Analysis of misalignment, twist, and bend in few-mode fibers using spatially and spectrally resolved imaging

Modal pattern and power in few-mode fibers, with dominant high-order modes by mode excitation at different core offset misalignments, were simulated and experimentally examined using the spatially and spectrally resolved imaging (S-2) technique. The far-field specklegram of the interference between the LP11 and LP21 modes was observed. Twisting and bending characteristics of the LP11 mode in a two-mode fiber (TMF) were presented by reconstructing the LP11 mode using the S-2 imaging technique. The experimental results showed that the LP11 mode specklegram rotated 76.4% of the fiber twist angle and 0.58 deg/m(-1) of the fiber curvature in the TMF. The relative peak amplitude was measured to decrease the fiber twist angle by 0.009 dB/deg and the curvature by 0.056 dB/m(-1). These characteristics allow for future use of the LP11 mode in fiber specklegram sensors. Additionally, the advantage of the S-2 method for specklegram sensors is that it does not require the excitation of only the LP11 mode in the fiber, which simplifies the experiment.