JP2006252638A - Polarization diffraction element and optical head apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarization diffraction element capable of attaining a narrow pitch and high diffraction efficiency, easily performing positioning and simplifying a manufacturing process and to provide an optical head apparatus. <P>SOLUTION: The polarization diffraction element whose diffraction efficiency is different according to the polarization direction of incident light by spatial and periodical change of the refractive index of an optically anisotropic medium having double refraction is provided with substrates 101 and 102, alignment layers 104a, 104b and 105a provided on the substrates 101 and 102 and a liquid crystal layer 103 wherein the optically anisotropic medium is constituted of a layer-shaped liquid crystal or the like. The alignment direction of the liquid crystal or the like is periodically changed in a first surface of the liquid crystal layer 103 and the alignment direction of second surface of the liquid crystal layer or the like is uniformly the same direction. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a diffraction element that changes diffraction efficiency according to the polarization direction of incident light (hereinafter referred to as a polarization diffraction element), and recording on an optical recording medium such as an optical disk or reproduction from the optical recording medium. The present invention relates to an optical head device that performs (hereinafter referred to as recording / reproduction).

  In recent years, as a method of manufacturing a polarization diffractive element, a method has been proposed in which birefringent media are spatially and periodically arranged so that the refractive index changes spatially and periodically according to the polarization direction of incident light. Yes. The polarization diffraction element uses a low-molecular liquid crystal as a birefringent medium and can change the diffraction efficiency electrically (for example, see Patent Document 1), and the diffraction efficiency is changed. There are polarization diffractive elements that cannot be used (hereinafter referred to as fixed polarization diffractive elements).

  Here, these polarization diffraction elements are used in optical communication devices, display devices, measurement devices, etc., and in particular, by using a polarization diffraction element in an optical head device, the light utilization efficiency can be improved, or a plurality of different standards can be used. An optical disk can be recorded and reproduced by one optical head device.

  In the prior art disclosed in the above Patent Document 1, an example is shown in which the diffraction efficiency can be changed by changing the voltage applied to the variable polarization diffraction element. Here, as a first method for realizing the function of changing the diffraction efficiency of the variable polarization diffractive element, a low polarization liquid crystal having two alignment states of a homogeneous type and a 90 degree twisted nematic type is alternately changed into a variable polarization diffractive element. The light is diffracted by periodically arranging them, and a voltage is applied to the low-molecular liquid crystal in the variable polarization diffractive element to align the liquid crystal molecules in the electric field direction (thickness direction), thereby reducing or reducing the diffraction efficiency to zero. There is a method. Hereinafter, the variable polarization diffraction element manufactured by the first method is referred to as a variable polarization diffraction element of the first example.

  As a second method, a striped transparent electrode is formed on one glass substrate of a pair of glass substrates sandwiching a homogeneously aligned low-molecular liquid crystal, and a transparent electrode ( Hereinafter, the counter transparent electrode is formed), and a voltage is applied between the striped transparent electrode and the counter transparent electrode to change the refractive index of the low-molecular liquid crystal, thereby controlling the diffraction or transmission. There is a way. Since the diffraction element according to the second method is a phase modulation type diffraction grating, the diffraction efficiency is higher than that of the diffraction element according to the first method. Hereinafter, the variable polarization diffraction element manufactured by the second method is referred to as a variable polarization diffraction element of the second example.

  On the other hand, as the fixed polarization diffraction element, the diffraction efficiency is fixed using a polymer liquid crystal obtained by polymerizing liquid crystal molecules while preserving the alignment state and the arrangement of the liquid crystal according to the first method. A diffraction grating (hereinafter referred to as a fixed polarization diffraction element of the first example) can also be realized. Further, a uniform liquid crystal thin film that is homogeneously oriented is etched to produce irregularities, and the concave portions are filled with an isotropic medium having no birefringence, and a fixed polarization diffraction element (hereinafter referred to as the second example of fixed polarization diffraction). A method for manufacturing an element is also proposed. The fixed polarization diffraction element of the second example can provide a polarization diffraction element that is very excellent in optical characteristics.

JP 2001-33734 A

  However, in such a variable polarization diffraction element of the first example of the prior art, in a state where no voltage is applied, no light is applied to the region where the liquid crystal is twist-aligned, and the light which is incident to the homogeneously aligned region is The effective optical path length when transmitting light is almost the same, there is no phase difference due to different regions, and light is diffracted by different optical path lengths in each region depending only on the polarization direction of the transmitted light In addition, there is a problem that the diffraction efficiency is very low as compared with a general phase modulation type diffraction grating. In addition, there is a problem that the polarization state of the transmitted light is different from the polarization state of the incident light, and the application is very limited.

  In the variable polarization diffraction element of the second example, which has higher diffraction efficiency than the variable polarization diffraction element of the first example, when a voltage is applied between the stripe electrode and the counter transparent electrode, the stripe electrode There is a problem that the phase grating boundary is blurred and the grating pitch cannot be reduced. In addition, the problem of electric field leakage is slightly solved by forming the stripe electrodes on the two substrates so as to face each other, but it is very difficult to properly align the positions of the two stripe electrodes with a diffraction element having a small grating pitch. In effect, there was a lower limit to the lattice pitch.

  Further, the fixed polarization diffraction element of the first example using the polymer liquid crystal has the same problem as the variable polarization diffraction element of the first example. On the other hand, in the fixed polarization diffraction element of the second example, the above problem has been solved, but it is necessary to etch the polymer liquid crystal thin film and fill it with an isotropic medium, which complicates the manufacturing process. There was a problem.

