Dual heterodyne polarization diversity(DHPD) scheme is demonstrated for the elimination of signal fading problems induced by the phase bias and polarization drift in Mach-Zehnder or Michelson interferometers. DHPD technique is based on different frequency shift for two orthogonal polarizations in one optical path of the interferometer. This scheme results in two heterodyned beat notes of different frequencies which do not fade simultaneously under any polarization variations. Since the phases of the beat notes are modulated by the measurand, signal fading due to phase bias is also avoided if the larger beat note is selected and demodulated. The dual frequency shift is realized by a novel fiber-optic device, namely a dual frequency shifter. The operating principle of the dual frequency shifter is very similar to that of well known fiber-optic single frequency shifter. The main difference is that the dual frequency shifter is made of high birefringence two mode fiber and two flexural acoustic waves are excited in the fiber. The periods of the flexural acoustic waves should be the same as the beat lengths between the two spatial modes for the two orthogonal eigen polarizations. Since it is difficult to separate the two beat notes due to very small frequency separation compared to the center frequencies, another frequency shifter may be introduced in the other optical path to reduce the carrier frequencies of the beat notes without changing the frequency separation. This triple frequency shift approach introduces an additional advantage when the efficiency of the dual frequency shifter is not unity. In the triple frequency shifted system the beat notes of our interest are generated only by the frequency shifted components and the effect of unshifted frequency components can be eliminated. In usual interferometric sensors, the sensitivity is limited by the largest noise which comes from the electronic or optical origin such as intensity noise and source phase induced intensity noise. In the dual frequency shifted system with a conversion efficiency of ~98%, the sensitivity was $~28μrad/\sqrt{Hz}$ which was limited by the source phase induced intensity noise due to the non-ideal optical pulse generator. In the triple frequency shifted system the measured sensitivity was $~50μrad/\sqrt{Hz}$. The sensitivity reduction was due to the low efficiency of the single frequency shifter of ~69%.
A depolarized Sagnac interferometer was also examined as an acoustic sensor array which inherently does not suffer from polarization induced signal fading. Since the phase responsivity of a Sagnac interferometer has very strong frequency dependence, they must be designed carefully for their specific use. Since the phase bias of the depolarized Sagnac interferometer output is always zero, quadrature phase bias was introduced by a phase modulator between the two interfering optical waves. Using this scheme the interferometer output showed a linear dependence on the acoustic signal with a good sensitivity of $~10μrad/\sqrt{Hz}$ in our experiment.
