Multiwavelength LP02 - LP21 intermodal interferometric temperature sensing using stimulated Raman scattering.

Modal interference in dual-mode or few-mode fibers has been widely investigated, largely for its application in interferometric fiber-optic sensors1. Generally the two or more modes are launched simultaneously in the fiber, their relative intensities depending on the light coupling conditions. We describe here a multiwavelength temperature sensor using the interference of LP02 - LP21 modes in a spectrally extended (538 - 600 nm) light produced largely in LP02 mode2,3 by stimulated Raman scattering (SRS) in a few-mode fiber. Unlike the examples referenced above, the second mode (LP21) is introduced by applying a mode-coupling perturbation in the form of a localized transverse stress close to the exit end of the fiber. As is known4, use of a single wavelength limits the useful range of the sensor to a value π of the phase difference change. The spectrally extended SRS light offers a convenient source for interferometric sensing at two or more wavelengths simultaneously, thus making it possible to sense unambiguously over an unlimited range.