Sesquioxide ceramics are promising candidates for high-power laser applications, yet achieving high optical quality remains challenging. This is primarily due to rapid grain-boundary mobility during the final stage of sintering and pump-induced photodarkening under high-power excitation, particularly when traditional tetravalent sintering additives (e.g., ZrO₂) are used. Here, approximately 1 : 1 (molar) Zr⁴⁺–Ca²⁺ codoping is introduced to regulate the sintering behaviors of Yb:Y₂O₃ ceramics, yielding dense microstructures with uniform elemental distributions. The charge compensation enabled by Zr–Ca codoping suppresses the formation of point defects, thereby significantly mitigating photodarkening. As a result, the optimized 0.02 at% Zr–Ca codoped sample delivers a maximum continuous-wave (CW) output power of 20.5 W at 1076 nm with a slope efficiency of 65.4%. These results demonstrate that Zr–Ca codoping is an effective strategy for fabricating high-performance sesquioxide laser ceramics with improved optical quality and enhanced resistance to pump-induced photodarkening.

The sintering trajectory of the Ho,Pr:Y₂O₃ ceramics could be effectively adjusted by sintering in a flowing oxygen atmosphere instead of vacuum. The final-stage grain growth was significantly suppressed by the use of oxygen atmosphere presintering, resulting in smaller average grain sizes than those of samples sintered under vacuum, while the same relative density was achieved. After hot isostatic pressing (HIP), the oxygen presintered Ho,Pr:Y₂O₃ ceramics achieved excellent optical quality, with transmittance exceeding 80% at a wavelength of 680 nm. The codoping of Pr³⁺ as deactivating ions effectively depopulated the lower energy level ⁵I₇ during the Ho³⁺:⁵I₆ → ⁵I₇ transition, thereby making the Ho,Pr:Y₂O₃ ceramics more conducive to promoting population inversion in the 2.9 μm laser wavelength range.

Highly transparent 0.5 and 1.0 at% Pr-doped Y₂O₃ ceramics were fabricated by vacuum sintering plus hot isostatic pressing (HIP) treatment. The selection of suitable pre-sintering temperatures and right microstructures before HIP was critical to obtain high density of the final sintered bodies. The well-densified ceramics had pore-free microstructures with an average grain size of about 1 μm. It was also found that the charge states of the Pr ions could be changed through regulating the annealing atmospheres, resulting in different absorption and emission characteristics in the visible wavelength region. Annealing in reducing atmosphere (5% H₂/95% Ar) favored the formation of Pr³⁺, resulting in stronger red emissions, while annealing in oxygen atmosphere led to the rise of lattice constant due to the concentration increase of oxygen interstitials. The H₂/Ar-annealed 0.5 at% Pr:Y₂O₃ ceramics exhibited strong red emission at 600-675 nm, which may be a promising gain material for red solid-state lasers.