JP3833876B2 - Optical pickup device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup device for reproducing an optical recording medium. More specifically, the present invention relates to an optical system for separating light emitted from a light source and return light from an optical recording medium.
[0002]
[Prior art]
As an optical pickup device for reproducing an optical recording medium such as a compact disc (CD), a ¼ wavelength phase difference plate (1/4 wavelength) is used for the light emitted from a laser light source and the return light from the optical recording medium. There is known a polarization type optical pickup device that is configured such that outgoing light and return light can be separated by passing through a plate. For example, as shown in FIGS. 10 and 11, a polarization beam splitter 11 (polarization separation element), a quarter-wave retardation plate 12 and an objective lens 16 are arranged in the middle of the optical path from the laser light source to the photodetector. In this case, the light emitted from the laser light source 13 composed of a laser diode passes through the polarization beam splitter 11 and the quarter-wave retardation plate 12, and is then irradiated onto the recording surface of the optical recording medium 5 as a light spot. The return light from the recording medium 5 is configured to pass through the quarter-wave retardation plate 12 and the polarization beam splitter 11 again. When the return light from the optical recording medium 5 passes through the ¼ wavelength phase difference plate 12, the return light is changed into laser light having a polarization direction different by 90 ° from the polarization direction of the light emitted from the laser light source 13. The light is guided to a photodetector 14 installed in a different direction from the light source 13.
[0003]
Further, as shown in FIGS. 1 and 2, a diffractive element 21 (hologram element) is disposed as a polarization separation element between the laser light source 13 and the quarter wavelength phase difference plate 12, and the diffractive element 21 is arranged. In some cases, the return light from the optical recording medium 5 may be guided to the photodetector 14.
[0004]
In any of these polarization type optical pickup devices 1A and 1B, the optical system is shown in FIGS. 12A and 12B, and the optical system is developed and the polarization state of the light is schematically shown. The linearly polarized light is converted into circularly polarized light by the ¼ wavelength phase difference plate 12, and the return light (circularly polarized light) from the optical recording medium 5 is emitted from the laser light source 13 by the ¼ wavelength phase difference plate 12. The light is converted into linearly polarized light having a polarization azimuth that is 90 ° different from that of the linearly polarized light, and is guided to the photodetector 14 installed in a direction different from the laser light source 13.
[0005]
Therefore, in this type of optical pickup device 1A, 1B, the light emitted from the laser light source 13 can be effectively applied to the optical recording medium 5, and the return light from the optical recording medium 5 can be applied to the photodetector 14 with high efficiency. There is an advantage that can be guided.
[0006]
[Problems to be solved by the invention]
However, the above explanation is that the effective light amount is 100% when the birefringence amount δ of the optical recording medium 5 is 0, and when the optical recording medium 5 itself has birefringence. The birefringence also causes a change in the polarization state of the light, and there is a problem that the effective light amount falls below 100%.
[0007]
For example, as shown in FIG. 12C, if the optical recording medium 5 itself has a birefringence amount δ corresponding to ¼ wavelength in a reciprocating manner, it is already a straight line when reflected by the optical recording medium 5. It becomes polarized light. As a result, when the return light reflected by the optical recording medium 5 passes through the quarter-wave retardation plate 12 again, the linearly polarized light becomes circularly polarized light, and the effective light amount is reduced to 50%. . Furthermore, as shown in FIG. 12D, if the optical recording medium 5 has a birefringence amount δ corresponding to λ / 2 (λ = wavelength) in a reciprocating manner, the return light is 1 When the light passes through the / 4 wavelength phase difference plate 12, it changes to linearly polarized light having the same polarization direction as the linearly polarized light emitted from the laser light source 13 at this time. As a result, the return light and the light emitted from the laser light source 13 cannot be separated, and the effective amount of light reaching the photodetector 14 becomes 0%.
[0008]
Such a relationship between the birefringence amount δ and the detected light amount of the optical recording medium 5 is expressed as a relationship as indicated by a dotted line L0 in FIG. That is, assuming that the amount of light detected by the photodetector 14 when the birefringence amount δ of the optical recording medium 5 is 0, light is not emitted until the birefringence amount δ of the optical recording medium 5 changes from 0 to λ / 2. When the signal intensity detected by the detector 14 decreases and the birefringence amount δ of the optical recording medium 5 becomes λ / 2, the signal intensity becomes zero.
[0009]
In general, the base of the optical recording medium 5 is manufactured by injection molding, and at this time, the resin flows radially outward from the center side of the optical recording medium 5, so that the optical recording medium 5 has a radial direction and a circumferential direction. It is easy to have birefringence that the refractive index differs between the two. In order to confirm the current state, in the optical recording medium 5, when the signal intensity is measured from the center side to the outside in the radial direction, the characteristic shown in FIG. There was a thing. In the characteristics shown in FIG. 13, the signal intensity is first at a very low level on the center side of the disk, shows a minimum value when going slightly outward from the radial direction, and the signal intensity from this position to the outermost circumferential side in the radial direction. Is getting higher. Considering this result, in the case of the characteristic shown in FIG. 13, there is a region where the birefringence amount δ is λ / 2 slightly outside from the center in the radial direction, and the birefringence amount δ gradually increases from there in the radially outward direction. You can see that it is getting smaller. Although this example is an extreme example, it can be assumed that disks manufactured by the same manufacturing method show the same tendency, and generally have a certain amount of birefringence in the entire radial direction. It is done.
