A new FOQELS setup is described for use in concentrated dispersions. A fiber-to-fiber coupler is introduced as an extra local oscillator source. This results in a reliable determination of the diffusion coefficient of concentrated dispersions where the heterodyne mode appertains at every concentration, This system is compared with others and results are presented of measurements of a 176-nm latex dispersion and a 230-nm titanium-dioxide dispersion. The diffusion coefficients measured with the new setup clearly differ from those measured with a single optical fiber probe, which are within the mixed homodyne-heterodyne regime; when taking this degree of mixing into account, the latter results can be transferred to the value of the true diffusion coefficients. For prolonged and robust measurement of highly concentrated dispersion, however, direct measurements with heterodyne probes are preferred.