Advances in All-Solid-State Passively Q-Switched Lasers Based on Cr 4+ :YAG Saturable Absorber

: All-solid-state passively Q-switched lasers have advantages that include simple structure, high peak power, and short sub-nanosecond pulse width. Potentially, these lasers can be applied in multiple settings, such as in miniature light sources, laser medical treatment, remote sensing, and precision processing. Cr 4+ :YAG crystal is an ideal Q-switch material for all-solid-state passively Q-switched lasers owing to its high thermal conductivity, low saturation light intensity, and high damage threshold. This study summarizes the research progress on all-solid-state passively Q-switched lasers that use Cr 4+ :YAG crystal as a saturable absorber and discusses further prospects for the development and application of such lasers. ∆ t s : the time difference between the establishment of the nth and mth longitudinal modes; t pump : pump pulse duration; τ f : ﬂuorescence lifetime of crystal Nd:YAG; σ k : the stimulated emission cross-section of the k th longitudinal mode, k = 1, 2, · · · · · · , n, · · · · · ·


Introduction
All-solid-state passively Q-switched lasers have the characteristics of simple structure, small size, and broad application potential in areas, such as high-precision processing, laser medical treatment, and laser communication. As these lasers do not require complex extra-cavity modulation devices or high stability of their light source in addition to also possessing high system reliability, they can be applied to various complex environments, including high precision ranging, space detection, and radar [1][2][3]. Materials commonly used for passive Q-switches are organic dyes, doped crystals, and semiconductors.
A Cr 4+ :YAG crystal has a wide absorption band and saturable absorption characteristics at 0.9-1.2 µm. When compared with other saturable absorbers, Cr 4+ :YAG crystal has the advantages of large ground-state absorption cross-section (~10 −18 cm 2 ), high doping concentration (~10 18 cm −3 ), good thermal conductivity, low saturated light intensity, high damage threshold (500 MW/cm 2 ), stable physical properties, and long service life. Therefore, it is an ideal Q-switch material for Nd 3+ -doped and Yb 3+ -doped solid-state lasers [4]. Cr 4+ :YAG has three main absorption bands: 0.48, 0.63, and 1.06 µm. The 0.48 and 0.63 µm absorption bands are in the visible light region, and the absorption coefficients of the two absorption bands are relatively large, but the absorption of Cr 4+ :YAG crystal in these two bands is due to the transfer of Cr 4+ charge in the crystal and the color center, rather than the saturable absorption characteristics of the Cr 4+ :YAG crystals. The absorption of Cr 4+ :YAG crystals in the 1.06 µm absorption band is caused by the energy level transition of the particles in Cr 4+ :YAG, so the 1.06 µm absorption band is an important basis for reflecting the saturable absorption characteristics of Cr 4+ :YAG crystals. The absorption spectrum of Cr 4+ :YAG is shown in Figure 1. Since the 1990s, the emergence of various saturable absorber doped with Cr 4+ ion has attracted attention, and a series of important advances have been made. Figure 2 illustrates the timeline of advances in all-solid-state passively Q-switched lasers based on Cr 4+doped crystals. This study summarizes the research progress on all-solid-state passively Q-switched lasers that use Cr 4+ :YAG crystal as the saturable absorber, including Nd 3+ -doped passively Q-switched laser, Yb 3+ -doped passively Q-switched laser, and other ion-doped lasers. Potential future developments and applications are also discussed.

Nd 3+ -Doped Passively Q-Switched Lasers
In recent years, LD-pumped Nd 3+ -doped lasers have been widely developed and have potential applications in scientific research, infrared remote sensing, etc. The energy level structure of an Nd 3+ ion in Nd:YAG is illustrated in Figure 3 [4]. The 1.06 μm transition provides the lowest threshold laser lines in Nd:YAG. The main laser transition occurs at 1064 nm due to the 4 F3/2→ 4 I11/2 transition, and the main pump band is shown in Figure  3. Since the 1990s, the emergence of various saturable absorber doped with Cr 4+ ion has attracted attention, and a series of important advances have been made. Figure 2 illustrates the timeline of advances in all-solid-state passively Q-switched lasers based on Cr 4+ -doped crystals. Since the 1990s, the emergence of various saturable absorber doped with Cr 4+ ion has attracted attention, and a series of important advances have been made. Figure 2 illustrates the timeline of advances in all-solid-state passively Q-switched lasers based on Cr 4+doped crystals. This study summarizes the research progress on all-solid-state passively Q-switched lasers that use Cr 4+ :YAG crystal as the saturable absorber, including Nd 3+ -doped passively Q-switched laser, Yb 3+ -doped passively Q-switched laser, and other ion-doped lasers. Potential future developments and applications are also discussed.

