A Ring-Core Optical Fiber Sensor With Asymmetric LPG for Highly Sensitive Temperature Measurement

A compact and highly sensitive temperature sensor based on mode conversion in the ring-core fiber (RCF) incorporating an asymmetric long-period grating (aLPG) is proposed and theoretically simulated. The RCF is composed of a hollow core and a high refractive index ring. The aLPG inscribed in the upper half ring generates mode conversion between modes with different azimuthal orders within the ring. A composite resonant dip containing dual-resonant dips which are generated by mode conversion from HE<inline-formula> <tex-math notation="LaTeX">$_{11x}$ </tex-math></inline-formula> to <inline-formula> <tex-math notation="LaTeX">$\mathrm{T}E_{01}$ </tex-math></inline-formula> and HE<inline-formula> <tex-math notation="LaTeX">$_{21x}$ </tex-math></inline-formula> can be observed in the transmission spectrum. The hollow core is infiltrated with a refractive index liquid whose material refractive index can be modulated via temperature, so the wavelength of the resonant dip can be modulated by temperature, which displays a high sensitivity of 12.92 nm/°C (<inline-formula> <tex-math notation="LaTeX">$32.3~\mu \text{m}$ </tex-math></inline-formula>/RIU). Furthermore, the relationship between 3-dB bandwidth of the resonant dip and temperature is also obtained by polynomial fitting with a high fitting degree of 99.99%.