JP3831090B2 - Optical device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for switching an effective numerical aperture of an optical system, particularly an optical pickup configured for different numerical apertures for DVDs and CD-ROMs in an optical pickup of a recent optical disc apparatus. It belongs to the technology that can be shared by two optical pickups.
[0002]
[Prior art]
In order to facilitate understanding of the prior art, the numerical aperture of the optical system will be briefly described. In an optical system designed with almost no aberration in terms of geometric optics, a point image is formed with an infinitely small spot, but in reality, the spot has a finite extent due to the influence of diffraction due to the wave nature of light. At this time, if the numerical aperture of the optical system that contributes to image formation or condensing is NA, the physical definition of the spread of the spot is expressed by k × λ ÷ NA. Here, λ is the wavelength of light, and k is a constant determined by the optical system, and usually takes a value of about 1 to 2. NA is proportional to the ratio D / f of the effective entrance pupil diameter D (generally the diameter of the effective light beam) of the optical system and the focal length f. The spread of the spot expressed by this equation becomes the theoretical resolution limit and is called the diffraction limit.
[0003]
As is clear from the above equation, the theoretical resolution of the optical system greatly depends on the numerical aperture. In general, in the case of an optical disk, the numerical aperture of a condensing (objective) lens of an optical pickup is about 0.45 for a CD or CD-ROM, and about 0.55 for a DVD (digital versatile disk). It is about 1.2 mm for CD and about 0.6 mm for DVD. The condensing lens of the optical pickup that needs to condense to the diffraction limit is designed taking into consideration the thickness of the optical disk substrate. Therefore, the optical pickup cannot be shared between CD or CD-ROM and DVD.
[0004]
Therefore, in order to solve this problem, a method in which two pickups are installed in one device, a method in which a hologram is engraved in a condensing lens of an optical pickup to make two focal points, or an effective entrance pupil using a liquid crystal shutter A method of switching the diameter is used.
[0005]
Next, FIG. 5 shows a conventional example closest to the present invention. This is premised on application to an optical disk. This will be described with reference to the drawings.
[0006]
The linearly polarized light 503 emitted from the linearly polarized laser light source 501 and converted into a plane wave by the collimator lens 502 has a Y axis direction in which the polarization axis 504 is parallel to the paper surface. The direction of the polarization axis 504 of the linearly polarized light 503 is rotated by 90 degrees by the 90-degree TN (twisted nematic) type liquid crystal element 505 and becomes the X-axis direction. The linearly polarized light 503 is condensed by the condensing optical system 506. At this time, it is assumed that a polarizing plate 507 whose center is cut out in a round shape is installed in front of the condensing optical system 506, and its linearly polarized light transmission axis is in the Y-axis direction. At this time, due to the optical shutter function of the 90-degree TN liquid crystal element 505 combined with the polarizing plate 507, only the linearly polarized light transmitted through the central portion of the polarizing plate contributes to the light collection. It is used in this state for CD playback.
[0007]
On the other hand, in reproducing a DVD, an electric field in the Z direction is applied to the 90 ° TN liquid crystal element 505 to bring it into a homeotropic state to be described later. In this state, the liquid crystal element loses optical rotation, so that the linearly polarized light 503 can pass through the polarizing plate and has a larger numerical aperture than the previous state. Even in this state, the amount of light is lost due to the light absorption action of the polarizing plate. In addition, since the central part of the polarizing plate is cut out, the phase difference of the light caused by the refractive index difference between the polarizing plate and the air is generated between the linearly polarized light that has passed through the polarizing plate and the linearly polarized light that is not, and the light is condensed to the diffraction limit. It becomes difficult to do. For this reason, if the same type of polarizing plate having the linearly polarized light transmission axis in the X-axis direction is installed in the central portion that is hollowed out, the problem of phase difference is solved, but the amount of light is further lost.
[0008]
The condensing spot 508 condensed by the condensing optical system 506 is reflected by the optical disk 509 and returns almost the same optical path as the incident light, and the light beam 511 separated by the light separating element 510 is condensed by another condensing optical system 512. The focused spot 513 is detected by the light detecting element 514.
