Spectral analysis of a phase locked laser at 891 GHz, an application of Josephson junctions in the far infrared

Overview & Synopsis

We have used a Josephson junction to investigate the spectral purity of an HCN laser which is used in an infrared frequency synthesis chain. To obtain a narrower linewidth from the laser it has been phase locked to a multiplied microwave reference chain. A calculated value for this linewidth was based upon the measured noise spectrum of the microwave source and a theory due to Middleton. One can take advantage of the unique properties of the Josephson junction as a frequency multiplier and mixer for use in measuring this linewidth. The Josephson junction is driven by an X-band signal which is derived from a specially designed cavity stabilized klystron system of high spectral purity. The 92nd harmonic of the X-band signal is generated in the Josephson junction. In addition, the Josephson junction acts as a mixer of the harmonic signals and the 891 GHz output of the HCN laser. The 92nd harmonic beat signal is taken from the Josephson junction, amplified, and sent to a spectrum analyzer for frequency domain analysis. Details of the experiment, results, and relation to predicted linewidths are presented.