Wavelength selective filter

Details Wavelength selective filter Wavelength selective filter

1997-07-24

2000-10-31

Davie, James W.

Coherent light generators

Particular beam control device

Tuning

359237, 359246, 359259, 372 94, 372 98, H01S 3107, G02B 520

Patent

active

061413615

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a wavelength selective filter and a tunable laser including it.


BACKGROUND ART

Many different types of optical filters for selecting light of a particular wavelength have been proposed in conjunction with wavelength division multiplexed optical systems for use in optical communication systems.
Also the use of a single fixed diffraction grating for passive optical wavelength separation is well understood. The spectral components of the input source are separated and distributed around an output plane so that selection of the correct spatial region allows any spectral component to be isolated. Thus, spatial filtering of the output plane produces spectral filtering of the source. Such a technique can be used to demultiplex a wavelength division multiplexed signal consisting of a number of data-modulated wavelength channels provided that the size of the output devices are correctly matched to the data bandwidths.
A single fixed diffraction grating only separates the different wavelength channels and allows no recombination of these channels in the output plane. Re-selection of the output wavelengths i.e. somehow tuning such a filter can only be achieved by changing the grating, rotating it or somehow moving the spatial filtering device in the output plane. In the past these have been implemented by some form of servo-mechanical system with a feedback control to cause physical movement of the grating or of a photodetector or optical fibre receiving light in the output plane.
An example of an optical crossbar switch using a variable hologram to route light in a particular direction between its input and output and so, in effect, cause switching between a particular input of the crossbar switch and a particular output is described in an article entitled "A Holographicly Routed Optical Crossbar: Theory and Simulation" by D. C. O'Brien, W. A. Crossland and R. J. Mears published in Optical Computing and Processing, 1991, volume 1, number 3, pages 233-243.


DISCLOSURE OF INVENTION

According to a first aspect of this invention a tunable optical wavelength selective filter comprises a dynamic holographic diffraction element in combination with a fixed diffraction grating or hologram.
Such a filter can be used in coherent and semi-coherent spread-spectra applications and is likely to be used principally within the visible and infra-red frequency band.
The filter in accordance with this invention relies upon the fixed diffraction grating or hologram to disperse light of different wavelength into its constituent spectral components or, for example, wavelength-multiplexed data streams, and then using the additional dynamic holographic diffraction pattern for tuning the filter so that by altering the holographic diffraction pattern present on the dynamic hologram it is possible to alter the wavelength or wavelengths which leave the particular angle of interest. The combination of a dynamic holographic diffraction element and a fixed holographic element may also be arranged arbitrarily to recombine various of the wavelengths in the output plane.
The dynamic diffraction element preferably is some type of opto-electronic interface which can take a number of forms depending upon the actual application. The dynamic diffraction element is preferably implemented as an electronically controlled image displayed on a pixellated spatial light modulator and in particular a spatial light modulator using opto-electronic integrated circuits fabricated using silicon VLSI technology and integrated with ferro-electric, nematic or electroclinic liquid crystals. Such spatial light modulators are well suited to telecommunication applications in terms of speed, reliability and ease of interfacing to control electronics. Such devices are readily controllable, typically via a computer to display one of a series of different holographic diffraction patterns. Typically such holograms are 2-dimensional optical phase and/or amplitude gratings which produce a controllable deviation and dispersion o

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patent: 5222071 (1993-06-01), Pezeshki et al.
patent: 5949804 (1999-09-01), Okazaki
N. Collings, et al., "Evolutionary Developement of Advanced Liquid Crystal Spatial Light Modulators," Applied Optics, Nov. 15, 1989, vol. 28, No. 22, pp. 4740-4747.
S. Warr et al., "Optically Transparent Digitally Tunable Wavelength Filter," Electronics Letters, Jan. 19, 1995, vol. 31, No. 2, pp. 129-130.
Dames et al., "Effecient Optical Elements to Generate Intensity Weighted Spot Arrays: Design and Fabrication," Applied Optics, Jul. 1, 1991, vol. 30, No. 19, pp. 2685-2691.
Barnes et al. "Phase-only Modulation Using a Twisted Nematic Liquid Crystal Television," Applied Optics, Nov. 15, 1989, vol. 28, No. 22, pp. 4845-4852.
International Search Report dated Jan. 12, 1996.

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