Optical Nanofiber Coupler Sensors Operating in the Cut-Off Wavelength Region

An ultrasensitive optical nanofiber coupler (NFC) for thermal and refractive index sensing with nanoscale dimension is proposed and experimentally demonstrated. The NFC operating near the cut-off region of the higher order modes is fabricated by fusing and pulling two standard telecom fibers using the modified flame brushing technique. Due to the submicron diameter, the NFC exhibits an anomalous behavior in close proximity to the odd supermode cut-off wavelength, where the high extinction ratio spectral oscillations exhibit very slow oscillations. The sensitivity of this region is exploited for thermal sensing over a broad range of temperatures (84 °C-661 °C). An average temperature sensitivity of 55 pm/°C and a maximum sensitivity of 60 pm/°C at the highest temperature were demonstrated. Owing to its small size, the thermal sensor exhibits a fast response time of ~7.2 ms. The sensor capability for detecting refractive index changes is also examined with different ethanol concentrations. A record sensitivity of 4.80 × 10
⁵
nm/RIU was achieved, which is the highest reported value in all optical fiber refractometers.