Self-Mixing Laser Sensor for Large Displacements: Signal Recovery in the Presence of Speckle

Abstract

Laser self-mixing (SM) sensors are successfully used to measure displacement in the absence of speckle. However, speckle deforms the SM signal rendering it unusable for standard displacement extraction techniques. This article proposes a new signal-processing technique, based on tracking the signal envelope, to remedy this problem. Algorithm was successfully employed to measure long-range displacements (25 mm), in the presence of speckle and the lateral movement of the target, both causing severe corruption of the SM signal. It therefore enabled the use of the sensor on noncooperative targets without the need for sensor positioning and/or alignment. The results have been obtained for SM signals in which the envelope amplitude is varied by a factor of 28, without the loss of interferometric fringes. The use of this technique effectively removes the need for opto/electromechanical components traditionally used to measure long-range displacement in the presence of speckle.

All Analog Processing of Speckle Affected Self-Mixing Interferometric Signals

“…3) due to its fading as a function of remote target's surface roughness which scatters the incoming laser light resulting in an interference pattern that is granular due to the superposition of random phases of the beams [21]. For small displacements, it does not affect largely but has a significant effect in the case of large displacements [19]. Let us present a brief literature review regarding speckle in the context of SMI.…”

“…Another DSP algorithm makes use of an envelope extraction technique and thus avoids the use of a constant threshold during the detection of amplitude modulated SMI fringes [19]. It thus employs a dynamic threshold based on local variations in SMI envelopes [19].…”

“…One approach is to avoid the Date of publication : 15 th of Sptember 2017 occurrence of speckle within SMI either by using cooperative target surface, or by adding electro-mechanical/optical/laser [15][16][17] components to the SMI sensor. The second approach uses additional DSP algorithms for signal recovery such as Hilbert transform [18], envelope extraction [19] and wavelet transform methods [20,21]. The later approach is thus preferable as it maintains the inherent simplicity of SMI sensor design.…”

“…The challenge, in the framework of analog phase unwrapping [23], was as to how speckle affected SMI signal could be processed in an all analog manner. The proposed work shows that the approach of SMI envelope extraction [19], [21] can be implemented in standard analog circuits so that all analog processing of speckle affected SMI signal can be successfully executed. Such an all analog processing of speckle affected SMI signal appears full of potential as it enables displacement extraction from SMI signals through standard analog circuits thereby removing constraints such as the requirement of highperformance analog to digital converter (ADC) and DSP hardware platforms.…”

Extraction of vibration parameters from optical feedback interferometry signals using wavelets

“…In order to keep track of speckle, Zabit et. al in [10] proposed the use of sliding window to determine the envelope of the OFI signal and locate the correct fringes for target reconstruction. Finding the envelope of the signal was performed twice -for positive and negative envelope separately.…”