What Is A Soliton Fiber Laser?

Although the generation of picosecond pulses in solid-state lasers mainly relies on saturable absorption, dispersion and nonlinear effects can significantly influence pulse evolution in fiber lasers, even for pulse durations of several picoseconds. Conceptually, the simplest approach is to use a soliton fiber laser, in which pulses circulate within the fiber cavity as quasi-solitons.

Passive mode-locking is commonly achieved using artificial saturable absorbers based on Kerr nonlinearity. One practical approach is nonlinear polarization rotation (NPR). In this process, the polarization state in a section of fiber undergoes complex evolution that depends on the optical power and is influenced by self-phase modulation (SPM) and cross-phase modulation (XPM). The pulse then passes through a polarization element, where the polarization changes are converted into transmission variations.

As shown in the figure above, an optical isolator is used as the polarization element in the laser resonator. By adjusting the fiber-loop polarization controller, the total loss at the pulse peak center becomes lower than that at the pulse wings. However, the optimum adjustment depends on fiber bending and temperature variations. Therefore, such lasers are not environmentally stable: they require highly stable temperature conditions or periodic realignment. Another type of artificial saturable absorber is the nonlinear optical loop mirror (NOLM), which can be incorporated into a figure-eight fiber laser configuration, as illustrated below.

The left side represents the actual laser resonator, while the right side is the nonlinear amplifying fiber loop. Pulses from the resonator are split into two paths and propagate through the nonlinear fiber loop in opposite directions. These two pulses experience different nonlinear phase shifts: one pulse is first amplified in the erbium-doped fiber and then passes through a longer section of nonlinear fiber, while the other propagates through the nonlinear fiber at a lower energy level. As a result, the interference condition when the two pulses recombine at the fiber coupler becomes power-dependent. At a specific power level, most of the optical power is coupled back into the resonator.

Figure-eight fiber lasers based on polarization-maintaining (PM) fibers offer excellent environmental stability. However, their fabrication is more complex because the polarization axes of multiple fiber splices must be accurately aligned.

Another possible solution is to use a SESAM (Semiconductor Saturable Absorber Mirror) for mode-locking, typically implemented in a fiber-coupled configuration.