Raman-driven destabilization of giant-chirp oscillators: Fundamental limitations to energy scalability

We report on the destabilization of a giant chirp oscillator (GCO), whose output displays features characteristic of noise-like pulses for sufficiently long laser cavities. The laser configuration studied here is schematically identical to that of Erkintalo et al (2012). We find that for increasing cavity lengths a secondary operation regime can abruptly manifest alongside with the primary, stable mode-locking regime. In this regime the laser output differs substantially from that reported by Erkintalo et al. Here we compare the optical spectra when the laser is operating in the stable (100 m long cavity) and in the unstable regime (200 m long cavity). We can see that whilst the spectrum in the primary mode of operation is highly structured, the spectrum in the secondary regime is considerably smoother and contains no fine features. This apparent smoothness is only due to the slow response time of the OSA, indeed, each wave packet has a different and much more structured spectrum leading to large shot-to-shot fluctuations. An even more substantial difference is the strong Stokes signal frequency which can be witnessed in the unstable regime.