Single-mode sapphire fiber Bragg grating high-temperature sensor

Sapphire fiber has been widely utilized in ultra-high-temperature environments. A sapphire fiber Bragg grating (SFBG) is a potential high-temperature sensor, but the multimodal behavior results in a broadband reflection spectrum and poor precision. In this article, we report a helical tapered depressed cladding waveguide (HTDCW) by using a femtosecond laser direct writing technique to realize single-mode operation of SFBG. The HTDCW was created using helical negative refractive index modification by forming a depressed cladding. The helical pitch was set as 0.7 μm, and the helical diameter linearly increased from 10 μm to 60 μm along the tapered length of 10 mm. We fabricated this structure into a coreless silica fiber and inserted it between a single-mode fiber (SMF) and an SFBG. According to the simulation result and mode field measurement, such a structure can effectively suppress the higher-order modes of sapphire fiber, and the fundamental mode distribution can be obtained eventually. In addition, the HTDCW can reduce the mode field mismatch between sapphire fiber and SMF. Based on this method, we successfully obtained a single-mode reflection spectrum with a signal-to-noise ratio of 11 dB and a narrow −3 dB bandwidth of 0.62 nm. In addition, such a single-mode operation SFBG sensor can be employed to measure temperatures ranging from 25°C to 1600°C, exhibiting much better repeatability and stability.