We demonstrate a simple and highly sensitive optical fiber relative humidity (RH) sensor based on a long-period fiber grating (LPFG) coated with polyethylene glycol (PEG)/polyvinyl alcohol (PVA) composite films. The resonance wavelength of the LPFG is sensitive to environmental humidity due to the change in effective refractive index caused by the strong surface absorption and desorption of the porous PEG/PVA coatings. The sensor is sensitive in a wide range from 50% to 95% RH, with a highest sensitivity of 2.485 nm/%RH in the range 50–75% RH. The proposed RH sensor has the advantages of compact size, good reversibility, and stability, which makes it attractive for high-humidity environments.

We demonstrate the fabrication of helical long-period gratings (HLPGs) in a polarization-maintaining fiber using CO2-laser. The maximum polarization dependent loss (PDL) of the grating was measured to be more than 30 dB at the resonance wavelength. The HLPG was found to have a high refractive index sensitivity of -7248.6 nm/RIU when the surrounding refractive index changes in the range of 1.4470-1.4600, temperature sensitivity of 158.8 pm/°C, and strain sensitivity of 3.17 pm/μE. The twist sensitivities based on intensity variation were measured to be 5.98 dB/ (rad/m) (co-directional twist) and -5.97 dB/ (rad/m) (contra-directional twist), respectively. The demonstrated grating devices have promising applications as high-sensitivity fiber sensors, PDL compensator etc.

A single-frequency distributed Bragg reflector (DBR) fiber laser at 1030 nm has been demonstrated using a 0.7 cm long homemade Yb:YAG crystal-derived silica fiber (YCDSF). The absorption and emission cross sections of the YCDSF are 4.93 × 10 − 20 cm 2 at 980 nm and 1.1 × 10 − 20 cm 2 at 1030 nm. Using this gain fiber, an over 255 mW continuous-wave lasing in the constructed laser has been obtained at single transverse and longitudinal mode operation. The slope efficiency and the pump threshold of the fiber laser were up to ∼35% and as low as 25 mW, respectively. The fiber laser also demonstrated an optical signal-to-noise ratio of 79 dB and a beam quality factor of 1.016 in two orthogonal directions. Its power fluctuation at 210.5 mW was less than 0.85% of the average power within 13 h. Moreover, the relative intensity noise and linewidth of the laser are also investigated at the different pump powers. The results indicate that the single-frequency DBR fiber laser has potential applications in high-quality seed sources and high-precision optical sensing.

We propose and demonstrate an optical fiber vector curvature sensor based on a long-period fiber grating (LPFG) inscribed in Panda type polarization-maintaining fiber (PMF) using CO ₂ -laser heating and tapering system. The uniform heating approach removes the necessity of CO ₂ -laser exposure alignment along the axis of PMFs during grating fabrication. A serial of PMF-LPFGs with grating period from 350 to 490 μm with a step of 20 μm have been fabricated experimentally. The curvature response of the fast and slow axis resonance dips for the LPFG with a period of 450 μm was investigated in the curvature range from 0 to 4.137 m ^-1 at four bending orientations. The maximum curvature sensitivities were measured to be 9.26 and -6.89 nm/m ^-1 for the fast and slow axis LP15 modes, respectively. The sensor can achieve 2-dimensional bending direction recognition. The proposed grating has potential application as a vector curvature sensor.

We demonstrated the fabrication of a chiral long-period fiber grating (CLPG) by CO2-laser heating of a twisted double-cladding fiber (DCF). The polarization and sensing characteristics of the DCF-CLPG are investigated experimentally. Due to the helical structure, the grating shows a higher torsion sensitivity of -0.40 nm/ (rad/m), and the torsion rate and torsion directions can be measured simultaneously using single DCF-CLPG. The temperature, strain, bending, and surrounding refractive index sensing characteristics are also investigated experimentally. The proposed CLPG has a promising application as high-sensitivity directional torsion sensor.

An opticalfiber vector magnetometer based on a thin-cladding polarization-maintaining fiber (TPMF) long-period fiber grating (LPFG) and magnetic fluid (MF) is proposed and experimentally demonstrated. The non-circular symmetric fiber structure and asymmetric evanescence field distribution of the LPFG induce a vector surrounding refractive index (SRI) response. Based on the magneto-optical properties of the MF nanoparticles and the directional SRI characteristics of the TPMF-LPFG, the resonance dip wavelength shift of the grating is dependent on the angle between the principal axis of TPMF and the external magnetic field direction when being applied to an external magnetic field. The achieved maximum sensitivity of the wavelength shift was −618 pm/mT when the magnetic intensity changes in the range of 0–10.5 mT. The achieved maximum orientation sensitivity of the sensor was 72 pm/degree, but the magnetic orientation can be detected only within the range of 0–90 degree. The minimum magnetic field intensity that can be detected is about 32.3 μT due to the 0.02 nm wavelength resolution of the optical spectrum analyzer. Our study proves that the high birefringence fiber based LPFG combined with MF can be developed as a high-performance vector magnetometer and such type of sensor is anticipated to find potential applications in magnetic or electric field systems.