Application prospects of 222nm laser in the field of photonic device fabrication 2
1.4 Selection of Technical Route for 222nm Laser
This section focuses on the research of a compact all-solid-state 222 nm UV light source. Based on the above literature review of LD-pumped deep ultraviolet (DUV) solid-state lasers, the characteristics of different Nd³⁺-doped quasi-three-level system gain media, LD pumping methods, resonant cavities, Q-switching methods, and nonlinear frequency-doubling crystals will be analyzed and selected to formulate a technical route for achieving 222nm UV output.
1.4.1 Laser Crystal
1. Characteristics of Different Types of Laser Gain Media
1) Rod-shaped Gain Medium
Solid-state lasers using rod-shaped crystals as gain media offer advantages such as simple and compact structure, low weight, good beam quality, high efficiency, and low cost. To date, rod-shaped gain media have been widely used in low-to-medium power solid-state lasers, including applications targeting far UVC lamp 222 nm technology. The diameter of rod-shaped gain media is generally a few millimeters, and the length usually ranges from a few millimeters to several hundred millimeters. Typically, the pumping methods of solid-state lasers with rod-shaped gain media are classified based on the relationship between the incident direction of the pump light and the propagation direction of the oscillating light, namely longitudinal pumping and transverse pumping. Their structures are shown in Figure 1.1 (a) and (b). Longitudinal pumping refers to the scenario where both the pump light and the oscillating laser propagate in the same longitudinal direction. Transverse pumping means the pump light is incident from the side of the laser medium, while the oscillating laser oscillates in the longitudinal direction. (Figure 1.1 Schematic diagrams of rod-shaped laser structures with longitudinal and transverse pumping, including (a) longitudinal pumping and (b) transverse pumping structures)
During laser operation, part of the pump light energy is not effectively converted into laser output but exists as heat inside the laser crystal. Rod-shaped laser crystals dissipate heat mainly through their sides, which causes a radial temperature gradient in the crystal. This gradient leads to changes in the refractive index gradient and thermal deformation of the crystal's end faces, ultimately resulting in thermal lensing effect, thermally induced birefringence, and thermally induced diffraction loss. These issues restrict the injection of high pump power and degrade the beam quality and power stability of the output laser, particularly in compact 222 nm UV light systems. In recent years, despite significant advancements in LD pump source technology [88], as well as improvements in heat dissipation technology and the quality of bonded laser crystals [89–90], the output performance of rod-shaped lasers has been greatly enhanced. However, rod-shaped lasers still suffer from the unavoidable adverse effect of thermal lensing. Therefore, rod-shaped gain media are generally suitable for low-to-medium power lasers.
2) Thin-disk Gain Medium
As shown in Figure 1.2, the thin-disk gain medium is fabricated into a very thin disk structure along the laser oscillation direction, with a thickness typically ranging from 100 to 300 μm and a diameter of 10 to 20 mm. The total reflection films for both pump light and oscillating light are coated on one end face of the thin-disk medium, which is then fixed in full contact with a heat sink and cooled by circulating water. (Figure 1.2 Schematic diagram of a thin-disk laser structure)
This structure provides a large contact area between the medium's end face and the heat sink, facilitating the rapid and sufficient dissipation of heat absorbed by the gain medium through this large contact area. The temperature gradient direction of the laser crystal in this structure is consistent with the propagation direction of the oscillating light in the cavity. Theoretically, this temperature gradient is small and does not affect the laser output efficiency or beam quality. Compared with rod-shaped crystal lasers, thin-disk laser crystals have a larger heat dissipation surface area and pump light volume, resulting in higher heat dissipation efficiency and suitability for high-power pump light. Due to the extremely small thickness of thin-disk laser crystals, the reflector for the pump light is usually designed into a parabolic structure to allow the pump light to pass through the thin-disk crystal multiple times, thereby improving the utilization efficiency of the pump light. Among thin-disk gain media, Yb:YAG crystals have advantages such as small quantum defect, simple energy level structure, and no parasitic effects (e.g., excited-state absorption (ESA), upconversion (ETU), and cross-relaxation) [91]. Thus, thin-disk gain media based on this material have been extensively studied in recent years and show considerable potential for high-power laser output, though less commonly for specialized 222nm bulb designs. TRUMPF GmbH (Germany) has developed a Yb:YAG thin-disk laser with a multi-disk gain medium structure, achieving an output power of the kilowatt level.
