Overview of Evolution:
A pulse under nonlinearity, normal dispersion, and gain develops a parabolic intensity and a linear up-chirp, asymptotically approaching a self-similar evolution known as an amplifier similariton. The remainder of the cavity is relatively insignificant except that the pulse must return to its original state for the next roundtrip.
Longer, less highly doped gain fibers (typ. 2-4 m) enhance convergence to the amplifier similariton pulse shape. Normal GVD in the gain is essential, but the net cavity dispersion may take on any value, with net normal (and particularly, all-normal) cavities producing the highest energies. Each time the pulse enters the gain fiber, it must be close enough to its ultimately parabolic form to reach that shape by the end of the gain. This can be accomplished by inserting a narrowband spectral filter into the feedback loop.
Few-nJ, few-ps pulses are obtainable, which dechirp cleanly to <100 fs. Using longer cavity lengths, 15 nJ, few-hundred-fs pulses can also be produced. The output chirp depends on the exact cavity design, but in optimized cavities, is somewhat less than the cavity dispersion.
The possibility of eliminating anomalous dispersion from the cavity permits higher energies than passive similariton oscillators, while the output pulses can be dechirped somewhat more cleanly than dissipative solitons.
Pulse energy and duration are primarily limited by gain narrowing and stimulated Raman scattering, which disrupt the evolution to an amplifier similariton.
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Animation credit: Walter Fu.