CO2 Laser Optics at the Forefront: Pushing Boundaries with CO2 Laser Lenses and CO2 Laser Mirrors

Author: Bryan Ng – Marketing Manager

Carbon dioxide (CO₂) lasers have become a cornerstone technology in numerous industries, including laser processing, medicine, and research. These lasers emit a beam of infrared light with a wavelength of 9.6 or 10.6 µm, enabling them to interact with a wide range of materials. However, the successful utilization of CO₂ lasers heavily relies on high-quality CO₂ laser optics.

1. The Importance of CO₂ Laser Optics in CO₂ Laser Systems

CO₂ laser optics play a vital role in beam delivery, control, and focusing, ultimately influencing the laser’s precision, efficiency, and overall performance. The specific optical components used in a CO₂ laser system will depend on the application. For example, a laser cutting system will typically use a focusing lens to focus the beam onto the workpiece, while an engraving system may use a beam expander to widen the beam.

The most common optical materials used in CO₂ laser optics are zinc selenide (ZnSe) and germanium (Ge). These materials are transparent to infrared light and have high thermal conductivity, which helps to dissipate the heat generated by the laser beam. The specific optical components used in a CO₂ laser will depend on the application.

The quality of the optical components used in a CO₂ laser system has a significant impact on the performance of the laser. High-quality optical components will ensure that the laser beam is transmitted and focused accurately, which will result in better cutting, welding, and engraving results.

2. Components of CO₂ Laser Optics

CO₂ laser optics encompass a diverse range of components that facilitate the manipulation and control of the laser beam. The primary components include CO₂ laser mirrors, lenses, windows, beam splitters, and attenuators, each playing a crucial role in optimizing the laser’s characteristics.

2.1 CO₂ Laser Mirrors

CO₂ laser mirrors are integral to the beam delivery and control in a CO₂ laser system. They are responsible for reflecting the laser beam and maintaining its alignment. Reflective CO₂ laser mirrors must have low reflection losses, high optical quality, and good resistance against extreme optical intensity.

Diameter Tolerance: +0/-0.13mm
Thickness Tolerance: ±0.25mm
Parallelism: ≤3 arc min
Clear Aperture: >90%
Surface Flatness: λ/4 per 1” Dia @ 632.8nm
Surface Quality: 40-20 S-D
Angle of Incidence: 45°

Table 1: Wavelength Opto-Electronic CO₂ Laser Mirrors

CO₂ laser cavity optics consist of a rear mirror and a front mirror (also called an output coupler or partial reflector ). Rear mirrors, typically ZnSe, with very high reflectivity (>99.7%) are key optical components in laser resonators. Output couplers are partially reflective mirrors to extract a portion of the laser beam from the laser resonator. They often require a slight wedge to prevent interference from multiple reflections inside the component. High-quality CO₂ laser mirrors minimize losses and ensure efficient beam delivery.

Diameter Tolerance: +0/-0.13mm
Thickness Tolerance: ±0.25mm
Centration: < 3 arc min
Clear Aperture: >90%
Surface Quality: 40-20 S-D
Angle of Incidence: 0°
LIDT: 5.1 MW/cm² @ 10.6µm

Table 2: Wavelength Opto-Electronic Cavity Optics

2.2 CO₂ Laser Lenses

CO₂ laser lenses are employed to shape and focus the laser beam. They’re typically made ZnSe material and can be either convex or concave, where their focal length determines the beam’s focus point. Plano-convex lenses have a positive focal length and are used to focus a collimated beam to a small spot size. Plano-concave lenses have a negative focal length and are used for diverging collimated beams. The curved surfaces of these lenses should face the source to minimize spherical aberration.

Diameter Tolerance: +0/-0.13mm
Thickness Tolerance: ±0.25mm
Focal Length Tolerance: ±2%
Edge Thickness Variation (ETV): ≤3 arc min.
Clear Aperture: >90%
Surface Flatness: λ/4 per 1″ Dia@632.8nm
Surface Quality: 40-20 S-DAR
Coating: R<0.2% per surface @10.6μm

Positive meniscus convex-concave lenses are converging lenses being thick at the center and thin at the edges and producing real images. The radius of the curvature of the concave side is greater than the convex side of the lens. The convex side of the lens should face the source to minimize spherical aberration. It is designed to minimize spherical aberration and produce minimum focal spot size for incoming collimated light. CO₂ laser lenses are essential for applications such as cutting, engraving, and welding, where precise control of the beam’s shape and intensity is crucial.

