Application prospects of 222nm laser in the field of photonic device fabrication 3
Electro-Optic Q-Switching Technology
The structure of an electro-optic Q-switched laser is shown in Figure 1.9. Electro-optic Q-switching employs an electro-optic modulator (electro-optic Q-switch) to control the cavity loss. When voltage is applied to the electro-optic modulator, the electro-optic effect (Pockels effect) modulates the polarization state of the light. Additionally, placing a polarizer or polarizing beam splitter in the resonant cavity allows control of the laser oscillation loss. Thus, electro-optic Q-switching achieves Q-switching triggered by an electrical signal, thereby generating laser pulses. For applications requiring precise deep-UV output, such as 222 nm far UVC light for sale, this technology ensures stable pulse control.
Figure 1.9 Structure of electro-optic Q-switched laser (mirror, laser crystal, polarizer, electro-optic Q-switch, output mirror).
The device that modulates the polarization state of light using the Pockels effect is called a Pockels cell. The structure of a Pockels cell is shown in Figure 1.10. A Pockels cell where the electric field direction is parallel to the laser oscillation direction is called a longitudinal device; one where the electric field is perpendicular to the laser oscillation direction is called a transverse device. In a longitudinal device, the electrode spacing is independent of the clear aperture size, and the required voltage is also independent of the aperture, making it suitable for fabricating large-aperture Pockels cells. In a transverse device, the electrode spacing depends on the aperture size, so it is not suitable for large-aperture Pockels cells but can reduce the voltage to some extent in small-aperture applications.
Figure 1.10 Pockels cell structure (a) Longitudinal device (electro-optic material, electrical signal) (b) Transverse device (electro-optic material, electrical signal)
Deep Ultraviolet 222 nm Solid-State Laser Technology
The voltage applied to a Pockels cell that produces a phase difference of π between the output and input light is called the half-wave voltage (λ/2 voltage), typically ranging from hundreds to thousands of volts. The voltage that produces a π/2 phase difference is called the quarter-wave voltage (λ/4 voltage). Applying these voltages to a Pockels cell yields effects equivalent to the corresponding waveplates. In an electro-optic Q-switch, inserting a polarizing beam splitter and applying a quarter-wave voltage causes the light to pass through the Pockels cell twice in a round trip, rotating the polarization direction by 90°. The polarizing beam splitter then blocks light propagation, thereby altering the cavity loss. This principle is critical in generating high-peak-power pulses for far UVC light 222 nm Amazon compatible disinfection systems.
Numerous crystals are used to fabricate electro-optic Q-switches:
For example, LiNbO₃ crystals have small variations in piezoelectric coefficients and a wide temperature adaptation range (−50 to 60 °C), making them commonly used in military applications.
KD₂PO₄ (KD*P) and KDP are commercially common crystals. Longitudinal devices are typically used to eliminate walk-off effects. KDP's piezoelectric coefficient is temperature-sensitive, prone to depolarization due to thermally induced birefringence, which can be compensated using a polarization rotator and two electro-optic crystals.
LGS and nonlinear crystals offer advantages such as resistance to deliquescence and wide transmission bands, showing great potential for electro-optic Q-switch fabrication in 222nm light sources.
Periodically poled lithium niobate (PPLN) crystals can function as equivalent Bragg modulators. As Q-switching devices, they feature low voltage (<200 V) and repetition rates up to 10 kHz, but their damage threshold needs improvement.
Currently, traditional electro-optic crystals are limited to repetition rates generally below 10 kHz due to factors such as conductivity and electro-optic coefficients. Emerging crystals like PPLN are immature and cannot sustain high-power continuous operation. However, RTP, BBO, and LGS can achieve high repetition rates (on the order of 100 kHz): RTP has a large electro-optic coefficient and a quarter-wave voltage of only 1 kV; BBO has a small electro-optic coefficient and a quarter-wave voltage up to 3 kV; LGS exhibits significant optical activity, complicating fabrication.
Table 1.6 Performance comparison of several common crystals for electro-optic Q-switching devices [99−100]
| Crystal | Advantages | Disadvantages |
|---|---|---|
| Lithium niobate (LiNbO₃) | High transmittance and extinction ratio, low half-wave voltage, small variation in piezoelectric coefficient | Difficult to grow large crystals, exhibits piezoelectric ringing, low damage threshold (10–50 MW/cm²) |
| Potassium dihydrogen phosphate (KDP) [101] | High electro-optic coefficient and high damage threshold | Deliquescent, piezoelectric coefficient highly temperature-dependent (e.g., 80 V/°C @ 1.06 μm) |
| Deuterated potassium dihydrogen phosphate (KD*P) | High electro-optic coefficient and high damage threshold | Deliquescent |
| Potassium titanyl phosphate (KTP) | High electro-optic coefficient and high damage threshold, no piezoelectric ringing | Prone to gray-track phenomenon and breakdown |
| Beta-barium borate (BBO) [102] | No piezoelectric ringing, high damage threshold, enables high-repetition-rate operation | Difficult to grow large crystals, small electro-optic coefficient |
| Rubidium titanyl phosphate (RTP) | Large electro-optic coefficient, high damage threshold, no piezoelectric ringing, enables high-repetition-rate operation | Difficult to grow large crystals, requires two crystals to compensate birefringence |
| Lanthanum gallium silicate (LGS) [103−104] | High transmittance over a wide wavelength range, can grow large crystals | Significant optical activity, complex electro-optic device fabrication |
Acousto-Optic Q-Switching Technology
The mechanism of acousto-optic Q-switching involves ultrasonic waves propagating through an acousto-optic medium to deflect the laser beam, thereby controlling cavity loss. The structure of an acousto-optic Q-switched laser is shown in Figure 1.11. Compared to electro-optic Q-switching, it requires only an acousto-optic modulator (acousto-optic Q-switch) to achieve pulse modulation, resulting in a more compact structure ideal for integrated 222nm UVC light systems.
