Abstract

An optical fiber frequency shifter is a key component for the implementation of optical fiber signal processing, communications and sensor systems. Several works reported in the literature address the various problems encountered when trying to fabricate a practical device.^1,2,3 The main driving force behind these efforts is the stringent specification requirements to be met for closed-loop operation of high-performance optical fiber gyroscopes. Desired features in a fiber frequency shifter for such systems include: (a) high sideband and carrier suppression, (b) road acoustic and optical bandwidth, (c) low driving power, (d) easy splicing into single-mode fiber systems, with low insertion loss, and (e) ruggedness and compacticity. The basic configuration of a two-mode fiber frequency shifter,⁴ employs mode conversion between the two lowest-order modes (LP₀₁ and LP₁₁) through scouto-optic interaction with a flexural wave traveling in the fiber, the freqeuncy-shifted output light is guided in the LP₁₁ mode, which requires an evanescent-field modal coupler and special fiber design to achieve connectivity into single-mode fiber systems. While our work was in progress some devices where reported addressing this issues as well as image sideband and carrier suppression.^2,3,5 The first two employ double conversion in two interaction regions where the acoustic flexural waves are excited by the same transducer, imposing a critical compromise between acoustic bandwidth and carrier suppression. Only up-shift by twice the acoustic wave frequency is obtained with this configuraiton. The latter utilizes a specially designed double-core fiber and so far represents a trade-off between carrier suppression and insertion loss.

© 1991 Optical Society of America