Lewis-Brønsted Acid Pairs in Ga/H-ZSM-5 To Catalyze Dehydrogenation of Light Alkanes.

The active sites for propane dehydrogenation in Ga/H-ZSM-5 with moderate concentrations of tetrahedral aluminum in the lattice were identified to be Lewis-Brønsted acid pairs. With increasing availability, Ga+ and Brønsted acid site concentrations changed inversely, as protons of Brønsted acid sites were exchanged with Ga+. At a Ga/Al ratio of 1/2, the rate of propane dehydrogenation was 2 orders of magnitude higher than with the parent H-ZSM-5, highlighting the extraordinary activity of the Lewis-Brønsted acid pairs. Density functional theory calculations relate the high activity to a bifunctional mechanism that proceeds via heterolytic activation of the propane C-H bond followed by a monomolecular elimination of H2 and desorption of propene.

Fiber-based optical parametric oscillator for high resolution coherent anti-Stokes Raman scattering (CARS) microscopy.

Imaging based on coherent anti-Stokes Raman scattering (CARS) relies on the interaction of high peak-power, synchronized picosecond pulses with narrow bandwidths and a well-defined frequency difference. Recently a new type of fiber-based CARS laser source based on four-wave-mixing (FWM) has been developed. In order to enhance its spectral resolution and efficiency, a FWM based fiber optical parametric oscillator (FOPO) is proposed in this work. The source delivers 180 mW with 5.6 kW peak power for the CARS pump and 130 mW with 2.9 kW peak power for the Stokes signal. CARS resonances around 2850 and 2930 cm(-1) can be resolved with a resolution of 1 cm(-1) enabling high-contrast, spectrally resolved CARS imaging of biological tissue.

Expanding multimodal microscopy by high spectral resolution coherent anti-Stokes Raman scattering imaging for clinical disease diagnostics.

Over the past years fast label-free nonlinear imaging modalities providing molecular contrast of endogenous disease markers with subcellular spatial resolution have been emerged. However, applications of these imaging modalities in clinical settings are still at the very beginning. This is because single nonlinear imaging modalities such as second-harmonic generation (SHG) and two-photon excited fluorescence (TPEF) have only limited value for diagnosing diseases due to the small number of endogenous markers. Coherent anti-Stokes Raman scattering (CARS) microscopy on the other hand can potentially be added to SHG and TPEF to visualize a much broader range of marker molecules. However, CARS requires a second synchronized laser source and the detection of a certain wavenumber range of the vibrational spectrum to differentiate multiple molecules, which results in increased experimental complexity and often inefficient excitation of SHG and TPEF signals. Here we report the application of a novel near-infrared (NIR) fiber laser of 1 MHz repetition rate, 65 ps pulse duration, and 1 cm(-1) spectral resolution to realize an efficient but experimentally simple SGH/TPEF/multiplex CARS multimodal imaging approach for a label-free characterization of composition of complex tissue samples. This is demonstrated for arterial tissue specimens demonstrating differentiation of elastic fibers, triglycerides, collagen, myelin, cellular cytoplasm, and lipid droplets by analyzing the CARS spectra within the C-H stretching region only. A novel image analysis approach for multispectral CARS data based on colocalization allows correlating spectrally distinct pixels to morphologic structures. Transfer of this highly precise but compact and simple to use imaging approach into clinical settings is expected in the near future.

A compact microscope setup for multimodal nonlinear imaging in clinics and its application to disease diagnostics.

The past years have seen increasing interest in nonlinear optical microscopic imaging approaches for the investigation of diseases due to the method's unique capabilities of deep tissue penetration, 3D sectioning and molecular contrast. Its application in clinical routine diagnostics, however, is hampered by large and costly equipment requiring trained staff and regular maintenance, hence it has not yet matured to a reliable tool for application in clinics. In this contribution implementing a novel compact fiber laser system into a tailored designed laser scanning microscope results in a small footprint easy to use multimodal imaging platform enabling simultaneously highly efficient generation and acquisition of second harmonic generation (SHG), two-photon excited fluorescence (TPEF) as well as coherent anti-Stokes Raman scattering (CARS) signals with optimized CARS contrast for lipid imaging for label-free investigation of tissue samples. The instrument combining a laser source and a microscope features a unique combination of the highest NIR transmission and a fourfold enlarged field of view suited for investigating large tissue specimens. Despite its small size and turnkey operation rendering daily alignment dispensable the system provides the highest flexibility, an imaging speed of 1 megapixel per second and diffraction limited spatial resolution. This is illustrated by imaging samples of squamous cell carcinoma of the head and neck (HNSCC) and an animal model of atherosclerosis allowing for a complete characterization of the tissue composition and morphology, i.e. the tissue's morphochemistry. Highly valuable information for clinical diagnostics, e.g. monitoring the disease progression at the cellular level with molecular specificity, can be retrieved. Future combination with microscopic probes for in vivo imaging or even implementation in endoscopes will allow for in vivo grading of HNSCC and characterization of plaque deposits towards the detection of high risk plaques.

