JP2009294604A - Optical modulation liquid crystal element and variable optical attenuator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulation liquid crystal element capable of adjusting light quantity stably and quickly over a wide wavelength range in accordance with an application voltage irrespective of a polarization state of incident light, and to provide a variable optical attenuator. <P>SOLUTION: The optical modulation liquid crystal element includes: a polarization separating element which deflects light of first linear polarization and light of second linear polarization by the same angle in axially symmetrical directions to each other with respect to polarization components of the first linear polarization and the second linear polarization vertical to each other of the incident light, on the basis of the light axis of incident light; and a reflection type liquid crystal element in which the polarization states of transmission light of the first linear polarization and the second linear polarization are changed in accordance with the application voltage. The reflection type liquid crystal element includes a polarization modulation element and a reflective layer, wherein the polarization modulation element is such a liquid crystal cell that a liquid crystal layer is sandwiched between substrates each having an electrode, and retardation value changes in accordance with the voltage applied between the electrodes. The optical modulation liquid crystal element is characterized in that the polarization separation element, the polarization modulation element and the reflective layer are arranged in the order from the incident light side. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a light modulation liquid crystal element and a variable optical attenuator that change the direction of light that reflects and travels in accordance with the magnitude of an applied voltage, without depending on the polarization state of incident light.

  When communicating light with a specific band in a communication field using a communication medium such as an optical fiber, for example, the amount of light transmitted as an optical element used between an optical fiber, between a semiconductor laser light source and an optical fiber, or between an optical fiber and a photodetector. There is known a variable optical attenuator capable of changing the above. The variable optical attenuator can control the amount of light transmitted according to the magnitude of the voltage applied to the liquid crystal layer by voltage control. The projection display apparatus projects an image by changing the amount of light to be projected temporally and spatially, and projects an image according to the magnitude of a voltage applied to the liquid crystal layer by voltage control.

  As an example of this conventional variable optical attenuator, a transmissive liquid crystal element 300 as shown in FIG. The liquid crystal element 300 includes a liquid crystal cell 310 in which a liquid crystal layer is sandwiched between substrates with electrodes, and the retardation value of transmitted light changes according to the magnitude of a voltage applied between the electrodes, and the light incident side of the liquid crystal cell 310 A first polarizing beam splitter 320 disposed on the liquid crystal cell 310 and a second polarizing beam splitter 330 disposed on the light exit side of the liquid crystal cell 310. Two orthogonal linearly polarized light incident on the liquid crystal element 300 are transmitted through the liquid crystal cell with different traveling paths depending on the polarization direction by the first polarizing beam splitter 320, and the liquid crystal cell 310 has a specific retardation value. Sometimes, the two linearly polarized light beams transmitted through the second polarizing beam splitter 330 are aligned with each other in the same traveling direction as the incident light of the liquid crystal element and are emitted from the liquid crystal element. Thus, in the liquid crystal element of the cited document 1, there is a description that specifically uses a polarizing diffraction grating as the first polarizing beam splitter 320 and the second polarizing beam splitter 330, and the emitted light Among them, there has been proposed a variable optical attenuator that condenses light that travels straight in the same direction as the incident direction and transmits only the light component that travels straight.

  As another example, Patent Document 2 proposes a projection display device 400 as shown in FIG. In the projection display device 400, light from a light source traveling in an oblique direction with respect to the optical axis of the projection lens 450 is transmitted in the order of the lens 410, the polarization separator 420, and the liquid crystal panel 430, and is reflected by the main mirror 440. It has a function of projecting the light reflected in this way and traveling in the optical axis direction of the projection lens 450. FIG. 19A and FIG. 19B show the state of light when different voltages are applied to the liquid crystal panel.

  Specifically, the polarizing diffraction grating 421 used in the polarization separator 420 has a function of transmitting either one of the first linearly polarized light and the second linearly polarized light orthogonal to each other in a straight line and diffracting the other. . FIG. 19A shows the state of light when the liquid crystal panel 430 applies a voltage for converting the first linearly polarized light into the second linearly polarized light and the second linearly polarized light into the first linearly polarized light. At this time, the first linearly polarized light and the second linearly polarized light transmitted and received through the liquid crystal panel 430 by the liquid crystal panel 430 and the main mirror 440 are respectively converted into the second linearly polarized light and the specific applied voltage shown in FIG. The incident light of the first linearly polarized light that has been converted into the first linearly polarized light and travels straight through the polarizing diffraction grating 421 in the forward path is first-order diffracted by the polarizing diffraction grating 421 in the backward path, and is polarized in the forward path. The incident light of the second linearly polarized light first-order diffracted by the diffraction grating 421 passes straight through the polarizing diffraction grating 421 on the return path. In FIG. 19B, the polarization state of the light traveling back and forth through the liquid crystal panel 430 does not change, and does the first linearly polarized light and the second linearly polarized light pass straight through the polarizing diffraction grating 421 in both the forward path and the backward path? Or, since the first-order diffraction is performed in both the forward path and the return path, the light does not travel in the Z direction. Then, the projection display device 400 is proposed in which the first and second linearly polarized lights are all aligned in the same direction and condensed on the projection lens 450 by the lens 410, and then the image of the liquid crystal panel is projected on the screen. Has been.

JP 2004-37480 A JP-A-4-230733

  However, in the configuration of Patent Document 1, in order to realize a variable optical attenuator with a high extinction ratio, the amount of change in the retardation value corresponding to the voltage applied to the liquid crystal cell 310 serving as the polarization modulation means becomes the wavelength λ of the incident light. On the other hand, λ / 2 or more is required, and the liquid crystal layer is relatively thick. In general, the response time of the liquid crystal cell corresponding to the applied voltage is proportional to the square of the thickness of the liquid crystal layer, so that there is a problem that the response time of the liquid crystal is long with respect to a change in voltage. In particular, an optical communication application used in the 1.55 μm wavelength band requires a large amount of change in the retardation value compared to the visible wavelength, and this response time is a limitation of the application as a variable optical attenuator.

  Further, in the liquid crystal element 300 of FIG. 18, ordinary light polarized light having a polarization direction with no difference in refractive index out of light incident on the first polarizing beam splitter 320 and the second polarizing beam splitter 330 is not diffracted. The extraordinary light polarization orthogonal to the ordinary light polarization is diffracted by the first polarizing beam splitter 320 and the second polarizing beam splitter 330, but is transmitted to the straight transmitted light according to the first-order diffraction efficiency. There is a problem that loss tends to occur, and as a result, the polarization dependence of the emitted light intensity tends to occur.

  In the configuration of Patent Document 2, the light emitted from the light source, transmitted through the projection lens 450 and projected onto the screen is first-order diffracted light that is diffracted once by the polarizing diffraction grating that constitutes the polarization separator 420. Therefore, when the incident light has a plurality of wavelengths or wavelength bands, the angle at which the incident light is emitted as the first-order diffracted light by the polarizing diffraction grating depends on the wavelength. As a result, there arises a problem that the condensing point by the lens 410 varies depending on the wavelength, and when used as a projection display device, the resolution of the projected image is deteriorated due to chromatic aberration. Further, as another optical system using the optical element described in Patent Document 2, light corresponding to a point light source emitted from a laser or an optical fiber is used as incident light, and the light emitted from the light modulation element is condensed to detect light. Assuming an optical system that receives light with a detector, the width of the angle at which light is diffracted and emitted with respect to light in a wide wavelength band becomes large, so that a photodetector with a large light receiving area is required, leading to deterioration in detection performance. Furthermore, when the optical element described in Patent Document 2 is received by an optical fiber and propagates the light, similarly, the condensing point position is shifted depending on the wavelength, so that the coupling efficiency with the optical fiber is lowered, and the wavelength dependence of the propagation efficiency is reduced. There was a problem that occurred.

  The present invention solves the problems in the conventional liquid crystal element and the variable optical attenuator using the same, responds at high speed, stabilizes the direction of outgoing light against changes in the wavelength of incident light, and changes the polarization state of incident light. It is an object of the present invention to provide a light modulation liquid crystal element in which the efficiency of linearly transmitted light changes according to the magnitude of an applied voltage, and a variable optical attenuator using the same.

The present invention has been made to solve the above problems, and is a light modulation liquid crystal element that modulates the light intensity of incident light. The light modulation liquid crystal element includes a polarization separation element, a reflective liquid crystal element, and the like. The polarization separating element is optically transparent with a birefringent material layer having a cross section made of a birefringent material on a translucent substrate and having a blazed shape or a pseudo-blazed shape approximating a blazed shape to a step shape. A polarization diffraction element made of an optical material layer or a Wollaston prism bonded so that the slow axes of the birefringent materials are orthogonal to each other, and the longitudinal direction of the grating having the blazed shape or the pseudo-blazed shape of the birefringent material layer Direction, or the longitudinal direction of the bonding interface of the Wollaston prism substantially coincides with the slow axis direction or the fast axis direction of the birefringent material, and the longitudinal direction of the birefringent material layer Alternatively, the polarization direction parallel to the longitudinal direction of the Wollaston prism is defined as a first polarization direction, and a polarization direction orthogonal to the first polarization direction is defined as a second polarization direction. The difference between the refractive index of the refractive material layer and the refractive index of the optical material layer, or the difference of the refractive index of the Wollaston prism with respect to the first polarization direction is Δn ₁ (≠ 0), and the difference with respect to the second polarization direction. When the difference between the refractive index of the birefringent material layer and the refractive index of the optical material layer, or the difference of the refractive index of the Wollaston prism with respect to the second polarization direction is Δn ₂ (≠ 0), Δn ₁ And Δn ₂ are substantially equal to each other, and the reflective liquid crystal element is incident on the liquid crystal layer, a polarization modulation element including an electrode, a liquid crystal layer that changes a retardation value according to a voltage applied from the electrode, and the liquid crystal layer Provided is a light modulation liquid crystal element including a reflective layer for regularly reflecting light.

