Megawatt pulses from an all-fiber and self-starting femtosecond oscillator

Henry Haig, Pavel Sidorenko, Robert Thorne, and Frank Wise, “Megawatt pulses from an all-fiber and self-starting femtosecond oscillator,” Opt. Lett. 47, 762-765 (2022)

Mamyshev Oscillators are a relatively new type of fiber laser with extraordinary pulse performance— these lasers generate the highest peak-power pulses from femtosecond fiber lasers by over a factor of 10. Mamyshev oscillators should be excellent tools for applications like microscopy and micromachining, but there’s a catch: most Mamyshev oscillators are impractical for applications outside laser labs since they need another mode-locked laser to start and are relatively complicated and expensive. In this project we designed a Mamyshev oscillator that solves these practical problems and comes in a totally fiber-integrated, ready-for-applications format. The laser generates pulses on par with those from much more complex Mamyshev oscillators, and remarkably starts with some simple electronics rather than an additional mode-locked laser. The pulse energy (80 nJ) and duration (40 fs) advances the state-of-the-art for all-fiber, self-starting lasers by 20x, and should be scalable by another factor of 5 with large-mode-area fiber.

Femtosecond optical parametric chirped-pulse amplification in birefringent step-index fiber

Michael L. Buttolph, Pavel Sidorenko, Chris B. Schaffer, and Frank W. Wise. “Femtosecond optical parametric chirped-pulse amplification in birefringent step-index fiber” Optics Letters Vol. 47, Issue 3, pp. 545-548 (2022)

While optical fiber is convenient for many applications, generating short pulses outside of the typical gain bandwidth of rare-earth dopants commonly used in fiber amplifiers and oscillators is challenging. There are relatively few dopants that are compatible with the silica glass host, which leads to significant spectral gaps in which it is difficult to generate strong ultrashort pulses. It is sometimes possible to use nonlinear wavelength conversion to generate pulses in these spectral gaps, however. Parametric amplification conveniently generates two sidebands simultaneously, though to this point compressed pulse durations have been limited to ~200 fs and non-standard fiber waveguide geometries have been necessarily employed in order to achieve phase-matching. In this work, we demonstrate optical parametric chirped-pulse amplification in commercially available birefringent step-index optical fiber, delivering tens-of-nanojoule pulses compressible to 60-70 fs at 900 nm and 1270 nm. The key advances in this work were using birefringence rather than dispersion engineering in order to achieve phase-matching, and furthermore realizing that pumping the system with an extremely broadband pulse (from a gain-managed nonlinear amplifier) would allow the generation of energetic pulses compressible to very short duration. In addition, as the amplifier works with chirped pulses, we believe that the pulse energy may further be increased by chirping the pulses to longer duration and/or by employing large-mode area fiber for parametric amplification. We plan to use this system for hyperspectral degenerate and non-degenerate two-photon excitation fluorescence microscopy in the near future, enabling studies of complex biological processes in vivo.

Starting dynamics of a linear-cavity femtosecond Mamyshev oscillator

Yi-Hao Chen, Pavel Sidorenko, Robert Thorne, Frank Wise “Starting dynamics of a linear-cavity femtosecond Mamyshev oscillator,” J. Opt. Soc. Am. B, 38, 743-748 (2021) 

This paper is chosen as Spotlight on Optics.

Mamyshev oscillator is a laser that not only generates strong pulses but is also capable of maintaining environmental stability. However, starting becomes a challenge due to the suppression of noise from continuous-wave (CW) lasing. Solutions to starting are to start with an external seed pulse, overlapped filter passbands to allow CW lasing, or self-seeding with a NPE starting arm described by Pavel et al. Here we proposed another solution to starting with pump modulation. It requires no mechanical flipping such as self-seeding and is demonstrated with full electronic control (Please watch the demonstration video here). Furthermore, it is demonstrated to reach a higher pulse energy by later increasing the filter separation. The laser is found to start reliably with pump modulation of a high repetition rate (>70 kHz) due to the emergence of a modulated mode-locked state. Besides, we found that damage from SBS constantly occurred in a linear cavity such that adding Faraday rotators is required.

Normal-dispersion fiber optical parametric chirped-pulse amplification

Walter Fu and Frank W. Wise, “Normal-dispersion fiber optical parametric chirped-pulse amplification,” Opt. Lett. 43, 5331-5334 (2018).

An ongoing limitation of fiber lasers is their lack of broad wavelength tunability. Here, we address this problem using fiber optical parametric chirped-pulse amplification (FOPCPA), which combines the energy capacity of chirped pulse amplification with the spectral flexibility of optical parametric amplification and the practical benefits of fiber. Notably, this is the first FOPCPA to be pumped in the normally-dispersive regime, which permits phase-matching far from the pump wavelength.

The system operates by coupling a stretched, broadband pump pulse and a continuous-wave signal into a photonic crystal fiber. At each point in time, the monochromatic signal interacts via four-wave-mixing with a different wavelength of the chirped pump, resulting in an idler that is chirped in exactly the same manner as the pump. Scalability follows from the timescale-invariance of this process: stretching the pump at constant peak power likewise stretches the idler at constant peak power, increasing the energy without affecting the dechirped duration. By exploiting this property, we are able to convert pulses from the Yb-band to the important bio-imaging window near 1300 nm, with energies of >100 nJ and femtosecond-scale durations.

Self-seeded, multi-megawatt, Mamyshev oscillator

P. Sidorenko, W. Fu, L. G. Wright, M. Olivier, and F. W. Wise, “Self-seeded, multi-megawatt, Mamyshev oscillator,” Opt. Lett. 43, 2672-2675 (2018).

As was shown by Liu et al., the pulses from a Mamyshev oscillator can be enhanced by increasing the spectral separation between the two bandpass filters. However, this comes at a cost: the same mechanism that strongly stabilizes the pulse against continuous-wave breakthrough also suppresses the weak electric field fluctuations that are needed to initiate pulse formation. Thus, a Mamyshev oscillator may be constructed that supports very high-energy pulses, but which can be mode-locked only with the aid of an external seed source. In this paper, we address this problem by showing how a simple auxiliary cavity–a “starting arm”–may be embedded into a Mamyshev oscillator, enabling the oscillator to seed itself at the flip of a mirror. A video of this process can be viewed here. We have furthermore scaled part of the cavity to fiber with a 10-micron core diameter. The result is a fiber oscillator with self-starting-like behavior that can deliver 190-nJ, 35-fs pulses without any external amplification, for an unprecedented peak power of 3 MW after dechirping.

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).