A thermally scanned all-fiber interferometer based on vanadium-doped fiber

Abstract

This paper presents an all-optically controlled, all-fiber optical path-length modulator. The presented system takes advantage of the heating effect induced within vanadium-doped fiber through laser excitation. It can be applied in various applications, for example in white-light interferometry. The system consists of a Michelson interferometer with vanadium-doped fiber in one arm, a 980 nm excitation high-power laser diode, and 1310/1550 nm signal sources or channels. Due to the spectral-absorption properties of vanadium ions in silica, the absorbed optical power emitted by the 980 nm source is mostly converted via a non-radiative relaxation process into heat within the vanadium-doped fiber. A rise of fiber core temperature causes the fiber core refractive index to change and consequently, a change in the optical path difference of the interferometer. The extinction laser diode operates in pulse mode for continuous scanning of the white-light interferometer. The vanadium-doped fiber is, therefore, periodically heated and self-cooled. The optical path difference of the scanning interferometer is simultaneously measured using a high-coherence source that provides the needed reference trace. The achieved modulated optical path is over 150 µm, with a system time constant of below 1 s. This all-optical configuration of the scanning interferometer allows for the remote and electrically passive control of the optical path length differences in various fiber-optic systems. In particular, the proposed design would be suitable as an interrogation system for various sensors, where an absolute optical path length variation/measurement is required.