Walter Fu

High-power femtosecond pulses without a modelocked laser

We demonstrate a fiber system which amplifies and compresses pulses from a gain-switched diode. A Mamyshev regenerator shortens the pulses and improves their coherence, enabling subsequent amplification by parabolic pre-shaping. As a result, we are able to control nonlinear effects and generate nearly transform-limited, 140-fs pulses with 13-MW peak power-an order-of-magnitude improvement over previous gain-switched diode sources. Seeding with a gain-switched diode results in random fluctuations of 2% in the pulse energy, which future work using known techniques may ameliorate. Further development may allow such systems to compete directly with sources based on modelocked oscillators in some applications while enjoying unparalleled robustness and repetition rate control.

Several new directions for ultrafast fiber lasers [Invited]

Ultrafast fiber lasers have the potential to make applications of ultrashort pulses widespread - techniques not only for scientists, but also for doctors, manufacturing engineers, and more. Today, this potential is only realized in refractive surgery and some femtosecond micromachining. The existing market for ultrafast lasers remains dominated by solid-state lasers, primarily Ti:sapphire, due to their superior performance. Recent advances show routes to ultrafast fiber sources that provide performance and capabilities equal to, and in some cases beyond, those of Ti:sapphire, in compact, versatile, low-cost devices. In this paper, we discuss the prospects for future ultrafast fiber lasers built on new kinds of pulse generation that capitalize on nonlinear dynamics. We focus primarily on three promising directions: mode-locked oscillators that use nonlinearity to enhance performance; systems that use nonlinear pulse propagation to achieve ultrashort pulses without a mode-locked oscillator; and multimode fiber lasers that exploit nonlinearities in space and time to obtain unparalleled control over an electric field.

Limits of femtosecond fiber amplification by parabolic pre-shaping

We explore parabolic pre-shaping as a means of generating and amplifying ultrashort pulses. We develop a theoretical framework for modeling the technique and use its conclusions to design a femtosecond fiber amplifier. Starting from 9 ps pulses, we obtain 4.3 μJ, nearly transform-limited pulses 275 fs in duration, simultaneously achieving over 40 dB gain and 33-fold compression. Finally, we show that this amplification scheme is limited by Raman scattering, and outline a method by which the pulse duration and energy may be further improved and tailored for a given application.

Self-similar pulse evolution in a fiber laser with a comb-like dispersion-decreasing fiber.

We demonstrate an erbium fiber laser with self-similar pulse evolution inside a comb-like dispersion-decreasing fiber. We show numerically and experimentally that the comb-like dispersion-decreasing fiber works as well as an ideal one, and offers major practical advantages. The existence of a nonlinear attractor is verified by the invariant pulse chirp over a wide range of net cavity dispersion in experiments. The laser generates 1.3 nJ pulses with parabolic shapes and linear chirps, which can be dechirped to 37 fs. Comb-like dispersion-decreasing fiber should enable the generation of high-energy few-cycle pulses directly from a fiber oscillator.

Normal-dispersion fiber optical parametric chirped-pulse amplification

We demonstrate normal-dispersion fiber optical parametric chirped-pulse amplification pumped with high-power, chirped pulses and seeded with continuous-wave light. We generate 47 nJ, 1310 nm idlers compressible to 210 fs, and anticipate further energy scaling.

13-MW 140-fs pulses from a gain-switched diode seeded fiber amplifier (Conference Presentation)

Mode-locked oscillators are fantastic scientific instruments. Today, femtosecond pulses are now in demand for applications outside physics and chemistry laboratories, most notably for precision materials processing and nonlinear optical microscopy in biomedical laboratories and clinics. These applications require short pulses with high peak power and excellent beam quality. However, they also require devices that are cheap, reliable, and – crucially -- flexible. For example, synchronization of pulses with scanning optics is crucial for high efficiency machining and imaging alike. Lasers in materials processing are often required to exhibit pulse-on-demand operation in order to deposit energy only in specific locations. In biology, such behavior would enable region-of-interest monitoring, allowing e.g. ultrahigh resolution monitoring of a neuronal circuit’s dynamics. This kind of behavior is not compatible with the regular pulse train of a mode-locked oscillator. To this end, we seed an ultrafast fiber amplifier with a gain-switched diode. This a robust, integrated device that can be electronically triggered at virtually any repetition rate, or on-demand, yielding partially-coherent, ~10 ps duration pulses. Remarkably, a single Mamyshev regenerator (nonlinear spectral broadening in fiber followed by offset spectral filtering) allows isolation of a coherent component of the diode pulse. After subsequent nonlinear pulse shaping, amplification to the µJ-regime, and linear compression, we obtain transform-limited ~140-fs pulses with 13 MW peak power. As will be discussed, such a source cannot replace all mode-locked oscillators (e.g., for frequency comb applications), but is extremely attractive for the key mainstream applications of ultrashort pulses.

Energy-scalable, 150-fs fiber source seeded by a gain-switched diode

We generate 150-fs pulses from a fiber system based on a gain-switched diode. A combination of nonlinear spectral-temporal filtering and amplification by parabolic pre-shaping compresses 14-ps pulses by nearly 100-fold with transform-limited pulse quality.