Volume 57, number 2 OPTICS COMMUNICATIONS 15 February 1986 COMBINED INTERFEROMETRIC-POLARIMETRIC FIBRE OPTIC SENSOR CAPABLE OF REMOTE OPERATION P. AKHAVAN LEILABADY, J.D.C. JONES and D.A. JACKSON Physics Laboratory, The University, Canterbury, Kent, CT2 7NR, UK Received 18 October 1985 A monomode fibre sensor is described in which the measurand-induced changes in both state of polarisation and phase are simultaneously recovered. The miniature fibre sensing element is deployed at the end of a fibre cable which may be of arbitrary length and is measurand-insensitive. The technique combines high resolution and wide unambiguous measurement range, and has been demonstrated here as a temperature sensor. 1. Introduction The use of monomode fibre sensors for the precise measurement of slowly varying physical quantities, such as temperature, is of considerable current inter- est (see ref. [1 ], for example). Within the sensing ele- ment, the measurand modulates both the phase and state of polarisation of the guided beam; instruments based on phase modulation are generally termed inter- ferometric, and those utilising polarisation state mo- dulation, polarimetric. Speaking broadly, interferom- etric sensors offer higher resolution, whereas polarim- etric ones give a greater unambiguous measurement range. We have recently described optical processing schemes [2] which may be applied to the monomode fibre sensing element in order to simultaneously re- cover phase and polarisation state information, thus realising a combined interferometric and polarimetric sensor. We have applied these techniques in the mea- surement of temperature [3,4] and strain [5]. In each case, the usefully extended measurement range of the polarimetric sensor was achieved, without compromis- ing interferometric resolution. However, in these previously reported schemes, a number of conven- tional optical components were required in the vicin- ity of the measurement volume. In this paper we re- port a development of the technique which allows the fibre sensing element to be situated remotely from the 0 030-4018/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) conventional optical components, but to be connected to them via a fibre optic cable. An essential feature of the scheme is that this connecting lead is measurand- insensitive. We report results of experiments carried out in which the system was configured as a tempera- ture sensor, using a 50 m fibre connecting lead. 2. Theory and experimental In the present experiments both the sensing and connecting fibres were highly linearly birefringent, characterised by two orthogonal linear polarisation eigenstates. The change in the phase retardances ex- perienced by each eigenmode due to a temperature change AT is given by Ad?[ = (27r/X)(n[ aLl/aT + L i an[/aT)a T , i=f,s, ]=0,1, (1) where the subscripts f and s refer to the fast and the slow eigenaxes of the fibre respectively, and the super- scripts 0 and 1 refer to the connecting and the sensing fibres respectively. L ! is the length of the fibre section, X is the vacuum wavelength of the source, and n / is the effective refractive index of the ith eigenmode. Similar- ly, the differential phase response of the fibre to a change in temperature, AT, is given by 77