[0010]
On the other hand, by directing the direction of the anisotropic axis of the retardation plate 12 (hereinafter simply referred to as “azimuth”) to the birefringence direction of the optical recording medium 5, the optical recording medium 5 itself and the retardation plate are aligned. 12 is configured to function as one retardation plate, and the retardation plate 12 is replaced with a quarter wavelength, and the amount corresponding to the birefringence amount of the optical recording medium 5 itself. It is conceivable to use a phase difference having a phase difference that deviates from a quarter wavelength.
[0011]
With such a configuration, the retardation plate 12 and the optical recording medium 5 itself function as a quarter-wave retardation plate, so that the light emitted from the laser light source 13 is duplicated by the optical recording medium 5. Even if it has refraction, after the return light passes through the phase difference plate 12, it changes to linearly polarized light having a polarization azimuth that differs by 90 ° from the linearly polarized light emitted from the laser light source 13. Therefore, even if the optical recording medium 5 has birefringence, an optical pickup device with a high effective light amount can be configured.
[0012]
When such a configuration is adopted, the polarization direction of the laser light emitted from the laser light source 13 and the axial direction of the phase difference plate 12 are usually set to an angle of 45 °. Such setting is generally performed by rotating the laser light source 13 around the outgoing optical axis and adjusting its angular position. However, in such an adjustment method, as shown in FIGS. 10 and 11, when the laser light source 13 and the photodetector 14 are separate, the laser light source 13 is rotated alone around the output optical axis. However, when the laser light source 13 is integrally formed with the photodetector 14 as the light source unit 20 as in the optical pickup device 1A as shown in FIGS. 1 to 3, such an adjustment method is used. There is a problem that the arrangement of the optical system is greatly restricted, such as being unable to be employed.
[0013]
In view of the above problems, an object of the present invention is to perform stable light detection without greatly restricting the arrangement of the optical system even if the optical recording medium itself has birefringence. An object of the present invention is to provide an optical pickup device that can be used.
[0014]
[Means for Solving the Problems]
In order to solve the above problems, in the present invention, based on the phase difference plate that converts the polarization state of the emitted light from the laser light source and the return light from the optical recording medium, and the polarization state of the emitted light and the return light, In an optical pickup device having a polarization separation element that separates the return light from an optical axis from a laser light source toward the optical recording medium and guides it to a photodetector, the birefringence amount of the optical recording medium and the detector The phase difference and the anisotropic axis of the phase difference plate appear so that the peak of the signal intensity appears while the amount of birefringence changes from 0 to ¼ wavelength when the relationship with the detected signal intensity is measured. The direction (hereinafter referred to as the direction of the retardation film) is set.
[0015]
In the present invention, the retardation and orientation of the retardation plate are adjusted so as to absorb the development of birefringence in the optical recording medium according to the direction in which the resin flows when the optical recording medium substrate is manufactured by injection molding. Set. That is, unlike the prior art, regardless of the direction in which the optical recording medium actually has birefringence, the direction of the laser light source is first fixed, and the polarization direction of the emitted light emitted from the light source is Assuming that the optical recording medium is fixed, a phase difference plate is used so that the peak of the signal intensity detected by the photodetector appears before the birefringence amount of the optical recording medium changes from 0 to ¼ wavelength. Set the phase difference and direction. Here, normally, the amount of birefringence of an optical recording medium to be manufactured is equivalent to a quarter wavelength at most. Therefore, in the present invention, if the birefringence amount of the optical recording medium is between 0 and ¼ wavelength, the return light from the optical recording medium is reflected regardless of the birefringence direction of the optical recording medium. Since it can detect with high intensity | strength, an optical pick-up apparatus with a high effective light quantity can be comprised. In addition, the laser light source can be arranged in an optimum state from the viewpoint of design, regardless of which direction the birefringence of the optical recording medium is actually directed. Therefore, even in an optical pickup device in which a laser light source and a detector are integrated, stable signal detection can be performed from any optical recording medium and from any location in the radial direction of the optical recording medium. it can.
[0016]
  In the present invention, the polarization separation element generates either the first linearly polarized light or the first linearly polarized light from the return light including the first linearly polarized light and the second linearly polarized light whose directions of polarization are orthogonal to each other. Also has a partial polarization property that is emitted toward the photodetector at a certain ratio..
[0017]
With this configuration, even if the optical recording medium has a birefringence amount and the return light incident on the polarization separation element includes both the first linearly polarized light and the second linearly polarized light, A part of any linearly polarized light is detected by a photodetector. Therefore, even if all of the linearly polarized light included in the return light is the second linearly polarized light, the level of signal intensity is suppressed to some extent, but the return light is controlled by the birefringence of the optical recording medium. Even if all of the linearly polarized light contained in is the first linearly polarized light, a signal is detected with a certain intensity. Here, in the optical pickup device, when detecting the return light, if the detected signal intensity is high to some extent, there is not much mess even if it is higher than that, and if the detected signal intensity is low, it is at which level Is more important. However, in the present invention, signals can be detected at a certain level due to the partial polarization property of the polarization separation element even under the condition that the signal intensity is zero in the conventional case. Therefore, signal detection can be reliably performed no matter what birefringence the optical recording medium has. In addition, the signal can be reliably detected from any location in the radial direction of the optical recording medium.
[0018]
The present invention can also be defined as follows. That is, the phase difference plate that converts the polarization state of the light emitted from the laser light source and the return light from the optical recording medium, and the light source from the laser light source toward the optical recording medium based on the polarization state of the emission light and the return light In the optical pickup device having a polarization separation element that separates the return light from the optical axis and guides it to a photodetector, the retardation plate is 50 degrees to 60 degrees with respect to the laser light emitted from the laser light source. It is set so that it may become the azimuth | direction.