Nd 3+ -Doped Passively Q-Switched Lasers
In recent years, LD-pumped Nd 3+ -doped lasers have been widely developed and have potential applications in scientific research, infrared remote sensing, etc. The energy level structure of an Nd 3+ ion in Nd:YAG is illustrated in Figure 3 [4]. The 1.06 μm transition provides the lowest threshold laser lines in Nd:YAG. The main laser transition occurs at 1064 nm due to the 4 F3/2→ 4 I11/2 transition, and the main pump band is shown in Figure  3. This study summarizes the research progress on all-solid-state passively Q-switched lasers that use Cr 4+ :YAG crystal as the saturable absorber, including Nd 3+ -doped passively Q-switched laser, Yb 3+ -doped passively Q-switched laser, and other ion-doped lasers. Potential future developments and applications are also discussed.

Nd 3+ -Doped Passively Q-Switched Lasers
In recent years, LD-pumped Nd 3+ -doped lasers have been widely developed and have potential applications in scientific research, infrared remote sensing, etc. The energy level structure of an Nd 3+ ion in Nd:YAG is illustrated in Figure 3 [4]. The 1.06 µm transition provides the lowest threshold laser lines in Nd:YAG. The main laser transition occurs at 1064 nm due to the 4 F 3/2 → 4 I 11/2 transition, and the main pump band is shown in Figure 3.
According to this diagram, the emission wavelengths of the Nd 3+ ions coincide with the absorption wavelengths of the Cr 4+ :YAG crystal, specifically in the commonly used 1.06 μm band. Therefore, using Nd 3+ -doped passively Q-switched lasers with Cr 4+ :YAG crystal as the saturable absorber has garnered scientific interest.

Research Progress on Passively Q-Switched Lasers in the Infrared Band
In 2016, Tasi et al. first reported spatiotemporal chaos under a stationary spatial distribution of polarization. A period-doubling route to chaos was observed in a passively Q-switched Nd:GdVO4 laser beam with azimuthal polarization [14]. Figure 4 demonstrates the testing setup and output results. Due to stringent emission norms, increasing demands for higher efficiency and reliable engine operation, and rapid depletion of petroleum resources, extensive research work is being conducted to explore advanced combustion and ignition technologies as well as the use of alternative fuels for internal combustion engines [15,16]. Laser ignition allows precise control of ignition energy deposited in ignition plasma and therefore increases the efficiency of the engine, which has led to increased research on laser ignition in the last few years [17][18][19][20][21][22].
With the development of crystal growth technology, studies have found that controllable Hermite-Gauss (HG), Ins-Gauss (IG), and Laguerre-Gauss (LG) mode lasers can be obtained from different orientations of Nd:YVO4 crystals [23,24]. Dong et al. have pumped According to this diagram, the emission wavelengths of the Nd 3+ ions coincide with the absorption wavelengths of the Cr 4+ :YAG crystal, specifically in the commonly used 1.06 µm band. Therefore, using Nd 3+ -doped passively Q-switched lasers with Cr 4+ :YAG crystal as the saturable absorber has garnered scientific interest.

Research Progress on Passively Q-Switched Lasers in the Infrared Band
In 2016, Tasi et al. first reported spatiotemporal chaos under a stationary spatial distribution of polarization. A period-doubling route to chaos was observed in a passively Q-switched Nd:GdVO 4 laser beam with azimuthal polarization [14]. Figure 4 demonstrates the testing setup and output results. According to this diagram, the emission wavelengths of the Nd 3+ ions coincide with the absorption wavelengths of the Cr 4+ :YAG crystal, specifically in the commonly used 1.06 μm band. Therefore, using Nd 3+ -doped passively Q-switched lasers with Cr 4+ :YAG crystal as the saturable absorber has garnered scientific interest.

