Unification of input and output ends in polarization-maintaining optical fiber stress sensor by synthesis of optical coherence function (Invited Paper)

We have developed a functional distributed fiber-optic stress sensor by synthesis of the optical coherence function (SOCF). The technique determines the location of stress-induced polarization mode coupling in a polarizationmaintaining fiber (PMF) by measuring the optical path difference (OPD) between the fast mode and the slow mode in PMF with SOCF. By modulating the frequency of the lightwave from a super-structure-grating distributed Bragg reflector laser diode (SSG-DBR-LD) in a stepwise waveform, the coherence function is synthesized into a series of periodical peaks in the meaning of time-integration. The period is controlled to allow only one coherence peak enter the range of the PM fiber under test. Then we can measure the polarization mode coupling at the position corresponding to the peak. The position of the peak is adjusted by using a phase modulation proportional to the frequency modulation. Therefore, polarization mode coupling distribution along the fiber can be obtained. Up to date, one end of the sensing fiber is used as the input end, and the other as the output end. This scheme is generally not convenient for remote applications. In this presentation, we report two new effective methods that unify the input and the output to one end of the fiber. In one scheme, a mirror plus a polarizer is attached to the far end of the PM fiber. The other scheme employs a polarization beam splitter (PBS) attached to the far end of the PM fiber. The light beam output from the PBS in one polarization direction is fed back into the fiber through the PBS in the perpendicular polarization direction. Experimental demonstrations for both schemes are presented.