Technologies are described for enabling VCSELs to transmit phase or frequency modulated signals, which may have a substantially smaller transmission bandwidth and are more robust to transmission in a highly dispersive media such as multi-mode fibers. By applying an electric, magnetic, or electromagnetic field across one or more reflector layers of the VCSEL, a refractive index of the reflector layer(s) may be modified. The VCSEL may then be excited and the laser beam generated by the VCSEL phase-modulated by modifying a magnitude of the applied field.