Research Progress on Passively Q-Switched Lasers in the Infrared Band
In 2016, Tasi et al. first reported spatiotemporal chaos under a stationary spatial distribution of polarization. A period-doubling route to chaos was observed in a passively Q-switched Nd:GdVO4 laser beam with azimuthal polarization [14]. Figure 4 demonstrates the testing setup and output results. Due to stringent emission norms, increasing demands for higher efficiency and reliable engine operation, and rapid depletion of petroleum resources, extensive research work is being conducted to explore advanced combustion and ignition technologies as well as the use of alternative fuels for internal combustion engines [15,16]. Laser ignition allows precise control of ignition energy deposited in ignition plasma and therefore increases the efficiency of the engine, which has led to increased research on laser ignition in the last few years [17][18][19][20][21][22].
With the development of crystal growth technology, studies have found that control-  Due to stringent emission norms, increasing demands for higher efficiency and reliable engine operation, and rapid depletion of petroleum resources, extensive research work is being conducted to explore advanced combustion and ignition technologies as well as the use of alternative fuels for internal combustion engines [15,16]. Laser ignition allows precise control of ignition energy deposited in ignition plasma and therefore increases the efficiency of the engine, which has led to increased research on laser ignition in the last few years [17][18][19][20][21][22].
YAG and YVO 4 crystals are the most commonly used materials for Nd 3+ ion doping. However, the output wavelength range of the Nd:YAG, Nd:YVO 4 passively Q-switched laser is limited and cannot produce the specific wavelengths required in various fields. As crystal growth technology has developed, studies have investigated new substrates for Nd 3+ ion doping, aiming to improve the laser's performance [27][28][29][30][31][32][33]. In addition, studies have shown that doping by multiple ions in the same substrate can increase the energy level life of the laser crystal, the crystal's heat dissipation capacity, and enhance its absorption bandwidth as well as increasing the utilization rate of the pump laser [34][35][36][37].
To further improve the overall performance of the all-solid-state passively Q-switched laser and solve the problems of thermal effects and timing jitter, studies have covered improvements in pump source [38,39], crystal position [40,41], cavity structure [22,42], Q-switching method [43], and temperature control [44][45][46][47][48][49]. In 2016, Zheng et al. proposed fast-pulse pumping technology to solve the thermal effect and timing jitter issues [50]; Chen et al. proposed a set of analytical expressions (1) to analyze the time difference between adjacent longitudinal modes of passively Q-switched lasers. An Nd:YAG/Cr 4+ :YAG laser was used for experimental verification, and a single longitudinal mode output was realized [51].
In 2017, Croitoru et al. investigated air breakdown in a static chamber using a passively Q-switched Nd:YAG/Cr 4+ :YAG laser. The energy transfer characteristics from a laser pulse to plasma were estimated by investigating the total input energy and local transfer energy [52]. In 2019, Villafana et al. used a periodic simulation model to analyze Nd:YVO 4 /Cr 4+ :YAG passively Q-switched lasers. This model is suitable for controlling the laser stability in different harmonic ranges [53]. Bai et al. reported a passively Qswitched Nd:YAG/Cr 4+ :YAG laser that operates stably at temperatures between −74 and (approximately) 50 • C, or over 100 • C. The experiment used a semiconductor laser diode array for side pumping, which enabled stable laser operation despite large temperature fluctuations [54].
Although all-solid-state passively Q-switched lasers have been utilized in differing fields (scientific research, military, etc.), many applications place strict requirements on the laser's wavelength and overall volume. For example, the field of nonlinear frequency conversion requires single longitudinal mode pulsed laser, and in a vehicle-mounted laser ignition device, the volume of the laser cannot exceed that of the spark plug. To improve the portability of the laser, the gain medium and saturable absorber can be thermally bonded and then used to directly coat both ends of the composite crystal. These design improvements reduce the volume of the laser while ensuring stable laser output [55][56][57]. Further studies have led to the development of a single longitudinal mode Q-switched laser with increased stability and improved repression of the spatial hole burning effect. These improvements were realized by increasing the pump power, changing the crystal's temperature, adding a modulator, and adopting a ring cavity [49,[58][59][60][61][62].
The relaxation time of the first excited state of Cr 4+ :YAG is~3.4 µs, which is much longer than the pulse oscillation time for most cases. Therefore, with low intracavity intensity, most of the population in Cr 4+ :YAG crystal is in the ground state, and the transition to higher-lying levels is rather weak. In this condition, Cr 4+ :YAG crystal acts as an effective saturable absorber only for Q-switching operation. While the intracavity laser intensity is large enough to reach the saturable absorption of the excited state, the relaxation time from high energy level to the first excited state is in the sub-nanosecond range. Qswitched mode-locking can then be generated with a single mode-locked pulse duration near the maximum of the Q-switched envelope in the order of picoseconds [63][64][65][66][67][68].
The gain medium of the all-solid passively Q-switched laser is usually round rod or block, but there are serious thermal lens effects and birefringence effects under high-power pumping, which make the gain medium produce strong optical distortion and limit the output power of the laser. The waveguide laser improves the power by providing onedimensional control of the thermal lens effect and increasing the width and length of the waveguide doped region. Therefore, combining the waveguide structure with a passive Q-switched crystal can obtain higher peak power pulsed laser output [69][70][71].