  With this configuration, the light having the first polarization direction and the light having the second polarization direction out of the light incident by the polarization separation means are separated in an axial symmetry with respect to the incident optical axis, and the two proceeded separately. The polarized light of one component passes through the polarization modulation means, is regularly reflected by the reflection means, and travels in the opposite direction. At this time, by changing the retardation value of the liquid crystal according to the voltage applied to the polarization modulation means, the traveling direction of the light emitted from the light modulation liquid crystal element having the polarization of these two components is changed without depending on the polarization component. The amount of light emitted in a specific direction can be controlled. Furthermore, since the light reciprocates through the light modulation liquid crystal element and expresses diffraction twice, the amount of light emitted in a specific direction with respect to light in a wide wavelength band can be controlled. Note that “substantially equal” here means that the deviation is within ± 10%.

The polarization separation element is the polarization diffraction element, and the polarization diffraction element includes one optical material layer so as to fill at least the concave portion of the one birefringent material layer and the birefringent material layer. And the optical material layer of the polarization diffraction element is an optically isotropic homogeneous refractive index transparent material layer, wherein the ordinary refractive index of the birefringent material layer is n _o , and the extraordinary refractive index is n _e, wherein when the refractive index of the homogeneous refractive index transparent material layer and n _s, n _s is and having a substantially intermediate value between n _o and n _e, 1-order diffraction efficiency is maximum among the incident light when the wavelength lambda ₀ the wavelength at which the sectional blaze shape composed of birefringent _{material, 0.7 × λ 0 / | n} e -n s | ≦ d ≦ 1.3 × λ 0 / | n _e -n s _| a filled, the cross-section pseudo blazed shape composed of the birefringent material, 0. _{5 × λ 0 / | n e} -n s | ≦ d ≦ 1.15 × λ 0 / | n e -n s | to meet, the depth d of the polarization diffraction grating consisting of the birefringent material layer The light modulation liquid crystal element described above is provided.

In addition, when the depth d is substantially equal to λ ₀ / | n _e −ns _s , 0.77 × λ ₀ ≦ λ ≦ 1.43 × λ when the cross section made of the birefringent material is blazed. The light modulation liquid crystal element according to the above, in which light having a wavelength of λ satisfying 0.87 × λ ₀ ≦ λ ≦ 1.18 × λ ₀ is incident when the cross section made of the birefringent material satisfies ₀ and has a pseudo-blazed shape. provide.

The polarization separation element is the polarization diffraction element, and the polarization diffraction element includes a pair of the light-transmitting substrates having the birefringent material layer and a pair of the birefringent material layers. The optical material layer has the same longitudinal direction and fills at least the concave portion of the birefringent material layer, and the optical material layer of the polarization diffraction element is an optically isotropic homogeneous refractive index transparent material layer a is, ordinary refractive index n _o of the birefringent material _layer, the extraordinary refractive index n _e, the refractive index of the homogeneous refractive index transparent material layer when the n _s, a pair of the birefringent material layer the refractive index n _s with the slow axis direction are perpendicular to each other, as well as substantially equal to one of the ordinary refractive index n _o or the extraordinary refractive index n _e, is the first-order diffraction efficiency of the incident light when the wavelength lambda ₀ the wavelength at which the maximum value, the double The cross section blaze shape composed of folding _{material, 0.7 × λ 0 / | n} e -n o | ≦ d ≦ 1.3 × λ 0 / | n e -n o | a filled, the birefringent material the pseudo blazed shape made of cross _{section, 0.85 × λ 0 / | n} e -n o | ≦ d ≦ 1.15 × λ 0 / | n e -n o | so as to satisfy, the birefringent material layer The light modulation liquid crystal element according to the above, in which the depth d of the polarization diffraction grating is set.

When the depth d is substantially equal to λ ₀ / | n _e −n _o |, the cross section made of the birefringent material is 0.77 × λ ₀ ≦ λ ≦ 1.43 × λ when the cross section made of the birefringent material is blazed. The light modulation liquid crystal element according to the above, in which light having a wavelength of λ satisfying 0.87 × λ ₀ ≦ λ ≦ 1.18 × λ ₀ is incident when the cross section made of the birefringent material satisfies ₀ and has a pseudo-blazed shape. provide. Note that the term “substantially equal” and “substantially equal” here means that the deviation is within ± 10%.

  With this configuration, since the light incident on the polarization separation means is emitted with a high first-order diffraction efficiency, the light utilization efficiency is high, and controllability to change the amount of light emitted in a specific direction can be improved. A light modulation liquid crystal element with a high degree of design freedom can be realized.

  In addition, the polarization separating means includes a triangular prism-shaped right-angle prism in which both the birefringent material layer and the optical material layer are optically birefringent, and the slow axes of the two right-angle prisms are mutually connected. The light modulation liquid crystal element described above is a Wollaston prism that is orthogonal.

  With this configuration, light in the first polarization direction and the second polarization direction can be refracted in a direction that is an axis object with respect to the optical axis, and polarization separation can be increased with high efficiency.

The polarization modulation means is a liquid crystal cell composed of the single liquid crystal cell and a phase plate, and when the voltage applied to the single liquid crystal cell is V ₁ and the light having the wavelength λ ₀ reciprocates through the liquid crystal cell. with the retardation value of is λ _0/2, when the voltage applied to the liquid crystal alone cell is _{V 2 (V 1 ≠ V 2} ), the retardation value when the light of the wavelength lambda ₀ to reciprocate the liquid crystal cell The light modulation liquid crystal device according to the above, wherein is zero.

  With this configuration, it is possible to reduce the difference between the voltage value applied to the liquid crystal and the minimum voltage value when the amount of light emitted in a specific direction is maximum, so low-voltage driving is possible. It becomes. In addition, since the liquid crystal layer can be made thinner than the transmissive liquid crystal single cell, the response time when the amount of light is varied can be shortened. As a result, a light modulation liquid crystal element with better controllability can be realized.

  Further, the light modulation liquid crystal element described above in the traveling direction of the light transmitted through the light collecting lens, the light source, the condensing lens, and the light in a specific traveling direction reflected and emitted by the light modulation liquid crystal element. A variable optical attenuator comprising a light receiving means for receiving light, wherein the amount of light received by the light receiving means changes according to the magnitude of a voltage applied to the light modulation liquid crystal element.

  This configuration realizes a variable optical attenuator with short response time and good controllability that takes out only the light emitted from the light modulation liquid crystal element in a specific direction and receives or propagates it, regardless of the polarization state of the incident light. can do.

  The present invention can control the magnitude of the amount of light reflected and traveling in a specific direction by the magnitude of the applied voltage regardless of the polarization state in incident light with a wide wavelength band, and can respond quickly. A light modulation liquid crystal element and an optical attenuator can be provided.

(First embodiment)
A basic configuration of the light modulation liquid crystal element 100 of the present invention is shown in FIGS. The light modulation liquid crystal element 100 includes a polarization separation element 101, a polarization modulation element 102, and a reflection layer 16 in this order from the light incident side. FIG. 1 and FIG. 2 show how light travels in the Z direction. Among the polarization directions of incident light, the light in the X direction polarization direction (hereinafter referred to as S polarization) and the polarization direction in the Y direction shown in FIG. And light (hereinafter referred to as P-polarized light). Here, the traveling direction of the S-polarized light and the traveling direction of the P-polarized light in the light modulation liquid crystal element 100 are controlled to be different from each other, reflected by the reflective layer 16, and emitted from the light modulation liquid crystal element 100 to be specified. The amount of light traveling in the direction is controlled by a light shielding means or the like. These elements and the like may be arranged separately. However, it is preferable that they are integrated because the miniaturization can be realized and the characteristics can be stabilized. In the present embodiment, the possible forms of the polarization separation element 101, the polarization modulation element 102, and the reflection layer 16 will be described below. Note that the polarization modulator 102 and the reflective layer 16 are collectively referred to as a reflective liquid crystal element 104.

1. Polarization Separation Element Specifically, the function of the polarization separation element 101 that is a component of the light modulation liquid crystal element 100 will be described with reference to FIG. Light traveling in the −Z direction is incident on the polarization separation element 101 and is separated so that the traveling directions of the S-polarized light and the P-polarized light are different as described above. At this time, the traveling direction of the incident light ( S-polarized light and P-polarized light are deflected by the same angle θ in directions that are axially symmetric with respect to the optical axis). In addition, when light reflected by a reflection layer (not shown in FIG. 3) enters the + Z direction from the opposite direction at an angle θ, the light is deflected by the same angle θ and emitted in a traveling direction parallel to the optical axis.

