Measurement of the Raman gain coefficient via inverse Raman scattering
Spotlight summary: Sir C.V. Raman could not have imagined the extent to which his namesake’s effect has impacted so many fields of optics. Raman scattering holds a vital place in an amazingly diverse range of technologies from internet communications to cancer diagnosis to stand-off detection of explosives. It is indeed a gift to optics that just keeps on giving. However, being a rather weak optical effect, accurately quantifying its strength in a sample material is challenging. Large probe intensities, long interactions lengths or highly sensitive photon detection are techniques often used to gather signals. The standard technique for measuring the Raman gain coefficient or Raman scattering cross-section is through measurement of the conversion rate of a pump beam into light at the first Stokes-shifted frequency. The work by Schneebeli and co-authors examines an alternative approach based on measurements of the loss of a probe beam at the anti-Stokes frequency, a process referred to as inverse Raman scattering (IRS).
Although the coupled equations that describe IRS are analogous to those for stimulated Raman scattering, the procedure is shown to have some interesting practical benefits. Since the measurement principle is based on the loss of optical power of a probe beam, the technique avoids the threshold requirement often integral to measurements based on stimulated Raman scattering; signal loss is also a benefit in keeping the probe signal small along the entire interaction length, thus helping reduce complications in the analysis due to other nonlinear effects such as self-phase modulation. The procedure avoids problems that might be introduced by background fluorescence since measurements are made at a higher frequency than the pump; furthermore, when using broadband probes, the resulting loss spectrum provides detailed information on the Raman response function. All these properties make the method highly adaptable to a wide range of samples of widely varying Raman scattering cross-section.
The paper describes a measurement procedure to extract the Raman gain coefficient and explores its merits using test samples such as silica optical fibers and several liquid samples placed in hollow core fibers. Their experimental setup employs a modelocked erbium fiber laser at 1.5 microns as the pump laser. The anti-Stokes probe pulses were generated splitting a portion of the beam from the pump laser and using it to generate supercontinuum radiation in a piece of nonlinear fiber. The effects of dispersion and Kerr nonlinearities are taken into consideration using a numerical model and the results verified against values obtained by conventional methods. The end result is a novel fiber-based system for determining Raman gain coefficients. The approach presented in this paper may in the future enable easier characterization of materials important in the many fields that exploit Raman scattering.
Technical Division: Light–Matter Interactions
ToC Category: Nonlinear Optics
|OCIS Codes:||(060.4370) Fiber optics and optical communications : Nonlinear optics, fibers|
|(160.4330) Materials : Nonlinear optical materials|
|(190.5650) Nonlinear optics : Raman effect|
|(300.6450) Spectroscopy : Spectroscopy, Raman|
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