Discrete electroabsorption modulators with enhanced modulation depth

In this paper, we present an investigation into factors that limited the median modulation depth of a batch of packaged discrete waveguide EA modulators to 23 dB at a wavelength of 1.55 μm. Results from detailed measurements of the DC absorption and photocurrent spectra are used to show how stray parasitic light can perturb the absorption characteristic and reduce the modulation depth of these high-speed multiple-quantum-well modulators. A novel ridged deeply etched buried heterostructure EA modulator design is presented in which stray light is removed from the immediate vicinity of the guided mode. The key structural difference between these and the previous devices is that they employ a much thicker Fe-doped InP current blocking layer that was grown by atmospheric pressure MOVPE using PCl₃ for planarization. Detailed measurements of the DC absorption spectra of a packaged ridged deeply etched buried heterostructure device confirm that stray light causes only a minor perturbation on its absorption characteristics. Consequently the new batch of EA modulator modules have a higher median modulation depth of 40 dB, as well as lower fiber-to-fiber insertion losses and picosecond pulse generation capabilities that are very similar to the previous devices which were used in 40 Gb/s optically time division multiplexed experiments