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A nonlinear index medium introduced into a passively or actively mode-locked system can lead to shorter pulses. The bandwidth limitation by gain dispersion is partially overcome by spectral broadening via the nonlinear index medium. We have found that the pulse shortening in an actively mode-locked system is limited to roughly a factor of 2 by the onset of an instability which occurs when an excessive amount of nonlinear medium is added. The instability appears as an oscillation in the pulse shape and energy. The study suggests that active mode locking of Nd YAG lasers could be improved by such means. It is also likely that the colliding pulse mode-locked system is affected by nonlinear index processes. We have found a pulse shortening of roughly the same magnitude as in the actively mode-locked system, again limited by the onset of instability.