Advances in and Future Perspectives on High-Power Ceramic Lasers

Abstract

Advancements in laser glass compositions and manufacturing techniques has allowed the development of a new category of high-energy and high-power laser systems which are being used in various applications, such as for fundamental research, material processing and inertial confinement fusion (ICF) technologies research. A ceramic laser is a remarkable revolution in solid state lasers. It exhibits crystalline properties, high yields, better thermal conductivity, a uniformly broadened emission cross-section, and a higher mechanical constant. Polycrystalline ceramic lasers combine the properties of glasses and crystals, which offer the unique advantages of high thermal stability, excellent optical transparency, and the ability to incorporate active laser ions homogeneously. They are less expensive and have a similar fabrication process to glass lasers. Recent developments in these classes of lasers have led to improvements in their efficiency, beam quality, and wavelength versatility, making them suitable for a broad range of applications, such as scientific research requiring ultra-fast laser pulses, medical procedures like laser surgery and high-precision cutting and welding in industrial manufacturing. The future of ceramic lasers looks promising, with ongoing research focused on enhancing their performance, developing new doping materials and expanding their functional wavelengths. The ongoing progress in high-power ceramic lasers is continuously expanding the limits of laser technology, therefore allowing the development of more powerful and efficient systems for a wide range of advanced and complex applications. In this paper, we review the advances, limitations and future perspectives of ceramic lasers.

1. Introduction

Solid state, high-power lasers have tremendous advantages over free-electron lasers (FELs) and gas lasers because of their substantially smaller sizes, mobility and superior performance. Examples of these include disk and rod lasers made from rare earth doped single crystals [1,2]. They have been widely used in fundamental research [3,4], material processing [5], micro-machining [6], medical applications [7] and optical transmission and telecommunication systems [8,9] and have showed potential for ICF applications [10]. The available laser sources provide a vast range of wavelengths, pulse durations, and output powers. Despite their many characteristics, lasers generally consist of three basic components (see Figure 1): a gain medium (solid, liquid or gas), a pump source and a resonator. Single crystals and glass are the most commonly utilized solid state gain mediums in high-power laser systems. Laser crystals, such as yttrium aluminum garnet (YAG) [11], $YV{O}_4$ [12] and Ti:sapphire [13], are commonly used due to their superior thermomechanical properties. Among these, Nd:YAG crystals show better general qualities than other laser crystals. However, the technological and financial constraints of creating a single-crystal laser gain medium for future high-power laser applications still remain quite difficult [14,15]. The factors which directly influence the production of individual laser crystals include investment costs, overall laser source efficiency, dependability, system capabilities to power transmission, and total maintenance costs. However, due to its high energy storage capacity and long fluorescence lifetime, glass lasers are used in the field of large-pulse laser oscillation as a driver for producing nuclear fusion reactions in ICF [10]. Nonetheless, because of its substantially lower thermal conductivity than single crystals, laser glass is not suitable for high repetition rate or continuous-wave (CW) laser operations [16]. Various types of lasers have been developed using ceramics doped with rare earth elements, which are now employed in a wide range of applications. Figure 1 describes the application of ceramic lasers in various areas of science, technology, industry and defense [15,17,18,19,20,21].

In recent times, there has been a significant focus on polycrystalline ceramic laser materials due to their notable improvement in optical qualities. These materials have demonstrated highly efficient laser oscillations, with efficiencies comparable to or, in some cases, even greater than those of single crystals [22,23]. Polycrystalline ceramics are composed of many crystalline grains, each having a random orientation in relation to the adjacent grains [24]. These laser ceramics offer significant advantages compared with laser crystals, making them an attractive choice. Firstly, ceramics possess the advantageous characteristic of being capable of being produced in vast quantities, making them extremely attractive for the purpose of producing high-power lasers. Second, the composite laser ceramics with complex structures can be produced, which is unachievable with single crystals. Furthermore, laser ceramics can be doped with a high concentration of dopants in a homogeneous way, eliminating the presence of optically distinct regions like facets and cores observed in single crystals. Additionally, ceramics can be transformed into single crystals with extended lifetimes and superior resistance against laser-induced damages through the process of sintering [25]. YAG ceramics with equivalent transmittance to single crystals can be mass produced, have larger sizes, easy to bond with other materials or YAG with other dopants and have homogeneous and high dopant concentrations (up to Nd: 4 at.%). These crystals hold great potential for use in high power density lasers [26]. The scaling of meter-sized plates is the most significant advantage in the development of high-power ceramic lasers [27,28]. As a result, ceramic lasers represent the most potential active medium for laser fusion drivers.

High-quality and transparency Nd:YAG ceramics with extremely low scattering losses have already been manufactured. Studies have shown that their optical absorption, emission and fluorescence spectra, as well as the physical and laser properties, are comparable to Nd:YAG single crystals, and qualitative analysis revealed nearly equivalent superiority features [29,30]. This demonstrates Nd:YAG ceramics’ potential to be used as gain media for high-efficiency and high-power lasers. A slope efficiency of 68% has already been reported when using Nd:YAG ceramics under an end-pumping disk laser [14]. A polycrystalline ceramic laser medium with application performance equivalent to single crystal materials offers many advantages, including simplified grinding, higher production yields and secondary processing due to better process control and repeatability, in addition to greater mechanical toughness for high shock resistance [31,32]. Fundamentally, ceramic lasers are more than just a potential alternative for traditional single-crystal lasers; they can also be used to produce super high-power lasers [27,28]. The present article reviews the developments, properties and future perspectives of ceramics as laser materials.

2. Overview of Laser Ceramics

Since the discovery of solid state ruby lasers, researchers have proposed that dense polycrystals of isotropic and pure materials are optically equivalent to a single crystal of the same material [33]. In 1964, ceramics were used as laser gain media under cryogenic circumstances. Hot-pressed Ca$F_2$-doped dysprosium ($D{y}^{2+}$) was the first polycrystalline material reported to achieve laser oscillation [34,35]. After that, for several years, no significant progress had been made. The challenge in producing laser materials from ceramics lies in their polycrystalline nature, which leads to increased light scattering within the material due to various features such as lattice defects, pores, grain boundaries and composition gradients. This increases the substance’s opacity, making it unsuitable for laser action. After 9 years, in 1973, a transparent ceramic, Nd-doped Yttralox (NDY, 1 mol%$N{d}_2$$O_3$, 10 mol%$Th{O}_2$ and 89 mol%$Y_2$$O_3$), was successfully developed and utilized for pulsed laser oscillation [36,37]. In 1974, researchers successfully manufactured polycrystalline Nd:YAG ceramic laser rods with a lasing efficiency of ∼0.32%. This achievement was made possible by carefully controlling the powder composition, preparation, processing and cooling rate of the rods [38]. In 1995, researchers successfully achieved CW laser oscillation using a polycrystalline Nd:YAG ceramic laser gain medium, as mentioned in a study by Ikesue et al. [39]. Their findings indicate that ceramic materials have the potential to overcome the technical and economic difficulties associated with producing single crystals through melt-growth methods. In 1997, researchers successfully demonstrated a microchip laser [40] and single-mode oscillation [41] using strongly doped Nd:YAG ceramics. Shoji et al. [40] found that a 3.4 at.% Nd:YAG ceramic microchip laser generates twice the output power compared with a Nd:YAG crystal microchip laser of equal dimensions. In 1999, Kenichi Ueda and his research team [42,43] made notable advancements in Nd:YAG ceramics by combining liquid-phase chemical processes with vacuum sintering procedures. This allowed them to create nanoparticles of a comparable size for ceramic manufacturing. Figure 2 illustrates the two primary methods employed for the fabrication of transparent ceramic laser materials from powder, mentioned as procedure A and B in the figure [44].

