Far-infrared laser frequency modulation by refraction index variation in low-density plasma

Experimental results are reported of frequency modulation of an optically pumped far-infrared laser obtained by varying the electron density in an ionized gas plasma confined in a dielectric waveguide external to the laser cavity and electromagnetically coupled to its oscillating mode. A modulation depth of 6 kHz at 403.7 GHz (HCOOH laser line) is reported. This was obtained with a weakly ionized plasma (≈10¹⁰ e/cm³) at a gas pressure of 0.1 Torr in a 0.3-m-long cell, with a relative electron density variation of 90%. The modulation bandwidth was determined by plasma recombination time and turned out to be 20 kHz at this pressure in nitrogen.