the irradiated light beam, and the optical axis correcting means is disposed at a position where the light beam reflected by the optical information recording medium is incident. A diffractive optical element that corrects the optical axis of the incident light beam and a diffractive optical element that is in close contact with the diffractive optical element so as to irradiate a predetermined position of the optical information detection means, and is adapted to the applied voltage A refractive index variable member that changes a refractive index and a refractive index that adjusts a refractive index of the refractive index variable member by changing a voltage application state to the refractive index variable member according to a light beam emitted from the light emitting means. Rate adjusting means.

  According to the optical pickup device, the refractive index of the refractive index variable member is adjusted according to the light beam of each light beam emitted from each light source of the light emitting means, and the diffraction efficiency of the light beam in the diffractive optical element is adjusted. In this state, the direction of the optical axis is corrected in the diffractive optical element. As a result, the light beam is appropriately applied to a predetermined position of the optical information detection unit, and the optical information detection unit can accurately read the recorded information based on the light beam reflected by the optical information recording medium. . Further, such an effect is achieved by a simple configuration mainly including a diffractive optical element, a refractive index variable member, and a refractive index adjusting unit that constitute the optical axis correcting unit.

  The refractive index adjusting means can adjust the refractive index of the refractive index variable member by changing the voltage application state to the refractive index variable member so as to increase the diffraction efficiency of the light beam in the diffractive optical element. . Thereby, the light beam emitted from each light source is efficiently utilized.

  The diffractive optical element may be a binary diffractive optical element. If a binary type diffractive optical element is used, the direction of the optical axis of the light beam emitted from each light source can be easily corrected.

  The diffractive optical element can correct the optical axis of light beams emitted from a plurality of light sources in an appropriate direction, and adjusts the diffraction efficiency according to the refractive index of the refractive index variable member. It can include everything that can be done.

  The light emitting means can emit three or more light beams having different wavelengths, and the refractive index adjusting means changes the voltage application state to the refractive index variable member in accordance with the light beam emitted from the light emitting means. The refractive index of the variable member can also be adjusted.

  When light beams are emitted from three or more light sources, it tends to be particularly difficult to maintain high diffraction efficiency with a single diffractive optical element. The present invention is sufficiently applicable even when light beams emitted from three or more light sources are used. Therefore, when the light emitting means includes three or more light sources, the use significance of the present invention is particularly great. Needless to say, even when two light sources are included, the present invention is useful.

  Each of the light beams that can be emitted from the light emitting means may belong to a different color spectral range. The “spectral range” here refers to a range of wavelengths exhibiting the same color, for example.

  In this way, when each of the light beams that can be emitted from the light emitting means belongs to a spectral range of different colors and the wavelength is greatly different between the light beams, high diffraction efficiency is maintained by a single diffractive optical element. Tend to be particularly difficult. The present invention can sufficiently cope with a case where the wavelengths of light beams emitted from the respective light sources are greatly different. Therefore, when the light beams emitted from the respective light sources belong to spectral ranges of different colors and the wavelengths are greatly different between the respective light beams, the use significance of the present invention is particularly great.

  A light beam intensity detecting means for detecting the intensity of the light beam applied to the light information detecting means, wherein the refractive index adjusting means takes into account the detection result of the light beam intensity detecting means; You may adjust the voltage application state with respect to a variable member.

  In this case, the voltage application state to the refractive index variable member adjusted by the refractive index adjusting means is adjusted in a feedback manner according to the detection result of the light beam intensity detecting means.

  It should be noted that any combination or recombination of the above-described constituent elements and what expresses the present invention as a method are also effective as an aspect of the present invention.

  As described above, according to the present invention, the optical axes of light beams emitted from a plurality of light sources are corrected by a simple structure of a diffractive optical element, a refractive index variable member, and a refractive index adjusting means, and an optical information detecting means. The predetermined position is appropriately irradiated. For this reason, the optical information detection means can accurately read the recorded information from the light beam reflected by the optical information recording medium, regardless of which light source the light beam is emitted from.

  Embodiments of the present invention will be described below with reference to the drawings. In order to facilitate understanding of the contents, each drawing may include a portion represented as an image and a portion represented as an equivalent.

(First embodiment)
FIG. 1 is a diagram illustrating an overall configuration of an optical pickup device 10 according to the present embodiment.
The optical pickup device 10 according to the present embodiment is provided so that CDs, DVDs, and HD DVDs can be reproduced interchangeably among optical information recording media.

  The optical pickup device 10 includes a laser light source 12, a recording medium placement unit 16 on which an optical information recording medium 14 is placed, a collimator lens 18 placed between the laser light source 12 and the recording medium placement unit 16, and a collimator A polarizing beam splitter 22 and a quarter-wave plate 24 disposed between the lens 18 and the recording medium placement unit 16; a reflecting mirror 26 disposed between the quarter-wave plate 24 and the recording medium placement unit 16; An objective lens 28 disposed between the reflecting mirror 26 and the recording medium placement unit 16 and a servo mechanism 29 for driving the objective lens 28 are provided. The optical pickup device 10 includes a photodiode 30 disposed on the optical path of the laser beam reflected by the polarization beam splitter 22, a condensing lens 32 disposed between the polarization beam splitter 22 and the photodiode 30, and a collecting lens. And an optical axis correction unit 20 disposed between the optical lens 32 and the photodiode 30.

