A phase reflecting timing deviations by relating them to the pulse period.

In the literature, the timing Jitter of a mode-locked Laser is often specified as the power spectral density not of the timing deviation Δt, but rather of the timing phase.

This definition is inspired by considering the emitted pulse train as a (usually highly anharmonic) oscillation of the optical power, which in the noiseless case can be seen as consisting of a sinusoidal signal and integer harmonics thereof, i.e., as a Fourier series. One pulse period corresponds to a change of the timing phase by 2Ï€.