Feed-forward comb-assisted coherence transfer to a widely tunable DFB diode laser

The transfer of phase coherence from an ultrastable master laser to a distributed feedback diode laser, using an optical comb as a transfer oscillator, is obtained via a new scheme allowing continuous scanning across the whole tuning range of the slave laser together with absolute frequency determination. This is accomplished without phase lock loops, through a robust high–bandwidth feed–forward control acting directly on the slave laser output radiation. The correction is obtained by means of a dual–parallel Mach–Zehnder interferometer used as optical single–sideband modulator. Coherence transfer across a master–slave frequency gap of 14 THz yields a ∼ 10 kHz linewidth providing high injection efficiency of an optical cavity with finesse 250 000. This allows to demonstrate a cavity ring–down absorption spectrum of low–pressure ambient air over a 300 GHz spectral window.