Optical techniques to solve the signal fading problem in fiber interferometers

Abstract

In single-mode optical fiber interferometer sensors environmental effects such as ambient temperature fluctuations and static pressure changes result in signal fading. Three different optical techniques which eliminate the signal fading problem are presented and experimentally verified. Comparison of the three techniques in terms of their technical merits is presented.

Wide dynamic range homodyne interferometry method and its application for piezoactuator displacement measurements

“…However, interferometry is also invariably corrupted by large quasi-static phase random drifts due to external environmental disturbances. This phenomenon is well known as signal fading [10,11].…”

“…A large number of phase-demodulation techniques have been described in the literature to detect the induced phase shift under fading: active homodyne [12], heterodyne [13], phase quadrature [14], and synthetic heterodyne [15], among many others [10,11]. There is general agreement that heterodyne techniques, with picometer resolution, are useful when the displacement exceeds an interference fringe, while homodyne techniques are often employed when the displacement does not exceed the fringe spacing [16].…”

Digital Demodulation of Interferometric Signals

“…A final digital demodulation technique we will review is based on the use of a 3x3 fiber optic coupler in the optical system (Kwon et al, 1999;Sheem, 1981;Sheem et al, 1982). Again, the demodulation method is classified as a passive homodyne technique.…”

A simple method for measuring dynamic phase changes in a homodyne interferometric fiber-optic sensor

“…The phase difference may be divided into two components: a slowly changing random phase shift induced mainly by ambient temperature fluctuations, and a rapidly changing dynamic phase modulation induced by a harmonically varying measurand [3]. The amplitude of the output signal will vary unpredictably due to the random phase shift, which makes the readout of the dynamic phase shift in an interferometric fiber-optic sensor more difficult [4][5][6][7][8].…”

“…To recover the dynamic phase shift, many methods have been developed [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. These schemes mainly include phase tracking and compensating methods, a passive homodyne method using a 3 Â 3 directional coupler, phase generated carrier (PGC) modulation and demodulation, and various heterodyne methods.…”