  Next, FIG. 4 shows a schematic diagram of a polarization diffraction element 101a using a polarizing diffraction grating as a polarization separation element. The polarization diffraction element 101a has a diffraction grating shape having a periodic cross section and an optically birefringent material layer having a birefringent material layer, and an optical element that fills at least the concave portion of the polarization diffraction grating 6. The homogeneous refractive index transparent material layer 7 made of a homogeneous refractive index transparent material having uniform characteristics is sandwiched between a pair of translucent substrates 1 and 2 arranged in parallel. Further, although the slow axis direction of the polarization diffraction grating 6 shown in FIG. 4 is the X direction parallel to the longitudinal direction of the diffraction grating, the slow axis direction may be the Y direction. The diffraction grating shape is a so-called blazed shape in which the cross-sectional shape is a sawtooth shape, or a pseudo blazed shape in which the blazed shape approximates a step shape, and can deflect and separate polarization components of S-polarized light and P-polarized light with high efficiency.

Birefringent material forming the polarization diffraction grating 6 has a refractive index anisotropy [Delta] n, [Delta] n is the absolute value of the difference between the ordinary refractive index n _o and extraordinary refractive index _{n e (Δn = | n e} - n _o |). The refractive index n _s of the homogeneous refractive index transparent material is set to be substantially equal average value _{(n o + n e) /} 2 of the ordinary refractive index n _o and extraordinary refractive index n _e. In this way, the difference between the refractive index of the polarization diffraction grating 6 in S-polarized light and the refractive index of the homogeneous refractive index transparent material layer 7 is Δn _s (≠ 0), and the polarization diffraction grating 6 in P-polarized light has a difference. If the difference between the refractive index and the refractive index of the homogeneous refractive index transparent material layer 7 is Δn _p (≠ 0), Δn _s and Δn _p are substantially equal to each other. And the same diffraction order, the diffraction efficiencies of the same diffraction orders are equal.

As long as the transparent substrates 1 and 2 are transparent to incident light, various materials such as a resin plate and a resin film can be used. However, optically isotropic materials such as glass and quartz glass are used. When used, it is preferable because it does not affect the birefringence of the transmitted light. The birefringent material may be composed of a polymer liquid crystal in which a liquid crystal monomer is disposed on an alignment film (not shown) and cured with ultraviolet rays or the like, but is not limited thereto, and is not limited to quartz or LiNbO ₃ (niobic acid). Alternatively, a single crystal having birefringence such as lithium) may be processed, a resin film having birefringence may be processed, or an injection molded product of resin may be used. The alignment film may be necessary in the manufacturing process, but may not be provided when processing the birefringent material. Further, the homogeneous refractive index transparent material may be an organic material used as an adhesive or a filler, or a SiO _x N _y film (where x and y are the atomic ratios of O and N to Si), SiO ₂ film, Si ₃ An inorganic material such as an N ₄ film or an Al ₂ O ₃ film may be used.

Here, the design of the blaze shape will be specifically described. A specific wavelength of incident light is set to a wavelength λ _0, and a polarization direction in which the light of this wavelength λ ₀ feels an ordinary refractive index (a fast axis direction of a birefringent material) and a polarization direction to sense an extraordinary light refractive index (birefringence) The first-order diffraction efficiency with respect to the slow axis direction of the active material is maximized. Further, when the depth of the unevenness, that is, the maximum thickness of the polarization diffraction grating 6 is d, the phase difference of the uneven portion is
2π × | n _o −n _s | × d / λ ₀ ≈2π,
2π × | n _e −n _s | × d / λ ₀ ≈2π,
Design to be The blazed shape may be a pseudo-blazed diffraction grating shape approximated by a staircase shape. In that case, it is preferable to have a staircase-like pseudo-blazed shape with 4 to 16 steps.

  Next, the shape of the polarization diffraction grating 6 for increasing the first-order diffraction efficiency of the diffracted light will be described. The first-order diffraction efficiency when efficiently expressing the first-order diffracted light is preferably 75% or more, and more preferably 90% or more. Thus, in order to make the first-order diffraction efficiency a large value of 75% or more, it is realized by adjusting the depth (thickness) d of the polarization diffraction grating 6 made of the blazed birefringent material layer. it can.

Here, when light of wavelength λ ₀ is incident, the calculation result of the first-order diffraction efficiency for | n _o −n _s | × d / λ ₀ is shown in FIG. When the first-order diffraction efficiency is designed to be the maximum at the wavelength λ ₀ , the calculation result of the first-order diffraction efficiency with respect to the ratio λ / λ ₀ of the wavelength λ ₀ and the wavelength λ is shown in FIG. Show. Thus, the wavelength λ ₀ can be defined as the wavelength at which the first-order diffraction efficiency is maximum. FIG. 5A shows characteristics in the case of a diffraction grating having a blazed cross section and a pseudo blazed diffraction grating having a number of steps (N) = 4, 8, and 16 in cross section. In the cross-sectional schematic diagram of the blaze shape, the depth of the blaze shape is d as shown in FIG. 5C, and the cross-sectional schematic diagram of the pseudo blaze shape is also the pseudo blaze shape as shown in FIG. 5D. Calculated as the depth d of the blaze shape. As shown in FIG. 5D, the physical thickness d ′ of the polarization diffraction grating 6 when processing into a pseudo-blazed shape is d × (N−1) / N when the number of steps is N. Equivalent to. The number of steps indicates the number of different steps in one period of the lattice, and the width of one step is the same.

At this time, when the cross section is a pseudo blazed shape (N = 4), in order to realize a first-order diffraction efficiency of 75% or more when light having a wavelength λ ₀ is incident, the depth d is:
_{0.85 × λ 0 / | n e} -n s | ≦ d ≦ 1.15 × λ 0 / | n e -n s | ··· (1)
By satisfying the above, it is preferable because loss of light due to diffraction other than the first-order diffracted light can be reduced. As described above, it is preferable to satisfy the expression (1) at an arbitrary wavelength in the wavelength band to be used because high diffraction efficiency is obtained in the entire wavelength band to be used. In the following formulas, it is preferable to satisfy the formulas at any wavelength in the wavelength band to be used. Further, when designed so that λ ₀ / | n _e −n _s | = d, a first-order diffraction efficiency of 75% or more can be obtained with respect to a wide band wavelength as will be described later. In the case of a pseudo blazed shape having a number of steps larger than N = 4, the first-order diffraction efficiency becomes larger when the depth d is the same at N = 4 in FIG. 5A. For example, N = 8 As shown by the number of steps of 16, the characteristics of the first-order diffraction efficiency in the blaze shape are approached.

Further, when the cross section is a blazed shape, in order to realize a first-order diffraction efficiency of 75% or more, the depth d is:
_{0.7 × λ 0 / | n e} -n s | ≦ d ≦ 1.3 × λ 0 / | n e -n s | ··· (2)
In order to realize a first-order diffraction efficiency of 90% or more, it is preferable that the depth d satisfies the above expression (1), so that the loss of light due to diffraction other than the first-order diffracted light can be reduced. .

Then, from FIG. 5 (b), 1-order diffraction efficiency is designed diffraction grating so as to maximize with respect to the wavelength lambda _0, the wavelength lambda ₀ and polarization grating 6 for the ratio lambda / lambda ₀ of the wavelength lambda In order to set the first-order diffraction efficiency to a value of 75% or more, when the cross section has a pseudo-blazed shape (N = 4),
0.87 × λ ₀ ≦ λ ≦ 1.18 × λ ₀ (3)
It is preferable that the wavelength λ satisfies the condition that light loss due to diffraction can be reduced. Note that equation (3), polarization grating depth d = λ ₀ / _| a possible range of the incident wavelength lambda at conditions | n e -n _s.

In order to realize a first-order diffraction efficiency of 75% or more when the cross section is a blazed shape,
0.77 × λ ₀ ≦ λ ≦ 1.43 × λ ₀ (4)
In order to realize a first-order diffraction efficiency of 90% or more, it is preferable that the depth d satisfies the above expression (3), so that the loss of light due to diffraction other than the first-order diffracted light can be reduced. .

In the polarization diffraction element 101a composed of the polarization diffraction grating 6 and the homogeneous refractive index transparent material layer 7 thus obtained, polarized light that senses ordinary light refractive index (P-polarized light) and polarized light that senses extraordinary light refractive index. Direction light (S-polarized light) is incident, as schematically shown in FIG. 4, P-polarized light and S-polarized light in the YZ plane are symmetrical with respect to the traveling direction of the incident light. Are emitted with the same angle θ deflected and separated. In addition, although the material which has a homogeneous refractive index was demonstrated as an optical material with which the recessed part of the polarizing diffraction grating 6 was filled, it does not restrict to this. For example, implemented by may be a material having birefringence, in this case, to adjust the direction of the optical axis so as to make the [Delta] n _s and [Delta] n _p as described above are substantially equal Can do.

Next, FIGS. 6A and 6B are schematic diagrams of polarization diffraction elements 101b and 101c having other configurations using a polarization diffraction grating. The polarization diffraction element 101b is formed so that the polarization diffraction gratings 8 and 9 are opposed to each other with the longitudinal direction of the grating shape being coincident, and the blazed inclination directions are the same when opposed. Furthermore, the polarization diffraction grating 8 and 9 made of a birefringent material layer in the same manner as the polarization diffraction grating 6, one period of a periodic diffraction grating pattern with each having a ordinary refractive index n _o and extraordinary refractive index n _e Are also equal to each other. Further, the slow axis of the birefringent material layer is set to be in the X-axis direction in the polarization diffraction grating 8 and in the Y-axis direction in the polarization diffraction grating 9. Further, in the gap between the polarizing diffraction gratings 8 and 9 opposed to each other, a homogeneous refractive index transparent material layer made of a material having a refractive index n _s substantially equal to the ordinary refractive index n _o of the birefringent material layer as a homogeneous refractive index transparent material. 10 is filled, and these are further sandwiched between the light-transmitting substrates 1 and 2. Further, the polarization diffraction element 101c has a structure in which the polarization refractive gratings 8 and 9 are adjacent to each other without the homogeneous refractive index transparent material layer 10.

