Ultrafast single-mode Zn-diffusion VCSELs at the 850 nm wavelength for energy-efficient linking via graded-index single-mode fiber transmission

The pursuit of more energy-efficient optical linking at higher data rates is an ongoing goal to ensure rapid developments in AI technology. A breakdown of the total energy consumption in the modern 400 and 800 Gbit/s transceiver modules shows that the largest proportion of power is consumed for the retiming of the digital signal processing integrated circuits (DSP ICs), which are used to compensate for the limited modulation bandwidth of the light source through the fiber channel. This work demonstrates that, by combining a novel Zn-diffusion single-mode (SM) 850 nm VCSEL with a 1 km length graded-index single-mode fiber (SMF), which is two-mode at 850 nm window with high modal bandwidth, we can obtain a significant direct modulation bandwidth enhancement (22 to 39 GHz), with an optical coupling efficiency as high as 60.5%, between the SM VCSEL output and a SMF-28 ultra fiber through a single lens on the fiber tip. Error-free 56 Gbit/s transmissions over 1 km of such fiber under a low bias current of 4 mA are obtained without using any power-hungry retiming of the DSP ICs.