With a semiconductor saturable absorber mirror (SESAM), a simple folded cavity has been designed to achieve mode-locking with a single output. The plane mirror is employed as a cavity mirror as well as an output coupler. The grin lens is employed as the optical coupler to shape the single diode laser, the stable continuous wave mode locking is achieved with the pulse width of 5.2 ps and the repetition rate of 100 MHz. The average output power is 800 mW with the pump power of 3 W, and the optical-optical conversion efficiency is 27%. The characteristics of the laser cavity are analyzed by computer.