| The optical characteristics and corresponding devices based on side-written long period fiber grating (LPFG) induced by high frequency CO2 laser pulses were studied in our research group during 2000 to 2003. This thesis is an extension of the above mentioned basic study. In order to strengthen the environmental response sensitivities, overcome the cross-affections of sensing and realize the multi-parameters measurement, three types of specific structures of LPFG are explored and proposed, including the model of refractive index, the theoretical analysis of transmission spectrum, fabrication methods, environmental response characteristics and potential applications. The main contents are as follows:1) The transverse refractive index distribution of LPFG with asymmetric refractive index modulation in the cross-section is divided into multilayer circular waveguides, which are then discretized according to the magnitude of index. The mode field of LPFG can be approximately calculated by making use of multilayer circular waveguide theory and numerical method. The mode coupling characteristics of this asymmetric LPFG is discussed by adopting coupled-mode equation sets and the revised Bragg condition. The numerical method of mode coupling for asymmetric LPFG provides theoretical foundation for the exploration of special LPFGs.2) The release of residual stress, rapid solidification, fiber densification and fused distortion are discovered to be the main mechanism of forming gratings induced by CO2 laser, which plays different role for different fiber, scanning method and laser power. The core/cladding index change caused by different inducing position and laser power is measured in this thesis. It provides the basis of experimental fabrication for special LPFGs.3) The structure of edge-written LPFG (E-LPFG) is proposed and fabricated by using high frequency CO2 laser pulses, for the first time to our knowledge. The mode coupling features are investigated and environmental response characteristics are studied experimentally. It's indicated that: (1).The environmental response sensitivities of E-LPFG with refractive index modulation in the fiber cladding were enhanced because the cladding mode fields were easily affected by the change of the environmental parameters. For example, the environmental index response sensitivity is increased by a factor of 4 compared with general LPFG, and the strain sensitivity of LPFG with periodical grooves si up to ? 100p m /μεfor0 ~100με. (2). E-LPFG has strong orientated coupling behavior, based on which optical communication devices, such as rotated optical switch, can be designed. (3).The PDL of E-LPFG is the greatest, and then is the double-edge-written LPFG. The multi-edge-written grating has the least PDL, which can be as less as 0.22dB.4) The structure of asymmetric LPFG with rotary refractive index modulation (R-LPFG) along the fiber axis is proposed and fabricated by using high frequency CO2 laser pulses, for the first time to our knowledge. The mode coupling features are investigated and environmental response characteristics are studied experimentally. It's observed that: (1) The transmission spectrum has different features according to the rotation angle. For small angle, resonance peak is unique, however, when the angle becomes greater, the resonance peak will be split into two, which is different from the transmission spectrum of previous all LPFGs. (2). For R-LPFG with greater rotation angle, the two split peaks have almost the same temperature sensitivity. (3). For R-LPFG with greater rotation angle, the two split peaks have opposite wavelength shift direction when it was applied strain. Besides, the sensitivity of R-LPFG with rotary grooves along fiber axis can be up to～50 pm /μεfor0 ~ 600με. (4). For R-LPFG with greater rotation angle, When co-twisted, the two peaks keep close and combine into one peak finally; when counter-twisted, the two peaks walk off. (5). The high sensitivity torsion meter and strain meter without temperature compensation can be designed by adopting the wavelength spacing between the two split peaks to measure the environmental variables.5) For the first time, the structure of asymmetric ultra long period fiber grating (ULPFG) is proposed to our knowledge. Also, the method of combining sub-fiber-gratings linearly with different frequency is firstly proposed. Two new grating structures of ULPFG--asymmetric edge-written ULPFGs (E-ULPFGs) and ULPFGs with rotary refractive index change are formed (R-ULPFGs). The above three ULPFGs are induced by high frequency CO2 laser and their environmental response characteristics are studied systematically. It shows that: (1). ULPFG has more resonance peaks than general LPFGs in that core mode can couple with more cladding modes. (2). Because the environmental response characteristics of each resonance peak has close relationship with the coupled level of sub-fiber gratings and the order of cladding modes, the environmental index and torsion sensitivity of ULPFG are different for each resonance peaks. (3). Based on the features of E-ULPFGs, the optical features of ULPFG can be strengthened, for example E-ULPFGs can be used to measure strain and index with high sensitivity. (4) Based on the features of R-ULPFGs, the variables such as strain and torsion can be measured by using the spacing of split peaks.6) Based on the specific LPFGs, new devices partly applicable to optical fiber communciation or sensor area are studied. (1). Grating coupler is produced by using two special LPFGs. (2). The EDFA gain equalizer is produced by using R-LPFG. (3). The temperature and index can be measured simultaneously by using ULPFG. (4). For R-LPFG, the strain and torsion measurement without temperature compensation can be realized by using R-LPFG with greater rotation angle. (5). The temperature, strain and torsion measurement can be realized simultaneously by using ULPFG. (6). In addition, adjustable band rejection filter can be designed by using E-LPFG or R-LPFG with periodical grooves along the fiber axis. (7). The bandpass filter with high performance can be devised by adopting LPFGs with specific transmission spectrum and micro-processing methods.
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