Observation of Fibre Laser Instabilities Through Modulation of the Cavity Optical Length

A Nd-doped, single-mode, double-clad fibre laser (FL) is coiled around a resonant piezoelectric ceramic ring; this induces modulation of the cavity optical length and two distinct classes of laser intensity instabilities are observed as the ring drive voltage V P-P is increased. The first class involves a slowly varying intensity, with characteristic time scale in the microsecond range. For fixed drive-voltage frequency f m ≈ 20 kHz, we consistently observe subharmonics f m /2 and f m /4 in the FL output intensity spectrum; noise in the system precludes the observation of higher subharmonic bifurcations. Small-amplitude random relaxation oscillations of the unmodulated FL are strongly amplified as V P-P is increased and transition to a state of large-amplitude chaotic relaxation oscillations occurs for V P-P ≈ 4 V; a value of 2·6 has been measured for the correlation dimension of the chaotic attractor. The second class of laser instability occurs simultaneously for V P-P ≳ 10 V, and the FL output consists of a train of pulses about 2 ns wide at a repetition rate close to the cavity frequency f cav ≈ 15 MHz and modulated by the slowly varying chaotic envelope described above.