InModelocking free-running operation without frequency-selecting elements, the output from laser resonators is a superposition of those longitudinal modes that experience a net gain for a resonator round trip (Fig. 13.1). A longitudinal mode is defined by the condition that an integer multiple of half the wavelength matches the optical resonator length. In cw-operation, and without optical elements that influence the phase, these longitudinal modes oscillate independently from one another, resulting in a random phase relationship.