  Aberrations and multiple scattering in biological tissues critically distort
light beams into highly complex speckle patterns. In this regard, digital
optical phase conjugation (DOPC) is a promising technique enabling in-depth
focusing. However, DOPC becomes challenging when using fluorescent guide-stars
for four main reasons: The low photon budget available, the large spectral
bandwidth of the fluorescent signal, the Stokes shift between the emission and
the excitation wavelength, and the absence of reference beam preventing
holographic measurement. Here, we demonstrate the possibility to focus a laser
beam through multiple-scattering samples by measuring speckle fields in a
single acquisition step with a reference-free, high-resolution wavefront
sensor. By taking advantage of the large spectral bandwidth of forward multiply
scattering samples, Digital Fluorescence Phase Conjugation (DFPC) is achieved
to focus a laser beam at the excitation wavelength while measuring the
broadband speckle field arising from a micron-sized fluorescent bead.
