Comparison of different tuning and modulation techniques for F.I.R. lasers

A review is presented of the techniques to tune and frequency modulate an optically pumped FIR laser. Experimental results obtained on a versatile structure by modulating different parameters are presented, along with the analysis of the laser output frequency variations thereby induced by a FIR synthesizer. The problem of frequency modulation of these lasers is not solved, and the Stark modulation provides a wide band and convenient modulation depth, but is not usable for all molecules and all lines. A variable reactance, however, or a variable electron density plasma coupled to the resonator will operate in all cases, but have their own defects because of coupling or bandwidth problems.