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.
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.
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.
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).
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.
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