JP2008185768A - Wavelength plate and optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wide-band wavelength plate having excellent reliability and to provide an optical device provided with the wavelength plate. <P>SOLUTION: The wavelength plate (1/2 wavelength plate) 25 is structured by laminating 2 birefringent plates 25A, 25B each comprising KTP (KTiOPO<SB>4</SB>) crystal to cross each in-plane optical axes D1, D2 each other at a right angle. The KTP crystal is biaxial birefringent crystal transparent in a range from visible region to near-infred region. When the KTP crystal is used as a birefringent material, the change of the birefringence in the visible light region of 400-700 nm is about 0.005 at the maximum which is 1/2 of that of quartz. Then, the degree of the phase shift in 400-700 nm wavelength is minimized to attain stable transmission characteristics over the wide-band. Because the KTP has smaller wavelength dependency compared to that of the quartz, laminated plates are reduced. As a result, a wide-band zero-order wave plate having high reliability is obtained at a low cost. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a wavelength plate suitable for use in, for example, a polarization conversion element incorporated in an optical system of a liquid crystal projector, and an optical apparatus including the same.

  In recent years, a liquid crystal projector that projects a light beam from a light source such as an ultra-high pressure mercury lamp onto a screen through a light modulation element composed of a reflective or transmissive liquid crystal light valve and a projection lens is widely used as a flat screen television or a presentation tool. ing. Since the liquid crystal device is a polarization rotation device, the brightness control of the screen is effective only for linearly polarized light. In the projector optical system, since the light from the lamp is non-polarized light, a polarization conversion element (PBC) that converts the light into constant linearly polarized light is used.

  On the other hand, with the progress of thin display device technology, demands for higher image brightness and contrast are increasing in recent years. In the projector system, it is necessary to increase the power of the light source in order to increase the brightness of the projected image. There is a problem that when the amount of light increases, the heat generated in the optical component increases. The effects of organic materials such as retardation plates are significant, especially in light-absorbing devices such as polarizers, where heat generation due to absorption is large, and thermal destruction of the polarizer, which is an organic material, occurs. There is.

  The wave plate used in the polarization conversion element is irradiated with high-power optical power in the vicinity of the light source, which causes problems such as destruction due to increased absorption due to material deterioration and peeling due to deterioration of the adhesive layer. . In addition, even if it is not obvious deterioration due to alteration of these materials, there is a problem that the retardation plate placed on the liquid crystal panel causes uneven color of the screen and deterioration of contrast due to the change of the birefringence due to heat generation. . Therefore, when high luminance is required, it is considered that an organic material is avoided and an inorganic birefringent crystal such as quartz is used as described in Patent Document 1 below.

The inorganic wave plate can be obtained by cutting a birefringent material so that the crystal axis is in the surface direction and polishing it to a thickness that provides a desired phase shift. For example, in the case of using quartz, at λ (wavelength) = 546 nm, ne (refractive index of extraordinary ray) = 1.55534, no (refractive index of ordinary ray) = 1.54617, and birefringence is 0.00917. is there. In order to obtain a quarter wave plate, the thickness is t,
(Ne-no) t = λ / 4
When λ = 546 nm, the thickness t is 14.8 μm.

However, since this thickness is too thin and difficult to handle, usually when the wavelength range to be used is narrow,
(Ne−no) t = {m + (1/4)} λ
The phase shift condition is satisfied only in the vicinity of the center wavelength. For example, if m = 9, the thickness t is 551 μm.

  Here, m is the order. A wave plate with m ≠ 0 is called a high-order wave plate, and a wave plate with m = 0 is called a zero-order wave plate. The 0th-order wave plate can be produced not only by reducing the thickness but also by bonding two birefringent materials so that their anisotropic axes are orthogonal to each other. In this case, the phase shift code is canceled for the first and second sheets. In the case of this configuration, it is necessary to make the difference in thickness between the two birefringent materials to be bonded coincide with the zero-order wave plate thickness of a single plate.

JP 2004-170853 A

  The wavelength dependence of the wave plate can be broadly divided into an intrinsic one based on the relationship between the wavelength and the thickness, and an intrinsic material due to dispersion of the refractive index and birefringence. The polymer material can be designed to cancel the essential part of the former by material optimization or to reduce the part caused by the latter material. However, in the case of an inorganic material such as an optical crystal, the direction and magnitude of refractive index dispersion (the shorter the wavelength, the greater the refractive index and birefringence) cannot be changed greatly. It is an important factor that determines the bandwidth.

