Time-frequency signal processing for a Self-Mixing laser sensor for vibration measurement

In this paper, a novel time-frequency signal processing approach is presented for a Self-Mixing (SM) interferometric Laser Diode (LD) sensor that enables measurement of harmonic and arbitrarily shaped vibrations. The proposed time-frequency technique ameliorates the measurement precision of a previously published time-domain based displacement retrieval technique called the Phase Unwrapping Method (PUM). By incorporating a frequency-domain analysis to the PUM, we not only improve the measurement precision but also recover information about target movement harmonics that can be used for modal analysis applications. In addition, the time-frequency processing has been found to be robust in case of variations in optical feedback coupling factor. The time-frequency technique has thus provided a precision of approx. 15nm rms (while that of PUM is approx. 40nm rms) for micrometric harmonic and arbitrarily shaped vibrations by using a SM sensor based on a LD emitting at 785nm.