JP2009176394A - Optical head device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical head device capable of reducing crosstalk during playback of a multilayer optical disk. <P>SOLUTION: In the optical head device wherein light outputted from a light source is reflected by an information recording layer of an optical disk and guided to an optical detector, in an optical path from the optical disk to the optical detector, a light reducing element, which is provided with a first region having a high transmittance, a second region having a low transmittance and a third region having a medium transmittance between the high transmittance and the low transmittance, is arranged, and return light which is form a layer different from the information recording layer and causes crosstalk is reduced by the optical detector. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  In the present invention, it is necessary to perform recording and reproduction on an optical recording medium (hereinafter referred to as “optical disk”) such as a CD, a DVD, a BD, and an HD-DVD, particularly a multilayer optical disk having a plurality of information recording layers. The present invention relates to an optical head device.

  Optical discs include single-layer optical discs having a single information recording layer and multi-layer optical discs having a plurality of layers. For example, when recording / reproducing information with respect to a two-layer optical disc having two recording layers, the return light reflected by the optical disc and returning to the photodetector is a desired information recording in which the light emitted from the light source is condensed. Not only the light reflected by the layer (hereinafter referred to as “signal light”) but also the light reflected by the adjacent information recording layer or the like (hereinafter referred to as “stray light”). An optical head device that performs recording / reproduction of a multi-layer optical disk needs to be configured so that the crosstalk component due to light reflected from such different recording layers does not affect the servo signal. In this specification, recording or reproduction with respect to an optical disc, or recording and reproduction is generically expressed as “recording and reproduction”.

  FIG. 15 shows a schematic diagram of an optical path during reproduction of a double-layer optical disc in a conventional optical head device that performs recording and reproduction of a multilayer optical disc. A layer close to the light incident surface of the two-layer optical disk is defined as an L1 layer, and a layer far from the light incident surface is defined as an L2 layer. For example, the light 204 reflected by the L2 layer as the 201 surface is positioned in front of the light 206 with respect to the light 206 received by the photodetector during reproduction with the L1 layer as the 202 surface. On the other hand, the light 205 reflected with the L1 layer as the 203 plane is positioned behind the light 206 with respect to the light 206 received by the photodetector during reproduction with the L2 layer as the 202 plane.

  At the time of reproduction of the L1 layer, the return light from the L1 layer (own layer) is condensed on the detection surface of the photodetector by 0th order transmitted light and ± 1st order diffracted light by diffraction of the diffraction element. Return light reflected from the L2 layer (other layers) with the L1 layer as a reference is irradiated as stray light on the detection surface of the photodetector, although the beam diameter is large and the light density is low, and the L1 layer (own layer) ) And interference on the photodetector. If the light interference condition changes due to a change in the distance between the information recording layers or the light source wavelength, the signal intensity changes, causing a problem of deterioration in reading performance. In particular, in an optical head device using the three-beam method, ± 1st-order diffracted light, which is a sub-beam of signal light, is smaller in amount of light than the main beam, and thus is more susceptible to stray light interference.

  As a countermeasure, for example, an optical head device as shown in Patent Document 1 has been proposed. This is because a hologram element 210 as shown in FIG. 16 is arranged in the light beam, and a diffraction grating is provided in the region 211 so as to diffract part of the return light from the optical disk, so that the ± first-order diffracted light that becomes a sub-beam becomes light. It removes stray light from the area irradiated to the detector.

JP 2005-203090 A

  In the configuration disclosed in Patent Document 1, the light transmitted through the region 112 without the diffraction grating of the hologram element 110 is guided to the photodetector with high transmittance. On the other hand, the light transmitted through the region 111 where the diffraction grating is located is diffracted (hereinafter referred to as “phase grating diffraction”), so that light in a region with low transmittance is guided to the photodetector. However, if the boundary between a high transmittance region and a low transmittance region is mixed in the light beam guided to the photodetector, light intensity modulation occurs in the light beam, and this intensity modulation causes the light to wrap around diffraction (hereinafter referred to as “intensity modulation diffraction”). Say). Due to this intensity-modulated diffraction, stray light from the other layers of the optical disk irradiates and irradiates the sub-beam photodetector, so that stray light cannot be effectively removed. For this reason, when the light from the own layer interferes with the light from the other layer on the photodetector, and the light interference condition changes due to the change in the distance between the information recording layers and the light source wavelength, the signal intensity changes and reads. There was a problem that performance deteriorated. As a countermeasure, if the area of the phase grating diffraction grating region is increased so that stray light intensity-diffracted and diffracted does not reach the photodetector, not only stray light from other layers but also light from its own layer from which information is originally read is generated by the hologram. There is a problem in that the intensity of the signal light entering the photodetector is also reduced due to phase grating diffraction by the element.

  The present invention has been made to solve the above-described problems of the prior art, and is capable of sufficiently removing stray light components on a photodetector and recording / reproducing a multi-layered optical disk without lowering the signal intensity. It is an object of the present invention to provide an optical head device capable of performing

The present invention includes a light source, an objective lens that condenses light emitted from the light source on an information recording surface of an optical disc, and a plurality of light receiving areas that detect return light that is collected and reflected by the information recording surface of the optical disc. An optical head device comprising: the optical detector having a light path; wherein the optical path of the return light from the optical disc to the optical detector is reduced or transmitted through a surface on which the return light is incident. A dimming element having a function of diffracting is disposed, and at least the effective area of the dimming element where the return light is incident is divided into three areas including a first area, a second area, and a third area. The outer edge of the second region is on the inner side not in contact with the outer edge of the third region, or on the inner side partially in contact with the outer edge of the third region, and the outer edge of the third region is In contact with the outer edge of the first region The ratio of the light that is inwardly transmitted through the return light incident on the dimming element and incident on the photodetector. when, T ₁ the transmittance of the returning light of the first _region, T ₁ when the transmittance of the returning light of the second region and T ₂ are greater than T _2, the third region The return light has a transmittance smaller than T ₁ and larger than T ₂ , and is reflected from the surface of the optical disc different from the information recording surface on which the light from the light source is collected and guided to the photodetector. Provided is an optical head device that reduces the amount of stray light that reaches at least a part of the light receiving area of the light detector by at least part of the stray light flux entering the second region of the dimming element. .

  With this configuration, the transmittance changes smoothly due to the presence of the third region from the first region to the second region in the plane of the dimming device on which light is incident. The influence of stray light wraparound due to intensity-modulated diffraction of transmitted light generated by the distribution can be suppressed. In particular, it is possible to provide an optical head device that can reduce stray light wraparound to a photodetector that receives a sub beam and can record and reproduce a multilayer optical disk with less interference between signal light and stray light.

Further, when the light attenuation element is T _{3 in} which the transmittance of the return light in the third region is uniform, the difference between T ₁ and T ₃ of the light attenuation element, and the light attenuation The optical head device according to the above, wherein the difference between T ₃ and T ₂ of the element is greater than 0% and 60% or less.

