Seven infinite classes of relative difference families with variable block sizes are presented explicitly. In particular, a balanced (gv,g,K,1)-DF withg=Σk∈K[(k2-k)/2] is explicitly given for: (i)K={3,4,5} and everyvcoprime to 6; (ii)K={3,4,6}, {3,5,6} or {3,4,5,6} and everyvcoprime to 30. As far as the authors are aware, these difference families can be viewed as the first explicit constructions of infinite classes of optimal variable-weight optical orthogonal codes with more than two weights. It is observed, however, that there are infinitely many values ofvfor which an optimal (v,W,1,Q) -OOC exists, whatever the set of weightsWand the weight distribution sequenceQare.