This paper reviews device design and performance of high-speed vertical cavity surface emitting laser (VCSEL) arrays for next-generation short-reach 400 Gbit/s applications in data centers using the advanced PAM-4 modulation format. The laser beam is emitted perpendicular to the top surface of the. Laser technology is the most expensive part of an optical transceiver, roughly 50% of the module's total cost. Picking the wrong one means you're either overpaying or underperforming, so it's worth understanding what each type actually does well. These include: These features make VCSELs better suited to a wide range of applications than conventional edge-emitting diode lasers and LEDs.