[0019]
  In the present invention, the polarization separating element converts the s-polarized light and the p-polarized light at a certain ratio from return light including linearly polarized light of s-polarized light and p-polarized light whose polarization directions are orthogonal to each other. Partially polarized light that separates toward the detector.
[0020]
In this case, when the transmittance and diffraction rate for the s-polarized light are Ts and Rs, respectively, both the transmittance Ts and the diffraction rate Rs for the s-polarized light are approximately 0.3 or more, and the p When the transmittance and diffraction rate for polarized light are Tp and Rp, respectively, it is preferable that both the transmittance Tp and the diffraction rate Rp for the p-polarized light are approximately 0.3 or more.
[0021]
In the present invention, the ratio of the transmittance Ts to the s-polarized light and the refractive index Rs is set to about 0.5: 0.5, and the ratio of the transmittance Tp to the p-polarized light and the refractive index Rp is set to about 0. .5: 0.5 is preferable.
[0022]
In the present invention, the retardation of the retardation plate is preferably set within a range of about 20 degrees centering on 90 degrees.
[0023]
  The present invention can be further defined as follows. That is, the phase difference plate that converts the polarization state of the light emitted from the laser light source and the return light from the optical recording medium, and the light source from the laser light source toward the optical recording medium based on the polarization state of the emission light and the return light In an optical pickup device having a polarization separation element that separates the return light from the optical axis and guides it to a photodetector, the polarization direction of the light emitted from the laser light source is 45 degrees with respect to the radial direction of the optical recording medium. And the orientation of the retardation plate is shifted within a range of about 5 degrees to about 15 degrees with respect to 45 degrees.Even in such a configuration, the polarization separation element separates the first linearly polarized light and the second linearly polarized light whose polarization directions are orthogonal to each other toward the photodetector at a certain ratio. It has such a partial polarization property.
[0024]
In the present invention, it is preferable that the retardation of the retardation plate is set to approximately 90 degrees, and the orientation of the retardation plate is set within a range of 50 degrees to 60 degrees. For example, it is preferable that the orientation of the retardation plate is set to about 55 degrees.
[0025]
  In the present invention, it is preferable that the retardation of the retardation plate is set within a range of about 20 degrees centering on 90 degrees, and the orientation of the retardation plate is set within a range of 50 degrees to 60 degrees. . For example, it is preferable to set the direction of the retardation plate to about 55 degrees..
[0026]
In the present invention, for example, a diffractive element can be used as the polarization separation element, and in this diffractive element, a diffraction grating may be formed by a polydiacetylene derivative film.
[0027]
In the present invention, the retardation plate may also be formed of a polydiacetylene derivative film or a dielectric film obliquely deposited on a substrate.
[0028]
The present invention is effective when applied to an optical pickup device in which the polarization separation element is formed integrally with the retardation plate and the laser light source and the photodetector are integrated as a light source unit.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described with reference to the drawings. In the following description, an example in which a diffractive element is used as a polarization separation element will be described. Nevertheless, the optical pickup device of this embodiment is basically the same as the optical pickup device described with reference to FIGS. Since the configuration is common, corresponding elements will be described with the same reference numerals.
[0030]
[overall structure]
FIG. 1 and FIG. 2 are respectively an explanatory diagram showing the main part of an optical disk drive device to which the present invention is applied, and an explanatory diagram showing the arrangement of the optical system.
[0031]
As shown in FIGS. 1 and 2, the optical disk drive device includes a motor 6 that rotationally drives an optical recording medium 5 and an optical pickup device 1A that reads information from the optical recording disk. In the optical pickup device 1A, as an optical system, a polarizing diffractive element 21 (hologram element / polarized light separating element) is provided on the optical path from the laser light source 13 formed of a laser diode to the recording surface 51 of the optical recording medium 5. The phase difference plate 12, the raising mirror 17, and the objective lens 16 are arranged in this order. A photodetector 14 made of a photodiode is disposed in the vicinity of the laser light source 13. For the objective lens 16, a lens actuator 7 that drives the objective lens 16 in the tracking direction and the focusing direction is formed as usual, and the lens actuator 7 and the optical system move in the radial direction of the optical recording medium 5. It is configured on a possible frame 8.
[0032]
3A and 3B are a perspective view and a plan view showing the internal structure of the light source unit used in the optical pickup device shown in FIG. 2, respectively.
[0033]
As shown in FIGS. 3A and 3B, the laser light source 13 is formed as the light source unit 20 on the same substrate as the photodetector 14. In the light source unit 20, a plurality of lead frames 22 protrude from the package 23, and a laser light source 13 made of a laser diode and a laser beam emitted from the laser light source 13 are raised to 90 ° on a substrate in the package 23. A mirror 24 and a photodetector 14 formed of a split type photodiode formed on the side of the laser light source 13 are formed.
[0034]
In FIG. 2, the diffractive element 21 has a diffraction grating 210 formed on one surface thereof. This diffraction grating 210 diffracts only ordinary light and allows extraordinary light to pass through. As the diffractive element 21, various elements described later can be used, but lithium niobate subjected to proton exchange can also be used.
[0035]
Since this type of diffractive element 21 is well known, a detailed description is omitted, but a diffraction grating 210 is formed on the surface of a lithium niobate crystal plate which is a birefringent crystal substrate. , Proton ion exchange regions are formed in a lattice shape with a certain width and depth on the surface of a lithium niobate crystal plate which is a birefringent crystal plate. A non-proton ion exchange region where proton ions are not exchanged remains between adjacent proton ion exchange regions. On the surface of this aprotic ion exchange region, a dielectric film of a certain thickness, such as SiO₂The surface of the proton ion exchange region is exposed as it is. Here, on the surface of the lithium niobate crystal plate, proton ion exchange regions and non-proton ion exchange regions are alternately (periodically) formed. In the proton ion exchange region of the crystal plate, the refractive index ne for extraordinary light is increased by about 0.11, and the refractive index no for ordinary light is decreased by about 0.04.