  For this reason, even a zero-order wave plate cannot operate over a wide band of wavelengths from 400 to 700 nm. On the other hand, as disclosed in Patent Document 1, it is possible to cancel the wavelength dependency between the first and second sheets by bonding the wave plates so that their anisotropic axes have a predetermined relationship. It is.

  However, if the two wave plates to be bonded are of higher order, the wavelength dependence due to the higher order is remarkably large, so the effect of the bonding is reduced. When the two wave plates are both in the 0th order, a wave plate operating in a wide band of wavelengths from 400 to 700 nm can be obtained. However, when two birefringent plates are used to form the 0th wave plate, In total, four birefringent plates are required, which is very expensive.

  On the other hand, in the orthogonally bonded type zero-order wave plate, the major factor that determines the operating band is the refractive index dispersion of the material. By using a material with small dispersion, the wave plate can be widened. However, the most commonly used quartz crystal as the inorganic birefringence material has a birefringence change of about 0.01 at the maximum in the visible light band of 400 to 700 nm. The wavelength dependence of the phase shift amount when a quarter wavelength plate is configured is 180 degrees or more at a wavelength of 400 to 700 nm, and a highly reliable broadband wavelength plate cannot be obtained.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a broadband wave plate excellent in reliability and an optical apparatus including the same.

  In solving the above problems, the wave plate of the present invention is a wave plate formed by bonding a plurality of birefringent plates so that their optical axes are orthogonal to each other, and the birefringent plate is made of a KTP crystal. It is characterized by becoming.

The KTP (potassium phosphate titanate: KTiOPO ₄ ) crystal is a biaxial birefringent crystal that is transparent from the visible region to the near infrared region, and the refractive indexes of the a-axis, b-axis, and c-axis at a wavelength of 588 nm are expressed as na. : 1.7898, nb: 1.76809, nc: 1.87429. The birefringence is 0.106 between the b-axis and the c-axis and 0.0108 between the a-axis and the b-axis. When a KTP crystal is used as the birefringent material, the maximum birefringence change in the visible light band of 400 to 700 nm is about 0.005, which is ½ of quartz. Therefore, the amount of phase shift at a wavelength of 400 to 700 nm can be kept small, and stable transmission characteristics can be obtained over a wide band. In addition, since KTP has less wavelength dependency than quartz, the number of bonded sheets can be reduced. This makes it possible to obtain a highly reliable broadband 0th-order wave plate at low cost.

  The wave plate according to the present invention can be configured as a half-wave plate, a quarter-wave plate, or the like. For example, when two half-wave plates are bonded together to produce a zero-order half-wave plate, the thicknesses of the two half-wave plates are made different from each other, and the difference Δt in thickness is , (Ne−no) Δt = (1/2) λ.

  As an application example of the half-wave plate, a polarization conversion element (PBC) or the like in an optical apparatus such as a liquid crystal projector is preferable. Since the wave plate according to the present invention is made of an inorganic material, even if it is disposed in the vicinity of the light source, a change in birefringence due to heat absorption hardly occurs. In addition, since the wavelength dependency of the refractive index is smaller than that of quartz or the like, highly reliable optical characteristics can be obtained, and a wave plate can be formed with a reduced number of bonded sheets, so that the element size can be reduced. Contributes to downsizing / thinning and cost reduction.

  As described above, according to the present invention, it is possible to obtain a broadband wave plate excellent in reliability at a low cost.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of an optical device 10 according to an embodiment of the present invention. The optical device 10 of the present embodiment is configured as a liquid crystal projector (image display device).

  In the optical device 10, a light source 11 made of, for example, an ultrahigh pressure mercury lamp irradiates white illumination light L toward an integrator lens 12 made of a pair of fly-eye lenses. The integrator lens 12 makes the light amount distribution of the illumination light L uniform and makes it incident on the polarization conversion element 13. The polarization conversion element 13 converts the P-polarized component of the incident illumination light L into an S-polarized component and makes it incident on the condenser lens 14. The condenser lens 14 converts the light emitted from the polarization conversion element 13 into light that matches the optical path length of the color separation optical system at the next stage and emits the light.

