The invention relates to a non-linear optical filter capable of transmitting an electromagnetic pulse with a duration of less than about 1 picosecond, provided with means for generating an electromagnetic field E with a linear polarization from this pulse, and with a crystal C1 of cubic geometry capable of generating an electromagnetic field E'.sub.1 from E with a linear polarization orthogonal to that of E. It comprises means for generating at least one other electromagnetic field E'.sub.2 capable of producing constructive interferences with the electromagnetic field E'.sub.1.