 The active medium of the laser is optical fiber with its core doped with rare-earth ions.It is placed asymmetrically with respect to the resonator loops to make a nonlinear difference in phase between opposite waves, ensuring mode locking.In 1992 a figure-8 laser was built with a smaller loop length of 1.6 m and a larger loop length of 60.8 m for generation of 315 fs pulses with repetition rate 125 MHz.