Light sources with a broad spectral output and diffraction limited beam quality have a wide variety of present and future applications. A few of particular interest are hyperspectral laser radar for environmental monitoring, active hyperspectral imaging for detection and identification of objects, and speckle-free illumination. With the exception of systems based on amplified femto- or picosecond lasers, which are large and extremely complicated, pulse energies from supercontinuum laser sources have been limited to <10 microJoules which is generally not sufficient for the applications listed above. We present a simple technique to generate broadband light spanning several hundred nanometers in the near infrared with pulse energies of ~1 mJ, an improvement of approximately two orders of magnitude. The system is comprised of a Q-switched Nd:YAG laser and a very large mode area photonic crystal fiber. A combination of cascaded stimulated Raman scattering, four wave mixing, and self-phase modulation is responsible for the spectral broadening. Possibilities of scaling the output to the ~10 mJ level as well as extending the spectral coverage to the visible and mid-infrared will also be discussed.
Noncritical temperature-tuned phase-matching and large nonlinear coefficients make potassium niobate an attractive material for frequency doubling tuneable near-infrared radiation. We have mounted a KNbO3 crystal intracavity in an argon ion pumped, continuous wave Ti:Sapphire ring laser to increase the power level of the second harmonic. Wavelength selection at the fundamental frequency is accomplished with a birefringent filter. By using the crystal orientation that defines the d32 coefficient of KNbO3 we have obtained a blue second harmonic output tuneable from 425-445 nm. The laser is also characterized by the narrow linewidth of the Ti:Sapphire ring oscillator and good temporal stability. A continuous wave, frequency doubled Ti:sapphire laser is well suited to excite the resonance Raman spectrum in heme proteins with strong absorption bands in the range of 400 to 450 nm. We demonstrate the feasibility of such a setup for Raman studies of ligand binding to myoglobin. The Raman bands yield information on the reaction dynamics and on conformational changes near the linkage between the heme and the protein. In particular, a shift of the stretch frequency of the iron- histidine bond with high pressure may be attributed to a protein conformational change.
Fluorophospates and fluorovanadates are efficient solid state laser materials with promising new applications, especially for diode pumping of microchip lasers. High quality, low loss Nd doped Sr5(PO4)3F, Ca5(PO4)3F, Sr5(VO4)3F and Ca5(VO4)3F crystals were grown by the Czochralski technique. The lattice vibrations of the host material are studied by polarized Raman spectroscopy. The Raman active modes are determined by the irreducible representations of the space group of the crystal, and they provide insight to the crystal symmetry and chemical bonding. In all four compounds the internal and external vibrations are separated in the Raman spectra indicating that the internal binding of the (PO4)-3 and (VO4)-3 tetrahedra is different from the other ionic forces in the crystals. The group of internal modes in the fluorovanadates is shifted to lower frequencies by more than 100 cm-1 compared with the fluorophosphates, though the x-ray structures are isomorphous. In addition there are remarkable changes in intensity, which may be attributed to interaction among the tetrahedral subunits in the hexagonal unit cell. The substitution of Sr for Ca causes frequency shifts in the vibrational modes. These are discussed with respect to the mass effect and changes in the coupling strength due to the differences between alkaline-earth cation binding and oxygen binding in the tetrahedral subunits. A comparison of the various substitutions makes assignments of the vibrational modes possible.
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