Narrow Transmission Bandpass Filters Using Bragg Grating-Assisted Mode Conversion

Abstract

Fibre Bragg gratings are proving to be instrumental in enabling the introduction of WDM systems. They offer highly selective bandpass filtering needed for the transmission of multiple closely-spaced wavelengths. These gratings, however, tend to work in reflection since the large k-vector allows coupling from forward travelling modes to backward travelling modes. This has proved to be a major disadvantage since to operate in transmission requires the use of optical recirculators. However, gratings which operate using phase-matching in the forward direction tend to have long periods, determined by the beat length between two modes. Fundamental mode conversion [1,2] requires stripping of the higher order mode to achieve a loss bandpass. This is more readily achieved when conversion is to cladding modes [3]. Whilst these filters have very low reflections, they operate as loss filters and not transmission bandpass devices. Further, the loss bandwidth tends to be broad, on the order of 20nm, because of the small differences in modal dispersion [1-3]. On the other hand, grating dispersion has been used to achieve mode conversion in reflection over a small wavelength range [4]. In all these cases transmission notch filters are generally produced, whereas in most cases the opposite - a transmission bandpass filter - is desired.