We report a novel long-period fiber grating (LPFG) fabricated by using a new writing technique that is mainly based on the thermal shock effect of focused high-frequency CO ₂ laser pulses at several kilohertz. A number of unique characteristics of such a LPFG, such as bend, torsion, and transverse load, are observed by experiments, for the first time, to our knowledge. Based on these unique features, a novel bend-insensitive LPFG sensor that could solve the problem of cross-sensitivity between bend and other measurands, a novel torsion sensor that can realize absolute measurement of twist rate, and a load sensor that can achieve simultaneous measurement of transverse load and temperature using a single LPFG element are proposed and demonstrated. These unique features of the LPFGs are mainly due to the asymmetrical distribution of the refractive index on the cross section of the LPFG induced by high-frequency CO ₂ laser pulses.

We investigate the microhole collapse property of different photonic crystal fibers (PCFs) and its effect on the splice loss using an electric arc fusion splicer. The physical mechanism of the splice loss for different kinds of PCFs is studied, and a guideline for splicing these PCFs and conventional single-mode fibers (SMFs) is proposed. We demonstrate a low-loss fusion splicing of five different PCFs with SMFs, including large-mode PCF, hollow-core PCF, nonlinear PCFs, and polarization-maintaining PCF.

The refractive index sensing characteristics of the side-polished photonic crystal fiber (PCF) surface plasmon resonance (SPR) sensor are detailed investigated in this paper. We used the finite element method (FEM) to study the influences of the side-polished depth, air hole size, lattice constant, and the refractive index (RI) of the PCF material on sensing performance. The simulation results show that the side-polished depth, air hole size, lattice pitch have significant influence on the coupling strength between core mode and surface plasmon polaritons (SPPs), but have little influence on sensitivity; the coupling strength and sensitivity will significant increase with the decrease of RI of the PCF material. The sensitivity of the D-shaped PCF sensor is obtained to be as high as 21700 nm/RIU in the refractive index environment of 1.33-1.34, when the RI of the PCF material is controlled at 1.36. It revealed a new method of making ultra-high sensitivity SPR fiber sensor. Then we experimental demonstrated a SPR refractive sensor based on the side-polished single mode PCF and investigated the sensing performance. The experimental results of the plasmon resonance wavelength sensitivity agree well with the theoretical results. The presented gold-coated D-shaped PCF SPR sensor could be used as a simple, cost-effective, high sensitivity device in bio-chemical detection.

We demonstrate a sub-micron silica diaphragm-based fiber-tip Fabry–Perot interferometer for pressure sensing applications. The thinnest silica diaphragm, with a thickness of $\sim 320\mathrm{nm}$, has been achieved by use of an improved electrical arc discharge technique. Such a sub-micron silica diaphragm breaks the sensitivity limitation imposed by traditional all-silica Fabry–Perot interferometric pressure sensors and, as a result, a high pressure sensitivity of $\sim 1036\mathrm{pm}/\mathrm{MPa}$ at 1550 nm and a low temperature cross-sensitivity of $\sim 960\mathrm{Pa}/°\mathrm{C}$ are achieved when a silica diaphragm of $\sim 500\mathrm{nm}$ in thickness is used. Moreover, the all-silica spherical structure enhanced the mechanical strength of the micro-cavity sensor, making it suitable for high sensitivity pressure sensing in harsh environments.

This paper presents a systematic review of long period fiber gratings (LPFGs) written by the ${\text{CO}}_2$ laser irradiation technique. First, various fabrication techniques based on ${\text{CO}}_2$ laser irradiations are demonstrated to write LPFGs in different types of optical fibers such as conventional glass fibers, solid-core photonic crystal fibers, and air-core photonic bandgap fibers. Second, possible mechanisms, e.g., residual stress relaxation, glass structure changes, and physical deformation, of refractive index modulations in the ${\text{CO}}_2$ -laser-induced LPFGs are analyzed. Third, asymmetrical mode coupling, resulting from single-side laser irradiation, is discussed to understand unique optical properties of the ${\text{CO}}_2$ -laser-induced LPFGs. Fourthly, several pretreament and post-treatment techniques are proposed to enhance the efficiency of grating fabrications. Fifthly, sensing applications of the ${\text{CO}}_2$ -laser-induced LPFGs are investigated to develop various LPFG-based temperature, strain, bend, torsion, pressure, and biochemical sensors. Finally, communication applications of the ${\text{CO}}_2$ -laser-induced LPFGs are investigated to develop various LPFG-based band-rejection filters, gain equalizers, polarizers, and couplers.