Optical: systems and elements – Polarization without modulation – Polarization variation over surface of the medium
Reexamination Certificate
2002-03-26
2004-12-21
Juba, Jr., John (Department: 2872)
Optical: systems and elements
Polarization without modulation
Polarization variation over surface of the medium
C359S485050, C353S020000
Reexamination Certificate
active
06833953
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarizer for converting incident light beam into a linear polarization beam and an optical device using the polarizer.
2. Description of Related Art
Conventionally, a projector having a light source, an electric optical device for modulating the light beam irradiated by the light source in accordance with image information, and a projection optical system for enlarging and projecting the light beam modulated by the electric optical device has been used as an optical device using the electric optical device.
Liquid crystal device is known as the electric optical device, which includes an electric optical element having a pair of transparent substrates between which electric optical material such as liquid crystal is sealed and enclosed, and two polarization plates as a polarizer disposed on the light-incident side and light-emission side of the electric optical device.
In a conventional arrangement, the polarizer is made of organic material such as PVA (polyvinyl alcohol) including iodine and colorant, which is made by sandwiching a film oriented in a predetermined direction with a support body such as glass plate or by adhering the film to the support body.
However, since such conventional polarizer is made of organic material, the polarizer is weak against high temperature and is decolorized after continuous use in an environment of more than 70° C. to lose polarizing function.
Accordingly, a structural birefringent polarizer has been proposed as a polarizer made of inorganic material. The structural birefringent polarizer is constructed by forming a birefringent portion where a plurality of stripe-shaped minute linear convex treads of metal such as aluminum are arranged on a surface of a transparent substrate such as glass, which uses diffraction of the space between the convex treads to convert the incident light beam to the linear polarization beam.
However, since such structural birefringent polarizer absorbs a part of the incident light beam on the birefringent portion, the glass substrate is distorted on account of heat, thus failing to conduct appropriate polarization conversion. Especially, when the glass substrate is disposed on the light emission-side, the polarization axis of the polarized light beam converted by the birefringent portion is revolved in the glass substrate, thereby causing light transmission failure where a part of the light does not pass through the glass substrate.
SUMMARY OF THE INVENTION
According to an aspect of the present invention, a specific base material of structural birefringent polarizer is selected in order to provide a polarizer having great durability and capable of conducting appropriate polarization conversion and an optical device using the polarizer.
Specifically, a polarizer according to an aspect of the present invention is for converting incident light beam into a linear polarization light beam, which includes: a base plate of a material having light transmittance of more than 97%; and a birefringent portion formed by arranging minute metal concave treads in stripe on a light-emission surface of the base plate.
The example of the base material having light transmittance of more than 97% is sapphire, LBC3N (manufactured by HOYA OPTICS CORPORATION), and Neoceram (manufactured by Nippon Electric Glass Co., Ltd).
The base plate may preferably be made of material having linear expansivity of less than 4.8*10
−7
/K. Example of such material is silica glass, Neoceram, CLEARCERAM (manufactured by Ohara Inc.), in which crystallized glass and silica glass may more preferably be used.
Concave treads made of aluminum may be used as the metal concave tread constituting the birefringent portion, where polarization properties of the polarizer is determined in accordance with the pitch, height and duty ratio of the concave tread. For instance, a plurality of concave treads of 65 nm width and 120 to 170 nm height may be arranged in 144 nm pitch to constitute the birefringent portion.
According to the present aspect, since the base plate is located on light-incident side, the revolution of polarization axis of the light beam after polarization conversion caused on account of influence of thermal distortion of the base plate can be prevented, thereby conducting appropriate polarization conversion. Since the material of low linear expansivity is used for the base material, distortion is not likely to be generated even when the heat absorbed by the birefringent portion affects on the base material, light transmission failure of the polarizer can be prevented. Further, since the birefringent portion is made of metal, sufficient durability can be secured.
The base plate may preferably be made of a material having thermal conductivity of more than 6.21 W/(m·K).
In the above arrangement, specific base material may be sapphire or crystal.
According to the above arrangement, since the material of high thermal conductivity is used as the base material, even when the heat absorbed by the birefringent portion affects on the base material, the heat can be immediately radiated through the holding frame etc. of the polarizer, so that thermal distortion is not likely to generated on the base plate, thereby preventing light transmission failure of the polarizer.
The base plate may preferably be made of a material having phtoelasticity constant of less than 0.43*10
−12
/Pa.
The photoelasticity constant is a proportional constant representing relationship between a stress applied to the base plate and an optical-path difference caused by birefringence of the light transmitting through the base plate while the stress being applied. Specifically, when the optical-path difference by birefringence is represented as &dgr; (nm), a component of internal stress of the base plate orthogonal with advancement direction of light is represented as Ps(*10
5
Pa), and thickness of the base plate is represented as d (mm), following relationship as shown in the following formula (1) is established.
&dgr;=
B·Ps·d/
10 (1)
The proportional constant B of the formula (1) is the photoelasticity constant, which is normally represented according to unit of 10
−12
/Pa.
The base material having the above specific photoelasticity constant is, for instance, low photoelasticity glass LBC3N manufactured by HOYA OPTICS CORPORATION.
According to the present arrangement, since the base material is made of a material of low photoelasticity constant, as evident by the formula (1), even when the internal stress Ps is generated on account of thermal distortion caused when the heat absorbed by the birefringent portion is applied on the base plate, the optical-path difference &dgr; can be restrained to a small value on account of small photoelasticity constant B, so that light transmission failure etc. can be prevented.
In the above arrangement, antireflection treatment may preferably be conducted on a light-incident surface of the base plate.
Since the antireflection treatment is conducted, utilization ratio of incident light beam can be improved by reducing the reflected light, so that the loss of light transmitting through the polarizer can be reduced.
In the above polarizer, a protection plate covering the birefringent portion may preferably be provided. The material of the protection plate may preferably be the same as the base plate having the above-described birefringent portion. Specifically, crystallized glass, silica glass, sapphire, crystal etc. may be used.
As described above, since the birefringent portion is constructed by arranging metal concave treads such as aluminum in stripe, the concave treads can be deteriorated under high temperature and high humidity. Provision of the protection plate prevents deterioration of the concave treads, so that durability of the polarizer can be further enhanced and the polarizer is not deteriorated even when the optical device including the polarizer is exposed to high temperature and high humidity during transportation and use.
Hashizume Toshiaki
Miyazawa Atsushi
Jr. John Juba
Seiko Epson Corporation
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