Noise of the optical phase of a beam or of an electric signal.
The output of a single-frequency Laser is not perfectly Monochromatic but rather exhibits some phase noise. This leads to a finite Linewidth of the laser output. The fundamental origin of phase Noise is quantum noise, in particular spontaneous Emission of the Gain medium into the Resonator modes, but also quantum noise associated with optical losses. In addition, there can be technical noise influences, due to vibrations of the cavity mirrors or to temperature fluctuations. In many cases, there is also a Coupling of Intensity noise to phase noise. Phase noise may occur in the form of a continuous Frequency drift, or as sudden phase jumps, or as a combination of both.