  The present invention has been made to solve such problems, and can realize a narrower pitch and higher diffraction efficiency than the conventional polarization diffraction element, and can facilitate alignment and simplify the manufacturing process. A polarization diffraction element and an optical head device are provided.

  Considering the above points, the invention according to claim 1 diffracts according to the polarization direction of the incident light by spatially periodically changing the refractive index of the optically anisotropic medium having birefringence. In the polarization diffraction element having different efficiencies, the optically anisotropic medium includes a liquid crystal layer formed of a liquid crystal or a polymer liquid crystal obtained by polymerizing the liquid crystal, and the two liquid crystal layers facing each other. In the first surface of the two surfaces through which the light passes, the orientation direction of the molecules of the liquid crystal or the polymer liquid crystal changes periodically, and the two surfaces facing the liquid crystal layer Thus, the second surface of the two surfaces through which the light passes has a configuration in which the orientation directions of the molecules of the liquid crystal or the polymer liquid crystal are uniformly the same.

  With this configuration, the alignment direction is periodically changed on one surface of the liquid crystal layer, and the alignment direction is uniformly the same on the other surface, so that the optical path length is changed according to the polarization direction of the light. Therefore, it is possible to realize a polarization diffraction element that can realize a narrower pitch and higher diffraction efficiency than the conventional polarization diffraction element, and that can facilitate alignment and simplify the manufacturing process.

  According to a second aspect of the present invention, in the first aspect of the present invention, in the first surface of the liquid crystal layer, the orientation direction of the molecules of the liquid crystal or the polymer liquid crystal is perpendicular to the in-plane direction of the liquid crystal layer. And a region where the alignment direction of molecules of the liquid crystal or the polymer liquid crystal is horizontal to the in-plane direction of the liquid crystal layer is periodically arranged, and the liquid crystal or the liquid crystal The alignment direction of the molecules of the polymer liquid crystal is uniformly perpendicular to the in-plane direction of the liquid crystal layer, or uniformly horizontal to the in-plane direction of the liquid crystal layer.

  With this configuration, in addition to the effect of the first aspect, since the alignment direction is perpendicular or horizontal to the surface of the liquid crystal layer, a polarization diffraction element capable of further improving the diffraction efficiency can be realized.

  According to a third aspect of the present invention, in the first or second aspect, the first alignment film is aligned in the alignment direction on the first surface of the liquid crystal layer, and the alignment is performed on the second surface of the liquid crystal layer. A second alignment film that is aligned in a direction, a first substrate formed so that the first alignment film is in contact with the first surface of the liquid crystal layer, and the second alignment film is the liquid crystal layer. And a second substrate formed so as to be in contact with the second surface.

  According to this configuration, in addition to the effect of the first or second aspect, since the molecules of the liquid crystal layer are aligned using the alignment film, a polarization diffraction element in which the alignment direction can be easily controlled can be realized.

  The invention according to claim 4 is the invention according to claim 3, wherein the first substrate and the second substrate are opposed to each other, and are formed on each of the two surfaces through which the light passes. An electrode for applying a voltage between the liquid crystal layers is provided.

  With this configuration, in addition to the effect of the third aspect, since a voltage can be applied between the liquid crystal layers through the electrodes to change the orientation direction, a polarization diffraction element capable of electrically changing the diffraction efficiency is provided. realizable.

  The invention according to claim 5 is the optical anisotropy sandwiched between the third substrate and between the first substrate or the second substrate and the third substrate in claim 3 or 4. And an additional liquid crystal layer composed of a liquid crystal layer as a medium, and the alignment direction of molecules of the additional liquid crystal layer is uniform in the same direction.

  With this configuration, in addition to the effect of the third or fourth aspect, an additional liquid crystal layer having a uniform alignment direction and the same direction can be integrated as a new optical element. Therefore, a polarization diffraction element capable of reducing the number of parts is provided. realizable.

  The invention according to claim 6 detects a light source, an objective lens for condensing the light emitted from the light source onto an optical recording medium, and light collected by the objective lens and reflected by the optical recording medium. An optical head device comprising a photodetector, wherein the polarization diffraction element according to any one of claims 1 to 5 is disposed in an optical path between the objective lens and the light source. Have.

  With this configuration, an optical head device having the effect of any one of claims 1 to 5 and capable of improving light utilization efficiency can be realized.

  In the present invention, since the alignment direction periodically changes on one surface of the liquid crystal layer and the alignment direction is uniformly the same on the other surface, the optical path length is changed according to the polarization direction of light. Therefore, it is possible to realize an optical head device that can realize a narrower pitch and higher diffraction efficiency than the conventional polarization diffraction element, and can easily align and simplify the manufacturing process.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 is a cross-sectional view showing a conceptual configuration of a polarization diffraction element according to the first embodiment of the present invention. A polarizing diffraction element 100 shown in FIG. 1A includes two transparent substrates 101 and 102 made of glass, resin, etc., a liquid crystal layer 103 sandwiched between the two substrates 101 and 102, a substrate 101, And alignment films 104a, 104b, and 105a provided at the interface between the liquid crystal layer 103 and the liquid crystal layer 103. In the following, it is assumed that the liquid crystal layer 103 is made of a polymer liquid crystal as an optically anisotropic medium having birefringence.

