For shorter pulse durations, with correspondingly higher soliton pulse energy, the soliton periods quickly become too short, and the pulse formation becomes unstable due to too-high nonlinear phase shifts per round-trip. The pulse duration of a soliton fiber laser is typically a few picoseconds, and the soliton pulse energy is in the picojoule domain. However, this implies significant manufacturing efforts because the polarization axes at many fiber joints need to be aligned.Īnother possibility is to use a SESAM, possibly in fiber-coupled form, for mode locking. The standard approach is to use magneto-optical materials that possess an asymmetric permittivity tensor1117. To build an optical isolator one needs a mechanism to break Lorentz reciprocity9,10. Therefore, the interference condition for recombining the two pulses at the fiber coupler in the middle becomes power-dependent: for a certain power level, much of the power can be sent downward in the laser resonator.Ī figure-eight laser can be made with polarization-maintaining fiber so that it becomes environmentally stable. e C) l l (a) ransmlssion i' Soliton Average T O lnput1Peak Poi/ver p 980 nm Pump Input ' Optical Q 1536 nm isolator Pulse input (a) Polarization. points to an important limitation on the use of nonlinear optical isolators for signal processing and for laser protection. These pulses experience different nonlinear phase shifts, as one is first amplified in the erbium-doped fiber and then propagates through a longer piece of nonlinear fiber, while the other one propagates through that fiber with a lower energy. Pulses from the laser resonator are split into two copies, propagating through the nonlinear loop in opposite directions. The actual laser resonator is seen on the left side, whereas the ring on the right side is a nonlinear amplifying fiber loop.