[0009]
[Problems to be solved by the invention]
However, installing two pickups in one device complicates the device configuration and leads to an increase in cost. Further, when a hologram is engraved on the condensing lens to make two focal points, either one of the unnecessary condensing spots is always generated, so that the light utilization efficiency is lowered. This is a problem in a device that requires a large amount of light such as a DVD-RAM, that is, a rewritable DVD. Similarly, the same problem occurs in a method using a liquid crystal shutter.
[0010]
[Means for Solving the Problems]
Therefore, in the present invention, the linearly polarized light and the condensing optical system for condensing the linearly polarized light, the reflecting member for reflecting the condensing spot by the condensing optical system, the light separating element for separating the reflected light from the incident light path, and the separation by the light separating element In an optical device comprising a light detection element for detecting light, a 90-degree optical rotation optical element is installed in the incident optical path, and a linearly polarized wave detection element is installed in the separation optical path.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment according to the invention is shown in FIG. The linearly polarized light 103 emitted from the linearly polarized laser light source 101 and converted into a plane wave by the collimator lens 102 is in the Y-axis direction whose polarization axis 104 is parallel to the paper surface. After the linearly polarized light 103 is transmitted through the optical rotatory optical element 105, the direction of the polarization axis 104 is rotated by the optical rotatory function of the optical rotatory optical element 105. The optical rotatory optical element 105 is composed of a portion 106 (circular region indicated by oblique lines) that rotates incident linearly polarized light 90 degrees in the X-axis direction and a portion 107 that rotates 0 degrees.
[0012]
The linearly polarized light 108 transmitted through the portion 106 that rotates 90 degrees and rotated 90 degrees is incident on a substantially circular region 112 (indicated by oblique lines) centered on the optical axis 111 of the condensing optical system 110. This circular area 112 is a part of the effective light beam 113 incident on the condensing optical system 110, and it can be seen that it is smaller than the numerical aperture formed by the effective light beam 113. Here, the aperture by the effective light beam 113 is set for DVD, and the aperture by the circular region 112 is set for CD. In FIG. 1, the diaphragm or the like that restricts the light beam incident on the condensing optical system 110 is not used, so that the effective light beam 113 coincides with the light beam of the linearly polarized light 103 transmitted through the optical rotation optical element 105.
[0013]
The linearly polarized light 109 transmitted through the portion 107 that rotates at 0 ° and rotated at 0 ° is incident on the area other than the circular region 112. It can be seen that the polarization axes of the linearly polarized light in the circular region 112 and the region other than the circular region 112 are orthogonal to each other.
[0014]
The condensing spot 114 reflected by the optical disk 115 returns almost through the same optical path as the incident optical path, passes through the condensing optical system 110, and is separated by the light separation element 116. If the optical disk 115 does not have strong birefringence or polarization dependency of diffraction, the incident polarization state is preserved. In general, the optical disk 115 has a birefringence of 20 nm or less and practically hardly causes polarization dependence due to diffraction.
[0015]
The separated light beam 117 is condensed again by another condensing optical system 118, and the condensing spot 119 is detected by the light detection element 120. If the linearly polarized light detecting element 121 is installed in the separated light beam 117 with its direction (direction through which the linearly polarized light is transmitted) 90 degrees (X axis direction), it is reflected by the optical disk 115 of the linearly polarized light 109 rotated by 0 degrees. Components are shielded from light. This is because the polarization axis of the linearly polarized light 109 rotated by 0 degrees is orthogonal to the direction of the linearly polarized light detecting element 121. This state can be used for reproduction of a CD or CD-ROM. That is, the linearly polarized light 109 rotated by 0 degree is a portion having a large numerical aperture, which is a light beam passing through the outer peripheral portion of the condensing optical system 110. Originally, it is a part of the portion used for DVD, and when used for CD or the like, it has a different thickness from the disc substrate for DVD, and thus becomes a reflected light beam with large aberration. This reflected light beam breaks the shape of the focused spot 119, and becomes signal noise when the light detection element 120 obtains a symmetrical signal of the focused spot 119. However, there is no problem if the light beam component that has passed through the outer periphery of the condensing optical system 110 is shielded in front of the light detection element 120. Obviously, in order to use this optical apparatus for DVD, the portion 106 that rotates at 0 degree may be switched to 90 degrees. That is, all the components of the effective light beam 113 are used in the separated light beam 117 to form a focused spot 119.