  With this configuration, the transmittance of incident light changes smoothly depending on the transmittance distribution of the three regions of the light reducing element, so that the influence of intensity modulation diffraction can be controlled. In particular, the stray light of the photodetector that receives the sub-beams Therefore, it is possible to provide an optical head device capable of efficiently reducing the influence of the wraparound and recording / reproducing the multilayer optical disc with less interference between the signal light and the stray light.

In addition, the third region is divided into _m regions R _{1 to} R _m (an integer of m ≧ 2), and the outer edge of the region R _m is inside not in contact with the outer edge of the first region. Or the outer edge of the region R _x-1 is on the inner side not in contact with the outer edge of the region R _x when _x is an integer between 2 and _m. , The outer edge of the region R _x-1 is partially in contact with the outer edge of the region R _x-1 , and the outer edge of the second region is not in contact with the outer edge of the region R ₁ , or the inner surface is partially in contact with the region R ₁ located, the region _{R 1,} region _{R 2,} ..., the transmittance of the returning light transmitted through or diffracts region _{R m} each _{T r1,} T r2, _..., when the _{T rm, T r1 <T r2} < The optical head device according to claim 1, wherein <T _rm is satisfied.

Further, the difference between T ₁ and T _rm of the dimming element, the difference between T _rx and T _rx−1 of the dimming element, and the difference between T _r1 and T ₂ of the dimming element are 0%. The optical head device according to the above, which is larger and 40% or less.

  With this configuration, the light transmittance distribution from the first region to the second region changes more smoothly, so that the intensity-modulated diffraction of the light incident on the dimming element can be further suppressed. In particular, it is possible to provide an optical head device capable of further reducing stray light sneaking into a photodetector that receives a sub-beam and capable of recording / reproducing a multilayer optical disk with less interference between signal light and stray light. With the configuration as described above, stray light can be controlled without increasing the area of the second region having low transmittance, so that recording / reproduction of a multi-layer optical disc can be performed without greatly reducing the signal intensity.

  In the above optical head device, the dimming element includes at least the second region and the third region including an optical multilayer film or a cholesteric phase liquid crystal layer that reduces the amount of incident return light. provide.

  With this configuration, it is possible to adjust the transmittance of incident light for each area of the dimming element, and further, a dimming element having a high degree of freedom by utilizing the characteristic that the transmittance varies depending on the wavelength of the incident light. The function can be realized.

  The optical head device according to the above, wherein the dimming element includes a diffraction grating structure in which at least the second region and the third region diffract the incident return light and reduce the light that passes straight through. provide.

  With this configuration, it is possible to change the diffraction grating structure for each region, and to control the transmittance of light that is transmitted in a straight line (hereinafter referred to as “0th-order transmitted light”). It is possible to reduce stray light wraparound. Further, since the efficiency of light that is transmitted in a straight line (hereinafter referred to as “0th-order transmittance”) can be changed depending on the wavelength of incident light, the wavelength of stray light can be selected and reduced.

  In addition, the dimming element has a diffraction grating structure that expresses diffracted light that diffracts the return light that is incident on at least the first region and the third region, and a photodetector for the optical path of the diffracted light The above-described optical head device for receiving light is provided.

  With this configuration, interference due to crosstalk can be reduced even in the optical system of the optical head device that guides the diffracted light to the photodetector, so that the degree of freedom in design is increased. In addition, when the diffracted light is guided to the photodetector, it is preferable that the light emitted from the second region does not reach the diffraction direction. Therefore, the cross-sectional shape of the second region may not be a diffraction grating structure.

  The diffraction grating structure of the dimming element is formed of a birefringent material having refractive index anisotropy, and the diffraction grating structure is substantially equal to the ordinary light refractive index or the extraordinary light refractive index of the birefringent material. The above-described optical head device is provided which is filled and flattened with an isotropic material having a refractive index.

  With this configuration, the dimming element is arranged in a common optical path from the light source of the optical head device to the optical disk (hereinafter referred to as “outward path”) and the optical path from the optical disk to the photodetector (hereinafter referred to as “return path”). Even so, the light in the forward path can be transmitted almost 100% and the light in the backward path can be diffracted and attenuated, so that the light in the forward path can be efficiently guided to the optical disc. Further, the degree of freedom of arrangement of the dimming element is increased.

  The dimming element includes a modulator and a polarizer that change at least a part of the polarization state of incident light in order of the traveling direction of the incident light, and the polarizer is a first polarized light. Light in a second polarization state orthogonal to the first polarization state is transmitted and light emitted from the first region is transmitted by the modulation element as light in the first polarization state. The light that passes through the polarizer and exits the second region becomes the second polarization state by the modulation element and does not pass through the polarizer, and the light that exits the third region is The optical head device according to the above, wherein the first polarization state and the second polarization state are mixed in the modulation element and only the light in the first polarization state is transmitted.

  With this configuration, light can be prevented from being emitted from the second region, so that the amount of stray light can be greatly reduced by setting the transmittance to approximately 0, and interference due to crosstalk can be greatly reduced. . In addition, light that becomes noise can be reduced by using a light absorbing polarizer. Although the modulation element will be described later, the polarization state may be changed by a wave plate. By using an optical rotator, the optical rotation angle is changed by the thickness, and the incident linearly polarized light is changed in different directions for each region. It may be converted into linearly polarized light and emitted.

  INDUSTRIAL APPLICABILITY The present invention can provide an optical head device that has the effect of sufficiently removing stray light components on a photodetector and recording and reproducing a multi-layer optical disc without greatly reducing the signal intensity. Is.

(First Embodiment)
FIG. 1 is a diagram showing a conceptual configuration of an optical head device 10 according to the present embodiment. The optical head device 10 includes a light source 11 that emits a light beam having a predetermined wavelength, a diffraction element 12 that diffracts a part of the light beam emitted from the light source 11 to generate three beams of a main beam and two sub beams, and an incident light. The collimator lens 14a that converts the emitted light beam into parallel light and the three beams emitted from the collimator lens 14a are transmitted in the direction of the optical disc 16, and the three beams reflected by the information recording surface 16a of the optical disc 16 are transmitted. A beam splitter 13 for deflecting and separating the return light to the photodetector 17; an objective lens 15 for condensing the three beams on the information recording surface 16a of the optical disc 16; and a photodetector for the return light of the three beams. A collimator lens 14b for condensing the light, a photodetector 17 for detecting the return light of the three beams, and a light reducing element 18a. Rui and a 18b.

  The dimming element of the present invention is disposed in a position where the forward path and the return path are the same optical path, or in a return path optical path where the forward path and the return path are different. In FIG. 1, the dimming element 18b is arranged in the optical path only for the return path, and the dimming element 18a is arranged in the optical path common to the forward path / return path. The dimming element is not limited to the configuration arranged in the two optical paths, and may be arranged only in one of the optical paths.