Recently, there have been many attempts to develop laser materials which have a wide emission range in order to generate and enhance ultrashort laser pulses. The $Y{b}^{3+}$ ion is a highly effective dopant for lasers due to its several beneficial features, which have played a significant role in its success in high-intensity, short-pulse laser applications. However, in most hosts, the emission bandwidth of $Y{b}^{3+}$ is narrower compared with Ti:sapphire. This makes it difficult to generate and amplify laser pulses with durations of less than 100 femtoseconds (fs). To address this challenge, extensive research has focused on mixed hosts, which are solid solutions composed of chemically compatible materials. The presence of a disordered lattice structure caused by the solid solution might cause an inhomogeneous broadening of the absorption and emission spectra. Several research publications on displaying the broad absorption and emission spectra of Yb:YAG crystals have been published [45,46]. The significant absorption band in the near-infrared (IR) region is highly suitable for laser diode (LD) excitation, while the broad emission band allows for the production of extremely short pulses [36]. On the other hand, an Yb:YAG laser is commonly known as a three-level laser or a quasi-four-level laser. In this laser, there is a limited number of particles at the Stark level of the lower manifold ²F_7/2, which is where the laser transition ends. To prevent reabsorption from the lower level of the laser, it is necessary to use high-intensity pumping and an efficient heat removal technique [47,48,49].

Several $Y{b}^{3+}$-doped compositions, including ceramics [50,51,52] and crystals [53,54], have been examined by researchers. For example, Beil et al. [55] conducted a study on crystalline Yb-doped mixed sesquioxides and discovered a considerable widening of the emission spectra, particularly in $Yb$:${(Lu,Sc)}_2$$O_3$ matrices. Takaichi et al. [56] performed a study on the absorption and emission spectra of an Yb:YAG ceramic containing 1 at.% Yb. This is the first instance of a ceramic laser that is powered by diode ends for Yb:YAG, with a CW output power of 345 mW and a slope efficiency of 26%. In 2001 [57], a Nd:YAG ceramic rod was successfully used to generate lasing at 1.47 kW, despite having a 15% poorer oscillation efficiency than a single crystal. In 2002, Ueda’s team [57], in partnership with Toshiba and Konoshima Co., successfully showcased the generation of 1.46 kW of power when utilizing a Nd:YAG ceramic. The slope efficiency achieved was 42%, which was slightly less than the 49% attained with a single crystal. Sato et al. [58] and Saikawa et al. [59] demonstrated the oscillations of ultrashort pulsed lasers (ranging from picoseconds (ps) to fs) using Nd:YSAG (yttrium scandium aluminum garnet) and Yb:YSAG ceramics, respectively. In 2006, the Lawrence Livermore National Laboratory (LLNL) showcased a 67 kW quasi-continuous wave (QCW) laser utilizing a large-scale Nd:YAG ceramic laser gain medium [60]. Northrop Grumman Corporation (NGC) presented a 105 kW end-pumped Yb:YAG ceramic slab laser, while Textron developed a zigzag Nd:YAG ceramic thin-slab laser with a power output above 100 kW [61]. Some Nd:YAG ceramic rods have lasing efficiencies substantially identical to that of a high-quality temporal Nd:glass laser rod. Ceramic laser materials have garnered much interest in recent decades because of manufacturing improvements such as extremely transparent, nanocrystalline YAG doped with $L{n}^{3+}$ activators, specifically $N{d}^{3+}$ ions.

In summary, ceramic lasers have advanced from early experiments in the 1970s to the production of multi-kilowatt laser systems, making them a cornerstone in high-power laser applications in industry, research and defense.

Table 1. Summary table for important achievements and progress in ceramic lasers.

Year	Achievements and Developments	Laser Properties	Description	Power Levels	Strength	Weeknesses	Comments
1964 [34,35]	First ceramic laser, hot-pressed $Ca{F}_2$ doped with dysprosium ($D{y}^{2+}$)	CW laser oscillation	Early attempts using ceramics as laser gain media	Low	Potential for high efficiency	High scattering losses	Laid the foundation for the use of ceramics in high-power lasers
1995 [39]	Development of Nd: YAG ceramic laser	High efficiency, CW and pulsed operation	Nd: YAG ceramics demonstrated comparable performance to single crystals	Moderate	High efficiency, good thermal properties	Fabrication challenges	CW laser oscillation using a polycrystalline ceramic laser gain medium
2001 [57]	Kilowatt (kW)-class power lasers	High efficiency	Development of Nd ceramic lasers capable of exceeding 1 kW of output power with 50% efficiency	High	High power output, good thermal conductivity	Thermal management issues	Demonstrated the scalability of ceramic lasers for high-power industrial applications
2006 [60]	Multi-kW ceramic lasers	Near-IR	QCW ceramic Nd: YAG laser with 67 kW power	Extremely high	High peak power and pulse energy	Limited duty cycle, complex cooling requirements	Large-scale Nd:YAG ceramic laser gain medium
2010 [61]	Development of 100 kW zigzag Nd:YAG ceramic thin-slab laser	Near-IR	Improvements in ceramic processing reduced scattering losses, leading to lasers with better beam quality	Extremely high	Enhanced beam quality, efficient thermal management	Limited duty cycle, limited to high-power applications	Zigzag path of laser beam inside thin slab averages out thermal gradients, reduces thermal lensing and enhances beam quality
2015 [62,63]	Advances in sesquioxide ceramic lasers (e.g., $Yb$:$S{c}_2$$O_3$)	High thermal conductivity, high power	Sesquioxides offering superior thermal properties	Extremely high	Excellent thermal properties, high power	High cost of raw materials	High cost of production, requires complex cooling infrastructure for high powers
2020 [64,65]	Development of efficient glass-ceramic lasers	Broad emission spectrum, high efficiency	Glass-ceramics combining properties of glass and ceramics	Moderate	Broad emission spectrum, ease of fabrication	Limited thermal conductivity	High doping flexibility, less favorable for short-pulse systems
2023 [66,67,68,69]	Achievements in Tm- and Ho-doped ceramic lasers	High efficiency, mid-IR emission	Tm- and Ho-doped ceramics for specific applications	High	High efficiency, specific wavelength emission	Limited availability of dopants	Mid-IR emission, broad absorption and tunability

summarizes the advances in ceramic laser technology over the past few decades and provides a concise overview of the significant progress and accomplishments achieved in ceramic lasers.

