Sidebands in the Spectrum of certain mode-locked lasers, related to periodic disturbance of solitons
In a Soliton mode-locked laser, a soliton pulse is circulating in the cavity. IF the effects of Dispersion and nonlinearity per cavity round trip are weak, the pulse basically experiences their average values plus some weak periodic disturbance due to the discrete nature of dispersion and nonlinearity. Additional disturbances result from the periodic amplification in the Laser cavity.
The mentioned periodically occurring disturbances couple the soliton to the copropagating dispersive wave. This does often not have strong effects, since the relative phase of soliton and Dispersive Wave is constantly changing. However, particularly in Fiber lasers a kind of resonant Coupling (or phase matching) can occur, where for some optical frequencies the relative phase of soliton and dispersive wave changes by an integer multiple of 2Ï€ per cavity round trip. This results in the formation of narrow peaks superimposed on the soliton spectrum. Such peaks are called Kelly sidebands. From their positions in the obtained optical spectrum one can retrieve information on the spectral profile of the Chromatic Dispersion of the fiber.