Basic Information On Laser Systems
Common Laser Sources:
- CO2 (9.4μm and, most commonly, 10.6μm)
- Crystal (Nd:YAG, Nd:YVO @ around 1.064μm)
- Fiber/MOPA @ around 1.064μm).
We will be referencing the 2 most common sources in this section: CO2 and fiber. CO2 is best for organics (acrylic, wood, leather, glass, etc…) while fiber is best on polymers, metals, and such. CO2 will not, reasonably, ablate metals but you can bond pigments to it at the molecular level with special marking compounds. Fiber lasers can etch and cut metals.
CO2 lasers sources are typically either water-cooled glass tubes that are DC excited or metal tubes that are air-cooled and RF excited. You will find metal air-cooled RF tubes in most US made lasers. DC excited laser tubes have a minimum excitation threshold meaning they will not fire below a certain power. This threshold goes up with the tube wattage.
Gantry lasers have rails and a carriage and steer the beam with mirrors and lenses. Galvanometers (galvos) steer the beam with oscillating mirrors (think Walmart scanner at the checkout). Not all fiber lasers are galvo and not all CO2 lasers are gantry. You can get either source in either beam delivery method and there are also dual-source units (mostly gantry like epilog fusion).
Thunder Laser USA does not currently offer any fiber lasers nor any galvo lasers. The remainder of this article will refer specifically to CO2 Gantry laser engravers.
This section covers a bit of laser theory as well as the practical application of the optics systems in a typical gantry CO2 laser. This is a beginner level article. There will be more advanced articles generated at a future time.
A laser consists of a gain medium, a mechanism to energize it, and something to provide optical feedback. The gain medium is a material with properties that allow it to amplify light by way of stimulated emission. Light of a specific wavelength that passes through the gain medium is amplified.
For the gain medium to amplify light, it needs to be supplied with energy in a process called pumping. The energy is typically supplied as an electric current or as light at a different wavelength. Pump light may be provided by a flash lamp or by another laser.
The most common type of laser uses feedback from an optical cavity with a pair of mirrors on either end of the gain medium. Light bounces back and forth between the mirrors, passing through the gain medium and being amplified each time. Typically one of the two mirrors, the output coupler, is partially transparent. Some of the light escapes through this mirror.
The section of the beam nearest the coupler is very coherent and parallel. In the far-field, the beam begins to diverge. The beam diameter at the far end is dependent on the length of the beam but, in the case of the typical laser engraver, is about 0.24″. The beam is bounced through the mirrors to the head that houses the focus lens.
The steering mirrors direct the laser beam and allows the movement of the head and gantry transport system. Laser mirrors are designed with a high degree of reflectivity for a specific wavelength, or range of wavelengths, using various lens materials and coatings, or a combination of the two.
Mirrors are ideal for laser applications where space is limited, as a beam can be precisely directed multiple times to fit within a particular area. We will briefly touch on some common mirror types and their properties:
Si Mirror: Silicon Glass Gold coated, great reflective index, not good for anything over 80 watts.
Mo Mirror: Molybdenum, very tough but the lowest reflective index, great for 80-watt sources or above.
Cu Mirror: Copper, Below gold Si for the reflective index but tough and better index than Mo.
K9 Mirror: Close to worthless gold-coated glass (likely from molten milk bottles).
Si will last around 9 months to a year, Cu’s will go a year or more but scratch easily and require constant cleaning/polishing. Mo will last up to 3 years or so, K9….well..you really must keep them spotlessly clean and avoid ANY scratches or imperfections.
The focusing lenses most commonly found in laser engravers are plano-convex zinc selenide (ZnSe) lenses. They are curved on one side and flat on the other. With a plano-convex lens, the curved side goes up (toward the beam) and the flat side goes down (toward the work). In other words, the laser beam needs to pass through the curved part of the lens first. Lenses are typically referenced by their focal point. The most common lens and the one that is shipped with Thunder Lasers is the 2.0″ or 50.8mm lens. This refers to the focal point or distance between the lens and the work. it does not refer to the diameter. The diameter of a standard Thunder lens is 20mm.
Here is a diagram showing some common lenses and their focusing properties. The tolerance is the focal plane or the part of the beam that has an almost uniform spot size. The focal length is from the lens to the exact middle of the focal plane.