Vertical-Cavity Surface-Emitting Laser Transmitter Frontends for High-Speed Optical Links
This work presents the design of high-speed, power efficient optical transmitter frontends based on vertical-cavity surface-emitting laser (VCSEL). The integrated circuits are implemented using three different processes: two types of highly scaled bulk CMOS (28 nm and 90 nm) technologies and one high-speed 130 nm SiGe BiCMOS process. The technology of choice depends on the project and application of the driver. Since the transmitted data rate has to be maximized, bandwidth extension techniques and multilevel modulation are implemented in the drivers’ designs.
Direct modulated lasers are often the bandwidth bottleneck of high-speed optical transceivers. The VCSEL model allows the designer to optimise the driver and equalizer in order to compensate the VCSEL non-linearities and extend the system bandwidth in a power efficient manner. The model of a 850 nm commercially available high-speed VCSEL is presented in this work. It is based on non-linear rate equations and provides an excellent agreement between the simulation results of the model and the measurement of the VCSEL.
Within the scope of this work new passive on-chip components are developed with the aim of improving the performance of the transmitters. A particular attention is given to the design of a new type of on-chip vertical inductor. The difference between conventional inductors and the proposed vertical inductor is that in the latter the spiral is oriented vertically to the chip substrate saving chip area. Vertical inductors are not only designed and measured, but also patented and implemented in the high-speed VCSEL drivers.
Several VCSEL drivers are designed during this thesis work. The main goal is to advance the state of the art in regard to power efficient, high-speed optical transmitters. The drivers are wire bonded to commercially available VCSELs and are measured using a wafer prober. Thanks to the design of a power efficient asymmetric 3-tap feed-forward equalizer, the fastest driver reaches an error-free optical data rate of 50 Gbit/s with a power consumption of only 190 mW using a 20 GHz bandwidth VCSEL and a 22 GHz linear receiver. This driver is currently the most power efficient NRZ driver for data rates higher than 40 Gbit/s. Multilevel amplitude modulation is an attractive alternative to NRZ in VCSEL-based optical transceivers. In this work the challenges of such modulation are studied and applied for the first time in a high-speed VCSEL driver.