Figure 1.11 Structure of acousto-optic Q-switched laser (mirror, laser crystal, acousto-optic Q-switch, output mirror).
The structure and working principle of a typical acousto-optic modulator are illustrated in Figure 1.12:
The crystal is typically fused quartz or tellurite glass, with antireflection coatings on the optical faces.
The electro-acoustic transducer converts a high-frequency electrical signal into ultrasonic waves, causing periodic variation in the refractive index of the acousto-optic medium, forming an equivalent volume grating.
When the Bragg diffraction condition is satisfied, the laser is diffracted, resulting in high cavity loss and low Q-value, preventing laser oscillation. After the acoustic field is removed, cavity loss decreases rapidly, forming a laser pulse. Periodic modulation of the Q-value produces pulsed laser output.
Figure 1.12 Structure and working principle of a typical acousto-optic modulator (a) RF off (input beam, transducer, absorber, diffracted beam) (b) RF on (input beam, transducer, acoustic wave, absorber, diffracted beam)
In acousto-optic Q-switching, single-pass cavity loss is approximately 50%, and round-trip loss is about 75%. Ultrasonic frequencies reach the order of 100 MHz, with the electro-acoustic transducer driven by watt-level RF signals. Large modulators require ~10 W RF power and water cooling. Crystals with high elasto-optic coefficients can reduce RF power but have lower optical damage thresholds than fused quartz. One side of the crystal is typically equipped with an acoustic absorber to maintain traveling-wave acoustic propagation. The beam deflection angle in acousto-optic Q-switching is approximately 5°, and the maximum laser repetition rate can reach the MHz level-suitable for high-speed 222 nm far UVC light for sale applications.
Passive Q-Switching Technology
Passive Q-switching uses a saturable absorber to control cavity loss, with optical transmittance varying with absorbed laser intensity. The structure of a passive Q-switched laser is shown in Figure 1.13, requiring only a saturable absorber crystal, making it the simplest configuration for compact far UVC light 222 nm Amazon modules.
Saturable absorbers are divided into transmissive type (transmittance increases with optical power) and reflective type (reflectance increases with optical power). The working process of a typical transmissive device: initially, transmittance is low, leading to high cavity loss; as laser oscillation builds up and optical power increases, transmittance rises to saturation, reducing cavity loss and generating a laser pulse; after pulse emission, intracavity power decreases, and transmittance also drops, completing one Q-switching cycle.
Figure 1.13 Passive Q-switched laser (mirror, laser crystal, saturable absorber crystal, output mirror).
Common saturable absorbers include Cr⁴⁺:YAG, V³⁺:YAG, SESAM, quantum-dot-based lead sulfide glass, graphite coatings, and single-walled carbon nanotubes; the latter two are typically used for passive mode-locking.
Comparison and Selection of Different Q-Switching Methods
| Comparison Dimension | Electro-Optic Q-Switching | Acousto-Optic Q-Switching | Passive Q-Switching |
|---|---|---|---|
| Pulse Width | Several nanoseconds | ~10 ns (optimizable to a few ns) | Depends on laser gain intensity, not actively controllable |
| Drive Requirements | High drive voltage | RF signal drive, no high voltage | No drive power required |
| Repetition Rate | Generally < 100 kHz | Easily > 100 kHz, even 1 MHz | Depends on laser gain intensity |
| Polarization Requirement | Requires modulation of laser polarization | No polarization requirement | None |
| Structural Complexity | Relatively complex | Compact | Simplest |
| Cost | Higher | Cost advantage | Low |
| Pulse Energy | Relatively high | Medium | Lower |
In summary, acousto-optic Q-switching is more suitable for low- to medium-power lasers, including those producing 222nm light for disinfection. This book selects an acousto-optic Q-switch as the Q-switching solution for the laser system.
Nonlinear Frequency-Doubling Crystals
Nonlinear optical frequency conversion relies on the interaction between light and matter, producing second-order nonlinear effects such as second harmonic generation (SHG), sum-frequency generation (SFG), difference-frequency generation (DFG), and optical rectification (OR). Among these, SHG is the most common application: two photons of wavelength λ nonlinearly combine to produce a photon of wavelength λ/2-essential for generating 222nm UVC light from 444 nm fundamentals.
