Cross-phase modulation (XPM) is a technique for adding information to a light stream by modifying the phase of a coherent optical beam with another beam through interactions in an appropriate non-linear medium. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0 In Physics, coherence is a property of waves that enables stationary (i This article describes the use of the term nonlinearity in mathematics This technique is applied to fiber optic communications. Fiber-optic communication is a method of transmitting information from one place to another by sending Light through an Optical fiber.