We develop a model to compute the line shape of an injected gas laser.
The model describes the competition that occurs between the injected
field and the usual resonant field of the laser, which are both consid
ered as separate dynamic variables. A small Signal stability analysis
of the stationary solutions is used to study this competition: the las
er field may be destroyed due to gain quenching by the injected field,
which explains the frequency locking phenomenon. We give the connecti
on with the standard Adler model. Theoretical and experimental results
that describe the line shape inside or outside the locking range are
given for the typical He-Ne laser operating at 3.39 mu m.