Characteristics Of Multi-wavelength Second-harmonic Generation In Quasi-periodic Optical Superlattices

Second-order nonlinear frequency conversion is one of the most important research fields in nonlinear optics and has important applications in the fields of spectroscopy,laser pulse,all-optical communication and biomedical detection.The quasi-phase matching（QPM）technique,which uses high voltage electric field to periodically modulate the second-order nonlinear coefficient of nonlinear crystal,can realize the phase matching simply and effectively,so as to realize the second-order nonlinear frequency conversion with high conversion efficiency.Nowadays,the realization of a single QPM on a single device can no longer meet the needs of people.Various optical superlattices with novel structures can meet multiple QPM conditions and realize frequency conversion of multiple wavelengths at the same time.In this paper,a simple approach is presented to generate the dual-wavelength second-harmonic with flexible conversion efficiency ratio and wavelength interval in nested quasi-periodic optical superlattice（NQOS）structure.The characteristics of dual-wavelength second-harmonic generation（SHG）is studied with a 10-mm-long NQOS devices base on 5%Mg O doped Li Nb O₃（5mol%Mg O:Li Nb O₃）crystal.The simulation results show that,in NQOS devices with period of16.40μm,dual wavelength SHG with fundamental wavelength of 1.187μm and 1.55μm is realized,and the ratio of the conversion efficiency of two wave peaks can be adjusted by altering the incident position of NQOS devices.By analyzing the relationship between fundamental wavelength and temperature and QPM period of four different QPM modes,using the combination of type-0（o+o→o）and type-I（e+e→o）QPM,the dual-wavelength SHG around 1.55μm is achieved in the NQOS device with 16.11μm period.By adjusting the operating temperature,the interval of two wave peaks in dual-wavelength SHG devices can continuously adjusted with the range of 0-352 nm.Furthermore,the central fundamental wavelength and the maximum interval range of dual-wavelength SHG devices depend on the the period of the NQOS.Meanwhile,we propose a complementary duty-cycle optical superlattice（CDOS）structure,which utilizes the frequency doubling characteristic of type-I（e+e→o）QPM to realize the full-wavelength SHG conversion in multiple wavebands at certain band by adjusting the temperature.We also realize the full-wavelength SHG conversion of 0.85μm,1.31μm,1.55μm,which are the common bands of optical communication.Then,the broadband frequency doubling characteristics of temperature gradient CDOS and segmented CDOS structure are analyzed respectively.In temperature gradient CDOS device,the wideband frequency doubling output of three bands is realized simultaneously.Based on type-I（e+e→o）QPM with an initial period of 6.5μm,the wideband frequency doubling conversion with 178 nm and 391 nm bandwidth are achieved in the segmented CDOS devices with 40 segments and chirp steps of 25 nm and 50 nm,respectively.