  A reproduction circuit 34 for reproducing information recorded on the optical information recording medium 14, a liquid crystal circuit 36, and a servo circuit 38 are connected to the photodiode 30. The liquid crystal circuit 36 is also connected to the optical axis correction unit 20, and the servo circuit 38 is also connected to the servo mechanism 29. The laser light source 12 and the liquid crystal circuit 36 are connected to a switch device (not shown).

  On each information recording surface of an optical information recording medium such as a CD used in the optical pickup device 10 of the present embodiment, a planar land and a plurality of pits formed so as to protrude from the land portion. And are formed. Each pit is spirally arranged on the information recording surface, and predetermined information is optically recorded on the optical information recording medium 14 such as a CD by the arrangement of the pits.

  The laser light source 12 functions as a light emitting means including two or more light sources capable of emitting a light beam in response to a signal from a switch device (not shown). Specifically, the laser light source 12 emits a P-polarized wave laser beam having a first wavelength λ1 and a first light source 40 that emits a P-polarized wave laser beam having a second wavelength λ2. It is configured as a so-called 1CAN3LD type light source having two light sources 42 and a third light source 44 that emits a P-polarized laser beam having a third wavelength λ3. The laser beam of the first wavelength λ1 is 655 nm infrared light that is preferably used for DVD, and the laser beam of the second wavelength λ2 is 408 nm of blue light that is preferably used for HD DVD. The laser beam having the third wavelength λ3 is 780 nm red light which is preferably used for CD. Thus, the laser beams emitted from the respective light sources belong to different color spectral ranges.

  The first light source 40 to the third light source 44 diffract the laser beam emitted from the first light source 40 as −1st order diffracted light having a diffraction order of −1 in the diffractive optical element of the optical axis correction unit 20 described later. The laser beam emitted from the second light source 42 is diffracted as 0th order diffracted light with a diffraction order of 0, and the laser beam emitted from the third light source 44 is diffracted as + 1st order diffracted light with a diffraction order of +1. It is provided as follows. Specifically, the second light source 42 is disposed on the central axis of the optical system including the collimator lens 18 and the like, and the first light source 40 and the third light source 44 are separated from the second light source 42 by the distance d1 and It is arranged at a position away from d2. In the present embodiment, the distance d1 between the first light source 40 and the second light source 42 is set to 110 μm, and the distance d2 between the second light source 42 and the third light source 44 is set to 132 μm.

  The collimator lens 18 aligns the laser beam emitted from the laser light source 12 with parallel light and prevents the laser beam from diffusing.

  The polarization beam splitter 22 has a function of transmitting the P-polarized laser beam and reflecting the S-polarized laser beam. The quarter-wave plate 24 converts the P-polarized wave laser beam emitted from the laser light source 12 into a circularly-polarized wave, and reflects the incident circularly-polarized wave reflected by the optical information recording medium 14 such as a CD. It has a function of converting to a polarized wave. Accordingly, since the laser beam is converted from the P-polarized wave to the S-polarized wave by passing through the quarter-wave plate 24 twice as described above, the reflected light from the optical information recording medium 14 is polarized beam splitter. 22 is reflected.

  The reflecting mirror 26 has a function of reflecting the laser beam, and reflects regardless of the polarization state of the laser beam. The reflecting mirror 26 of the present embodiment applies a laser beam that is emitted from the laser light source 12 and passes through the optical axis correction unit 20, the collimator lens 18, the polarization beam splitter 22, and the ¼ wavelength plate 24 to approximately 90. Reflected so as to be folded back and guided to the recording medium placement unit 16, and the laser beam reflected by the optical information recording medium 14 placed on the recording medium placement unit 16 was reflected so as to be folded back by about 90 °. It arrange | positions so that it may guide | induced to the quarter wavelength plate 24 and the polarizing beam splitter 22. FIG.

  The objective lens 28 is disposed so as to face the information recording surface of the optical information recording medium 14, and is provided so as to be movable in the radial direction and the rotation axis direction of the optical information recording medium 14 by the servo mechanism 29. Yes. The objective lens 28 focuses the laser beam reflected by the reflecting mirror 26 on the information recording surface of the optical information recording medium 14 arranged in the recording medium arrangement unit 16. The laser beam reflected by the information recording surface of the optical information recording medium 14 travels toward the objective lens 28 and the reflecting mirror 26 with the traveling direction being changed to the reverse direction.

  The condensing lens 32 condenses the laser beam reflected by the optical information recording medium 14, the reflecting mirror 26, and the polarization beam splitter 22 on the photodiode 30.

  The optical axis correction unit 20 functions as an optical axis correction unit that corrects the optical axis of the laser beam that is reflected by the optical information recording medium and passes through the reflecting mirror 26, the quarter wavelength plate 24, and the condenser lens 32. . Specifically, it has a structure as shown in FIG.