  The color separation optical system includes a first dichroic mirror 15A that reflects blue band light and transmits red and green band light, and reflects green band light and reflects red illumination light L emitted from the condenser lens 14. It is composed of a second dichroic mirror 15B that transmits light in the band.

  The blue light Lb reflected by the first dichroic mirror 15A is totally reflected by the mirror 16, the optical path is converted by 90 degrees, and enters the blue spatial modulation element 17b. On the other hand, the green light Lg reflected by the second dichroic mirror 15B is incident on the green spatial modulation element 17g, and the red light Lr transmitted through the second dichroic mirror 15B is totally reflected by the mirrors 18 and 19 so that the optical paths are respectively changed. After the 90 degree conversion, the light enters the red spatial modulation element 17r.

  Each of the blue spatial modulation element Lb, the green spatial modulation element Lg, and the red spatial modulation element Lr includes a transmissive liquid crystal display panel, and synthesizes the spatially modulated blue light Lb, green light Lg, and red light Lr. The light is emitted to the prism 20. The combining prism 20 combines the incident color lights and projects the image light PL through a projection lens system 21 onto a screen (not shown).

  Next, details of the polarization conversion element 13 in the optical device 10 of the present embodiment configured as described above will be described. FIG. 2 is a schematic diagram illustrating the configuration and function of the polarization conversion element 13.

  The polarization conversion element 13 is an optical element that converts the non-polarized illumination light L into constant linearly polarized light (S-polarized light). The polarization conversion element 13 has a structure in which a half-wave plate 25 is provided at a predetermined position of a prism array 24 in which an optical thin film 23 is formed on the slope of the prism. The optical thin film 23 is made of, for example, an optical multilayer film, and has an optical characteristic of transmitting the P-polarized component and reflecting the S-polarized component. The half-wave plate 25 is selectively installed in the exit region of the P-polarized component that is transmitted through the optical thin film 23 on the exit surface of the prism array 24, and has a phase difference of 180 degrees with respect to the incident P-polarized component. And fulfills the function of emitting as S-polarized light.

FIG. 3 is a schematic configuration diagram of the half-wave plate 25. The half-wave plate 25 of the present embodiment has two birefringent plates 25A and 25B made of KTP (potassium phosphate titanate: KTiOPO ₄ ) crystal and their optical axes (optical axes) D1 and D2 are orthogonal to each other. It is configured by pasting together. The birefringent plates 25A and 25B are configured as two half-wave plates having different thicknesses, and are bonded together via a transparent adhesive.

  The two birefringent plates 25A and 25B are bonded so that the in-plane optical axes D1 and D2 are orthogonal to each other, so that the sign of the phase shift is canceled in the same thickness range. At this time, the birefringence plates 25A and 25B are set so that the thickness difference Δt (t1−t2) between the birefringence plates 25A and 25B satisfies the relationship of (ne−no) Δt = {m + (1/2)} λ. Thickness is selected. When m = 0, a zero-order half-wave plate is obtained, and when m ≠ 0, a high-order half-wave plate is obtained. In the present embodiment, Δt is designed so as to coincide with the 0th-order wave plate thickness. As a result, it is possible to easily construct a zeroth-order half-wave plate with a thickness that does not impair the handleability.

  The KTP crystal is a biaxial crystal that is transparent from the visible range to the near-infrared range, and the refractive indexes of the a-axis, b-axis, and c-axis at a wavelength of 588 nm are na: 1.78989, nb: 1.76809, nc : 1.87429. The birefringence is 0.106 between the b-axis and the c-axis and 0.0108 between the a-axis and the b-axis.

  4 and 5 show the wavelength dependence of the birefringence change and the phase shift amount in the visible light band measured for the KTP crystal (between the ab axis) and the quartz crystal, respectively. In each figure, the solid line represents the KTP crystal, and the alternate long and short dash line represents the crystal. FIG. 5 shows experimental results when a 0th-order quarter-wave plate is used as a sample.

  As shown in FIG. 4, when a KTP crystal is used as the birefringent material, the maximum birefringence change in the visible light band of 400 to 700 nm is about 0.005, and the birefringence change (about 0.01 ) Is really half. Therefore, as shown in FIG. 5, the phase shift amount at a wavelength of 400 to 700 nm is also suppressed to as small as ± 30 degrees. Thereby, high PS polarization conversion efficiency can be obtained over a wide band.