Analysis of passively combined divided-pulse amplification as an energy-scaling concept.

The energy scaling of ultrashort-pulse systems employing simultaneously the techniques of chirped-pulse amplification and passively combined divided-pulse amplification is analyzed both experimentally and numerically. The maximum achievable efficiency is investigated and fundamental limitations originating from gain saturation, self-phase modulation and depolarization are discussed. A solution to these limitations could be an active stabilization scheme, which would allow for the operation of every single fiber amplifier at higher pulse energies.

Widely tuneable fiber optical parametric amplifier for coherent anti-Stokes Raman scattering microscopy.

We present a narrow-bandwidth, widely tunable fiber laser source for coherent anti-Stokes Raman scattering (CARS) spectro-microscopy. The required, synchronized, two-color pulse trains are generated by optical-parametric amplification in a photonic-crystal fiber (PCF). The four-wave-mixing process in the PCF is pumped by a 140ps, alignment-free fiber laser system, and it is seeded by a tunable continuous-wave laser; hence, a high spectral resolution of up to 1cm(-1) is obtained in the CARS process. Since the PCF is pumped close to its zero-dispersion wavelength, a broad parametric gain can be accessed, resulting in a large tuning range for the generated signal and idler wavelengths. CARS spectroscopy and microscopy is demonstrated, probing different molecular vibrational modes within the accessible region between 1200cm(-1) and 3800cm(-1).

Alignment-free, all-spliced fiber laser source for CARS microscopy based on four-wave-mixing.

An environmentally-stable low-repetition rate fiber oscillator is developed to produce narrow-bandwidth pulses with several tens of picoseconds duration. Based on this oscillator an alignment-free all-fiber laser for multi-photon microscopy is realized using in-fiber frequency conversion based on four-wave-mixing. Both pump and Stokes pulses for coherent anti-Stokes Raman scattering (CARS) microscopy are readily available from one fiber end, intrinsically overlapped in space and time, which drastically simplifies the experimental handling for the user. The complete laser setup is mounted on a home-built laser scanning microscope with small footprint. High-quality multimodal microscope images of biological tissue are presented probing the CH-stretching resonance of lipids at an anti-Stokes Raman-shift of 2845 cm(-1) and second-harmonic generation of collagen. Due to its simplicity, compactness, maintenance-free operation, and ease-of-use the presented low-cost laser is an ideal source for bio-medical applications outside laser laboratories and in particular inside clinics.

Fiber-based source for multiplex-CARS microscopy based on degenerate four-wave mixing.

We present a fiber-based laser source for multiplex coherent anti-Stokes Raman scattering (CARS) microscopy. This source is very compact and potentially alignment-free. The corresponding pump and Stokes pulses for the CARS process are generated by degenerate four-wave mixing (FWM) in photonic-crystal fibers. In addition, an ytterbium-doped fiber laser emitting spectrally narrow 100 ps pulses at 1035 nm wavelength serves as pump for the FWM frequency conversion. The FWM process delivers narrow-band pulses at 648 nm and drives a continuum-like spectrum ranging from 700 to 820 nm. With the presented source vibrational resonances with energies between 1200 cm-1 and 3200 cm-1 can be accessed with a resolution of 10 cm-1. Additionally, the temporal characteristics of the FWM output have been investigated by a cross-correlation setup, revealing the suitability of the emitted pulses for CARS microscopy. This work marks a significant step towards a simple and powerful all-fiber, maintenance-free multiplex-CARS source for real-world applications outside a laboratory environment.

