A system for passively scrambling and unscrambling a, pulse optical signal transmitted through a multi-mode optical fiber is provided. The system includes a scrambling unit connected between a signal receiving end of said transmission fiber and an optical signal source that includes an optical fiber which creates a differential delay between two groups of optical modes of the signal that is at least one bit period long such that said optical signal is passively scrambled, and an unscrambling unit connected to a signal transmitting end of said transmission fiber having an optical fiber that counteracts said differential delay between said two groups of optical modes of the signal such that said optical signal is passively unscrambled. The scrambling unit optical fiber has an alpha profile with a complementary perturbation that creates the differential delay between two groups of optical modes of the signal, and the unscrambling unit optical fiber has an alpha profile with reversal complementary perturbation that counteracts the differential delay between said two groups of optical modes of the signal. Alternatively, the scrambling and unscrambling units may have the combination of a mode splitter that splits the signal into two different groups of optical modes, and a pair of optical fibers that conduct the two groups of optical modes in parallel, the fibers being of different lengths to create or to counteract a differential delay in the two groups of modes.