  In the photodetector 17, a read signal, a focus error signal, and a tracking error signal of information recorded on the information recording surface 16 a reproduced from the optical disc 16 are detected. The optical head device 10 controls the lens in a direction substantially perpendicular to the optical axis based on the focus servo (not shown) that controls the lens in the optical axis direction based on the focus error signal and the tracking error signal. A tracking servo (not shown).

  The light source 11 is configured by a semiconductor laser that emits a linearly polarized divergent light beam having a wavelength band of 650 nm, for example. The light source 11 used in the present invention is not limited to light in the 650 nm wavelength band, and may be, for example, light in the 400 nm wavelength band, light in the 780 nm wavelength band, or light in other wavelength bands. Here, the 400 nm wavelength band, the wavelength 650 nm wavelength band, and the 780 nm wavelength band are in the ranges of 385 nm to 430 nm, 630 nm to 690 nm, and 760 nm to 800 nm, respectively.

  The light source 11 may be configured to emit light beams of two or three types of wavelengths. As a light source having such a configuration, a so-called hybrid two-wavelength laser light source or three-wavelength laser light source in which two or three semiconductor laser chips are mounted on the same substrate, or two light sources emitting different wavelengths of light are used. Alternatively, a monolithic type two-wavelength laser light source or three-wavelength laser light source having three light emitting points may be used.

  FIG. 2A, FIG. 2B, FIG. 2C, and FIG. 2D are schematic plan views of the dimming elements 20a, 20b, 20c, and 20d in the first embodiment. The dimming element 20a includes a first area 21a including the outer frame of the dimming element, a third area 23a inside the outer edge of the first area 21a, and a second area inside the outer edge of the third area. Are divided into regions 22a. Here, the outer edge is an outermost boundary line constituting the region. The outer edge of the second region does not necessarily need to be inside the outer edge of the third region, and these outer edges may partially touch each other as shown in FIGS. 2 (b) and 2 (c). Further, for example, as shown in FIG. 2B, the outer edge of the second region 22b is in contact with two non-continuous outer edges of the third region 23b, and the third region is separated into two. The two are combined into a third region 23b, and the outer edge of the third region is uniquely determined. In FIG. 2C, even if the second region 22c and the third region 23c are in contact with two locations on the outer edge of the first region 21c, the outer edge is determined uniquely in the same manner. In the example as shown in FIG. 2D, the first region 21d is a combination of the two, and the outer edge of the first region is a part of the second region 22d and the third region 23d. It shall be uniquely determined as a thick line including some outer edges.

When the transmittance of light passing through the first region, the second region, and the third region is T ₁ , T _2, and T ₃ , respectively,
T ₁ > T ₃ > T ₂
Set to the relationship. In particular, it is preferable to set the difference between T ₁ and T ₂ to be large because stray light transmitted through the light reducing element can be suppressed. The transmittance of each region can be adjusted by using characteristics such as light absorption, reflection, and diffraction, or by combining these characteristics. As will be described later, the light received by the photodetector is not limited to light that passes straight through the light reducing element, but in the case of a light reducing element having a diffraction grating structure, for example, even if + 1st order diffracted light having different diffraction efficiency is received for each region Good. In this case, since the + 1st order diffraction efficiency corresponds to the above-described transmittance, hereinafter, in an optical system in which diffracted light is received by a photodetector, the diffraction efficiency is also included in the transmittance. Similarly, when the above-described 0th-order transmitted light is received by the photodetector, the 0th-order transmittance is also included in the transmittance.

If the transmittance in the plane of the dimming element changes smoothly like a Gaussian distribution from the first region to the third region and the direction of the second region, intensity modulation diffraction is suppressed, This is preferable because the S / N ratio due to signal light and stray light can be increased. In the first embodiment, the third region has a substantially uniform transmittance. However, it is more preferable that the third region has a continuous transmittance change such as a Gaussian distribution. Further, even if the transmittance of the third region is substantially uniform, intensity modulation diffraction can be suppressed if the transmittance approximates a Gaussian distribution. FIG. 3A shows a graph of transmittance change according to the configuration of the second embodiment. The X axis represents an arbitrary distance that goes straight to the second area with the boundary between the first area and the third area as the origin (X = 0), and the Y axis when T ₁ is normalized (= 1) Represents the transmittance distribution of the third region. Solid line is Gaussian distribution, dotted line is Gaussian approximate distribution of T ₃ / T ₁ when T ₂ / T ₁ = 0, and alternate long and short dash line is Gaussian approximation of T ₃ / T ₁ when T ₂ / T ₁ = 0.1 Distribution. This approximation is calculated by averaging the Gaussian distribution. When T ₂ / T ₁ is increased, stray light is prevented from being sufficiently attenuated, so at least 0.1 is set so that stray light reaching the photodetector is 10% or less as compared with the case where no dimming element is inserted. It was. With this configuration,
T ₂ / T ₁ ≦ 0.1
When
0.3 ≦ T ₃ / T ₁ ≦ 0.7
Since it can be approximated to a Gaussian distribution if it is designed in the range of
0.4 ≦ T ₃ / T ₁ ≦ 0.6
It is more preferable in the range.

For example, by designing so that T ₁ is of 80% or more, preferably it is possible to guide the signal light efficiently photodetector, and more preferably is 90% or more. In addition, since the second region removes stray light that reaches the photodetector, the amount of stray light can be attenuated to less than half by designing the T _{2 to} be 50% or less. In order to substantially block stray light, a design in which T ₂ is substantially 0% is preferable. However, if the difference between T ₁ and T ₃ and the difference between T ₃ and T ₂ are large, Since intensity-modulated diffraction of light increases at the interface, it is preferably 60% or less so that stray light does not wrap around the photodetector. Further, the transmittance T ₃ of the _third region is preferably designed between the transmittance T ₁ of the first region and the transmittance T ₂ of the second region, and is designed to be a substantially intermediate value. It is more preferable.

  Although the dimming element of this embodiment has been described with respect to a three-beam optical head device, it is naturally applicable to a one-beam optical head device. In any of the above methods, at least the effective region, which is the region where the signal light is incident on the dimming element, includes the first region. The effective area is an area having a light intensity of 10% or more with respect to the maximum light intensity of the incident signal light. In addition, as the proportion of the area of the first region in the effective region that is the region where the signal light is incident on the dimming element is larger, the light use efficiency is reduced without reducing the amount of the signal light collected on the photodetector. Get higher. Therefore, it is preferable that the first region occupies 70% or more of the effective region effective region. Further, the second region is required to be at least an area smaller than 30% with respect to the effective region so as not to greatly reduce the light utilization efficiency. Also, if the area of the second region with respect to the effective region is made too small, stray light may reach the position where the signal light is condensed without being reduced due to fluctuations in the optical axis and the S / N may be lowered. In consideration of this range, the area ratio to the effective region may be 1% or more.

  As described above, by providing the third region having the intermediate transmittance between the second region having the low transmittance and the first region having the high transmittance, the change in the transmittance at the region interface is reduced. Therefore, intensity-modulated diffraction of transmitted light caused by the transmittance distribution of the dimming element can be suppressed. As a result, the wraparound of stray light onto the photodetector that receives the sub-beam can be reduced, so that interference between the signal light and the stray light can be suppressed.

