On combining the beam path of similar wavelength lasers for dual-laser Thomson scattering

A novel method for the path combination of two lasers with very similar wavelengths has been implemented to enable the evaluation of a dual-laser calibration Thomson scattering (TS) technique. The first experimental test of this TS technique has been performed in a RFX-mod plasma device, where, due to experimental constraints, the combination of a Nd:YAG (λ = 1064 nm) and a Nd:YLF (λ = 1053 nm) lasersystem was the only viable choice available. The method requires that the beam path of both lasers be combined into a single path with the same polarization. This presents a unique challenge due to the small difference between the two laser wavelengths. In this paper, we describe two methods for beam combination: first via a prism, eventually dismissed as unpractical, and second via a polarizing beam splitter in reverse with a dual-wavelength waveplate. We detail the optical setup, waveplate design, and successful implementation of this second method.