  The liquid crystal layer 103 constituting the polarization diffraction element 100 shown in FIG. 1A has a configuration in which two types of regions 1031 and 1032 having different alignment states of the liquid crystal molecules 106 are periodically repeated. In the first region 1031, both the alignment film 104 a and the alignment film 105 a align the liquid crystal molecules 106 in the same direction substantially parallel to the substrates 101 and 102. It is in an oriented homogeneous alignment state.

  On the other hand, in the second region 1032, the alignment film 104 b aligns the liquid crystal molecules 106 substantially perpendicularly to the first substrate 101, and the alignment film 105 a aligns the liquid crystal molecules 106 substantially parallel to the second substrate 102. It is like that. Therefore, the liquid crystal molecules 106 are aligned almost perpendicularly to the first substrate 101 in the vicinity of a first surface (hereinafter referred to as a first interface) 107 that is the surface of the liquid crystal layer 103 on the first substrate 101 side. The so-called hybrid alignment state in which the liquid crystal layer 103 is aligned substantially parallel to the second substrate 102 in the vicinity of the second surface (hereinafter referred to as the second interface) 108a that is the surface on the second substrate 102 side. It has become. As described above, the alignment state is different in the vicinity of both the interfaces 107 and 108 a of the liquid crystal layer 103, and in the vicinity of the first interface 107, the alignment state is substantially parallel to the first substrate 101 and substantially the same as the first substrate 101. The vertical alignment state is periodically repeated.

  In such a configuration, when linearly polarized light parallel to the cross section of the polarization diffraction element shown in FIG. 1A is transmitted, the alignment of the liquid crystal molecules 106 in the first region 1031 and the second region 1032 of the liquid crystal layer 103 is performed. Therefore, the light transmitted through the liquid crystal layer 103 undergoes phase modulation due to a periodic change in the refractive index based on the periodicity of the regions 1031 and 1032 and is diffracted.

  That is, the optical path length of the light transmitted through the first region 1031 is the product ne × d of the extraordinary refractive index ne of the polymer liquid crystal and the thickness d of the liquid crystal layer 103 made of the polymer liquid crystal. On the other hand, in the second region 1032, the alignment state of the polymer liquid crystal smoothly changes between the first interface 107 and the second interface 108 a, and the optical path of light transmitted through the second region 1032. The length is (no + Δn · A) d. Here, Δn is the difference between the extraordinary refractive index ne and the ordinary refractive index no. A is a function of the ordinary light refractive index no and the extraordinary light refractive index ne. As a result of detailed examination, it was found that A can be approximated by A = 1/2 + 0.037Δn.

  As a result, the difference between the optical path lengths of light passing through the first region 1031 and the second 1032 (hereinafter simply referred to as optical path length difference) is Δn · (1−A) · d, which is transmitted through the liquid crystal layer 103. The received light undergoes wavefront modulation and is diffracted. Here, the polarization state of the transmitted light does not change. In this case, it is preferable that the optical path length difference Δn · (1−A) · d coincides with the product of ½ of the wavelength λ to be used and the integer i within a range of ± 20%. .

More preferably, it is within the range of ± 10%, more preferably within the range of ± 5%, and further, Δn · (1−A) · d = i · λ / 2 is preferable for the same reason. In other words, it is preferable to make it equal to (Δn / 2−0.037Δn ² ) · d = i · λ / 2 for the same reason.

  On the other hand, when linearly polarized light perpendicular to the cross section of the polarization diffraction element shown in FIG. 1 (a) is transmitted, the refractive index of the polymer liquid crystal is ordinary light refraction in both the first region 1031 and the second region 1032. Since the light is transmitted through the liquid crystal layer 103 without being phase-modulated, the diffraction hardly occurs. In this way, it is possible to realize a polarization diffraction element having a diffraction efficiency different depending on the polarization direction of light incident on the liquid crystal layer 103.

  FIG. 1B shows an example of a polarization diffraction element in which the alignment state of the liquid crystal molecules 106 in the liquid crystal layer 103 is different. In this example, in the polarization diffraction element 110, the alignment film 105b provided at the interface between the second substrate 102 and the liquid crystal layer 103 aligns the liquid crystal molecules 106 substantially perpendicularly to the second substrate 102. It is like that. Like the polarization diffraction element 100, the polarization diffraction element 110 having such an alignment film 105b has a configuration in which two types of regions 1033 and 1034 having different alignment states of the liquid crystal molecules 106 are periodically repeated in the liquid crystal layer 103. It has become.

  In the third region 1033, both the alignment film 104b and the alignment film 105b align the liquid crystal molecules 106 substantially perpendicularly to the first substrate 101, and the liquid crystal molecules 106 are aligned uniformly. It is in the vertical alignment state. On the other hand, in the fourth region 1034, the alignment film 104 a aligns the liquid crystal molecules 106 substantially parallel to the first substrate 101, and the alignment film 105 b aligns the liquid crystal molecules 106 approximately perpendicular to the second substrate 102. It is like that.

  Therefore, the liquid crystal molecules 106 are aligned substantially parallel to the first substrate 101 in the vicinity of the first interface 107 with the first substrate 101, and the second interface 108b in the vicinity of the second interface 108b with the second substrate 102. It is in a hybrid alignment state in which it is aligned substantially perpendicular to the substrate 102. As described above, the alignment state is different in the vicinity of both the interfaces 107 and 108 b of the liquid crystal layer 103, and the alignment state substantially parallel to the first substrate 101 and the first substrate 101 is approximately in the vicinity of the first interface 107. The vertical alignment state is periodically repeated.