  Here, the function and design of the blaze-shaped polarization diffraction gratings 8 and 9 in the polarization diffraction element 101b will be specifically described. When the incident light traveling in the Z direction is S-polarized light (X-axis direction), a refractive index difference is generated between the polarization diffraction grating 8 and the homogeneous refractive index transparent material layer 10, so that the light is diffracted. Since there is no difference in refractive index between 10 and the polarizing diffraction grating 9, the light passes straight through. On the other hand, in the P-polarized light (Y-axis direction), there is no difference in the refractive index between the polarization diffraction grating 8 and the homogeneous refractive index transparent material layer 10, so that the light travels straight, and the uniform refractive index transparent material layer 10 and polarization diffraction. Since a difference in refractive index occurs between the grating 9 and the grating 9, diffraction occurs. In the polarization diffraction element 101b, both the polarization diffraction gratings 8 and 9 are made of a birefringent material layer, and both the S-polarized light and the P-polarized light have a refractive index difference, so that they are diffracted at an angle θ that is axially symmetrical with each other. .

Further, when the cross sections of the polarization diffraction elements 101b and 101c have a pseudo-blazed shape (N = 4), the primary diffraction of the polarization diffraction grating 8 with respect to S polarization and the polarization diffraction grating 9 with respect to P polarization of 75% or more. to achieve the folding efficiency may represent a n _s X-axis in FIGS. 5 (a) in the same manner as described above by replacing the n _{o,
0.85 × λ 0 / | n e} -n o | ≦ d ≦ 1.15 × λ 0 / | n e -n o | ··· (5)
The blazed shape thickness d satisfying the condition is preferable because light loss due to diffraction can be reduced. In the case of a pseudo blazed shape having a number of steps larger than N = 4, the first-order diffraction efficiency becomes larger when the depth d is the same at N = 4 in FIG. 5A. For example, N = 8 As shown by the number of steps of 16, the characteristics of the first-order diffraction efficiency in the blaze shape are approached.

Further, when the cross sections of the polarization diffraction gratings 8 and 9 are blazed, the polarization diffraction grating 8 achieves a first-order diffraction efficiency of 75% or more for S-polarization and the polarization diffraction grating 9 for P-polarization. The depth d is
_{0.7 × λ 0 / | n e} -n o | ≦ d ≦ 1.3 × λ 0 / | n e -n o | ··· (6)
In order to realize a first-order diffraction efficiency of 90% or more, it is preferable that the depth d satisfies the above expression (5), so that the loss of light due to diffraction other than the first-order diffracted light can be reduced. .

Next, the primary diffraction efficiency is designed diffraction grating so as to maximize with respect to the wavelength lambda _0, the wavelength lambda and the wavelength lambda ₀ and the polarization S-polarized light in the diffraction grating 8 against a ratio lambda / lambda ₀ of polarization In order to achieve a first-order diffraction efficiency of 75% or more with respect to P-polarized light in the diffraction grating 9, when the cross section has a pseudo-blazed shape (N = 4),
0.87 × λ ₀ ≦ λ ≦ 1.18 × λ ₀ (7)
It is preferable that the wavelength λ satisfies the condition that light loss due to diffraction can be reduced. Note that equation (7), polarization grating depth d = λ ₀ / _| a possible range of the incident wavelength lambda at conditions | n e -n _o.

In order to achieve a first-order diffraction efficiency of 75% or more when the cross section is blazed,
0.77 × λ ₀ ≦ λ ≦ 1.43 × λ ₀ (8)
In order to realize a first-order diffraction efficiency of 90% or more, it is preferable that the depth d satisfies the above formula (7), so that the loss of light due to diffraction other than the first-order diffracted light can be reduced. .

The configuration of the polarization diffraction elements 101b and 101c is not limited to this, and the slow axis of the birefringent material layer may be the Y-axis direction with the polarization diffraction grating 8 and the X-axis direction with the polarization diffraction grating 9; The refractive index of the refractive index transparent material 10 may be equal to the extraordinary light refractive index of the birefringent material layer. The inclination direction of the blaze shape is also right-down on the paper surface in FIG. 6, but may be right-up on the paper surface. In FIG. 6, the refractive index of the polarization grating 8 in the S-polarized light The difference between the refractive index of the homogeneous refractive index transparent material layer 10 is Δn _s (≠ 0), and the difference between the refractive index of the polarization diffraction grating 9 and the refractive index of the homogeneous refractive index transparent material layer 10 in the P-polarized light is Δn _p. (≠ 0), and Δn _s and Δn _p are substantially equal to each other, they are deflected and separated from each other by the same angle θ in the axially symmetric direction with respect to the optical axis, and the diffraction of the same diffraction order. Efficiency is equal.

  Since the polarization diffraction element 101a has one blaze-shaped polarization diffraction grating, it has an advantage that it can be easily manufactured. On the other hand, the polarization diffraction elements 101b and 101c have two blazed polarization diffraction gratings. Therefore, when the same birefringent material is used, the depth d of the polarization diffraction gratings 8 and 9 is less than that of the polarization diffraction grating 6. Even if the pitch is narrow, the fabrication is easy and high first-order diffraction efficiency is easily realized, so that the diffraction angle, that is, the polarization separation angle θ can be increased.

Next, the configuration of the polarization separation element other than the polarization diffraction element will be described with reference to FIG. FIG. 7 is a schematic diagram showing a configuration using a Wollaston prism 101d as polarization separation means. The Wollaston prism is composed of two rectangular prisms 4 and 5 having a triangular prism shape made of an optically birefringent material such as calcite, yttrium vanadate (YVO ₄ ) crystal and rutile (TiO ₂ ) crystal. As viewed from the XY plane, the slow axis of the right-angle prism 4 and the slow axis of the right-angle prism 5 are integrated so as to be orthogonal to each other. Note that the boundary line formed by the right-angle prisms 4 and 5 constituting the Wollaston prism 101d shown in FIG. 7 is right-down on the page, and is the same on any plane (YZ plane) orthogonal to the X axis. Here, if the direction in which the boundary line continues is the longitudinal direction of the bonding interface between the right-angle prism 4 and the right-angle prism 5, the longitudinal direction of the bonding interface coincides with the X-axis direction. From FIG. 7, the longitudinal direction of the interface of the Wollaston prism corresponds to the direction orthogonal to the direction in which the light before separation is incident.

Here, the slow axis of the right-angle prism 4 is the Y-axis direction, the slow axis of the right-angle prism 5 is the X-axis direction, the fast axis of the right-angle prism 4 is the X-axis direction, and the fast axis of the right-angle prism 5 is the Y-axis. The direction. By setting in this way, the S-polarized light parallel to the X-axis is angled with respect to the optical axis by the law of refraction at the interface between the high-refractive index medium of the right-angle prism 5 and the low-refractive index medium of the right-angle prism 4. It advances in the direction inclined in the + Y direction at θ. On the other hand, the P-polarized light parallel to the Y-axis is in the −Y direction at an angle θ with respect to the optical axis according to the law of refraction at the interface between the low-refractive index medium of the right-angle prism 5 and the high-refractive index medium of the right-angle prism 4. Proceed in a tilted direction. In other words, the S-polarized light and the P-polarized light are deflected and separated by the same angle θ in the YZ plane in the axially symmetric direction with respect to the traveling direction of the incident light. Here, since the right-angle prism 4 and the right-angle prism 5 is made of the same material, the ordinary refractive index n _o and extraordinary refractive index n _e is also having the same characteristics. The slow axis direction of the Wollaston prism is not limited to this, and the slow axis of the right-angle prism 4 may be the X-axis direction, and the slow axis of the right-angle prism 5 may be the Y-axis direction.

2. Reflective liquid crystal elements (polarization modulation elements and reflective layers)
The polarization modulation element 102 and the reflective layer 16 constituting the reflective liquid crystal element 104, which are components of the light modulation liquid crystal element 100 of FIG. 1, will be described. The polarization modulation element has a function of changing the polarization state of incident light. Specifically, it is controlled by the magnitude of the voltage applied to the liquid crystal as a means of changing the polarization state, operates at a relatively low voltage, consumes little power, is small and lightweight, and is excellent in mass productivity. Therefore, it is preferable to use a liquid crystal element in which a liquid crystal layer is sandwiched between substrates with electrodes and a retardation value changes according to a voltage applied between the electrodes. As a configuration other than the liquid crystal element, an electro-optical crystal or a magneto-optical crystal may be used. In FIG. 1, the polarization modulation element 102 can modulate the polarization state of incident light in accordance with an electromagnetic field or the like applied from the voltage control device 103, and the incident light is reflected by the reflection layer 16 to be polarized. The polarization state is also modulated when passing through 102.

  A specific configuration will be described with reference to the reflective liquid crystal element 104a in FIG. FIG. 8 is a schematic diagram showing a configuration in the case where the single liquid crystal cell 102a and the reflective layer 16 are used as the polarization modulation element of the reflective liquid crystal element 104a. The single liquid crystal cell 102a includes a transparent substrate 2b having a transparent electrode 11a and an alignment film 12a formed on one side, and a transparent substrate 3 having a reflective layer 16a, the transparent electrode 11b, and an alignment film 12b formed on one side. The liquid crystal layer 13 is sandwiched and sealed to a desired thickness using a seal 14 therebetween. The single liquid crystal cell 102a indicates a structure including at least an electrode for applying a voltage to the liquid crystal layer 13. For example, the reflective layer 16a is a conductive material and is electrically connected to the voltage controller 103. In this case, the liquid crystal single cell is formed including the reflective layer 16a.

