In view of the complexity and bulkiness of most current random number generation schemes

we theoretically propose and demonstrate an all-optical physical random number generation method based on Vertical-Cavity Surface-Emitting Laser（VCSEL） mode hopping. Through periodically restarting a VCSEL operating in a bistable regime with a square-wave modulation signal

and balancing the asymmetry between two polarization modes utilizing spontaneous emission noise

a random number stream can be directly generated. Numerical results show that 2.5 Gbit/s physical random numbers can be obtained with verified randomness. This scheme can be simply implemented by only a solitary VCSEL and provide a new solution for photonic integrated random number generator.