[0036]
Here, in order to prevent the extraordinary light from being diffracted, a dielectric film having a predetermined thickness is formed on the surface of the aprotic ion exchange region, and the extraordinary light is exchanged between the proton ion exchange region and the aprotic ion exchange region. The phase difference generated when passing through the region is canceled out. The phase of ordinary light advances when it passes through the proton ion exchange region. However, the phase is relatively delayed when passing through the aprotic ion exchange region, and the phase is further delayed by the dielectric film formed on the surface thereof. Therefore, when ordinary light passes through the diffractive element 21, a phase difference is generated and it undergoes a diffracting action. On the other hand, the extraordinary light component travels straight through without being diffracted because it undergoes the same phase change when passing through any region.
[0037]
In the optical pickup device 1A configured in this way, basically, after the laser light is emitted from the laser light source 13, the linearly polarized light that has passed through the diffractive element 21 is circularly polarized by the quarter wavelength phase difference plate 12. In addition to conversion into light, the return light (circularly polarized light) from the optical recording medium 5 is converted into linearly polarized light having a polarization azimuth that is 90 ° different from that of the linearly polarized light emitted from the laser light source 13 by the quarter-wave retardation plate 12. Thus, the diffractive element 21 guides the return light to the photodetector 14 installed in a direction different from the laser light source 13.
[0038]
[Configuration of retardation plate]
Such a principle is established without problems when the optical recording medium 5 is not birefringent. However, as described above, the optical recording medium 5 used in the optical pickup device 1A is manufactured by injection molding. Since the resin generally flows in the radial direction from the inner peripheral side, the optical recording medium 5 has a birefringence in the radial direction as indicated by an arrow D in FIG. Therefore, in the present invention, in order to prevent a decrease in detection sensitivity due to the birefringence, the following two measures are taken.
[0039]
FIG. 4 shows the relationship between the amount of birefringence of the optical recording medium and the intensity of the signal detected by the photodetector when the phase difference amount and direction of the retardation plate are changed in the optical pickup device shown in FIG. It is a graph. FIG. 5 shows the relationship between the amount of birefringence of the optical recording medium and the intensity of the signal detected by the photodetector, showing the effect when the first countermeasure described later is taken in the optical pickup device shown in FIG. It is a graph. FIG. 6 shows the birefringence amount of the optical recording medium and the signal detected by the photodetector showing the effect when the first countermeasure and the second countermeasure described later are taken in the optical pickup apparatus shown in FIG. It is a graph which shows the relationship with an intensity | strength.
[0040]
First, the first countermeasure is an attitude designed so that the optical recording medium 5 has a radial birefringence even if it is unclear which direction the birefringence direction of the optical recording medium 5 is actually. The light source unit 20 is disposed. That is, the light source unit 20 is arranged so that the polarization direction of the laser light emitted from the optical laser light source 13 is 45 ° with respect to the radial direction of the optical recording medium 5. Further, the diffractive element 21 is arranged in a direction corresponding to the light receiving position of the photodetector 14 arranged on the light source unit 20. Next, regarding the retardation plate 12, when the birefringence amount of the optical recording medium 5 is changed from 0 to ¼ wavelength (the birefringence amount is determined to exist in the range of 0 to ¼ wavelength). ), The phase difference Φ and the azimuth θ of the phase difference plate 12 are set so that the peak of the signal intensity detected by the detector 14 appears in the meantime.
[0041]
In setting the phase difference Φ and the azimuth θ of the phase difference plate 12 under such conditions, each sample shown below
Sample 1 Phase difference of phase difference plate 12 Φ = 90 °, orientation θ = 45 °
Sample 2 Phase difference of phase difference plate 12 = 98.4 °, orientation θ = 45 °
Sample 3 Phase difference of phase difference plate 12 Φ = 120 °, direction θ = 45 °
Sample 4 Phase difference of retardation plate 12 Φ = 90 °, orientation θ = 55.4 °
Sample 5 Phase difference of phase difference plate 12 Φ = 90 °, orientation θ = 70 °
Sample 6 Phase difference of phase difference plate 12 Φ = 98.4 °, orientation θ = 55.4 °
The signal intensity was measured using the phase difference plate 12.
[0042]
FIG. 4 shows the result of measuring the signal intensity when these samples were used to change the birefringence of the optical recording medium 5 from 0 to 3/4 wavelength. Here, each data of each sample 1-6 is shown by the lines L1-L6 in FIG. 4, respectively.
[0043]
As is clear from the results shown in FIG. 4, when the phase difference Φ and the azimuth θ of the phase difference plate 12 are changed, the relationship between the amount of birefringence of the optical recording medium 5 and the signal intensity changes.
[0044]
However, as shown by the lines L2 and L3 in FIG. 4, the characteristics of the samples 2 and 3 are based on the sample 1 corresponding to the conventional example (the characteristics are represented by the line L1 in FIG. 4) as a reference. In the case where only the phase difference Φ of the phase difference plate 12 is changed from 90 ° to 98.4 ° and 120 ° while the azimuth θ of the light is set to 45 °, the birefringence amount of the optical recording medium 5 is 0 to 1. It can be seen that the peak of the signal intensity does not appear while changing to / 4 wavelength.
