Temperature sensing in E.M.D. environment with periodically poled lithium niobate devices

A temperature sensor immune to electromagnetic noise is designed and fabricated. The sensor key element is a periodically poled lithium niobate (PPLN) substrate. PPLN allows a direct and efficient frequency conversion of lightwave through the quasi-phase matching (QPM) of the pump radiation propagating at the fundamental and second harmonic wavelengths. For these devices, the efficiency of second harmonic generation (SHG) depends on the QPM condition, and it strongly changes with respect to the wavelength and the temperature. The effect of temperature variation on the SHG in periodically poled lithium niobate annealed proton exchange (APE) channel waveguides (WG) is theoretically modeled via a home-made computer code and experimentally validated via a suitable measurement set-up. A lot of simulations have been performed to test the temperature sensor feasibility and to identify its optimal configuration. Another sensor configuration made by two waveguides with suitable gratings of inverted ferroelectric domains is designed and refined, too. For an optimised PPLN-WG device, which could be fabricated through electric field poling and annealed proton exchange or titanium diffusion, a sensitivity S≡0.03μW/°C for the temperature range equal to 100 °C is demonstrated by using an input power at a fundamental wavelength equal to 40 mW. Similar evaluations and measurements, performed on bulk substrates, allowed us to design a layout of a sensor particularly suited for rugged in-field applications.