Informatii elemente optice – Tutoriale
(sursa : Ophir Optronics)
A) FOCUSING LENSES
Collimating and focusing lenses
Aspherical surfaces on collimating and focusing lenses provide improved performance over conventional, spherical surfaces in high-power industrial fiber laser and direct diode laser systems. The aspherical optics’ shape reduces spherical aberration resulting in a smaller spot size, a uniformed spot shape and greater depth of focus.
Lens Design – Meniscus Lenses
- Meniscus lenses have one concave surface and one convex surface.
- Creates a smaller beam diameter, reducing the spherical aberration and beam waste when precision cutting or marking.
- Provides a smaller spot size with increased power density at the work piece.
- Although meniscus lenses are more expensive than Plano-Convex lenses, they offer a higher accuracy of the cut therefore increasing cutting speed by up to 5%.
Meniscus lenses are usually used by European customers and machine manufacturers (OEM’s).
- Plano-convex lens has one flat surface and one convex surface.
- More suitable for high turnover cutting applications, and when the cost is more important than the level of accuracy.
- When cutting steel and other thick materials, a Plano-convex lens provides a greater cut width, enabling the laser’s oxygen assist gas to aid the cutting process.
When cutting steel and other thick materials, a Plano-Convex lens provides a greater width of the cut enabling the laser’s Oxygen assist to enter and ease the cutting process.
In addition, Plano-Convex lenses give a greater depth of field needed to maintain a taperless edge when cutting thicker materials. This kind of lens is more popular in American and Japanese machinery.
The focusing lens is the last optic in the laser path, before it hits the workpiece. Its main role is to focus the laser beam to a specific focal length (FL) – depending on the application. Therefore, the focal length – that is dictated by the radiuses and curvatures of the lens – is its most important feature.
The focusing lens is normally made of Zinc Selenide (ZnSe) using an anti-reflective coating. Focusing lenses are either Plano-Convex lenses or Meniscus lenses.
In order to ensure that your laser operates at maximum efficiency it is crucial to select the right lens. The type to be used with each machine is usually specified in the machine manufacturer’s manual.
Ophir offers a wide range of standard and special lenses assuring that the most suitable and effective lens be fitted to your laser cutting machine.
Cutting Head Optics
Focal length and mounting distance
In general, there are two types of focusing CO2 laser lenses: Plano convex lenses which have one convex surface (convex = dome-like curvature) and one flat surface, and meniscus lenses which have one convex surface and one concave surface (concave = hollow curvature). In most CO2 laser cutting machines, meniscus lenses are used because they produce a smaller focus diameter (see next section). In some CO2 laser machines, CO2 laser lenses type plano-convex are used because their production costs are a little bit lower. For a laser user who thinks about replacing a plano-convex lens by a meniscus lens, it is important to check if the focus position can be adjusted correctly. Even if a plano-convex lens and a meniscus lens have same diameter, thickness and focal length, the focus position of the meniscus lens can be several mm higher if compared to the plano-convex lens. Reason is that the focal length of a lens is defined as the distance between the focus and the so-called principal plane. The principal plane is defined according to a scientific rule and is located somewhere inside the lens. For checking the position of the focus in a CO2 laser cutting head, it is much more useful to know the “Mounting distance” of the lens. It is defined as the distance between the edge of the lower surface and the focal plane and therefore connected directly to the position of the focus within the cutting head. If the mounting distance of replacement optics is different from the mounting distance of the original ZnSe lens for CO2 laser, it might happen that the focus position is shifted such that it cannot be corrected within the adjustment range of the cutting head. On the other side, it is possible to extend the adjustment range by using lenses with different mounting distances. All CO2 laser lenses are made of Zink Selinide (ZnSe lenses)
Spherical aberration means that the focus position of the outer portion of the laser beam is closer to the lens than the focus position of the inner portion (see picture). As a consequence, the focus diameter is not zero, but has some extension which can be calculated by the following formulas:
df = 0.0286 (din)3 / (FL)2 (plano-convex lenses)
df = 0.0187 (din)3 / (FL)2 (meniscus lenses)
df = focus diameter, din = diameter of incoming beam,
FL = focal length
din = 20 mm,
FL = 3.75″:
>>> df = 0.025 mm (plano-convex lens)
>>> df = 0.017 mm (meniscus lens) The example shows that meniscus lenses produce smaller focus diameters than plano-convex lenses. The difference is significant especially at large beam diameters and short focal lengths. In order to minimize this effect, meniscus lenses are used in most CO2 laser cutting systems. In most practical applications, however, there is a second and much more important effect which influences the focus diameter. It is called diffraction and described in the next section.
A laser beam is an electromagnetic wave and therefore has properties similar to water waves or sound waves. One consequence of this wave-like nature is that a laser beam cannot be focussed to a sharp point. Instead the focus has a spot size which can be calculated as follows:
df = (4/Π) M2 λ FL / din
df = focus diameter, M2 = beam quality, λ= laser wavelength,
FL = focal length of focussing lens, din = diameter of incoming beam
Examples: ( λ = 10.6 µm, M1 = 2)
din = 20 mm, FL = 7.5″ >> df = 0.13 mm
din = 20 mm, FL = 3.75″ >> df = 0.065 mm
First of all, the example shows that focus diameters are much larger than the values calculated in the section above. This means that in most cutting applications, spherical aberration can be neglected. Diffraction is therefore the most important effect concerning focus diameters. In general, the formula shows that by decreasing the focal length, the focus diameter is decreased as well, with the consequence that the intensity of the laser beam is increased. As high laser intensity is useful in most cutting applications, focal length should be as short as possible. However, a short focal length has the disadvantage that the beam diameter increases rapidly above and below the focus. Therefore, maximal thickness of materials which can be cut efficiently is very limited, and the optimal focal length increases with increasing thickness of material.