FIG. 2 illustrates the configuration of the optical axis correction unit 20.
The optical axis correction unit 20 is provided on the exit side of the condenser lens 32, and includes a binary diffractive optical element 46 disposed at a position where the laser beam reflected by the optical information recording medium 14 is incident, And a liquid crystal body 48 disposed in close contact with the optical element 46.

  The diffractive optical element 46 has a function of correcting the optical axis of the incident light beam so that the laser beam is irradiated to a predetermined light detection position of the photodiode 30. The diffractive optical element 46 of the present embodiment has a binary step 50 of three steps, and is provided so that the binary step 50 is disposed on the laser light source 12 side. The height (hereinafter referred to as “step height”) t of each step of the binary step portion 50 is 1.71 μm. Note that the number of steps of the binary step unit 50 here is determined based on the number of steps of the stepped portion of the binary step unit 50.

  Generally, in the binary type diffractive optical element 46, the maximum diffraction efficiency ηmax is determined according to the number of steps m of the binary step portion 50, and specifically, the maximum diffraction efficiency ηmax is determined by the following equation (1). For example, when the binary step portion 50 has 3 steps, the maximum diffraction efficiency ηmax is 68%, and when it has 3 steps, the maximum diffraction efficiency ηmax becomes 81%.

  ηmax = {sin (π / m) / (π / m)} ^ 2 Formula (1)

In the diffractive optical element 46, the diffraction angle θ _{+ 1 of the} laser beam emitted from the first light source 40 is 0.38 °, and the diffraction angle θ- _{1 of the} laser beam emitted from the third light source 44 is 0.45 °. The lattice spacing Λ is 100 μm. The diffractive optical element 46 is formed of a glass material of SK1 having a refractive index represented by the following formulas (2) to (4) with respect to the laser beam of each wavelength.

In the case of a laser beam of 408 nm n1 = 1.628 Formula (2)
In the case of a 655 nm laser beam n1 = 1.607 Formula (3)
In the case of a 785 nm laser beam, n1 = 1.603 Formula (4)

  The liquid crystal body 48 is provided so as to be in close contact with the binary step portion 50 of the diffractive optical element 46, and is a refractive index variable member that changes the refractive index n0 according to the applied voltage. The refractive index n0 of the liquid crystal body 48 is one of the factors that influence the diffraction efficiency of the laser beam in the diffractive optical element 46, as will be described later. The liquid crystal body 48 can be made of any material that can change the refractive index in accordance with the applied voltage.

  The photodiode 30 generates a voltage signal corresponding to the intensity of the laser beam irradiated to a predetermined light detection position with the optical axis corrected by the optical axis correction unit 20 while being condensed by the condenser lens 32. To the reproduction circuit 34, the liquid crystal circuit 36, and the servo circuit 38. The voltage signal generated in this way is a signal indicating information recorded on the optical information recording medium 14 and a signal indicating an irradiation state such as the intensity of the laser beam irradiated to the photodiode 30. It is. Therefore, the photodiode 30 functions as an optical information detection unit that detects information recorded on the optical information recording medium 14, and also detects a laser beam intensity detection unit that detects the intensity of the laser beam applied to the photodiode 30. Will function as well.

  The reproduction circuit 34 converts the voltage signal sent from the photodiode 30 into a digital signal, and reproduces information recorded on the optical information recording medium 14 arranged in the recording medium arrangement unit 16.

  The servo circuit 38 detects the irradiation state of the laser beam on the photodiode 30 based on the voltage signal sent from the photodiode 30. Then, the servo circuit 38 controls the servo mechanism 29 based on the detected irradiation state of the laser beam to adjust the position of the objective lens 28, and with respect to the optical information recording medium 14 arranged in the recording medium arrangement unit 16. Adjustments are made so that the laser beam is irradiated properly. For example, the servo circuit 38 adjusts the distance between the objective lens 28 and the optical information recording medium 14 so that the focal point of the laser beam is correctly connected on the information recording surface of the optical information recording medium 14 without blurring. The servo circuit 38 adjusts the position of the objective lens 28 so that the laser beam is irradiated to the center of the pit array by the objective lens 28. As described above, the voltage signal sent from the photodiode 30 to the servo circuit 38 is used as feedback information for appropriately irradiating the optical information recording medium 14 with the laser beam.

  The liquid crystal circuit 36 adjusts the refractive index n0 of the liquid crystal body 48 by changing the voltage application state of the optical axis correction unit 20 to the liquid crystal body 48 according to the wavelength of the laser beam emitted from the laser light source 12 and the diffraction order. To do. Specifically, the liquid crystal circuit 36 changes the voltage application state to the liquid crystal body 48 so as to increase the diffraction efficiency of the laser beam in the diffractive optical element 46 of the optical axis correction unit 20. At this time, information on the laser beam actually emitted from the laser light source 12 is sent to the liquid crystal circuit 36 from a switch device (not shown), and the liquid crystal circuit 36 uses the liquid crystal body 48 based on the information from the switch device. The voltage application state with respect to is adjusted.

