Project 1.3

The project will design and fabricate hybrid lasers based on innovative resonant cavities. These building blocks will be exploited as sources in the QEPAS and PTS systems. ESR 1.3 will work closely with ESR 2.1. ESR 1.3 will focus more on the laser design and ESR 2.1 on the integration techniques.

To minimise crosstalk between the heat dissipated by the laser and the heat generated by light absorption in the gas, the gain element will be positioned far from the sensing volume (i.e. well away from the forks in the case of QEPAS) and the light will be carried by the silicon nitride waveguide which will be on the order of 5mm long. As a result, the longitudinal modes of the laser cavity will be very densely spaced, and to ensure single mode operation, a high Q-factor photonic crystal cavity resonant mirror will be realised such that only a single mode falls inside the reflection band. Slotted waveguides or subwavelength grating waveguides will be implemented to provide a waveguide mode with an evanescent field in air that will excite the gas. Coupling structures will be designed to ensure efficient coupling between the different components of the laser.

Traditional lasers have an intrinsic sensitivity to fluctuations in the ambient temperature, which necessitates the use of thermo-electric coolers to regulate the temperature of the laser. Silicon nitride has a lower thermo-optic coefficient than the III-V materials typically used in lasers. This property will be used to stabilise the centre wavelength of the resonant mirror. The longitudinal modes of the laser will be stabilised through cladding part of the SiN waveguide with a material with a negative thermo-optic coefficient. The length of this section will be chosen to balance the positive thermo-optic coefficient of the gain chip. The composite laser cavity will have a net zero thermo-optic coefficient overall, freezing the wavelength of the longitudinal mode. The ease of use the laser will be greatly improved allowing wider deployment.

The design of the nanostructures will be carried using Finite Difference Time Domain modelling and the lasing performance using the VPI software. Fabrication will be carried out on the silicon nitride platform. Commercial gain chips will be used at the 1.35 and 1.6 μm wavelengths.