This paper will discuss the vertical cavity surface emitting laser (VCSEL) bandwidth and noise performance needed to support 106 Gbd line rates with PAM-4 modulation for 200Gb/s per lane multimode optical links. Despite their low manufacturing costs, diffraction-limited, narrow-band emission and excellent modulation capability, VCSELs were only used for optical data transmission. In this chapter we will deal with major principles of vertical-cavity surface-emitting laser (VCSEL) operation. Basic device properties and generally applicable cavity design rules are introduced. 2 The Honeywell HFE-4080 ion implanted 850 nm VCSEL as well as a series of.