High-speed vertical-cavity surface-emitting lasers (VCSELs) are essential for future data communications with high speed, low cost, and low power consumptions. For exceeding intrinsic modulation bandwidth of VCSELs limited by the relaxation oscillation frequency, manipulation of electron spin polarizations in VCSELs has been attracting attention in recent years. In this study, we theoretically and experimentally investigate modulation characteristics of 1.55-m VCSELs under the spin polarization modulation to obtaining tailored modulation characteristics suitable for high-speed data communications. Spin-flip rate equation analyses reveal that a short spin relaxation time is suitable for a flat modulation response under the spin polarization modulation, and a 100-Gbs operation is expected in InAlGaAs quantum well (QW) VCSELs with a spin relaxation time of 10 ps, linear birefringence of 100 GHz, and dichroism of 50 GHz. A wide 3-dB bandwidth of 23 GHz determined by a frequency split between two orthogonal polarization modes (∼19.4 GHz) is experimentally confirmed under optical spin polarization modulations by using a commercially-available InAlGaAs QW VCSEL whose relaxation oscillation frequency is ∼3 GHz. These results support the idea that the ultrahigh- speed optical signal generation is available at the telecom wavelength of 1.55 m by applying the spin polarization modulation to VCSELs. Additionally, a modulation format conversion technique for output lights from the polarization modulation to phase modulation by using a polarizer is suggested and confirmed by the spin-flip rate equation analyses.