Ka Fai Mak

Tunable vacuum-UV to visible ultrafast pulse source based on gas-filled Kagome-PCF.

An efficient and tunable 176-550 nm source based on the emission of resonant dispersive radiation from ultrafast solitons at 800 nm is demonstrated in a gas-filled hollow-core photonic crystal fiber (PCF). By careful optimization and appropriate choice of gas, informed by detailed numerical simulations, we show that bright, high quality, localized bands of UV light (relative widths of a few percent) can be generated at all wavelengths across this range. Pulse energies of more than 75 nJ in the deep-UV, with relative bandwidths of ~3%, are generated from pump pulses of a few μJ. Excellent agreement is obtained between numerical and experimental results. The effects of positive and negative axial pressure gradients are also experimentally studied, and the coherence of the deep-UV dispersive wave radiation numerically investigated.

Multi-mW, few-cycle mid-infrared continuum spanning from 500 to 2250cm-1

The demand for and usage of broadband coherent mid-infrared sources, such as those provided by synchrotron facilities, are growing. Since most organic molecules exhibit characteristic vibrational modes in the wavelength range between 500 and 4000 cm−1, such broadband coherent sources enable micro- or even nano-spectroscopic applications at or below the diffraction limit with a high signal-to-noise ratio1, 2, 3. These techniques have been applied in diverse fields ranging from life sciences, material analysis, and time-resolved spectroscopy. Here we demonstrate a broadband, coherent and intrinsically carrier-envelope-phase-stable source with a spectrum spanning from 500 to 2250 cm−1 (−30 dB) at an average power of 24 mW and a repetition rate of 77 MHz. This performance is enabled by the first mode-locked thin-disk oscillator operating at 2 μm wavelength, providing a tenfold increase in average power over femtosecond oscillators previously demonstrated in this wavelength range4. Multi-octave spectral coverage from this compact and power-scalable system opens up a range of time- and frequency-domain spectroscopic applications.

Modulation Instability in Xenon-Filled Hollow-Core Photonic Crystal Fiber

Abstract: We experimentally access the modulation instability regime in xenon-filled kagome PCF. Soliton orders ~100 are obtained with few-μJ, 490 fs pulses at 800 nm. Numerical simulations confirm pulse breakup into ultrashort solitons.

Kerr-lens mode-locked Ho:YAG thin-disk oscillator at 2.1 μm

High power ultrafast laser sources at 2 μm are highly desirable as primary sources in many fields such as mid-IR generation, remote sensing, Lidar systems, and medical use. Ho:YAG is an excellent gain crystal for such kind of sources due to its low quantum defect, good crystal quality and broad emission bandwidth. To date, Ho:YAG has been utilized in q-switched and actively mode-locked systems [1, 2]. Very recently passive mode locking has also been demonstrated using semiconductor saturable absorber mirrors (SESAMs) [3, 4]. However, the output power and pulse duration were limited to only several hundred milliwatts and few picoseconds, respectively. Since the invention of thin disk technology, great progress has been made in power and energy scaling of thin disk lasers mode-locked by both SESAM and Kerr-lens mode locking (KLM) [5, 6]. However, KLM shows great advantages compared to SESAM mode-locking in generating shorter pulses with high power due to its fast response time, broad bandwidth operation and higher damage threshold. Here we present a KLM Ho:YAG thin disk oscillator working at 2.1 μm for the first time. It delivers 220 fs pulses with average power up to 20 W, which is, to the best of our knowledge, the shortest pulse duration ever obtained in a Ho:YAG oscillator and highest average power of any mode locked ultrafast oscillator in 2 μm range.

7-W, 2-cycle self-compressed pulses at 2.1 micron from a Ho:YAG thin disk laser oscillator

High peak power, few cycle pulses at 2 um are highly sought after for their ability to generate ultrabroad, passively CEP stabilized continua in the mid-infrared (IR) [1] using highly nonlinear non-oxide crystals, which promises numerous applications in physics, environmental and life sciences. Recently, a Kerr-lens mode-locked Ho:YAG thin disk (based on TRUMPF technology) oscillator has directly generated high power femtosecond pulses at 2.1 micron [2], and the output can be further compressed for applications requiring few-cycle pulses. Soliton self-compression is an elegant scheme that can deliver such pulses directly from the fiber, and has previously been applied to pulse compression of a Yb:YAG thin-disk oscillator [3] and to ∼2 micron fiber sources [4,5]. Here we demonstrate the compression of 260 fs pulses down to 15 fs using a silica glass fiber, delivering, to the best of our knowledge, the shortest pulses from any holmium or thulium-based laser system.

Vacuum-UV dispersive wave emission using gas-filled hollow-core PCF

Vacuum-UV radiation between 145-155 nm is generated from 40 fs, 800 nm 6.8 μJ pump pulses in a 34 μm core-diameter kagomé-photonic crystal fibre (PCF) filled with 20-25 bar neon. Simulations confirm the mechanism as resonant dispersive-wave emission.

Tunable sources from the visible to vacuum-UV based on gas-filled hollow-core photonic crystal fibers

High-energy, single-mode, coherent, ultrafast pulses of light - tunable from the vacuum-UV to the visible spectral region - can be generated in gas-filled hollow-core photonic-crystal fibers through a simple experimental scheme.

Efficient Broadband Vacuum-Ultraviolet Generation in Gas-Filled Hollow-Core Photonic Crystal Fibers

We report two techniques for the efficient generation of tunable ultrafast pulses in the vacuum-ultraviolet, covering at least 117-200 nm, by pumping gas-filled kagome-style photonic crystal fibers with few-µJ, 35 fs, 800 nm laser pulses.

Compression of µJ-level pulses from 250 fs to sub-10 fs at 38 MHz repetition rate using two gas-filled hollow-core kagomé-PCF stages

Compression of thin-disk oscillator µJ-level pulses from 250 fs to sub-10 fs using a two-stage gas-filled hollow-core kagome-PCF setup is demonstrated with 18 W average output power. Further compression to sub-5 fs is predicted.

Frequency conversion and compression of ultrashort pulses in gas-filled hollow-core photonic crystal fibres

By combining the anomalous dispersion of hollow-core photonic-crystal-fibres with the Kerr and photo-ionization nonlinearities of a filling gas, many remarkable soliton dynamics can be accessed, including pressure tunable wide-band frequency conversion, and few-cycle pulse compression.