Gas Sensor Based on Large Hollow-Core Photonic Bandgap Fiber

One concern in using photonic band-gap fiber (PBGF) as a gas sensor is the response time. In this type of the gas sensors, response time is the time required for gas to diffuse into the hollow-core. So considering a large hollow-core PBGF (HC-PBGF), the response time can be significantly reduced. But in the large HC-PBGF, the fundamental issue is the presence of higher order modes (HOMs). Sometimes the leakage loss of the HOMs is comparable to those of the fundamental mode. So in order to suppression of the HOMs, six small-cores with reasonable radius were incorporated in the cladding of the proposed fiber. In other words, due to resonant-coupling mechanism of HOMs in central core with fundamental mode of outer cores, the leakage loss of the HOMs can be enhanced. Considering optimum parameters such as hollow-core radius, air filling factor, and the distance between the center-to-center of two adjacent air holes, the small-cores are surface-mode-free and proposed structure can be considered effectively single mode. So at the wavelength of 1550nm the relative sensitivity of the gas sensor was improved to 97%. The results proved the ability of proposed design as a sensitive gas sensor with low response time.