3) Slab Gain Medium
Similar to thin-disk lasers, slab lasers are also developed to address thermal effects encountered in high-power laser generation. However, their structure differs significantly from that of thin-disk gain media: the thickness of the slab gain medium is reduced in the direction perpendicular to the oscillating light, typically to a few millimeters. In 1972, W. S. Martin et al. [92] proposed the typical Zig-Zag slab laser, whose structure is shown in Figure 1.3. It adopts a side pumping method, where the temperature gradient direction in the gain medium is the same as the pump light propagation direction (i.e., perpendicular to the large face of the crystal). To solve the thermal lensing effect, increase the transmittance of the oscillating light, and reduce the reflection loss of the oscillating light on the light-transmitting face of the slab, the light-transmitting face of the gain medium is usually cut at the Brewster angle. This allows the oscillating laser to propagate through multiple total reflections on the large faces of the gain medium, thereby offsetting the thermal effects of the laser crystal in the pump light direction. The slab structure plays an important role in the field of medium-to-high power lasers. (Figure 1.3 Schematic diagram of a slab laser structure)
4) Fiber Gain Medium
Fiber lasers are developed based on fiber amplifiers, using rare-earth-doped glass fibers as gain media. Due to the large surface area-to-volume ratio of fiber gain media, fiber lasers have inherent advantages in heat dissipation performance and promising application prospects in the field of high-power lasers. Compared with traditional single-mode fiber structures, double-clad fibers have an additional inner cladding, consisting of four parts: core, inner cladding, outer cladding, and protective layer. Figure 1.4 shows its typical structural schematic. The core serves as the medium for laser oscillation, and single-mode or multi-mode laser output can be obtained depending on the core size. (Figure 1.4 Schematic diagram of a fiber laser structure)
Typically, irregular structures (e.g., elliptical, D-shaped, square, and plum blossom-shaped) are used to couple the pump light into the inner cladding, with no requirements on the input mode of the pump light. The pump light propagates back and forth between the inner and outer claddings and is absorbed by the core after multiple passes. The outermost layer is the protective layer of the fiber. Among rare-earth ions such as Nd³⁺, Er³⁺, Yb³⁺, and Tm³⁺, Yb³⁺-doped double-clad fibers have many advantages (e.g., extremely low thermal load, small quantum defect, and a wide wavelength output range (975–1180 nm) [93–94]), making them the most widely studied fiber lasers currently.
5) Comparison of Characteristics of Different Laser Gain Media
Although thin-disk, slab, and fiber gain media are conducive to high-power laser output, lasers with thin-disk or slab gain media have relatively complex structures, large volumes, and high costs. Similarly, fibers are long, and the structure of Nd³⁺-doped quasi-three-level system rod-shaped fibers for the 0.9 μm band is complex, bulky, and expensive. Given the goal of this study-to develop a compact far UVC lamp 222 nm system-the Nd³⁺-doped rod-shaped gain media are preferred as the laser crystal.
2. Nd³⁺-doped Quasi-three-level System Gain Media
Nd:YVO₄ and Nd:GdVO₄ laser crystals have a main emission wavelength of 0.91 μm in their quasi-three-level systems. As important gain media for generating 222nm UV via quadruple frequency doubling, they exhibit excellent physical and optical properties. Neodymium-doped yttrium vanadate (Nd:YVO₄) crystal has a tetragonal structure and belongs to the uniaxial crystal system. Its birefringence Δn ranges from 0.2225 to 0.254, and its transparent wavelength range is 0.45–4.8 μm. With hardness close to that of glass, it is not easily deliquescent and easy to process and coat. Thus, it is one of the most commonly used laser crystals currently. Table 1.4 lists the physical properties of the Nd:YVO₄ crystal. The emission cross-section of Nd:YVO₄ at 0.914 μm (σ₉₁₄ₙₘ) is approximately 19.5×10⁻²⁰ cm². Near the 808 nm band, the Nd:YVO₄ crystal has a wide absorption band (about 21 nm). The absorption efficiency of the Nd:YVO₄ crystal for pump light is related to the polarization direction between the pump light and the output laser, with the highest absorption efficiency when the two directions are the same. For a-axis cut crystals, the E-vector of the laser is parallel to the π-polarization direction of the crystal's optical axis and perpendicular to the σ-polarization direction of the optical axis. However, compared with σ-polarization, the crystal exhibits stronger absorption of pump light and stronger radiation in the π-polarization direction. Meanwhile, the frequency doubling efficiency of polarized light is relatively higher during frequency doubling. Therefore, a-axis cut Nd:YVO₄ crystals with π-polarization are mostly used.
