High-Sensitivity Detection of Fiber Bends: 1-μm-Band Mode-Detection OTDR

We propose a measurement system based on optical time-domain reflectometry (OTDR) that offers high-sensitivity detection of bend location in a standard single-mode (SM) fiber. This technique, which employs 1-μm-band probe pulse, i.e., below the cut-off wavelength of SM fiber, enables us to individually observe the fundamental (LP
₀₁
) and the second-order (LP
₁₁
) modes of the backscattered light generated in the two-mode region of the SM fiber, using a key technical advance is a mode-selective coupler designed for optimal 1-μm-band operation. Analyzing the loss distribution of the backscattered LP
₁₁
mode realizes the high-sensitivity detection of the fiber bend location without suffering degradations from the modal dispersion. We perform experiments for the four kinds of widely-used SM fibers, and demonstrate the proof-of-concept; a comparison is made with the conventional OTDR operating at the wavelength of 1650 nm.