Harmonic analysis in a large ring laser with backscatter-induced pulling

Analytical solutions for the beam intensity, interferogram, and instantaneous phase are found for a ring laser with backscatter. The harmonics of these signals form geometric progressions, whose ratios depend differently on pumping, pulling, and the net backscatter phase. The latter phase depends on the time-reversal symmetry of the scatterers and on their location. We also report confirmatory results in the Canterbury ring laser system, with a quality factor up to 7.5×${10}^{10}$ and a present frequency resolution of 140 nHz, and where thermal changes in the net backscatter phase dominate other causes of drift.