Table 1.4 Physical Properties of Nd:YVO₄ Crystal
| Atomic Density / cm³ (Nd³⁺ 1.0%) | Crystal Structure | Density | Mohs Hardness | Thermal Expansion Coefficient (300K) | Thermal Conductivity (300K) |
|---|---|---|---|---|---|
| 1.26×10²⁰ | Tetragonal System | 4.22 g/cm³ | 4–5 | aₐ=4.43×10⁻⁶/K a_c=11.37×10⁻⁶/K | //c: 5.23 W/(m·K) ⊥c: 5.10 W/(m·K) |
The energy level structure of the Nd:YVO₄ crystal is shown in Figure 1.5. The Nd:YVO₄ crystal has high absorption rates for light near the 808 nm and 879 nm bands. Particles in the ground energy level absorb the pump light and transition to the ⁴F₅/₂ energy level. However, the lifetime of particles in the ⁴F₅/₂ energy level is very short (approximately 10⁻¹⁰ s), and they quickly transition to the metastable energy level ⁴F₃/₂ through non-radiative relaxation. (Figure 1.5 Schematic diagram of the energy level structure of Nd:YVO₄ crystal)
The lifetime of particles in the metastable energy level ⁴F₃/₂ is relatively long (approximately 10⁻⁴ s), which provides the condition for population inversion. The Nd:YVO₄ crystal has four main transition energy levels: ⁴F₃/₂→⁴I₁₁/₂, ⁴F₃/₂→⁴I₁₃/₂, ⁴F₃/₂→⁴I₁₅/₂, and ⁴F₃/₂→⁴I₉/₂, corresponding to emission wavelengths of 1064 nm, 1342 nm, 1839 nm, and 914 nm, respectively. At room temperature, the 1064 nm spectral line has the largest emission cross-section and highest gain coefficient, followed by the 1342 nm spectral line, while the 1839 nm and 914 nm spectral lines have the smallest. Among the above four energy level transitions, the ⁴F₃/₂→⁴I₉/₂ transition belongs to the quasi-three-level system of the Nd:YVO₄ crystal. Under the influence of the host lattice field, each energy level of Nd³⁺ ions undergoes Stark splitting. The laser ⁴F₃/₂ energy level splits into two sub-levels (R₁ and R₂), which are very close to each other. The particle populations of the R₁ and R₂ sub-levels follow the Boltzmann distribution law, accounting for 55% and 45% of the total particles in the ⁴F₃/₂ energy level, respectively. The lower laser energy level ⁴I₉/₂ also undergoes Stark splitting, forming 5 sub-levels (Z₁~Z₅), and the particle population in each sub-level also follows the Boltzmann distribution law. The 914 nm spectral line is generated by the transition of particles from the R₁ sub-level of ⁴F₃/₂ to the Z₅ sub-level of ⁴I₉/₂. According to the Boltzmann distribution, the particle population in the lower laser sub-level Z₅ accounts for 5% of the total particles in the ⁴I₉/₂ energy level. These particles in the lower sub-level cause reabsorption of the 914 nm laser generated by the ⁴F₃/₂→⁴I₉/₂ laser energy level transition, which seriously affects the threshold power and output slope efficiency of the laser. Therefore, in-depth research is needed to effectively suppress the influence of reabsorption in the quasi-three-level system and improve the output performance of quasi-three-level system lasers targeting 222 nm UV light.