  In such a configuration, when linearly polarized light perpendicular to the cross section of the polarization diffraction element shown in FIG. 1B is transmitted, the liquid crystal molecules 106 in the third region 1033 and the fourth region 1034 of the liquid crystal layer 103 are transmitted. Therefore, the light transmitted through the liquid crystal layer 103 undergoes phase modulation due to a periodic change in the refractive index based on the periodicity of the regions 1033 and 1034 and is diffracted.

  That is, the optical path length of the light transmitted through the third region 1033 is the product no × d of the extraordinary refractive index no of the polymer liquid crystal and the thickness d of the polymer liquid crystal. On the other hand, in the fourth region 1034, the alignment state of the polymer liquid crystal changes smoothly between the first interface 107 and the second interface 108b, and the optical path of the light transmitted through the fourth region 1034 The length is (no + Δn · A) d.

As a result, the optical path length difference of the light transmitted through the third region 1033 and the fourth region 1034 becomes Δn · A · d, and the light transmitted through the liquid crystal layer 103 undergoes wavefront modulation and is diffracted. Here, the polarization state of the transmitted light does not change. In this case, it is preferable that the optical path length difference Δn · A · d is made to coincide with the product of ½ of the wavelength λ to be used and the integer i within a range of ± 20%. More preferably, it is within the range of ± 10%, more preferably within the range of ± 5%, and further, Δn · A · d = i · λ / 2 is preferable for the same reason. In other words, it is preferable to make it equal to (Δn / 2 + 0.037Δn ² ) · d = i · λ / 2 for the same reason.

  On the other hand, when linearly polarized light perpendicular to the cross section of the polarization diffraction element shown in FIG. 1B is transmitted, the refractive index of the polymer liquid crystal is ordinary light refraction in both the third region 1033 and the fourth region 1034. The light becomes a rate no, and light transmitted through the liquid crystal layer 103 is not subjected to phase modulation, so that almost no diffraction occurs. In this way, it is possible to realize a polarization diffraction element having a diffraction efficiency different depending on the polarization direction of light incident on the liquid crystal layer 103.

  As described above, either the polarization diffraction element 100 shown in FIG. 1A or the polarization diffraction element 110 shown in FIG. 1B transmits or diffracts incident light according to the polarization direction and transmits or diffracts it. The polarization direction of the diffracted light can be maintained in the polarization direction when incident.

  As described above, the alignment direction of the liquid crystal molecules 106 in the liquid crystal layer 103 is preferably controlled by providing an alignment film at the interface of the liquid crystal layer 103. Specifically, the alignment state in the vicinity of the first interface 107 is periodically changed by periodically and repeatedly arranging the alignment film 104a for horizontally aligning the liquid crystal molecules 106 and the alignment film 104b for vertically aligning the liquid crystal molecules 106. Controlled to change.

  Further, the alignment state in the vicinity of the second interface 108a shown in FIG. 1A is controlled by using the alignment film 105a for horizontally aligning the liquid crystal molecules 106, and in the vicinity of the second interface 108b shown in FIG. The alignment state is controlled by using an alignment film 105b that vertically aligns the liquid crystal molecules 106. By using a single alignment film in this way, the alignment state in the vicinity of the second interfaces 108a and 108b is made uniform.

  The two types of alignment films 104a and 104b are alternately and periodically formed on the first substrate 101, and the alignment film 105a or the alignment film 105b having the one type of alignment characteristics is uniformly formed on the second substrate 102. The alignment layer on the first substrate 101 and the alignment layer on the second substrate 102 are made to face each other, and a liquid crystal is polymerized by inserting a liquid crystal between them. 103 can be realized. At this time, since the alignment film provided on the second substrate 102 is homogeneous, it is not necessary to align the patterns of the first substrate 101 and the second substrate 102, and the yield is reduced due to misalignment at low cost. A polarization diffraction element can be realized without deterioration of characteristics.

  Here, as the film for the alignment film, an organic film such as polyimide and polyvinyl alcohol (PVA), an inorganic film such as SiO, and the like can be used. As a method for imparting alignment characteristics to each of the above films so that liquid crystal molecules are aligned in a predetermined direction, there are a method of rubbing each of the above films and a method of photo-alignment by irradiating ultraviolet rays or the like.

  Here, the photo-alignment refers to irradiating the film on the substrate with light having optical anisotropy such as polarized light to induce rearrangement of molecules in the film on the substrate or anisotropic chemical reaction. Thus, anisotropy is given to the film on the substrate to align the liquid crystal molecules. Photo-alignment is preferable because an alignment pattern can be formed by partially irradiating ultraviolet rays. Also, for the same reason, it is also preferable to change the orientation property of the polyimide to be horizontally oriented to a vertical orientation by partially applying ultraviolet rays or heat.

  As described above, in the polarization diffraction element according to the first embodiment of the present invention, the orientation direction periodically changes on one side of the liquid crystal layer, and the orientation direction is uniformly uniform in almost the same direction on the other surface. As a result, the optical path length can be changed according to the polarization direction of light, and a narrower pitch and higher diffraction efficiency can be realized than the conventional polarization diffraction element, and alignment can be facilitated and the manufacturing process can be simplified. .

