Envelope Tracking of Laser Self-Mixing Signals for Robust Fringe Detection

Abstract

Self-Mixing Signals are used for measurement of target’s distance/displacement and vibrations. SMI theory [1] is based on change in Laser’s characteristics due to optical feedback from a remote target. These changes including the change in Laser power can be recorded at receiver end (at the receiver photo diode). Fringe corresponds to the portion of SMI signal corresponding to half of wavelength change in the external cavity length. By analyzing the detected fringes, target’s movement can be estimated. Thus, fringe detection is the fundamental step in SMI processing. Conventional methods proposed lay foundation for SMI signal detection from targets with smooth surface and efforts have been invested to avoid speckle, mostly by tracking the strong signal by means of electro mechanical actuators. However, in case of irregular surface of the target, speckle is introduced in the signal. Techniques have been proposed in literature for fringe detection for speckle affected signal e.g. [2] Proposes fringe detection matched filter, [3] by three envelope method etc. This thesis proposes a methodology of fringe detection for speckle affected SMI signal by using envelope tracking based on Hilbert transform. Results obtained by the proposed technique indicate an improvement over conventional fringe detection method for both simulated SMI signal as well as experimentally acquired signal