Study On Special Structure Fiber Gratings And Their Applications

Fiber gratings of special and complex structure play important roles in optical active and passive devices,and will be widely applied in optical communication and sensor systems,because of their unique spectrum properties and excellent integratability. This thesis is mainly devoted to the detailed researches on several kinds of fiber gratings of special structures:new characteristics,improvement on research method and new applications.And the main achievements of this thesis are listed as follows:1.Spectral characteristics of chirped phase-shifted fiber gratings are analyzed by the transfer matrix method combined with resonance theory.Unique properties that are different from and superior to uniform phase-shifted fiber gratings are found and experimentally demonstrated,which make chirped phase-shifted gratings an ideal choice to realize a comb-like filter and having a good application prospect in multi-channel filters and multi-wavelength fiber lasers.Sectional apodization is used on chirped phase-shifted grating to obtain appropriate filtering properties:the amplitude and the phase responses of the filter are improved,and this method brings great advantages of flexible design and easy fabrication for chirped phase-shifted fiber gratings.2.Several chirped moir(?) gratings with different amounts of transmission peaks are experimentally fabricated by the dual-phase-mask exposure method.A simple multi-wavelength fiber ring laser based on a chirped moir(?) fiber grating and a semiconductor optical amplifier(SOA) is proposed,and stable triple-wavelength lasing oscillations at room temperature are experimentally demonstrated.The lasing oscillation shows a narrower bandwidth and a higher optical signal-to-noise ratio than SOA-based multi-wavelength fiber lasers utilizing some other kinds of wavelength selective components.Meanwhile,the wavelength stability and the power stability of the lasing are tested to be good.3.A novel wavelength-switchable erbium-doped fiber ring laser based on a chirped moir(?) fiber grating and a wavelength-tunable uniform fiber grating is proposed,and stable wavelength lasing oscillations at room temperature is experimentally demonstrated.The wavelength stability and the power stability of each lasing are tested to be good,and high optical signal-to-noise ratio and uniform output power of each lasing are obtained.The optical signal-to-noise ratio is increased greatly by introducing a wavelength-tunable Sagnac loop interferometer(FSI) filter into the ring laser,on the premise that the lasing wavelength and the output power are stable. The cost of the laser is cut down due to the absence of the optical circulator.4.A simplified coupled-mode theory(CMT) approach to the analysis of radiation-mode coupling in tilted fiber gratings is proposed for the first time.Based on consideration of the vectorial phase-matching conditions and the phase terms of the complete CMT equations,the complex two-element differential equations are simplified to a simple formula,which is suitable for both reflective(tilt angle≤45 degrees) and transmissive(tilt angle≥45 degrees) tilted gratings.The validity and accuracy of the simplified CMT method is demonstrated by comparing its simulation results with that of the complete CMT equations and the volume current method(VCM).5.With the simplified approach,the radiation-mode coupling in tilted fiber gratings is investigated in detail,and variation of loss spectrum characteristics with the grating parameters is analyzed.Based on the polarization dependence property,an analysis is performed on how to obtain a high-performance in-fiber polarizer that involves a compromise between polarization extinction and insertion loss.