Characterization of refractive index change and fabrication of long period gratings in pure silica fiber by femtosecond laser radiation

Index change quantification using Bragg grating Fabry–Perot cavity structure.Pulse filamentation based fabrication of long period grating in pure silica fiber.Characterization of LPG in pure silica fiber for refractive index sensing.

Ultrafast laser induced refractive index (RI) change in the core of a standard telecommunication fiber is quantified using the spectral shift of an in-fiber Bragg grating (FBG) based Fabry–Perot cavity. Measured RI change is used to design and then fabricate long period grating (LPG) in pure silica core single mode fiber (SMF) employing identical laser irradiation conditions used in core index characterization. A core length of 100 µm within the 10 mm long cavity structure is scanned with ultrafast laser pulses, and the corresponding spectral shift is used to calculate index modification. The index change of 0.000449 found in characterization process is used to simulate the LPG in pure silica fiber. Identical index modulation written in pure silica fiber by femtosecond laser radiation provides a rejection band that is in good agreement with the simulation results. The fabricated LPG sensors are also characterized for ambient temperature and RI.