Fiber-optic sensors for measuring voltage, strain and temperature have been demonstrated by using a Mach-Zehnder interferometer and a two-mode fiber-optic interferometer. While a Mach-Zehnder interferometer is the most straight forward approach, a two-mode fiber-optic interferometer allows the realization of very useful sensors that are difficult to make using conventional single-mode fiber-optic technology.
A fiber-optic voltage sensor based on the Mach-Zehnder interferometer has been demonstrated with a new signal processing scheme, which displays the magnitude as well as the waveform of applied voltage with immunity from the signal frequency change. The sensing element was formed using a PZT phase modulator. The voltage was measured by counting the number of fringe shift during a half period of applied signal. The sensor showed a linear response to the applied voltage. The temperature dependence of the sensor output was experimentally evaluated over the temperature range from -20℃ to 80℃. It was demonstrated that the detrimental polarization modulation effect could be overcome by using polarization maintaining fibers or a half-wave plate in the phase modulator. The polarization signal fading could be avoided by using a rotating polarizer at the interferometer output.
For two-mode fiber-optic interferometric sensors, the polarization and modal characteristics of a two-mode fiber with a highly elliptical core were studied. We measured the cutoff wavelengths of the second-order mode($LP_{11}^{even}}$) for two polarization states, the beatlengths between two spatial modes (${rm LP_{01}}$ and ($LP_{11}$) for two polarization states, and the polarization beatlength of the fundamental mode. Large polarization dependances of the cutoff wavelength and the beatlengths were measured. The large difference of the cutoff wavelengths for two polarization states can be utilized for a new type polarizer.
The differential phase shift between two modes induced by strain and temperature change was measured. We observed a large nonlinear response of the differential phase shift between the two spatial modes to the fiber elongation when the optical wavelength is near the cutoff wavelength $(λ_c)$ of the $LP_{11}}$ mode. It has been analyzed experimentally and theoretically that the nonlinearity increases dramatically as the wavelength approaches the cutoff wavelength. The nonlinear strain response is an important factor for the design of two-mode fiber-optic strain sensors and new applications may be possible based on the effect.
