Fig. 1. (a) Exaggerated view of a fan-out QPM grating; (b) schematic setup of broadband SHG in spatially chirped QPM, which consists of two diffraction gratings (G₁, G₂) and two concave mirrors (M₁, M₂) arranged in a zero-dispersion 4f configuration; f is the focal length of the concave mirrors.

Fig. 2. (a) Spatial-spectral and (b) spatiotemporal intensity profiles of the 1ω laser in the Fourier-plane; (c) spatial-spectral and (d) spatiotemporal intensity profiles of the 2ω laser in the Fourier-plane calculated under the assumption of perfect phase matching.

Fig. 3. (a) Local acceptance bandwidth of a 1 mm-long PPLN with a poling period of Λ₀ = 2.81 µm; (b) optimum space-dependent poling period to match the spatial chirp of the 1ω beam (black line), in comparison with its linear approximation (red line).

Fig. 4. (a) Characterization of the spectral amplitude modulation arising from broadband frequency conversion using a multi-period PPLN; (b) second-harmonic spectra output from the PPLNs with the poling periods of Λ₁ = 2.80 µm (magenta), Λ₂ = 2.81 µm (blue), and Λ₃ = 2.82 µm (green), respectively; (c) second-harmonic spectrum output from a multi-period PPLN with gratings of the three different periods calculated in (b).

Fig. 5. (a) Conversion efficiency curve calculated for the broadband SHG in a spatially chirped QPM; (b), (c), (d) spectral, temporal, and spatial intensity profiles of the 2ω pulse right at the output plane of the fan-out PPLN; (e), (f), (g) spectral, temporal, and spatial profiles of the 2ω pulse right at the output of the second diffraction grating, G₂.

Fig. 6. (a) Optimum space-dependent poling period Λ_opt (black line) calculated for the spatially chirped 1ω wave according to Eq. (16), in comparison with the local poling period Λ (red line) of the fan-out PPLN; (b) spectral intensity (black line) and wavelength-dependent phase mismatch (red line) of the 2ω pulse; (c) temporal intensity of the 2ω pulse corresponding to the 2ω spectrum shown in (b).