A Fast, Flexible, and Compact Terahertz Time-Domain System

The term "terahertz radiation" denotes electromagnetic radiation in the frequency range between microwaves and infrared light. For a long time, it was not possible to generate intense, directional terahertz radiation, nor was this type of radiation easily detected. This left a gap in the electromagnetic spectrum, known as the "terahertz gap." In the last 30 years, technical progress paved the way towards the development of today's terahertz systems. For instance, the invention of ultrashort pulse lasers and the advent of fast photoconductive materials played an important role in the development of terahertz time-domain systems. These systems, which generate and detect terahertz pulse traces, have recently gained increasing attention for both scientific and industrial applications (e.g., spectroscopy and the non-destructive testing of materials).
Terahertz time-domain systems generally rely on a so-called "pump and probe" technique: at the receiver, the incident terahertz pulse is sampled with a time-shifted laser pulse. This concept necessitates a delay line, which usually presents the bottleneck in terms of measurement rate. Conventional terahertz TD systems generally employ a mechanical delay, which—if carefully designed—enables broadband measurements of up to several 100 terahertz pulse traces per second. This is still insufficient for some applications (e.g., thickness measurement in industrial applications that require measurement rates well above 1000 traces/s).
This thesis describes a fast, flexible, and compact terahertz time-domain system based on electronically controlled optical sampling (ECOPS).