In this research, first, the Z-scan technique is used to measure the nonlinear optical properties of reduced graphene oxide (rGO) to indicate the high nonlinear coefficients. Second, a novel, to the best of our knowledge, vertically pumped, all-optical modulator is produced based on a rGO-coated multimode optical microfiber. The effect of the microfiber curvature, microfiber diameter, and substrate materials is investigated and optimized. Also, a simulation based on the finite-difference time-domain (FDTD) method is performed. The modulation depth increased to 4.2 dB by the external low-power ultraviolet pump laser (300 mW) for modulators based on the multimode microfibers. The presented process is a simple, cost-effective route to fabricate, and it is easy to use the device.