A Study On Electro-optically Tunable Super-broadband Filter Based On Long Period Grating In Ti:LiNbO₃ Waveguide

Long period waveguide grating?LPWG?and long period fiber grating have the similar working principles and applications such as sensing,optical filtering,and intensity modulation.Each has its advantages and disadvantages.In comparison,the LPWG has the advantage in the choice for materials,geometry and type of both waveguide and grating.This article preliminarily studies on the influence of photorefractive effect of LPWG,and successfully fabricates electro-optically tunable super-broadband filter based on long period grating in Ti:Li NbO₃ waveguide.The main works and results,1.Fabricating a special waveguide structure with strip waveguide embedded in a planar waveguide based on LiNbO₃ by twice titanium-diffusion method in order to make up the LPWG,and utilizing prism coupling system to measure mode refractive index in order to design the pitch of LPWG,then fabricating the buffer layer and interdigital type electrodes onto the waveguide surface.Till then the LPWG is finished.2.Building experimental system to characterize the resistance properties of LPWG,and further studying the relations that rejection band of LPWG including resonant wavelength,contrast,3dB width dependence of driving voltage.3.The experimental results show that the contrast of the rejection band can be adjusted linearly within 0-30 d B by utilizing the electro-optic effect of LiNbO₃ and changing the driving the voltage,and the linearity is similar for the two cases of 675 and 880?m pitches.Higher than 300 V,the contrast decreases due to over-coupling effect.Accompanying the contrast modulation,the resonant wavelength is simultaneously linearly tuned by utilizing the photorefractive effect.For the 675?880??m pitch,the tuning range is 160?200?nm in the 1.1-1.3?1.4-1.6??m spectral region.By using two parallel LPWGs with different pitches 675 and 880?m,the electro-optically tunable filter can realize 360 nm super-broadband filtering in the 1.1-1.6?m spectral region.This paper work provides the basis of tunable super-broadband filter based on LiNbO₃ waveguides.