FIR Laser Modulation by an External Variable Reactance

Fast frequency modulation of an FIR laser by means of an external variable reactance is presented. In particular, the use of the varactor effect in a micrometer-size Schottky-barrier diode and of the electron density modulation in a cold plasma is considered. Experiments carried out on an optically pumped HCOOH line at 403.7 GHz are described and the results are compared with the theory developed. Peak-to-peak frequency deviation Δfpp of about 15 kHz and Q-limited bandwidth have been obtained with the varactor effect, whereas the plasma modulator yielded Δfpp of 6 kHz and a 20-kHz bandwidth.