The alignment films 12a and 12b are arranged so that the alignment vectors (molecular alignment axes) of the liquid crystal molecules of the liquid crystal layer 13 are aligned in a specific direction in a plane parallel to the substrate (for example, a direction of 45 ° with respect to the X axis in the XY plane). Is provided with an orientation function. As the alignment film, a rubbed polyimide film, an obliquely deposited SiO film, an obliquely finely etched SiO ₂ film, or the like is used as an inorganic material. When the parallel alignment treatment is performed in which the alignment vectors of the liquid crystal molecules are the same in the alignment films 12a and 12b, the alignment of the liquid crystal molecules is aligned in that direction. The alignment treatment is not limited to this, and may be twist alignment treatment in different directions.

The liquid crystal used for the liquid crystal layer 13 may be nematic liquid crystal, but may be smectic liquid crystal or cholesteric liquid crystal. When a liquid crystal having a positive dielectric anisotropy is used, the liquid crystal alignment when no voltage is applied between the transparent electrodes 11a and 12a is substantially parallel to the translucent substrates 2b and 3 with a pretilt angle of several degrees. Thus, an alignment film is formed. The thickness of the liquid crystal layer 13 is such that when the voltage is not applied, the polarization state of the light reciprocally transmitted through the liquid crystal layer 13 is such that S-polarized light is converted to P-polarized light and P-polarized light is converted to S-polarized light. D _LC is adjusted.

For this purpose, when the refractive index anisotropy of the liquid crystal is Δn, Δn × d _LC is approximately λ with respect to the wavelength λ of the incident light when the alignment films 12a and 12b are aligned in parallel with each other. / (4 × Δn), and in the case of 90 ° twist alignment in which the alignment films 12a and 12b are aligned perpendicular to each other, Δn × d _LC may be about 1.5 times. As the voltage applied to the liquid crystal layer 13 is increased, the alignment in the major axis direction of the liquid crystal molecules of the liquid crystal layer 13 is inclined in the vertical direction with respect to the light-transmitting substrate. The retardation value is zero. At this time, even if the S-polarized light and the P-polarized light are transmitted through the liquid crystal layer 13, they maintain the same polarization state as the incident light. As described above, the ratio of the S-polarized light and the P-polarized light converted into the P-polarized light and the S-polarized light respectively changes depending on the magnitude of the voltage applied to the liquid crystal layer 13. The polarization state can be adjusted by controlling.

On the other hand, when a liquid crystal having a negative dielectric anisotropy is used, the alignment in the major axis direction of the liquid crystal molecules when no voltage is applied almost forms a pretilt angle of several degrees with respect to the normal lines of the translucent substrates 2b and 3. An alignment film is formed to achieve vertical alignment. At this time, the retardation value of the liquid crystal layer 13 is substantially zero, so that the S-polarized light and the P-polarized light maintain the same polarization state as the incident light even though they pass through the liquid crystal layer 13. As the voltage applied to the liquid crystal layer 13 increases, the alignment of the liquid crystal molecules in the liquid crystal layer 13 in the major axis direction becomes the alignment direction defined by the alignment films 12a and 12b in the horizontal direction with respect to the light-transmitting substrate. To tilt. The thickness of the liquid crystal layer 13 is such that S-polarized light is converted to P-polarized light and P-polarized light is converted to S-polarized light when the alignment in the major axis direction of the liquid crystal molecules is aligned in the horizontal direction. d Adjust _LC . Similarly, the ratio of S-polarized light and P-polarized light converted to P-polarized light and S-polarized light changes according to the magnitude of the voltage applied to the liquid crystal layer 13, respectively. The polarization state can be adjusted by controlling the thickness.

  Next, the reflective layer 16 in FIG. 1 will be described. The reflective layer 16 has a function of efficiently and regularly reflecting incident light, and includes a metal film made of a metal material such as Au or Al, a dielectric film having a relatively low refractive index, and a large dielectric film. A dielectric multilayer reflective film is used in which the optical film thickness is alternately laminated so that the optical film thickness is approximately λ / 4 with respect to the wavelength λ of the incident light. In the case of a metal film, it also functions as an electrode for applying a voltage to the liquid crystal layer 13, and in this case, the transparent electrode 11b is unnecessary. When the dielectric multilayer reflective film is used, the transparent electrode 11b may be formed on the light incident side of the dielectric multilayer reflective film as shown in FIG. Alternatively, a dielectric multilayer reflective film may be formed on the surface of the translucent substrate 3 on which the transparent electrode 11b is formed, except for the connection portion of the voltage control device 103. The reflective layer 16a may be disposed so as to be in contact with the transparent electrode 11b, but may be formed, for example, on the surface of the translucent substrate 3 opposite to the light incident side.

  Next, the configuration of the polarization modulation element 104b for lowering the voltage for adjusting the retardation value of the liquid crystal layer 13 will be described with reference to FIG. When a liquid crystal having positive dielectric anisotropy is used and the alignment direction of liquid crystal molecules when no voltage is applied is parallel alignment aligned with a specific direction in a plane parallel to the translucent substrates 2b and 3, the liquid crystal is increased as the applied voltage is increased. The retardation value approaches zero because the liquid crystal molecule alignment in the layer 13 is inclined in the direction perpendicular to the substrate surface, but the liquid crystal molecules on the alignment film surface are bound to the surface, so that the retardation value becomes completely zero. For example, a high applied voltage of 20 Vrms or higher is required as an effective value of applied voltage of rectangular wave alternating current. In order to reduce such a high applied voltage, the liquid crystal cell 102b is formed by laminating a phase plate 15 for canceling the retardation value of the liquid crystal layer 13 on the light incident side of the liquid crystal single cell 102a. As the phase plate 15, a birefringent crystal such as quartz, a birefringent film obtained by stretching an organic film such as polycarbonate, a polymer liquid crystal obtained by polymerizing and curing a liquid crystal monomer in a specific direction and irradiating light such as ultraviolet rays is used. It is done. The liquid crystal cell 102b may include a translucent substrate 2a.

  FIG. 10 shows a schematic diagram of the reflective liquid crystal element 104b as seen from the light incident surface (XY surface). In FIG. 10, in order to apply a voltage to the liquid crystal layer 13 by the transparent electrodes 11a and 12a, the extraction electrode formed on one side of the transparent electrode 11a and the translucent substrate 3 through a seal 14 mixed with conductive fine particles. 11c is connected to the voltage control device 103. Further, the transparent electrode 11 b and the extraction electrode 11 d that is electrically connected are connected to the voltage control device 103.

Further, the phase plate 15 is arranged so that the slow axis direction is orthogonal to the slow axis direction when no voltage is applied to the liquid crystal layer 13 using liquid crystal having positive dielectric anisotropy. Then, in certain applied voltage is not zero, and retardation value R _LC when transmitted through the liquid crystal layer 13 twice, designed to be substantially equal to the retardation value R _WP when passing through the phase plate 15 twice As a result, the total retardation value R = R _LC -R _WP of the liquid crystal layer 13 and the phase plate 15 can be offset to zero.

In the reflective liquid crystal element 104b having such a configuration, for example, the change amount of the total retardation value R is λ / 2 when both the liquid crystal layer 13 and the phase plate 15 are transmitted twice with an applied voltage change of 10 Vrms or less. If R _LC and R _WP are set to, a variable optical attenuator capable of obtaining a high extinction ratio with a low applied voltage can be realized as will be described later.

The voltage applied to the liquid crystal layer 13 and the retardation value in the reflective liquid crystal element 104b will be described. Here, an applied voltage at which the total retardation value R when the light is transmitted twice through the reflective liquid crystal element 104b is λ / 2 is V ₁ , and an applied voltage at which the total retardation value R is zero is V ₂ . In addition, FIGS. 11A and 11B schematically show the state of light when the voltages are V ₁ and V ₂ , respectively. Thus, in V ₁ , the retardation value becomes λ / 2 when the light of wavelength λ reciprocates, so that the S-polarized light and the P-polarized light are emitted as P-polarized light and S-polarized light, respectively. . On the other hand, since the retardation value is zero at V ₂ , S-polarized light and P-polarized light are emitted without changing the polarization state.

FIG. 12 shows a schematic diagram of the light modulation liquid crystal element 110 in which the polarization diffraction element 101a and the reflective liquid crystal element 104b are integrated. Further, a state of light when the applied voltage is set to _{V 1} and _{V 2} to the liquid crystal layer 13, respectively schematically shown in FIG. 13 (a), (b) and FIG. 14 (a), (b) . In the following description, an optical path from the incidence of light until reaching the reflection layer 16a is defined as an outward path, and an optical path from the reflection of the reflection layer 16a to emission is defined as a return path. As shown in FIG. 13A, at the applied voltage V ₁ , P-polarized light perpendicularly incident on the light modulation liquid crystal element 110 (−Z direction) is transmitted in the −Y direction by the polarization diffraction element 101 a in the forward path. Diffracted. Then, the light that is regularly reflected by the reflective layer 16a and reciprocates through the reflective liquid crystal element 104b becomes S-polarized light, reenters the polarization diffraction element 101a in the return path, is diffracted in the + Y direction, and is the same as the incident light. The light is emitted in the opposite direction (+ Z direction).

