A high-speed laser diode driver (LDD) for vertical-cavity surface-emitting laser (VCSEL) with direct non-return-to-zero (NRZ) modulation and adjustable energy efficiency is presented. The LDD is designed in a 0.13 μm SiGe BiCMOS technology and assembled with a commercial 850 nm VCSEL with a bandwidth of 20GHz in common cathode (CC) configuration. Optimized driver operation is studied for constant extinction ratio (ER) and highest energy efficiency at different speeds. The highest error-free optical data rate (DR) with a bit error rate (BER) of less than 10^-13 was measured to 45Gbit/s. Since the driver does not incorporate any signal equalization or pre-emphasis, only 1.8 pJ/bit are required at this DR. To the best of the authors' knowledge, this is the highest reported DR for a direct VCSEL modulation without any kind of equalization at the transmitter as well as the receiver side. The best energy efficiency of 1.17 pJ/bit is achieved at 30Gbit/s by adopting the modulation levels of the LDD. By adaptively tuning the VCSEL modulation, the power consumption of the LDD can be reduced by 80% and the energy efficiency can be improved by 500%. Furthermore, no area consuming peaking inductors are used for bandwidth enhancement leading to a compact design with an active area of only 0.15 × 0.12 mm² which makes the design well suited for low-cost multi-channel optical interconnects.