The Application of Super-Resolution Adaptive Algorithms to Fringe Order Estimation in All-Optical-Fibre Interferometric Sensors

Abstract

Optical-fibre white-light interferornetry (WLI) has recently been attracting significant attention in the research area of optical-fibre sensors¹. In this application it is necessary to identify the centre fringe from an interference fringe pattern which is the output of an optical fibre interferometer incorporating a broadband optical source. As a result, the unambiguous range of the output signal is no longer limited to within half a fringe, and an absolute phase measurement over a large operating range can be achieved. The output fringe pattern (Figure 1) of such a system takes the form of a Gaussian amplitude-modulated cosinusoidal pattern, but the centre fringe and adjacent may not be easily distinguishable from the rest of the pattern. This problem is further exacerbated when the signal is buried in noise. Because of the difficulty of pinpointing the centre fringe, long-range absolute phase detection with a resolution in the subfringe region may not be possible. However, modern adaptive digital filtering techniques coupled with parametric estimation, which have been successfully applied in such diverse fields as communications, radar, sonar, seismology, and biomedical engineering, can be utilized to enhance the spatial fringe pattern in the presence of strong additive Gaussian noise even when such a pattern is buried in noise. This implies that it is possible to multiplex intrinsic sensors distributed along a fibre without stringent constraints on power requirements.