3. Types of Laser Ceramic Materials

Laser fabrication may utilize a diverse range of materials, with the selection of potential materials expanding continuously. In 1964, the first ceramic material used in a laser was $D{y}^{2+}$: $Ca{F}_2$ [34]. Subsequently, both ceramic materials treated with doping agents and those that have not been treated have undergone testing. The materials which have been tested over the years include YAG ceramics [17], composite ceramics [18], sesquioxide ceramics [70] and non-cubic doped fluorapatite ceramics [71]. Each of these materials exists in both doped and undoped forms, with various dopants which display distinct properties. Additionally, noticeable differences can be observed between the doped and undoped variants of the same material. The subsequent subsections provide a description of the various categories of laser ceramic materials.

3.1. Glass-Ceramic Lasers

Glass-ceramics are a two-phase system consisting of crystals that are intentionally formed within the host glass by applying a suitable heat treatment. For a number of years, researchers have focused on enhancing the materials’ transparency when examining the optical characteristics of glass-ceramic systems [72]. This enhancement can be obtained by carefully controlling the crystal size and composition caused by the ceramming process. Refractive indices that are comparable to the surrounding glass matrix and crystal sizes smaller than the wavelength of light can be formed to minimize scattering losses, especially in the infrared region of the spectrum [73,74,75]. As a result, crystalline bodies of glass may form with peculiar characteristics. When the right heat treatment was used, a transparent glass-ceramic system with extremely low levels of scattering losses was demonstrated in single-mode fiber form [76]. In 1972, Rapp and Chrysochoos reported the first pulsed lasing transparent glass-ceramic host [77]. With the inclusion of Ti$O_2$ and Zr$O_2$ for fine nucleation initiation and $N{d}_2$$O_3$ as a doping ion, the starting glass was in the ($L{i}_2$O-$A{l}_2$$O_3$-$S{i}_2$O) basic system. The small average size of the crystallites (100–300 Å) and the good matching between the refractive indices of crystallites and glass result in a transparent glass-ceramic. A comparison was made between the laser characteristics of the parent glass and glass-ceramics at 1064 nm [78]. The glass-ceramic laser rods had an output mirror reflectivity of 74.5%, which meant that their threshold was approximately three times higher than that of glass laser rods. Glass-ceramic rods have a significantly lower lasing efficiency than glass rods. Light scattering brought on by the material’s composite structure is the cause of these poor properties of glass-ceramics. Figure 3 shows the microstructure differences between glass, glass-ceramics and ceramics.

Pulsed laser emission in Nd-doped glass-ceramics was achieved by Müller and Neuroth [79] in 1973. $T{a}_2$$O_5$ was added as a new nucleating agent, which caused the $\beta$-quartz solid solution to precipitate synchronously with two other phases and ensure low thermal expansion. The partially crystalline arrangement was expected to have a distinctive influence on the activating ions for the first time, but this was not proven. Auzel et al. [80] developed Yb:Er oxyfluoride glass-ceramics in 1975. The fluoride crystal phase composed of $Pb{F}_2$ was the primary partitioning medium for rare earth ions. When comparing this highly crystalline material to $La{F}_3$ single crystals, it was more efficient for infrared upconversion but still opaque. Because the rare earth ions completely segregated into the fluoride nanocrystallites, which made up 1–3% of the total volume, Mortier et al. [81] were able to create erbium (Er)-doped ultra-transparent glass-ceramics in an oxyfluoride system with optical qualities resembling the crystals in 1999. When comparing the optical absorption spectra of Er-doped glass-ceramics to the original glass, it was found that the inhomogeneous linewidth was reduced by approximately 50%, but at room temperature, the maximum absorption cross-section of some transitions increased to 100%. Using the co-doped material, Malyarevich et al. [82] demonstrated the Q-switched pulses from the Er:glass lasers.

The first new lasing glass-ceramic system was published only in 2001 by the group of Tick at Corning [83]. Though they reported a slope efficiency of 28% with the glass-ceramic fiber compared with 33% with the glass fiber, they were able to realize a Nd-doped glass-ceramic fiber laser. A double crucible procedure was used to create the single-mode glass-ceramic fibers from glasses comprising 30$Si{O}_2$, 15$A{l}_2$$O_3$, 29$Cd{F}_2$, 17$Pb{F}_2$ and 4$Y{F}_3$ doped with neodymium. The fiber’s core was thermally treated at 450 °C for 30 min to create 10 nm-sized crystals of $Cd{F}_2$-$Pb{F}_2$-$Y{F}_3$, which constituted less than 10% by volume and caused scattering losses of less than 1 dB/m. The Nd-ions’ spectroscopic characteristics were significantly impacted by their partitioning into the crystal phase, primarily by the induced inhomogeneous line narrowing. To ensure various optical functions in laser set-ups, glass-ceramic materials’ optical qualities should also be utilized. Initially, reports of saturable absorber development have occurred when using glass-ceramics. In a MgO-$A{l}_2$$O_3$-Si$O_2$ glass, Malyarevitch et al. [82] produced $C{o}^{2+}$ magnesium-aluminum spinel nanocrystallites. They produced 80 ns pulses with 5.5 mJ of energy at a repetition rate of 1 kHz by Q-switching an Er:glass laser.

Moreover, a number of glass-ceramic systems, including $CsLi{B}_6$$O_{10}$, which contains glass-ceramics, have recently been found to exhibit second harmonic generation (SHG) [84]. The highest SHG intensity was similar to what is often found in Y-cut quartz. Glass ceramics have been applied as optical amplifiers and random lasers, as demonstrated by Araújo et al. [85] in a review. Kowalska et al. [86] reported the near-IR luminescence properties of specific rare earth ions ($E{r}^{3+}$, $H{o}^{3+}$, $P{r}^{3+}$ and $T{m}^{3+}$) in titanate-germanate glasses under $Y{b}^{3+}$ excitation. Energy transfer in co-doped titanate–germanate glass is fairly well reported for the resonant $Y{b}^{3+}$ → $P{r}^{3+}$ and $Y{b}^{3+}$ → $E{r}^{3+}$ and non-resonant $Y{b}^{3+}$ → $T{m}^{3+}$ and $Y{b}^{3+}$ → $H{o}^{3+}$. At 1.3 $\mathsf{\mu }$m ($P{r}^{3+}$: ¹G₄ → ³H₅), 1.5 $\mathsf{\mu }$m ($E{r}^{3+}$: ⁴I_13/2 → ⁴I_15/2), 1.8 $\mathsf{\mu }$m ($T{m}^{3+}$: ³F₄ → ³H₆) and 2 $\mathsf{\mu }$m ($H{o}^{3+}$: ⁵I₇ → ⁷I₈), the near-IR emission bands are enhanced when $Ti{O}_2$ is introduced into germanate glass [86]. Research conducted on a large scale showed that while the lifetimes of $Y{b}^{3+}$ ions decrease as the $Ti{O}_2$ content increases, the energy transfer efficiency varies greatly depending on the pair of $Y{b}^{3+}$/$L{n}^{3+}$$(Ln=Pr,Er,Tm,Ho)$ in titanate–germanate glass. Figure 4 shows the absorption spectra of titanate–germanate glasses co-doped with $Y{b}^{3+}$/$P{r}^{3+}$, $Y{b}^{3+}$/$E{r}^{3+}$, $Y{b}^{3+}$/$T{m}^{3+}$ and $Y{b}^{3+}$/$H{o}^{3+}$. In their studies, Pawlik et al. [87] reported on the structural and photoluminescence of silicate glass-ceramics with $Ca{F}_2$ nanocrystals co-doped with $T{b}^{3+}$/$E{u}^{3+}$ ions. The sol–gel process, a good substitute for photonic materials, was used to create a series of glass-ceramics containing $Ca{F}_2$ nanocrystals [88,89]. The excitation or emission spectra observations demonstrated the existence of an energy transfer process between $T{b}^{3+}$ and $E{u}^{3+}$. Specifically, partial separation of $T{b}^{3+}$ and $E{u}^{3+}$ ions inside the $Ca{F}_2$ nanocrystals generated upon a regulated heat treatment process was demonstrated in the emission decay curve analysis.

