Abstract
Electroabsorption modulated lasers (EMLs) is sensitive to the residual facet reflection (RFR), which can cause a low-frequency drop (LFD) in the modulation response. The LFD may result in output waveform distortion, especially for high-data rate and multi-level modulation. We verify by simulation and experiments that the effect of LFD on the EML performance and propose to use LFD as one of the key performance indices for qualifying EMLs in high-speed optical transceivers. LFD can be suppressed by incorporating partial-corrugated-grating (PCG) in the laser section. With PCG-DFB structure, the EMLs can maintain high single-mode yield (SMY) and an excellent quality factor even with a strong modulator reflection from EAM section. This provides the robustness in applying the PCG-DFB to achieve a flat intensity modulation response or to enhance the output waveform under large signal modulation. By designing the PCG-EML to have about 60% of grating region, it can reach >80% of SMY and improve the average Q-value from the reduced LFD in the modulation response. The PCG-EML with reduced LFD can enhance the eye-opening through reduced waveform undershoot and overshoot for transmitting 56-Gbaud/s or beyond PAM-4 signals.
Highlights
The rapid expansion of cloud computing, big data, and applications with smart mobile devices, intra- and inter-data center traffics are growing remarkably with the extension of next-generation high-speed light sources and 5G mobile network technology
We proposed here the PCG-electroabsorption modulated lasers (EMLs) to accomplish a smaller low-frequency drop (LFD) and be relatively invulnerable to the change of residual facet reflection (RFR) for transmitting ultra-high-speed signals
To justify the performance enhancement, the measured eye margin for EMLs with different grating ratios is depicted in Fig. 4(b), which is extracted from measured eye diagrams under 28Gb/s NRZ modulation
Summary
The rapid expansion of cloud computing, big data, and applications with smart mobile devices, intra- and inter-data center traffics are growing remarkably with the extension of next-generation high-speed light sources and 5G mobile network technology. A conventional EML, which is based on the uniform grating DFB laser (UG-EML), has relatively poor resistance to residual facet reflection (RFR) that can cause output waveform distortion [8] Such residual reflection would interfere with the laser resonance and appear as low-frequency drop (LFD) or resonant peaks in the smallsignal frequency response. We proposed here the PCG-EMLs to accomplish a smaller LFD and be relatively invulnerable to the change of RFR for transmitting ultra-high-speed signals. They will provide better single-mode and waveform stability under the phase fluctuation of the rear facet. We verify that the PCG-EML can reduce the LFD and enlarge the eye opening for high-data-rate modulation
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