Overview of Evolution:
The basis for this evolution is the well-known Mamyshev regenerator . Two non-overlapping spectral filters serve as an effective saturable absorber, suppressing continuous wave lasing while allowing a high-intensity pulse to sustain itself. An intense pulse experiences spectral broadening between each filter. When this broadening is sufficient for significant pulse energy to pass through both spectral filters repeatedly, a stable, periodic evolution can occur, where the pulse spectrally “oscillates” between the two offset filters. Self-similar pulse evolution between the filters was proposed early on as a factor in achieving optimal performance, while more recent work has suggested the role of a new regime known as amplification under gain-managed nonlinearity .
Right: cartoon illustrating the effect of the Mamyshev oscillator on the pulse spectrum.
The Mamyshev ring oscillator can be thought of as comprising two arms, each with its own gain fiber, output coupler, and offset spectral filter. Within each arm, many of the general considerations that apply to other normal-dispersion evolutions (e.g., dissipative solitons or amplifier similaritons) remain valid. For examples of cavity designs, see [3-5].
The spectral offset between the two filters is closely linked to the laser’s performance. Larger separations allow the pulse spectrum to oscillate more widely, accommodating greater energy and bandwidth up to a certain point. However, initiating modelocking will also become more difficult, with techniques such as pump power modulation or external seeding quickly becoming necessary. For instance, the 4-nm bandpass filters in the animation shown are separated by 10 nm, which is a moderate offset; as a consequence, we would not expect self-starting for this example. Obtaining self-starting behavior without compromising on performance is the subject of ongoing research. For instance, a simple, auxiliary, “starting arm” can be embedded into the main cavity to allow the laser to seed itself with a flip of a mirror .
Because the Mamyshev regenerator mechanism provides strong effective saturable absorption, other types of saturable absorbers such as nonlinear polarization evolution are unnecessary. This allows the cavity to be constructed entirely out of polarization-maintaining fibers, rendering it highly environmentally stable.
Mamyshev oscillators remain early enough in their development that it is difficult to give an idea of their “typical” performance. By optimizing the pulse evolution and taking advantage of fibers with a 10-micron core diameter, nearly 200-nJ pulses have been obtained directly from the oscillator, which can be dechirped using a standard grating pair to 140-nJ, 35-fs pulses with 3-MW peak power . Greater compactness and robustness can be achieved by sacrificing some performance, and 15-nJ, ~150-fs pulses have been reported from a completely fiber-integrated Mamyshev oscillator . While effects such as multiple-pulsing, gain narrowing, and Raman scattering have been implicated in limiting Mamyshev oscillators, the ultimate performance ceiling and its causes remain to be determined.
1. P. V. Mamyshev. “All-optical data regeneration based on self-phase modulation effect.” European Conference on Optical Communication (ECOC) (IEEE, 1998), p. 475.
2. P. Sidorenko, W. Fu, and F. Wise. “Nonlinear ultrafast fiber amplifiers beyond the gain-narrowing limit.” Optica 6, 1328 (2019).
3. K. Regelskis, J. Želudevičius, K. Viskontas, and G. Račiukaitis. “Ytterbium-doped fiber ultrashort pulse generator based on self-phase modulation and alternating spectral filtering.” Optics Letters 40, 5255 (2015).
4. I. Samartsev, A. Bordenyuk, and V. Gapontsev. “Environmentally stable seed source for high power ultrafast laser.” Proc. SPIE 10085, 100850S (2017).
5. Z. Liu, Z. Ziegler, L. G. Wright, and F. W. Wise. “Megawatt peak power from a Mamyshev oscillator.” Optica 4, 649 (2017).
6. P. Sidorenko, W. Fu, L. G. Wright, M. Olivier, and F. W. Wise. “Self-seeded, multi-megawatt, Mamyshev oscillator.” Optics Letters 43, 2672 (2018).
Animation credit: Walter Fu.