Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-10-16
2001-08-07
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S198000
Reexamination Certificate
active
06271899
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a tuneable filter, in particular for use in the visible or near infrared wavelength regions. The filter may be used, for example, in protection against laser radiation, or as a high contrast tuneable filter for display purposes. The filter may also be used for wavelength division multiplexing in telecommunications applications.
2. Discussion of Prior Art
In a conventional tuneable filter based on a Fabry Perot etalon, the cavity length may be varied in order to tune the filter to the required wavelength. This may be done by mechanically varying the cavity length, using piezoelectric drivers attached to the glass substrates at the end of the cavity. Temperature tuning techniques may also be employed, using thermo-optic effects in the material contained within the cavity. Alternatively the cavity can be tuned to a particular wavelength by varying the angle at which light is incident on the filter.
Tuneable filters may be particularly useful as a means of protecting against laser radiation, especially where the precise laser wavelength is unknown. A suitable protective device for any optical imaging system or detection system (including the human eye) must transmit in-band wavelengths so that vision or detection is not significantly obscured even when activated.
SUMMARY OF THE INVENTION
The present invention relates to a tuneable filter and active material which is operable in the visible or near infrared regions. As the filter is transparent to visible and near infrared wavelengths in all operating states it does not significantly obscure vision and may therefore be incorporated within imaging equipment or viewing aids, such as a pair of binoculars.
The following prior art documents describe background art to the present invention; Feiling Wang and Gene H. Haertling, “Large Electrooptic Modulation Using Ferroelectric Thin Films . . . ”, Proc. of the Ninth IEEE Int. Symp. on Applications of Ferroelectrics (ISAF), University Park, Pa., Aug. 7-10, 1994, Symp. 9 pp. 683-686 and EP 282 963 A.
According to the invention, a tuneable filter having a transmission-wavelength characteristic with at least one maximum or minimum comprises;
a nano phase polymer dispersed liquid crystal (PDLC) material contained within and forming part of a resonant cavity, wherein said material is polarisable by means of an applied electric field,
at least one layer of dielectric material on each side of the nano phase PDLC material, said dielectric material having a refractive index different from that of the nano phase PDLC material and
means for applying a variable electric field across the nano phase PDLC material,
wherein variation of the electric field applied across the nano phase PDLC material gives rise to variation of the wavelengths at which the maximum or minimum occurs in the transmission-wavelength characteristic.
The liquid crystal material may be a nematic material. Preferably the droplets of liquid crystal material within the PDLC material have a diameter within the range of 10 nm and 50 nm.
In a further preferred embodiment, the resonant cavity has substantially parallel, facing input and output glass substrates.
The transmission-wavelength characteristic of the filter may have a plurality of maxima and minima. For a tuneable filter operable in the visible or near infrared wavelength region, at least one maximum or minimum is in the visible or near infrared wavelength region.
According to the invention, a tuneable filter comprises;
a nano dispersed composite material contained within and forming part of a resonant cavity, wherein said material is polarisable by means of an applied electric field and has a transmission-wavelength characteristic with at least one maximum or minimum;
at least one layer of dielectric material on each side of the nano dispersed composite material, said dielectric material having a refractive index different from that of the nano dispersed composite material and
means for applying a variable electric field across the nano dispersed composite material
wherein variation of the electric field applied across the nano dispersed composite material gives rise to variation of the wavelengths at which the maximum or minimum occurs in the transmission-wavelength characteristic.
The nano dispersed composite material may be a nano phase polymer dispersed liquid crystal (PDLC) material.
The liquid crystal material may be a nematic material. For use of the filter in a wavelength region including a wavelength &lgr;, droplets of liquid crystal material within the PDLC material may have a diameter of between 0.02&lgr; and 0.12&lgr;. Typically, the droplets of liquid crystal material within the PDLC material have a diameter of less than 0.1 &mgr;m. For use of the filter in the visible wavelength region, the droplets of liquid crystal material may typically have a diameter of between 10 nm and 50 nm.
In a preferred embodiment, the resonant cavity has substantially parallel, facing input and output substrates which may typically be glass substrates.
The transmission-wavelength characteristic of the filter may have a plurality of maxima and minima. For a tuneable filter operable in the visible or near infrared wavelength region, at least one maximum or minimum is in the visible or near infrared wavelength region.
Some of the layers of dielectric material may be indium tin oxide (ITO). The ITO layers may conveniently be coated on the inward facing surfaces of the input and output substrates such that the ITO layers form electrodes to which the electric field is applied.
The filter may also comprise at least one other layer of dielectric material on either side of the polarisable material for enhanced spectral contrast, said layers being contained within and substantially parallel to the facing substrates. Preferably, the filter comprises layers of dielectric material of high (H) and low (L) refractive index, contained within the facing substrates in an alternate HL layer structure. For example, the material of high refractive index may be titanium oxide (TiO
2
) and the material of low refractive index may be silicon oxide (SiO
2
).
The filter may comprise alternate layers of ITO and polarisable material, said layers being contained within and substantially parallel to the input and output facing substrates. The filter may further comprise means for applying a separate voltage to each individual ITO layer.
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patent: 4779959 (1988-10-01), Saunders
patent: 4869847 (1989-09-01), Leslie et al.
patent: 5068749 (1991-11-01), Patel
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patent: 5150236 (1992-09-01), Patel
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patent: 0 260 073 A (1988-03-01), None
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IEEE Photonics Technology Letters, vol. 4, No. 6, Jun. 1, 1992, pp. 597-599, XP000275547 Katsuhiko Hirabayashi et al: “Tunable wavelength-selective liquid crystal filters for 600-channel FDM systems” see the whole document.
Journal Of Applied Physics, vol. 67, No. 9, May 1, 1990, pp. 4253-4259, XP002035040 M.J. Sansone et al. “Large Kerr effects in transparent encapsulated liquid crystals” see the whole document.
“Large Electrooptic Modulation Using Ferroelectric Thin Films In a Fabry-Perot Cavity”, Feiling Wang & Gene H. Haertling, Proc.. of the Ninth IEEE Int. Symp. On Applns of Ferroelectrics (ISAF), University Park, PA, Aug. 7-10, 1994, Symp. 9 pp. 683-686.
Blacker Richard S
Lewis Keith L
Mason Ian R
Sage Ian C
Dudek James A.
Nixon & Vanderhye P.C.
The Secretary of State for Defence in Her Brittanic Majesty&apos
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