The modulators used in the experiment were Mach-Zender-type interferometers* designed and fabricated in Leti labs and cleanrooms in partnership with STMicroelectronics. The components were designed for integration with a hybrid III–IV/silicon laser to form a complete integrated transmitter.
Silicon modulators have been a popular topic among researchers seeking ways to boost performance (50 Gb/s modulation, integrated electronics). However, the laser source is often external to the component, and there have been few cases in which a transmitter with both a laser and modulator integrated on the same silicon substrate has been used. And the most recent such cases are limited to 12.5 Gb/s. Here, the goal is to align performance with the 100GBASE-LR4 standard for datacom applications with four wavelengths at 25 Gb/s and a minimum extinction rate of 4 dB.
OOK transmission at 25 Gb/s was demonstrated at 2.5 Vpp [see Figure] on each of the components’ arms, polarized at 1.25 V.
Figure: Shape of the electrical input signal at 25 Gb/s (NRZ format).
And the modulators could achieve even higher transmission rates if clock frequency is increased or if a more complex modulation format, like PAM4, is used. Finally, the design methods developed to create the component were validated and can now be reused to make new modulators with even higher bandwidth and lower optical losses.
*The Mach-Zehnder interferometer was created by Ludwig Mach, son of Ernst Mach, and Ludwig Zehnder. It consists of two mirrors and two half-silvered mirrors used to split and then recombine a collimated beam of light. The shape of the interference between the two beams provides very accurate measurements.