Table 3: Wavelength Opto-Electronic ZnSe Focusing Lenses

2.2.1 ZnSe Aspheric Lenses

Diameter Tolerance: +0/-0.13mm
Thickness Tolerance: ±0.25mm
Focal Length Tolerance: ±2%
Edge Thickness Variation (ETV): ≤ 3 arc min
Clear Aperture: >90%
Aspheric Accuracy: ≤1µm P-V
Surface Quality: 40/20 60/40
AR Coating: R<0.2% per surface @ 1030-1090nm
Material: Fused Silica, Suprasil 313, Corning 7980, Si, Ge, ZnS, ZnSe, Chalcogenides
Coating: Options Available

CO₂ laser lenses also come in other shapes such as aspheric, spherical, cylindrical, and axicon. Compared to conventional ZnSe spherical lenses, the most significant advantage of ZnSe aspherical lenses is that they can perform spherical aberration correction. Aspheric lenses allow the designer to correct the aberration with fewer optical lenses than spherical lenses, so the optical system can be lower cost and more compact in size.

Table 4: Wavelength Opto-Electronic ZnSe Aspheric Lenses

2.2.2 ZnSe Cylindrical Lenses

Dimension Tolerance: +0/-0.13mm
Thickness Tolerance: ±0.25mm
Focal Length Tolerance: ±2%
Edge Thickness Variation (ETV): ≤3 arc min.
Clear Aperture: >90%
Surface Flatness: λ/4 per 1″Dia@632.8nm
Surface Quality: 60-40 S-DAR
Coating: R<0.2% per surface @10.6μm

ZnSe cylindrical lenses are used to focus an incoming beam in a single focal line than at a single focal point, it comes with both positive and negative focal lengths. They are either round or rectangular objects with cylindrically shaped surfaces, either in plano-concave or plano-convex. They differ from spherical lenses in that they focus a beam on a focal line rather than a focal point.

Table 5: Wavelength Opto-Electronic ZnSe Cylindrical Lenses

2.2.3 ZnSe Axicon Lenses

ZnSe axicon focus lenses have one conical surface and are used to produce a ring focus beam. Typically, axicon focus lenses have a second flat surface and are used in combination with a focusing lens. They’re made from a manufacturing process suited to the axicon angle and the accuracy required. For small-angle, high-accuracy lenses, the manufacturing process involves diamond machining.

Standard Axicon lens – Axicon cone angle equal to 180°-2α

(+/-0.01 deg): α (°)
Cone Angle 140°: 20°
Cone Angle 160°: 10°
Cone Angle 165°: 7.5°
Cone Angle 170°: 5°
Cone Angle 175°: 2.5°
Cone Angle 179.5°: 0.25°= 15′

Table 6: Wavelength Opto-Electronic ZnSe Axicon Lenses

2.2.4 Medical Laser Lenses

There is also another range of CO₂ laser lenses used for varieties of medical laser systems such as CO₂, Q-switched ND:YAG, ER:YAG, Ruby and Alex Laser systems. These optics have been used as replacements in most well-known medical systems such as Continuum-Biomedical, ESC, Sharplan, Candela, and Coherent.

Diameter Tolerance: +0/-0.13mm
Thickness Tolerance: ±0.25mm
Focal Length Tolerance: ±2%
Edge Thickness Variation (ETV): <=3 arc min.
Clear Aperture: >90%
Surface Flatness: λ/4 per 1″Dia @632.8nm
Surface Quality: 40-20 S-DAR
Coating: R<0.2% per surface

Table 7: Wavelength Opto-Electronic ZnSe Medical Laser Lenses for Er:YAG

2.3 CO₂ Laser Windows

CO₂ laser windows are typically flat, transparent plates made of materials with excellent optical properties, acting as a protective barrier between the laser system and the external environment. They’re designed to transmit light with minimal distortion, scattering, or absorption. CO₂ laser windows are typically made of materials with low absorption at the CO₂ laser wavelength, such as ZnSe or Ge.

Dimension Tolerance: +0/-0.13mm
Thickness Tolerance: ±0.25mm
Parallelism: ≤3 arc min.
Clear Aperture: >90%
Surface Flatness: λ/4 per 1″Dia@632.8nm
Surface Quality: 60-40 S-DAR
Coating: R<0.2% per surface @10.6μm
Angle of Incidence: Brewster Angle @ 10.6μm

ZnSe laser windows are commonly used in high-power CO₂ laser systems. They’re available in either coated or uncoated form and come in a wide variety of shapes and sizes. Ge laser windows isolate different physical environments while allowing light to pass through.

Table 8: Wavelength Opto-Electronic ZnSe Laser Windows

2.3.1 ZnSe Thin Film Polarizers

Thin film polarizers are used to split a laser beam into two parts with S and P Polarizations. Meanwhile, they can also be used to combine two beams with S and P polarizations. They consist of a coated plate, which is oriented at Brewster’s angle with respect to the incoming beam. The thin-film coating is able to enhance the reflectivity of the s-polarized component of the beam and maintain high transmission of the p-polarized component.

