An optical extending device for use in transmission of optical signals which comprise at least one sequence of periodic optical signals, said optical device comprising: a first fiber optic having a characteristic dimensional propagation coefficient equal to .beta..sub.1 and adapted to be connected to a single mode second fiber optic having a length equal to L.sub.0 and a characteristic dimensional propagation coefficient equal to .beta..sub.0, wherein L.sub.p, the length of said first fiber optic is substantially equal to {[T.sup.2/.PI.-L.sub.0*.beta..sub.0]/.beta..sub.1}*{1-MOD(L.sub.0/{T.sup.- 2/.PI.-L.sub.0*.beta..sub.0]/g(b)}} and wherein: n is an integer 1, 2, 3 . . . and is selected in accordance L.sub.0, the length of the single mode second fiber optic; T is a time period of the periodic optical signals; and MOD is the remainder obtained from dividing 1.sub.0 by {[T.sup.2/.PI.-L.sub.0*.beta..sub.0]/.beta..sub.1}.