Fig. 4: Phase-matched CW harmonic generation in MNFs.
From: High-power continuous-wave optical waveguiding in a silica micro/nanofibre
a, b Experimentally measured THG (a) and SHG (b) spectra in a 779-nm-diameter, 7-cm-length MNF waveguiding a 5-W-power light. To search the optimal phase matching condition, the wavelength of the light is scanned from 1570 nm (1557 nm) to 1575 nm (1559 nm) for THG (SHG). Insets, optical microscope images of output spots of the TH and SH at the endface of the standard fibre connected with the MNF. c Calculated effective refractive indices (neff) of the fundamental (HE11) and high-order (HE12 and HE21) modes showing phase-matching condition at two intersection points with nearly the same diameter of ∼779 nm at wavelengths of λ1 = 1572.5 nm for THG (blue lines) and λ2 = 1558.2 nm for SHG (red lines), respectively. Inset, calculated field intensity profiles of phase-matched waveguiding modes for THG and SHG, respectively. d Dependence of the output power (PHG) of THG/SHG on the waveguided power (Pin). The fitting slopes for the THG and SHG are 3.11 ± 0.09 and 1.98 ± 0.06, respectively. ξ, conversion efficiency of harmonic generation
