Characterization of the hybrid integration of a shared dispersive element laser and a modulator

This paper characterizes a ten-channel waveguide grating router multifrequency laser (MFL) with 200-GHz channel spacing coupled to a 2.5-Gb/s electroabsorption modulator on a separate chip. We show theoretically and experimentally that in long-cavity semiconductor lasers such as the MFL, light reflected back into the laser perturbs the device wavelength a negligible amount compared to the inherent modulator dynamic chirp but can cause multimode lasing. However, if the ratio of the net external reflectivity to the laser facet reflectivity is kept below approximately 1/(4α²+1) where α is the linewidth enhancement parameter in the laser, the lasing spectrum is immune to the back-reflected light. We also show that placing a simple “isolator” that only has to rotate the polarization of the back-reflected light rather than provide full isolation can easily provide the immunity