Passively Q-Switched Green Lasers
Green pulsed lasers (490-580 nm) have aroused significant interest because of their potential application to many fields. Three established methods exist for all-solid-state passively Q-switched lasers to achieve visible laser output. One method uses blue-pumped Pr 3+ , Sm 3+ , Tb 3+ , Dy 3+ , Ho 3+ , and Er 3+ passively Q-switched lasers. The second method involves generating a~1 µm pulsed laser, which irradiates a frequency-doubling crystal, and a green laser is emitted [72]. Although this method can generate high peak power, the frequency conversion step from the near-infrared to the green spectral range requires highly stable fundamental frequency laser. This condition leads to overly complex systems. The third method is self-frequency-doubled Nd 3+ -doped or Yb 3+ -doped lasers. In this method, a high peak power pulsed laser is obtained by inserting a saturable absorber in the self-frequency-doubled lasers. When compared with the first two methods, the self-frequency-doubled lasers are more compact and effective [73,74].

Passively Q-Switched Raman Lasers
Compact solid-state lasers that can simultaneously emit multiple wavelengths have a wide range of applications, including optical communications, environmental monitoring, and medical instruments. Stimulated Raman scattering enables traditional lasers to achieve stable, concurrent multi-wavelength output. Combining stimulated Raman scattering and LD-pumped passively Q-switched lasers can produce dual-wavelength Raman laser [75][76][77].
All-solid-state passively Q-switched Raman lasers are the most compact and effective Raman lasers, because the laser medium and the Raman medium are the same crystal. The passive Q-switched Raman lasers have high peak power and narrow pulse width, effectively realizing multi-wavelength output. Owing to its excellent Raman gain, Nd:YVO 4 crystal is often used as a self-Raman medium. In 2016, Lin et al. reported a multi-wavelength passively Q-switched self-Raman laser. Using c-cut Nd:YVO 4 as the laser and Raman medium, and Cr 4+ :YAG crystal as the saturable absorber, the self-Raman laser wavelength outputs were 1067.4, 1097.9, 1130.1, and 1163.6 nm [78]. This team achieved 1066.7 and 1168.6 nm Raman laser output using equipment based on Lin et al.'s study [79]. In 2019, Chen et al. reported a c-cut Nd:YVO 4 /Cr 4+ :YAG passively Q-switched self-Raman laser, with a maximum average output power of 2.5 W; the highest reported value for a passively Q-switched Raman laser [80]. In 2020, Li et al. reported a multi-wavelength passively Q-switched intracavity Raman laser. In the study, c-cut Nd:GdVO 4 and c-cut Nd:YVO 4 crystals were used as the self-Raman and Raman medium, respectively. The laser generated simultaneous wavelengths of 1096, 1176, and 1177 nm, with average output power of 47.6, 27, and 44.2 mW, respectively [81].

