  A multilayer GaAs/AlAs heterostructure forming a Fabry--Perot microcavity
with a narrow resonance at 1.1~$\mu$m was produced by molecular-beam epitaxy.
Under nanosecond pulsed laser radiation, a blue shift of the resonant line,
associated with a photo-induced negative change in refractive index in GaAs,
was experimentally registered by using an optical parametric oscillator. The
spectral shift was accompanied by a reduction in peak transmittance, associated
with nonlinear intracavity absorption. Such a cavity can be used as an optical
limiter at the resonant wavelength when both the spectral shift and the
transmittance reduction contribute to the limiting effect. An exceptionally low
limiting threshold of about 1 mJ/cm$^2$ was observed in the experiment.