[0045]
Further, as shown by the line L5 in FIG. 4, the characteristic of the sample 5 is indicated by a line L5, and the orientation θ of the phase difference plate 12 is set while the phase difference Φ of the phase difference plate 12 is set to 90 ° with reference to the sample 1 as a conventional example. Is changed to 70 °, a signal intensity peak appears while the birefringence amount of the optical recording medium 5 changes from 0 to ¼ wavelength, but the signal intensity decreases.
[0046]
On the other hand, as shown by the line L4 in FIG. 4, the characteristic of the sample 4 is based on the sample 1 as a conventional example, while the phase difference Φ of the phase difference plate 12 is set to 90 °. Is set to 55.4 °, a peak of signal intensity appears while the amount of birefringence of the optical recording medium 5 changes from 0 to ¼ wavelength, and a high signal intensity is obtained. can get.
[0047]
Further, as shown by the line L6 in FIG. 4, the characteristic of the sample 6 is set with the orientation θ of the phase difference plate 12 set to 55.4 ° with respect to the sample 1 as a conventional example, and the phase difference of the phase difference plate 12 is set. When the phase difference Φ is set to 98.4 °, the peak of the signal intensity appears while the birefringence amount of the optical recording medium 5 changes from 0 to ¼ wavelength, and the signal intensity is also high. Moreover, in the sample 6, the birefringence, which is expected to be most likely for the optical recording medium 5, shows a signal intensity peak at the 1/8 wavelength position. Therefore, when the optical pickup device 1B is configured so as to correspond to the sample 6, FIG. 5 compares the data of the sample 1 which is the conventional example and the data of the sample 6 which is the optimum embodiment of the present invention. As shown, a peak appears in the signal intensity between the birefringence of the optical recording medium 5 ranging from 0 to ½ wavelength, so that the birefringence of the optical recording medium 5 ranges from 0 to ½ wavelength. It is possible to prevent the occurrence of errors due to the birefringence of the optical recording medium 5 within a condition range narrower than the above range.
[0048]
As described above, the peak of the signal intensity appears while the birefringence amount of the optical recording medium 5 changes from 0 to ¼ wavelength, and the range where the signal intensity is high is the range of the retardation plate 12. This is a range in which the azimuth θ is set in a range of 50 ° to 60 °, and the phase difference of the retardation plate is set in a range of about 20 ° centering on 90 °.
[0049]
However, even in the optical pickup device 1A configured as described above, as shown in FIG. 4, the signal intensity is changed while the birefringence of the optical recording medium 5 changes from ½ wavelength to ¾ wavelength. There is a condition that becomes zero. In general, the birefringence of the optical recording medium 5 is considered to be in the range from 0 to ¼ wavelength, but the birefringence of the optical recording medium 5 is from ½ wavelength to ¾ wavelength. In order to be able to cope even if it is between, the following measures should be taken.
[0050]
The second countermeasure in the present invention is to use the diffractive element 21 used as a polarization separation element for s-polarized light (first linearly polarized light) and p-polarized light (second straight line) whose directions of polarization are orthogonal to each other. In other words, the s-polarized light and the p-polarized light are separated from the returning light including the polarized light toward the photodetector 14 at a certain ratio.
[0051]
That is, in the diffractive element 21, when the transmittance and diffraction rate for s-polarized light are Ts and Rs, the ratio between the transmittance Ts and the diffraction rate Rs for s-polarized light is 0: 1. The ratio of the transmittance Ts to the polarized light and the refractive index Rs is set to 0.5: 0.5. Conversely, when the transmittance and the diffraction rate for p-polarized light are Tp and Rp, the ratio of the transmittance Tp to the p-polarized light and the diffraction rate Rp was 1: 0. The ratio between the transmittance Tp and the diffraction rate Rp is 0.5: 0.5.
[0052]
For example, when a polarization beam splitter 11 composed of a cubic prism is used as the polarization separation element of the optical pickup device as shown in FIGS. 10 and 11, the transmittance and reflectance for s-polarized light are expressed as Ts and Rs. In this case, the ratio between the transmittance Ts and the reflectance Rs for s-polarized light is set to 0.5: 0.5. In other words, when the transmittance and reflectance for p-polarized light are Tp and Rp, the ratio of the transmittance Tp and reflectance Rp for p-polarized light is 0.5: 0.5.
[0053]
The ratio between the transmittance T and the reflectance R is preferably equal to 0.5: 0.5, but even if it is not equal, a certain ratio according to the necessity of design, that is, It is preferable that both sides ensure approximately 0.3 or more.
[0054]
Therefore, in the optical pickup device 1A of this embodiment, as indicated by a solid line L7 in FIG. 6, due to the birefringence of the optical recording medium 5, the return light incident on the diffractive element 14 is converted into s-polarized light and p-polarized light. Both of the lights are included, and a part of the polarized light is incident on the photodetector 14. Accordingly, when all of the polarized light included in the return light is, for example, s-polarized light, the signal intensity level is suppressed to some extent, but the return due to the birefringence of the optical recording medium 5 is reduced. Even when all the polarized light contained in the light is p-polarized light, the signal can be detected with a certain intensity. Here, the optical pickup device 1A has no problem if the detected signal intensity is high to some extent when detecting the return light.
[0055]
Therefore, in this embodiment, it is possible to detect a signal at a somewhat high level even under a condition where the signal intensity is zero in the conventional case. Therefore, due to the fact that the optical recording medium 5 has birefringence, even if the return light contains only p-polarized light, it is more than a certain level due to the partial polarization property of the diffractive element 21. Can always be detected. Therefore, signal detection can be reliably performed no matter what birefringence the optical recording medium 5 has.
