Precise estimation of linewidth reduction in wavelength-detuned DFB semiconductor lasers

The linewidth reduction of DFB semiconductor lasers due to the wavelength detuning is estimated precisely. Firstly, the Schawlow-Townes linewidth, which excludes the enhancement due to the carrier effect, is determined from the AM noise spectrum. Secondly, the actual linewidth including the carrier effect is measured. Lastly, the contribution from the carrier effect is determined from both of the linewidths. The experimental result shows that the linewidth enhancement due to the carrier effect is four times larger in a 1.3 μm laser with a positive detuning (Δλ=+15 nm) than in a laser with a negative detuning (Δλ=-15 nm)