Study On Characteristics Of LPFGs With Radial Asymmetric Structures Written By High-Frequency CO₂ Laser Pulses

Prof. Yunjiang Rao, the supervisor, invented the fabrication method of long-period fiber gratings (LPFGs) in the single-side of fiber by using high-frequency CO2 laser pulses in 2000. In the past several years, the fabrication method, formation mechanism, basic characteristics and applications have been investigated and gained international reputation and prominent achievements. In this thesis, some kinds of LPFGs with asymmetric refractive index along the radial of the fiber are designed and fabricated based on Prof. Rao's invention. The characteristics of these LPFGs are studied by a series of experiments. The main work is provided as follows:1. Based on the previous studies on LPFGs written in single-side of the fiber (called S-LPFGs) by high-frequency CO2 laser pulses, the formation mechanism, spectral properties and applications are further studied: (1) experimental results show the residual stress release is one of the most important reasons for the forming of LPFGs. (2) a novel dynamic axial strain sensor based on the bend characteristics of the S-LPFG has been demonstrated. (3) the S-LPFG has good high-temperature and strain characteristics, and can be used to measure high-temperature and strain simultaneously. (4) the transverse load characteristics of twisted S-LPFGs are studied. It is observed that such a S-LPFG weakens the direction-dependence of transverse load and increases its sensitivity comparing with the traditional untwisted S-LPFGs reported previously. Finally, an intensity demodulating method is demonstrated when the twisted S-LPFG is used as a transverse load sensor.2. A new kind of LPFGs written in the twisted fiber (called T-LPFG) by high-frequency CO2 laser pulses is designed and investigated for the first time to our knowledge. (1) it is discovered that the interaction between linear and circular birefringence leads to the different spectral structures of T-LPFGs in the fibers with different twisted rates. (2) the particular spectral properties, including transverse load, temperature and twisting characteristics are studied and proposed to fabricate novel optic fiber gratings sensors.3. A new kind of LPFGs written in the edge of the fiber cladding (called E-LPFGs) by high-frequency CO2 laser pulses is reported for the first time to our knowledge. (1) the E-LPFGs with different induced depths in the edge of the fiber cladding can be fabricated by controlling the exposure position of laser pulses. (2) it is discovered that such an E-LPFG is more sensitive to temperature and strain, and most importantly it has very high sensitivity to refractive index due to the different cladding mode coupling with the fundamental mode of the fiber core.