[0056]
[Other embodiments]
In the above-described embodiment, the diffractive element 21 as the polarization separation element and the retardation plate 12 are separately used. However, as shown in FIG. 7A, the diffractive element 21 or FIG. A composite optical element 25 in which a polarization separation element such as a cubic prism-shaped polarization beam splitter 11 shown in FIG. 11 is integrated with the phase difference plate 12 may be used. With this configuration, the diffraction element 21 or the cubic prism-shaped polarization beam splitter 11 and the phase difference plate 12 may be handled as the composite optical element 25. Therefore, the assembly work of the optical pickup device is highly efficient, and the diffraction type. In forming such a composite optical element 25 capable of improving the efficiency of adjusting the birefringence direction of the element 21 and the orientation of the retardation plate 12 and reducing the size of the component, There is a method in which the polarization separation element and the phase difference plate 12 are formed separately and then bonded with an adhesive.
[0057]
As will be described below, if the retardation plate 12 is formed on the diffractive element 21 or the polarizing beam splitter 11 in the form of a cubic prism to form the composite optical element 25, the assembly work of the optical pickup device can be increased. In addition to improving efficiency, adjusting the birefringence direction of the diffractive element 21 and the orientation of the retardation plate 12, it is possible to reduce the cost of components in addition to reducing the size of the components.
[0058]
In order to manufacture such a composite optical element 25, for example, as shown in FIGS. 7B and 7C, the opposite side to the side where the diffraction grating 210 is formed on both sides of the diffractive element 21. A dielectric film 121 is obliquely deposited on the side surface 21 b, and the retardation film 12 is formed by the dielectric film 121. In this case, inorganic oxides such as tantalum pentoxide, tungsten oxide, bismuth trioxide, and titanium oxide are obliquely deposited from the direction of arrow A that forms a predetermined angle with respect to the normal direction H of the diffractive element 21. The birefringent film 121 made of a dielectric film formed in this way is adjusted, for example, by setting the angle formed by the normal direction H and the crystal axis of the birefringent film to, for example, 70 ° and adjusting the film thickness. The obliquely vapor-deposited birefringent film 121 performs an optical function as the quarter-wave retardation plate 21. Further, by adjusting the angle formed by the normal direction H and the crystal axis of the birefringent film and the film thickness, various retardation plates 21 can be formed without being limited to the quarter-wave plate. In this case, the phase difference amount can be controlled by the deposited film thickness, and the direction can be controlled by the deposition direction.
[0059]
In order to form the retardation film 12 with respect to the diffractive element 21, a polydiacetylene derivative film may be used instead of the oblique deposition of the dielectric film. In this case, for example, as shown in FIG. 8A, for example, a PET film (polyethylene terephthalate film) 125 or a polyimide film is applied to the back surface side of the diffractive element 21 so as to have a predetermined thickness. Then, a rubbing treatment is performed with a fiber such as polyester. Next, a polydiacetylene derivative film 122 having birefringence is formed on the surface of the PET film 125 by vacuum deposition. Here, as can be seen from FIG. 8A, the polydiacetylene derivative film 122 is aligned in the XY plane according to the rubbing direction with respect to the PET film 125, and the main chain direction (alignment direction) is indicated by an arrow YH. As shown, it is in the Y-axis direction. The polydiacetylene derivative film 122 thus formed has birefringence, and the refractive index (ne) in the alignment direction YH is different from the refractive index (no) in the direction YI perpendicular to the alignment direction YH. Here, the film thickness d of the polydiacetylene derivative film 5 is, for example, the following formula:
2πΔnd / λ = π / 2
If it determines so that it may satisfy | fill, the quarter wavelength phase difference plate 12 can be formed.
[0060]
In the above equation, λ and Δn are the wavelength of light incident on the polydiacetylene derivative film 5 and the refractive index difference (ne-no) between extraordinary light and ordinary light, respectively. The extraordinary light is polarized light that vibrates in the alignment direction YH of the polydiacetylene derivative film 5, and the ordinary light is polarized light that vibrates in the direction YI perpendicular to the alignment direction YH of the polydiacetylene derivative film 5.
[0061]
In such a polydiacetylene derivative film 122, as shown in FIG. 8 (B), the orientation of the polydiacetylene derivative film 5 with respect to linearly polarized light traveling while oscillating in the Y′-axis direction in the Y′-Z ′ plane. When the direction YH is tilted in the X′-Y ′ plane so as to be in the direction of 45 degrees with respect to the Y ′ axis, the linearly polarized light incident on the phase difference plate 12 has a component of the alignment direction YH and the alignment direction YH. A phase difference of ¼ wavelength occurs between the components perpendicular to the light and is emitted as circularly polarized light. When this circularly polarized light is reflected by the optical recording medium and passes through the polydiacetylene derivative film 122 again, it is emitted as linearly polarized light having a 90-degree vibration direction different from that of the first incident linearly polarized light. That is, the retardation plate 12 of this example functions as a quarter wavelength plate. Various retardation films can be formed by adjusting the film thickness. In the retardation plate 12 formed in this way, the orientation can be controlled by the rubbing direction, and the phase amount can be controlled by the film thickness.
