Signal Pick-Up Technique for Sapphire Fiber External Fabry-Perot Interferometer Using Tapered Multimode Fibers

A tapered multimode fiber (MMF) to sufficiently and stably pick up the interference signal of a sapphire fiber external Fabry-Perot interferometer (EFPI) is proposed and investigated. The tapered MMF was fabricated following the adiabaticity criteria for the local tapering angle such that the excited fundamental mode in MMF barely couples to the higher-order modes, ensuring a high-visibility interference output from the sapphire fiber EFPI. Tapered MMF also guarantees a robust fusion splice between the leading single-mode fiber and the sapphire fiber. Experimental results show that using tapered MMF the excitation coefficient of fundamental mode can be improved by more than 3.5 times comparing with conventional technique using non-tapered MMF, and the fringe contrast of the interference spectrum can increase by 2.3 times. The response of the collected interference spectra from the sapphire fiber EFPI using tapered MMF was tested when ambient pressure and temperature was changed from 0.1 MPa to 6.1 MPa and 17 °C to 302°C respectively. The results validate that tapered MMF can provide a high stability against environmental pressure and temperature variations.