3.2. Transparent YAG Ceramic Lasers

Polycrystalline ceramics are well suited for high-power laser materials due to their ability to produce transparent ceramics with a diverse range of shapes, sizes and geometries [90]. The ceramics created often possess physical characteristics that are similar to—or in certain instances even superior to—those of single crystals [91]. Because grains and grain boundaries distinguish a ceramic material from a single crystal, it is crucial to ascertain how the grain structure affects particular attributes which affect lasing performance in high-energy laser (HEL) applications. The factors which affect an HEL consist of the mechanical strength, optical losses, and laser-induced damage threshold (LIDT) [32]. These qualities are actually on par with single crystals, and they might even be superior for ceramics. For the creation of translucent ceramics, the starting powder’s purity is crucial. The phenomenon of scattering at grain boundaries, also known as Rayleigh scattering, has traditionally been considered a significant problem for ceramic laser gain media. This is mostly owing to the presence of dislocations in ceramics, which lead to changes in the crystal orientations. Grain boundaries have no effect on the creation and amplification of a coherent beam, as demonstrated by the use of ceramic gain media in single-mode lasing demonstrations. Excellent results of successful laser oscillation from ceramic gain media have also been reported by other research teams [15,18,92,93].Figure 5 shows the scattering and attenuation of light in transparent ceramics [94].

Several studies in the past few years have been conducted to study the lasing capabilities of transparent crystalline ceramics. For example, Quarles [95] performed a study on the scattering loss of high-quality Nd:YAG ceramics and discovered that the optical scattering was reduced when compared with single crystals. He attributed this to the high degree of homogeneity of rare earth ions in the powder that persisted in the ceramics. In another investigation, Feldman et al. [96] found that the tensile strength of ceramic YAG was roughly 1.4 times higher than that of single-crystal YAG. Other researchers have also found similar results, linking the increased strength mostly to the ceramic material’s improved ability to resist fractures [97,98]. The Hall–Petch equation in ceramics also states that strength is inversely proportional to grain size [99]. As a result, lowering the grain size increases the strength even more. Furthermore, eliminating impurities and holes from grain boundaries improves toughness and optical performances. Ueda et al. [100] found that the LIDTs for rare earth ion-doped and undoped YAG ceramics are equal. As a result, if the ceramic is properly produced, then it has outstanding optical and mechanical qualities that are suited for HEL applications.

Over the years, efforts to decrease scattering losses in $N{d}^{3+}$-doped YAG ceramics have resulted in higher output powers becoming achievable. Polycrystalline $N{d}^{3+}$-doped YAG ceramics can have a doping content of up to 9%. High amounts of $N{d}^{3+}$ ions have been efficiently integrated into YAG ceramics to overcome the limiting absorption length of $N{d}^{3+}$-doped YAG single crystals. Chretien et al. [101] investigated the reactivity during reaction sintering in $Y_2$$O_3$-$A{l}_2$$O_3$-$N{d}_2$$O_3$ mixed powder and found improved optical characteristics for high laser performance. Chang et al. [102] investigated the impact of attenuation loss on the laser characteristics of Nd:YAG transparent ceramics. The laser studies conducted utilizing the 885 nm direct end-pumped technique yielded a notable optical efficiency of 62.5% and achieved a maximum output power of 144.8 W when the absorbed pump power was 231.5 W. This demonstrates a remarkable level of optical conversion efficiency achieved in a Nd:YAG ceramic laser. Li et al. [103] introduced a manufacturing technique for producing laminar-structured YAG/1.0 at.% Nd:YAG/YAG transparent ceramics by utilizing a solid state reaction method and vacuum sintering. Figure 6 compares the optical efficiency of Nd:YAG single crystals and ceramics.

3.3. Sesquioxide Ceramic Lasers

Sesquioxides are chemical compounds consisting of three oxygen ions for every two metal ions. Ceramics and their solid solutions have tremendous potential as hosts for solid state laser gain materials because of their strong compatibility with various rare earth ions, comparable physical properties to single crystals, and substantially lower preparation temperature [70]. They can also create complete solid solution series with each other in the chemical formula of ($L{u}_x$$S{c}_y$$Y_z$)₂O₃, where $x+y+z$ = 1, such as ${\left(LuSc\right)}_2$$O_3$ [105], ${\left(LuY\right)}_2$$O_3$ [106] and ${\left(YSc\right)}_2$$O_3$ [107] in order to achieve a wide gain bandwidth by disrupting the crystalline structure [70]. This has the potential to generate ultra-short pulse lasers [108]. Sesquioxides exhibit a wide range of optical transparency wavelengths, spanning from about 0.2 to 8.5 $\mathsf{\mu }$m [109]. The sesquioxides have a broad energy gap of roughly 6.0 eV, making them ideal as laser host materials for various active ions [70]. Out of many sesquioxides, $Y_2$$O_3$ was chosen as the best material to be mixed with $E{r}^{3+}$ for mid-IR laser application because of its lowest maximum phonon energy of 597 cm⁻¹ [110], while $S{c}_2$$O_3$ has the best thermal conductivity among pure sesquioxides, although this value decreases significantly as the doping concentration increases. Although $L{u}_2$$O_3$ has the lowest thermal conductivity, the value remains nearly constant after substantial doping with various rare earth active ions. Because the masses of Lu ions and doping ions are similar, there is only a modest distortion in phonon propagation, resulting in weaker heat transport in laser gain [111]. Sesquioxide ceramic materials are renowned for their exceptional thermal conductivity, surpassing those of any other laser material. It has been observed that the thermal conductivities of lanthanide sesquioxides ($L{n}_2$$O_3$) are approximately two times greater than those of YAG. This could be useful for high-power laser applications. Figure 7 shows an energy level diagram for pump and laser transitions, as well as the probable mechanisms involved in laser operations in sesquioxide materials for various rare earth ions [70,112].