  Next, the arrangement of the dimming elements will be described. FIG. 4 shows a schematic cross-sectional view of the state of light when the dimming element 32 is disposed in the optical path between the collimating lens 31 and the photodetector 33. FIGS. 4A and 4B show the stray light that is not condensed on the photodetector, and FIG. 4C shows the condensing state of the signal light. The dimming element 32 has three separated second regions 32a and 32b. It is assumed that there is a third region (not shown) around each second region. As will be described later, 32a is a second region for a sub beam, and 32b is a second region for a main beam. The photodetector 33 has a light receiving area 33a for the sub beam and a light receiving area 33b for the main beam.

  First, stray light that is not focused on the photodetector 33 will be described. In FIG. 4A, the stray light 34 having a focal point behind the photodetector reaches the photodetector 33 without being largely collected in the optical path. At this time, the stray light beam 35 passing through the center of the second region 32a is indicated by a one-dot chain line. The light beam 35 is guided to the center of the light receiving area 33a. In addition, the stray light 36 having a focal point in front of the photodetector in FIG. 4B reaches the photodetector 33 with an increased width of the light. At this time, although the stray light beam 36 passing through the center of the second region 32a is indicated by a one-dot chain line, it is similarly guided to the center of the light receiving area 33a. When the second region 32b is provided for the main beam, the second region 32b may include the optical axis of the main beam so as to be focused on the light receiving area 33b for the main beam, and the second region 32b. More preferably, the optical axis is aligned with the center of the light receiving area 33b.

  In this way, stray light passing through the second region of the light reducing element is guided to the sub-beam light receiving area 33a, so that the stray light reaching the light receiving area is reduced and reaches, and further has a third region. Thus, stray light can be effectively reduced. The stray light is generated by the reflection of the main beam and the sub beam by the optical recording medium, but is not collected on the photodetector, and the sub beam stray light has a lower intensity than the amount of stray light of the main beam. The stray light can be generally considered as reflected light by the main beam. In addition, it is preferable that the light receiving area and the second region have a similar outer edge because the light use efficiency increases. FIG. 4 (c) shows the condensing state of the signal light. The sub beams 39a and 39b are condensed on the light receiving area 33a for the sub beam, and the main beam 38 is condensed on the light receiving area 33b for the main beam. Led.

(Second Embodiment)
FIG. 5A shows a schematic plan view of the light reducing element when the third region is further divided into a plurality of parts. The light attenuating element 40 shown in FIG. 5A is divided into a first region 41 having high transmittance, two second regions 42 and 44, and two third regions 43 and 45. The third regions 43 and 45 are further composed of three divided regions 43a, 43b and 43c and 45a, 45b and 45c, respectively. The number of divisions of the third region is not limited to 3, and may be 2 or 4 or more, and may have a distribution that continuously changes between the transmittances of the first region and the second region. . In this example, the light reducing element sets two second regions in accordance with two sub beams diffracted by the three beam method. In addition, the present embodiment is not limited to the region configuration in which the second region and the third region are concentrically distributed, and may have a shape including a polygon or an arbitrary curve, and the outer edge of each region is a different region. There may be a portion in contact with the outer edge of. The second embodiment is a configuration of a dimming element that acts on a sub-beam that is easily affected by crosstalk due to stray light. Also good.

The transmittance of the first region 41 is T ₁ , and the transmittance of the second regions 42 and 44 is T ₂ . Also, the transmittance of the third regions 43a and 45a is T _r1 , the transmittance of the regions 43b and 45b is T _r2 , and the transmittance of the regions 43c and 45c is T _r3 . At this time, the relationship between the respective transmittances is as _follows : T ₁ > T _r3 > T _r2 > T _r1 > T ₂
Is preferable because the transmittance increases stepwise from the region 2 toward the outer region, and the intensity-modulated diffraction of stray light at the region boundary can be suppressed. As described above, the suppression effect is further improved by further dividing the third region and designing so that the transmittance is finely changed stepwise or continuously.

Next, a method for setting the value of the transmittance difference between regions having different transmittances when the third region is divided into a plurality of regions will be described with reference to FIG. As an example, the dimming element 36 is divided into regions as shown in FIG. 3B, the third region 39 is divided into regions 39a and 39b, and the widths of these regions are equal to d. And FIG. 3B shows a graph of transmittance change when the third region is divided into two regions. The X axis represents an arbitrary distance heading straight from the boundary between the first region 37 and the region 39b to the boundary between the second region 38 and the region 39a with the origin (X = 0), and the Y axis represents T _1. Represents the transmittance distribution of the third region when is normalized (= 1). The solid line is the Gaussian distribution, the dotted line is the Gaussian approximate distribution of the normalized third region when T ₂ / T ₁ = 0, and the alternate long and short dash line is the normalized third region when T ₂ / T ₁ = 0.1 Represents the Gaussian approximate distribution of the region. This approximation is calculated by averaging the Gaussian distribution. With this configuration,
T ₂ / T ₁ ≦ 0.1
, The maximum value of the normalized transmittance difference between regions having different transmittances is 0.6 of (T _r2 −T _r1 ) / T ₁ . Therefore, it is preferable that the normalized transmittance difference between regions having different transmittances across one boundary is greater than 0 and less than or equal to 0.7, and more preferably greater than 0 and less than or equal to 0.6. Further, when the third region is divided into three or more regions so that the transmittance changes stepwise, the normalized transmittance difference can be made smaller than 0.6 as the number of divisions increases, and more Gaussian. Approach the distribution change.

Further, by making the difference in transmittance T ₁ −T _r3 , T _r3 −T _r2 , T _r2 −T _r1 , and T _r1 −T ₂ to be 40% or less, diffraction due to the difference in transmittance between regions can be further suppressed. preferable.

  Next, a specific configuration for operating the dimming element common to the first and second modes will be described. FIG. 6 is a schematic cross-sectional view of a light reducing element 50 in which each region is formed of an optical multilayer film having a light reflecting action. FIG. 6 is a schematic cross-sectional view cut along a straight line passing through the center points of the two second regions in the schematic plan view of FIG. 5A, and the following schematic cross-sectional views are also the same. In this case, the three divided regions 52a, 52b, and 52c constituting the second region 51 and the third region 52 are each composed of a multilayer film having different transmittances due to the reflection action in stages. As described above, the transmittance is designed such that each of the second regions has the lowest transmittance, and the region outside the second region has a higher transmittance. In other words, the second region has the highest reflectance, and the region outside the second region has a lower reflectance.

  The optical multilayer film can be composed of inorganic oxides such as Si, Ta, Nb, Ti, Ca, and Mg, fluorides, nitrides, or organic materials. Further, it is preferable to change the laminated structure such as the film thickness of these materials for each region because the reflectance can be changed. In order to set a light shielding region where the transmittance is approximately 0%, a metal such as Al or Cr or a Cr oxide may be used. Further, the multilayer film is not limited to the configuration of being laminated on the glass substrate 53, and may be a translucent material such as a plastic resin. In order to improve reliability, a protective film or the like may be stacked on the multilayer film. Furthermore, you may comprise with a single layer light shielding film like a coloring film.