  In addition, since the alignment direction is substantially perpendicular or substantially horizontal to the surface of the liquid crystal layer, the diffraction efficiency can be further improved.

  In addition, since the liquid crystal layer is aligned using the alignment film, the alignment direction can be easily controlled.

(Second Embodiment)
FIG. 2 is a sectional view showing a conceptual configuration of the polarization diffraction element according to the second embodiment of the present invention. In FIG. 2, the polarization diffraction element 200 has the same alignment state as the polarization diffraction element 110 shown in FIG. 1B, and the second substrate 102 and the alignment film 105b constituting the polarization diffraction element 110 are removed. It has a configuration. In general, since many liquid crystal molecules are vertically aligned at the interface with air or vacuum, using the liquid crystal having such characteristics and providing an alignment film on one side as shown in FIG. A polarization diffraction element 200 having a similar orientation state can be realized.

  In addition to the effects of the first embodiment of the present invention, the polarization diffraction element according to the second embodiment of the present invention can be manufactured easily by reducing the number of manufacturing steps, and the number of members can also be reduced. To preferred. In addition, by adding a surfactant to the liquid crystal, the characteristics of the liquid crystal can be changed, and the liquid crystal molecules can be horizontally aligned at the interface with air or vacuum. Accordingly, a polarization diffraction element having the same orientation state as that of the polarization diffraction element 100 shown in FIG.

(Third embodiment)
In many cases, a polarization grating and a wave plate (for example, a quarter wave plate or a half wave plate) that changes the polarization state of light are used in combination. A polarization diffraction element in which the polarization diffraction element according to the present invention and the wave plate are integrated is preferable because the number of parts can be reduced. This wave plate can be made of a polymer film such as polycarbonate, an organic material such as polymer liquid crystal, a single crystal such as quartz, LiNbO _3, or the like.

  FIG. 3 is a cross-sectional view conceptually showing a schematic configuration example in which the polarization diffraction element and the wave plate according to the present invention are integrated. Hereinafter, an element obtained by integrating a polarization diffraction element and a wave plate is referred to as a polarization diffraction element with a wave plate. In FIG. 3, the polarization diffraction element with a wave plate is provided between three substrates 101, 102, and 303, a liquid crystal layer 103 that is sandwiched between the substrates 101 and 102, and the alignment state changes periodically, and the substrates 102 and 303. A liquid crystal layer (hereinafter referred to as an additional liquid crystal layer) 304 that is sandwiched and has a uniform alignment state is formed.

  The additional liquid crystal layer 304 has a uniform alignment state (an alignment state substantially parallel to the substrate in FIG. 3), and functions as a wave plate that changes the polarization state of incident light. The other liquid crystal layer 103 functions as a polarization diffraction grating as described above. This configuration example is preferable because a polarizing diffraction element with a wavelength plate can be produced by forming an alignment film having an alignment characteristic according to each of the substrates 101, 102, and 303 and sandwiching liquid crystal in one step to cause polymerization. .

  As described above, in the polarization diffraction element according to the third embodiment of the present invention, in addition to the effect of the first embodiment of the present invention, an additional liquid crystal layer having a uniform alignment direction in the same direction is newly added. Since it can be integrated as a simple optical element, the number of parts can be reduced.

(Fourth embodiment)
Next, a case where a low molecular liquid crystal is used as the optical anisotropic medium will be described. A low-molecular liquid crystal changes its alignment state by applying a voltage or changing the applied voltage, so that a variable polarization diffraction element having a variable diffraction efficiency can be realized. FIG. 4A is a cross-sectional view showing an example of a conceptual configuration of a variable polarization diffraction element according to the fourth embodiment of the present invention.

  4A, the variable polarization diffraction element 400 includes substrates 401 and 402, electrodes 403 and 404 formed on the substrates 401 and 402, and alignment films 405a and 405b formed on the electrodes 403 and 404, respectively. 406 and a low-molecular liquid crystal layer 407 sandwiched between the alignment films 405a, 405b, and 406. Here, as the electrodes 403 and 404, when the variable polarization diffraction element 400 is used as a reflection type diffraction element, a transparent electrode such as ITO is used, and when it is used as a reflection type diffraction element, an electrode that reflects light such as metal. Can be used.

  The alignment films 405a and 405b are formed to have the same alignment characteristics as the alignment films 104a and 104b constituting the polarization diffraction element 110 (see FIG. 1B), and the alignment films 405a and 405b. The liquid crystal molecules of the low-molecular liquid crystal layer 407 sandwiched between the alignment film 406 and the alignment film 406 are aligned substantially parallel to the first substrate 401 in the vicinity of the interface between the low-molecular liquid crystal layer 407 and the alignment films 405a and 405b. The alignment state substantially perpendicular to the first substrate 101 is periodically repeated (see FIG. 4A).

  Although not shown in FIG. 4A, a sealant is provided so that the sandwiched liquid crystal does not leak, or insulation is provided between the electrodes 403 and 404 and the alignment films 405a, 405b, and 406 in order to improve insulation. A film may be provided. The electrodes 403 and 404 of the variable polarization diffraction element 400 thus configured are connected to the power source 408 via the switch 409, and operate as follows when a predetermined voltage is applied.

