Period-doubling and total mode-locking, and chaotic pulsations in a GaAs ridge wave-guide external-cavity diode laser

We report on the experimental observations of a perioddoubling route to chaos and a total mode-locking between two lowest-order lateral modes of a GaAs ridge wave-guide diode laser at 795 nm. A selfstarting passively mode-locking was achieved in an extended-cavity diode laser (ECDL) operating in a gain saturation regime. In the period-doubling mode-locked state, the RF spectra associated with the coherently coupled lateral TE₀ and TE₁ modes, consisted of harmonics (nf_rep/2, n is an integer) of the half of pulse repetition rate f_rep/2 and the period (2T) of the pulse train was two-times longer than the pulse round-trip time T. On the other hand, in the total mode-locked state, the RF spectra and the pulse train corresponding to the TE₀ and TE₁ modes had exactly the same features compared to those of the period-doubling mode-locked state, except the RF spectra and the pulse train of the TE₁ mode were shifted by f _ref/4 in frequency and by T in time, respectively, indicating the pulses actually traveled alternatively through two lateral modes. The total mode-locking and also chaotic pulsations were observed at slightly different operation parameters, e.g. at different feedback angle of the grating, which was used as an output coupler of the ECDL.