Passively Q-Switched Vortex Lasers
Helical beams with spiral phase and orbital angular momentum are widely used in particle manipulation, material processing, quantum information, optical imaging, and optical communication. By adding passive Q-switching technology to the vortex laser, more effective vortex beam power scaling can be achieved. In 2017, Chen et al. analyzed the optimal cavity length required to achieve passive Qswitching in an off-axis pumped Nd:YVO 4 /Cr 4+ :YAG laser. Additionally, using an external cylindrical mode converter, high-order eigenmodes and geometric modes generated vortex beams with large orbital angular momentum [82]. The experimental and theoretical results are shown in Figure 5. In 2017, Chen et al. analyzed the optimal cavity length required to achieve passive Qswitching in an off-axis pumped Nd:YVO4/Cr 4+ :YAG laser. Additionally, using an external cylindrical mode converter, high-order eigenmodes and geometric modes generated vortex beams with large orbital angular momentum [82]. The experimental and theoretical results are shown in Figure 5. In 2018, Tuan et al. studied the output performance of off-axis pumped Nd:YAG/Cr 4+ :YAG lasers under different degenerate cavities and obtained a stable output for passively Q-switched laser. Various high-energy vortex beams, with large angular momentum and diverse phase structures, were successfully realized using an astigmatic mode converter (AMC) [83]. In 2019, Pan et al. reported a dual-vortex passively Qswitched microchip laser with controllable orientation and separation. The Nd:YAG/Cr 4+ :YAG composite crystal was pumped by an off-center ring beam to achieve a dual-vortex laser pulse with a peak power of 5 kW and pulse width of 3.6 ns. By adjusting the position of the collimating lens, the direction and distance between the two holes in the dual-vortex laser pulse can be varied. The experimental setup, experimental results, and theoretical analysis are shown in Figure 6 [84].

Yb 3+ -Doped Passively Q-Switched Lasers
When compared with Nd 3+ -doped lasers, Yb 3+ -doped lasers have the advantages of absorption bandwidth, long fluorescence lifetime, large emission cross-section, and low In 2018, Tuan et al. studied the output performance of off-axis pumped Nd:YAG/Cr 4+ :YAG lasers under different degenerate cavities and obtained a stable output for passively Qswitched laser. Various high-energy vortex beams, with large angular momentum and diverse phase structures, were successfully realized using an astigmatic mode converter (AMC) [83]. In 2019, Pan et al. reported a dual-vortex passively Q-switched microchip laser with controllable orientation and separation. The Nd:YAG/Cr 4+ :YAG composite crystal was pumped by an off-center ring beam to achieve a dual-vortex laser pulse with a peak power of 5 kW and pulse width of 3.6 ns. By adjusting the position of the collimating lens, the direction and distance between the two holes in the dual-vortex laser pulse can be varied. The experimental setup, experimental results, and theoretical analysis are shown in Figure 6 [84]. In 2017, Chen et al. analyzed the optimal cavity length required to achieve passive Qswitching in an off-axis pumped Nd:YVO4/Cr 4+ :YAG laser. Additionally, using an external cylindrical mode converter, high-order eigenmodes and geometric modes generated vortex beams with large orbital angular momentum [82]. The experimental and theoretical results are shown in Figure 5. In 2018, Tuan et al. studied the output performance of off-axis pumped Nd:YAG/Cr 4+ :YAG lasers under different degenerate cavities and obtained a stable output for passively Q-switched laser. Various high-energy vortex beams, with large angular momentum and diverse phase structures, were successfully realized using an astigmatic mode converter (AMC) [83]. In 2019, Pan et al. reported a dual-vortex passively Qswitched microchip laser with controllable orientation and separation. The Nd:YAG/Cr 4+ :YAG composite crystal was pumped by an off-center ring beam to achieve a dual-vortex laser pulse with a peak power of 5 kW and pulse width of 3.6 ns. By adjusting the position of the collimating lens, the direction and distance between the two holes in the dual-vortex laser pulse can be varied. The experimental setup, experimental results, and theoretical analysis are shown in Figure 6 [84].

Yb 3+ -Doped Passively Q-Switched Lasers
When compared with Nd 3+ -doped lasers, Yb 3+ -doped lasers have the advantages of absorption bandwidth, long fluorescence lifetime, large emission cross-section, and low