  First, when no voltage is applied between the electrodes 403 and 404, the alignment state of the liquid crystal molecules periodically changes as shown in FIG. 4A, so that light is diffracted according to the polarization direction of incident light. Can do. Next, when a voltage for aligning liquid crystal molecules is applied between the electrodes 403 and 404 with a force stronger than the alignment film 405a, 405b, and 406, the alignment direction of the liquid crystal molecules is shown in FIG. As shown in the figure, the incident light cannot be diffracted. By gradually changing the voltage applied between the electrodes 403 and 404 within a predetermined voltage range, the diffraction efficiency can be changed according to the voltage.

  In the above description, an example in which the liquid crystal molecules in the variable polarization diffraction element 400 are aligned in the same manner as the liquid crystal molecules in the polarization diffraction element 110 illustrated in FIG. The liquid crystal molecules in the polarization diffractive element 100 shown in FIG. 1 (a) without applying voltage by giving the same alignment characteristics as the alignment film 105a constituting the polarization diffractive element 100 shown in FIG. 1 (a). It is possible to realize a variable polarization diffractive element in the orientation state.

  In this case, an element that does not diffract light when a voltage is applied can be created by using a liquid crystal having a negative dielectric anisotropy so that liquid crystal molecules are in a direction substantially parallel to the substrate when a voltage is applied. it can. In the two variable polarization diffraction elements configured as described above, since the alignment film of the substrate 402 is uniform, it is not necessary to align the patterns of the substrate 401 and the substrate 402. It is possible to produce a polarization diffraction element that is free from deterioration and the like at low cost.

  Here, as the films for the alignment films 405a, 405b, and 406, organic films such as polyimide and polyvinyl alcohol (PVA), inorganic films such as SiO, and the like can be used. Further, similarly to the polarization diffraction element using the liquid crystal layer 103, the polarization diffraction element and the wave plate can be integrated.

  As described above, the polarization diffraction element according to the fourth embodiment of the present invention is not limited to the effects of the first embodiment of the present invention and the third embodiment of the present invention. Since a voltage can be applied between the liquid crystal layers to change the orientation direction, the diffraction efficiency can be electrically changed.

  An additional liquid crystal layer 304 and a substrate 303 constituting the polarization diffraction element according to the third embodiment of the present invention are added to the polarization diffraction element according to the fourth embodiment of the present invention on the substrate 402 side. Alternatively, it may be integrated with an optical element such as a wave plate. Thereby, the number of parts can be reduced.

(Fifth embodiment)
Next, an example in which the polarization diffraction element according to the present invention is used in an optical head device will be described. FIG. 5 is a diagram conceptually showing a configuration example of an optical head device using the polarization diffraction element according to the present invention. In FIG. 5, an optical head device includes a light source 501 that emits a light beam having a predetermined wavelength, a polarization diffraction element 502 with a wavelength plate in which any of the polarization diffraction elements 503 and the wavelength plate 504 described above are integrated, A collimator lens 505 that converts incident light into parallel light, an objective lens 506 that condenses incident light on the optical disk 507, and light that is reflected by the optical disk 507 and returns to the polarization diffraction element 502 with a wavelength plate (hereinafter referred to as return light). .) Is detected by a photodetector 508. Here, as the light source 501, for example, a semiconductor laser can be used.

  The light emitted from the light source 501 is transmitted through the polarization diffraction element 502 with a wave plate, and is condensed on the optical disk 507 by the collimator lens 505 and the objective lens 506. Return light from the optical disk 507 sequentially passes through the objective lens 506 and the collimator lens 505, and reaches the photodetector 508 by the polarization diffraction element 502 with a wave plate. Information recorded on the optical disc 507 can be read based on the return light that has reached the photodetector 508.

  Here, the polarization diffraction element 502 with a wavelength plate according to the present invention is used to guide the return light from the optical disk 507 to the photodetector 508, and the polarization diffraction element 502 with a wavelength plate transmits the return light to the photodetector 508. It is comprised so that it may diffract toward. As the polarization diffraction element 502 with a wave plate, for example, a quarter wave plate may be used as the wave plate 504, and the wave plate 504 and the polarization diffraction element 503 may be integrated.

  At this time, when light linearly polarized in the direction of high transmittance (low diffraction efficiency) of the polarization diffraction grating 503 is emitted from the light source 501, the emitted light is transmitted linearly without being diffracted by the polarization diffraction grating 503. Then, it is converted into circularly polarized light by the wave plate 504 that is a quarter wave plate, and reaches the optical disk 507. The return light from the optical disk 507 becomes linearly polarized light orthogonal to the polarization direction of the incident light in the forward path by the wave plate 504 which is a quarter wave plate.

  As described above, the return light whose polarization direction is changed in the direction orthogonal to the polarization direction of the incident light by the wave plate 504 becomes a polarization direction in which the diffraction efficiency of diffraction by the polarization diffraction element 503 is high. Light reaches the detector 508. This is preferable because the optical disk 507 can be irradiated with light with a small loss and the return light can be efficiently guided to the photodetector 508.

  Further, the use of the variable polarization diffraction element described in the fourth embodiment of the present invention for an optical head device means that, for example, a CD, a DVD, and a large capacity optical disk using a light source having a wavelength of 400 nm band, In the case of recording and reproducing with one optical head device using a light source that emits light, it is desirable that the diffraction efficiency of the variable polarization diffraction element can be changed for three beams according to the type of the disk and the wavelength of the light source.

