We present an experimental scheme that produces Doppler-free spectra of the 5s → 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 allow 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 perfom an all-optical measurement of the hyperfine splittings of the 6p 3/2 manifold in rubidium.