## 2 Experimental Methods  
A YLF crystal doped with 0.5 at.% Pr\({}^{3+}\) ions was grown using the Czochralski method. Four Pr:YLF a-cut samples with dimensions (a \(\times\) a \(\times\) c) of 1 \(\times\) 14 \(\times\) 10 mm\({}^{3}\) were prepared from this crystal for ion implantation at Tandetron Laboratories in Rez, Czech Republic. YLF is a well-known fluoride crystal that is less susceptible to non-radiative multi-phonon decay and excited-state absorption (ESA) at 5d levels at both pump and laser wavelengths compared to oxide crystals. A beam of carbon ions (C\({}^{3+}\)) with high energies of 10 MeV was used to irradiate the surface of the crystals. The samples were tilted 7\({}^{\circ}\) from the normal direction to minimize the ion tunneling effect during implantation. The crystals were implanted with fluences (doses) of 2 \(\times\) 10\({}^{14}\), 6 \(\times\) 10\({}^{14}\), 2 \(\times\) 10\({}^{15}\), and 6 \(\times\) 10\({}^{15}\) ions/cm\({}^{2}\) and labeled Pr:YLF-1 to Pr:YLF-4 with an increasing dose. The effective refractive indices of guided modes of the formed planar waveguides were measured at a wavelength of 532 nm using a prism coupler (Metricon 2010/M) for both TM (\(\sigma\), ordinary wave, \(n_{0}\)) and TE (\(\pi\), extraordinary wave, \(n_{\mathrm{e}}\)) polarizations. Annealing treatment after ion implantation leads, on the one hand, to a decrease in optical propagation losses through the reduction in ion-induced electronic defects, such as scattering or color centers, but, on the other hand, it can also partially restore the birefringence of uniaxial crystals, which is often reduced before by implantation, to the previous values. The first effect is desirable, but the second should only occur to a limited extent in order not to reduce the waveguiding properties too much. Therefore, the samples were annealed in air at temperatures of up to 250 \({}^{\circ}\)C, starting from 100 \({}^{\circ}\)C and increasing the temperature stepwise in 50 \({}^{\circ}\)C increments, with each temperature being maintained for 30 min. After each annealing step, the prism coupler measurements were repeated until a change in the mode spectrum was observed. Lateral light confinement was achieved by cutting ridges with a 13-25 \(\upmu\)m width, a 15 \(\upmu\)m height, and a 14 mm length perpendicularly to the c-axis of the fabricated planar waveguides using a Disco DAD322 precision diamond saw, as described in [14, 15]. Sincechipping occurs easily during the cutting of YLF, a soft resin-bonded blade (Disco G1A853 SD6000 R21B01) with a 200 \(\upmu\)m width was used at 25,000 rpm with a 0.2 mm/s feed speed to cut the ridges. For this reason, the end facets were also prepared with the same blade, making 50 \(\upmu\)m deep so-called polishing cuts. We want to note that broader ridge widths were chosen here to achieve a better overlap in future laser and amplifier experiments, where a multimode diode with an astigmatic beam profile is planned to be used for pumping. Propagation loss measurements in the ridge waveguides were conducted with the transmission method using a helium-neon (HeNe) laser at a 632.8 nm wavelength. The coupling efficiency was estimated by calculating the overlap integral of the transmitted waveguide mode intensity and that of the measured HeNe laser beam focal spot. For fluorescence measurements, bulk and ridge waveguide samples were excited using a tunable continuous-wave frequency-doubled Ti:sapphire laser (MSquared SolsTiS Double, UK).