Electronic speckle pattern interferometry using diode lase stroboscopic illumination

Abstract

An electronic speckle pattern interferometer (ESPI) using stroboscopic illumination from a diode laser, and its application in vibration measurement, are described. Diode lasers are readily frequency (FM) and power modulated (AM) via control of their injection current; they are inexpensive and compatible with the single-mode optical fibres used in the described system. However, their modest peak power output demands the use of high-duty-cycle pulses, so that it was necessary to synchronize the illumination with the turning points of the vibration. This was achieved by operating the ESPI system in conjunction with a laser vibrometer. Injection-current-induced FM of diode lasers is non-uniform for modulation frequencies less than about 10 MHz, and to achieve successful stroboscopic illumination an electronic equalization stage was included to shape the current pulse, and hence compensate for the non-uniform FM.

<title>ESPI-FEM hybrid system for studies of time-dependent stress characteristics in small components</title>

“…This process can be represented by the following equations: Il = Io+Ir+2AoArcos(çboiçbr) (5) '2 '0 + Jr + 2AoAr cos (q0 _ r) (6) I = '2 '1 = 2A0Ar C05 (ço r) 2A0Ar cos (q° -çr) = = 4A0Ar sin (r o + ) sin , (7) where Ii and 12 are intensities in the image plane before and after loading, respectively, and Li = qo2 -cO1 the phase contribution due to the displacement of the object under the load. For the retroreflective illumination case, (x,y) = d(x,y), (8) where d(x, y) is the displacement of the point (x, y) of the object in z direction.…”

Multi-point vibrometer based on high-speed digital in-line holography

“…[30][31][32][33][34][35]. In the past, speckle interferometry (also similar to "digital image-plane holography" [36]), was demonstrated to provide an efficient tool in combination with time-averaging and digital image processing [37], fiber optics and acousto-optic stroboscopic illumination [38], laser-diode modulation [39], and pulsed laser coupled to a classical vibrometer [40]. In the past, the use of stroboscopic [41][42][43] or pulsed light [44][45][46][47][48] resulted in the direct measurement of the vibration amplitude.…”

Evaluation of surface acoustic waves on the human skin using quasi-time-averaged digital Fresnel holograms

“…A solution may lie in the use of stroboscopic [37,38] or pulsed light [39][40][41][42][43][44], which results in the direct measurement of the vibration amplitude. Such methods are quite useful for modal analysis and the determination of structural intensity [45,46], to measure 3D vibrations [47], surface acoustic wave [48], the high amplitude autooscillations of a clarinet reed [49] or 3D displacement on a cat's post-mortem tympanic membrane [50].…”