Optical fiber grating

#This invention relates to an optical fiber grating defined on an optical fiber.

#There is a demand for optical fiber gratings that can be designed and manufactured such that their response is tailored to ideal responses.

#In many applications such as dense wavelength division multiplexing (WDM) transmission systems and satellite communications, optical amplifiers and transmitters, optical filters having ideal amplitude and phase responses are desired in order to maximize bandwidth or to optimize the filtering function provided by the fiber Bragg grating.

#Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:

#FIG. 1 is a diagram of an embodiment of the present invention;

#FIG. 2 is a diagram of an embodiment of the present invention depicting line spacing and strength

#We have developed a fiber grating synthesis method which uses a synthesis algorithm based on a differential method. More particularly, the algorithm is based on the differential IS method, wherein the medium is inverted in the time domain. It should be stressed that, in contrast with the iterative integral IS algorithm, the iteration process of the present invention is serial and, as a consequence, each grating section is fully reconstructed in a single step and used for the determination of subsequent segments (layers). The inverse scattering principle relies on the synthesis in the time domain of the grating impulse response. The first step is to build a physically realizable impulse response that closely corresponds to the required spectral response of the filter. At each time instant t0 and corresponding maximum penetration length L(t0), the impulse response is renormalised, by subtracting the contributions of all the possible previous multiple reflections within the grating (past history), providing uniquely the grating coupling constant at L(t0). The algorithm can be practically implemented in different ways. Initially, the grating has to be adequately discretised. Then, either mixed time-frequency or purely time leapfrogging strategies can be used. Preferably, a transfer matrix method is used to solve the propagation problem in both cases.

#FIG. 1 shows an optical waveguide 1 defining a Bragg grating 2 wherein the Bragg grating 2 is designed using an iterative process in accordance with the present invention. The optical waveguide 1 can comprise an optical fiber or a planar waveguide. The planar waveguide can be constructed from glass with indiffused waveguides. The optical waveguide 1 can comprise a core 9 and a cladding 10 and at least part of at least one of the core 9 and cladding 10 are photosensitive. Input light 6 of a wavelength 7 launches an optical mode 11 which propagates along the optical waveguide 1 with a defined group velocity 12. The Bragg grating 2, which is defined on the core 9, induces reflection of the input light 6 yielding reflected light 8. The Bragg grating 2 is defined by a first end 3 and a second end 4 and a length 5 along the grating measured from the first end 3. The Bragg grating 2 has an overall length 13 as shown in FIG. 1.

#The iterative process is preferably started at the first end 3 of the Bragg grating 2 moving along the grating 2 in incremental steps. Alternatively, the iterative process can be commenced at an arbitrary position within the Bragg grating 2, and elements are added to the Bragg grating 2 in a random or sequential fashion.