Determination of the spectrally resolved gain profile of HeSe/sup +/ laser lines from the beat frequency spectrum

Several laser lines are selected from the multiline spectrum of a CW HeSe/sup +/ laser by an intracavity birefringent filter. The different axial modes of a laser line move across the gain profile because of statistical variations of the resonator length. The laser modes are recorded as beat frequencies in the GHz region by a fast photodiode and a spectrum analyzer. Variable losses are inserted into the resonator by a pair of Brewster plates. From the vanishing of the axial modes with increasing losses spectrally resolved, gain profiles are determined. The measured Doppler profiles correspond to a selenium ion temperature of 1500+or-100 K. A population density difference of the order of 10/sup 9/ cm/sup -3/ is estimated from the unsaturated gain of the 522 nm line.>