Effect of VCSEL Characteristics on Ultra-High Speed Sigma-Delta-Over-Fiber Communication Links

State-of-the-art high-speed multi-mode vertical-cavity surface-emitting-lasers (VCSELs) have a 3-dB analog bandwidth of ∼25 GHz and can operate up to 55 Gbps error-free, without need of equalization electronics. This makes them strong candidates as optical sources for ultra-high speed sigma-delta-over-fiber (SDoF) communication links. This paper examines the effects of VCSEL characteristics on a 32-Gbps SDoF communication link with a 50 m, OM4, multimode optical fiber. Three high-speed, 850-nm-wavelength VCSELs with oxide aperture sizes of 8.3, 6.6, and 4.6 μm were tested. The effects of different modulation voltage amplitudes and bias currents were also investigated. The link performance and VCSEL power consumption were used as figures-of-merits. By avoiding sub-laser threshold modulation for the VCSEL, each of three VCSELs could provide similar link performance. We show that, compared to conventional datacom links, eased bit-error-rate requirements enabled by bi-level nature of sigma-delta modulation makes it possible to use significantly lower VCSEL bias currents. This in turn enables a reduced VCSEL power consumption (50% lower heat-to-bit ratio) and a potential longer VCSEL lifespan. For a SDoF modulated, 160-Mbaud, QAM64 signal centered at 12 GHz an error vector magnitude of -28 dB was achieved with the VCSEL having oxide aperture size of 4.6 μm operating at only 2 mA bias current and 0.25 V modulation amplitude. This results in a heat-to-bit ratio of only 0.1 mW/Gb/s (or equivalently 0.1 pJ/b), and a current density in the VCSEL that is less than 10 kA/cm².