Transmission Spectrum Characteristics for a Single-Mode-Multimode-Single-Mode Fiber Filter

We designed and demonstrated a single-mode-multimode-single-mode fiber filter using a step-indexed large-mode-area fiber, which has a critical wavelength at 1046 nm. A broad transmission peak or trough at 1046 nm is very sensitive to strain or temperature variation. The interference fringes on either side of the critical wavelength show blue or red shift, respectively, when the filter is subjected to strain and temperature changes. We simulated the transmission spectrum based on the interference of two modes, LP₀₁ and LP₀₂, and their propagation constant difference. The simulated spectrum matches reasonably well with the experimental spectrum. The filter can be used as a variable output coupler or for fiber optic sensors.