This investigation presents a simple, widely tunable, and ultrasensitive sensor that is based on a leaky-guided multimode fiber interferometer (MMFI) operated under refractive-index-matched coupling. By use of a material with an appropriate dispersion profile around the MMFI as a cladding yields strong index-matched coupling, which performs ultrasensitive sensing in variations of the surroundings. Index matching at a single wavelength yields a coupling wavelength dip with a narrow bandwidth and a high extinction ratio of over 25 dB. The wavelength dip can also be effectively tuned, greatly shifting with a variation in temperature ($T$) or refractive index (RI), when the index-matched condition is satisfied. This work demonstrates that the proposed sensor responds sensitively to $T$ with an extremely high tuning efficiency of $50\mathrm{nm}/°\mathrm{C}$ and an excellent sensitivity to RI of 113,500 nm per RI unit.

© 2012 Optical Society of America

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