Photo-thermal induced wide-range and high-precision tunable dispersion turning point in tapered fiber coupler

A tapered fiber coupler (TFC) whose dispersion turning point (DTP) can be widely and precisely tuned by a 980 nm laser was designed and fabricated in this research. It offers the potential application advantage of adapting to environmental changes while providing both a wide detection range and high sensitivity. Theoretical studies indicate that the operational region near the DTP can achieve an infinitely high sensitivity for refractive index (RI) sensing. To further enhance these properties, the surface of the TFC (r = 1.15 µm) was decorated with an erbium-ytterbium co-doped silica film, which exhibits significant photo-thermal properties for tuning. Experimental results show that the DTP can be linearly tuned with a sensitivity of 0.95832 nm/mA in the wavelength range of 1500-1634 nm. The maximum control sensitivities for the 1-order and 1'-order dips were 27.608 nm/mA and 30.884 nm/mA, respectively. The temperature sensitivities of the 1-order and 1'-order dips within the temperature range of 40.0-41.0 °C were measured to be 85.06 nm/°C and 88.257 nm/°C, respectively. The research results showcase that the DTP can be tuned on demand, enabling the DTP within a specific wavelength range to be utilized for achieving wide-range measurements, and the dips near the DTP to be employed for ultra-high sensitivity measurements, which has significantly laid a solid foundation for the engineering applications of fiber-optic platform-based sensors in fields such as photo-thermal integrated diagnosis and therapy, ultra-fast desorption, multi-physical quantity sensing, thermophysical property measurement, and low-concentration molecular detection.