  Further, the liquid crystal circuit 36 adjusts the voltage application state with respect to the liquid crystal body 48 in consideration of the voltage signal indicating the detection result of the intensity of the laser beam in the photodiode 30. Thus, the voltage signal sent from the photodiode 30 to the liquid crystal circuit 36 is used as feedback information for efficiently diffracting the laser beam in the diffractive optical element 46.

Next, the operation of the optical pickup device 10 of the present embodiment will be described.
First, the overall operation of the optical pickup device 10 when reproducing information recorded on the optical information recording medium 14 will be described.

  When a user or the like sets a desired optical information recording medium 14 among the CD, DVD, and HD DVD in the recording medium placement unit 16 and turns on the switch device, the optical set in the recording medium placement unit 16 is set. A laser beam having a wavelength corresponding to the formula information recording medium 14 is emitted from the laser light source 12.

  As shown in FIG. 1, the laser beam emitted from the laser light source 12 is converted into parallel light by the collimator lens 18, passes through the polarization beam splitter 22, and is circularly polarized from the P-polarized wave by the quarter wavelength plate 24. Converted into waves. Then, the laser beam is folded back approximately 90 ° by the reflecting mirror 26, and then focused on the information recording surface of the optical information recording medium 14 by the objective lens 28, and reflected on this information recording surface.

  At this time, the laser beam irradiated on the land portion of the information recording surface is reflected while maintaining the light intensity, but the laser beam irradiated and reflected on the pit portion of the information recording surface spreads. The light intensity is weakened. Therefore, the laser beam reflected on the information recording surface of the optical information recording medium 14 includes information recorded on the optical information recording medium 14 by the pits in a form converted into light intensity. .

  Then, the laser beam reflected by the optical information recording medium 14 is returned to parallel light by the objective lens 28, folded back approximately 90 ° by the reflecting mirror 26, and directed toward the quarter-wave plate 24 and the polarizing beam splitter 22. proceed. Since the laser beam is converted into an S-polarized wave by the quarter wavelength plate 24, it is reflected by the polarizing beam splitter 22 without being transmitted. Then, the laser beam reflected by the polarization beam splitter 22 is condensed by the condenser lens 32 and the optical axis is corrected by the optical axis correction unit 20 and irradiated to a predetermined light detection position of the photodiode 30. .

  FIG. 3 is a diagram showing optical axis correction of a laser beam in the optical axis correction unit 20. FIG. 3 shows a laser beam emitted from the first light source 40 as an example. In FIG. 3, the solid line indicates the actual optical path of the laser beam, the dotted line indicates the optical path of the laser beam when the optical axis correction unit 20 is not disposed, and the alternate long and short dash line indicates the optical system such as the condenser lens 32. The center axis of is shown.

  The laser beam emitted from the second light source 42 is emitted in a state where the optical axis substantially coincides with the central axis of the optical system, is reflected by the optical information recording medium 14, and then enters the optical axis correction unit 20. And exit. At this time, this laser beam is applied to a predetermined light detection position of the photodiode 30 as 0th-order diffracted light without being diffracted by the diffractive optical element 46 of the optical axis correction unit 20.

  On the other hand, the laser beam emitted from the first light source 40 or the third light source 44 is emitted in a state where the optical axis is deviated from the central axis of the optical system and reflected by the optical information recording medium 14, and then the light beam. The light is diffracted by the diffractive optical element 46 of the axis correction unit 20. The direction of the optical axis of the laser beam diffracted by the diffractive optical element 46 is corrected as a −1st order diffracted light or a + 1st order diffracted light, and is irradiated around a predetermined light detection position 30a of the photodiode 30 (FIG. (See solid line part 3). These laser beams are emitted from the light source when the optical axis is not corrected by the optical axis correction unit 20, so that the optical axis is shifted from the central axis of the optical system. The light is irradiated around a position 30b deviated from the position 30a (see the dotted line portion in FIG. 3).

  As described above, each laser beam emitted from the laser light source 12 has its optical axis corrected by the optical axis correction unit 20 and is irradiated to the predetermined light detection position 30a of the photodiode 30 with high accuracy. Thereby, the photodiode 30 can appropriately detect the laser beam reflected on the optical information recording medium 14.

  The photodiode 30 generates a voltage signal corresponding to the intensity of the laser beam emitted to a predetermined light detection position. This voltage signal is sent to the reproduction circuit 34 and also to the servo circuit 38 and the liquid crystal circuit 36.

  In the reproduction circuit 34, a digital signal is generated from the voltage signal sent from the photodiode 30, and information recorded on the optical information recording medium 14 is reproduced. The servo circuit 38 detects the irradiation state of the laser beam on the photodiode 30 based on the voltage signal sent from the photodiode 30 and drives and controls the objective lens 28. The liquid crystal circuit 36 detects the irradiation state of the laser beam to the photodiode 30 based on the voltage signal sent from the photodiode 30 and adjusts the voltage applied to the liquid crystal body 48 of the optical axis correction unit 20. The diffraction efficiency in the diffractive optical element 46 is adjusted.

