External Cavity Quantum Cascade Lasers

In this thesis we deploy mid-infrared quantum cascade lasers (QCLs) in external
linear cavity (EC) and external ring cavity (ERC) configurations, with the aim to
develop the platform as a source for high resolution gas spectroscopy.
Initially, the temporal evolution of pulsed ERC-QCL system is compared to that
of standard Fabry Perot (FP) QCLs to provide insight into the role played by the
additional external feedback provided by ECs and ERCs. Time-resolved spectral
measurements show that external feedback promotes single longitudinal mode operation on a shorter timescale than is the case for FP-QCLs. A room temperature
wavelength tunable external ring cavity QCL is presented which shows improved
performance over its linear counterpart due to its ability to support unidirectional
emission regimes. Spectral comparisons between linear and ring cavities show that
these regimes lead to improved performance due to the suppression of spatial hole
burning (SHB), an instability common to linear laser resonators.
An active modulation scheme based on a number of bespoke power amplifiers
and bias-tees is developed. These systems are used to produce mode-locked pulse
trains in external cavity and external ring cavity QCLs by modulating the QCL gain
medium’s drive current at frequencies matching the cavity’s round trip frequency.
Spectral and temporal comparisons are made between these two cases. Asymmetric
pulse propagation is observed in the case of the ERC-QCL which has been suggested
as promising platform for ultrashort mode-locked mid-infrared pulses.