  FIG. 7 is a schematic cross-sectional view of a light reducing element 60 in which each region is formed of a cholesteric phase liquid crystal having a light reflecting action. The cholesteric phase liquid crystal molecules are continuously rotated by a helical axis parallel to the thickness direction of the light reducing element. In this way, cholesteric phase polymer liquid crystals that are irradiated with ultraviolet rays and solidified in a spiral state are used. Is preferred.

The light reflection effect of the cholesteric phase liquid crystal will be described. Cholesteric phase liquid crystal molecules have a spiraling property, and when two substrates subjected to uniform alignment treatment are injected into the opposed gap, they are uniformly spiraled in the thickness direction of the substrate. A cholesteric phase liquid crystal is the same as the twist direction of liquid crystal molecules in light incident parallel to the helical axis direction when the helical pitch P is about the same as the product of the wavelength λ of incident light and the refractive index n of the cholesteric liquid crystal phase. The circularly polarized light in the rotational direction is substantially reflected, and the circularly polarized light in the reverse rotational direction has a circular polarization dependency that is substantially transmitted. Central wavelength lambda _c of the wavelength band showing the reflection characteristic, the helical pitch P, and ordinary refractive index of the liquid crystal n _o, shown the extraordinary refractive index in relation to when the n _e (1) formula. Further, the reflection bandwidth Δλ is expressed by the relationship of the expression (2). Hereinafter, (λ _c ± Δλ) is defined as a reflection wavelength band.

When circularly polarized light having the same rotational direction as the liquid crystal molecules in the reflection wavelength band is incident, it is reflected in the cholesteric phase polymer liquid crystal layer. Further, when light having a wavelength different from the reflection wavelength band of (λ _c ± Δλ) is incident, it has a characteristic of transmitting even circularly polarized light having the same rotational direction as the liquid crystal molecules.

  In the dimming element 60 of FIG. 7, the three divided regions 62a, 62b, and 62c constituting the second region 61 and the third region 62 are each composed of cholesteric phase polymer liquid crystals having different transmittances due to reflection effects. The At this time, the spiral direction and the spiral pitch P of the liquid crystal molecules in any region are the same, but the thickness of each region is different. In this case, since the reflectivity increases as the thickness increases, the third region 62 also has the regions 62c, 62b, and 62a, respectively, so as to increase the thickness in the order of the third region 62 and the second region, respectively. The transmittance distribution is designed so that the thickness increases in the order of. It is preferable that the dimming element 60 is sandwiched between the opposing glass substrates 63 and 64 because the reliability is improved. In addition, it is preferable that the space corresponding to the first region is filled with a translucent material because the transmittance is increased.

  When the light attenuating element 60 using the cholesteric phase liquid crystal is used as the light attenuating element 18a of the optical head device 10 in FIG. 1, the light emitted from the light source 11 converts the polarization of the forward light toward the optical disk 16 into circularly polarized light. A quarter wavelength plate (not shown) is disposed in the optical path between the light reducing element 18 a and the beam splitter 13. The cholesteric phase liquid crystal is disposed so as to exhibit high transmittance in all regions with respect to the forward circularly polarized light. By doing so, the return light reflected by the optical disk 16 becomes circularly polarized light that rotates in the opposite direction to the forward path, and this light changes its light amount depending on the transmittance (reflectance) that varies depending on each region of the light reducing element 18a. Through. Therefore, it is possible to realize a dimming element that has high transmittance with respect to the forward light and has different reflectivity (difference in transmittance) depending on the region with respect to the light of the backward path, and the forward / return paths are arranged in a common optical path. However, it is preferable because the outgoing light can be efficiently guided to the optical disk.

  Also, for example, in an optical head device that uses light of two types of wavelengths, one for a single-layer optical disk and one for a multilayer optical disk, the reflection wavelength band of the cholesteric phase liquid crystal of the dimming element is set to include the wavelength for the multilayer optical disk To do. Furthermore, by transmitting almost 100% of light having a wavelength for a single-layer optical disk that is less affected by crosstalk, it becomes a wavelength-selective dimming element, and an optical head device having a high degree of freedom can be configured.

  Next, the configuration of the light-attenuating element as a region where the light-attenuating element is composed of a light diffraction effect will be described with reference to the schematic cross-sectional view of FIG. The three divided regions 72a, 72b, and 72c constituting the second region 71 and the third region 72 of the dimming element 70 in FIG. 8A are diffraction gratings having periodic irregularities formed on the surface of the region. It has a structure. Different diffraction characteristics due to the diffraction grating having the uneven period can be used in each region to change the zero-order transmittance of incident light. At this time, the 0th-order transmittance is designed to increase in the order of the second region, the third region, and the first region as described above.

  The zero-order transmittance of light incident on the diffraction grating structure in each region can be adjusted by changing the depth of the diffraction grating structure unevenness formed on the surface of each region, or by changing the refractive index of the grating material of the unevenness. . Further, the ratio of the width of the convex portion to the concave portion (duty ratio) of the lattice may be changed, or the change in transmittance may be realized by a combination of depth, material, and the like. In addition, the diffraction grating structure is not limited to a rectangular cross section, and may have a structure in which the zero-order transmittance is different due to a diffraction action such as a saw blade shape.

When the grating is formed by photolithography, the duty ratio can be realized by changing the opening width of the grating of the photomask for each region, which is preferable because it can be realized at low cost. Also, when changing the duty ratio in a method of changing the grating depth or changing the material of the grating, such as when producing a narrow pitch grating, the line width becomes very narrow and difficult due to process limitations. Is also preferable. FIG. 8B shows an enlarged schematic view of the cross section of FIG. Each region is formed of a material which exhibits birefringence, the uneven structure of the surface is filled with a substantially equal material 73 to the ordinary refractive index of the birefringent material (n _o) or the extraordinary refractive index (n _e) It is preferable that it is planarized.

  When the light reducing element 70 is used as the light reducing element 18a of the optical head device 10 in FIG. 1, a quarter-wave plate that converts the polarized light of the light traveling from the light source 11 toward the optical disk 16 into circularly polarized light. (Not shown) is disposed in the optical path between the light reducing element 18 a and the objective lens 15. At this time, the dimming element 18a is arranged so as to exhibit high transmittance in all regions with respect to the linearly polarized light in the forward path. On the other hand, the return light reflected by the optical disk 16 passes through a quarter-wave plate (not shown) and becomes linearly polarized light that is orthogonal to the forward linearly polarized light. Transmits light in a straight line while changing the amount of light with different zero-order transmittance. Therefore, even if the forward / return paths are arranged on a common optical path, it is preferable because the forward light can be efficiently guided to the optical disc.

