Confirmation of observation of the Risken-Nummedal-Graham-Haken instability

Summary form only given. The Risken-Nummedal-Graham-Haken (RNGH) instability, described 30 years ago, was never before observed clearly, in contrast to all other types of instabilities derived from simple laser models. The prediction is that beyond a second threshold cw lasing would give rise to intensity-modulated lasing, where the modulation occurs with a frequency equal to the mode spacing, and the second threshold is at least a factor of 9 above the lasing (first) threshold. It was recently conjectured that the intensity modulation often observed in erbium doped fiber lasers was in fact attributable to it, even though the instability was observed significantly below the predicted threshold R>9. We performed experiments designed to test this prediction. Our fibre laser is fitted with an electrooptic amplitude modulator which allows us to vary losses.