Absorption and thermal lensing
During laser operation with several kilowatts, the focusing lens is heated because it absorbs a small portion of the laser power. Absorption takes place mainly in the AR coatings and at dirt on the lens. At a new clean lens with standard AR coating, absorption is typically 0.2% of the incoming laser power. A CO2 laser lens such as Ophir’s BLACK Magic is a low absorption lens (lower than 0.15%). During use in a CO2 laser cutting machine, absorption increases gradually due to increasing amounts of dirt on the lower surface. When the lens needs to be replaced, absorption usually is in the range 0.3 to 0.4%. Heating of the lens causes additional surface curvature due to thermal expansion and increases the refractive index of the lens material (ZnSe lens). As a consequence of these effects, the lens focal length becomes shorter, and the focus position cannot be predicted exactly because it depends on many parameters like laser power, intervals laser on/off, cleanliness of lens, and others. Therefore, use of low absorption CO2 laser lenses with can make the focal length more stable and therefore improve reliability of the cutting process. If there are dirt particles on the lens, the lens material is not heated uniformly, but mainly at the areas close to these dirt particles. As a consequence, focusing properties become worse; focus diameter increases, and cutting quality decreases. So if a certain “critical” amount of dirt has accumulated on the lens, it needs to be replaced. However, it might still work fine at reduced laser power.
Video : Find out whether your CO2 laser lens is
Ophir demonstrates a simple way to identify a CO2 lens surface (concave, convex or plano) using the len’s surface as a mirror.
B) BEAM DELIVERY OPTICS
n the beam delivery section of a Co2 laser catting machine, the laser beam is transferred from the laser cavity to the working head. In principle, two Co2 laser moveable mirrors would be sufficient in a 2D-machine for guiding the laser beam to any point on the worksheet. In modern 2D-machines and especially in 3Dmachines, however, the beam delivery section has additional functions which require additional Co2 optical mirrors with specific properties.
Plano Cu & Si Mirrors
In order to optimize function of these mirrors, different substrate materials are used – the most common ones are silicon(Si) and copper (Cu). Silicon mirrors have light weight and are therefore preferred in flying optics where high accelerations are needed. Copper has high thermal conductivity, and channels for cooling water can be included directly into the Co2 laser mirrors. Therefore, copper mirrors are preferred if best-possible cooling is important, for example in Co2 laser machines with very high laser power. – The optical properties of a mirror (reflectance, phase shift, etc) are determined by its coating. So in order to realize different mirror functions, different coatings are needed.
Water Cooled Cu Mirrors
As the output power of the Co2 laser is getting higher throughout the years, the technical demands and damage threshold the Co2 laser optics need to withstand is more challenging. As a solution to those challenges optics manufacturers added cooling system to the Co2 laser Cu mirror, by that the mirror will be less inclined to thermal expansion, and a proper cooling of the optical surface is ensured. The cooling of the Co2 laser optic surface is achieved by drilling of channels close to the surface, by that there is less damage to the coating as in is exposed to less heat. This drilling is not applicable to Si mirrors due to the brittleness of the material .
In many applications, the small diameter of the laser beam produced in the Co2 laser cavity is not convenient because the beam has high divergence and high power density. In order to avoid subsequent problems, the beam diameter can be increased by using a Co2 laser optics telescope consisting of two mirrors – one with a convex surface and the other with a concave surface. Such Co2 laser optics telescopic mirrors are usually made of copper.
|ZPS||Si & Cu||99.50%||–||–||630-670nm≥80%||0°±2°|
|90PS||Si & Cu||98%||–||–||630-670nm≥80%||90°±2°|
|90PS-HR||Si & Cu||99%||–||–||≥80%||90°±2°|
|ATFR-HR||Si & Cu||–||99%||≤1%||–||0°±2°|
C) CAVITY OPTICS
The output coupler usually has reflectance in the range 40% to 70%. In order to optimize power and spatial profile of the laser beam, the output coupler and rear mirror have optical surfaces with well-defined radii of curvature. The output coupler is typically composed of ZnSe in order to minimize absorption and thermal distortion while transmitting a high-power laser beam.
The end mirror usually has reflectance 99.5% which means that the transmitted laser beam has low power. However, because absorption in the coating is higher than in the output coupler, substrate materials, such as Germanium or Gallium Arsenide, with high thermal conductivity are used in most lasers.
Total Reflectors are quite similar to turning mirrors. However, durability of the coating is more critical because the total reflectors are exposed to the gas discharge and the extremely high laser power density inside the cavity. Therefore, the coating must have best-possible environmental resistance and low absorption in order to minimize thermal distortion.
|MMR||Si & Cu||99.80%||99.90%||99.75%||40%||0o±2o|
|MMR-A||Si & Cu||99.80%||99.85%||99.60%||40%||0o±2o|
|MMR-H||Si & Cu||99.70%||99.80%||99.60%||650±20nm>80%||–|
|MMR-P||Si & Cu||–||99.90%||99.80%||900-1000nm≥55%||0o±2o|