  As described above, the optical system including the collimator lens 18, the polarizing beam splitter 22, the quarter wavelength plate 24, and the reflecting mirror 26 allows the laser beam emitted from the laser light source 12 to be optical information such as a CD. By the optical system configured to irradiate the recording medium 14 and includes the reflecting mirror 26, the quarter-wave plate 24, the polarizing beam splitter 22, the condenser lens 32, the optical axis correction unit 20, and the photodiode 30, The recording information of the optical information recording medium 14 is optically read from the laser beam reflected by the optical information recording medium 14.

  Next, the operation of the liquid crystal body 48 of the optical axis correction unit 20 will be described in detail.

First, an example in which the liquid crystal body 48 is not provided in the optical axis correction unit 20 will be described. In this case, the laser beam reflected by the optical information recording medium enters the diffractive optical element 46 having a constant refractive index n1 from air having a constant refractive index n0 and is diffracted. At this time, the diffraction efficiency of the laser beam in the diffractive optical element 46 is such that the step height t of the binary step 50 of the diffractive optical element 46, the refractive index n0 of the medium that enters the convex concave portion of the binary step 50, and the diffractive optical element. It is theoretically known to be determined based on the refractive index n1 of 46. Specifically, the diffractive optical element 46 has a step height t ₊₁ represented by the following expression (5) for the + 1st order diffracted light and a step height represented by the following expression (6) for the 0th order diffracted light. With respect to t ₀ and −1st order diffracted light, the diffraction efficiency of the laser beam in the diffractive optical element 46 can be maximized when it has a step height t ₋₁ represented by the following equation (7).

t _{+ 1} = (1/3) * λ / (n1-n0) Equation (5)
t ₀ = λ / (n1-n0) Equation (6)
t ₋₁ = (2/3) * λ / (n1−n0) Equation (7)

The refractive index n1 of the diffractive optical element 46 is n1 = 1.46 with respect to a laser beam with a wavelength λ1 of 655 nm diffracted as −1st order diffracted light, and with respect to a laser beam with a wavelength λ2 of 408 nm diffracted as 0th order diffracted light. N1 = 1.47, n1 = 1.45 for a laser beam of wavelength λ3 of 785 nm diffracted as + 1st order diffracted light, and if the refractive index n0 of air is n0 = 1, the diffraction efficiency is The step heights t ₊₁ , t ₀ , and t ₋₁ to be maximized are values represented by the following formulas (8) to (10), respectively.

t _{+ 1} = 0.581 μm (8)
t ₀ = 0.868 μm Equation (9)
t ₋₁ = 0.949 μm Formula (10)

  FIG. 4 shows the relationship between the diffraction efficiency and the step height when the liquid crystal body 48 is not provided in the optical axis correction unit 20 for each laser beam having a different wavelength. FIG. 4 shows 780 nm infrared light diffracted as + 1st order diffracted light and 408 nm diffracted as 0th order diffracted light in the diffractive optical element 46, similarly to the laser beam used in the optical pickup device 10 of the present embodiment. Data relating to a laser beam of 655 nm red light diffracted as blue light and −1st order diffracted light is shown.

As shown in FIG. 4, when using a plurality of laser beams having different wavelengths, it is difficult to select a single step height that keeps all the diffraction efficiencies of the laser beams having different wavelengths good. It becomes very difficult when there are three or more different laser beams. In FIG. 4, for example, as 0.815μm step height t, when the same embodiment other conditions, + 1 diffraction efficiency of diffracted light eta _{+1, 0} diffraction efficiency eta _0-order diffracted _light, and -1 The diffraction efficiency η ₋₁ of the next diffracted light is a value represented by the following formulas (11) to (13).

η _{+ 1} = 40.4% Formula (11)
η ₀ = 90.5% (12)
η ₋₁ = 53.7% Formula (13)

  Thus, when the binary step 50 of the diffractive optical element 46 is exposed to an air-like medium whose refractive index cannot be changed, all laser beams having different wavelengths depending on the single step height. It is difficult to keep good diffraction efficiency. When the number of steps of the binary step 50 is increased or the step height is increased, the only step height that can satisfactorily maintain the diffraction efficiency of all laser beams having different wavelengths is selected. Sometimes it is possible. However, as the number of steps increases, the manufacturing cost of the diffractive optical element 46 increases, and in particular, the diffractive optical element 46 having a number of steps of 5 steps or more becomes very expensive. Therefore, it is preferable that the diffraction efficiency of all laser beams having different wavelengths is satisfactorily maintained by the diffractive optical element 46 having four or fewer steps. Further, when the step height is increased, it becomes difficult to keep the manufacturing accuracy of the diffractive optical element 46 constant, and it becomes difficult to manufacture the diffractive optical element 46 with high accuracy.

