Self-phase-modulated coherent pulse

Phase-modulated coherent pulse such as due to self-phase modulation has a common feature with incoherent light in that it has broad band-width whose reciprocal is much shorter than the light duration. It, therefore, is expected to play the same role as incoherent light in some nonlinear transient spectroscopy. This expectation has also been confirmed by preliminary theoretical and experimental study. 

The spectral width Aty can be further increased by focusing the laser pulses into a special optical fiber, which consists of a photonic crystal (Fig. 9.88) where by self-phase modulation the spectrum is considerably broadened and extends over one decade (e.g., from 1064 nm to 532 nm) (Fig. 9.89). This corresponds to a frequency span of 300 THz [1327] It was found by interference experiments, that the coherence properties were preserved in this broadened spectmm, i.e. the nonlinear processes in the optical fiber did not destroy the coherence of the original frequency comb. 

Soliton ring fiber lasers can be also realized with active mode locking by polarization modulation [11.77], or by additive-pulse mode locking (APM). In the latter technique the pulse is split into the two arms of an interferometer and the coherent superposition of the self-phase modulated pulses results in pulse shortening [11.55]. 

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