Abstract
This paper deals with the transduction of strain accompanying elastic waves in solids by firmly attached optical fibers. Stretching sections of optical fibers changes the time required by guided light to pass such sections. Exploiting interferometric techniques, highly sensitive fiber-optic strain transducers are feasible based on this fiber-intrinsic effect. The impact on the actual strain conversion of the fiber segment’s shape and size, as well as its inclination to the elastic wavefront is studied. FEM analyses show that severe distortions of the interferometric response occur when the attached fiber length spans a noticeable fraction of the elastic wavelength. Analytical models of strain transduction are presented for typical transducer shapes. They are used to compute input-output relationships for the transduction of narrow-band strain pulses as a function of the mechanical wavelength. The described approach applies to many transducers depending on the distributed interaction with the investigated object.
Highlights
Ultrasonic waves are convenient means to investigate the structural integrity of extended objects [1,2,3,4,5,6,7,8]
Elastic waves may be visualized in a contactless manner by laser Doppler vibrometry, but they are typically probed with attached transducers that are sensitive to strain variations induced by elastic waves in the specimen [13]
The modeling of wave strain conversion using fiber-optic interferometry and compact strain pulses was successfully performed for several shapes of attached fiber segments at a broad range of ultrasonic wavelengths
Summary
Ultrasonic waves are convenient means to investigate the structural integrity of extended objects [1,2,3,4,5,6,7,8]. Piezoelectric strain transducers suffer from interferences by electromagnetic fields generated, for example, during piezoelectric actuation of the elastic waves to be detected [14,15,16]. Fiber Bragg grating (FBG) transducers are very common devices for the transduction of elastic waves [8]. Interferometric fiber-optic transducers rely on the strain dependence of fiber-intrinsic properties [19,20] Such transducers comprise typically two fiber arms, i.e., a segment of an optical waveguide that is firmly attached to the investigated specimen and an unperturbed reference fiber. They are complemented by an optical arrangement that ensures interference between the light waves of measurement and reference waveguide.
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