[0016]
【Example】
FIG. 2 shows an embodiment of the present invention. This is an example for confirming the principle, and the scale is different from that of a generally used optical pickup optical system. Basically, it is the same as the embodiment shown in FIG. 1, but a 90 degree twisted nematic type liquid crystal is used as the optical rotation optical element. First, in order to facilitate understanding of the present embodiment, an optical rotation effect using a 90-degree twisted nematic liquid crystal will be described.
[0017]
FIGS. 3A and 3B schematically show the optical rotation function of a general 90-degree twisted nematic liquid crystal element that can be electrically controlled. Liquid crystal molecules 302 are sandwiched between glass substrates 301 coated with transparent electrodes. The glass substrate on the incident side has the alignment axis direction 303 in the Y-axis direction, and the glass substrate on the output side has the alignment axis direction 303 in the Y-axis direction and the lower half in the X-axis direction. The liquid crystal molecules 302 are arranged in parallel on the upper half side as shown in FIG. 3A due to the property of aligning the major axis direction with the alignment axis direction and the property of acting as a continuum, and this is called homogeneous. In the lower half, the liquid crystal molecules 302 rotate 90 degrees gradually and smoothly. This is called 90 degree twisted nematic.
[0018]
When the incident linearly polarized light 304 enters the liquid crystal element, the polarization axis of the incident linearly polarized light 304 propagates along the major axis direction of the liquid crystal molecules 302 due to the dielectric anisotropy of the liquid crystal molecules. That is, the polarization axes of the outgoing linearly polarized light 305 are perpendicular to each other with the upper half in the Y direction and the lower half in the X direction. When the refractive index in the major axis direction of the liquid crystal molecules is n1, the refractive index in the minor axis direction is n2, and the thickness of the liquid crystal layer is d, the optical path length of the incident linearly polarized light 304 traveling in the liquid crystal layer is expressed by n1 × d both above and below. You can see that
[0019]
Strictly speaking, in order for the incident linearly polarized light 304 to be emitted as strictly linearly polarized light, the direction of the polarization axis of the incident linearly polarized light 304 coincides with the direction of the alignment axis 303 on the incident side, that is, the liquid crystal molecule major axis, and 2 × ( It is known that n1−n2) × d ÷ λ needs to be any square root of 3, 15, 35, and the like. Here, λ is the wavelength of incident light. However, even if the wavelength of the light used, the refractive index of the liquid crystal molecules, and the thickness of the liquid crystal layer do not strictly observe the above formula, there is no inconvenience. At this time, the direction of the polarization axis of the incident linearly polarized light 304 may be intentionally shifted somewhat from the orientation axis direction 303 on the incident side.
[0020]
When an electric field in the Z-axis direction is applied to the liquid crystal element through the transparent electrode coated on the glass substrate, the major axis of the liquid crystal molecules 302 is aligned in the Z-axis direction, which is the direction of the electric field, as shown in FIG. Quiesce. This state is called homeotropic. At this time, the outgoing linearly polarized light 305 does not change and is in the same Y-axis direction as the incident linearly polarized light 304. That is, the optical rotation is lost. At this time, it can also be seen that the optical path length of the incident linearly polarized light 304 traveling in the liquid crystal layer is n2 × d.
[0021]
An embodiment will be described with reference to FIG. 1 is basically the same as the embodiment shown in FIG. 1, but a 90-degree twist nematic type liquid crystal element 205 is used as an optical rotatory optical element, and the basic structure and operation of the liquid crystal element 205 are shown in FIG. Same as (b) but does not use homogeneous orientation. The direction of the alignment axis of the liquid crystal element 205 on the side on which the linearly polarized light 203 is incident is substantially the same as the direction of the polarization axis 204 and is the Y-axis direction.
[0022]
The linearly polarized light 203 emitted from the linearly polarized laser light source 201 and converted into a plane wave by the collimator lens 202 is incident on the liquid crystal element 205 with its polarization axis 204 in the Y-axis direction parallel to the paper surface. The liquid crystal element 205 is divided into a functional region into a homeotropic region 207 and a 90-degree twisted nematic region 206 by an electric signal. That is, as is apparent from the description of FIG. 3, a sufficient electric field is applied to the liquid crystal molecules in the homeotropic region 207 through the transparent electrode by an electric signal. The homeotropic region is a region outside a circle centered on the optical axis 211.
