Optical waveguides – With optical coupler – Particular coupling function
Patent
1998-11-05
2000-10-31
Healy, Brian
Optical waveguides
With optical coupler
Particular coupling function
385 24, 385 31, 385 37, 356345, G02B 626, G01B 902
Patent
active
061414697
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a multi-band-pass filter having particular but not exclusive application to wavelength division multiplexed (WDM) communication systems for filtering individual WDM channels.
BACKGROUND OF THE INVENTION
Hitherto, it has been proposed to select narrow pass bands from a relatively broad band optical source such as an optical laser, using Bragg-matched reflection gratings, for example recorded in an optical fibre. For a general review, reference is directed to Photosensitive Optical Fibres: Devices and Applications, R. Kashyap, Optical Fibre Technology 1, 17-34 1994. As explained in this review paper, band pass filters can be constructed for filtering out several wavelengths in a narrow range of a few nanometers based on a Michelson Interferometer as proposed by D. C. Johnson et al, "New Design Configuration for a Narrow Band Wavelength Selective Optical Tap and Combiner" Electron. Lett. Vol. 23, 668 (1987). In this configuration, a pair of Bragg matched reflection gratings are carefully positioned in the arms of the Michelson Interferometer so that the optical path lengths to and from the gratings are identical. Thus, the device is carefully tuned in terms of the matching of the gratings and the path length.
In WDM bandpass filters, it is usually necessary to select a number of different pass bands centred on different wavelengths. Hitherto, it has been proposed to include a number of pairs of identical gratings in each arm of the interferometer, as discussed by Kashyap, supra. However, the resulting structure is inflexible because the wavelengths of the pass bands are fixed by the characteristics of the individual pairs of tuned gratings, and the device is difficult to set up for use with desired pass bands.
SUMMARY OF THE INVENTION
The present invention provides an alternative approach, in which the filter is deliberately de-tuned in order to provide a plurality of separate pass bands.
More particularly, in accordance with the invention there is provided a multiband-pass filter comprising: a splitter operative to split optical radiation incident thereon into first and second optical paths, a combiner to combine optical radiation from the first and second paths to provide an optical filter output, and reflective means in the paths, to reflect optical radiation from the splitter to the combiner, the reflective means being de-tuned so that the filter output provides a plurality of separate, wavelength spaced pass bands for the incident radiation.
The reflective means may be de-tuned in different ways. The optical path lengths presented to the radiation travelling from the splitter to the combiner may be sufficiently different to produce the plurality of separate pass bands in the filter output. Also, the reflective means may exhibit respective different pass bands for the incident radiation, in reflection, for the first and second paths, which on combination in the combiner, result in the plurality of separate pass bands in the filter output.
The splitter may be operative to split incident relatively broadband optical radiation into the first and second paths, and the reflective means may include first and second reflective elements in the paths respectively, each having a relatively narrow band reflective characteristic for reflecting radiation from the splitter to the combiner. The plurality of pass bands in the filter output may be disposed within the envelope of the reflection pass band of at least one of the reflective elements. The path lengths presented by the first and second paths may differ by a length greater than that corresponding to a wavelength of the radiation reflected by the individual reflective elements.
The reflectivity to incident radiation presented in the filter output may be such that the reflectivity in each of the pass bands has substantially the same maximum value, within at least a given range of the output.
Stated differently, the invention provides a multiple pass band optical filter comprising an interferometer with relati
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British Telecommunications puplic limited company
Healy Brian
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