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Mode locking through nonlinear frequency broadening and spectral filtering

The propagation of symmetrical laser pulses in a Kerr medium produces a broadening of the pulse spectrum while that of nonsymmetrical pulses leads also to a spectral shift. Such a spectral reshaping can be used to mode lock a laser by inserting two spectral filters with different transmission profiles at the ends of a laser cavity including a Kerr material; it can be shown that short pulses travelling in such a laser experience a higher feedback per round trip than do lower power multimode signals. Numerical simulations tend to indicate that the pulses so produced have a nonsymmetrical shape; their spectrum is shifted with respect to that of multimode oscillations. Different considerations suggest that the method may not be self- starting and that another mode-locking technique (active or passive) may be needed to initiate mode locking.