Research On Theories, Sensing And Band-pass Filter Characteristics For The Long Period Fiber Grating Having Cladding Refractive Index Perturbation

Optical fiber grating is one of the fastest growing passive optic components in the fields of fiber sensing and communication over the last 20 years. Fiber grating sensor/sensing technology shows many unique properties, such as high sensitivity, immune to electromagnetic interference and good compatibility with existing optoelectronic devices, etc., which makes it to be one of effective methods to realize intelligent material and structure. Long period fiber grating(LPFG) can promote the coupling or energy exchange between the guided core mode and cladding modes, resulting in discrete resonance in the transmission spectrum at the resonance wavelength. Currently, studies on the LPFG based devices focus on the conventional LPFG having core refractive index perturbation with band-refection characteristics and there are quite perfect theories reported for the analysis of the LPFG based devices. However, as the architectural construction and environmental safety have received increasing attention, exploring more types of fiber-optic based sensing techniques with high sensitivity to the variation of the architectural construction and environment is the people’s urgent hope. In this thesis, we carry out research work on the mode coupling characteristics as well as sensing characteristics of novel type of LPFG having cladding refractive index perturbation(C-LPFG) and its related devices in details. The main innovations are gained as follows:1. By dividing and discretizing the cross section of the fiber into multilayer circular waveguides and ring sectors based on the transverse refractive index distribution of LPFG, the dispersion equation and the multilayer coupled mode theory(MCMT) for the linearly polarized modes are presented for the three-layer open waveguide fiber grating. The combination of multilayer index perturbation with full-vector complex coupled mode theory(CCMT) extends greatly the application of MCMT and CCMT. Both the MCMT and the CCMT facilitate significantly the analysis of the LPFG with complicated refractive index perturbation.2. The cladding refractive index can be characterized by three parameters: ?θ, ?r and clδn. Non-uniform refractive index in fiber cladding causes the LPFG coupling the guided core mode with both low-order and high-order cladding mods(LP0j and LP vj(v≥1) modes), resulting in resonances of LP01,0j and LP01,vj(v≥1) in the transmission spectrum. It’s indicated that, the resonance of LP01,vj(v≥1) is synthetized by two resonances of01,C vj LP and01,S vj LP corresponding to two linearly polarized modes of C vj LP and S vj LP. As a result, the resonance of LP01,vj(v≥1) features the periodic variation with ?θ. We show that there are some turning points/extreme points that depend on the coupling phase as well as the mode distribution for the variation of resonance peak01,res vj T(in absolute value) with Δr. The resonance wavelength01,res vjλ shows red shift(or blue shift) when ?θ(or ?r) increases. The increase in clδn gives rise to the linear increase in the resonance peak01,res vj T. It is found that the resonances of LP01,0j and LP01,vj(v≥1) exhibit similar refractive index sensing characteristics with the sensitivity of S01,vj ≈760nm/ RIU. In addition, the SRI sensing characteristics of the tilted LPFG(TLPFG) is investigated. It is shown that the SRI sensitivity of 01,0S j =1918.4nm/ RIU is obtained for the TLPFG, which is much higher than that for the non-tilted LPFG.3. A novel SPR sensing platform based on the LPFG only having cladding refractive index modulation with a sliver overlay coating(C-LPFG based SPR) is proposed and investigated theoretically using CCMT. Systematic studies are conducted on characteristics of the SPR mode, including the real and imaginary part of wavelength dependent dispersion and its derivative, mode field and fill factor of mode field. The results reveal the mechanism of exciting the SPR mode and the sensing principle of the metal coated C-LPFG. It is found that the SPR mode is transited from the higher EH cladding mode with the real part of ERI close to the SRI. The dispersion curve of the SPR mode shifts monotonously but retains its shape unchanged as the SRI changes, which leads to large shift in the resonance wavelength. Therefore, the C-LPFG based SPR with high sensitivity is achieved. It is obtained that as the cladding radius of the C-LPFG is reduced from 62.5cl r =μm to 22.25 cl r =μm, the sensitivity is increased from 2900 /SRI S ≈nm RIU to 4900 /SRI S ≈nm RIU for the corresponding C-LPFG based SPR(the SRI changes in the range of 1.33 1.337SRI≤n ≤), which is higher than that for the conventional LPFG based SPR.4. A novel SRI sensing platform based on the band-pass C-LPFG is proposed and investigated theoretically. The band-pass C-LPFG is the C-LPFG that the incident light is directly coupled into the fiber cladding of the input end to excite only cladding modes which then couple with the guided core mode in the grating region written in the cladding region after satisfying the PMC and finally propagate within fiber core. Therefore, the C-LPFG is characterized by the band-pass transmission spectrum. The results show that high index sensitivity is reached for the band-pass C-LPFG based sensor working around the TAP. For the cladding radius reduced band-pass C-LPFG( 14.6cl r =μm), the maximum average sensitivity and local sensitivity as high as SRI =4757.6nm/ US RI and 6000 /SRI=n S m RIU is achieved respectively( 1.33 1.36SRI≤n ≤), which is much higher than the conventional band-rejection LPFG based sensor.5. A novel SRI sensing platform based on the band-pass film coated C-LPFG(CC-LPFG) is proposed and investigated theoretically. As the mode is transited to the film-guided mode, the corresponding mode profile is distributed mainly within the film, which increases the interaction between the fiber modes and the surrounding medium. It is shown that the maximum average sensitivity and local sensitivity as high as 4S SRI2.12 ×=10 nm/ RIU and 3.5 104 /SRI= ×nm IUS R are obtained respectively( 1.33 1.34SRI≤n ≤) for the band-pass CC-LPFG based sensor( 17.56 cl r =μm), which exhibits significant advantages over the conventional LPFG based sensor.6. Preliminary studies are conducted on the band-pass filter based on the band-pass C-LPFG. The results provide the theoretical basis for designing and realizing tunable wide band-pass filter as well as narrow band-pass filter.