Self-Mixing in Terahertz Quantum Cascade Lasers

Terahertz (THz) quantum cascade lasers (QCL) have stimulated significant interest in THz laser imaging systems due to their compact size, broad spectral coverage (~1.2-5.2 THz) and high output power (>1 W). Due to their continuous wave (CW) narrowband emission and quantum noise limited linewidths, THz QCLs are particularly suited to coherent detection, but the majority of previously reported imaging systems have employed incoherent detection. Furthermore THz detectors typically fall into one of two categories (thermal or electrical), both of which have downsides (slow response rate or limited frequency range respectively). Self-mixing (SM) can be seen as a solution to these problems while also gaining the advantages of a reduced experimental set-up and cost, and increased sensitivity. SM occurs when radiation emitted from a laser is re-injected into the laser cavity by reflection from a remote target. The re-injected field interferes with the intracavity field, resulting in perturbations to both the measured output power and laser terminal voltage that depend on both the amplitude and phase of the reflected field. In this work, new SM imaging and modulation techniques were developed for both two- (2D) and three-dimensional (3D) imaging systems, including improvements leading to improved acquisition speed and depth resolution. Other techniques were developed to identify parameters of the QCL spectral emission and tunability, and SM was also exploited for extraction of optical parameters of explosive materials; a precursor to identification of such materials, something that is very important to national security and public safety. Further work was also developed in the areas of phase-nulling for the purpose of vibromacy measurements and extraction of laser parameters, and near-field (NF) spectroscopy, which has led to a massively improved lateral imaging resolution (~1 um).