Developing an effective method of quantifying defects in the bulk of transparent ceramics is a challenging task that could facilitate their widespread use as a substitute for single crystals. Conventionally, SEM analysis is used to examine the microstructure but it is limited to the material surface. On the other hand, optical transmittance assesses material quality, but does not provide information on the size and concentration of defects. In this study, we illustrate the use of a digital optical microscope for the non-destructive, precise, and rapid analysis of residual porosity in transparent ceramics. YAG-based ceramics doped with Yb have been selected for this study because they are used as laser gain media, an application that requires virtually defect-free components. Different production processes were used to produce YAG samples, and the digital optical microscope analysis was used to compare them. This analysis was shown to be effective and precise to measure the size and concentration of the residual pores. In addition, the comparison of samples obtained with different production processes showed that the size and distribution of the residual porosity is affected by the drying step of the powders before shaping by pressing, as well as by the sintering aids used to ease the densification. It also showed that the transmittance is influenced by both the total volume and the concentration of the pores.

Planar polycrystalline laser waveguides with the structure YAG/Yb:YAG/YAG were produced by ceramic processing. Transparent ceramics represent a valid alternative to single crystals as the flexibility of the ceramic process paves the way to new application opportunities. The implementation of different ceramic shaping techniques allows the production of complex or multicomponent (composite) structures that are generally difficult or about impossible to obtain with single crystals. A practical example are the planar structures presented here, characterized by a strict control of the dopant distribution obtained already during the shaping process, without further machining or bonding. The waveguides are composed of a laser active Yb:YAG layer obtained by tape casting with a thickness between 100 and 250 μm placed between dopant-free YAG cladding obtained by oxide powder mixing. The refractive index of Yb:YAG is higher than YAG thus realizing a multimode waveguide. Two types of geometries were tested: 1. simple planar waveguide with a sandwich structure (confinement in one direction); 2. waveguide with a rectangular cross section (confinement in two directions). The waveguides were tested for laser emission in a cavity end pumped by a fiber coupled diode laser, under quasi-CW pumping conditions (rectangular pump pulses with 10 ms duration, 10 Hz), delivering up to 60W at 936 nm. The planar waveguide has shown a maximum output power of 14.4 W (slope efficiency 39.1%, optical efficiency 27.4%) whereas the 2D waveguide had a maximum output power of 11.6 W (slope efficiency 38.4%, optical efficiency 22.4%). Lasing wavelength was 1030 nm.

The potential of the field‐assisted sintering technique (FAST) as a novel postdensification method for densifying pre‐sintered transparent Yb‐doped yttrium aluminum garnet (Yb:YAG) ceramics was evaluated. Specifically, the unconventional FAST method was compared with hot isostatic pressing. Achieving transparency in a ceramic requires a precise process control to attain a fully pore‐ and defect‐free microstructure. Conventionally, YAG‐based transparent ceramics are prepared through double‐step sintering, such as vacuum sintering followed by HIP. In this work, various combinations of pressure and temperature were explored using the FAST process as an alternative to HIP. The resulting microstructures were analyzed and compared. Carbon contamination was observed in samples processed by FAST. Transmittance after the FAST and HIP treatments, and after air annealing treatments was compared. The findings indicate that FAST effectively reduces residual porosity, yielding transparent materials, and can serve as a viable alternative to HIP.