  Therefore, in the present embodiment, by changing the voltage application state to the liquid crystal body 48 according to the laser beam, the refractive index of the liquid crystal body 48 is adjusted, and the diffraction efficiency of all laser beams having different wavelengths is satisfactorily maintained. ing.

  In the present embodiment, since the diffractive optical element 46 has a refractive index n1 as expressed by the above formulas (2) to (4), the liquid crystal circuit 36 is actually emitted from the laser light source 12. The voltage applied to the liquid crystal body 48 is adjusted so that the refractive index n0 of the liquid crystal body 48 becomes a value represented by the following formulas (14) to (16) in accordance with the laser beam.

In the case of a laser beam of 408 nm, n0 = 1.39 (14)
In the case of a 655 nm laser beam, n0 = 1.352 (15)
In the case of a 785 nm laser beam, n0 = 1.45 (16)

  In this case, the relationship between the diffraction efficiency of the laser beam of each wavelength and the step height is as shown in FIG.

FIG. 5 illustrates the relationship between the diffraction efficiency and the step height when the liquid crystal body 48 is provided in the optical axis correction unit 20 for each laser beam having a different wavelength. In this case, the step heights t ₊₁ , t ₀ , and t ₋₁ that maximize the diffraction efficiency η of the laser beam in the diffraction grating are values represented by the following formulas (17) to (19), which are substantially the same. To do.

t _{+ 1} = 1.710 μm Formula (17)
t ₀ = 1.714 μm (18)
t ₋₁ = 1.712 μm Formula (19)

  When the step height t is t = 1.71 μm and the refractive index of the liquid crystal body 48 is adjusted as described above, the diffraction efficiency of the laser beam of each wavelength is expressed by the following equations (20) to (22). It becomes the value to be.

η _{+ 1} = 68% Formula (20)
η ₀ = 100% Formula (21)
η ₋₁ = 68% Formula (22)

  As described above, according to the present embodiment, the diffraction efficiency of the laser beam in the diffractive optical element 46 is dramatically improved as compared with the conventional pickup device having the diffraction efficiency represented by the equations (11) to (13). Can be improved.

  As described above, according to the present embodiment, the optical axis of each laser beam emitted from a plurality of light sources is corrected by the optical axis correction unit 20 including the binary diffractive optical element 46 and the liquid crystal body 48. Thus, it is possible to appropriately irradiate the photodiode 30 which is the optical information detection means. In particular, by changing the refractive index of the liquid crystal body 48 according to the laser beam emitted from the light source, high diffraction efficiency is ensured for any laser beam emitted from each light source, and the light in the diffractive optical element 46 is obtained. This prevents a decrease in strength. Thereby, the utilization efficiency of the laser beam emitted from each light source can be increased, and the information recorded on the optical information recording medium 14 can be read with high accuracy.

  Further, since the laser beam emitted from the laser light source 12 is irradiated to the optical information recording medium 14 without passing through the diffractive optical element 46, the optical information is not affected by the energy loss in the diffractive optical element 46. The recording medium 14 is irradiated. For this reason, the laser beam emitted from the laser light source 12 is used efficiently, and a laser beam with high light intensity is required, particularly when information is recorded on the optical information recording medium 14. In this case, the optical pickup device 10 of the present embodiment can be suitably used.

  Such an effect is obtained even when the 1-can type laser light source 12 whose degree of freedom in mechanism design is easily limited is used, the liquid crystal body 48 and the diffractive optical element 46 constituting the optical axis correction unit 20 and the liquid crystal. This is achieved by the simple structure of the circuit 36.

  In addition, by correcting the optical axes of laser beams emitted from a plurality of light sources, information recorded on each of different types of optical information recording media 14 such as CD, DVD, and HD DVD can be changed. A single photodiode 30 can be detected with high accuracy. Thereby, the structure of the optical pick-up apparatus 10 can be simplified, the number of parts can be reduced, and size reduction can be achieved.

  In the present embodiment, since the diffraction efficiency in the diffractive optical element 46 is adjusted by adjusting the refractive index of the liquid crystal body 48, it is not necessary to strictly adjust the thickness of the liquid crystal body 48. The dimensional accuracy regarding the thickness of the body 48 and the reproduction accuracy during mass production are not strictly required. Accordingly, the optical axis correction unit 20 including the liquid crystal body 48 can be manufactured relatively easily, and the manufacturing cost can be reduced. In addition, the liquid crystal body 48 can be formed relatively thin. When the liquid crystal body 48 is formed thin, the response speed can be improved and the amount of applied voltage can be reduced when changing the refractive index.

Next, a modification of the present embodiment will be described.
In the above-described embodiment, the case where the binary type diffractive optical element 46 having the binary step unit 50 of three steps is used as the diffractive optical element 46 is described. However, other diffractive optical elements 46 may be used. Is possible. Hereinafter, a case where a binary type diffractive optical element 46 having a four-step binary step portion 50 is used will be described.