[0062]
Furthermore, a polydiacetylene derivative film may be used in forming the diffractive element 15. In this case, first, as shown in FIG. 9A, for example, a PET film (polyethylene terephthalate film) 212 or a polyimide film is applied to the back surface side of the retardation film 12 so as to have a predetermined thickness. A rubbing treatment is performed with a fiber such as polyester. Next, as shown in FIG. 9B, a polydiacetylene derivative film 213 is formed by vacuum-depositing a diacetylene monomer on the surface of the PET film 212 and then polymerizing it by irradiating ultraviolet rays. At this time, the polydiacetylene derivative film 213 is spontaneously oriented along the rubbing direction of the PET film 212. Next, as illustrated in FIG. 9C, a light shielding mask 214 is disposed on the surface of the polydiacetylene derivative film 213. The light shielding mask 214 is formed of a material having an ultraviolet blocking property such as chromium, and a fine periodic grating pattern is formed as the ultraviolet transmitting portion 214a. After the light-shielding mask 214 is disposed, high-intensity ultraviolet rays 215 are irradiated from the surface side. As a result, the irradiated light is transmitted through the ultraviolet transmissive portion 214 a of the light shielding mask 214, so that the surface of the polydiacetylene derivative film 213 is exposed to a region corresponding to the fine periodic lattice pattern formed on the light shielding mask 214. The Here, although the polydiacetylene derivative film 213 is colored, when the ultraviolet ray 215 is irradiated, the irradiated portion is decomposed and transparentized. Further, the region irradiated with the ultraviolet ray 215 loses birefringence and becomes isotropic having a refractive index substantially equal to the ordinary refractive index in the colored portion. Therefore, if the film thickness of the transparent part is equal to the film thickness of the colored part, a completely polarizing diffraction grating is obtained.
[0063]
Here, when the ultraviolet irradiation amount is adjusted, the transparent portion (region irradiated with ultraviolet rays) contracts and becomes thinner than the colored portion (region not irradiated with ultraviolet rays). As a result, as shown in FIG. 9D, a periodic grating is formed by the convex portions 213c and the concave portions 213b. Therefore, even when normal light is incident, diffraction occurs due to the level difference on the surface, so that the diffractive element 21 having partial polarization properties is obtained instead of perfect polarization characteristics.
[0064]
The diffractive element 21 formed in this way can be made completely polarizable or partially polarized by adjusting the amount of ultraviolet irradiation. Also, when changing the diffraction characteristics for ordinary light, it can be dealt with by changing process conditions. Furthermore, since a very fine diffraction grating can be formed only by changing the exposure pattern without using a process such as etching, the lattice element 21 is inexpensive and has a high degree of freedom in designing the layout of the optical system. Can be manufactured.
[0065]
In the above embodiment, the optical pickup apparatus 1A using the diffractive element 21 as the polarization separation element has been described as an example. However, as shown in FIGS. 9 and 10, the cubic prism-shaped polarization beam splitter 11 is polarized and separated. The present invention may be applied to the optical pickup device 1B used as an element.
[0066]
【The invention's effect】
As described above, in the optical pickup device according to the present invention, the birefringence amount of the optical recording medium is from 0 without necessarily assuming that the orientation of the retardation plate is aligned with the birefringence direction of the optical recording medium. The phase difference and azimuth of the phase difference plate are set so that the peak of the signal intensity at the photodetector appears while changing to ¼ wavelength.
[0067]
Further, the optical recording medium is irradiated through a phase difference plate having an orientation of 50 degrees to 60 degrees with respect to the laser light emitted from the laser light source.
[0068]
Further, the polarization direction of the light emitted from the laser light source is arranged so as to face 45 ° with respect to the radial direction of the optical recording medium, and the direction of the retardation plate is set to about 45 degrees with respect to the direction of 45 degrees. It is made to shift in the range of 5 to 15 degrees.
[0069]
Therefore, in the present invention, as long as the birefringence amount of the optical recording medium is between 0 and ¼ wavelength, the direction of the retardation plate is not oriented in the birefringence direction of the optical recording medium. Since the return light from the recording medium can be detected with high intensity, an optical pickup device with a high effective light amount can be configured. Further, the light source can be arranged in an optimum state from the viewpoint of the design, regardless of which direction the birefringence of the optical recording medium is actually directed. Therefore, even in an optical pickup device in which a light source and a detector are integrated, the birefringence of the optical recording medium can be absorbed and stable signal detection can be performed.
[Brief description of the drawings]
FIG. 1 is an explanatory view schematically showing a planar arrangement of the main part of an optical disc drive apparatus using a diffractive element as a polarization separation element of an optical pickup device.
2 is an explanatory view schematically showing an arrangement of an optical system of an optical pickup device used in the optical disk drive device shown in FIG. 1. FIG.
3A and 3B are a perspective view showing an external appearance of a light source unit used in the optical pickup device shown in FIG. 2 and a plan view showing an internal structure thereof, respectively.
4 shows the relationship between the amount of birefringence of the optical recording medium and the intensity of the signal detected by the photodetector when the phase difference amount and orientation of the phase difference plate are changed in the optical pickup device shown in FIG. It is a graph.
5 is a graph showing the relationship between the amount of birefringence of an optical recording medium and the intensity of a signal detected by a photodetector, which show the effect when the first countermeasure is taken in the optical pickup device shown in FIG. is there.
6 is a graph showing the relationship between the amount of birefringence of the optical recording medium and the intensity of the signal detected by the photodetector, which shows the effect when the first countermeasure and the second countermeasure are taken in the optical pickup device shown in FIG. 2; It is a graph which shows a relationship.
FIGS. 7A, 7B, and 7C are explanatory views showing a main part of an optical system of an optical pickup device using a composite optical element in which a retardation plate and a diffractive element are integrated, respectively; It is explanatory drawing which shows the structure of this composite optical element, and explanatory drawing which shows a mode that a dielectric film is diagonally vapor-deposited in order to manufacture this composite optical element.
