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OPERATION OF A COMPENSATED POLARIMETRIC SENSOR WITHSEMICONDUCTOR LIGHT SOURCEJ.P. DAKIN, S. BRODERICK, D.C. CARLESS, C.A. WADEPlessey Electronic Systems ResearchRoke Manor, Romsey, U.K.INTRODUCTIONThe conventional polarimetric sensor, first devised byRashleigh and Ulrich (1), is a simple transducer usingpolarisation maintaining fibre which, utilises changesin the differential optical delay between the twoorthogonally -polarised modes of the fibre to sensephysical parameters. The relative delay is convenientlyobserved by monitoring the state of polarisation of thelight at the far end of the fibre (fig. 1(a)).The conventional polarimetric sensor has two mainproblems, firstly, a highly coherent light source isrequired in order to avoid phase noise at the output ofthe unbalanced fibre interferometer, and secondly thesensor is sensitive to ambient changes in both pressureand temperature. Both these problems have been solvedusing the compensated polarimetric technique (2), whichenables a differential sensor to be constructed bysplicing two equal lengths of polarisation -preservingfibre, after a relative twisting of one fibre by 90°about its axis with respect to the other. In this mannera balanced interferometer is formed, thus in principleallowing the use of relatively incoherent sources. |