  Further, when a plurality of types of optical discs are recorded and reproduced by a single optical disc device, for example, a single light source is used to switch the diffraction efficiency of diffraction by the variable polarization diffractive element to generate a light beam for each type of optical disc described above. Such a configuration is preferable because the generated beam can be changed by ON / OFF of the voltage applied to the variable polarization diffraction element and adjustment of the applied voltage. In addition to the use of generating a plurality of beams such as three beams, a configuration in which one light source emits a light beam having a plurality of wavelengths for these plural types of optical discs can be used to correct aberrations due to chromatic aberration, disc thickness, etc. It is very effective to use a variable polarization diffraction element having a variable diffraction efficiency.

  As described above, in the optical head device according to the fifth embodiment of the present invention, in addition to the effects of any one of the first embodiment to the fourth embodiment of the present invention, the optical disc The light utilization efficiency of the light incident on the optical disk or the return light from the optical disk can be improved.

  In the fifth embodiment of the present invention, the optical head device having the configuration using the collimator lens has been described. However, the collimator lens is not necessarily a component necessary for the optical head device of the present invention. Even if the lens is omitted, the same effect as described above can be obtained.

  Further features of the polarization diffraction element and the optical disk apparatus of the present invention will be specifically described by the following examples.

  FIG. 1A is a cross-sectional view showing a conceptual configuration of the polarization diffraction element according to the first embodiment of the present invention. First, alignment films 104a, 104b, and 105a for aligning liquid crystals are formed on opposing surfaces of two transparent substrates 101 and 102 made of glass, resin, or the like. In the first substrate 101, an alignment film 104b capable of obtaining an alignment state substantially parallel to the substrate 101 and an alignment film 104a capable of obtaining an alignment state substantially perpendicular to the substrate 101 are alternately cycled in a stripe pattern. Form.

  The periodic repetition of the stripe shape is not limited to a linear repetition, and may be a concentric ring-shaped repetition. Further, it may be curved and curved. The direction of the diffracted light and the wavefront shape can be changed by the stripe shape. On the other hand, an alignment film 105 a that can obtain an alignment substantially parallel to the substrate 102 is formed on the second substrate 102. Hereinafter, the alignment obtained by the alignment films 104a, 104b, and 105a is referred to as parallel alignment.

  In the case of parallel orientation, the orientation directions in the planes of the substrates 101 and 102 are the same between the first substrate 101 and the second substrate 102, or are directions reversed by 180 degrees. The alignment direction in the planes of the substrates 101 and 102 can be realized by a rubbing method or an optical alignment method. The two substrates 101 and 102 are stacked with a gap so that the alignment films 104a and 104b and the alignment film 105a face each other.

  The gap is preferably in the range of 0.5 μm to 50 μm, more preferably 1 μm to 20 μm. In addition, the alignment accuracy when the two substrates 101 and 102 are stacked does not need to be performed with high accuracy as in the past, and even a very rough alignment as compared with the conventional method can be used as a polarization diffraction element. Function can be obtained.

  Next, liquid crystal is sandwiched in this gap by a vacuum injection method or the like. In the second region 1032, the liquid crystal molecules 106 have a hybrid alignment that changes from vertical alignment to horizontal alignment in the thickness direction, and in the first region 1031, the liquid crystal molecules 106 have horizontal alignment in the entire thickness direction. By comprising in this way, the orientation state of a liquid crystal can be changed periodically. As this liquid crystal, it is possible to polymerize, for example, by using a liquid crystal to which an acrylic group is added, it becomes possible to polymerize by irradiation with ultraviolet rays or the like, and the liquid crystal layers 103 having different alignment states periodically. Can be formed.

When a polarization diffractive element having a large diffraction efficiency at a wavelength λ of 650 nm is produced, a polymer liquid crystal having a difference Δn between the extraordinary refractive index ne and the ordinary refractive index no is 0.1, and the thickness of the liquid crystal layer 103 is increased. It is preferable to set the length d to 6.6 μm so that the relationship of approximately (Δn / 2−0.037Δn ² ) · d = λ / 2 is satisfied.

  The incidence of linearly polarized light in the same direction as the alignment direction of the polymer liquid crystal (the direction of extraordinary light) is incident on the polarization diffraction element produced as described above, so that the diffraction efficiency of ± 1st order diffracted light is about 40%. can get. On the other hand, when light having a polarization direction (ordinary refractive index direction) perpendicular to the alignment direction of the polymer liquid crystal is incident, a high zero-order transmittance of 95% or more can be obtained, and diffraction hardly occurs. In this way, a polarization diffraction grating having different diffraction efficiency depending on the polarization direction can be realized. Furthermore, by producing a ¼ wavelength plate using a polymer liquid crystal, a polarization diffraction element with a wavelength plate as shown in FIG. 3 can be produced.

  Hereinafter, an example in which the polarization diffraction element with a wave plate is used in an optical head device will be described. FIG. 5 is a diagram conceptually illustrating a configuration example of an optical head device using the polarization diffraction element according to the present embodiment. The light emitted from the semiconductor laser as the light source 501 passes through the polarization diffraction element 502 with a wave plate, and is condensed on the optical disk 507 by the collimator lens 505 and the objective lens 506. Return light from the optical disk 507 sequentially passes through the objective lens 506 and the collimator lens 505, and reaches the photodetector 508 by the polarization diffraction element 502 with a wave plate. Information on the optical disk 507 can be read based on the return light that has reached the photodetector 508.