Yb 3+ -Doped Passively Q-Switched Lasers
When compared with Nd 3+ -doped lasers, Yb 3+ -doped lasers have the advantages of absorption bandwidth, long fluorescence lifetime, large emission cross-section, and low quantum defects. In addition, the Yb 3+ -doped crystal's simple electronic structure, combined with low quantum defects, significantly reduces the thermal load in the ma- terial [85]. At low temperatures, Yb 3+ -doped crystals can be used as a four-level laser medium with excellent thermo-optical properties [86]. Figure 7 demonstrates the Yb 3+ energy level structure of Yb:YAG. The laser has a quasi-three-level operating mechanism because its upper energy level cannot have excited state absorption and up-conversion; particularly, the laser only has a 2 F 7/2 ground state and 2 F 5/2 stimulated multiple states with a spacing of 10,000 cm −1 [87]. quantum defects. In addition, the Yb 3+ -doped crystal's simple electronic structure, combined with low quantum defects, significantly reduces the thermal load in the material [85]. At low temperatures, Yb 3+ -doped crystals can be used as a four-level laser medium with excellent thermo-optical properties [86]. Figure 7 demonstrates the Yb 3+ energy level structure of Yb:YAG. The laser has a quasi-three-level operating mechanism because its upper energy level cannot have excited state absorption and up-conversion; particularly, the laser only has a 2 F7/2 ground state and 2 F5/2 stimulated multiple states with a spacing of 10,000 cm −1 [87]. In 2017, Šulc et al. studied the influence of temperature on passively Q-switched lasers. Studies have shown that the highest pulse repetition rate of the output light was 38 kHz, the maximum single pulse energy was 0.4 mJ, the highest peak power was 325 kW, and the pulse width reached the minimum value of 2.2 ns at 80, 180, 220 and 230 K [88]. In 2019, this team studied the effects of temperature and pump radius on pulsed lasers. The best output performance was achieved when the pulsed laser's temperature was 140 K and the pump light radius was 0.87 mm. The laser's pulse energy was 2.41 mJ, and the corresponding peak power was 1.67 MW [89]. In 2018, Rodenko et al. reported a 343 nm solid-state laser, produced using Yb:YAG/Cr 4+ :YAG, to generate a fundamental frequency laser, then frequency doubling is performed by two lithium triborate (LBO) crystals [90]. In 2020, Kim et al. successfully constructed a diode laser pumped passively Q-switched Yb:YAG laser ignitor, which had low power, long pulse width, and a quasi-MW output power [91].
Generally, two temperature controllers are used in passively Q-switched lasers to cool the laser medium and saturable absorber. These increase the volume of the laser and generate additional losses. The volume of the laser and cavity loss can be reduced, and the laser output performance can be improved by using thermal bonding technology to combine the laser medium and saturable absorber into a composite crystal. By direct coating at both ends of the composite crystals, a passive Q-switched microlaser can be formed, which generates pulsed lasers with a high repetition rate and high peak power. The main advantages of the microlaser are its simple structure, low maintenance requirements, and that no fine alignment is required [92][93][94].
The solid-state waveguide laser is a miniature laser source based on the optical waveguide of the laser material. It has the advantages of small size, high stability, and easy integration, in addition to the waveguide structure being able to effectively reduce the thermal effect on the laser. The combination of Yb 3+ -doped waveguide laser and Cr 4+ :YAG Studies have shown that the highest pulse repetition rate of the output light was 38 kHz, the maximum single pulse energy was 0.4 mJ, the highest peak power was 325 kW, and the pulse width reached the minimum value of 2.2 ns at 80, 180, 220 and 230 K [88]. In 2019, this team studied the effects of temperature and pump radius on pulsed lasers. The best output performance was achieved when the pulsed laser's temperature was 140 K and the pump light radius was 0.87 mm. The laser's pulse energy was 2.41 mJ, and the corresponding peak power was 1.67 MW [89]. In 2018, Rodenko et al. reported a 343 nm solid-state laser, produced using Yb:YAG/Cr 4+ :YAG, to generate a fundamental frequency laser, then frequency doubling is performed by two lithium triborate (LBO) crystals [90]. In 2020, Kim et al. successfully constructed a diode laser pumped passively Q-switched Yb:YAG laser ignitor, which had low power, long pulse width, and a quasi-MW output power [91].
Generally, two temperature controllers are used in passively Q-switched lasers to cool the laser medium and saturable absorber. These increase the volume of the laser and generate additional losses. The volume of the laser and cavity loss can be reduced, and the laser output performance can be improved by using thermal bonding technology to combine the laser medium and saturable absorber into a composite crystal. By direct coating at both ends of the composite crystals, a passive Q-switched microlaser can be formed, which generates pulsed lasers with a high repetition rate and high peak power. The main advantages of the microlaser are its simple structure, low maintenance requirements, and that no fine alignment is required [92][93][94].
The solid-state waveguide laser is a miniature laser source based on the optical waveguide of the laser material. It has the advantages of small size, high stability, and easy integration, in addition to the waveguide structure being able to effectively reduce the thermal effect on the laser. The combination of Yb 3+ -doped waveguide laser and Cr 4+ :YAG can effectively improve the thermal effect of the laser and increase the peak power of the output laser [69,70,95,96].
