2. Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong 999077, China;
3. Beijing Institute of Spacecraft Environment Engineering, Beijing 100094, China
Overview: In this paper, the unique properties and some recent sensing applications of hollow-core photonic bandgap fibers (HC-PBFs) are reviewed. Different to conventional all-solid fibers based on the principle of total internal reflection, in HC-PBF, most of light propagates in a hollow core region inside the fiber (typically > 95%). Hence, the core region of HC-PBF can be a contamination-free light-matter interaction channel with low loss, high energy density and long interaction distance. The air-propagation of light in HC-PBF would also reduces the impacts of fiber material properties (such as infrared absorption, thermos-optical effect) on the propagating light, hence offers an efficient platform for the sensing applications such as trace gas/liquid detection, optical fiber gyro sensing. Many high-sensitive single point and distributed/quasi-distributed gas sensing techniques based on HC-PBFs have been developed in recent years. Based on a photothermal interferometric detection method, the near-infrared HC-PBF acetylene sensing system can reach a detection limit of few ppb (parts per billion) level in noise equivalent concentration, and a dynamic range of about six orders of magnitude. The response time of long HC-PBF gas sensing systems can be improved by drilling side-holes along the fiber by using femtosecond laser. The average loss of the holes has been optimized to about 10-2 dB per hole. Liquids with different properties can be filled in the core or cladding region for a functional modification or extension. For example, the bandgap of HC-PBF can be adjusted by filling the liquid with specific refractive-index into the fiber. The fine silica-structure in HC-PBF exhibits novel mechanical and thermal properties, which would be beneficial to the sensing applications such as sound wave and vibration detection. The HC-PBF's porous structure can also be locally modified by applying various post-processing techniques, such as local heat treatment, micro-machining and selective filling. This would enable building novel in-fiber devices, for example long period gratings, polarizer and polarization interferometer et al. At present, the development of HC-PBF sensing technology has greatly expanded the sensing ability and application range of optical fiber. It is an important direction for the development of all optical devices and optical integration technology.