  Here, the polarization diffraction element 502 with a wave plate of the present invention is used to guide the return light from the optical disk 507 to the photodetector 508, and the return light is diffracted to reach the photodetector 508. The optical head device according to the present embodiment is an example using a polarization plate 502 having a wavelength plate in which a polarization diffraction grating 503 of the present invention and a wavelength plate 504 that is a quarter wavelength plate are integrated.

  At this time, when light linearly polarized in the direction of high transmittance (low diffraction efficiency) of the polarization diffraction grating 503 is emitted from the light source 501, the emitted light is transmitted linearly without being diffracted by the polarization diffraction grating 503. Then, it is converted into circularly polarized light by the wave plate 504 that is a quarter wave plate, and reaches the optical disk 507. The return light from the optical disk 507 becomes linearly polarized light orthogonal to the polarization direction of the incident light in the forward path by the wave plate 504 which is a quarter wave plate.

  As described above, the return light whose polarization direction has been changed to linearly polarized light by the wave plate 504 becomes a polarization direction with high diffraction efficiency of diffraction by the polarization diffraction element 503, so that the light efficiently reaches the photodetector 508. This is preferable because the optical disk 507 can be irradiated with light with a small loss and the return light can be efficiently guided to the photodetector 508.

  The diffraction element and the optical head device according to the present invention can realize a narrower pitch and higher diffraction efficiency than the conventional polarization diffraction element, and also have a useful effect of facilitating alignment and simplifying the manufacturing process. It is useful as an optical head device or the like.

1 is a cross-sectional view showing a conceptual configuration of a polarization diffraction element according to a first embodiment of the present invention. Sectional drawing which shows the notional structure of the polarization | polarized-light diffraction element which concerns on the 2nd Embodiment of this invention. Sectional drawing which shows notionally the structural example which integrated the polarization | polarized-light diffraction element and wavelength plate which concern on this invention. Sectional drawing which shows an example of a notional structure of the variable polarization | polarized-light diffraction element which concerns on the 4th Embodiment of this invention. The figure which showed notionally one structural example of the optical head apparatus using the polarization | polarized-light diffraction element which concerns on this invention.

Explanation of symbols

100, 110, 200, 503 Polarization diffraction element 101, 102, 303, 401, 402 Substrate 103, 407 Liquid crystal layer 1031 First region of liquid crystal layer 1032 Second region of liquid crystal layer 1033 Third region of liquid crystal layer 1034 Fourth region of liquid crystal layer 104a, 104b, 105a, 105b, 405a, 405b, 406 Alignment film 106 Liquid crystal molecule 107 First surface (first interface) of liquid crystal layer
108a, 108b Second surface (second interface) of liquid crystal layer
300, 502 Polarization diffraction element with wave plate 304 Additional liquid crystal layer 400 Variable polarization diffraction element 403, 404 Electrode 408 Power supply 409 Switch 501 Light source 504 Wave plate 505 Collimator lens 506 Objective lens 507 Optical disk 508 Photo detector

Claims (6)

In the polarization diffraction element in which the diffraction efficiency varies depending on the polarization direction of the incident light, because the refractive index of the optically anisotropic medium having birefringence changes periodically in space,
The optically anisotropic medium comprises a liquid crystal layer comprising a liquid crystal or a polymer liquid crystal obtained by polymerizing a liquid crystal.
The alignment direction of the molecules of the liquid crystal or the polymer liquid crystal is periodically changed on the first two of the two surfaces of the liquid crystal layer that face each other and through which the light passes.
The alignment direction of the molecules of the liquid crystal or the polymer liquid crystal is uniformly the same in the second surface of the two surfaces of the liquid crystal layer that face each other and through which the light passes. A polarized light diffraction element characterized by comprising:   A region in which the alignment direction of the molecules of the liquid crystal or the polymer liquid crystal is perpendicular to the in-plane direction of the liquid crystal layer on the first surface of the liquid crystal layer, and the alignment of the molecules of the liquid crystal or the polymer liquid crystal A region whose direction is horizontal to the in-plane direction of the liquid crystal layer is periodically arranged, and on the second surface of the liquid crystal layer, the alignment direction of the molecules of the liquid crystal or the polymer liquid crystal is 2. The polarization diffraction element according to claim 1, wherein the polarization diffraction element is uniformly perpendicular to the in-plane direction or uniformly horizontal in the in-plane direction of the liquid crystal layer.   A first alignment film that is aligned in the alignment direction on the first surface of the liquid crystal layer, a second alignment film that is aligned in the alignment direction on the second surface of the liquid crystal layer, and the first alignment film A first substrate formed so as to be in contact with the first surface of the liquid crystal layer, and a second substrate formed so that the second alignment film is in contact with the second surface of the liquid crystal layer The polarization diffraction element according to claim 1, comprising:   An electrode for applying a voltage between the liquid crystal layers formed on each of two opposing surfaces of the first substrate and the second substrate and through which the light passes. Item 4. The polarization diffraction element according to Item 3.   A third substrate; and an additional liquid crystal layer formed of a liquid crystal layer of an optically anisotropic medium sandwiched between the first substrate or the second substrate and the third substrate. The polarization diffraction element according to claim 3 or 4, wherein the orientation directions of the molecules are uniformly in the same direction.   In an optical head device comprising: a light source; an objective lens that condenses light emitted from the light source onto an optical recording medium; and a photodetector that detects light collected by the objective lens and reflected by the optical recording medium. An optical head device, wherein the polarization diffraction element according to any one of claims 1 to 5 is disposed in an optical path between the objective lens and the light source.

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