Length and pressure dependence of the tuning spectrum produced by CH3F double Raman lasers

We have measured the spectrum produced by a tunable Raman FIR laser operated in a single-pass waveguide configuration using waveguide lengths of 50, 120 and 200 cm. For each waveguide a range of pressures was used that brackets the pressure at which the maximum output occurs. For ν₃ R-branch transitions in ¹²CH₃F and ¹³CH₃F with 15 ⩽ J ⩽ 20, the shape of the interline spectral profile could be changed significantly with only a modest sacrifice of output power. Using a 200-cm waveguide, for example, the usual spectral asymmetry weighted towards the higher J number transition can be eliminated or reversed by controlling the pressure. A 4-level density matrix model that incorporates pump depletion and signal growth along the waveguide reproduces some of the trends displayed by the experimental data but is unable to account fully for the large observed changes in the interline spectral profile.