[0023]
The linearly polarized light 209 transmitted through the homeotropic region 207 of the liquid crystal element 205 and rotated by 0 degree enters the region other than the substantially circular region 212 centered on the optical axis 211 of the condensing optical system 210. At this time, the circular region 212 is a part of the effective light beam 213 incident on the condensing optical system 210, and it can be seen that it is smaller than the numerical aperture constituted by the effective light beam 213. Here, the opening by the effective light beam 213 is set for DVD, and the opening by the circular region 212 is set for CD. 2 does not use a diaphragm or the like that restricts the light beam incident on the condensing optical system 210, the effective light beam 213 matches the light beam of the linearly polarized light 203 that passes through the liquid crystal element 205. The effective light beam 213 that has passed through the condensing optical system 210 forms a condensing spot 214.
[0024]
The linearly polarized light 208 transmitted through the 90-degree twisted nematic area 206 and rotated by 90 degrees enters the circular area 212. It can be seen that the polarization axes of the linearly polarized light in the circular region 212 and the region other than the circular region 212 are orthogonal to each other.
[0025]
FIG. 4 shows the shape of the liquid crystal element actually used. A liquid crystal sealing region 401 having a square shape with an outer shape of approximately 15 mm and a diameter of 10 mm is provided at the center. A homeotropic region 402 is formed by an electrical signal from the electrode portion 405 in addition to a circular region having a diameter of 3 mm at the center of the liquid crystal sealing region 401, and a 90-degree twist nematic region 403 is not applied to the circular region. Will remain. The alignment axis direction 404 of the liquid crystal molecules on the light incident side is the Y-axis direction. Further, the optical axis Z is a direction that proceeds perpendicular to the paper surface. By removing the electric signal, the entire liquid crystal sealing region 401 can be returned to the 90-degree twisted nematic region. In the liquid crystal element having this configuration, it is not necessary to draw out the lead wire from the central circular region, so that it is not necessary to form a dead space for the lead wire from the circular region in the transparent electrode of the region 402 that is homeotropic. This liquid crystal element substantially satisfies the above-described 15 square root for light having a wavelength of 633 nm.
[0026]
In the liquid crystal element 205, the homeotropic region 207 has the same basic effect because the polarization axis does not rotate even if there is no liquid crystal layer. However, in this case, since the optical path length changes compared to a region where the liquid crystal layer is present, phase modulation of incident linearly polarized light occurs, and therefore there is a possibility that correction must be performed using the condensing optical system 210 or another optical system. The same effect can be obtained by homogenous or homeotropic alignment of the homeotropic region 207 from the beginning. However, a 90 degree twisted nematic alignment and a homeotropic alignment or a homogenous alignment can be used when manufacturing a liquid crystal element. This requires mask rubbing, that is, a relatively complicated alignment technique such as masking the other alignment region during one alignment.
[0027]
The condensing spot 214 returns almost the same optical path as the incident light by the optical disk 215 installed at substantially the same position on the optical axis 211, passes through the condensing optical system 210, and is separated by the light separation element 216. The separated light beam 217 is condensed by another condensing optical system 218 to form a condensing spot 219. The focused spot 219 is detected by the light detection element 220. By installing it in the optical path of the light beam 217 separated with the direction of the linearly polarized light detecting element 221 (the direction through which the linearly polarized light is transmitted) as the X-axis direction, only the component transmitted through the circular region 212 can be taken out and CD reproduction becomes possible. Become. Further, if the electrical signal in the homeotropic region 207 of the liquid crystal element 205 is removed to achieve 90 degree twisted nematic orientation (that is, the entire area of the liquid crystal element 205 becomes 90 degree twisted nematic), the entire component of the effective light beam 213 can be extracted, so Can be used.
[0028]
In an actual optical system, a polarizing plate is used as the linearly polarized wave detecting element 221, and the effective light beam 213 has a diameter of 5 mm. Alternatively, only the component penetrating the circular region 212 through the central portion of the polarizing plate may be passed through. Since the polarizing plate has light absorption, the light utilization rate is improved. However, a photodiode often used as the light detection element 220 has a relatively high sensitivity and is not a problem.
[0029]
The liquid crystal element 205 is used as an optical rotatory optical element and does not use a polarizing plate or the like in the incident optical path. In actual measurement, the light loss was about 15%, but it can be reduced to 10% or less by applying a non-reflective coating to the liquid crystal glass substrate.
