Tunable Filters Based On Periodically Poled Lithium Niobate

Lithium niobate(LN)crystals have a series of excellent physical properties,such as electro-optic,nonlinear optical,birefringence,acousto-optic,photorefractive and other properties,which have become one of the most popular materials in optical research.The electro-optic(EO)effect is a phenomenon in which the refractive index of the materials changes obviously under the action of a DC electric field(or a low frequency electric field),and the optical properties of the material are changed accordingly.This paper is mainly based on the EO effect of quasi-phase matched(QPM)periodically poled lithium niobate(PPLN)crystals,and we analyze the transmission spectrums of different structures of PPLN crystals by the method of theory and experiment.When the electric field is applied along the Y axis of the LN crystal,the refractive-index ellipsoid deforms,and the Y axis and the Z axis will rotate a small angle 0 around the X axis.When the length of each domain satisfies the QPM condition,the optical axis of the positive and negative domains will rotate right and left from +? to-? successively owing to the periodic EO coefficient change in PPLN,so the polarization state of the incident light will change after a propagation distance.In the QPM condition,PPLN could supply reciprocal vectors to compensate the phase mismatch of the involved waves,and the coupling efficiency between ordinary and extraordinary light is increased.For the normal structure of the PPLN crystal,we set the duty cycle of the positive and the negative domain is 50%,and the thickness of each domain is equivalent to a half-wave plate.When the electric field is applied to the Y-plane of the crystal,the PPLN crystal acts like a folded Solc filter,forming one peak transmission spectrum at the matching wavelength.When the intensity of the applied electric field is changed,the intensity of different wavelengths of the light can be modulated,and then electro-optical modulation single-wavelength filters can be achieved.When the crystal temperature is changed,the center wavelength in the transmission spectrum will shift,so that the temperature tunable electro-optic single-wavelength filter is achieved.The full width at half maxima of the spectrum will change by changing the intensity of the applied electric field and the period numbers of PPLN.In this paper,we also introduce a special structure of PPLN crystal with a structure defect in the middle of the crystal.When the electric fields are applied along the Y axis of the crystal,light satisfying QPM condition(?0)will undergo polarization rotation but then return to the initial polarization state,keeping the original polarization output.However,since the light at the wavelength of ?0±?? does not fully satisfy the QPM condition,the polarization state cannot be restored to the initial state at the time of output,so that a dual-valley transmission spectrum is obtained.Since the LN crystal is very sensitive to temperature,the matching wavelength will be changed when the temperature changes,so we can apply this structure to the application of temperature tunable electro-optic dual-wavelength filter.The bandwidth between the two peaks of the dual-wavelength filter will change by changing the intensity of the applied electric field,the number of N and the length of the defect.We also analyzed the relationship between the group delay dispersion and the applied electric field and the defect length.When the defect structure is not in the center of the crystal,the crystal structure is asymmetric.In a certain condition,a flat-top transmission spectrum appears,and this structure can be extended to the application of the flat-top filters.