Material: ZnSe
Surface Flatness: λ/4 per 1″Dia@632.8nm
Surface Quality: 40-20 S-D
Coating: Tp =97%+/-0.5%@10.6um | Rs =97%+/-0.5%@10.6μm
Angle of Incidence: 67.3˚ (brewster angle@10.6μm)

Table 9: Wavelength Opto-Electronic ZnSe Thin Film Polarizers

2.4 CO₂ Dichroic Mirrors

CO₂ Dichroic mirrors, typically consisting of beam combiners and beam splitters, are ZnSe filters that selectively transmit a certain wavelength range of light while reflecting another wavelength based on their coating layers’ properties. 

2.4.1 ZnSe Beam Combiners

Diameter Tolerance: +0/-0.13mm
Thickness: ±0.25mm
Surface Flatness: λ/4 per 1″Dia@632.8nm
Surface Quality: 40-20 S-D
AOI: 45°

ZnSe beam combiners merge two or more beams into one and are used for CO₂ laser system alignment. They usually transmit a long-wavelength beam and reflect a short-wavelength beam whereas reverse beam combiners transmit a short-wavelength beam and reflect a long-wavelength beam. CO₂ beam combiners are designed at a 45° angle of incidence and transmit a laser beam, combining the beam with the 90° reflected visible alignment beam.

Table 10: Wavelength Opto-Electronic ZnSe Beam Combiners

2.4.2 ZnSe Beam Splitters

Diameter Tolerance: +0/-0.13mm
Thickness: ±0.25mm
Surface Flatness: λ/4 per 1’’ Dia@632.8nm
Surface Quality: 60-40 S-D

Unlike beam combiners, ZnSe beam splitters are utilized to divide the laser beam into multiple paths, allowing for simultaneous processing or monitoring. They can also redirect a portion of the beam for alignment or calibration purposes. ZnSe beam splitters are often coated to achieve a specific reflectance or transmission ratio at the CO₂ laser wavelength.

The transmission and reflection of the light depend upon various parameters such as the incident angle, state of polarization, and wavelength of the input beam. There is a substantial difference in the transmittance and reflectance values of s and p polarization at a 45° incident angle, so the ZnSe beam splitters designed are for this angle.

Table 11: Wavelength Opto-Electronic ZnSe Beam Splitters

2.5 Laser Attenuators

Laser attenuators are employed to control the laser power by reducing its intensity. They are useful when fine-tuning the energy delivered to the workpiece, especially in applications where excessive power may cause undesirable effects. Laser attenuators utilize filters or variable mechanisms to achieve the desired power adjustment.

Wavelength: 355/532/1064nm
Type: Transmission Mode
Clear Aperture: 14mm
Beam Shift: 0.5mm
Extinction Ratio: >200:1
Attenuation Range: 0.5%-95%
Damage Threshold: >5J/cm²@1064nm, 20ns, 20Hz
Weight: <300g

Item No.: ATTN-10600-WC-V1
Wavelength: 10.6μm
Clear Aperture: Up to 19mm (Customizable)
Transmission Range: 10 – 90%
Damage Threshold: 1MW/cm^²
Resolution: 5%

Table 12: Wavelength Opto-Electronic Laser Attenuators

3. CO₂ Laser Optics Applications

The significance of CO2 laser optics lies in their ability to optimize beam quality, control beam parameters, and enhance overall system performance. High-quality optics ensure minimal losses, reduced beam divergence, and improved focusability, ultimately resulting in increased process efficiency, reduced downtime, and higher-quality output. CO₂ laser optics find extensive use in a wide range of applications due to their ability to deliver high-power laser beams with excellent precision. Some notable applications include:

3.1 Material Processing

CO₂ lasers are extensively employed for cutting, engraving, marking, and welding various materials, including metals, plastics, wood, and ceramics. High-quality optics ensure accurate focusing, beam shape control, and enhanced process efficiency.

3.2 Medical

CO₂ lasers are utilized in surgical procedures, dermatology, and ophthalmology due to their precise tissue ablation capabilities. Optics enable surgeons to deliver highly controlled laser energy for procedures such as skin resurfacing, tumor removal, and eye surgery.

3.3 Scientific Research

CO₂ lasers are indispensable tools in scientific research, particularly in fields such as spectroscopy, atmospheric monitoring, and particle acceleration. Optics enable precise beam control, alignment, and manipulation, facilitating accurate data acquisition and analysis.

4. Conclusion

CO₂ laser optics are integral components in harnessing the full potential of CO₂ laser systems across various industries. Through careful selection and implementation of mirrors, lenses, windows, beam splitters, and attenuators, these optics enable precise control, manipulation, and delivery of high-power laser beams. As technology advances, further advancements in CO₂ laser optics are expected, opening up new possibilities for enhanced precision, efficiency, and innovation in diverse applications.

Overall, CO₂ laser optics are a critical component of CO₂ lasers. They play a vital role in the transmission and focusing of the laser beam, which ultimately determines the performance of the laser. With proper care and maintenance, CO₂ laser optics can provide years of reliable service. Wavelength Opto-Electronic design and manufacture various CO₂ laser optics like CO₂ laser lenses and CO₂ laser mirrors for your CO₂ laser systems.

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