Optically passive fiber sensors for dynamic studies using Doppler velocimeter and white-light interferometry

This Dissertation introduces several optical fiber-based measurement techniques designed for use in shock loading environments. The first technique is an optical fiber strain-rate sensor that uses a Mach-Zehnder interferometer to convert strain-rate induced Doppler wavelength shifts in the light propagating in the fiber to phase changes. The Mach-Zehnder read-out interferometer is passively demodulated using a 3 x 3 coupler and yields one independent measure of phase related to the strain-rate field. While this sensor is not localized, the results show that the concept of using in-fiber velocimetry for strain-rate measurement is feasible. Following the same idea, a simple all-fiber Doppler velocimeter that is lightweight, compact, and requires no optical alignment other than launching the light into the fiber is described. This approach uses passive 3 x 3 coupler optical fiber demodulation to obtain unambiguous Doppler-induced optical phase changes, which are related to velocity. The theory shows that the sensitivity of this technique scales with the readout interferometer path-imbalance, so that the sensitivity can be adjusted by simply "plugging-in" different lengths of optical fiber. A classical Hopkinson bar experiment was used to test the all-fiber velocimeter, and the results showed good agreement with velocity distributions calculated using one-dimensional elastic wave propagation theory combined with resistance strain gage measurements. The third class of optical fiber sensing techniques that was developed are strain sensors that use passive quadrature signal processing of Fabry-Perot white-light interferometers. These techniques are based on path-matched differential interferometric (PMDI) configurations, 3 x 3 optical coupler, and two intracore Bragg-gratings of spectral decoding. Using the amplified spontaneous emission (ASE) from an erbium doped fiber amplifier (EDFA) as a low coherence high power broad-band source, the three passive detection techniques are demonstrated for use with in-fiber Fabry-Perot sensors (IFP/EFPI/ILFE) in high strain-rate dynamic studies. Fabry-Perot sensors have been successfully applied to monitor the strain under dynamic loading conditions produced by one-dimensional stress wave propagation.; The sensors applied successfully in the Hopkinson bar tests were tested under explosive loading environments. The short sensor gage length ({dollar}sim{dollar}100 {dollar}mu{dollar}m) was to improve the signal-noise ratio in the optical fiber system and the polarization maintaining fiber was to overcome the significant intensity and polarization fading that occurs during explosive loading. These improvements along with an intensity compensation technique developed for these experiments proved successful in reducing intensity fading to the point where accurate strain measurements were possible.