A laser apparatus has a multipath solid-state slab laser rod and an
excitation source that excites it, and uses a solid-state slab laser rod
that has a trapezoid or parallelogram-shaped cross section along the
optical path, that is provided with six or more faces and that has a
light amplifying effect or a nonlinear optical effect. The light that is
incident to the laser rod is totally reflected at the incident-light end
face and exit end face of the laser rod, passes back through the interior
of the laser rod a plurality of times, e.g., three times, and then exits.
For light amplification, excitation light is irradiated from the side
face. In addition, for nonlinear optical effects, the z-axis of a
nonlinear optical crystal is disposed so as to be orthogonal to the plane
of the optical path, and the temperature of the solid-state slab laser
rod is kept at the phase-matching temperature. This laser apparatus of
this configuration solves drawbacks of prior-art laser rods so that the
high conversion rate from excitation light to laser light is obtained at
a high laser gain.