Differential length measurement using low coherence coupled tandem interferometry

This paper presents the use of low coherence coupled tandem interferometry to measure the differential length of two independent Fabry-Perot (F-P) type microcavities. The two discrete F-P type microcavities are formed between the cleaved end of a fibre and a reflective surface, which could for example, be a pressure sensing membrane or any other component of a transducing element. The technique is an all-optical fibre based sensing configuration in which the sensing cavities are at widely separated locations in an environment where strong temperature gradients may exist.

The sensing system is based on two sequential cavities arranged in tandem. The lengths of the cavities are probed by a temperature stabilised fibre based Michelson interferometer operating with a broadband light source. One arm of the probing Michelson interferometer is scanned using a piezo fibre stretcher resulting in an optical path length difference (OPD) between the two arms.

The optical interconnecting leads from the probing Michelson interferometer to the two F-P locations are not an active part of the sensor configuration and therefore this configuration is largely insensitive to temperature and strain effects on these interconnecting leads. It is only the probing Michelson interferometer which has to be temperature stabilised. This arrangement allows the F-P measurement cavities to be separated by distances in the range of tens of meters.