Our understanding of the physics/chemistry of the interstellar medium increased since we got the capacity to develop heterodyne spectroscopy tools in the THz frequency range. For instance, an example of an important emission line in astronomy is the fine structure of the molecular deuterated hydrogen at 2.675 THz.
Heterodyne detection requires local oscillator sources that operate a few GHz away from the frequency of interest. THz quantum cascade lasers (QCL) emerge therefore as suitable sources. The combination of quantum cascade laser as local oscillator and ultra-sensitive hot electron bolometers for the mixing is so far the sole solution available in order to realise a compact and ultra-sensitive heterodyne detection system.
The first building block of our heterodyne detector is a spectrally single mode, low power consumption THz QCL operating at a specified target frequency. We developed devices with low threshold driving currents (<30mA). Their power dissipation, when operated in CW mode, stays below 250mW over the whole operation range. These characteristics make the components compatible for compact integration.
Despite the small beam divergence of the 3rdorder DFB architecture employed, the emission pattern is not perfectly Gaussian. We have therefore developed a solution to re-shape the QCL’s output beam into a Gaussian one, using a dielectric hollow waveguide (DHW). We have realized a full study to perfect the coupling between the QCL and the DHW, as the coupling losses are the limiting factor. This solution stands out as the most efficient for our heterodyne system.
Finally, the low-power-dissipation QCL was combined with a hot-electron superconducting bolometer, to yield an ultra-compact heterodyne detector. Characterization of the heterodyne detector unit, obtained with a hot and a cold blackbody calibration set-up, will be presented during the talk.