Phase-locking terahertz quantum cascade lasers for high dynamic range heterodyne imaging

Abstract

Terahertz radiation exhibits unique properties that make it an attractive candidate for safe imaging. Current barriers to its commercial implementation include the shortage of efficient terahertz detectors and convenient, high-powered terahertz sources. These factors combined with the strong atmospheric absorption of terahertz radiation necessitate the development of an imaging system capable of extracting small terahertz signals from the background noise. This can be achieved using a combination of heterodyne detection and a phase-locked loop to selectively lower the system noise floor while preserving signal power. In this thesis, an optical phase-locked loop using a Schottky diode mixer and 3rd order DFB QCLs is implemented to achieve a maximum SNR of 60 dB, limited by residual phase error from the high free-running linewidth of the QCLs. System limitations and alternative phase-locking configurations are discussed towards the ultimate goal of achieving a 110 dB dynamic range terahertz QCL imaging system..