A single-isotope He—Ne laser gyro has been constructed and operated, and the results have been compared with those obtained from a multi-isotope system. Strong mode competition in the single-isotope ring laser has been found in a region 10–30 MHz near line center. The sense of the mode competition has been found to depend on the direction of rotation of the laser cavity. Hence, by mechanical dithering of the laser, the time-averaged effect of the mode competition can be zero. However, with a current unbalance, a large dispersion-type null shift is found near line center. Unlike the linear laser and the multi-isotope ring laser, the single-isotope ring laser is characterized by strong mode competition such that only a single longitudinal mode oscillates over most of the free spectral range.
Frederick Aronowitz, "Single-Isotope Laser Gyro," Appl. Opt. 11, 405-412 (1972)