  We present an experimental scheme that produces Doppler-free spectra of the
$5s \to 6p $ second resonance transition in atomic rubidium. The experiment
uses the saturation of the cascade fluorescence that occurs when thermal
rubidium atoms interact with two counterpropagating $420 $ nm laser beams of
comparable intensity. Part of the excited atomic population goes through the
$5p_{3/2}$ level which then decays by emission of $780$ nm photons. Narrow dips
appear in this otherwise broad $780$ nm fluorescence, which allows resolution
of the $6p_{3/2}$ hyperfine structure. A rate equation model is used to
interpret the spectra. It is also shown that these narrow peaks can be used to
lock the frequency of the $420 $ nm laser. Using a second beam modulated in
frequency produces three sets of spectra with known frequency spacings that can
be used to perform an all-optical measurement of the hyperfine splittings of
the $6p_{3/2} $ manifold in rubidium.
