Waveguide coupler with diffraction grating which selectively inh

Details Waveguide coupler with diffraction grating which selectively inh Waveguide coupler with diffraction grating which selectively inh

1996-10-16

1999-11-02

Sanghavi, Hemang

Optical waveguides

With optical coupler

Input/output coupler

385 28, 385 30, 359130, G02B 634

Patent

active

059785306

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION



Background of the Invention

This invention relates to waveguide couplers.
Waveguide couplers are employed to transfer electromagnetic radiation between two or more coupled waveguides. For example, optical fibre waveguide couplers couple two or more optical fibers together so that light launched into one fibre is at least partially coupled into the other fibre(s). These techniques are described in the publication "Optical Waveguide Theory", (Chapman and Hall, London, 1983).
Attempts have been made to combine waveguide couplers with optical fibre gratings to produce wavelength-selective splitting of an optical signal. Such an arrangement would be very useful in splitting, for example, wavelength-division multiplexed optical communication signals, and would replace the present use of bulk (non-fibre) optical devices such as optical circulators. In one form, these grating-couplers use a 50:50 2.times.2 directional coupler (i.e. one in which half of the light launched into one of two coupled fibers is coupled into the other fibre) with a fibre grating on one of the output ports of the coupler. The fibre grating acts in effect as a wavelength selective mirror, reflecting light at or near to a particular wavelength (referred to as the "Bragg" wavelength), and transmitting light at other wavelengths. The reflected light passes back into the coupler and is split once again between the two fibers.
The result of this arrangement is that light launched into the first fibre at the Bragg wavelength is reflected back to one of the output ports of the second fibre, but is diminished in intensity by 75% (having passed through the 50:50 coupler twice). The transmitted light also suffers a loss of 50%. In, for example, a communications system involving signal transmission along long lengths of optical fibre, these additional losses impose greater constraints on the length of optical fibre which can be used, the signal to noise ratio of the received optical signal, and/or the number of optical channels which can be propagated along a single fibre.
In another form, as described in the publication entitled "Compact all-fibre narrowband transmission filter using Bragg gratings" (European Conference on Optical Communication, ECOC '93 post-deadline paper 12.8, page 29, 1993), the grating-couplers use a 50:50 2.times.2 directional coupler with fibre gratings on each of the output ports of the coupler. The fibre gratings reflect light at or near to the Bragg wavelength, and transmit light at other wavelengths. The reflected light passes back into the coupler arid is recombined in such a way as to re-emerge from the second input port of the coupler. For this to work, the recombination must be interferometrically exact, making the device potentially highly sensitive to environmental changes.
EP-A-0 234 828 discloses a coupler in which a grating disposed between the two fibers causes wavelength-selective contradirectional coupling.


SUMMARY OF THE INVENTION

This invention provides an optical fibre coupler comprising at least a first optical fibre coupled at a coupling region to a second optical fibre such that at least a part of radiation propagating along the first optical fibre is coupled into the second optical fibre; grating disposed within the core of the second optical fibre at the coupling region to inhibit coupling of radiation from the first optical fibre into the second optical fibre at wavelengths characteristic of the diffraction grating.
The invention addresses the above problems by providing a wavelength selective coupler in which the coupling from one optical fibre to another can be selectively inhibited by the use of a diffraction grating disposed at the coupling region in the recipient optical fibre. This arrangement can therefore be referred to as a "grating frustrated coupler". The wavelengths inhibited from coupling can be similar to those lying within the "stop band" of the grating. By increasing the strength of the grating, the inhibition to coupling can be increased. The wav

REFERENCES:
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Prohaska et al, "Magnification of Mask Fabricated Fiber Bragg Gratings", Electronics Lett., vol. 29, No. 18, pp. 1614-1615, Sep. 1993.
A.S. Svakhin et al., "Narrow-band Bragg reflecting filter based on a single-mode fiber", Jun. 1987, pp. 701-702.
J.P. Weber, "Spectral characteristics Bragg-reflection tunable optical fiber", Oct. 1993, pp. 275-284.

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