All posts by Walter Fu

Normal-dispersion fiber optical parametric chirped-pulse amplification

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

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.

High-power femtosecond pulses without a modelocked laser

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).

Schematic of the demonstrated system.