Compact chirped-pulse amplification systems based on highly Tm3+-doped germanate fiber.

We report the fabrication of a dual cladding large mode area thulium-doped germanate fiber (TDGF). The fiber has a core diameter of 20 µm, a high Tm3+ ion concentration of 3cm3×1020/cm3, and a hexagonal inner cladding to enhance pump absorption when cladding-pumped. Using a short fiber length, we demonstrate a compact 300 fs chirped-pulse amplification system operating at 1925 nm, investigating both core- and cladding-pumped implementations. By cladding pumping a 65 cm long fiber we produced an average power of 14.1 W (limited by thermally induced damage) and a peak power of 2.17 MW at a pulse repetition rate of 15.7 MHz. Core pumping a 19 cm length of TDGF produced 2.3 W of average-power and 16 MW peak-power pulses at 0.39 MHz. The performance is already comparable to the state-of-the-art success achieved with flexible silica fibers. Considering the rapid improvements in glass quality and the scope for further increasing the doping concentration, this fiber type holds great potential for pulsed fiber lasers in the 1.5-3 µm wavelength region.

Widely-tunable synchronisation-free picosecond laser source for multimodal CARS, SHG, and two-photon microscopy.

We demonstrate a continuous wave (CW) seeded synchronization-free optical parametric amplifier (OPA) pumped by a picosecond, 1 µm laser and show its performance when used as a simple yet powerful source for label-free coherent anti-Stokes Raman scattering (CARS), concurrent second harmonic generation (SHG), and two-photon fluorescence microscopy in an epi-detection geometry. The average power level of above 175 mW, spectral resolution of 8 cm-1, and 2 ps pulse duration are well optimized for CARS microscopy in bio-science and bio-medical imaging systems. Our OPA is a much simpler setup than either the "gold-standard" laser and optical parametric oscillator (OPO) combination traditionally used for CARS imaging, or the more recently developed OPA systems pumped with femtosecond pulses [1]. Rapid and accurate tuning between resonances was achieved by changing the poled channels and temperature of the periodically-poled lithium niobate (PPLN) OPA crystal together with the OPA seed wavelength. The Pump-Stokes frequency detuning range fully covered the C-H stretching band used for the imaging of lipids. By enabling three multiphoton techniques using a compact, synchronization free laser source, our work paves the way for the translation of label-free multi-photon microscopy imaging from biomedical research to an imaging based diagnostic tool for use in the healthcare arena.

Noise-related polarization dynamics for femto and picosecond pulses in normal dispersion fibers.

We report how the complex intra-pulse polarization dynamics of coherent optical wavebreaking and incoherent Raman amplification processes in all-normal dispersion (ANDi) fibers vary for femto and picosecond pump pulses. Using high temporal resolution vector supercontinuum simulations, we identify deterministic polarization dynamics caused by wavebreaking and self-phase modulation for femtosecond pulses and quasi-chaotic polarization evolution driven by Raman amplification of quantum noise for picosecond pulses. In contrast to cross-phase modulation instability, the Raman-based polarization noise has no power threshold and is reduced by aligning the higher energy polarization component with the lower index axis of the fiber. The degree of polarization stability is quantified using new time domain parameters that build on the spectrally averaged degree of coherence used in supercontinuum research to quantify the output spectral stability. We show that the spectral coherence is intrinsically linked to polarization noise, and that the noise will occur in both polarization maintaining (PM) and non-PM fibers, spanning a broad range of pulse energies, durations, and fiber birefringence values. This analysis provides an in-depth understanding of the nonlinear polarization dynamics associated with coherent and incoherent propagation in ANDi fibers.

Compact, high repetition rate, 4.2 MW peak power, 1925 nm, thulium-doped fiber chirped-pulse amplification system with dissipative soliton seed laser.

We report the demonstration of a high average- and peak-power, 1925 nm, thulium-fiber based chirped pulse amplification (CPA) system. A compact, dissipative soliton thulium-fiber, mode-locked seed produced pre-chirped pulses with 25 ps duration, 45 mW output power and repetition rate of 15.7 MHz. After stretching to 105 ps in 83 m of normal dispersion fiber, the pulses were amplified in a core-pumped pre-amplifier and a cladding pumped power amplifier to average output powers of 28 W and 30 W with forward and backward pumping, respectively, with the output power limited only by the available pump power. After a pair of fused silica transmission gratings with an efficiency of 71%, the amplified pulses were re-compressed to 297 fs yielding pulses with a peak power of 4.2 MW (backward pumped) and a pulse energy of 1.27 µJ.

Demonstration of arbitrary temporal shaping of picosecond pulses in a radially polarized Yb-fiber MOPA with > 10 W average power.

High precision surface processing has an unmet demand for picosecond pulses with arbitrary temporal profiles in radial polarization states and at high average powers. Here, simultaneous spatial and arbitrary temporal shaping of chirped 10 - 100 picoseconds pulses is demonstrated with an Yb-doped fiber laser system generating an output power of more than 10 W at 40 MHz repetition frequency. The closed-loop control algorithm carves the pulses using a commercial, rugged, and fiberized optical pulse shaper placed at the front end of the system and uses feedback from the output pulse shapes for optimization. Arbitrary complex temporal profiles were demonstrated using a dispersive Fourier transform based technique and limits set by the system were investigated. Pulse shaping in the spatial domain was accomplished using an S-waveplate, fabricated in-house, to change the linearly polarized fundamental mode into a doughnut mode with radial polarization. This was amplified in a final-stage few-mode large-mode area fiber amplifier. Placing both temporal and spatial shaping elements before the power-amplifier avoids complex and potentially lossy conversion of the spatial mode profile at the output and provides an efficient route for power-scaling. The use of properly oriented quarter- and half-wave plates, which have both low loss and high power handling capability, enabled the output to be set to pure radial or azimuthal polarization states. Using commercial off-the-shelf components, our technique is able to immediately enhance the versatility of ultrashort fiber laser systems for high precision material processing and other industrial applications.

