We report a new distributed fiber optic sensing technique using optical carrier based microwave interferometry. The concept has been demonstrated using different types of optical fibers including singlemode fiber, multimode fiber, single crystal sapphire fiber and polymer fiber. Using the microwave-photonic technique, many fiber interferometers with the same or different optical path differences were interrogated and their locations could be unambiguously determined. The distributed sensing capability was demonstrated using cascaded low-finesse Fabry-Perot interferometers fabricated by fs laser micromachining. Spatially continuous, fully distributed temperature and strain measurements were used as examples to demonstrate the capability of the proposed concept.
A multimode fiber (MMF) based cascaded intrinsic Fabry-Perot interferometers (IFPIs) system is presented and the distributed strain sensing has been experimentally demonstrated by using such system. The proposed 13 cascaded IFPIs have been formed by 14 cascaded reflectors that have been fabricated on a grade index MMF. Each reflector has been made by drawing a line on the center of the cross-section of the MMF through a femtosecond laser. The distance between any two adjacent reflectors is around 100 cm. The optical carrier based microwave interferometry (OCMI) technique has been used to interrogate the MMF based cascaded FPIs system by reading the optical interference information in the microwave domain. The location along with the shift of the interference fringe pattern for each FPI can be resolved though signal processing based on the microwave domain information. The multimode interference showed very little influence to the microwave domain signals. By using such system the strain of 10-4 for each FPI sensor and the spatial resolution of less than 5 cm for the system can be easily achieved.