Reshaping of ultrashort pulses is predicted to occur when a pulse train from a mode-locked laser is incident upon a nonlinear Fabry–Perot cavity whose length is matched to the period of the pulse train. The temporal shape of the transmitted pulses depends on the relaxation time of the nonlinearity of the Kerr medium inserted into the Fabry– Perot cavity. When the pulse duration is shorter than the Kerr relaxation time, considerable pulse narrowing (by factors of 101–103) is predicted. If the Kerr relaxation time is longer than the period of the pulse train, the analysis shows the existence of two temporal shapes of the output pulse, leading to the possibility of bistability between these two states. A Kerr nonlinearity with an instantaneous response can be used to generate square output pulses. Both the transient and the permanent regimes are investigated, and analytical expressions for the narrowing factors are found.
© 1988 Optical Society of America
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