Abstract

Raman wavelength converters based on hollow core microstructured fibres filled with gases or liquids have been widely studied in the last few decades [1,2]. In these converters the wavelength of a laser pump beam is red-shifted by stimulated Raman scattering in the material filling the core of the fibre, giving rise to new discrete wavelengths that can be used in many domains of applications (biophotonics, environment ...). The possibility to strongly confine the pump light over long distances is an asset to get a high conversion efficiency. However if no particular care is taken unwanted non linear effects can occur at the expense of the stimulated forward Raman scattering. These effects are the Raman and Brillouin back-scatterings. In this work we are interested in the optimization of liquid-core Raman converters operating with pulses of a few nanoseconds or less. In these temporal regimes competition between the abovementioned effects are known to be very strong. We validate here a solution to minimize Raman and Brillouin back-scatterings by an appropriate choice of the fibre length. The effective length of interaction L_eff in the backward direction is estimated to be cΔt / 2n, where Δt is the pulse width duration and n the refractive index. In the forward direction, as the dispersion is negligible, this effective length is the length of the fibre L. In liquids, as backward and forward Raman gains are nearly the same, L has just to be slightly higher than L_eff to avoid Raman back-scattering. In most liquids and for long pulses Brillouin gain is much higher than Raman gain (typically one order of magnitude). To decrease Brillouin scattering L has to be subsequently higher than L_eff. We demonstrate experimentally these trends by building two Raman converters differing only by their lengths. Our initial aim was to realize a Raman converter emitting in the near IR range. The pump source is a microlaser at 532 nm delivering 900 ps pulses at a repetition rate of 4.5 kHz. We choose ethanol as the Raman liquid. When pumped at 532 nm, the first Stokes order of ethanol is at 630 nm and the second Stokes order is at 772 nm, i.e the near IR wavelength we want to generate. A Kagome fibre was used to realize the two converters. When empty this fibre guides in the near IR range over a 1 µm wide transmission band. When filled with ethanol this transmission band is blue-shifted and then covers the pump and the Stokes wavelengths. In our configuration L_eff≈0.1 m. The lengths of the two converters are 0.5 m and 1.5 m. The results are shown in figure 1. As expected we observed no Raman backscattering in both cases and Brillouin effect is strongly attenuated for the longer fibre. For instance at the critical point (when the output Raman Stokes energy equals the transmitted pump energy), the Brillouin energy is 66% of the forward Raman energy for the 0.5 m fibre and is only 9% for the 1.5 m fibre. As a consequence, the output energy at 772 nm is highly improved and we obtain an output energy of 0.9 µJ at 772 nm for an injected energy of 5.8 µJ.

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