Proceedings Article | 15 May 2010
KEYWORDS: Optical filters, Reflectivity, Mirrors, Bandpass filters, Apodization, Electronic filtering, Reflectors, Wavelength division multiplexing, Neodymium, Fiber Bragg gratings
A theoretical analysis of a new type of bandpass fiber filter is presented in this paper, which has the favorable
transmission characteristics such as low loss, high optical rejection, flat top and steep edge etc.. These parameters are
very important for the Wavelength Division Multiplexing (WDM) telecommunication systems. In our structure, a
continuum spectrum is achieved thanks to a resonator cavity, formed by one mirror and a Linearly Chirped Fiber Bragg
Grating (LCFBG). This grating is apodized along the fiber by a Super-Gaussian window function in order to make the
output curves much smoother. Furthermore, the cavity length is calculated precisely by the continuum oscillation
condition and this fiber filter is mathematically modeled and resolved by Transfer Matrix Method (TMM). The more
efficient transmission of filter in our structure is observed when the reflector reflectivity at the both sides of the resonant
cavity is symmetrical, in that case, the maximum output is 1. So, we could vary the coupling coefficient κ to control the
maximum grating reflectivity for the purpose of making it be same as that of mirror's. Rather, there are other input
parameters, for instance, the chirped value C, the length of grating LB, and the mirror reflectivity RM, which could
influence the output results such as the grating reflectivity RB, the transmission of filter TF, the optical rejection of filter
τop, and the bandwidth of filter ΔλF. We have firstly plotted some schematics to find out the relations between the input
and output parameters, and then with the restricted conditions about the input parameters, we have found finally the
preferable input and output values for our structure. Besides, we observed that the bandwidth of filter could be also
changed by apodization, in other words, the Super-Gaussian apodization plays an important role not only to smoothen
the curves but to vary the width of bandpass as well.