Ultraviolet Laser Nonlinear Frequency-Doubling Crystals
China has achieved significant progress in nonlinear ultraviolet crystals. The following are commonly used crystals:
LBO (LiB₃O₅, lithium triborate): Developed by the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Transmission range: 160–2600 nm; easy to grow (>5 cm³ crystals possible); wide acceptance angle, high optical homogeneity, low walk-off; phase matching tunable by angle or temperature; high damage threshold, non-deliquescent. Widely used in high-average-power lasers for SHG, THG, fourth harmonic generation (FOHG), SFG, and DFG.
BBO (β-BaB₂O₄, beta-barium borate): Developed by the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Transmission range: 190–2500 nm; high damage threshold, good temperature stability, wide phase-matching band, large birefringence, low dispersion; but small acceptance angle, large walk-off, slightly deliquescent (requires coating protection). Industrially widely used in ultraviolet lasers for SHG, THG, FOHG, SFG, and DFG, including 222nm light systems.
CBO (CsB₃O₅, cesium triborate): Developed by the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Transmission range: 170–3000 nm; high damage threshold, large nonlinear optical coefficient, small walk-off angle. Potential applications in laser THG.
CLBO (CsLiB₆O₁₀, cesium lithium borate): Developed by Osaka University, Japan. Transmission range: 180–2750 nm; easy to grow large high-quality crystals, small walk-off, large acceptance angle, low birefringence, low pump beam quality requirements; but highly deliquescent (requires sealing or high-temperature storage). Mostly used in experimental research.
BIBO (BiB₃O₆, bismuth triborate): Monoclinic biaxial crystal. Transmission range: 270–2600 nm; high effective nonlinear coefficient, high damage threshold, low walk-off, wide transmission band, non-deliquescent. Promising in visible and ultraviolet regions, but difficult to grow.
KBBF (KBe₂BO₃F₂, potassium fluoroberyllium borate): Developed by the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Deep-UV nonlinear crystal. Transmission range: 155–3700 nm; moderate birefringence, wide phase-matching range; can achieve shortest UV SHG wavelength of 163.4 nm. Highly promising for UV/deep-UV applications, including 222 nm far UVC light for sale, but growth technology needs improvement.
KABO (K₂Al₂B₂O₇, potassium aluminum borate): Developed by the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Transmission range: 180–3600 nm; birefringence 0.074, small effective nonlinear coefficient; acceptance and walk-off angles better than BBO but inferior to CLBO.
RBBF (RbBe₂BO₃F₂, rubidium fluoroberyllium borate): Reported by the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Transmission range: 160–3550 nm; overcomes KBBF's growth difficulties and cracking tendency, yields large crystals, deliquescence-resistant, chemically stable; deep-UV nonlinear performance slightly inferior to KBBF.
Additionally, novel crystals such as YCOB, GdCOB, ReCOB, and LB4 are under research but not widely applied due to growth technology and damage threshold issues.
Selection of Nonlinear Frequency-Doubling Crystals
For negative uniaxial nonlinear crystals, phase-matching methods mainly include angle phase matching and temperature phase matching:
Angle phase matching: Achieved by selecting the light propagation direction (combining polarization and crystal cut angle). Divided into Type I (o+o→e) and Type II (o+e→e). Prone to beam walk-off; if incident light is perpendicular to the crystal optic axis (θ=90°), walk-off can be eliminated.
Temperature phase matching: Utilizes temperature sensitivity of crystal birefringence and dispersion to achieve phase matching at θ=90°.
Conversion efficiency expression (no walk-off, plane wave, small-signal approximation): η=P3P1=ϵ0cn1n2n3λ32A8π2deff2L2P2A⋅sinc2[∣Δk∣L2]\eta = \frac{P_3}{P_1} = \frac{\epsilon_0 c n_1 n_2 n_3 \lambda_3^2 A}{8\pi^2} d_{\text{eff}}^2 L^2 \frac{P_2}{A} \cdot \operatorname{sinc}^2\left[\frac{|\Delta k| L}{2}\right]η=P1P3=8π2ϵ0cn1n2n3λ32Adeff2L2AP2⋅sinc2[2∣Δk∣L] where deffd_{\text{eff}}deff is the effective nonlinear coefficient, n1,n2n_1, n_2n1,n2 are refractive indices of fundamental light, n3n_3n3 is the refractive index of the sum-frequency light, LLL is the optical path length in the crystal, Δk\Delta kΔk is the phase mismatch, and P2/AP_2/AP2/A is the fundamental light power density.
To improve conversion efficiency, select crystals with high deffd_{\text{eff}}deff under Δk=0\Delta k = 0Δk=0 (phase matching), appropriately increase fundamental light power density, and extend crystal length-critical for efficient 222nm UVC light generation in practical systems.
