Transverse mode structure and output stability of the p-Ge terahertz laser

The p-Ge terahertz laser with flat external cavity mirrors exhibits intensity modulations at well-defined nonequidistant frequencies in the 8-80-MHz range, resulting from beating of transverse modes. A theoretical description of the mode structure of this cavity is given. Results compare well with experimental data on beat frequencies and far-field beam profiles obtained using a metal mesh out-coupling window. Based on calculations, a cavity with a reduced size back mirror is proposed to suppress higher order transverse modes. Experiments prove that a reduction of the mirror to about 70% of the end-face surface leads to an effective suppression of the intensity modulations. Such a cavity exhibits a strongly improved pulse-to-pulse stability of the output and a much better shape of the micro pulse train under mode-locked operation.