Tunable Holographic Filter Based On The Photorefractive Grating

Tunable optical flters play a key role in next generation’s high speedcommunication systems and they are important devices to achieve highcapacity and high speed optical network communication. Optical tunablegrating filter based on photorefractive effect of lithium niobate （LiNbO₃）has been researched in this article.Firstly, we study the volume holographic grating filter. The couplingconstant and the so-called off-Bragg parameter based on asymmetricrecording and readout are theoretically researched. The factors that affectdiffraction efficiency and bandwidth are analysed. When the grating lengthis10mm, and refractive index modulation Dn is7.1×10^-5, diffractionefficiency of80%and the bandwidth of less than0.08nm are obtained bynumerical simulation with the ideal condition. The photorefractive volumegrating is written in LiNbO₃with two beams interference by controlling therecording angle of the two beams in experiment. We get the experiment result that the diffraction efficiency is about10%and the bandwidth is4nmwhen d is10mm and Dn is2.8×10^-5with the wavelength of1548nm.Secondly, the Ti: LiNbO₃diffusion waveguide photorefractive gratingfilter is researched. A scheme for realizing filtering of photorefractivegrating is proposed that the two beams interference on Ti: LiNbO₃diffusionwaveguide forms the gratings. It is found that diffraction efficiency can beincreased and the building time decreased by suitable intensity and dopant.The results based on coupled wave theory show that diffraction efficiencycan be increased and bandwidth narrowed when the grating length isincreased, and diffraction efficiency can be increased but bandwidthswidened with the increasing refractive index modulation. When refractiveindex modulation is larger than8×10^-5and the grating length is12mm,diffraction efficiency more than91%and bandwidth narrower than0.08nmcan be obtained. Two-beam interference method for holographic recordingis experimently carried out based on the scheme and theoretical research.The experimental feasibility of the waveguide and fiber’s end surfacecoupling for realizing the infrared read-out of the grating filter are alsoanalyzed. We establish the preliminary recording and read out filteringsystem used in the CWDM network. Then the feasibility of wholeexperiment schemes is researched.At last, the tunable filtering performances and experiment schemes forrealizing tunable filter are studied. We analyse the photorefractive dynamics of waveguide grating tunable filter and obtain that thephotorefractive response time of re-established grating which is related tothe variation of the grating period is millisecond order of magnitude. Ascheme of two methods for CWDM is proposed to realize tunable filter.And then the feasibility of the devices is theoretical analysed.