  Although the 0th-order transmittance has been described as the characteristic of the diffraction grating structure, a photodetector may be disposed in the optical path of diffracted light such as ± 1st order. In the case of an optical system in which the photodetector is arranged in accordance with the diffracted light of the dimension to be used, stray light can be reduced by receiving the diffraction efficiency with a similar light amount distribution for each region. In addition, the dimming element that guides the diffracted light other than the 0th-order transmitted light to the photodetector can be arranged in an optical path that is common to the forward path and the return path so that the optical path can be different from the forward path. Since the function of a beam splitter is also included, it is preferable.

  The light dimming elements as described above are arranged in one or both of 18a and 18b of the optical head device 10, and the light guided to the photodetector 17 is shown as a schematic plan view in FIG. The light emitted from the light source 11 is converted into three lights by the diffraction element 12 as described above. Return light from the optical information recording layer 16 a of the optical disk 16 is guided to the photodetector 17 by the beam splitter 13. One main beam 84 and two sub beams 85 and 86 are led to the light receiving areas 81, 82 and 83, respectively, which are divided into the photodetector 17. These light receiving areas may be further divided as shown in FIG.

  On the other hand, light reflected by a layer (not shown) different from the information recording layer 16a is not focused on the layer, and therefore becomes stray light 87 having a large diameter in the photodetector 17. At this time, when the dimming element is not arranged in the optical path, the stray light 87 reaches the light receiving areas 81, 82, and 83, and overlaps with the signal light from the information recording layer 16a to interfere. Therefore, by using the dimming element of the present invention in the optical path, a region where stray light does not reach as shown in 88 and 89 can be formed, so that interference with signal light can be reduced. FIG. 9 is a schematic diagram when a geometric optical simulation is performed without considering intensity-modulated diffraction due to transmittance modulation in the light-reducing element.

  In this example, the light receiving areas 82 and 83 of the sub beam of the photodetector shown in FIG. 9 are arranged corresponding to the second and third areas of the light reducing element, respectively. Further, as described above, it is further preferable that the light receiving area 81 for the main beam is provided with regions corresponding to the second region and the third region in the light reducing element so that interference between the main beam and stray light can be reduced. . The region may have a shape that envelops a plurality of light receiving areas, or a shape similar to the shape of the light receiving areas.

  Thus, the shape of the second region and the third region is the light receiving area of FIG. 9 among the stray light flux that is the return light from the other layer of the multilayer optical disk that transmits the light reducing element 18a or 18b of FIG. 82 or 83, or in a region where a light beam reaching the light receiving areas 81, 82, and 83 is transmitted. In the case of a multilayer optical disc having four or more information recording layers, the other layer preferably corresponds to a layer adjacent to the own layer. This is because the light density of stray light from adjacent layers is particularly problematic as high crosstalk on the photodetector.

(Third embodiment)
FIG. 10 shows a third embodiment including a dimming element in which a modulation element and a polarizer are combined. As a method of dimming incident light, the polarization direction of incident light is modulated by a modulation element, and light components in a specific polarization state are blocked. The modulation element may be one that changes the polarization state of the emitted light with respect to the polarization state of the incident light by the polarizing plate, or may be one that rotates the polarization state of the incident light and emits it like an optical rotator. . Here, a mode in which a wave plate is used as the modulation element will be described. FIG. 10A is a schematic plan view of the polarizer 97, and the polarizer includes a transmission region 98 and a polarization blocking region 99. The polarization blocking region 99 has a function of preventing a specific component of incident light from being transmitted straight through reflection or diffraction. For example, linearly polarized light parallel to the XX ′ linear direction in the plane of the polarizer 97 in FIG. 10A is s-polarized light, and perpendicular to the XX ′ linear direction in the plane of the polarizer 97. Linearly polarized light is defined as p-polarized light, and the polarization blocking region 99 has a function of not transmitting only the p-polarized component. Of course, it may be a polarization blocking region corresponding to s-polarized light.

  Moreover, the shape of the area | region which comprises the wavelength plate 96 has the same shape as the plane schematic diagram of Fig.5 (a), attaches | subjects the same code | symbol to the same part, and avoids duplication description. FIG. 10B shows a cross-sectional view of the dimming element 94 when cut in the linear direction of XX ′. When the light reducing element 94 is configured by overlapping the wave plate 96 and the polarizer 97, stray light emitted from at least the second region 61 and the third region 62 of the wave plate 96 is a polarization blocking region of the polarizer 97. When viewed from the direction of the optical axis so as to be incident on the optical axis 99, it may be positioned within the polarization blocking region 99. Here, the polarization blocking region 99 is square, but the outer edge shape of the polarization blocking region is not limited as long as the light emitted from the second region and the third region is incident.

  The second region 61 and the third region 62 are made of a material exhibiting optically birefringence, and the retardation value of each region is adjusted by adjusting the thickness. By providing the retardation value in this way, the polarization state of light incident in a uniform polarization state can be changed for each region where light is emitted from the wave plate. For example, when linearly polarized light having 100% s-polarized light is incident on the dimming element 94, the light emitted from the first region of the wave plate is emitted as s-polarized light without changing the polarization state, and the second region 51 Is designed to have a retardation value of (2n + 1) λ / 2 with respect to the wavelength λ of the incident light (n ≧ 0). That is, light incident with 100% s-polarized light is emitted with a p-polarized component of almost 100%. The third region 62 is further divided into three divided regions 62a, 62b, and 62c, and is designed so that the ratio of the component of light emitted as s-polarized light increases stepwise in the order of 62a, 62b, and 62c.

  The light emitted from the respective regions of the wave plate enters the polarizer 97, blocks the p-polarized component at the polarization blocking region 99, and emits the s-polarized component. Since the light (s-polarized light) emitted from the dimming element 94 is emitted with different light intensity for each region, the influence of crosstalk between signal light and stray light can be reduced in the optical head device. In this case, the polarization blocking region 99 is arranged corresponding to the sub beam, but may be arranged also in the region including the main beam. When 100% s-polarized light is incident as in the above example, the polarization blocking region may be provided over the entire effective region where the light is incident, and the light emitted from the first region is greatly reduced by the polarizer. Since the dimming element 94 is emitted without being illuminated, the same effect can be obtained. The polarizer 97 may be one that transmits s-polarized light using a diffraction grating and diffracts p-polarized light in a direction different from the straight direction, and can be realized using liquid crystal as a birefringent material. Further, when the liquid crystal is sandwiched between transparent electrodes so that a voltage can be applied to the liquid crystal, for example, it can function as a polarizer when a non-voltage is applied, and can be switched to transmit when a voltage is applied. In this case, it is possible to increase the light utilization efficiency by applying a voltage to the liquid crystal at the time of recording / reproduction of a single-layer optical disc with little influence of crosstalk. In addition, the function of dimming can be similarly switched by applying a voltage to the wave plate as well as the polarizer.