GdVO₄ and YVO₄ are host crystal materials with the same structure. The ⁴F₃/₂→⁴I₉/₂ energy level transition of the Nd:GdVO₄ crystal's quasi-three-level system generates 912 nm laser. Table 1.5 compares the key performance parameters of the Nd:YVO₄ and Nd:GdVO₄ crystals in their quasi-three-level systems, including fundamental wavelength, frequency-doubled wavelength, stimulated emission cross-section, upper-level lifetime, absorption cross-section, absorption bandwidth, and thermal conductivity.
Table 1.5 Comparison of Performance Parameters of Nd:YVO₄ and Nd:GdVO₄ Crystals in Quasi-three-level Systems
| Crystal | Fundamental Wavelength λₐ / nm | Frequency-doubled Wavelength λₐ / nm | Stimulated Emission Cross-section σₐ / (10⁻²⁰ cm²) | Upper-level Lifetime τ / μs | Absorption Cross-section σₐ / (10⁻²⁰ cm²) | Absorption Bandwidth / nm | Thermal Conductivity / (W/(cm·K)) |
|---|---|---|---|---|---|---|---|
| Nd:YVO₄ | 914 (π) 915 (σ) | 457 | 4.8 (π) 4.3 (σ) | 100 | 60.1 (π) 12.0 (σ) | 20 | 0.0532 |
| Nd:GdVO₄ | 912 | 456 | 6.6 (π) 5.6 (σ) | 95 | 54.6 (π) 12.3 (σ) | 4 (π) 5.8 (σ) | 0.117 |
Through the above comparison of physical and optical properties, the Nd:YVO₄ and Nd:GdVO₄ crystals each have their own characteristics as candidate gain media. Compared with Nd:GdVO₄, the Nd:YVO₄ crystal has a wider absorption bandwidth near the 808 nm band, which reduces the requirements for pump sources and temperature control technology and allows for relatively relaxed external operating conditions of the laser. Additionally, it has a slightly longer upper-level lifetime (suitable for pulsed lasers) and advantages in cost. Therefore, the Nd:YVO₄ crystal is selected as the laser gain medium in this study for achieving efficient 222nm bulb prototypes.
1.4.2 Pumping Method
In LD-pumped solid-state lasers, there are multiple methods to couple the emitted light of LD into the laser gain medium. Typically, these methods are classified into two types based on the relationship between the propagation directions of the pump light and the oscillating laser: end pumping (longitudinal) and side pumping (transverse) (see Figure 1.1). The structural characteristics of these two pumping methods are analyzed and compared below.
1. End Pumping
LD end-pumped low-to-medium power solid-state lasers have the advantages of simple and compact structure, good beam quality, and high efficiency, making them the most widely used lasers, especially in 222nm UV generation setups. They mainly consist of three parts: LD pump source, optical coupling system, and solid-state laser, with their structure shown in Figure 1.1 (a). The laser emitted by the LD is transmitted for pumping along the direction of the oscillating light in the cavity, and the beam is focused into the gain medium. By optimizing the parameters of the resonant cavity, an appropriate spot size ratio between the pump light and the oscillating light (i.e., mode matching) can be achieved. This ratio has a significant impact on the pumping efficiency of the pump light and the output performance of the laser. In addition, the pump light propagates a long distance in the laser crystal and is fully absorbed by the crystal, resulting in a low pump threshold power and high slope efficiency of the laser with this pumping method. Therefore, LD end-pumping technology is widely used in low-to-medium power solid-state lasers with high beam quality and high conversion efficiency. There are two main types of end-pumping technology:
The emitted light of the laser diode is directly injected into the laser crystal through an optical coupling system. By optimizing the design of the resonant cavity and coupling system parameters, the optimal overlap between the pump beam and the oscillating beam can be achieved.
The output light of the LD is first coupled into an optical fiber, and then injected into the laser gain medium through the fiber output. This method not only isolates the solid-state laser from the laser diode to reduce the mutual influence of thermal effects but also allows the fiber to shape the output light of the LD, which is conducive to achieving mode matching.