On the other hand, as shown in FIG. 13B, the S-polarized light incident perpendicularly to the light modulation liquid crystal element 110 is diffracted in the + Y direction by the polarization diffraction element 101a in the forward path. Then, the light that is regularly reflected by the reflective layer 16a and reciprocates through the reflective liquid crystal element 104b becomes P-polarized light, reenters the polarization diffraction element 101a in the return path, is diffracted in the −Y direction, and is the same as the incident light. The light is emitted in the opposite direction (+ Z direction). Thus, at the applied voltage V ₁ , the light incident on the light modulation liquid crystal element 110 and the light emitted from the light modulation liquid crystal element 110 are traveling in the same direction but in opposite directions regardless of the polarization state of the light.

Next, the state of light reciprocating in the light modulation liquid crystal element 110 at the applied voltage V ₂ will be described with reference to FIGS. First, as shown in FIG. 14A, at the applied voltage V ₂ , P-polarized light that is perpendicularly incident on the light modulation liquid crystal element 110 is diffracted in the −Y direction by the polarization diffraction element 101 a in the forward path. . The light that is regularly reflected by the reflective layer 16a and reciprocates through the reflective liquid crystal element 104b is re-incident on the polarization diffraction element 101a in the return path as P-polarized light, is diffracted in the −Y direction, and is different from the incident light. To exit. When the diffraction angle at the polarization diffraction element 101a is θ, the angle between the incident light and the outgoing light is approximately 2θ.

On the other hand, as shown in FIG. 14B, the S-polarized light incident perpendicularly to the light modulation liquid crystal element 110 is diffracted in the + Y direction by the polarization diffraction element 101a in the forward path. Then, the light that is regularly reflected by the reflective layer 16a and reciprocated through the reflective liquid crystal element 104b remains incident on the polarization diffraction element 101a in the return path as S-polarized light, is diffracted in the + Y direction, and is in a direction different from the incident light. Exit. Thus, the applied voltage V _2, the light emitting light and a light modulating liquid crystal element 110 entering the light modulation liquid crystal element 110, regardless of the polarization state of light, the different traveling directions.

When the applied voltage is between V ₁ and V ₂ , a state in which the outgoing light component in the same direction as the incident light and the outgoing light component in the direction different from the incident light are mixed, that is, FIG. The light traveling state shown in FIG. 14 is mixed, and the ratio of the amounts of these light components changes depending on the magnitude of the applied voltage. Thus, for example, by receiving only the outgoing light component in the same direction as the incident light and in the opposite direction, a light modulation liquid crystal element in which the amount of received light can be changed according to the applied voltage can be obtained.

(Second Embodiment)
FIG. 15 shows an example of a schematic diagram of a variable optical attenuator 200 including the optical fiber 18, the condensing lens 17, and the light modulation liquid crystal element 110. FIG. 15A and FIG. 15B show how light modulated liquid crystal element 110 propagates when voltages applied to the liquid crystal layer are V ₁ and V ₂ , respectively. In the variable optical attenuator 200, the divergent light emitted from the optical fiber 18 is transmitted through the condenser lens 17 to become parallel light, reflected by the light modulation liquid crystal element 110, and again reflected by the condenser lens 17 among the reflected light. 18 has a function of propagating light condensed. The optical fiber 18 is installed at the focal position of the condenser lens 17. Light (hereinafter referred to as random polarization) in which S-polarized light and P-polarized light emitted from the optical fiber 18 are mixed and converted into parallel light by the condenser lens 17 and enters the light modulation liquid crystal element 110.

FIG. 15A is a schematic diagram showing a state when the applied voltage is V ₁ , and the light incident on the light modulation liquid crystal element 110 is reflected and travels in the opposite direction parallel to the traveling direction. The light is condensed at a position where light is emitted from the optical fiber 18 by the condensing lens 17 and propagates through the optical fiber 18 in the opposite direction. On the other hand, FIG. 15B is a schematic diagram showing a state when the applied voltage is V ₂ , and the light incident on the light modulation liquid crystal element 110 is reflected and becomes emitted light in a direction different from the incident direction. In this way, since the light is condensed by the condensing lens 17 at a position different from the position where the light is emitted from the optical fiber 18, it cannot propagate through the optical fiber. Further, the function of the light modulation liquid crystal element 110 realizes an optical attenuator in which the amount of light returning the optical fiber is variable depending on the magnitude of the applied voltage regardless of the polarization state of the incident light.

In the above example, since the position of the light emitted from the optical fiber is arranged at the focal position on the optical axis of the condensing lens, the reflected light is collected at the same optical fiber position, but other arrangements may be used. Absent. For example, the position of the light emitted from the optical fiber is shifted from the position of the optical fiber 18 in FIG. 15 by a distance A (not shown) in the + X direction. Since the center of the light emitted from the optical fiber does not pass through the center of the condenser lens 17, the light axis is incident on the light modulation liquid crystal element 110 with an inclination. For this reason, when the applied voltage V ₁ is applied, the light reflected by the reflective layer 16a and emitted from the light modulation liquid crystal element 110 is transmitted again through the condenser lens 17, and the same distance in the −X direction, that is, the optical fiber 18 of FIG. The light is condensed at a position shifted by a distance A in the −X direction with respect to the position. By arranging a light receiving optical fiber different from the optical fiber on the light emitting side at the condensing point, the light propagates to the light receiving optical fiber.

Further, as an optical system (element) on the light emitting side, a semiconductor laser or the like may be used as a light source instead of an optical fiber, and an optical device using a photodetector on the light receiving side may be used. As described above, when the light modulation liquid crystal element 110 is used as an optical attenuator that varies the amount of light according to the applied voltage, in order to obtain a high extinction ratio, the applied voltage V ₁ is fed back to a predetermined condensing point. as well as improving the amount of light, at an applied voltage V ₂ is possible to reduce the amount of light fed back to the same focusing point becomes important.

In particular, when the applied voltage V ₂ as shown in FIG. 15 (b), the can better amount of light fed back to the same point as the position of the light emitted from the optical fiber 18 is small to obtain a high extinction ratio. At this time, as the light returning to the optical fiber 18, Fresnel reflected light generated due to the difference in the refractive index of the material forming the interface exists at the interface parallel to the reflective layer 16 constituting the light modulation liquid crystal element 110. Therefore, a higher extinction ratio can be realized by reducing the amount of the Fresnel reflected light as follows.

  Specifically, in the light modulation liquid crystal element 110 shown in FIG. 12, air (medium on which light is incident) and the transparent substrate 1, the transparent substrate 1 and the polarization diffraction grating 6, the homogeneous refractive index transparent material 7 and the transparent material. It is necessary to reduce the Fresnel reflected light generated at the interface between the optical substrate 2a, the translucent substrate 2a and the phase plate 15, the phase plate 15 and the translucent substrate 2b, and the translucent substrate 2b and the transparent electrode 11a. As a measure for reducing the reflected light generated at these interfaces, for example, an antireflection film or a light diffusing surface with fine irregularities may be formed on the surface of the translucent substrate.

  In addition, a configuration for reducing the amount of light received by the optical fiber of Fresnel reflected light generated at the interface will be described using the light modulation liquid crystal element 120 shown in FIG. 16, but the same optical components as those of the light modulation liquid crystal element 110 have the same reference numerals. To avoid duplication of explanation. Here, the point that the reflection surface of the reflection layer 16 is inclined with respect to the light modulation liquid crystal element 110 is different. In the light modulation liquid crystal element 120, a reflective layer 16b is formed on one surface of a transparent substrate 3a with a wedge having an inclination of an angle α to form a reflective liquid crystal element 104c, and the reflective surface is inclined in combination with the polarization diffraction element 101a. The light modulation liquid crystal element 120 is used. By doing so, it is possible to reduce the Fresnel reflected light that reaches the position (such as an optical fiber) that receives the light reflected by the reflective layer 16b, so that a higher extinction ratio can be realized.

  In addition, in the light modulation liquid crystal element 130 shown in FIG. 17, the interface between the air having a large amount of Fresnel reflection light generated due to the large difference in refractive index and the translucent substrate 1a can be inclined with respect to the reflection surface. It is valid. Here, a polarizing diffraction element comprising an antireflection film (not shown) formed on the light incident surface of a transparent substrate 1a with a wedge having an inclination of angle β, and comprising a polarizing diffraction grating 6 and a homogeneous refractive index transparent material layer 7 The light modulation liquid crystal element 130 has an incident surface inclined to the reflection surface of the reflection layer 16a.

  The light modulation liquid crystal element and the variable light attenuator of the present invention can realize a light modulation liquid crystal element in various forms including polarization separation means, polarization modulation means, and reflection means other than those described in the above embodiments.

(Example 1)
A method for manufacturing the light modulation liquid crystal element 110 of the present invention will be described with reference to FIGS. First, a method for manufacturing the polarization diffraction element 101a as a polarization separation element will be described. Polyimide is applied to one side of a quartz glass substrate as the translucent substrate 1 and cured, and rubbed in one direction to form an alignment film (not shown). Another translucent substrate (not shown) on which an alignment film is formed is prepared, a sealing material having a certain thickness is arranged with the alignment films facing each other so that the alignment directions are parallel, and a liquid crystal is formed in the gap formed. After injecting and filling the monomer solution, the liquid crystal molecules are aligned so that the alignment vector (molecular alignment axis) of the liquid crystal molecules is aligned in a specific direction (X-axis direction in FIG. 12) in the plane parallel to the substrate. Irradiate to cure. The other light-transmitting substrate (not shown) is subjected to a release treatment, and a polymer liquid crystal having a thickness of 14 μm and a normal light refractive index n _o = 1.55 and an extraordinary light refractive index n _e = 1.75 in light having a wavelength of 1.56 μm. A layer is formed and processed into a staircase-like pseudo-blazed diffraction grating shape approximated by 7 steps and 8 levels (N = 8) using a photolithography and etching technique with a grating pitch of 40 μm. The longitudinal direction of the pseudo blazed shape is made parallel to the alignment direction of the polymer liquid crystal layer.

