RESUMO
Understanding of pump-induced lensing in laser amplifiers is essential for the optimized design of high-power lasers with high spatial quality, but there is usually incomplete knowledge of the interplay between thermal and population induced lensing mechanisms, lensing under lasing and non-lasing conditions, and transient lensing effects under pulsed operation. This paper provides quantitative insight of pump-induced lensing effects by using experimental transient pump-probe measurements in an alexandrite laser amplifier end-pumped by a short pulse pump beam with Gaussian spatial intensity distribution. Lensing results are presented showing a large difference in lensing under both non-lasing and lasing conditions and distinction of the population lens and thermal lens contributions from their different response time. Different pump beam sizes are used to show the variation of the relative strength of the lensing mechanisms. Comparison of experimental results with the analytical transient theory developed in this paper for the Gaussian pump beam gives excellent agreement and quantitative information on the thermal and population contributions to the amplifier lens. This paper provides a methodology for quantitative investigation of pump-induced lensing in general laser amplifier systems, and potentially other classes of optical materials with residual optical absorption.
RESUMO
The availability of high-power and high-brightness blue diode lasers makes them attractive as low-cost pump sources for broadly tunable Alexandrite lasers. In this paper we investigate the performance of an Alexandrite laser pumped by a high-power fiber-delivered blue diode module. Output power 1.84 W is achieved, the highest power from blue diode pumped Alexandrite to date. Excellent pump absorption is demonstrated of scrambled pump polarization on both a-axis and b-axis of Alexandrite crystal. Wavelength tuning and dual wavelength operation is produced using the self-birefringent filtering of the Brewster-cut Alexandrite crystal. An analysis is made of laser efficiency and mode formation including the creation of higher-order Laguerre-Gaussian vortex modes (LG01 and LG02). Performance is compared to red diode pumping and prospects for further optimization and power-scaling are discussed.
RESUMO
The Raman scattering of light by molecular vibrations is a powerful technique to fingerprint molecules through their internal bonds and symmetries. Since Raman scattering is weak1, methods to enhance, direct and harness it are highly desirable, and this has been achieved using optical cavities2, waveguides3-6 and surface-enhanced Raman scattering (SERS)7-9. Although SERS offers dramatic enhancements2,6,10,11 by localizing light within vanishingly small hot-spots in metallic nanostructures, these tiny interaction volumes are only sensitive to a few molecules, yielding weak signals12. Here we show that SERS from 4-aminothiophenol molecules bonded to a plasmonic gap waveguide is directed into a single mode with >99% efficiency. Although sacrificing a confinement dimension, we find a SERS enhancement of ~103 times across a broad spectral range enabled by the waveguide's larger sensing volume and non-resonant waveguide mode. Remarkably, this waveguide SERS is bright enough to image Raman transport across the waveguides, highlighting the role of nanofocusing13-15 and the Purcell effect16. By analogy to the ß-factor from laser physics10,17-20, the near-unity Raman ß-factor we observe exposes the SERS technique to alternative routes for controlling Raman scattering. The ability of waveguide SERS to direct Raman scattering is relevant to Raman sensors based on integrated photonics7-9 with applications in gas sensing and biosensing.
