Highly Sensitive Intensity-Interrogated Gas Refractometer Using Slotted Photonic Crystal Fiber

A fiber-optic gas refractometer based on a slotted photonic crystal fiber (PCF) is developed for gas refractive index (GRI) measurement. The sensor consists of a six-air-hole grapefruit PCF sandwiched between two single-mode fibers, which forms a Mach-Zehnder interferometer by a slight offset splicing at the lead-in junction. One air hole of the PCF is open and exposed to external gas sample via two micro slots by femtosecond laser micromachining. Due to a large overlapping of the mode field and gas sample, a high GRI sensitivity of -827.94-dB/refractive index unit is obtained. Temperature change only shifts the interference wavelength and the cross-sensitivity of fiber deformation is also negligible. The proposed sensor with a good mechanical strength can be a promising candidate for highly sensitive GRI measurement by simple intensity interrogation.