Recently, it has been demonstrated that several mixed sesquioxides have much lower melting temperatures than their constituents [113]. This allows them to develop using the known Czochralski process, resulting in rare earth-doped mixed sesquioxides of good optical quality. Mixed sesquioxides exhibit significant potential for generating and amplifying ultrashort pulses as a result of their gain spectra, which are enlarged in an inhomogeneous manner due to their intrinsic disorder. $H{o}^{3+}$-, $E{r}^{3+}$-, $T{m}^{3+}$- or $Y{b}^{3+}$-doped sesquioxide gain materials enabled exceptional laser performance in CW and mode-locked laser operations [70,114]. However, the development of these materials is troublesome because they have extremely high melting temperatures exceeding 2400 °C. As a result, despite past successful demonstrations of the manufacture of high-quality sesquioxide crystals, sesquioxide crystals are not yet available commercially. Ceramic sesquioxides, on the other hand, may be produced at lower temperatures, and their commercial availability is increasing.

On the other hand, in mixed sesquioxides, it is suggested that rare earth cations are randomly distributed across the crystallographic cation sites in the cubic bixbyite structure [115], leading to inhomogeneous broadening. The produced spectra combine the spectroscopic characteristics of the constituents, leading to a substantial increase in the range of wavelengths absorbed and emitted. The features of rare earth-doped mixed sesquioxide crystals allow for precise adjustment of the emission bands. This is especially useful for ultra-fast mode-locked lasers, which require a smooth and wide emission band in order to achieve the shortest pulse duration. The phenomenon of lasing in host materials which contain a mixture of sesquioxides was first reported in the case of $Nd$:$YSc{O}_3$ [115]. Since then, it has taken more than 30 years for other findings on lasing in mixed sesquioxides to be published [116]. Recently, several works have been published on lasers using rare earth-doped ceramic mixed sesquioxides [70,113,117]. The latest study on $Y{b}^{3+}$:($L{u}_x$$Y_{1-x}$)₂O₃ ceramics revealed that the extended laser emission was initially quantified, achieving a maximum slope efficiency of 68.1% in the QCW state. The wavelength tunability ranged from 994 to 1089 nm [118]. In the same group, $Y{b}^{3+}$-doped ${(Y,Sc)}_2$$O_3$ transparent ceramics displayed highly efficient laser action, with a maximum output power of 5 W [119]. CW with concurrent dual-wavelength Q-switching operation [106] and semiconductor saturable absorber mirror (SESAM) mode $T{m}^{3+}$:${(Lu,Y)}_2$$O_3$ ceramics were used to create a wavelength-tunable laser near 2.05 $\mathsf{\mu }$m, as reported in [120]. In addition, Xu et al. [63] published the first study on the CW and passively Q-switched laser operations of $Tm$:$LuSc{O}_3$ ceramics. Jing et al. [121] discovered that a ceramic laser composed of 1.4 at.% $H{o}^{3+}$:${(Lu,Sc)}_2$$O_3$ generated an output of 187 mW in QCW mode at a wavelength range of 2.114–2.135 $\mathsf{\mu }$m. The laser also exhibited a slope efficiency of 7.6%. Wu and his group [122] recently created mixed sesquioxide ceramics using $Tm$:$(L{u}_{0.8}$$S{c}_{0.2}$)₂O₃ and successfully showcased a CW laser operating at 2090 nm with a power output of 1.88 W and a slope efficiency of 24.6%. Figure 8 shows the important energy transfer mechanisms, absorption and fluorescence spectra of Tm- and Ho-doped materials [69].

3.4. Other Ceramic Lasers

The success of Nd:YAG ceramic lasers has inspired research into other doped ceramic materials for various laser applications. These new ceramic lasers are produced using the same technologies as Nd:YAG ceramics. YAG laser materials (available since 1995 [39]) are currently the most extensively explored and used materials. They have been utilized in ultra-short pulsed lasers (≤${10}^{-12}$ s), microchip lasers and some of the most powerful lasers (in terms of kilowatts of output power) ever constructed. Ionic solids, including fluorides and oxides, are clear ceramic laser media. Recently, numerous groups have demonstrated $C{r}^{2+}$:ZnSe ceramic lasers [124]. The $C{r}^{2+}$ ion in tetrahedral symmetry is an interesting activator when inserted into diverse chalcogenide crystals such as ZnS, ZnSe and CdSe. It lases in the mid-infrared range at 2300–2500 nm with a considerable tunability covering the 2000–3100 nm range. In the near future, such lasers could be useful for high-resolution spectroscopy, medical and Lidar investigations or pumping optical parametrical oscillators (OPOs) to longer wavelengths [124]. Because they are one of the more recent laser ceramic materials, their application has still not become particularly widespread.

In summary, glass laser ceramics provide an economical option with quite satisfactory optical quality and thermal conductivity. Despite their ease of fabrication and ability to be manufactured in huge quantities, the below-average performance of these ceramics limits their use in medium-power applications. Conversely, transparent Nd:YAG laser ceramics are known for their exceptional laser efficiency, remarkable transparency and excellent thermal conductivity. High-power applications, such as industrial machining and medical equipment, find Nd:YAG ceramics to be highly suitable. However, the manufacturing process is complex and costly, particularly for large dimensions. Sesquioxide laser ceramics are distinguished by their exceptional thermal conductivity, higher melting temperatures and optical quality, making them quite suitable for ultra high-power applications. These lasers are ideal for cutting-edge, high-performance laser systems [117,125]. Nevertheless, their production is costly and challenging, resulting in limited commercial availability.

Table 2. Comparative table summarizing the achievements, developments, and limitations of glass laser ceramics, transparent Nd:YAG laser ceramics and sesquioxide laser ceramics.

