Optical: systems and elements – Diffraction – From grating
Patent
1991-08-14
1993-03-09
Hannaher, Constantine
Optical: systems and elements
Diffraction
From grating
359572, G02B 532, G02B 518, G02B 526
Patent
active
051930277
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical filter tuning apparatus and optical filtering method.
2. Description of Related Art
In order to filter light emitted from an optical fibre a chirped dichromated gelatin (DCG) filter may be used. A filter of this type is described in a paper entitled "Position--tunable holographic filters in dichromated gelatin for use in single - mode - fiber demultiplexers", Optical Letters, Vol. 10, page 303, Jun. 1985. Light is normally directed from an input optical fibre onto the chirped grating filter using a lens arrangement. As the filter is a chirped grating the wavelength of light reflected therefrom is dependent on the position of incidence of the light from the input fibre. To adjust the filtering characteristics of this filter arrangement, which is described in detail hereinafter, the filter must be moved relative to the lens and input fibre so as to alter the position of incidence. To effect this adjustment regard must be had to very stringent angular tolerances in two planes. The restrictions imposed by the tolerances necessitate the use of an elaborate and expensive support mechanism on which the filter arrangement must be mounted. Hence, it is desirable to provide a filter apparatus and filtering method which reduces the angular tolerances that must be set.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided an optical tuning apparatus comprising: wavelength being determined by the position of incidence of said light on said filter; and reflect light from an optical source to said position of incidence on said filter and reflect light of said predetermined wavelength to optical receiving means; adjustable by moving said retroreflector with respect to said filter.
In accordance with the present invention there is also provided an optical filtering method comprising: filter using a retroreflector, the wavelength of light reflected from said filter being dependent on the position of incidence of light on said filter and said retroreflector being moveable with respect to said filter so as to adjust said position of incidence; and receiving means using said retroreflector.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will now be described, by one example only, with reference to the accompanying drawings wherein:
FIG. 1 is a schematic diagram of a known filter arrangement;
FIG. 2 is a schematic diagram of a first preferred embodiment of a filter tuning apparatus according to the present invention tuned to a short wavelength;
FIG. 3 a schematic diagram of the apparatus of FIG. 2 tuned to a long wavelength;
FIG. 4 is a plan schematic view of a second preferred embodiment of a filter tuning apparatus according to the present invention;
FIG. 5 is side schematic view of the apparatus of FIG. 4;
and FIG. 6 is an end schematic view of the apparatus of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A known optical filter arrangement 2, as shown in FIG. 1, includes an input optical fibre 4 and an output optical fibre 6, a chirped grating dichromated gelatin (DCG) filter 8 and a lens 10 disposed between the fibres 4 and 6 and the filter 8. The input optical fibre 4 carries light 12 to be filtered which is emitted from the end 14 of the input fibre 4 and directed onto the filter 8 by the lens 10. The relative positions of the input fibre 4, the lens 10 and the filter 8 determines the point of incidence 16 of the light 12 on the filter 8.
The filter arrangement 2 further includes an output optical fibre 6 which is aligned with and parallel to the input fibre 4. The output fibre 6 is used to receive light 18 reflected from the filter 8. The reflected light 18 is directed into the output fibre 6 by the lens 10. The wavelength of the reflected light 18 is dependent on the position of the point of incidence 16. By moving the filter 8 with respect to the lens 10 in one of the directions indicated by the arrows 20 and 22, without altering the lateral dist
REFERENCES:
patent: 4569569 (1986-02-01), Stewart
patent: 4669811 (1987-06-01), McQuoid
patent: 4671603 (1987-06-01), McQuoid et al.
patent: 5039201 (1991-08-01), Liu
Optics Letters, vol. 10, No. 6, Jun. 1985, Optical Society of America, D. J. McCartney et al: "Position-tunable holographic filters in dichromated gelatin for use in single-mode-fiber demultiplexers".
British Telecommunications PLC
Glick Edward J.
Hannaher Constantine
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