Femtosecond laser pulses induced ultra-long-period fiber gratings for simultaneous measurement of high temperature and refractive index

We have fabricated ultra-long-period fiber gratings (ULPFGs) by femtosecond laser pulses for simultaneous measurement of high temperature and refractive index. ULPFGs induced by femtosecond laser pulses can survive in higher temperature. Both theories and experiments show that different resonant peaks of ULPFGs have different temperature and refractive index sensitivities. Fiber sensors for simultaneous measurement of high temperature and refractive index are proposed based on two resonant peaks of ULPFGs with periods of 4 mm. It is expected that they can be widely used in petroleum industry to monitor the temperature and refractive index of distillate simultaneously.