Megawatt peak power from a Mamyshev oscillator
Zhanwei Liu, Zachary M. Ziegler, Logan G. Wright, and Frank W. Wise. “Megawatt peak power from a Mamyshev oscillator” Optica, Vol. 4, Issue 6, pp. 649-654 (2017).
Historically, it has been really tough to make an ultrafast fiber laser that is both environmentally stable and that has good performance (i.e., it has similar performance as a Ti:sapphire oscillator). Recently, several groups have realized that a pair of spectral filters, each offset from the center of the laser gain spectrum, can be used as an effective saturable absorber. An intense pulse will experience nonlinear spectral broadening within fiber in between the filters, and can oscillate stably in a ring cavity formed in this way – a laser we call a ‘Mamyshev oscillator’ (see figure). Low-intensity pulses, or continuous-wave lasing, are meanwhile strongly attenuated. This mechanism, first proposed by Pavel Mamyshev for signal regeneration in telecommunications, is fully compatible with environmentally-stable laser designs. In this paper, we show that the Mamyshev oscillator can, when combined with the self-similar evolution of parabolic pulses, actually support extraordinary performance. Our initial experiments already show 10 times higher peak power than the previous state-of-the-art, and we are optimistic about further improvements.
High-power femtosecond pulses without a modelocked laser
Walter Fu, Logan G. Wright, and Frank W. Wise. “High-power femtosecond pulses without a modelocked laser” Optica, Vol. 4, Issue 7, pp. 831-834 (2017).
Modelocked lasers have long been a mainstay of ultrafast optics. However, they face ongoing challenges regarding long-term reliability, and can only emit pulses at regular intervals. Here, we present an alternative approach by seeding a fiber amplifier with a gain-switched diode. Gain-switched diodes emit pulses that are much longer and less coherent than those from modelocked oscillators. We address these issues using fiber nonlinearities: a Mamyshev regenerator isolates a coherent component of the pulse, and subsequent parabolic amplification allows the pulses to be compressed to 140 fs with 13 MW of peak power. Starting with a gain-switched diode means our system is highly robust and can in principle be electronically triggered in arbitrary pulse patterns. This flexibility may facilitate machining or microscopy sources (where pulses must be synchronized to scanning optics) or enable new types of functional neuroimaging (where specific neurons must be illuminated without saturating an entire sample).
Self-similar pulse evolution in a fiber laser with a comb-like dispersion-decreasing fiber
Yuxing Tang, Zhanwei Liu, Walter Fu, and Frank W. Wise. “Self-similar pulse evolution in a fiber laser with a comb-like dispersion-decreasing fiber” Optics Letters, Vol. 41, Issue 10, pp. 2290-2293 (2016).
We demonstrate an erbium fiber laser with self-similar pulse evolution inside a comb-like dispersion-decreasing fiber (DDF), which has the potential of generating nJ-level few-cycle pulses directly from a fiber oscillator. A passive DDF is formally equivalent to a fiber with constant gain, and can thus support self-similar pulse evolution but without any bandwidth limitation. Considering the challenges to fabrication of DDF, we try to imitate an ideal DDF with a comb-like DDF based on segments of ordinary fibers, which offers major practical advantages. The laser generates 1.3 nJ pulses with parabolic shapes and linear chirps, which can be dechirped to 37 fs. This constitutes a 4-fold increase in pulse energy compared to previous reports of this pulse duration.
Ultrafast fiber lasers based on self-similar pulse evolution: a review of current progress
A. Chong, L. G. Wright and F. W. Wise “Ultrafast fiber lasers based on self-similar pulse evolution: a review of current progress ” Rep. Prog. Phys. 78, 113901 (2015).
We summarize the state of research on lasers based on self-similar pulse evolutions, including passive similariton, amplifier similariton, and others. Self-similar fiber lasers are conceptually different from other kinds of short-pulse lasers. This distinction allows for exciting new laser design options.
Characteristic steady-state round trip evolutions of the pulse chirp for different mode-locking regimes. Solid lines indicate the chirp of the pulse, while dashed lines indicate the local dispersion of the cavity. In the highlighted plot, the lines show the difference of the pulse from a parabolic pulse.
Divided Pulse Lasers
We show that divided-pulse amplification can be used within a laser cavity to increase the pulse energy of a soliton fiber laser. In divided-pulse amplification, pulses are split up N times prior to amplification. After amplification, they are recombined into a single pulse. By reducing the peak intensity within the gain fiber, each split copy can be amplified to the single-pulse limit, and therefore the final recombined pulse can have N times higher energy. This work was featured in Spotlight on Optics.
Diagram of a general divided pulse laser. Pulses are divided before ampification in the gain fiber, then recombined before being output. A saturable absorber mirror (SAM) is used for mode-locking, while a dispersive delay (DD) can provide dispersion.