Since Albert Einstein established the theoretical foundation for lasers, they have gained increasing relevance in our everyday lives, being used in vital tools across many industries. A laser’s power and effectiveness, however, depends on its beam source and optics.
As components that help a system reach its best performance, optics are necessary elements across all types of laser systems; however, there are important differences in the specifications and materials of the optics depending on the laser type and its application.
Importance of optics and role within a laser
The acronym for “LASER” suggests the important role of optics: Light Amplification by Stimulated Emission of Radiation. In order for light to be amplified and focused in a precise manner, high-quality optics are necessary.
Optical components across various laser systems perform similar roles. The mirrors or output couplers located inside the laser cavity, also called resonator optics, are responsible for generating and amplifying the beam.
Beam delivery optics located outside of the cavity ensure the precise delivery of a beam onto a workpiece, such as sheet metal, and are usually comprised of mirrors, beam expanders and focusing lenses.
An array of fiber laser optics made from clear materials such as BK7, Fused Silica, and various types of optical glass.
CO2 optics vs. fiber optics
Although they may function similarly within a system, laser optics will differ in material and coating depending on the laser source. Due to differences in wavelengths at which CO2 and fiber lasers operate, optics in each system have unique characteristics.
Along the spectrum of light, CO2 lasers operate in the Mid-Wave Infrared (MWIR) region at 10.6 microns, whereas fiber lasers operate in the Near Infrared (NIR) region. That wavelength is around 1 micron, considerably shorter than that of the CO2 laser.
Due to differing wavelengths, one of the biggest factors differentiating optics in a CO2 laser versus those in a fiber laser is the material from which they are fabricated. While certain materials transmit well in one region of the spectrum, their transmission will significantly drop off in other regions.
CO2 laser optics are fabricated from special infrared materials that transmit at the 10.6 micron region: zinc selenide (ZnSe), germanium (Ge), galium arsenide (GaAs) and silicon (Si). Meanwhile, optics for fiber lasers are made from optical-grade glass materials, such as BK7 and fused silica, which transmit at a wide range from 400 nm to 2 microns. One easy way to visually distinguish CO2 and fiber laser optics is by their colors – CO2 laser optics are yellow or metallic while fiber laser optics are clear.
In general, one can expect CO2 laser optics to be more expensive than fiber laser optics. IR material technology is more specialized and is produced in smaller quantities while optical-grade glass is mass produced. This price differentiation applies to the optical coatings of CO2 laser optics and fiber laser optics, as well.
Damage, wear and replacement
In both CO2 and fiber laser systems, resonator optics will need to be replaced more often than beam delivery optics since they are used within the laser cavity and thus are exposed to much higher levels of energy. In the case of a CO2 laser system, the cavity is pressurized with gases and electrodes – this combination of pressure, radiation and electrical discharge can quickly degrade an optic’s performance.
High levels of energy within the laser cavity for a fiber laser system may also have similar damaging results on its resonator optics, but it is often to a lesser extent. And some fiber laser systems do not even contain resonator optics at all, as the ends of the fiber can act as a substitute for resonating mirrors.
Ideally, transmitting optics (including focusing lenses) should have a 100 to 0 transmission-to-absorbency ratio – where all of the light from the beam is being transmitted through the optic and none of it is being absorbed. A low absorbency rate means less risk of light being trapped in the optic. When light is absorbed, it will cause the optic to warp, burn or be damaged. If this happens, the optic must be replaced in order for the system to run at peak performance.
Optics in a CO2 laser system will have a higher absorption rate than that of their fiber counterparts; therefore causing higher frequency of replacement. For example, a ZnSe focusing lens with an AR coating will have a higher absorption rate than a BK7 lens with a silica coating due to natural limitations in the ZnSe material and coating technology. Because of this, it is extremely important for CO2 optics to have the highest grade material with an optimized coating so that its absorption rate is minimized, resulting in the optic performing better and having a longer lifespan within the system.
The focusing lens, being the closest optic to the workpiece, is prone to replacement due to “splatter” from a workpiece or other environmental factors. In the case of a fiber laser, a cover glass is used to protect the focusing lens from environmental stress. Cover glass is by far the most replaced optical component of a fiber laser system. In the case of a CO2 laser, a ZnSe window may be used as a protective piece. Although made from the same material, a ZnSe window is less expensive to replace than a ZnSe focusing lens, as it is easier to make (flat and not curved like a lens) and requires less material.
Strengths and applications
As fiber lasers continue to have a growing influence in the industrial laser market, many fabricators are considering – or have already purchased – fiber laser systems. Meanwhile, CO2 lasers have been the most commonly used for industrial applications for decades and still maintain the largest install base in the industrial market. While some shops use one laser specifically, others may use a combination of both fiber and CO2 or even run other laser type.
Among laser technologies, both CO2 and fiber lasers are the most prominently used for a wide range of applications such as cutting, welding and marking. Due to their ability to be scaled up with more ease and at a lower cost than other laser types, they are especially strongly suited for use in high-power cutting applications.
As mentioned before, the primary difference between CO2 and fiber systems is their inherent laser wavelength (10.6 micron for CO2 vs. 1 micron for fiber) and the unique absorption characteristics of these wavelengths on a given material. In other words, the effectiveness of material processing (cutting, welding and marking) for any given wavelength is dependent on the material of the workpiece. In general, shorter wavelengths have better absorption on a wider range of materials, can focus to a smaller spot and also produce less heat.
But this does not automatically mean that shorter wavelengths are better since the ultimate result is still application specific. For example, CO2 lasers are ideal for engraving on wood, leather, many plastics and glass. When it comes to processing highly reflective metal surfaces, however, 1 micron lasers such as Fiber or YAG (in the DPSS family) have become the industry standard.
In the case of stainless steel cutting applications, both CO2 and fiber laser systems are extremely effective. This is due to the unique combination of absorption, heat and high power levels from each laser on this particular material. However, reports from numerous end users seem to suggest that for thinner stainless steel work pieces (less than 0.25 in.) fiber lasers are more efficient and therefore cut faster than CO2 lasers, which may be more efficient at cutting thicker workpieces (greater than 0.25 in.).
Overall, both systems have unique strengths and weaknesses that vary based on application. Regardless of laser type, high-quality optics and coatings are essential for maximizing the effectiveness of any system and for reliable peak performance – increased power delivery, focusing ability and overall efficiency – for any given application.