Long period fiber grating fabricated by defocused CO₂ laser for refractive index sensing

Abstract

Long period fiber grating (LPFG) has many applications in optical fiber communications and optical fiber sensors. It can be fabricated by many methods. Recently, an edge written LPFG with large resonant peak loss and apparent asymmetric cladding mode coupling has been produced by focusing the high frequency CO2 laser pulses directly on the fiber cladding surface. Such a LPFG exhibits only a small area of refractive index change on the fiber cladding surface without any physical deformation. In this work, we developed a new LPFG fabrication method by use of defocused CO2 laser pulses, i.e. to move the laser beam focus slightly away from the fiber cladding surface. No apparent physical damage is observed after the LPFG fabrication. With the increase of the moving distance from the fiber cladding surface, the induced average refractive index change in the fiber cladding becomes smaller, which also leads to a decreased polarization dependent loss when compared with the LPFG made by the focused CO2 laser pulses. The LPFG developed in this work also shows a higher sensitivity of the resonant wavelength to the external refractive index around the fiber cladding than that made by use of focused CO2 laser pulses. It is expected that this new type of LPFGs have important applications in refractive index sensors.