“…When the pump frequency is the same, it is degenerate four wave mixing (DFWM). The nonlinear phenomenon of FWM can be used in a variety of applications, such as temperature sensing [8.9], magnetic field sensing [10], strain sensing [11,12], supercontinuum generation [13,14], and so on. When FWM occurs in an optical fiber, changes in the external RI result in wavelength shifts of the signal and idler waves, which can be used for RI detection.…”