Experimental observation of noise precursors for virtual Hopf phenomena near period-doubling bifurcations in modulated diode lasers

We present an experimental analysis of the effect of a rf amplitude modulat ion of the injection current in a distributed-feedback diode laser at 1.55 mum near the first period-doubling bifurcation. First experimental results of a virtual Hopf phenomenon were observed when two moving bumps with Loren tzian shapes appeared in the spectrum before the onset of the first period- doubling bifurcation occurred. We characterized this observation by increas ing the amplitude of the current modulation of the diode laser until period -doubling bifurcation took place. The shape and position in the spectrum of these Lorentzian bumps were studied by the noise precursors theory, which predicts that the presence of noise will act as a virtual Hopf precursor in systems undergoing a period-doubling bifurcation. (C) 2001 Optical Society of America.