  The dimming element 94 is not limited to the configuration in which the wave plate 96 and the polarizer 97 are overlapped and integrated, but they may be arranged apart from each other. For example, when the wave plate 96 is disposed immediately after the collimator lens 14b and the polarizer 97 is disposed immediately before the photodetector in the return optical path, the main beam of the signal light is condensed and transmitted through the polarizer 97. Since the light passes through 98, not only the s-polarized component but also the p-polarized component reaches the photodetector without being dimmed, so that the light utilization efficiency of the main beam increases.

  In addition, the operation in the case where the polarization state of the light incident on the dimming element 94 is linearly polarized light has been described, but the incident polarization state may be circularly polarized light or elliptically polarized light. The light may be incident on the polarizer in a form having a function of converting into a polarizer. In addition to the action of blocking a specific component of linearly polarized light, the polarizer may have a function of blocking circularly polarized light in a specific direction by using a cholesteric liquid crystal. The dimming element including the wave plate for changing the polarization state of the incident light is not an optical path common to the forward path / return path in the optical head apparatus, but the dimming element 18b serving as the optical path of the return path in the optical head apparatus of FIG. Place in position.

Example 1
With the configuration of the light reducing element 40 shown in FIG. 5A, the transmittance of each region at a wavelength of 405 nm is set as follows. In each region, a multilayer film of SiO ₂ and Ta ₂ O ₅ is laminated on a glass substrate by vacuum deposition, and the transmittance is adjusted by changing the total film thickness for each region. In particular, an antireflection film is laminated on the first region 41 that requires a high transmittance to achieve a transmittance close to about 100%. Further, in the regions 42 and 44 having a transmittance of about 0%, an Al film is laminated on the glass substrate by vacuum deposition. By the above method, the transmittance of the first region 41 is about 100%, the transmittance of the third regions (region R ₃ ) 43c and 45c is about 90%, and the third region (region R ₂ ) 43b and 45b. The transmittance of the third region (region R ₁ ) 43a and 45a is about 10%, the transmittance of the second regions 42 and 44 is about 0%, and the transmittance of each region changes. give.

Here, the effective diameter of the signal light incident on the polarizing element 40 is about 4 mmφ, the diameter of the second region is about 800 μmφ, and the widths of the divided regions R ₁ , R ₂ and R ₃ constituting the third region are each 75 μm. , 50 μm and 75 μm.

  FIG. 11 is a diagram showing the intensity of stray light of the main beam received by the photodetector 17 when the dimming element 40 is disposed on the optical head device 18a or 18b of FIG. The darker the position, the stronger the light. In FIG. 9, areas 88 and 89 correspond to the areas 101a and 101b in FIG. In this manner, stray light corresponding to the sub-beam optical receivers in the regions 101a and 101b can be sufficiently suppressed. Moreover, although the solid line of FIG. 14 is a figure which shows the intensity distribution of a stray light in the cross section which passes along the center of area | region 101a, 101b, it turns out that the stray light to a photodetector can be suppressed small from now on.

Here, the overlap of the signal light and the stray light that reaches the region serving as the light receiving area of the photodetector is evaluated using the following equation.
I = ∫I ₁ · I ₂ dS
I ₁ represents the intensity of the signal light, I ₂ represents the intensity of the stray light, and I is derived by integrating this product with the area. That is, as the value of I increases, the amount of signal light and stray light overlap to reach the light receiving area, and is more susceptible to interference. When the light receiving area of one sub-beam is evaluated, the value of I is 1.9% when the case where the dimming element 40 is not installed is 100%.

(Example 2)
In the same configuration of the dimming element 40 as in the first embodiment, the effective diameter of the signal light incident on the dimming element 40 is about 4 mmφ, the diameter of the second region is about 800 μmφ, and the divided region R ₁ constituting the third region. , R ₂ and R ₃ have widths of 495 μm, 330 μm and 495 μm, respectively. Other transmittance conditions are the same as those in the first embodiment.

  At this time, the overlap between the signal light and the stray light that reaches the region serving as the light receiving area of the photodetector is evaluated in the same manner as described above. As a result, the value of I is 1.4%.

(Example 3)
Similarly, in the configuration of the dimming element 40, the effective diameter of the signal light incident on the dimming element 40 is about 4 mmφ, the diameter of the second region is about 560 μmφ, and the divided regions R ₁ and R ₂ constituting the third region. And the width of R ₃ is 75 μm, 50 μm and 75 μm, respectively. The transmittance of the first region 41 is about 100%, the transmittance of the third regions (region R ₃ ) 43c and 45c is about 36%, and the third regions (region R2) 43b and 45b. The transmittance of the third region (region R ₁ ) 43a and 45a is about 4%, the transmittance of the second regions 42 and 44 is about 0%, and the transmittance of each region changes. give.

  At this time, the overlap between the signal light and the stray light that reaches the region serving as the light receiving area of the photodetector is evaluated in the same manner as described above. As a result, the value of I is 2.2%.

(Comparative example)
As shown in FIG. 12, a description will be given of a case in which a light reducing element 110 formed of a first region 111 having a transmittance of about 100% and second regions 112 and 113 having a transmittance of 0% is used. At this time, the third region having an annular shape in the embodiment is the same as the embodiment except that the width is divided into the second region and the first region, respectively. That is, the diameter of the second region is about 1 mmφ. As in the embodiment, the first region 111 having a transmittance of about 100% made of a multilayer film of SiO ₂ and Ta ₂ O ₅ is formed on a glass substrate, and the regions 112 and 113 having a transmittance of about 0% made of an Al film are formed on the glass substrate. Form.

  FIG. 13 is a diagram showing the intensity of the main beam stray light received by the photodetector 17 when the dimming element 110 is disposed in the optical head device 18a or 18b of FIG. The darker the color, the stronger the light. Regions 88 and 89 in FIG. 9 correspond to the regions 121a and 121b in FIG. Thus, it can be seen that stray light corresponding to the sub-beam optical receivers in the regions 121a and 121b wraps around the regions 121a and 121b by intensity modulation diffraction. 14 shows the intensity distribution of stray light in a cross section passing through the centers of the regions 121a and 121b. In particular, the broken lines show high intensity at the center of the regions 121a and 121b. This indicates that stray light cannot be sufficiently suppressed due to the influence of light wraparound due to intensity modulation between regions having different transmittances.

  Similar to the example, the value of I for evaluating the overlap of the signal light and the stray light is 8.7% when the dimming element 40 is not installed and is 100%. The stray light is not greatly reduced compared to the dimming element 40 provided with the region. As a result, the sub beam of the signal light and the stray light interfere with each other, causing a crosstalk that generates noise. In particular, in a detection system in which a photodetector is divided into a plurality of light receiving areas and a differential signal having a light quantity reaching each divided area is detected as an error signal, an error rate of a signal generated by increasing this I value. Therefore, the result of the example for the comparative example can be expected to greatly reduce the error rate.

  As described above, the optical head device according to the present invention has the stray light that is reflected by the multilayer optical disc in the light receiving area of the photodetector by arranging the dimming element in the optical path reflected from the multilayer optical disc to the photodetector. The amount of light can be reduced efficiently. Therefore, the influence of crosstalk with signal light can be reduced, which is useful.

