Transverse stress induced LP(02)-LP(21) modal interference of stimulated Raman scattered light in a few-mode optical fiber

Four-photon mixing followed by stimulated Raman scattering is observed in LP(02) mode in a 7.9 mu m core diameter optical fiber. A localized transverse stress efficiency couples LP(02) to the LP(21) mode with a macroscopic beat length of 1.8 mm. LP(02)-LP(21) modal interference is investigated by detecting the 550-590 nm SRS through a pinhole in the far field exit plane. Quantitative explanation of wavelength dependent intensity modulation results in a precise experimental determination of a partial derivative[beta(02)(lambda) - beta(21)(lambda)]/partial derivative lambda, for mode-propagation constants beta(02)(lambda) and beta(21)(lambda) of LP(02) and LP(21) modes respectively, as well as Delta, the relative core-cladding refractive index difference. The LP(02)-LP(21) modal interference is used for sensing of temperature between 50-300 degrees C.