Interferometric Correction of Delay Errors in an Ultra-High Repetition Rate Terahertz Time-Domain Spectroscopy System

A compact and economical terahertz time-domain spectroscopy system can be constructed by using a monolithic mode-locked laser diode (MLLD) as the light source and by employing an off-the-shelf optical delay unit (ODU). The sub-picosecond pulses emitted by the MLLD promise a system bandwidth of up to 2 THz. However, mechanical inaccuracy and wobble of the ODU often cause the delay axis of the detected terahertz signal to be non-linear. In this work, we show how the optical signal from the MLLD can be used in a Mach-Zehnder interferometer to generate a low-frequency reference signal that allows us to correct the delay axis with femtosecond-accuracy.