Flat-top temperature tuning response in periodically-poled nonlinear crystals

摘要

Second harmonic generation via periodically-poled nonlinear materials offers an efficient means of generating highquality visible light at wavelengths that would be otherwise unattainable with traditional laser sources. While this technology has the potential for implementation in many mass-industrial applications, temperature stability requirements, often as restrictive as 0.1℃, can make packaging with a pump source problematic. In this work we are investigating the use of synthesised response PPLN gratings to create crystals that are better suited to visible SHG. Our route towards addressing this issue is to convert the standard sine-shaped temperature-tuning response of a uniform grating to a flat-top temperature tuning function with widths of up to several degrees. We have achieved a computationally efficient means of designing such gratings with a required temperature tuning profile based on a simulated annealing algorithm using repeated local changes of grating layout and subsequent Bloembergen-style analysis of the second harmonic, successive iterations of which quickly lead to the desired temperature tuning profile. Using our high fidelity poling technique we have fabricated synthesised response PPLN with precise placement of poled domains in Lithium Niobate based on the designs from our mathematical models. Measurements on these initial devices provide more than 4℃ flat-top temperature stability, albeit with a corresponding loss in operational efficiency. Our aim is to implement improved designs in magnesium-doped Lithium Niobate for packaging with near-room temperature diode-based pump sources, as could be applied towards RGB TV and projector applications.