Spectral properties of strongly coupled 1.5 µm DFB laser diodes

Spectral measurements of strongly coupled DFB lasers operating at 1.5 μm are presented. The magnitude of the coupling coefficient
k
in these devices was determined to be 80 cm
^-1
for lasers with
\lambda = 1.12 \mu
m cladding layers and 160 cm
^-1
for devices with
\lambda = 1.3 \mu
m cladding layers. These values for
k
are believed to be the largest reported for 1.5 μm DFB lasers. CW spectral linewidths as low as 10 MHz at 15 mW output power were obtained, and the linewidth was observed to vary approximately as the inverse of the device length cubed. Spectral measurements performed under 2 Gbit/s direct modulation exhibited a side mode suppression ratio of >38 dB. The effects of transient wavelength chirping were also investigated in detail and the maximum wavelength deviation was found to be ≃1.5 Å.