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Laser Lenses, Optics, and Focus

This article covers a bit of laser theory as well as practical application of the optics systems in a typical CO2 laser. This is a beginner level article. There will be more advanced articles generated at a future time.

1. LASER BEAMS

20mm-lenses.jpg

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.

2. MIRRORS

laser_mirrors.jpgThe steering mirrors direct the laser beam to and allow 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 substrates, 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 reflective index but tough and better index than Mo.
K9 Mirror: Close to worthless gold-coated glass (likely from re-melted 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.

Here is a hack I performed on my ULS-25. I used old hard drive platters to fabricate an “emergency” mirror (which is still in the unit and working GREAT). The proper way to form the mirror is by cutting it out on a CNC mill to prevent warpage and deformation but I ended up drilling a series of small holes and using a pair of diagonal cutters to trim it out. It’s ugly but it worked like a charm:

3. FOCUSING LENSES

lens-laser.pngThe focusing lenses most commonly found in K40’s (as well as most others of similar design) 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 the K40, 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 K40 lens is 12mm. Many people replace the factory head with the LightObject head.

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 of the tolerance (focal plane).

click image to enlarge

4. CLEANING & MAINTENANCE

lens-clean.jpgNOTE: The manufacturer’s recommendations should be followed and they supersede our suggestions.

Dust and stains on Optics can cause scattering, and impurities on the optic surface can react with incident laser light to damage optical coatings. With proper handling and cleaning of your optics, you can prevent damage and ensure their continued performance.

If it’s not dirty, don’t clean it! Handling optics increases their chances of getting dirty or damaged, so you should clean optics only when necessary.

You should handle optics in a clean, low-dust environment. Since oil and debris from your hands can stain or damage optical coatings, you should not touch any transmissive or reflective surface of your optic.

Inspect an optic for dust and stains by holding it near a bright visible-light source. Viewing the optic at different angles allows you to see scattering from dust and stains.

Dusting is always the first step in cleaning your optics. Wiping a dusty optic is like cleaning it with sandpaper. So always dust with a canned air duster, compressed and filtered air, or nitrogen before wiping any optic.

If the dusted optic has no visible stains after you dust it, then remember: “If it’s not dirty, don’t clean it.” If it’s still not clean, proper use of solvents and lens tissue can often do the trick.

Glass-cleaning solvents will streak, and tissue paper or a t-shirt will scratch, so always clean optics with reagent- or spectrophotometric-grade solvent and a low-lint tissue manufactured for cleaning optics.

Always use lens tissue with a solvent, because dry lens tissue can scratch optical surfaces. A good solvent to use is a mix of 60% acetone and 40% methanol. The methanol slows the evaporation time, and also dissolves debris that acetone alone would not clean.

Isopropyl alcohol is safe and effective, but its relatively slow evaporation can leave drying marks on the optic.

Cleaning your optic’s edges before cleaning its faces prevents dirt from being drawn up onto the face. Wiping slowly allows the solvent to evaporate without streaking. Remember, slow and steady cleans the optic.

For softer coatings, which damage more easily, we recommend using the “immersion” technique. Simply remove any dust from the optic and then immerse it in acetone. If the optic is very dirty, you can use an ultrasonic bath.

Rinse and immerse the optic in fresh solvent a number of times until it’s clean. To dry the optic, carefully blow the solvent off from one direction to avoid leaving drying marks.

5. RAMP TEST

This is my take on the ramp test:

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Tech Bravo

My name is Brian and I am a full time geek! Before being pulled in to a real estate firm as their digital marketing guru I was Operations Manager/Senior Network Engineer/Lead Color Digital Imaging Technician for a medium sized Copier/Printer/IT firm. I have been taking things apart since i was old enough to handle a screwdriver and, over the years, have figured out how to put stuff back together. My toys growing up were oscilloscopes, Z80 based microprocessors, tools, and the like and I am heavily addicted to all things technical. I look forward to bringing hi-tech to you! Feel free to contact me if you wish.

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