Spectral properties of strongly coupled 1.5 µm DFB laser diodes

Abstract

Spectral measurements of strongly coupled DFB lasers operating at 1.5 μm are presented. The magnitude of the coupling coefficient <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</tex> in these devices was determined to be 80 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for lasers with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda = 1.12 \mu</tex> m cladding layers and 160 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for devices with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda = 1.3 \mu</tex> m cladding layers. These values for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</tex> 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.