We present an integrated optical wavelength meter based on a Si<sub>3</sub>N<sub>4</sub>/SiO<sub>2</sub> micro ring resonator (operating over a free spectral range of ≈ 2.6 nm) whose output response is immune to temperature changes. The wavelength meter readout is performed by a neural network and a non-linear optimization algorithm. This novel approach ensures a high wavelength estimation precision (≈ 50 pm). We observe a long-term reproducibility of the wavelength meter response over a time interval of one week. We investigate the influence of the ambient temperature on the estimated wavelength. We observe an immunity of the displayed output wavelength to temperature changes of up to several degrees. The temperature-drift immunity appears to be caused by deviations from the theoretically expected (perfect) transmission function of a ring resonator, i.e., caused by deviations that are usually undesired in spectroscopic devices.
We present for the first time second harmonic generation in amorphous stoichiometric Si<sub>3</sub>N<sub>4</sub> waveguides grown via low pressure chemical vapor deposition. An effective second-order susceptibility (χ <sup>(2)</sup>) is established via the coherent photogalvanic effect. A waveguide was designed to phase match a horizontally (parallel to the waveguide width) polarized hybrid EH<sub>00</sub> mode at 1064 nm with the higher-order hybrid transverse EH<sub>02</sub> mode at 532 nm. A mode-locked laser delivering 6.2-ps pulses at 1064 nm with a repetition rate of 20 MHz was used as pump. When pumped with a constant average power, it was found that the photoinduced χ <sup>(2)</sup> is established over a time of the order of 1000 s in as-manufactured waveguides, during which the second harmonic signal grows from below noise to a saturation value. The life-time of the photoinduced χ <sup>(2)</sup> is at least a week. In steady state, we obtain a maximum conversion efficiency close to 0.4% for an average pump power of 13 mW inside the waveguide. The effective second-order susceptibility is found to be 8.6 pm/V.