Fig. 1. (a) Microscopic image of the end-facet of the Nd,Gd:CaF₂ sample with 16 cladding WGs deeply embedded beneath the surface. (b)–(e) The cross sections of WG1–WG4.

Fig. 2. (a) Mode intensity distributions from the fabricated cladding WGs at 633 nm under TM polarization. (b) Propagation losses of WGs obtained under both TM and TE polarizations.

Fig. 3. (a) The room temperature µ-PL emission spectra of Nd³⁺ ions obtained from the WG area (blue), the filament (green), and the bulk of Nd,Gd:CaF₂ crystals (red). The inset of (a) shows the cross section of WG1. The spatial two-dimensional (2D) distributions of (b) intensity, (c) peak shift, and (d) FWHM of 867 nm emission line obtained from the WG cross section. 1D distributions of (e) the emitted intensity, (f) energy shift, and (g) FWHM corresponding to the regions indicated by dashed lines in the inset of (a).

Fig. 4. cw WG laser output power as a function of the input pump power with (a) 9% and (b) 70% output coupler. (c) Spectrum of WG laser at 1059.91 nm. The inset shows the spatial intensity distribution of the output laser with a single-mode profile.

Fig. 5. Photographs of visible UC luminescence generated from (a) WG1 and (b) the bulk. UC spectra obtained from WG1 under (c) TE and (d) TM polarizations with varying excitation powers.

Fig. 6. (a) UC spectra from WG1 obtained with a rotating half-wave plate. Luminescence intensities of emission lines centered at (b) 485 nm and (c) 585 nm as a function of the angles of the polarizer. The intensity ratios of (d) I₄₈₅/I₅₈₅ and (e) I₅₈₅/I₄₈₅ as a function of the angles of the polarizer.