Output spectra of the argon ion laser

The output spectra of an A+ laser in its λ 4879.90 Å transition has been observed at various levels above threshold, in a ring resonator, a two-mirror resonator of the same longitudinal mode spacing (107 Mc/s), and another two-mirror resonator of about three times that mode spacing. Oscillations were restricted to the fundamental (Gaussian) mode. The A+ laser spectra display three novel features, hitherto unobserved in the oscillation spectra of neutral-gas lasers [17], which appear to stem, at least in part, from its large natural line width: 1) At moderate levels above threshold, the spectra are extraordinarily stable, and usually characterized by gaps of many mode spacings between oscillations. 2) Above a critical level, oscillations occur in random sequence at all resonances, with violent amplitude fluctuations at each frequency. 3) The time-average envelope of the unstable "multimode" oscillations exhibits three well-defined maxima, which give way to a single maximum at a higher excitation level. The line shift of the laser transition has been measured to within about 10 percent, by comparing oscillation frequencies at the center of symmetrical spectra in the two counter-circulating waves of the ring laser.