Pulse-train instabilities in a mode-locked argon laser: experimental studies

The output train of pulses from an acousto-optically mode-locked argon-ion laser was studied as a function of the loss modulation frequency for small detunings from the optimum c/2L modulation. A shift away from the optimum frequency by as little as 1 part in 104 was found to produce several strong lower-frequency components that amplitude modulate the mode-locked pulse train. Notably, the strongest subharmonic is a frequency near fopt/4. Further detuning results in an irregular, slow (of the order of 105 Hz) amplitude envelope on the pulse train. The homodyne spectrum of this phase of operation represents a progressive filling-in behavior reminiscent of deterministically chaotic systems.