In 2017, Ye et al. reported a passively Q-switched frequency-switched laser based on a Yb:YAG/Cr 4+ :YAG/YAG composite crystal, which achieved simultaneous dualwavelength laser oscillation at 515 and 524.5 nm. The experimental setup is illustrated in Figure 8 [97]. can effectively improve the thermal effect of the laser and increase the peak power of the output laser [69,70,95,96]. In 2017, Ye et al. reported a passively Q-switched frequency-switched laser based on a Yb:YAG/Cr 4+ :YAG/YAG composite crystal, which achieved simultaneous dual-wavelength laser oscillation at 515 and 524.5 nm. The experimental setup is illustrated in Figure  8 [97].  Figure 9. By adjusting the pump power, the frequency interval can be varied [98]. As crystal growth technology has developed, studies have addressed the specific wavelength requirements of various fields while also improving laser performance. For example, yttrium-doped calcium borate compounds are an excellent active media for passively Q-switched lasers, which can be operated at room temperature [99]. The thermal conductivity of Lu2O3 ceramics is 12.5 W/mK (at 300 K), and the thermal conductivity of Lu2O3 ceramics will not change significantly even if laser ions with higher concentrations are added [100].
In 2016, Liu et al. reported a Yb:LuPO4/Cr 4+ :YAG passively Q-switched laser that achieved 1013.3 nm pulsed laser output at room temperature. Its pulse repetition rate, pulse energy, duration, and peak power are 23.8 kHz, 22.3 mJ, 4.0 ns, and 5.6 kW, respectively [101]. In 2017, Schoepp et al. reported a Yb:CaF2/Cr 4+ :YAG passively Q-switched laser that achieved a 1050 nm pulsed laser output. The M 2 of the output laser was 1.1, and the peak power, pulse energy, and pulse width were 8.9 kW, 0.7 mJ, and 78 ns, respectively [102]. Subsequently, the same team also reported a compact Yb:YAB passively Qswitched laser using a c-cut Yb:YAB crystal and with a laser output of 1041 nm [103]. In 2018, Liu et al. reported the first passive Q-switched laser based on a Yb:GSGG crystal. Its repetition rate, pulse duration, and single pulse energy are 20 kHz, 21 ns, and 38 μJ, respectively [104]. In 2019, David S.P. et al. reported a Yb:Y2O3/Cr 4+ :YAG ceramic passively  Figure 9. By adjusting the pump power, the frequency interval can be varied [98]. can effectively improve the thermal effect of the laser and increase the peak power of the output laser [69,70,95,96]. In 2017, Ye et al. reported a passively Q-switched frequency-switched laser based on a Yb:YAG/Cr 4+ :YAG/YAG composite crystal, which achieved simultaneous dual-wavelength laser oscillation at 515 and 524.5 nm. The experimental setup is illustrated in Figure  8 [97].  Figure 9. By adjusting the pump power, the frequency interval can be varied [98]. As crystal growth technology has developed, studies have addressed the specific wavelength requirements of various fields while also improving laser performance. For example, yttrium-doped calcium borate compounds are an excellent active media for passively Q-switched lasers, which can be operated at room temperature [99]. The thermal conductivity of Lu2O3 ceramics is 12.5 W/mK (at 300 K), and the thermal conductivity of Lu2O3 ceramics will not change significantly even if laser ions with higher concentrations are added [100].
In 2016, Liu et al. reported a Yb:LuPO4/Cr 4+ :YAG passively Q-switched laser that achieved 1013.3 nm pulsed laser output at room temperature. Its pulse repetition rate, pulse energy, duration, and peak power are 23.8 kHz, 22.3 mJ, 4.0 ns, and 5.6 kW, respectively [101]. In 2017, Schoepp et al. reported a Yb:CaF2/Cr 4+ :YAG passively Q-switched laser that achieved a 1050 nm pulsed laser output. The M 2 of the output laser was 1.1, and the peak power, pulse energy, and pulse width were 8.9 kW, 0.7 mJ, and 78 ns, respectively [102]. Subsequently, the same team also reported a compact Yb:YAB passively Qswitched laser using a c-cut Yb:YAB crystal and with a laser output of 1041 nm [103]. In 2018, Liu et al. reported the first passive Q-switched laser based on a Yb:GSGG crystal. Its repetition rate, pulse duration, and single pulse energy are 20 kHz, 21 ns, and 38 μJ, respectively [104]. In 2019, David S.P. et al. reported a Yb:Y2O3/Cr 4+ :YAG ceramic passively As crystal growth technology has developed, studies have addressed the specific wavelength requirements of various fields while also improving laser performance. For example, yttrium-doped calcium borate compounds are an excellent active media for passively Q-switched lasers, which can be operated at room temperature [99]. The thermal conductivity of Lu 2 O 3 ceramics is 12.5 W/mK (at 300 K), and the thermal conductivity of Lu 2 O 3 ceramics will not change significantly even if laser ions with higher concentrations are added [100].
In 2016, Liu et al. reported a Yb:LuPO 4 /Cr 4+ :YAG passively Q-switched laser that achieved 1013.3 nm pulsed laser output at room temperature. Its pulse repetition rate, pulse energy, duration, and peak power are 23.8 kHz, 22.3 mJ, 4.0 ns, and 5.6 kW, respectively [101]. In 2017, Schoepp et al. reported a Yb:CaF 2 /Cr 4+ :YAG passively Q-switched laser that achieved a 1050 nm pulsed laser output. The M 2 of the output laser was 1.1, and the peak power, pulse energy, and pulse width were 8.9 kW, 0.7 mJ, and 78 ns, respectively [102]. Subsequently, the same team also reported a compact Yb:YAB passively Q-switched laser using a c-cut Yb:YAB crystal and with a laser output of 1041 nm [103]

