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...dulated SM fringes when the SM signal is deformed bysmodulation and fluctuations (such as shown in Fig. 4). Underssuch conditions, standard fringe detection techniques such asszero-crossing detection =-=[1]-=-, SM derivative detection [9], orsSM hysteresis based technique [16] would fail as such a SMssignal is no more centered at zero, is devoid of hysteresissbased symmetry, has varying fringe amplitude an...

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... the laser wavelength (λ) [7]. For smallsdisplacements of the target, speckle may not affect the SMssignals if the displacement is much smaller than the typicalslongitudinal size of the speckle grain =-=[8]-=-. For largersdisplacements, speckle can cause amplitude fading (Fig. 1).sSuch an amplitude modulation thus demands additionalsThis work was supported in part by the Agence Nationale de la Recherches(A...

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...ponents. Anothersapproach uses two piezo-actuators to move a lens for a specklestracking technique [8]. An improved version of it adds a liquidscrystal attenuator to maintain moderate feedback regime =-=[11]-=-.sIn [12], a SM vibrometer able to detect the deformed fringesshas been presented though harmonic vibration results are onlysof an order of 10-5 m. Finally, a voltage controlled liquid lenssand a doub...

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...ee shaded area of Fig. 5). It can be done either bysusing fringe counting providing λ/2 resolution as proposed ins[18] or by unwrapping the SM signal providing at least λ/10sresolution as proposed in =-=[19]-=-. As the unwrapping of SMssignal requires a normalization of each and every SM fringe,sso fringe location information is used to segment the SMssignal P(t) for each and every detected SM fringe. A sca...

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...arge displacement SM sensors reported earliersindicates that the use of co-operative targets has allowedsavoiding speckle effect [5] and even helped in improvingsmeasurements by an order of magnitude =-=[9]-=-. On the othershand, long range SM displacement sensing has also beensachieved by electronically keeping the operating point of thesSM interferometer fixed at the half-fringe [10] though thessensor st...

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...re the sensor would undergosimportant extraneous mechanical movements while ensuringscorrect measurement. The influence of the extraneoussmovements can be corrected by using an embeddedsaccelerometer =-=[17]-=-. Nonetheless, such an arrangementsnecessitates improved processing of the SM signal obtainedsunder given conditions. This also incited us to test thesproposed long range SM sensor under similar condi...

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...o recover thesvariations in the optical output power of the laser diode P(t)sthough it is also possible to recover equivalent interferometricsinformation by using LD junction voltage, as suggested in =-=[13]-=-sfor LD packages having no built-in PD. A lens is then used tosfocus the laser beam on to the target. Finally, P(t) is used tosrecover the target displacement D(t) [1-2]. The SM sensor issbased on a H...

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... fact, variations in C causeschanges in the SM feedback regimes varying from weak tosmoderate to strong feedback, where each regime would requiresspecific signal processing for displacement retrieval =-=[16]-=-.sHence, correct displacement can be recovered easily as long assthe signal is not corrupted by the presence of speckle.sOn the other hand, this basic processing does not work forsspeckle affected SM ...

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...versa) which may require further signal processing [1-2].sA look into large displacement SM sensors reported earliersindicates that the use of co-operative targets has allowedsavoiding speckle effect =-=[5]-=- and even helped in improvingsmeasurements by an order of magnitude [9]. On the othershand, long range SM displacement sensing has also beensachieved by electronically keeping the operating point of t...

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...ct Identifier: 10.1109/JSEN.2012.2227718sI. INTRODUCTIONself-mixing (SM) or optical feedback interferometry [1-3]shas been regularly used for displacement sensing [4-5]sthough the presence of speckle =-=[6]-=- has often reduced the rangesor even the possibility of correct measurement. Traditionally,sefforts have been invested to avoid speckle, mostly by trackingsthe strong signal by means of electro-mechan...

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...issstrongly affected by speckle.sSpeckle caused amplitude modulation (see Fig. 1) of a SMssignal is influenced by the target surface roughness, the lasersspot size as well as the laser wavelength (λ) =-=[7]-=-. For smallsdisplacements of the target, speckle may not affect the SMssignals if the displacement is much smaller than the typicalslongitudinal size of the speckle grain [8]. For largersdisplacements...

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...an order of magnitude [9]. On the othershand, long range SM displacement sensing has also beensachieved by electronically keeping the operating point of thesSM interferometer fixed at the half-fringe =-=[10]-=- though thessensor still requires additional optical components. Anothersapproach uses two piezo-actuators to move a lens for a specklestracking technique [8]. An improved version of it adds a liquids...

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...Anothersapproach uses two piezo-actuators to move a lens for a specklestracking technique [8]. An improved version of it adds a liquidscrystal attenuator to maintain moderate feedback regime [11].sIn =-=[12]-=-, a SM vibrometer able to detect the deformed fringesshas been presented though harmonic vibration results are onlysof an order of 10-5 m. Finally, a voltage controlled liquid lenssand a double-headed...

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...tes how the cut-off value is used to help identifysthe segments indicating the presence of a distinct SM fringesfrom the segments devoid of this information. As compared tosthe algorithm presented in =-=[18]-=- which used a fixed cut-offsvalue (see Fig. 5 of [18]), the current algorithm uses a dynamicscut-off indicative of local variations of SM signal. This resultssin a better detection of deeply modulated...