FIGS. 8A and 8B are explanatory diagrams showing the configuration of a composite optical element in which a retardation plate using a polydiacetylene inducer film and a diffractive element are integrated, and the retardation plate of FIG. It is explanatory drawing which shows a principle.
9 (A), (B), (C), and (D) are all composite optical systems in which a partially polarizing diffractive element using a polydiacetylene derivative film is formed on a retardation plate. It is process sectional drawing for manufacturing an element.
FIG. 10 is an explanatory diagram schematically showing a planar arrangement of the main part of an optical disc drive device using a cubic prism-shaped polarization beam splitter as a polarization separation element of an optical pickup device.
11 is an explanatory diagram schematically showing the arrangement of the optical system of the optical pickup device used in the optical disk drive device shown in FIG.
12A is a development view of an optical system configured in an optical pickup device, and FIGS. 12B to 12D are diagrams schematically showing a reciprocating birefringence amount and a polarization state of light respectively possessed by the optical recording medium itself. It is explanatory drawing shown.
FIG. 13 is a graph showing the relationship between the position of the optical recording medium in the radial direction and the intensity of the signal detected by the photodetector from the position in the conventional optical pickup device.
[Explanation of symbols]
1A, 1B Optical pickup device
5 Optical recording media
11 Polarizing beam splitter Polarization separating element)
12 Phase difference plate
13 Laser light source
14 Photodetector
16 Objective lens
20 Light source unit
21 Diffraction element (polarized light separation element)
25 Compound optical elements
121 Birefringent film deposited obliquely
122,213 Polydiacetylene attractant membrane

Claims (13)

A phase difference plate that converts the polarization state of the light emitted from the laser light source and the return light from the optical recording medium; In an optical pickup device having a polarization separation element that separates the return light from above and guides it to a photodetector,
When the relationship between the birefringence amount of the optical recording medium and the signal intensity detected by the detector is measured, the signal intensity peak appears while the birefringence amount changes from 0 to ¼ wavelength. as, Ri Na set the direction of the phase difference and anisotropically axis of the retardation plate,
The polarization separation element has a certain ratio of the first and second linearly polarized light from the return light including the first linearly polarized light and the second linearly polarized light whose polarization directions are orthogonal to each other. An optical pickup device having partial polarization that emits toward the photodetector. A phase difference plate that converts the polarization state of the light emitted from the laser light source and the return light from the optical recording medium, and an optical axis that travels from the laser light source to the optical recording medium based on the polarization state of the emitted light and the return light In an optical pickup device having a polarization separation element that separates the return light from above and guides it to a photodetector,
The retardation plate is set to have an orientation of 50 degrees to 60 degrees with respect to the laser light emitted from the laser light source,
The polarization separation element directs the s-polarized light and p-polarized light from the return light including linearly polarized light of s-polarized light and p-polarized light whose directions of polarization are orthogonal to each other to the photodetector at a certain ratio. An optical pickup device having a partial polarization property to be separated. In Claim 2, when the transmittance and diffraction rate for the s-polarized light are Ts and Rs, respectively, both the transmittance Ts and the diffraction rate Rs for the s-polarized light are approximately 0.3 or more, and An optical pickup device characterized in that both the transmittance Tp and the diffraction rate Rp for the p-polarized light are about 0.3 or more when the transmittance and the diffraction rate for the p-polarized light are Tp and Rp, respectively. The ratio between the transmittance Ts and the diffraction rate Rs for the s-polarized light is approximately 0.5: 0.5, and the ratio between the transmittance Tp and the diffraction rate Rp for the p-polarized light is approximately An optical pickup device having a ratio of 0.5: 0.5. 5. The optical pickup device according to claim 2, wherein a phase difference of the retardation plate is set within a range of about 20 degrees centering on 90 degrees. A phase difference plate that converts the polarization state of the light emitted from the laser light source and the return light from the optical recording medium, and an optical axis that travels from the laser light source to the optical recording medium based on the polarization state of the emitted light and the return light In an optical pickup device having a polarization separation element that separates the return light from above and guides it to a photodetector,
The polarization direction of the light emitted from the laser light source is arranged to be 45 degrees with respect to the radial direction of the optical recording medium, and the direction of the retardation plate is about 5 degrees to about 15 degrees with respect to 45 degrees. Within the range of
The polarization separation element has a partial polarization property that separates the first linearly polarized light and the second linearly polarized light, whose polarization directions are orthogonal to each other, toward the photodetector at a certain ratio. An optical pickup device. 7. The optical pickup device according to claim 6, wherein the phase difference of the phase difference plate is set to approximately 90 degrees, and the orientation of the phase difference plate is set within a range of 50 degrees to 60 degrees. 8. The optical pickup device according to claim 7, wherein the direction of the retardation plate is set to about 55 degrees. In claim 6, the retardation of the retardation plate is set within a range of about 20 degrees around 90 degrees, and the orientation of the retardation plate is set within a range of 50 degrees to 60 degrees. A characteristic optical pickup device. The orientation of the retardation plate according to claim 9 is set to about 55 degrees. An optical pickup device characterized by the above. 11. The optical pickup device according to claim 1, wherein the polarization separation element is a diffractive element, and a diffraction grating of the diffractive element is formed of a polydiacetylene derivative film. 12. The optical pickup device according to claim 1, wherein the retardation plate is formed of any one of a dielectric film and a polydiacetylene derivative film deposited obliquely. 13. The polarization separation element according to claim 1, wherein the polarization separation element is formed integrally with the phase difference plate, and the laser light source and the photodetector are integrated as a light source unit. Optical pickup device.

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