The 4-step binary diffractive optical element 46 has + 2nd order diffracted light and + 3rd order diffracted light and + 1st order diffracted light when the step heights t ₊₃ and t ₋₁ represented by the following equation (23) are used. For the −2nd order diffracted light, when it has step heights t ₊₂ and t ₋₂ expressed by the following formula (24), the + first order diffracted light and the −3rd order diffracted light are expressed by the following formula (25). When the step heights t ₊₁ and t ₋₃ are used, the diffraction efficiency of the laser beam in the diffractive optical element 46 is changed with respect to the zero-order diffracted light when the step height t ₀ represented by the following equation (26) is provided. It is theoretically known that it can be maximized.

t ₊₃ = t ₋₁ = 3/4 * λ / (n1−n0) Equation (23)
t ₊₂ = t ₋₂ = 2/4 * λ / (n1-n0) Formula (24)
t _{+ 1} = t- ₃ = 1/4 * λ / (n1-n0) Equation (25)
t ₀ = λ / (n1-n0) Equation (26)

  Also in this modification, the diffractive optical element 46 has the refractive index n1 as expressed by the above formulas (2) to (4), so that the liquid crystal circuit 36 has the refractive index n0 of the liquid crystal body 48. The voltage applied to the liquid crystal body 48 is adjusted in accordance with the laser beam actually emitted from the laser light source 12 so that becomes a value represented by the following equations (27) to (29).

In the case of a laser beam of 408 nm, n0 = 1.4 Formula (27)
In the case of a 655 nm laser beam, n0 = 1.332 (28)
In the case of a 785 nm laser beam, n0 = 1.493 (29)

  In this case, the relationship between the diffraction efficiency of the laser beam of each wavelength and the step height is as shown in FIG.

FIG. 6 shows the relationship between the diffraction efficiency and the step height when the liquid crystal body 48 is provided in the optical axis correction unit 20 for each laser beam having a different wavelength. Step heights t ₊₁ to t ₋₁ that maximize the diffraction efficiency η of the laser beam in the diffraction grating with respect to the + 3rd order diffracted light to the −3rd order diffracted light are values represented by the following formulas (30) to (32). And approximately match.

t _{+ 1} = 1.784 μm Formula (30)
t ₀ = 1.789 μm Formula (31)
t ₋₁ = 1.786 μm Formula (32)

  When the step height t is t = 1.79 μm and the refractive index of the liquid crystal body 48 is adjusted as described above, the diffraction efficiency of the laser beam of each wavelength is expressed by the following equations (33) to (35). It becomes the value to be.

η _{+ 1} = 81.1% Formula (33)
η ₀ = 100% (34)
η ₋₁ = 81% Formula (35)

  Even when the binary step portion 50 of the diffractive optical element 46 has various number of steps as in this modification, the diffraction efficiency of the laser beam in the diffractive optical element 46 is improved and emitted from the laser light source 12. The utilization efficiency of the laser beam can be increased.

(Second Embodiment)
In the present embodiment, the same parts as those in the first embodiment described above are denoted by the same reference numerals, and detailed description thereof is omitted.

FIG. 7 is a diagram showing an overall configuration of the optical pickup device 10 of the present embodiment.
In the present embodiment, the optical axis correction unit 20 is arranged between the polarizing beam splitter 22 and the condenser lens 32 instead of being arranged between the condenser lens 32 and the photodiode 30. Therefore, the optical axis correction unit 20 is provided on the incident side of the condenser lens 32.

Other configurations are substantially the same as those of the first embodiment shown in FIGS.
Also in the optical pickup device 10 of the present embodiment, the optical axis direction of the laser beam emitted from each light source and reflected by the optical information recording medium 14 is corrected by the optical axis correction unit 20. At this time, each laser beam is diffracted by the diffractive optical element 46 of the optical axis correction unit 20 so that the optical axis coincides with the central axis of the optical system such as the condenser lens 32.

  Then, the laser beam whose optical axis is corrected is condensed by the condensing lens 32 and applied to a predetermined light detection position 30 a of the photodiode 30. At this time, any laser beam emitted from the laser light source 12 is incident on the condensing lens 32 in a state where the optical axis is corrected by the optical axis correction unit 20 and the optical axis and the central axis of the optical system substantially coincide with each other. To do. Therefore, the laser beam condensed by the condenser lens 32 and applied to the photodiode 30 is applied to the predetermined light detection position 30a with high accuracy.

  Further, by changing the refractive index of the liquid crystal body 48 according to the laser beam emitted from the light source, high diffraction efficiency is ensured for any laser beam emitted from each light source, and the diffractive optical element 46 has This prevents a decrease in light intensity.

  Thus, also in this embodiment, the utilization efficiency of the laser beam emitted from each light source is enhanced, and the information recorded on the optical information recording medium 14 is read with high accuracy.

  The present invention is not limited to the above-described embodiments, and various modifications such as design changes can be added based on the knowledge of those skilled in the art. Forms can also be included within the scope of the present invention.

  For example, in each of the above-described embodiments and modifications, the case where the binary step portion 50 and the liquid crystal body 48 of the binary type diffractive optical element 46 are provided on the incident side of the laser beam has been described. Even when 50 and the liquid crystal body 48 are provided on the laser beam emitting side, the same operations and effects as the above-described embodiments and modifications are achieved.

