Long-period gratings in photonic crystal fiber: Fabrication, characterization, and potential applications for gas sensing

摘要

Although long-period gratings (LPGs) and photonic crystal fibers (PCFs) have emerged at the same time and been around for almost ten years, the fabrications of fiber components in PCFs have attracted great attention in resent years. Post processing of PCFs with a CO_2 laser is very powerful and versatile method for making miniature compact fiber-based devices including LPGs and phase-shifted LPGs. This article will review our research work on fabrication of those gratings in PCFs by use of focused beam from a CO_2 laser and point-by-point writing fashion. Either the mechanical stress relaxation technique or surface deformation method is employed in the design and fabrication of the gratings. The characterizations of the inscribed LPGs in PCFs at high temperature and high strain are also described. The potential applications of PCF-LPG devices for gas sensing have been discussed. Unlike the PCF-based gas sensor that detects the analytes by the interaction of light with gases through the absorption of the evanescent wave in the holes of fiber cladding, the PCF-LPG gas sensing works by the interrogating of the shifts of different resonance wavelength and strength of core-cladding mode coupling in the transmission spectrum. The advantages of the PCF-LPG sensing devices are: (1) high temperature insensitive and stability; (2) compactness when packaged; (3) practical use under hazardous conditions and in high temperature environment.