Abstract

Inside a fiber resonator, pulse shaping due to dispersion and nonlinearity is modified considerably by cavity boundary conditions. For example it is well known that periodic modulation of a relatively broad pulse may emerge due to modulational instability in standard fiber; it was pointed out by Haelterman et. al.¹ that inside a cavity this modulation exhibits a threshold, its frequency differs from that in free propagation, and it can occur even for normal dispersion. We experimentally and numerically study a fiber ring resonator driven by picosecond pulses from a modelocked laser. The repetition rate of the driving pulses is chosen in synchronism with the round trip time of the resonator. The experiment allows flexible choice of fiber dispersion. This system has been shown before to exhibit period doubling and chaos for some parameters.² In other cases, an ensemble of solitons in perpetual relative motion (a soliton gas) is formed.³ This gas may transform into a regular lattice of solitons, a process closely related to modulational instability.⁴ In both experiment and numerical simulations we verify the predictions of Haelterman et al.¹ There is a threshold power for the onset of modulation, and the frequency of modulation is shifted with respect to the free propagation value. Close to the zero dispersion wavelength, however, we also find a modulation based on a different mechanism. Its frequency depends sensitively on cavity length fine tuning, but is insensitive to dispersion details. We find that this latter type of modulation is related to the temporal shift due to a mismatch between cavity length and drive pulse repetition time as described in Gatz and Herrmann.⁵

© 1998 Optical Society of America