Brief Summary
As the Cr 4+ :YAG crystal has a wide transmission range (0.9-1.2 µm) and a particularly strong absorption peak in the 1 µm band, it is frequently used as the passive Q-switch material for Nd 3+ -doped and Yb 3+ -doped lasers. Table 1 lists the research progress on Nd 3+ and Yb 3+ passively Q-switched lasers over the last five years. The table summarizes information about the doped ion, laser crystal, center wavelength, peak power, repetition rate, and pulse width. Notes. T 0 : the initial transmittances of Cr 4+ :YAG; P: peak power; f : repetition rate; FWHM: pulse width; "-" means no relevant data; "*" means the output parameters are measured after the amplifier.
When compared with the Nd 3+ -doped lasers, the main advantage of the Yb 3+ -doped lasers is that the pump and emission wavelengths are close, ensuring a low thermal effect. However, because the emission cross-section of Yb 3+ ions is much smaller than that of Nd 3+ ions, a higher pump intensity is required to achieve the given gain.

Summary and Outlook
Using Q-switched lasers with Cr 4+ :YAG crystal as the saturable absorber can provide high peak power, large pulse energy, high beam quality, and sub-nanosecond pulse width. They have the potential to be widely used in various fields, including high-precision laser processing, space exploration, laser medical treatment, and material analysis. Over the last five years, some unstable features of these lasers have been progressively analyzed and improved upon. In addition, researchers have successfully realized passively Q-switched pulsed laser output of Pr 3+ -doped lasers [121,122] and ceramic lasers [123,124]. Usually, to reduce the lasing threshold, the saturable absorber is placed near the beam waist of the resonator for higher power density to reach the saturated absorption. In addition, the saturable absorber is usually cooled by thermoelectric cooler (TEC) or water for long-term stable operation, especially in the case of high-power pumping.
In conclusion, an all-solid-state passively Q-switched laser can produce peak power output in the order of megahertz, and its pulse width can be compressed to the order of sub-nanoseconds. Their overall design is becoming more compact and stable. However, all-solid-state passively Q-switched lasers also have limitations, for example, the damage threshold of coating on both ends of crystals is much smaller than that of the laser medium, limiting the peak power and single pulse energy output. With the continuous progress in scientific research and technology, the demand for the output performance of passively Qswitched lasers will continue to increase. Future technical demands will require continued development of all-solid-state passively Q-switched laser performance through research into new doped ions, suitable laser matrices with high doping concentrations, and laser ceramics. Advances in these areas could rapidly lead to the development of lasers with higher energy, narrower pulse width, and better stability.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest:
The authors declare no conflict of interest.