Tunable Infrared Gas Lasers

The gain profiles of infrared transitions in low pressure gas lasers are 

The first uses the Zeeman shift of laser transitions in external magnetic fields and the second pressure broadening of the gain profiles to obtain a continuous overlap between adjacent transitions. 

Historically the Zeeman tuned gas lasers have been the first continuously tunable lasers used in high-resolution molecular spectroscopy [7.15a,b]. The discharge tube of a gas laser is placed in an axial magnetic field which causes a splitting of the gain profiles for the laser transitions into two 

Either one of the components can be selected with a X/4 plate and a polarizer. Unfortunately the magnetic field affects the gas discharge and the gain of the active laser medium. The laser intensity therefore depends on the magnetic field and drops to zero at some maximum field strength This restricts the tuning range to 

A number of infrared gas lasers can oscillate on many closely spaced rotational lines of a vibrational transition. Examples are CO2, N2O, HF, DF, or H O lasers. If the pressure in the gas discharge can be made sufficiently large to increase the pressure-broadened linewidth beyond the spacings between adjacent lines, a quasicontinuous gain profile is formed which allows continuous tuning over a larger spectral interval [7.17]. 

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