Optical spectrum analyzer having tunable interference filter

Abstract

An optical spectrum analyzer includes a polarization modifier that eliminates polarization-dependent wavelength spreading, enabling the analyzer to have high signal selectivity and high measurement sensitivity. The polarization modifier spatially separates orthogonal polarization components of the applied optical signal into separate optical beams and rotates the relative polarization components of the beams so that the beams are incident on a tunable interference filter in a multipass configuration at a single polarization state. The optical beams are directed through regions of the interference filter which lie on a contour of substantially equal center wavelength so that each of the multiple passes through the interference filter provides for corresponding narrowing of the filter bandwidth. Narrow bandwidth and low insertion loss are maintained over a wide tuning range by tilting the interference filter about a tilt axis intercepting the regions of the interference filter that lie on a contour of substantially equal optical thickness.