All-fiber heavily ytterbium-doped, passively mode-locked laser with the 456  MHz repetition rate

In this Letter, we report on, to the best of our knowledge, the first demonstration of ultrashort pulses generation with an all-fiber laser with the repetition rate of up to 456 MHz, without the use of additional nonlinear optical elements. It was possible to obtain this repetition rate thanks to the use of a specially manufactured heavily doped ytterbium fiber. The generation dynamics of the laser assembled according to the classical Fabry-Perot scheme with two fiber Bragg gratings was investigated. The formation of ultrashort pulses as a result of passive mode-locking (ML) is presented. Passive ML is attributed to the absorption saturation, while the role of the saturable absorber in the active optical fiber with a high ytterbium ion concentration plays itself.