2. Side Pumping
Although the end-pumping structure has the advantages of high efficiency and good beam quality, it cannot inject high pump power due to the limitations of the small pumping area of the laser crystal and the adverse effect of thermal lensing. With the increase in LD output power and improvements in heat dissipation technology, researchers have optimized the parameters of laser crystals using multiple LD arrays based on the structure of lamp-pumped lasers, injecting the pump light energy into the crystal from the surface of the cylinder or cuboid. This pumping method increases the pump light injection area and heat dissipation surface area of the laser crystal, thereby significantly improving both the injection power and output power. In addition, the increased size of the gain medium also contributes to a certain increase in the laser output power. The structure of side pumping is shown in Figure 1.1 (b). Currently, most all-solid-state lasers with output power ranging from hundreds of watts to kilowatts adopt the side pumping structure.
3. Comparison of the Two Pumping Methods
LD end pumping has the advantages of simple and compact structure, high efficiency, and good beam quality. Although LD side pumping is conducive to high-power laser output, it has a relatively complex structure and large volume. Therefore, the LD end-pumping method is adopted in this study for compact far UVC lamp 222 nm applications.
1.4.3 Laser Resonant Cavity
In all-solid-state lasers, the selection of the laser resonant cavity has a significant impact on the laser output performance. After determining the parameters of the laser crystal, a suitable resonant cavity needs to be selected based on the requirements for laser output performance. From the above analysis of the energy level system of the Nd³⁺-doped gain medium, the quasi-three-level system has a small emission cross-section and suffers from reabsorption. These adverse factors have a non-negligible impact on achieving high-power and high-beam-quality fundamental frequency light output. Therefore, when selecting the resonant cavity structure, two aspects need to be considered: it should be conducive to enhancing the output of fundamental frequency light and achieving high frequency doubling efficiency. From the above literature review of DUV solid-state lasers, for four-level systems, the simplest method to generate the fourth harmonic is to use extracavity second harmonic generation. However, for the quasi-three-level system adopted in this study, the frequency doubling conversion efficiency obtained by extracavity second harmonic generation will be quite low. Therefore, to effectively generate 222 nm UV light, it is necessary to comprehensively consider and select a resonant cavity structure that is easy to implement and enables high-efficiency frequency doubling.
To improve the frequency doubling efficiency, the frequency-doubling crystal is usually placed inside the laser cavity (i.e., intracavity frequency doubling). This allows the fundamental frequency light to pass through the frequency-doubling crystal multiple times (back and forth), thereby improving the utilization rate of the fundamental frequency light. The commonly used cavity types include straight-cavity intracavity frequency doubling, V-cavity intracavity frequency doubling, and Z-cavity intracavity frequency doubling, with their structures shown in Figures 1.6–1.8. A similar effective method is to use an extracavity resonant cavity to increase the circulating power of the fundamental wave, harmonic wave, or both.
The straight-cavity structure has the advantages of simple and compact design and easy adjustment, but its frequency doubling efficiency is low. This is because the spot sizes on the laser gain medium and the frequency-doubling crystal cannot be freely selected, making it impossible to achieve both beam mode matching and high frequency doubling efficiency. Therefore, the straight-cavity structure is not suitable for the Nd³⁺-doped quasi-three-level system gain medium.
The V-cavity resonant structure has two relatively independent beam waists on its two arms, which can simultaneously meet the conditions of mode matching and high-efficiency frequency doubling. However, it suffers from astigmatism, which degrades the beam quality of the laser output.
The Z-cavity has high frequency doubling efficiency and good thermal stability, with slightly better beam quality than the V-cavity. However, it has a complex structure and large intracavity loss.
For the extracavity resonant cavity, a broadband servo is required to precisely control the cavity length, and the transmittance of the input mirror for the incident light must be impedance-matched with the total cavity loss to achieve the highest conversion efficiency. This results in a relatively complex overall structure. (Figure 1.6 Schematic diagram of LD end-pumped straight-cavity intracavity frequency doubling structure) (Figure 1.7 Schematic diagram of LD end-pumped V-cavity intracavity frequency doubling structure) (Figure 1.8 Schematic diagram of LD end-pumped Z-cavity intracavity frequency doubling structure)
Based on the above analysis, the V-shaped laser resonant cavity with intracavity second harmonic generation is preferred in this study, and a relatively simple lens focusing method is adopted for the extracavity quadruple frequency doubling part to optimize 222nm bulb performance.