Aspect	Glass Laser Ceramics	Transparent YAG Laser Ceramics	Sesquioxide Laser Ceramics
Material Composition	Glass-based material generally embedded with crystalline particles and often doped with rare earth or transition metal ions (e.g., $E{r}^{3+}$, $Y{b}^{3+}$ or $N{d}^{3+}$)	Polycrystalline (Nd-doped YAG) ceramics with high optical transparency	Polycrystalline sesquioxides (e.g., Nd-doped $Y_2$$O_3$ or Yb-doped $L{u}_2$$O_3$)
Thermal Conductivity	Moderate (∼1–2 W/mK) [126]; lower than single crystals, limiting high-power applications	High (∼8–11 W/mK) [127]; close to that of single crystals, enabling effective heat dissipation	Extremely high (∼12–16 W/mK) [70]; excellent for high-power and high-temperature applications
Optical Quality	Good but generally lower than single crystals because of scattering and absorption in the glass medium	High comparability to single crystals; low scattering and high optical uniformity	Excellent comparability to single crystals, with low scattering and absorption losses
Fabrication Complexity	Easier and more cost-effective than single crystals; allows for large and complex shapes	Complex and costly; requires advanced sintering and HIP techniques	Highly complex and expensive; requires high-temperature vacuum sintering
Scalability	High and easy to produce in large sizes	High and large sizes can be achieved, but with increased complexity and cost	Limited; scaling up is difficult due to fabrication challenges and costs
Laser Efficiency	Moderate; limited by optical quality and thermal conductivity	High and efficient due to excellent optical and thermal properties	Extremely high; superior thermal management leads to excellent efficiency, especially in high-power lasers
Doping Uniformity	Can be challenging due to phase separation and inhomogeneity	Generally good; doping can be controlled well across large sizes	Good but challenging due to the high temperatures required for fabrication
Power Scaling	Moderate; suitable for medium-power applications	High; capable of scaling to high power values with proper heat management	Extremely high; suitable for high-power and ultra high-power applications
Thermal Shock Resistance	Moderate; glass phase can be vulnerable to thermal shock	High; polycrystalline nature provides good resistance to thermal shock	Extremely high; highly resistant to thermal shock
Melting Point	Relatively low (∼1500 °C) [128], limiting high-temperature applications	Higher (∼1970 °C for YAG) [129], allowing higher operational temperatures	Extremely high (above 2400 °C) [70], suitable for the most demanding high-temperature applications
Achievements	First successful use in solid state lasers [77], and high doping concentrations are achievable	Demonstrated high-power laser operations [17,130]; widely used in industrial and medical lasers	Record-breaking output power in high-power laser systems [131,132]; used in cutting-edge laser research
Developments	Continuous improvement in fabrication methods; enhanced doping techniques	Advances in reducing scattering and absorption; improvement in large-size production techniques	Progress in high-temperature fabrication methods; development of novel sesquioxide compositions
Limitations	Lower thermal conductivity and optical quality compared with transparent and sesquioxide ceramics; limited power scaling	Expensive and complex fabrication process; difficulties in achieving uniform doping	Extremely high fabrication costs and complexity; limited commercial availability
Applications	Medium-power lasers, medical devices and low-cost laser systems [133]	High-power industrial lasers, medical lasers and military applications [17]	High-power lasers, space applications and advanced research in ultra high-power lasers [70,117]

shows a detailed comparative table summarizing the achievements, developments and limitations of glass laser ceramics, transparent YAG laser ceramics and sesquioxide laser ceramics.

4. The Role of Rare Earth Dopants in Laser Matrix Materials

Rare earth and transition metal ions play a crucial role in laser materials due to their unique optical properties and can be doped in host materials for various laser applications [134]. The laser’s material properties are strongly dependent on the spectroscopic properties of the dopants in laser gain media [113]. These properties include sharp absorption and emission spectra, long energy level lifetimes and minimum sensitivity to external perturbations. Rare earth ions such as $N{d}^{3+}$, $Y{b}^{3+}$ or $E{r}^{3+}$ typically have long energy level lifetimes and often range from a few milliseconds to a few microseconds. This makes them ideal for storing and transferring the energy inside the gain media and allows them to be used for both continuous and pulsed operations [113]. They also have well-defined and sharp absorption and emission spectra, which makes them efficient for energy absorption from pump sources and emissions of radiation. For example, $N{d}^{3+}$ ions are typically used for their strong emissions at around 1064 nm, while $Y{b}^{3+}$ ions offer a broader absorption bandwidth and are efficient for high-power CW lasers due to their lower quantum defect. $E{r}^{3+}$ ions are well suited for eye-safe laser applications, with strong emission at 1.55 µm. Figure 7 and Figure 9 show descriptions of absorption and emission radiation in various rare earth ions in transparent YAG and sesquioxide ceramic hosts, respectively [32,70].

Laser host materials such as YAG and sesquioxides play a crucial role and determine how these dopants interact in terms of energy transfer, thermal conductivity and optical properties [69]. The high thermal conductivity of the host material is crucial for heat removal generated during lasing. In this regard, ceramic materials offer advantages of scalability as well as good thermal management, maintaining laser performance at higher powers. Hence, the effectiveness of the dopant and host matrix combination depends on how well the host stabilizes the dopant ions, minimizes the lattice defects and controls the energy transfer processes in the gain media. Therefore, a careful selection of dopant and host is essential for high lasing efficiency and optimal performance of the laser systems. A more detailed discussion on the interaction, optimization and performance of rare earth ions as dopants in ceramic materials can be found elsewhere [32,69,70,113].

5. Comparative Overview of Laser Properties among Polycrystalline Ceramics and Single Crystals

5.1. Optical Transmission

The absorption in the material can reduce the transmission of the radiation. Feldman et al. [96] investigated the optical characteristics of Nd:YAG single and polycrystalline specimens necessary for high-quality active laser components. They compared the optical transmission spectra of uncoated rods to those of comparable single-crystal rods and discovered that the optical transmission from Nd:YAG ceramic samples is only slightly lower than that from single crystals across the majority of the tested spectral range. Ikesue and Aung [23] reported the transmission spectra of commercially available 8 mm-thick, 0.6 at.% Nd:YAG single crystals and ceramics without any scattering sources. Generally, ceramics containing scattering centers have a lower transmission baseline than single crystals, which should increase as the wavelength lowers. The optical transmission spectrum for Nd:YAG ceramics were found to be identical, and they are shown in Figure 10. The absorption and fluorescence spectra of Nd:YAG ceramics and single crystals were reported to be almost similar [135].

5.2. Fluorescence Lifetime

The fluorescence lifetime of the lasing medium is an important parameter for developing a laser system. Also, the similarity between ceramics and single crystals can be determined by analyzing fluorescence lifetimes. For Nd:YAG systems, the $N{d}^{3+}$ ion’s extremely rich energy level diagram allows for a number of energy transfer processes within the Nd ion system, resulting in self-quenching of emissions of radiation from the metastable energy level ⁴F_3/2. These mechanisms may involve downconversion of excitation on intermediate levels by interaction between a $N{d}^{3+}$ ion excited in ⁴F_3/2 and a non-excited $N{d}^{3+}$ ion or upconversion through the contact of two Nd ions excited in the ⁴F_3/2 state. The proportion of these two categories of processes is dictated by the concentration of stimulated $N{d}^{3+}$ ions, which is then determined by the pumping intensity. The fluorescence lifetimes for single crystals and ceramics can be calculated by fitting the fluorescence decay curve and has been found to depend on the Nd concentrations [57]. Evidence demonstrated that the Nd ions within the grain exhibit identical conditions to those of a single crystals, and the discrepancy in fluorescence lifetimes is solely caused by the Nd ions situated around the grain boundaries.

