Currently deployed fiber and free-space optical communication systems use on-off keying (OOK) with direct detection, and some are beginning to use differential phase-shift keying (DPSK) with interferometric detection. Wave propagation is guided by optical fibres. Co pared to twisted pair and coaxial cable, it has a greater bandwidth efficiency. In simple terms, modulation means changing some property of the light (such as its intensity, phase, or frequency) in a controlled way to represent. Optical modulation is a crucial process that allows control over an optical wave or encoding of information on a carrier optical wave. Optical modulation enables many key functions in. Before the invention of the laser, long-range optical communication was envisioned using bright flashes of light produced by intense pulses of electric current passing through an incandescent fiber placed in the focal-plane of an optical reflector.