Fabrication of side-polished fiber Bragg grating for refractive index sensor

A new type of refractive index (RI) sensor based on a side-polished fiber Bragg grating (FBG) has been experimentally demonstrated. At first, a FBG was inscribed in a single mode fiber by means of 267 nm fs laser irradiation through a uniform phase mask, which has an excellent spectral shape with a transmission loss of -9 dB (i.e., a high reflectivity of 87.41%), a 3dB bandwidth of 0.78 nm. Then, the portion of the fiber cladding enclosing the FBG was side-polished into a D-shaped configuration, and the D-shaped FBG was employed as an RI sensor. The removal of the fiber cladding via side-polishing technique ensures that the propagating core-mode of the FBG can interact with the external medium, and hence the Bragg wavelength of the D-shaped FBG will be sensitive to the RI change of the external medium. A conventional unpolished FBG demonstrated a linear relationship between the Bragg wavelength and the surrounding RI with a low sensitivity of similar to 1.8 pm/RIU at an RI of 1.45. For comparison, the side-polished FBG exhibited a considerably increased RI sensitivity of up to similar to 10 nm/RIU at an RI of 1.45. Hence, it may be attractive for biochemical sensing applications.