Rocking Filters Written in Photosensitive Birefringent Fiber: Suppression of Sidelobes in Fiber Wavelength Response

Abstract

Photosensitivity [1] in optical fiber has been used to fabricate rocking filters (polarization mode converters) [2] in birefringent optical fiber by the external point-by-point writing technique [3]. Figure 1(a) shows the fabrication technique and 1(b) is a schematic of the rocking filter configuration. The point-by-point writing technique permits the fabrication of high transmission, narrow-band rocking filters that are resonant (i.e. when the rocking period Lr equals the birefringent beat period Lb) at preselected wavelengths. The spectral response of such filters typically exhibits the sinc2 like dependence on wavelength that is expected from forward (co-propagating mode) coupling in an un-chirped grating filter with a finite length and uniform coupling coefficient (see Fig. 2). This filter characteristic is not ideal for dense wavelength multiplexing because the presence of the sidelobes increases the frequency separation (guard-space) needed between optical carriers to reduce interchannel interference to acceptable levels. In this paper we demonstrate an effective technique for suppressing the sidelobes in the wavelength response of an externally written rocking filter.