1989-05-17
1990-08-07
Sikes, William L.
G02B 634, G02B 532
Patent
active
049462458
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to optical filters.
2. Related Prior Art
Volume reflection grating filters have been proposed for use in wavelength multiplexed optical systems. An example of such a filter is described in published U.K. Patent Application GB No. 2151036. However, such reflection filters are difficult to implement in practice since an incident light beam is reflected back towards the launch direction making it difficult to couple the filtered wavelength into a detector or other output device. Furthermore, with reflection filters, once a desired wavelength has been selected by the grating, the filtered wavelength must generally be separately focussed into an output optical fiber for onward transmission.
SUMMARY OF THE INVENTION
In accordance with the present invention, an optical filter comprises an optical waveguide within which is provided a volume diffraction grating of refractive index modulations, the grating having a thickness sufficient such that when an optical beam is incident on the grating from outside the waveguide, wavelengths in a predetermined range at or near the Bragg wavelength for the grating are diffracted and coupled into the waveguide, all but the first order diffractions being substantially eliminated, whilst wavelengths away from the Bragg wavelength pass through the waveguide substantially undiffracted.
The filter according to the invention makes use of the wavelength selectivity of a volume diffraction grating in which light at one wavelength can be efficiently diffracted whilst light of a second wavelength passes through the diffraction grating with no effect. The thickness of the grating enables diffraction of harmonic wavelength components to be eliminated (i.e. higher than first order diffractions are suppressed). Preferably Q is greater than about 10 where Q=2.pi..lambda.t
d.sup.2, d being the fringe spacing. Moreover, by appropriately varying the grating thickness, the bandwidth of the range of wavelengths at or near the Bragg wavelength which are efficiently diffracted by the grating at a given angle of incidence of the input beam may be conveniently adjusted as desired. The present filter is further distinguished from the conventional reflection gratings of GB No. 2151036 since the grating is provided in a waveguide. This enables a relatively thick (in the direction of grating normal) grating to be constructed providing the advantage of narrower bandwidth operation. In addition the direct coupling between the grating and waveguide results in a much simpler overall construction.
In order to achieve good coupling into the waveguide the grating modulations are conveniently established in planes extending at least in one direction normal to the preferred axis of propagation in the waveguide and extending in another direction, orthogonal to the one direction, at an angle .pi./2-.phi. with respect to the axis of propagation.
For coupling from an external optical beam into the waveguide the grating inclination .phi. should be less than .pi./2. In operation the optical beam should then be incident at an angle .pi./2-.theta. to the axis of propagation, where .theta.<.phi. and .theta.=2.phi.-.pi./2. For a given grating thickness, the coupled bandwidth is narrower the closer .theta. approaches to .pi./2. For narrowband operation, it is therefore generally preferable for .phi. (and thus .theta.) to be as close to .pi./2 as possible.
However, as .phi. approaches .pi./2, the required inclination .theta. of the input beam also approaches .pi./2. In these circumstances, as .theta. increases, in the absence of efficient, index-matched coupling between the input beam and the waveguide, an increasing proportion of the input beam is simply reflected away from the waveguide before interaction with the grating leading to a corresponding reduction in the amount of light available for coupling into the waveguide. Practically, therefore, it is necessary to find a compromise. Without taking special steps towards index matching, it ha
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Chamberlin Giles R.
McCartney David J.
Payne David B.
British Telecommunications public limited company
Sikes William L.
Ullah Akm E.
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