VCSEL modulation capacity: Continued improvements or physical limits?

The short-reach optical interconnects used in datacenters and high-performance computing systems are dominated by VCSEL and multimode fiber (MMF) links
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. The VCSEL-MMF technology is the most cost and power efficient and offers the smallest footprint. VCSELs operating at 25-28 Gbit/s are in production
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while research has extended the VCSEL modulation bandwidth to 30 GHz
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(Fig.1) and enabled OOK-NRZ data transmission up to 57 Gbit/s at 25°C
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and 50 Gbit/s at 85°C
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, without equalization or forward-error-correction (FEC). A VCSEL energy dissipation below 100 fJ/bit has been demonstrated at 25-50 Gbit/s
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(Fig.1). The need for higher interconnect capacity raises the question whether the speed and dynamics of VCSELs can be further improved or whether physical limits preventing this have been reached. Higher speed VCSELs would enable higher lane rates and therefore reduced number of lanes and increased bandwidth density for a given aggregate interconnect capacity.