Further, the surface of the quartz substrate provided with the pseudo-blazed diffraction grating is opposed to the quartz glass substrate as the translucent substrate 2a with an interval of 30 μm (at the lowest part of the grating). The ultraviolet curable composition used as a transparent material which has an isotropic refractive index after hardening | curing is inject | poured from an injection hole, and it hardens | cures by ultraviolet irradiation. Thus, in the light of the wavelength lambda ₀ = 1.56 .mu.m, the average value of the ordinary refractive index _{n o} and extraordinary refractive index _{n e} of the liquid crystal polymer _(n o _{+ n} e) / 2 corresponding to the refractive index _{n s} = 1.65 homogeneous refractive index transparent material layer 7 is formed to be a polarization diffraction element 101a.

Next, a method for manufacturing the reflective liquid crystal element 104b will be described. On one side of a quartz glass substrate as the translucent substrate 3, SiO ₂ with a refractive index of 1.45 and Ta ₂ O ₅ with a refractive index of 2.15 have an optical film thickness (refractive index × film thickness) with a wavelength of 1.56 μm. A dielectric multilayer reflective film having a reflectivity of 98% or more in the wavelength band of 1480 to 1620 nm is formed as the reflective layer 16 by alternately stacking the layers so as to be 1/4. Further, ITO serving as the transparent electrodes 11b and 11a is formed on one surface of the same quartz glass substrate as the surface of the dielectric multilayer reflective film and the translucent substrate 2b. Further, polyimide is applied to the surface of each ITO, and alignment films 12a and 12b are formed by rubbing treatment. The alignment films 12a and 12b are opposed to each other and arranged in parallel by using a seal 14 having a thickness of about 2.8 μm, and liquid crystal is injected into these gaps from an injection port (not shown) to be filled and sealed. Form.

As a liquid crystal, a nematic liquid crystal having a positive dielectric anisotropy having an ordinary refractive index n _o = 1.50 and an extraordinary refractive index n _e = 1.66 with respect to light having a wavelength of 1.56 μm is used. As shown, a liquid crystal unit cell 102a having a thickness d _LC of the liquid crystal layer 13 of 2.8 μm is produced. Here, the alignment films 12a and 12b are aligned so that the slow axis direction of the liquid crystal layer 13 is 45 ° to the Y-axis direction and parallel to the substrate as shown in FIG. At this time, for the light having a wavelength of 1.56 μm, the liquid crystal single cell 102 a in a state where no voltage is applied to the liquid crystal layer has a retardation value R _LC of 0.45 μm, and the voltage controller 103 applies a rectangular wave AC having an effective voltage of 5 Vrms. In a state where a voltage is applied, the retardation value R _LC of the liquid crystal single cell 102a is set to 0.06 μm.

Further, the ordinary light refractive index n _o = 1.55 and the extraordinary light refractive index n _e = 1.59 are formed on the surface of the translucent substrate 2a of the polarization diffraction element 101a having an antireflection film (not shown) formed on one side. Then, a phase plate 15 made of a polymer liquid crystal layer having a thickness d = 1.5 μm is bonded and integrated to a single liquid crystal cell to form a light modulation liquid crystal element 110. Here, the phase plate 15 is arranged so that the slow axis direction thereof is orthogonal to the slow axis direction of the liquid crystal layer 13 so that the retardation value _RWP is 0.06 μm. As a result, the retardation value 0.06 μm remaining in the liquid crystal layer 13 at the applied voltage V ₂ = 5 Vrms is canceled out, the retardation value R of the liquid crystal cell 102 b becomes zero, and the liquid crystal cell 102 b at the applied voltage zero (V ₁ = 0). The retardation value R is 0.39 μm.

In the light modulation liquid crystal element 110 manufactured in this way, when the applied voltage is zero, the S-polarized light and the P-polarized light incident on the polarization diffraction element 101a reciprocate through the liquid crystal cell 102b, respectively. It is converted into polarized light and emitted. On the other hand, when the applied voltage V ₂ = 5 Vrms, the S-polarized light and the P-polarized light that reciprocate through the liquid crystal cell 102b are emitted without changing the polarization state.

(Example 2)
An optical attenuator 200 as shown in FIG. 15 comprising the light modulation liquid crystal element 110 produced in Example 1, the optical fiber 18 having both functions of light emission and light reception, and the condenser lens 17 is configured. Random polarized light with a wavelength of 1.52 μm to 1.60 μm emitted from the optical fiber 18 is converted into parallel light by the condenser lens 17 and incident on the light modulation liquid crystal element 110, and the outgoing light reflected by the light modulation liquid crystal element 110 is reflected by the condenser lens 17. Thus, the light is condensed around the optical fiber 18. When the amplitude of the rectangular AC voltage applied to the liquid crystal layer 13 of the light modulation liquid crystal element 110 is changed from V ₁ = 0 Vrms to V ₂ = 5 Vrms, light that travels back and forth through the condensing lens 17 and the light modulation liquid crystal element 110 is transmitted through the optical fiber. A variable optical attenuator in which the intensity I (V) changes from the maximum to the minimum is realized.

At this time, the extinction ratio defined by the light intensity ratio I (0 Vrms) / I (5 Vrms) is as high as 22 dB. Further, the optical insertion loss I _L (0 Vrms) associated with the use of the light modulation liquid crystal element 110 is a low value of about −1 dB. Further, when the applied voltage is switched between V ₁ = 0 Vrms and V ₂ = 5 Vrms, the optical response time is 5 msec or less, and the response time is about 1 because the thickness of the liquid crystal layer is about half that of the conventional transmissive liquid crystal element. / 4 speed. Furthermore, even if the wavelength of the incident light changes from 1.52 μm to 1.60 μm, there is no change in the position of the condensing point collected by the condensing lens 17 on the optical fiber 18, and the intensity of the light transmitted through the optical fiber is stable. To do.

(Example 3)
As Example 3, an optical attenuator using a polarization separation element using a Wollaston prism is configured. Although the optical attenuator of the third embodiment is not shown, a Wollaston prism 101d made of a birefringent crystal shown in FIG. 7 is used instead of the polarization diffraction element 101a in the light modulation liquid crystal element 110 of the second embodiment.

The Wollaston prism 101d uses an yttrium vanadate (YVO ₄ ) crystal having an ordinary refractive index n _o = 1.945 and an extraordinary refractive index n _e = 2.148 as a birefringent crystal, and has a light transmission surface and a slope of 30. It is fabricated by processing into two right-angle prisms 4 and 5 having a triangular prism shape with an angle of °, and fixing the slopes of each other with an adhesive. The slow axes indicating the extraordinary refractive indexes of the right-angle prisms 4 and 5 are set so that the right-angle prism 4 is in the Y direction and the right-angle prism 5 is in the X direction in FIG. Here, the S-polarized light and the P-polarized light that are perpendicularly incident on the Wollaston prism 101d are refracted at the interface and the exit surface of the right-angle prisms 4 and 5, and the exit surface is quartz glass having a refractive index of 1.45. S-polarized light is emitted at an angle of θ = 4.6 ° in the −Y direction, and P-polarized light is emitted at an angle of θ = 4.7 ° in the + Y direction.

The Wollaston prism 101d thus fabricated is integrated with the reflective liquid crystal element 104b of Example 1 to obtain a light modulation liquid crystal element. In FIG. 15, the light modulation liquid crystal element according to the present embodiment is disposed instead of the light modulation liquid crystal element 110 according to the first embodiment, and the amplitude of the rectangular AC voltage applied to the liquid crystal layer 13 is from V ₁ = 0 Vrms to V ₂ = 5 Vrms. When it is changed, a variable optical attenuator is realized in which the light intensity I (V) transmitted through the optical fiber changes from the maximum to the minimum. At this time, the extinction ratio defined by the light intensity ratio I (0 Vrms) / I (5 Vrms) is as high as 35 dB. Further, the optical insertion loss I _L (0 Vrms) associated with the use of the light modulation liquid crystal element is a low value of about −0.3 dB.

  As described above, according to the present invention, a stable light modulation liquid crystal element having a low optical insertion loss and a high extinction ratio regardless of the polarization direction of incident light, and a variable optical attenuator using the same can be realized. Further, by adopting the configuration of the light modulation liquid crystal element of the present invention, a light modulation liquid crystal element having a high response speed and a variable optical attenuator using the light modulation liquid crystal element can be realized. Furthermore, it is possible to realize a variable optical attenuator that can obtain a stable extinction ratio with less wavelength dependency over a wide wavelength band of incident light.

