This thesis is focused on a new method for overcoming the polarization-induced signal fading(PSF) in interferometric sensors that are time division multiplexed(TDM) using short optical pulses.
The sensor array consists of a compensating interferometer that contains all active devices for signal processing, and remote sensors that are highly unbalanced Mach-Zehnder interferometers. A pair of highly unbalanced interferometers connected in series, which is a basic unit of TDM, makes it possible to form a passive remote sensing. When an optical pulse, whose width is shorter than the optical path length difference(OPD) of the interferometers, is launched into the input fiber of a basic unit of TDM, two light pulses come out of the compensating interferometer. When the two pulses pass through the sensing interferometer, four optical pulses will come out, two of which have complete overlap producing interference signal. A sensor array was constructed with a compensating interferometer and two sensing interferometers. A laser diode with single longitudinal mode (λ~830nm) was gated by an acousto-optic modulator to produce optical pulses. The width and repetition rate of the optical pulses were 160ns and 600kHz, respectively. The OPD's of the unbalanced Mach-Zehnder interferometers were about 70m and the difference between them was matched within 3cm. An optical switch or an electronic switch was used to separate an interference pulse of a specific sensor in the output pulses. Synthetic heterodyne signal processing technique was used to overcome the signal fading due to random phase drift. The relative intensity noise of the source at 25kHz was$\sim3\mu rad/\sqrt{Hz}$ and the shot noise level of the detector was $\sim0.7\mu rad/\sqrt{Hz}$. The measured sensitivity at signal frequencies of a few kHz was about $40\mu rad/\sqrt{Hz}$limited by source phase-induced intensity noise owing to the OPD mismatch between the two unbalanced interferometers. The cross talk between two sensors was below -50dB.
Signals from fiber-optic interferometric sensors formed with conventional single-mode fibers may be lost due to the random fluctuation of the polarization states of two interfering optical waves. When the state of polarizations(SOP) of two interfering pulses is orthgonal to each other, the fringe visibility becomes zero. This is so-called PSF. In this thesis, a new polarization switching method to suppress the PSF is proposed and demonstrated with a TDM-based sensor array. The principle is that when the PSF occurs, the visibility can be recovered if the SOP of one of the two interfering pulses is converted to the orthogonal SOP.
For the demonstration of the polarization switching method, a fiber-optic polarization switch is put in the longer arm of the compensating interferometer. The system was powered by an optical pulse with 160ns of the width and 940kHz of the repetition. The polarization switch was driven at 470kHz, a half of the input optical pulse repetition, in order to convert the SOP of every other input pulse to orthogonal SOP. From the ouput of the array for two successive input pulses, it has been clearly observed that when one interference pulse experienced the PSF, the visibility of the adjacent interference pulse became maximum as predicted. The two sets of signal pulses corresponding to the two SOP's can not be simultaneously experienced the PSF and can be separated and processed independently. Therefore the PSF can be successfully overcomed if the non-faded signal of the two is selected. The polarization switching does not degrade the sensitivity of the sensor array.