66 W average power from a microjoule-class sub-100 fs fiber oscillator.

Performance scaling of passively mode-locked ultrashort-pulse fiber oscillators in terms of average power, peak power, and pulse energy is demonstrated. A very-large-mode-area fiber laser in an all-positive group-velocity-dispersion ring cavity configuration with intracavity spectral filter, mode-locked by nonlinear polarization evolution, emits 66 W of average power at 76 MHz repetition rate, corresponding to 0.9 µJ pulse energy. The pulses are dechirped to 91 fs outside the cavity with an average power of 60 W remaining after the compressor. The generated pulse peak power is as high as 7 MW.

All-fiber laser source for CARS microscopy based on fiber optical parametric frequency conversion.

A novel approach for an all-fiber mono-laser source for CARS microscopy is presented. An Yb-fiber laser generates 100 ps pulses, which later undergo narrowband in-fiber frequency conversion based on degenerate four-wave-mixing. The frequency conversion is optimized to access frequency shifts between 900 and 3200cm-1, relevant for vibrational imaging. Inherently synchronized pump and Stokes pulses are available at one fiber end, readily overlapped in space and time. The source is applied to CARS spectroscopy and microscopy experiments in the CH-stretching region around 3000cm-1. Due to its simplicity and maintenance-free operation, the laser scheme holds great potential for bio-medical applications outside laser laboratories.

High average and peak power femtosecond large-pitch photonic-crystal-fiber laser.

We report on the generation of high-average-power and high-peak-power ultrashort pulses from a mode-locked fiber laser operating in the all-normal-dispersion regime. As gain medium, a large-mode-area ytterbium-doped large-pitch photonic-crystal fiber is used. The self-starting fiber laser delivers 27 W of average power at 50.57 MHz repetition rate, resulting in 534 nJ of pulse energy. The laser produces positively chirped 2 ps output pulses, which are compressed down to sub-100 fs, leading to pulse peak powers as high as 3.2 MW.

High-energy femtosecond photonic crystal fiber laser.

We report the generation of high-energy high-peak power pulses in an all-normal dispersion fiber laser featuring large-mode-area photonic crystal fibers. The self-starting chirped-pulse fiber oscillator delivers 11 W of average power at 15.5 MHz repetition rate, resulting in 710 nJ of pulse energy. The output pulses are dechirped outside the cavity from 7 ps to nearly transform-limited duration of 300 fs, leading to pulse peak powers as high as 1.9 MW. Numerical simulations reveal that pulse shaping is dominated by the amplitude modulation and spectral filtering provided by a resonant semiconductor saturable absorber.

Sub-80 fs dissipative soliton large-mode-area fiber laser.

We report on high-energy ultrashort pulse generation from an all-normal-dispersion large-mode-area fiber laser by exploiting an efficient combination of nonlinear polarization evolution (NPE) and a semiconductor-based saturable absorber mode-locking mechanism. The watt-level laser directly emits chirped pulses with a duration of 1 ps and 163 nJ of pulse energy. These can be compressed to 77 fs, generating megawatt-level peak power. Intracavity dynamics are discussed by numerical simulation, and the intracavity pulse evolution reveals that NPE plays a key role in pulse shaping.

The influence of index-depressions in core-pumped Yb-doped large pitch fibers.

Rare-earth doped photonic crystal fibers rely ideally on an index matching of the doped core to the surrounding glass to work properly. Obtaining a perfect index matching is technologically very challenging, and fiber manufacturers opt for targeting an index depression instead, which still ensures the influence of the photonic structure on the light propagation. In this paper the analysis of the influence of this core index depression on the higher-order mode discrimination and on the beam quality of the fundamental mode of different designs of core-pumped active large pitch photonic crystal fibers is discussed. The most promising design is evaluated in terms of mode area scaling with a view to mode field diameters above 100 µm. Detailed requirements on the accuracy of the core index matching are deduced.

Approaching microjoule-level pulse energy with mode-locked femtosecond fiber lasers.

We report on the generation of high-energy ultrashort pulses from a mode-locked Yb-doped large-mode-area fiber laser operating in the all-normal dispersion regime. The self-starting fiber laser emits 9 W of average output power at a pulse repetition rate of 9.7 MHz, corresponding to a pulse energy of 927 nJ. The laser produces positively chirped 8 ps output pulses, which are then compressed down to 711 fs. These compressed pulses exhibit megawatt-level peak powers. To our knowledge, this is the first time that a mode-locked fiber oscillator has generated femtosecond pulses with pulse energies approaching the microjoule level in combination with high average output power. Numerical simulations show excellent agreement with experimental results and reveal further scaling potential, which is discussed.