6. Advances in Laser Ceramics

Solid state lasers have limitations in terms of available wavelengths compared with other laser types, which can restrict their applicability in certain applications. Because of their high power output, solid state lasers may require cooling systems to manage heat dissipation, which adds complexity and cost. For example, the Nd:YAG single crystals created using the standard Czochralski method are quite expensive, time-consuming and small in size and have poor concentrations, limiting their usage in high-power lasers. Conversely, obtaining Nd-doped glass material in large diameters and high concentrations is quite simple. However, this material has limited thermal conductivity and gain, resulting in inadequate laser efficiency. Polycrystalline ceramics, unlike single-crystal and glass laser materials, are made up of crystalline grains which are arbitrarily oriented in relation to their neighbors. Ceramics, including single crystals, offer excellent conversion efficiency and heat conductivity and good optical properties. They can also grow to quite huge sizes. Ceramic laser media made using vacuum sintering [39] and nanocrystalline [57] technologies highly appealing materials since they have several significant benefits over single-crystal laser materials. Multiplayer and multi-functional ceramic laser materials are achievable due to ceramic polycrystallinity [137]. Ceramic laser materials have the potential to be substantially less expensive than single crystals due to their fast fabrication time and mass production capabilities. Furthermore, ceramic growing does not require complex facilities or essential processes.

Ceramic crystals creation is not limited to Nd-doped materials or YAG crystals. $E{r}^{3+}$, $Y{b}^{3+}$, $N{d}^{3+}$, $E{u}^{3+}$, $D{y}^{3+}$, $C{r}^{4+}$ and sesquioxide host crystals can also be produced. Ceramic laser materials, such as Nd:$Y_2$$O_3$ and Yb:$Y_2$$O_3$, offer a superior advantage compared with single crystals. The production of a single crystal of $Y_2$$O_3$ is highly challenging due to its high melting temperature of 2430 °C [70]. The sintering temperature of $Y_2$$O_3$ ceramics is about 700 °C below its melting point, suggesting that a vacuum sintering technique might be used to make large $Y_2$$O_3$ ceramics. Sesquioxides (e.g., $S{c}_2$$O_3$, $Y_2$$O_3$ and $L{u}_2$$O_3$) are more heat-conductive than YAG, making them suitable for high-power solid state lasers. Nevertheless, the considerable melting point of approximately 2400 °C poses a challenge in the production of large crystals from the molten state [138]. Sesquioxide ceramics have demonstrated good laser characteristics. For example, an uncoated 14 mm × 2.7 mm $Nd:Y_2$$O_3$ ceramic plate was end-pumped with a laser diode, resulting in CW one-micron laser oscillation with a 32% slope efficiency [139]. Sesquioxide transparent ceramics doped with $E{r}^{3+}$ or $H{o}^{3+}$ are highly desirable for high-power, eye-safe laser applications. A resonantly diode-pumped $Ho$:$Y_2$$O_3$ ceramic laser operated at 2.1 $\mathsf{\mu }$m, having a slope efficiency of 35% compared with the absorbed power and 2.5 W of CW output at 77 K [140]. Resonant pumping at 1535 nm produced an $Er$:$S{c}_2$$O_3$ ceramic laser at 1558 nm with a slope efficiency of more than 45%.

In recent years, non-oxide-based ceramic lasers have also evolved significantly. The use of active color centers and rare earth ion doping has been demonstrated in fluoride ceramic lasers [141]. Utilizing a diode pumping nanostructure, $F_2$:LiF color center ceramics was able to produce lasing at a wavelength of 1.117 $\mathsf{\mu }$m. The lasing process achieved a slope efficiency of up to 26% and an output power of approximately 3 mW [142]. By utilizing diode pumping, a fluoride ceramic laser composed of $Ca{F}_2$-Sr$F_2$-Yb$F_3$ achieved an oscillation with a slope efficiency of 45% and an output power of 1.5 W. An experiment by Alimov et al. [143] demonstrated the successful development of a CW laser using a neodymium-doped strontium fluoride ($Nd$:$Sr{F}_2$) ceramic material. The laser operated at a wavelength of 1037 nm and obtained a maximum slope efficiency of 19% when pumped with light at a wavelength of 790 nm. A ceramic laser operating at a wavelength of 639 nm in the visible spectrum was successfully produced by utilizing $Pr$:$Sr{F}_2$ fluoride ceramic material. The slope efficiency of gain-switched lasing in $C{r}^{2+}$:ZnSe samples was demonstrated to reach a maximum of 10%, with an output energy of up to 2 mJ [144]. A recent development in the field of ceramics is the creation of anisotropic fluorapatite (FAP) ceramics doped with rare earth elements. This advancement led to the successful demonstration of a diode-pumped Nd:FAP ceramic laser, as reported in [71]. Nd:YAG single crystals are commonly employed as the most extensively utilized laser medium. An Yb:YAG/$C{r}^{4+}$:YAG microchip laser, utilizing ceramics and operating in a passive Q-switched mode, was successfully fabricated. The laser emitted pulses with a duration of 380 ps. Recent developments in the design and manufacturing of polycrystalline ceramic laser media [18,23,145] have raised the power density and functionality of solid state lasers. The emergence of ceramic laser technology has, in recent decades, positioned it as a high-potential candidate due to its several advantages over single-crystal lasers. Hence, we can say that in the coming years, Nd:YAG ceramics will be a preferable alternative to Nd:YAG single crystals for high-intensity pulse laser applications. Figure 11 shows the progress and technological advancements in laser ceramics over the years.

7. Current Status of Ceramic Laser Applications

In recent years, ceramic-based lasers have gained significant attention, particularly in the field of high-power lasers [17,28,146] and their application in material processing [147,148,149]. Their ability to be produced in large sizes of laser gain media without sacrificing their performance has made them a preferable choice for industrial, defense and scientific applications. They are capable of processing a large range of materials including metals, polymers, semiconductors and composite materials, which makes them highly versatile for different research and industrial applications. Figure 12 shows the schematic of an experimental set-up for material drilling using high-power and high-precision lasers [150]. The set up uses combined laser pulses from nanosecond and millisecond lasers for high-quality and high-efficiency material drilling and processing. More details about this set up and its methodology can be be found elsewhere [149,150].

On the other hand, the outgoing wavelength of ceramic lasers can also be fine-tuned through doping materials, which further enhances their capabilities. For example, Yb:YAG lasers, because of their stability and tunability for specific wavelengths [151], are extensively used in the semiconductor industry. Moreover, laser ceramics have excellent thermal stability, which makes them suitable for handling continuous high-power operations without any performance degradation, making them suitable for high-energy laser weapon systems as well as high-speed cutting and welding processes (see Figure 1). Furthermore, they are also widely used in LiDAR and environmental monitoring systems for essential and accurate data collection [152].

8. Limitations and Possible Solutions

Despite all the advantages of laser ceramics described in above sections, the existing technology is not yet perfect. Ceramic materials face several challenges which restrict their widespread adoption in high-performance laser applications. The present section focuses on the key challenges faced by laser ceramic materials compared to single crystal lasers and what the possible potential solutions could be.

