β-BaB₂O₄ deep UV monolithic walk-off compensating tandem

The generation of watt-level cw narrow-linewidth sources at specific deep-UV wavelengths corresponding to atomic cooling transitions usually employs external cavity-enhanced second-harmonic generation (SHG) of moderate-power visible lasers in birefringent materials. Among the oxo-borate materials, barium borate (β-BaB₂O₄ or BBO) combines the highest UV band edge and largest nonlinearity but suffers from large walk-off angles that limits the nonlinear interaction length. Alternative quasi-phase-matched (QPM) ferroelectrics are hardly suited for cavity-enhanced operation due to their much larger UV absorption and associated photo-refractive and thermal lensing effects, in addition to the difficult fabrication of fine-pitch domain gratings for short UV coherence lengths. In this work, we investigate an alternative approach to cw deep-UV generation by employing the low-loss BBO in a monolithic walk-off compensating structure [J.-J. Zondy, Ch. Bonnin, D. Lupinski, J. Opt. Soc. Am. B 20 (2003) 1675] to simultaneously enhance the effective nonlinear coefficient while minimizing the UV beam ellipticity under tight focusing. As a preliminary step to cavity-enhanced operation, and in order to apprehend the design difficulties stemming from the extremely low acceptance angle of BBO, we investigate and analyze the single-pass performance of a L_c = 8 mm monolithic walk-off compensating structure made of 2 optically-contacted BBO plates cut for type-I critically phase-matched SHG of a cw λ = 570.4 nm dye laser. As compared with a bulk crystal of identical length, a sharp UV efficiency enhancement factor of 1.65 has been evidenced with the tandem structure, but at ∼-1 nm from the targeted fundamental wavelength, highlighting the sensitivity of this technique when applied to a highly birefringent material such as BBO. Solutions to angle cut residual errors are identified so as to match accurately more complex periodic-tandem structure performance to any target UV wavelength, opening the prospect for high-power, good beam quality deep-UV cw laser sources for atom cooling and trapping.