  In each of the above-described embodiments and modifications, the optical information recording medium 14 is irradiated with a laser beam and reflected by the optical information recording medium 14 by the polarizing beam splitter 22, the quarter wavelength plate 24, and the like. The laser beam is guided to the photodiode 30, but other devices can also be used. For example, it is possible to use a normal beam splitter or combine a dichroic prism that transmits a part of the incident laser beam and reflects the other part.

  In each of the above-described embodiments and modifications, the case where laser beams having wavelengths of 780 nm, 655 nm, and 408 nm used for CD, DVD, and HD DVD are used has been described. However, laser beams having other wavelengths are used. The optical pickup device 10 according to the present invention can also be applied when used. In this case, a voltage corresponding to the laser beam used in the optical pickup device 10 is applied to the liquid crystal body 48, and the liquid crystal body 48 is adjusted so that the diffraction efficiency of the laser beam in the diffractive optical element 46 is increased.

  Further, in each of the above-described embodiments and modifications, the liquid crystal circuit 36 acquires information about the laser beam actually emitted from the laser light source 12 based on information sent from a switch device (not shown), and the liquid crystal body 48. However, the liquid crystal circuit 36 can also acquire information on the laser beam emitted from the laser light source 12 by other methods. For example, a new mechanism for discriminating the type of the optical information recording medium 14 using a laser beam reflected by the optical information recording medium 14 may be provided. The liquid crystal circuit 36 can also adjust the voltage application state to the liquid crystal body 48 by specifying the type of the optical information recording medium 14 from the determination result of such a mechanism. When such a mechanism is used, the type of the optical information recording medium 14 can be appropriately specified, and a laser beam that needs to be emitted from the laser light source 12 can be easily selected.

It is a figure which shows the whole structure of the optical pick-up apparatus of 1st Embodiment. It is a figure which shows the structure of an optical axis correction | amendment part. It is a figure which shows the optical axis correction | amendment of the laser beam in an optical axis correction part. It is a figure which shows the relationship between the diffraction efficiency and step height in case the liquid crystal body is not provided in the optical axis correction | amendment part. It is a figure which shows the relationship between diffraction efficiency and step height in case a liquid crystal body is provided in the optical axis correction part. In the modification of 1st Embodiment, it is a figure which shows the relationship between the diffraction efficiency and step height in case a liquid crystal body is provided in the optical axis correction | amendment part. It is a figure which shows the whole structure of the optical pick-up apparatus of 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical pick-up apparatus, 12 Laser light source, 14 Optical information recording medium, 16 Recording medium arrangement | positioning part, 18 Collimator lens, 20 Optical axis correction | amendment part, 22 Polarization beam splitter, 24 1/4 wavelength plate, 26 Reflecting mirror, 28 Objective Lens, 30 photodiode, 32 condenser lens, 34 reproduction circuit, 36 liquid crystal circuit, 38 servo circuit, 40 first light source, 42 second light source, 44 third light source, 46 diffractive optical element, 48 liquid crystal body, 50 Binary step.

Claims (6)

A light emitting means including two or more light sources capable of emitting a light beam;
A light beam irradiating means for irradiating the optical information recording medium with a light beam emitted from the light emitting means;
Optical axis correction means for correcting the optical axis of the light beam reflected by the optical information recording medium;
An optical information detecting means for irradiating a light beam whose optical axis has been corrected by the optical axis correcting means and detecting information recorded on the optical information recording medium based on the irradiated light beam;
The optical axis correcting means is
A diffractive optical system that corrects the optical axis of the incident light beam so that the light beam reflected by the optical information recording medium is incident and the light beam is applied to a predetermined position of the optical information detecting means. Elements,
A refractive index variable member that is arranged in close contact with the diffractive optical element and changes a refractive index according to an applied voltage;
Refractive index adjustment means for adjusting a refractive index of the refractive index variable member by changing a voltage application state to the refractive index variable member according to a light beam emitted from the light emitting means. Optical pickup device.   The refractive index adjusting means adjusts the refractive index of the refractive index variable member by changing the voltage application state to the refractive index variable member so as to increase the diffraction efficiency of the light beam in the diffractive optical element. The optical pickup device according to claim 1.   The optical pickup device according to claim 1, wherein the diffractive optical element is a binary diffractive optical element. The light emitting means includes three or more light sources,
The refractive index adjusting means adjusts a refractive index of the refractive index variable member by changing a voltage application state to the refractive index variable member in accordance with a light beam emitted from the light emitting means. Item 4. The optical pickup device according to any one of Items 1 to 3.   5. The optical pickup device according to claim 1, wherein each of the light beams that can be emitted from the light emitting unit belongs to a spectral range of a different color. A light beam intensity detecting means for detecting the intensity of the light beam applied to the light information detecting means,
6. The light according to claim 1, wherein the refractive index adjusting unit adjusts a voltage application state to the refractive index variable member in consideration of a detection result of the light beam intensity detecting unit. Pickup device.

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