Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-12-18
2002-02-19
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S253000, C359S251000, C359S238000, C359S009000, C349S017000
Reexamination Certificate
active
06348990
ABSTRACT:
This is a national stage entry PCT/JP99/03225, international filing date Jun. 17, 1999.
TECHNICAL FIELD
The present invention relates to a spatial light modulating device which uses a spatial light modulator, which is an element for generating a particular modulation in a cross-section of a light beam, and to a spatial light modulating method which uses the spatial light modulator.
BACKGROUND ART
A spatial light modulating device is used for optical information processing and computer-generated holograms (CGH). Especially, a spatial light modulating device of a type, which prepares signal images according to electric addresses, is very useful because this type of spatial light modulating device can easily prepare those signal images. A device that can efficiently perform phase modulation is required for the above-described usage.
A device that employs a transmission type liquid crystal element can be considered as an example of this type of spatial light modulating device. Such a device has a merit of being purchasable at a relatively low price. However, such devices have been unable to achieve a phase modulation amount of greater than 2&pgr;, which is required for computer-generated holograms and the like.
In an example of another device that is capable of performing electric addressing is a spatial light modulating device that employs an optically-addressed type spatial light modulator and that uses an electrically-addressed type element for generating write light that will fall incident on the optically-addressed type spatial light modulator. For example, Technical Report of the Institute of Electronics, Information and Communication Engineers, (1997-10) p95-p99, discloses a spatial light modulating device that uses a transmission type liquid crystal element as an electrically-addressed type element and an optically-addressed type parallel-aligned nematic-liquid-crystal spatial light modulator (PAL-SLM) as an optically-addressed type spatial light modulator. In this device, a relay lens is disposed between the transmission type liquid crystal element and the optically-addressed type parallel-aligned nematic-liquid-crystal spatial light modulator. Write light from the transmission type liquid crystal element passes through the relay lens to reach the optically-addressed type PAL-SLM. It is possible to attain phase modulation of 2&pgr; or greater. Because phase-only modulation can be attained, it is also possible to attain high diffraction efficiency.
There is known another method, other than using a relay lens, to transmit write light from a transmission type liquid crystal element to an optically-addressed type spatial light modulator. In this method, the output end of the transmission type liquid crystal element and the write light input end of the optically-addressed type spatial light modulator are both configured from fiber optic plates, and these two fiber optic plates are optically connected. Alternatively, the output end of the transmission type liquid crystal element and the write light input end of the optically-addressed type spatial light modulator are configured from a single fiber optic plate (Japanese Patent Application Publication No. HEI-7-72503).
However, in the case of the above-described two spatial light modulating devices, a signal component that is generated as a result of a pixel structure of the transmission type liquid crystal element is not erased. There is a problem in that a signal image to be transmitted receives influences from this signal component.
It is an objective of the present invention to overcome the above-described problem and provide a spatial light modulating device and a spatial light modulating method, which transmits, by using write light, a signal image from an electrically-addressed type element, which has a pixel structure, to an optically-addressed type spatial light modulator, and which is capable of erasing the signal component caused by the pixel structure and suppressing degrades in the signal image.
DISCLOSURE OF THE INVENTION
In order to solve the above-described problems, a spatial light modulating device of the present invention comprises: an electrically-addressed type spatial light modulator, which has a pixel structure where a plurality of pixels are arranged at a predetermined pitch P, and which is addressed by electric signals representing information to be written, thereby modulating incident write light; an optically-addressed type spatial light modulator, which has an optical addressing layer and a light modulation layer between a pair of transparent electrodes and which modulates read light incident to the light modulation layer by the write light incident to the optical addressing layer; and an optical connecting element which guides the write light, which has been modulated by the electrically-addressed type spatial light modulator and which has been outputted therefrom, to the optical addressing layer of the optically-addressed type spatial light modulator, the optical connecting element having a predetermined critical transfer spatial frequency &ugr;
c
, the value of the critical transfer spatial frequency &ugr;
c
and the value of a pixel structure spatial frequency 1/P of the pixel structure in the electrically-addressed type spatial light modulator satisfying the following relationship: &ugr;
c
<1/P.
According to the spatial light modulating device described above, the relationship of &ugr;
c
<1/P is established between the critical transfer spatial frequency &ugr;
c
and the pixel structure spatial frequency 1/P. Accordingly, it is possible to erase, from write light that reaches the optically-addressed spatial light modulator, a signal component that is caused by the pixel structure of the electrically-addressed type spatial light modulator.
It is preferable that the value of the critical transfer spatial frequency &ugr;
c
of the optical connecting element and the value of the pixel structure spatial frequency 1/P further satisfy the relationship of 1/2P<&ugr;
c
. The spatial frequency of a signal image generated by the electrically-addressed type element is limited to be lower than or equal to the maximum spatial frequency 1/2P that is determined dependently on the pitch P of the pixel structure. By further applying the condition of 1/2P<&ugr;
c
to the critical transfer spatial frequency &ugr;
c
and the pixel structure spatial frequency 1/P, it is possible to transmit the write light without any degradation over the entire range of the spatial frequencies included in the signal image. It is therefore possible to attain a spatial light modulating device that can erase the signal component caused by the pixel structure and that can generate no degradation in the signal image.
In the case where the optically-addressed type spatial light modulator has sensitivity characteristic to write light wavelength, the sensitivity characteristic has a predetermined sensitivity to a predetermined wavelength &lgr; and has another sensitivity to a wavelength other than the predetermined wavelength &lgr;, and the predetermined sensitivity is higher than the other sensitivity, the value of the critical transfer spatial frequency &ugr;
c
of the optical connecting element is preferably determined based on a numerical aperture NA of the optical connecting element and the predetermined wavelength &lgr;. Because the value of the critical transfer spatial frequency &ugr;
c
is determined based on the numerical aperture NA of the optical connecting element and the predetermined wavelength &lgr; that mainly contributes to the modulation in the optically-addressed type spatial light modulator and because the thus determined critical transfer spatial frequency &ugr;
c
satisfies the relationship of 1/2P<&ugr;
c
<1/P, it is possible to attain an efficient spatial light modulating device whose output plane has very little influence from the pixel structure. The predetermined frequency &lgr; may be a wavelength region having a predetermined wavelength width.
It is preferable that the optical connecting elemen
Hara Tsutomu
Igasaki Yasunori
Toyoda Haruyoshi
Yoshida Narihiro
Epps Georgia
Hamamatsu Photonics K.K.
Oliff & Berridg,e PLC
Thompson Tim
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