Adaptive demodulation of dynamic signals from fiber Bragg grating sensors using two-wave mixing interferometry in indium phosphide:iron

A two-wave mixing (TWM) interferometer using photorefractive (PRC) InP:Fe crystal is configured to demodulate the dynamic wavelength (spectral) shift signals from Fiber Bragg Grating (FBG) sensors. The FBG is illuminated with a broadband source, and any strain in the FBG is encoded as a wavelength shift of the light reflected by the FBG. The wavelength shift is converted into a phase shift by means of an unbalanced TWM interferometer. TWM spectral demodulation is attractive for monitoring dynamic strains signals from acoustic emission and impact event because it is adaptive and multiplexable. Adaptivity implies that it can selectively monitor dynamic strains without active compensation of large quasi-static strains and temperature drifts that otherwise would cause the system to drift. Multiplexability implies that several FBG sensors can be simultaneously demodulated using a single demodulation unit. The TWM spectral demodulator is therefore a cost-effective method of demodulating multiple spectrally encoded FBG sensors for the purpose of monitoring acoustic emissions and impact events.; A theoretical model of the TWM gain in the 1550nm wavelength for InP:Fe is discussed in this dissertation and experiments carried out to characterize the TWM gain at this wavelength are reported. The working principle of the TWM interferometry is discussed and three different configurations of using InP:Fe are discussed and compared.; The TWM spectral demodulator for FBG sensors is experimentally demonstrated and so is its adaptivity to quasistatic strain and temperature drift. The TWM spectral demodulator is found to be a linear detection scheme with resolution on the order of a hundred nano-strains. The TWM spectral demodulator is also shown to be amenable to wavelength division multiplexing to simultaneously demodulate multiple FBG sensor signals. In a first step, a four-channel TWM spectral demodulator is experimentally demonstrated and is shown to have essentially negligible cross-talk between channels. The system is then scaled up to an eight-channel system which is then used to monitor impact events.