Selective Mode Excitation in a Few-Mode Photonic Crystal Fiber for Strain Sensing With Restrained Temperature Response

Photonic crystal fibers (PCFs) with various micro to nano-scale features are of intensive interest for fiber sensing. Flexible fiber structures enable the selective control of fiber properties, such as transmission modes, dispersion tailoring, trigger of multiple nonlinear effects, and so on. In this article, three pieces of few-mode PCFs (FM-PCFs) are designed and fabricated. Guided modes in each fiber are theoretically analyzed and experimentally observed through careful excitation. Later one of the PCFs (PCF1) is used as a fiber interferometer, based on the analyses of intermodal couplings/interferences of all PCFs. The intermodal interferences between linearly polarized (LP) core modes LP01/LP11 of PCF1 are experimentally investigated, using selective mode excitation technique. The fringe contrast and intensity of the interferences can be precisely manipulated by varying the relative energies of the LP01 and LP11 modes, as the basis for the intermodal interferometer. Different from most previous studies, the interferences are mainly limited to the two core modes, which intrinsically reduces the cross-influence from temperature variations, when this fiber is used for strain sensing. Experimentally, a strain sensor using this FM-PCF based Mach-Zehnder (MZ) interferometer, is demonstrated with a restrained temperature response. This work provides significant guidance for the construction of FM-PCF based fiber sensors with simple fabrication procedure and precisely controlled interference, and most critically, with reduced cross-sensitivity between temperature and strain.