The side surface schematic diagram which shows the basic composition of the light modulation liquid crystal element of this invention. FIG. 3 is a schematic plan view showing the basic configuration of the light modulation liquid crystal element of the present invention. The side surface schematic diagram which shows the function of the polarization separation element which is a component of the light modulation liquid crystal element of this invention. The cross-sectional schematic diagram which shows the structural example which used the polarization separation element which is a component of the light modulation liquid crystal element of this invention as the polarization | polarized-light diffraction element. The schematic diagram which shows the definition of the 1st-order diffraction efficiency characteristic with respect to the depth change of a diffraction grating, the 1st-order diffraction efficiency characteristic with respect to the wavelength fluctuation of the light which injects into a diffraction grating, and the depth of a blazed shape and a pseudo-blazed shape. The cross-sectional schematic diagram which shows the other structural example which used the polarization | polarized-light separation element which is a component of the light modulation liquid crystal element of this invention as the polarization | polarized-light diffraction element. The cross-sectional schematic diagram which shows the structural example which used the polarization separation element which is a component of the light modulation liquid crystal element of this invention as the Wollaston prism. The cross-sectional schematic diagram which shows the structural example which used the polarization | polarized-light modulation element and reflective layer which are the components of the light modulation liquid crystal element of this invention as the reflection type liquid crystal element. The cross-sectional schematic diagram which shows the other structural example which used the reflection type liquid crystal element as the polarization | polarized-light modulation element and reflective layer which are the components of the light modulation liquid crystal element of this invention. The plane schematic diagram of a reflection type liquid crystal element. The cross-sectional schematic diagram which shows the function of a reflection type liquid crystal element. The cross-sectional schematic diagram which shows the structural example of a light modulation liquid crystal element. FIG. 3 is a schematic cross-sectional view showing the function of a light modulation liquid crystal element (voltage = V ₁ ). FIG. 3 is a schematic cross-sectional view showing the function of a light modulation liquid crystal element (voltage = V ₂ ). The schematic diagram which shows the structural example of the variable optical attenuator which consists of the light modulation liquid crystal element of this invention. Sectional drawing which shows the structural example of Fresnel reflected light reduction in the light modulation liquid crystal element of this invention. Sectional drawing which shows the other structural example of Fresnel reflected light reduction in the light modulation liquid crystal element of this invention. The cross-sectional schematic diagram which shows the structural example of the conventional variable optical attenuator. FIG. 10 is a schematic cross-sectional view showing a configuration example of a conventional projector.

Explanation of symbols

1, 1a, 2, 2a, 2b, 3, 3a Translucent substrate 4, 5 Right angle prism 6, 8, 9 Polarization diffraction grating 7, 10 Homogeneous refractive index transparent material 11a, 11b Transparent electrode 11c, 11d Extraction electrode 12a, 12b Alignment film 13 Liquid crystal layer 14 Seal 15 Phase plate 16 Reflection layer 16a, 16b Reflection layer 17 Condensing lens 18 Optical fiber 100, 110, 120, 130 Light modulation liquid crystal element 101 Polarization separation element 101a, 101b, 101d, 101e Polarization diffraction element 101d Wollaston prism 102 Polarization modulation element 102a Liquid crystal single cell 102b Liquid crystal cell 103 Voltage control device 104, 104a, 104b, 104c Reflective liquid crystal element 200 Optical attenuator 300 Liquid crystal element (conventional)
310 Liquid crystal cell (conventional)
320 First polarization beam splitter 330 Second polarization beam splitter 400 Projector 410 Lens 420 Polarization separator 421 Polarizing diffraction grating 430 Liquid crystal panel 440 Main mirror 450 Projection lens

Claims (8)

A light modulation liquid crystal element for modulating the light intensity of incident light,
The light modulation liquid crystal element includes a polarization separation element and a reflective liquid crystal element,
The polarization separating element includes a birefringent material layer having a cross section made of a birefringent material on a translucent substrate and having a blazed shape or a quasi-blazed shape approximating a blazed shape like a staircase and an optically transparent optical material layer. Or a Wollaston prism that is bonded so that the slow axes of the birefringent materials are orthogonal to each other,
The longitudinal direction of the grating having the blazed shape or the pseudo-blazed shape of the birefringent material layer, or the longitudinal direction of the bonding interface of the Wollaston prism is substantially the slow axis direction or the fast axis direction of the birefringent material. Match
A polarization direction parallel to the longitudinal direction of the birefringent material layer or the longitudinal direction of the Wollaston prism is a first polarization direction, and a polarization direction orthogonal to the first polarization direction is a second polarization direction. The difference between the refractive index of the birefringent material layer and the refractive index of the optical material layer with respect to the first polarization direction, or the difference of the refractive index of the Wollaston prism with respect to the first polarization direction is expressed by Δn ₁ (≠ 0 ), the second difference between the refractive index of the optical material layer and the refractive index of the birefringent material layer with respect to the polarization direction or the relative said second polarization direction wollastonite a difference in refractive index of the emission prism [Delta] n _2, ( ≠ 0), Δn ₁ and Δn ₂ are substantially equal,
The reflective liquid crystal element includes an electrode, a polarization modulator including a liquid crystal layer that changes a retardation value depending on a voltage applied from the electrode, and a reflective layer that regularly reflects light incident on the liquid crystal layer. A light modulation liquid crystal element provided. The polarization separation element is the polarization diffraction element,
The polarization diffraction element has one optical material layer so as to fill at least the concave portion of the one birefringent material layer and the birefringent material layer,
The optical material layer of the polarization diffraction element is an optically isotropic homogeneous refractive index transparent material layer,
The ordinary refractive index of the birefringent material layer n _o, the extraordinary refractive index n _e, the refractive index of the homogeneous refractive index transparent material layer when the n _s,
When n _s is to be n _o and n substantially with an intermediate value of _e, the wavelength lambda ₀ the wavelength in which the primary diffraction efficiency is maximum among the light incident,
If the cross section made of the birefringent material is blazed,
_{0.7 × λ 0 / | n e} -n s | ≦ d ≦ 1.3 × λ 0 / | n e -n s | a filled,
When the cross section made of the birefringent material is a pseudo blazed shape,
0.85 × λ ₀ / | n _e −n _s | ≦ d ≦ 1.15 × λ ₀ / | n _e −n _s |, the depth of the polarizing diffraction grating composed of the birefringent material layer The light modulation liquid crystal element according to claim 1, wherein d is set. When the depth d is approximately equal to λ ₀ / | n _e −n _s |
If the cross section made of the birefringent material is blazed,
Satisfies 0.77 × λ ₀ ≦ λ ≦ 1.43 × λ ₀ ,
When the cross section made of the birefringent material is a pseudo blazed shape,
The light modulation liquid crystal device according to claim 2, wherein light having a wavelength satisfying 0.87 × λ ₀ ≦ λ ≦ 1.18 × λ ₀ is incident. The polarization separation element is the polarization diffraction element,
The polarization diffraction element has a pair of translucent substrates having the birefringent material layer, and the longitudinal directions of the pair of birefringent material layers match.
The optical material layer filling at least the recesses of the birefringent material layer;
The optical material layer of the polarization diffraction element is an optically isotropic homogeneous refractive index transparent material layer,
The ordinary refractive index of the birefringent material layer n _o, the extraordinary refractive index n _e, the refractive index of the homogeneous refractive index transparent material layer when the n _s,
The refractive index n _s with the slow axis direction are orthogonal to each other of the pair of the birefringent material layer is substantially with equal to one of the ordinary refractive index n _o or the extraordinary refractive index n _e, is incident When the wavelength at which the first-order diffraction efficiency is maximum among the light is the wavelength λ ₀ ,
If the cross section made of the birefringent material is blazed,
_{0.7 × λ 0 / | n e} -n o | ≦ d ≦ 1.3 × λ 0 / | n e -n o | the meet,
When the cross section made of the birefringent material is a pseudo blazed shape,
0.85 × λ ₀ / | n _e −n _o | ≦ d ≦ 1.15 × λ ₀ / | n _e −n _o |, the depth of the polarizing diffraction grating composed of the birefringent material layer. The light modulation liquid crystal element according to claim 1, wherein d is set. When the depth d is approximately equal to λ ₀ / | n _e −n _o |
If the cross section made of the birefringent material is blazed,
Satisfies 0.77 × λ ₀ ≦ λ ≦ 1.43 × λ ₀ ,
When the cross section made of the birefringent material is a pseudo blazed shape,
The light modulation liquid crystal element according to claim 4, wherein light having a wavelength satisfying 0.87 × λ ₀ ≦ λ ≦ 1.18 × λ ₀ is incident. The polarization separation element is a triangular prism-shaped right-angle prism in which both the birefringent material layer and the optical material layer are optically birefringent,
The light modulation liquid crystal element according to claim 1, which is a Wollaston prism in which slow axes of the two right-angle prisms are orthogonal to each other. The polarization modulation element is a liquid crystal cell composed of the single liquid crystal cell and a phase plate,
If the voltage applied to the liquid crystal alone cell is V _1, with a retardation value of lambda _0/2 when light of the wavelength lambda ₀ to reciprocate the liquid crystal cell,
The retardation value when the light of the wavelength λ ₀ reciprocates through the liquid crystal cell is zero when the voltage applied to the single liquid crystal cell is V ₂ (V ₁ ≠ V ₂ ). The light modulation liquid crystal element according to item. A light source;
A condenser lens;
The light modulation liquid crystal element according to any one of claims 1 to 7, in a traveling direction of light transmitted through the condenser lens;
A variable optical attenuator comprising light receiving means for receiving light in a specific traveling direction reflected and emitted from the light modulation liquid crystal element;
A variable optical attenuator in which the amount of light received by the light receiving means changes according to the magnitude of a voltage applied to the light modulation liquid crystal element.

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