Simple narrow-linewidth 1.5 μm InGaAsP DFB external-cavity laser

An external-cavity laser consisting of a 1.5 μm-wavelength antireflection-coated DFB laser chip, a microscope objective and a mirror provides a stable, single-frequency, narrow-linewidth (Δf=40 kHz) output. Because the laser operates in the strong feedback regime, it is immune to external optical perturbations. Single-longitudinal-mode operation is maintained by the DFB grating, and the linewidth is reduced by the extended passive cavity.