Terahertz quantum cascade laser application in local oscillator development

Terahertz (THz) quantum cascade lasers (QCLs) are high-quality THz sources in terms of power (>1W) and compact size, and the application of THz QCLs has been widely investigated. Spectroscopy is one of the most widespread application for THz QCLs. The project presented in this thesis is an application of THz quantum cascade laser.
The project is focused on development of a local oscillator (LO system) aimed to be used in Earth observation. A single mode THz QCL which produces continuous-wave signals is designed, fabricated and characterized. In order to improve the performance of the LO (THz QCL), it is integrated within a waveguide block, which is more mechanically robust than a normal packed QCL and a significant improvement in beam profile was obtained by the integration with little change in the electrical and thermal performance.
Next, the detector (Schottky diode detector) use in the application was investigated in terms of heterodyne detection and detection calibration. The heterodyne signal from the Schottky diode detector was used to study a Fabry–Perot (FP) QCL, whose neighbouring FP modes are coupled into the Schottky diode detector. The investigation gives a QCL emission linewidth and thermal equilibrium speed of the THz QCL. QCLs are used to calibrate a new Schottky diode detector designed by Rutherford Appleton Laboratory (RAL), which gives a 3.67 THz room temperature cut-off frequency.
Lastly, spectroscopy with the THz QCL is carried out. A single mode CW lasing QCL and a photomixer are used in this application. The system gives a clear demonstration of methanol spectroscopy. Different partial pressure and absolute pressure of methanol are investigated. The measured result is backed by the simulation result from a JPL database. The measured result also proves the possibility of obtaining absorption linewidth broadening by increase methanol pressure. The noise level of the system is also investigated, which gives a detection limitation of 6.0×〖10〗^18 molecules in the 73 cm long gas cell that can be detected for the system.