Figure-8 laser

A figure-8 laser is a fiber laser with a figure-8-shaped ring resonator. It is used for making pico- and femtosecond soliton pulses. The typical spectrum of such a laser consists of a wide central peak and a few narrow lateral peaks that are placed symmetrically around it. The amplitudes of the narrow peaks are the same as or less than that of the central peak.^[1]

Fiber laser with figure-8 ring resonator. In: pumping. Out: emitting light. 1: active fiber. 2: polarizer. 3: optical isolator. 4 WDM splitter. 50:50 splitter 50/50.

Both loops of the resonator work as Sagnac loops. 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.^[2] 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.^[3]