[0030]
【The invention's effect】
As is apparent from the above description, when the optical device using the optical rotatory optical element and the linearly polarized light detecting element according to the present invention is applied to an optical disc apparatus, the light amount for writing (or reading) is not lost in principle. The numerical aperture can be easily switched electrically. This is very effective as an optical system for reproducing a CD in an optical system of a DVD-RAM that is expected to be promising in the future, that is, an optical system of a writable or rewritable digital versatile disk device. This is because increasing the light output of the semiconductor laser as the light source is a difficult problem.
[0031]
Further, the liquid crystal element in the present invention is not particularly large in cost because it is small in size and very simple in structure compared with a liquid crystal display panel for a personal computer having a complicated structure. In addition, it is possible to use an inexpensive polarizing plate used in a commercially available liquid crystal device. The present invention can obtain the same effect even in a transmission type optical disc.
[Brief description of the drawings]
FIG. 1 is a structural example of an optical device according to the present invention.
FIG. 2 is an embodiment of an optical device according to the present invention.
FIG. 3 is a diagram illustrating an optical rotation function of an electrically controllable twist nematic liquid crystal element.
4 is a diagram showing a structure of a liquid crystal element used in the embodiment of FIG.
FIG. 5 is a diagram showing a prior art closest to the present invention.
[Explanation of symbols]
101, 201, 501, linearly polarized laser light sources 102, 202, 502, collimate lenses 103, 203, 503, linearly polarized light 104, 204, 504, polarization axis 105, optical rotation optical element 205, liquid crystal element 505, 90 degree TN type Liquid crystal element 106, portions 206, 403, 90-degree twisted nematic region 107, 90-degree twisted nematic region 107, sites 207, 402 that rotate 0-degree, homeotropic regions 108, 208, linearly polarized light 109, 209, 90-degree rotated Linearly polarized light 110, 118, 210, 218, 506, 512, condensing optical system 507, polarizing plates 111, 211, optical axes 112, 212, circular regions 113, 213, effective light beams 114, 119, 214, 219, 508, 513, condensing spots 115, 215, 509, optical disks 116, 216, 10, light separation elements 117, 217, and 511, separated light beams 120, 220 and 514, light detection elements 121 and 221, linear polarization detection element 301, glass substrate 302 coated with a transparent electrode, liquid crystal molecules 303 and 404, Orientation axis direction 304, incident linearly polarized light 305, outgoing linearly polarized light 401, liquid crystal sealing region 405, electrode portion

Claims (2)

Linearly polarized laser light source and the converging optical system of the light flux optical rotation in optical rotatory element and revolving optical element for the optical rotation of 90 degrees is incident linearly polarized light emitted from the linearly polarized laser light source for focusing light and the condenser optical system A light reflecting member installed in the vicinity of the focal point; a light separating element that separates reflected light reflected by the light reflecting member from incident light; and a light detecting element that detects a separated light beam separated by the light separating element. wherein the optical rotatory element optical rotation is controlled by an electric signal, an optical device sites revolving light resistance is controlled to act in the area of a portion in the effective light beam of the linearly polarized light to be utilized by the converging optical system in established the linearly polarized light detection element in an optical path of the beam splitter and the light detecting element, the orientation of the linearly polarized light detection element is substantially perpendicular to the orientation of the linearly polarized light, and 90 degrees as the optical rotatory element Tsuisutonema Using Ikku type liquid crystal device, generally circular region sites around the optical axis in the effective light beam of the linearly polarized light to be utilized by the converging optical system in which optical rotation of the 90-degree twisted nematic liquid crystal element is controlled an area other than the alignment direction of liquid crystal molecules of the linearly polarized light incident side of the 90-degree twisted nematic liquid crystal element is substantially coincident or substantially orthogonal to the polarization axis direction of the linearly polarized light, the 90-degree twisted nematic liquid crystal site optical rotation of the device is controlled optical device characterized by comprising a homeotropic orientation by an electric signal. The 90-degree twisted nematic liquid crystal device has a refractive index in the major axis direction of liquid crystal molecules as n1, a refractive index in the minor axis direction as n2, a liquid crystal layer thickness as d, and the wavelength of the linearly polarized light as λ,
2 × (n1-n2) × d ÷ λ
The optical apparatus according to claim 1, wherein a value of is substantially equal to a square root of any one of 3, 15, and 35 .

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