Enhancement of Terahertz Spectra by Model-Driven Spectral Shaping of a Mode-Locked Laser Diode in a Terahertz Time-Domain Spectroscopy System

In this work, we show first results of spectrally shaping the output of a mode-locked laser diode (MLLD) in a terahertz time-domain spectroscopy (THz-TDS) system with a programmable optical filter. The coefficients of the programmable optical filter are found with a genetic algorithm based on an analytical model that relates the detected terahertz spectrum to the optical spectrum of the MLLD. We define two different fitness functions for the genetic algorithm to deliberately impress desired properties on the detected terahertz spectrum. With the first fitness function, we achieve an increase in signal amplitude of up to 8 dB at the upper end of the terahertz spectrum. With the second fitness function, we flatten the terahertz spectrum between 150 and 950 GHz.