Detection of Multimodal Fringes for Self-Mixing-Based Vibration Measurement

Correct detection of interferometric fringes is necessary to enable high-resolution metric measurements using laser diode (LD)-based self-mixing (SM) interferometry. For noisy, experimentally acquired SM signals, fringe detection (FD) is a challenge even when the LD remains monomodal. This task becomes more complicated in case of multiple laser modes undergo SM. In this paper, a novel SM FD method is proposed which correctly processes multimodal as well as typically encountered monomodal SM signals. As SM fringes appear as local signal peaks in SM signals, so the proposed method uses peak detection to detect all-possible local peaks within the SM signal. Then, it successively refines its performance by removing false peaks (fringes) by analyzing corresponding SM fringe characteristics. In order to validate the proposed method for SM vibration sensing, several monomodal, bimodal, and trimodal SM signals were acquired by using two different LDs normally emitting at 637 and 650 nm, respectively. The proposed method is also tested on experimental SM signals corresponding to cases of variation in frequency as well as amplitude of remote target vibration. The proposed method has correctly detected 10786 fringes out of total 10844 fringes making up the data set, i.e., an accuracy of 99.46%. However, 55 false fringes were also detected (0.51%) while 58 true fringes were undetected (0.54%) by the proposed method.