In the examples provided herein, a data center transmission system includes a VCSEL (vertical-cavity surface-emitting laser) that lases in a single spatial mode with a side mode suppression ratio of at least 25 dB, where the VCSEL is formed on a substrate and lases at a wavelength transparent to the substrate, and further where an output of the VCSEL exits through the substrate. Also, the VCSEL is directly modulated. The system further includes an optical fiber having a first end to receive the output of the VCSEL for propagation along the optical fiber. The optical fiber supports a single spatial mode without supporting higher order spatial modes over a range of wavelengths between 1260 nm and 1360 nm. The system also includes a receiver to receive the directly modulated output of the VCSEL after propagation through the optical fiber.