  This inventor measured each polarization | polarized-light transmittance (transmittance average value of 400-700 nm) when a polarization conversion element was comprised using the half-wave plate which consists of a crystal | crystallization and a KTP crystal | crystallization. According to the experimental results, it was 74% for quartz and 88% for KTP crystals. If the phase shift amount deviates from 180 degrees with respect to the light in the visible light band, the polarization conversion becomes incomplete and the transmittance decreases. That is, the difference in polarization transmittance is due to the fact that the wavelength dependence of the birefringence of KTP is smaller than that of quartz, indicating the superiority of the present invention.

  In addition, since the wavelength dependence of the KTP crystal is smaller than that of quartz, there is no need to cancel the wavelength dependence by increasing the number of bonded sheets as in the case of using quartz, and the cost is reduced by reducing the number of bonded sheets. A small and thin half-wave plate 25 can be produced. In addition, since the influence of wavelength dependence decreases as the order increases, it is of course possible to produce a higher-order wavelength plate by laminating a plurality of KTP crystal substrates.

  In addition, since the KTP crystal is a biaxial crystal, the same analysis as the uniaxial crystal is performed under the condition that one of the optical axes is in the in-plane direction of the birefringent plate and the incident light is perpendicularly incident on the surface of the birefringent plate. Therefore, optical design of the wave plate is possible. In addition, since biaxial crystals have more optical axes than uniaxial crystals, a plurality of birefringences can be obtained by selecting the optical axis so that the degree of freedom in designing the wave plate can be improved. Become.

  As described above, according to the present embodiment, since the KTP crystal is used as the birefringent material constituting the half-wave plate 25, the wavelength dependence of birefringence in the visible light band is compared with the conventional quartz wavelength plate. It can be made lower than that. As a result, stable phase difference characteristics can be obtained over the entire visible light range, and high polarization conversion efficiency can be ensured. As a result, the polarization transmittance is improved, the light use efficiency is increased, the brightness of the image can be increased, and the overall power consumption of the optical device 10 can be reduced.

  The embodiment of the present invention has been described above, but the present invention is not limited to this, and various modifications can be made based on the technical idea of the present invention.

  For example, in the above-described embodiment, the example in which the wave plate according to the present invention is configured as a half-wave plate has been described. However, the wave plate according to the present invention may be configured as another retardation plate such as a quarter-wave plate or a ８-wave plate. Is also possible.

  The quarter-wave plate is an element that converts linearly polarized light in a target wavelength band into circularly polarized light or circularly polarized light into linearly polarized light. As an application example, in a liquid crystal projector, when the spatial modulation element is formed of a reflective liquid crystal display panel, a quarter wavelength plate installed on the front surface of the liquid crystal display panel can be cited. Alternatively, the present invention can also be applied to a quarter wavelength plate incorporated in an optical pickup device.

It is a schematic block diagram of the liquid crystal projector as an optical apparatus by embodiment of this invention. It is a figure explaining the structure and effect | action of the polarization conversion element in the liquid crystal projector of FIG. It is a perspective view which shows schematic structure of the wave plate by embodiment of this invention. It is a figure which shows the wavelength dependence of the birefringence in the visible broadband of KTP crystal and quartz. It is a figure which shows the wavelength dependence of the phase shift amount in the visible light band of a KTP crystal and quartz.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Optical apparatus (liquid crystal projector), 11 ... Light source, 13 ... Polarization conversion element, 15A, 15B ... Dichroic mirror, 17r, 17g, 17b ... Spatial modulation element (transmission type liquid crystal display panel), 25 ... 1/2 wavelength plate , 25A, 25B ... birefringent plates

Claims (6)

A wave plate formed by laminating a plurality of birefringent plates so that their optical axes are orthogonal,
The birefringent plate is made of a KTP (KTiOPO ₄ ) crystal. The wavelength plate according to claim 1, wherein the plurality of KTP substrates have different thicknesses. The wave plate according to claim 2, wherein the wave plate is a half wave plate. The wave plate according to claim 2, wherein the wave plate is a quarter wave plate. An optical device including a wave plate formed by laminating a plurality of birefringent plates so that their optical axes are orthogonal to each other,
The birefringent plate is made of a KTP (KTiOPO ₄ ) crystal. The optical device according to claim 5, wherein the wavelength plate is used as a polarization conversion element that converts one linearly polarized light into the other linearly polarized light.

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