Ceramics are made up of grains that are arranged in various crystal orientations [153], hence making it a significant issue for laser ceramic materials to obtain the optical quality and uniform gain media for high-performance laser systems. Poor optical quality because of light scattering at the grain boundaries, porosity and material inhomogeneities can result in higher laser operating thresholds, lower efficiency and poor beam quality. In contrast, single crystals are known for their outstanding optical homogeneity. To overcome all these difficulties with ceramic materials, reduce optical damage and improve the laser conversion efficiency, the grain boundaries must be fully removed from the material. This can be achieved by advances in sintering processes, grain boundary management and post-fabrication operations, such as annealing of the material [154]. Another challenge with laser ceramics is achieving uniform doping throughout the gain media. The problem of homogeneously doping a ceramic material, in comparison to single crystals, is difficult. A lack of homogeneous distribution of dopants in the ceramic gain media can cause variance in the optical and thermal properties of the gain media which can impact the performance of the laser [155]. The potential solution for this problem could be the development of more sophisticated doping methods which can achieve a more uniform distribution of dopants, such as controlled doping, co-precipitation or sol-gel procedures or diffusion annealing.

The complexity and cost of making high-quality laser ceramics creates an additional challenge in widespread acceptance of laser ceramics [156]. To solve this obstacle, a focus on automating the manufacturing activities and cutting-edge manufacturing processes, such as 3D printing [157], has the potential to reduce production costs as well as increase operational efficiency. Finally, power scaling and heat management provide important challenges for laser ceramics, particularly in high-power laser applications [44]. Thermal gradients and the resulting stress in ceramic materials can produce cracking or thermal lensing, restricting energy scalability in these systems [158]. In contrast, single crystals generally have more predictable thermal properties, making them more suitable for high-power laser applications. To resolve this challenge, sophisticated cooling technologies such as microchannel cooling may need to be developed. Hence, further research and developments in ceramic materials, their manufacturing methods and ceramic laser system designs can improve the feasibility and efficiency of ceramic lasers as an alternative to single-crystal lasers.

9. Summary and Future Perspective

The development of polycrystalline ceramic laser materials has made great progress since the first demonstration of lasing in 1964. Significant accomplishments have been achieved through advancements in powder synthesis and ceramic sintering, as well as the introduction of innovative ideas. These milestones include the introduction of the first Nd:YAG ceramic laser in 1995, which outperformed the 1 kW threshold in 2002, and the remarkable achievement of producing more than 100 kW of output power from a YAG ceramic laser system in 2009.

Recent advancements in laser technology include microchip lasers with high doping levels, ultra-short pulse lasers and the utilization of novel materials including sesquioxides, fluoride and composite ceramic lasers in the 2–3 $\mathsf{\mu }$m range. In addition, composite ceramic lasers have been produced to improve temperature control, and single-crystal lasers have been generated by utilizing polycrystalline ceramics. When compared with mono-crystalline media, they offer distinct advantages in terms of efficiency, allowing for previously unachievable solutions. For instance, exemplary, highly translucent Nd:YAG ceramics possessing little scattering loss exhibit significant promise for high-power laser applications.

In addition to comparable laser efficiency, the process of producing Nd:YAG ceramics is significantly less complex compared with the creation of single crystals. The advancement of highly detailed ceramic laser rods enabled the exhibition of CW laser emission in the 1.5 kW range by lamp pumping, as demonstrated in [57]. Diode laser pumping at a wavelength of 809 nm in the energy level ⁴F_5/2 of conventional or composite ceramic Nd:YAG lasers has shown impressive results. For example, a power output of 144 W was achieved with end-pumped, core-doped rods [159]. Side pumping resulted in a pulsed emission of 236 W [160], and a burst mode emission of 67 kW was achieved in a face-pumped Nd:YAG ceramic with a high heat capacity [161].

The most commonly used high-energy laser facilities, such as the National Ignition Facility (NIF) in the United States [162], Shenguang-III in China [163] and Laser Mega Joule (LMJ) in France [164], employ Nd:glass as their primary material. Nevertheless, these facilities have the capability to generate only one shot every few hours, providing sufficient time for the weak thermally conductive glass media to cool down. With recent advances in the fabrication of large-sized laser-quality transparent ceramics, a diode-pumped solid state laser with a pulse energy of 100 J at 10 Hz was produced at the HiLASE laser facility using Yb:YAG ceramics with Cr:YAG cladding [148].

For the production of laser ceramics of superior quality, careful monitoring is required at every step of the ceramic manufacturing process. Ensuring the absence of irregular portions and achieving near-perfect compaction while preparing powders is extremely crucial but quite difficult. Currently, residual micropores are the main cause of scattering in laser ceramics. After the elimination of optical scattering loss caused by pores, the primary cause of loss in laser ceramics becomes the defects and dislocations found at the interfaces between grains. The inclusion of sintering aids and other impurities has resulted in an unusually high density of dislocations at the grain boundaries.

To minimize the number of sintering aids, it is important to maintain an exceptionally clean experimental environment. The enhancement of ceramic laser efficiency and reduction in laser damage can only be achieved by minimizing optical scattering occurring at the grain boundaries. Undoubtedly, utilization of the solid state crystal growth (SSCG) method can greatly reduce the density of dislocations in single crystals. Producing single crystals of SSCG can yield gain media with superior performance compared with polycrystalline ceramics. Additionally, it allows for the acquisition of single crystals that are challenging to obtain when using conventional melt-growth methods, such as highly doped Nd:YAG, sesquioxides and composites.

Sesquioxide transparent ceramics, including $Y_2$$O_3$, $L{u}_2$$O_3$, and $S{c}_2$$O_3$, have been recognized as promising host materials for high-power solid state lasers. For mixed ceramics, the use of ultra-short pulse lasers with improved power and efficiency can result in enhanced optical quality. The properties of raw powders are essential in the production of high-quality transparent sesquioxide ceramics, and significant efforts have been made to improve these attributes. The manufacturing of nanopowders on a large scale, which possess an excellent capacity to fuse together, disperse evenly and have a consistent chemical composition, continues to be a challenging task. Essentially, resolving these problems and effectively manufacturing large-sized transparent ceramics with excellent optical quality will lead to a considerable advancement in the usage of sesquioxide laser ceramics in efficient solid state lasers. The results of ceramics constructed from two sesquioxides show that they have a broader emission bandwidth and a controllable wavelength range. This indicates that mixed sesquioxide ceramics have great potential for use in ultra-short pulse laser operations.

The recent advances in ceramic lasers have revolutionized their applications and opened up exciting new possibilities. Potential developments include putting them into portable devices, improving their environmental stability and lowering production costs, which could lead to increased application in a wide range of industries. Ceramic lasers are utilized in a wide range of applications, such as high-speed machining for metal cutting and welding, advanced medical devices used in surgical procedures and diagnostic tools, laser guidance systems, red, green and blue (RGB) light sources for projectors and laser television and laser drivers for nuclear fusion. Currently, a number of these applications are under consideration for potential product development. Ceramic laser research is still in its early phases on a global basis. Ceramic lasers are now being investigated and developed in various parts of the world, including Asia [23], America [147] and Europe [17,27,28]. The utilization of ceramic laser technology has garnered significant attention, and it is not an exaggeration to say that a new era centered around this technology has gradually developed for the advancement of future high-power solid state lasers.