Optical waveguides – With optical coupler – Input/output coupler
Patent
1997-10-16
2000-06-27
Ullah, Akm E.
Optical waveguides
With optical coupler
Input/output coupler
G02B 634
Patent
active
060816406
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to broadband Bragg gratings and to a method of forming the gratings. The invention has particular application to in-fibre Bragg gratings and is hereinafter described in the context of a germano-silicate fibre, having a photosensitive core, but it will be understood that the invention does have broader application, for example to planar wave guides and other light transmitting devices in which gratings may be formed.
BACKGROUND OF THE INVENTION
Bragg gratings are conventionally employed as filters in optical communication systems. The gratings are formed with regions of differing (i.e., high and low amplitude) refractive index in the direction of light propagation and they reflect light over a spectral range .delta..lambda..sub.B centred on a wavelength .lambda..sub.B =2nP where
The spectral region or bandwidth .delta..lambda..sub.B over which reflection occurs depends upon the strength of the grating (i.e., the amplitude or depth of refractive index modulation) and the length of the grating. In the case of a 10 cm long grating that is formed to provide a centre wavelength .lambda..sub.B of 1.55 .mu.m, the reflection bandwidth would be in the order of 0.01 nm.
The above described Bragg gratings are employed conventionally as optical filters which, in being formed within optical fibres, exhibit low insertion loss, and as transducers in strain or temperature responsive sensor devices. Also the gratings when chirped are employed as dispersion compensators and function as such to reflect different spectral components of light from different axially spaced positions along the length of the grating.
Chirping has the effect of expanding the width of the reflection peak and is achieved by varying the periodicity of the grating in the direction (Z) of light propagation. The chirping may be linear in which the grating period P=P.sub.O +.alpha.Z or non-linear in which case the grating period P=P.sub.O +.alpha.Z.sup.n for example.
One problem that is inherent in the above described gratings, including those that are chirped to exhibit an expanded spectral width, is that they cannot be tuned to a significant extent. However, it would be useful if a broadband grating of the described type could be produced for use in processing a single channel signal to facilitate a reduction in the degree of tuning required to match a source to the grating. This in turn would facilitate replacement of one source with another without there being a need for critical component matching. Another problem that is inherent in the known gratings is that they cannot be employed singly to perform filtering, dispersion compensation or other functions simultaneously on multiple channels at different wavelengths. In this respect it has been recognised by the inventors that it would be useful if a single grating could be employed to perform equivalent functions on multiple communication channels such as exist in wavelength division multiplexing (WDM) systems.
SUMMARY OF THE INVENTION
The present invention seeks to meet the above stated needs by providing a light transmitting device that comprises a Bragg grating which is formed in the direction of light propagation with grating regions of alternatingly high and low refractive index, the grating regions having periodicity P. Also, the grating regions are structured in a manner which varies periodically with a modulation period L>P, and either one or the other or both of the grating period P and the modulation period L is or are chirped over substantially the full extent of the grating.
The invention may also be defined in terms of a method of producing a broadband Bragg grating and which comprises forming grating regions of alternatingly high and low refractive index in the light transmitting device, the grating regions being formed in the direction of light propagation and having periodicity P. When forming the grating regions they are structured in a manner which varies periodically with a modulation period L>P, and chirping is imposed on e
REFERENCES:
patent: 5309260 (1994-05-01), Mizrahi et al.
Eggleton Ben
Hill Peter
Krug Peter
Ouellette Francois
Telstra Corporation Limited
The University of Sydney
Ullah Akm E.
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