  By using a blow-up construction, the nearby-cycle functor and l-adic Fourier
transform, Abbes and Saito are able to define a geometric measure of wild
ramification of l-adic sheaves on the generic point of any complete discrete
valuation ring of equal characteristic p with perfect residue field, where p is
different from l. In this paper, we adapt their construction to differential
modules over the field of formal Laurent series with coefficients in any
characteristic zero field K. For such a module M, we prove a formula relating
Abbes and Saito's construction to the differential forms occuring in the
Levelt-Turrittin decomposition of M. If K is algebraically closed, one recovers
a version of Laurent's micro-characteristic cycles.