Efficient high-harmonic generation from a stable and compact ultrafast Yb-fiber laser producing 100 µJ, 350 fs pulses based on bendable photonic crystal fiber.

The development of an Yb3+-fiber-based chirped-pulse amplification system and the performance in the generation of extreme ultraviolet (EUV) radiation by high-harmonic generation is reported. The fiber laser produced 100 µJ, 350 fs output pulses with diffraction-limited beam quality at a repetition rate of 16.7 kHz. The system used commercial single-mode, polarization maintaining fiber technology. This included a 40 µm core, easily packaged, bendable final amplifier fiber in order to enable a compact system, to reduce cost, and provide reliable and environmentally stable long-term performance. The system enabled the generation of 0.4 µW of EUV at wavelengths between 27 and 80 nm with a peak at ~45 nm using xenon gas. The EUV flux of ~1011 photons per second for a driving field power of 1.67 W represents state-of-the-art generation efficiency for single-fiber amplifier CPA systems, corresponding to a maximum calculated energy conversion efficiency of 2.4 × 10-7 from the infrared to the EUV. The potential for high average power operation at increased repetition rates and further suggested technical improvements are discussed. Future applications could include coherent diffractive imaging in the EUV, and high-harmonic spectroscopy.

Broadband telecom to mid-infrared supercontinuum generation in a dispersion-engineered silicon germanium waveguide.

We demonstrate broadband supercontinuum generation (SCG) in a dispersion-engineered silicon-germanium waveguide. The 3 cm long waveguide is pumped by femtosecond pulses at 2.4 µm, and the generated supercontinuum extends from 1.45 to 2.79 µm (at the -30 dB point). The broadening is mainly driven by the generation of a dispersive wave in the 1.5-1.8 µm region and soliton fission. The SCG was modeled numerically, and excellent agreement with the experimental results was obtained.

High-power, high repetition-rate, green-pumped, picosecond LBO optical parametric oscillator.

We report on a picosecond, green-pumped, lithium triborate optical parametric oscillator with record-high output power. It was synchronously pumped by a frequency-doubled (530 nm), pulse-compressed (4.4 ps), high-repetition-rate (230 MHz), fiber-amplified gain-switched laser diode. For a pump power of 17 W, a maximum signal and idler power of 3.7 W and 1.8 W was obtained from the optical parametric oscillator. A signal pulse duration of ~3.2 ps was measured and wide tunability from 651 nm to 1040 nm for the signal and from 1081 nm to 2851 nm for the idler was achieved.

Polarisation maintaining 100W Yb-fiber MOPA producing microJ pulses tunable in duration from 1 to 21 ps.

This paper demonstrates a single polarisation, 1.06 microm Yb-doped fiber MOPA, delivering 21 ps pulses in a diffraction limited beam at repetition rates of up to 908 MHz and average output power of 100 W. The maximum pulse energy was 1.7 microJ at a repetition rate of 56 MHz, with corresponding peak power of 85 kW. The 100 W power was limited by available diode pump power and scaling to higher power levels is discussed. We also report self-phase-modulation based pulse compression which produced pulse durations as short as 1.1 ps from an external grating compressor. Using 4.2 ps pulses at a repetition rate of 227 MHz enabled 26 W of visible laser power (50% SHG efficiency) to be demonstrated.

Picosecond fiber MOPA pumped supercontinuum source with 39 W output power.

We report a picosecond fiber MOPA pumped supercontinuum source with 39 W output, spanning at least 0.4-2.25 microm at a repetition rate of 114.8 MHz. The 2m long PCF had a large, 4.4 microm diameter core and a high-delta design which led to an 80% coupling efficiency, high damage threshold and rapid generation of visible continuum generation from the picosecond input pulses. The high and relatively uniform power density across the visible spectral region was approximately 31.7 mW/nm corresponding to peak power density of approximately 12.5 W/nm for the 21 ps input pulses. The peak power density was increased to 26.9 W/nm by reducing the repetition rate to 28 MHz. This represents an increase in both average and peak power compared to previously reported visible supercontinuum sources from either CW pumped or pulsed-systems.

Single-mode tellurite glass holey fiber with extremely large mode area for infrared nonlinear applications.

We report the fabrication of a large mode area tellurite holey fiber from an extruded preform, with a mode area of 3000microm(2). Robust single-mode guidance at 1.55microm was confirmed by both optical measurement and numerical simulation. The propagation loss was measured as 2.9dB/m at 1.55microm. A broad and flat supercontinuum from 0.9 to 2.5microm with 6mW output was obtained with a 9cm length of this fiber.

Ultrashort-pulse Yb3+-fiber-based laser and amplifier system producing >25-W average power.

We have demonstrated an ultrashort-pulse Yb3+-fiber laser and amplifier system that produces >400-nJ pulses at a repetition rate of 62 MHz (>25-W average power). The output pulses were recompressed to a duration of 110 fs with good pulse quality by use of a standard bulk grating-based compressor.