Conceptual configuration diagram of optical head device Planar schematic diagram of the light-reducing element (first embodiment) Transmittance distribution diagram of the third region Schematic cross-sectional view of the optical path that passes through the dimmer Planar schematic diagram of the light reduction element (second embodiment) Cross-sectional schematic diagram of a light-reducing element formed by an optical multilayer film Cross-sectional schematic diagram of a light-reducing element formed of a cholesteric phase liquid crystal material Cross-sectional schematic diagram of a dimming element with diffractive action Photodetection schematic diagram of photodetector Planar schematic diagram of polarizer and cross-sectional schematic diagram of dimming element (third embodiment) Example of intensity distribution of stray light received by photodetector Schematic diagram of the dimming element (comparative example) Intensity distribution diagram of stray light received by photodetector (comparative example) Received light intensity distribution in the sub-beam light receiving area Schematic diagram of the optical path when playing a two-layer optical disc Schematic diagram of diffraction element (conventional example)

Explanation of symbols

DESCRIPTION OF SYMBOLS 10: Optical head apparatus 11: Light source 12: Diffraction element 13: Beam splitter 14a, 14b, 31: Collimator lens 15: Objective lens 16: Optical disk 16a: Information recording surface 17, 33: Photo detectors 18a, 18b, 20a, 20b 20c, 20d, 32, 40, 46, 50, 60, 70, 94, 110: Dimming elements 21a, 21b, 21c, 21d, 41, 47, 111: First regions 22a, 22b, 22c, 22d, 32a, 32b, 42, 44, 48, 51, 61, 71, 112, 113: second region 23a, 23b, 23c, 23d, 43, 45, 49, 52, 62, 72: third region 33a, 33b, 81, 82, 83: light receiving areas 34, 36: stray light outer diameter 35, 37: stray light ray trajectories 38, 84 passing through the center of the second region: main beam Signal light)
39a, 39b, 85, 86: sub beam (signal light)
43a, 45a, 49a, 52a, 62a, 72a: region R ₁
43b, 45b, 49b, 52b, 62b, 72b: region R ₂
43c, 45c, 52c, 62c, 72c: region R ₃
53, 63, 64, 74: Glass substrate 73: Isotropic material 87: Stray light 88, 89: Area where stray light does not reach 96: Wave plate 97: Polarizer 98: Transmission area 99: Polarization blocking area 100: Photo detector Of stray light intensity distribution (Example)
101a, 101b, 121a, 121b: sub-beam detection regions 102, 122: stray light outer diameter 120: stray light intensity distribution simulation diagram of photodetector (comparative example)
201, 202, 203: Optical disc layers 204, 205, 206: Light reflected from the optical disc 210: Hologram element 211: Diffraction grating 212: Area without diffraction grating

Claims (9)

Light detection having a light source, an objective lens for condensing the light emitted from the light source on the information recording surface of the optical disc, and a plurality of light receiving areas for detecting the return light that is collected and reflected by the information recording surface of the optical disc An optical head device comprising:
In the optical path of the return light from the optical disc to the photodetector, a dimming element having a function of reducing or transmitting or diffracting the amount of the return light is disposed in a plane on which the return light is incident,
The effective region where at least the return light of the dimming element is incident is divided into three regions including a first region, a second region, and a third region,
The outer edge of the second region is on the inner side not in contact with the outer edge of the third region, or is on the inner side partially in contact with the outer edge of the third region,
The outer edge of the third region is on the inner side not in contact with the outer edge of the first region, or on the inner side partially in contact with the outer edge of the first region,
When the ratio of the light that travels straight through the return light incident on the dimming element and is incident on the photodetector is the transmittance, the transmittance of the return light in the first region is T ₁ , the second T ₁ is greater than T ₂ when the transmittance of the returning light region and T _2,
The return light transmittance of the third region is less than T _{1 and} greater than T ₂ ;
At least a part of the stray light beam reflected from the surface of the optical disc different from the information recording surface on which the light from the light source is collected and guided to the photodetector is the second of the dimming element. An optical head device that reduces the amount of stray light that enters the region and reaches at least a part of the light receiving area of the photodetector. When the light attenuation element is T _{3 in} which the transmittance of the return light in the third region is uniform, the difference between T ₁ and T ₃ of the light attenuation element and the light attenuation element _2. The optical head device according to claim 1, wherein a difference between T ₃ and T ₂ is greater than 0% and equal to or less than 60%. The third region is divided into _m regions R _{1 to} R _m (an integer of m ≧ 2),
Outer edge of the region R _m is either on the inside which is not in contact with the outer edge of the first region or is inside the contact outer edge and a portion of said first region,
the outer edge of the region R _x-1 when the integer between the x of 2~m is or are inside which is not in contact with the outer edge of the region R _x, is inside the contact part and the outer edge of said region R _x-1,
The outer edge of the second region is on the inner side not in contact with the outer edge of the region R ₁ , or is on the inner side partially in contact with the region R ₁ ,
The region _{R 1,} region _{R 2,} ..., region _{R m} transmission or diffraction that the transmittance of the returning light respectively _{T r1, T r2, ...,} when the _{T rm, T r1 <T r2} <... <T The optical head device according to claim 1, which is _rm . The difference between T ₁ and T _rm of the dimming element, the difference between T _rx and T _rx−1 of the dimming element, and the difference between T _r1 and T ₂ of the dimming element is greater than 0%. 4. The optical head device according to claim 3, wherein the optical head device is 40% or less.   5. The light-attenuating element according to claim 1, wherein at least the second region and the third region include an optical multilayer film or a cholesteric phase liquid crystal layer that reduces the amount of incident return light. The optical head device described.   5. The light attenuation element according to claim 1, wherein at least the second region and the third region include a diffraction grating structure that diffracts the incident return light and reduces light that passes straight through. The optical head device described.   The dimming element has a diffraction grating structure that expresses diffracted light that diffracts the return light incident on at least the first region and the third region, and is received by a photodetector in the optical path of the diffracted light. The optical head device according to any one of claims 1 to 4. The diffraction grating structure of the dimming element is formed of a birefringent material having refractive index anisotropy,
8. The optical head device according to claim 7, wherein the diffraction grating structure is filled and flattened with an isotropic material having a refractive index substantially equal to an ordinary light refractive index or an extraordinary light refractive index of the birefringent material. The dimming element has a configuration in which a modulation element and a polarizer that change at least a part of the polarization state of incident light in order of the traveling direction of the incident light are arranged,
The polarizer transmits light in a first polarization state and blocks light in a second polarization state orthogonal to the first polarization state;
The light emitted from the first region becomes light in the first polarization state by the modulation element, passes through the polarizer, and the light emitted from the second region is second by the modulation element. The light that is polarized and does not transmit through the polarizer and exits the third region is mixed with the first polarization state and the second polarization state in the modulation element. The optical head device according to claim 1, wherein only the light in the state is transmitted.

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