  The paper offers a discrete thermodynamic model of lasers. Laser is an open
system; its equilibrium is based on a balance of two thermodynamic forces, one
related to the incoming pumping power and another to the emitted light. The
basic expression for such equilibrium is a logistic map, graphical solutions to
which are pitchfork bifurcation diagrams. As pumping force increases, the
relative populations on the ground and lasing branches tend to zero and unity
correspondingly. An interesting feature of this model is the line spectrum of
the up and down transitions between the branches beyond bifurcation point. Even
in a simple case of 2-level laser with only 2 possible transition types (up and
down), the spectra look like sets of the line packets, starting well before the
population inversion. This effect is an independent confirmation of the
Einstein's prohibition on practical realization of 2-level laser. Multilevel
lasers may be approached by employing the idea of thermodynamic activity for
the emitting atoms. Considering coefficient of thermodynamic activity of the
lasing level atoms to be proportional to the ratio of life times on the upper
and lasing (the 3d) levels, one can derive a new basic map for the multilevel
laser system. For a modest ratio only of 0.01, spontaneous transitions between
levels are pushed to the area beyond population inversion, opening a space for
the functioning of laser.
