Why use a focus lens in a laser cutter?

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glass isn’t transparent to all light

The infrared laser beam used in a laser cutter does have a certain amount of power.

For example, in a good 40 Watt tube, the raw beam power is … 40 Watt.

Shining the beam of a 40 Watt tube onto a piece of wood will produce some smoke, some glowing, some ashes and… eventually you might even get a very ugly looking hole in your wood.

hey wait, I wanted quality

Suppose the diameter of a 40W laser is approximately 3mm. That makes the area of the beam about 7.06mm².

40 Watt raw power in a 7.06mm² beam means 5.67 Watt of power per square millimeter.

I’m pretty sure that won’t cut through wood. As a matter of fact, it will do “something” to the wood but is totally unusable for our purpose. You’ll probably also not be happy with a 3mm cut width.

You could use a higher wattage laser, but that does only make the cost higher and it will not make the beam smaller, so… not a good idea.

An easier way to have more power per square millimeter is to use a focal lens. There are different kinds of focal lenses available, but the most used lens material in our 40-100W range is a ZnSe (Zinc Selenide) lens. Lenses for co2 laser simply cannot be made out of glass because… glass is NOT transparent to the 10.6 µm wavelength of our beam.

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A focus lens is used to reduce the beam diameter. The most common focal length used in our cutters is 2″. This roughly means that the lens produces its focal point (smallest point achievable with that lens) at 2 inch distance from the lens. I will explain the difference between the different focal lengths in another article.

If we take our original 3mm diameter beam and use a focal lens to reduce our beam to a realistic diameter of 0.25mm, then it gets a lot more interesting:

3mm diameter = 7.06mm² area

0.25mm diameter = 0.04908mm² area

Our area is now 143.84 times smaller, so the intensity of our beam is 40 watt on 0.04908mm² or the full 814 watt per mm². Now we are talking baby! This thing can cut a lot of things.

 

Of course you could try to get an even smaller spot and increase the power even further, but there are limits to this. Both laws of physics and your wallet will limit the possibilities. A 0.2 to 0.25mm spotsize with a 2″ focus lens is both acceptable and payable.

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