Coexistence and noncoexistence of modes in a multi-mode laser

Mode competition effects in a cw multi-mode laser with a broad homogeneous emission line are studied theoretically, taking account of a Lorentzian gain profile. Explicit formulas are presented for the frequency range in which simultaneous oscillation of modes takes place, and the tuning range for a selected mode (the latter being privileged by a lower loss). Moreover, a novel spectroscopic technique introduced and successfully applied recently, is analysed. The experimental procedure consists in first restricting the oscillation range by means of filter and afterwards observing the additional spectral broadening of the laser output, which is caused by placing a narrow-band absorber inside the laser cavity. A useful formula allowing to estimate the sensitivity of this technique is derived.