When to Replace a Fiber Laser Protective Window, and How to Find the Right Size

Replace a fiber laser protective window as soon as it shows a burn pit, a crack, damaged coating or haze in the beam path that does not come off with cleaning. Loose dust and light spatter can be cleaned off; a burn mark cannot. When cut quality drops at settings that worked yesterday, the protective window is the first part to check, because it is the cheapest optic in the head and the one that takes every hit. To find the right replacement, match the part number first, then the diameter, the thickness and the power rating. Size alone is not enough: we list six different windows that all measure D34 x 5 mm.

What the protective window does

A fiber laser cutting head stacks its optics in a column. The collimating lens straightens the beam coming out of the fiber, the focusing lens brings it to a point at the nozzle, and the protective window sits below the focusing lens, between the optics and the cut. It is a flat disc of fused silica with an anti reflection coating on both faces. It has no optical power of its own. Its only job is to stop spatter, fume and dust from reaching the focusing lens.

That makes it a wear part. Every pierce throws molten metal and vapour back up through the nozzle, and the window catches it. The focusing and collimating lenses above it cost many times more and are cleaned and refitted; the window is the part you throw away.

Signs the window needs attention

A contaminated window absorbs part of the beam instead of passing it. The absorbed energy turns into heat in the glass, which degrades the beam further. The symptoms show up at the cut before they show up anywhere else:

• Dross on the underside of parts that used to cut clean, at the same speed, power and gas pressure.
• Heavier striations or a rougher cut edge.
• Slower piercing or failed pierces on thicker plate.
• A wider kerf or a cut that no longer drops through at the end.
• Head temperature or capacitance alarms that come back after a reset.

Before changing cutting parameters to chase any of these, pull the window and look at it. Raising power to compensate for a dirty window heats the window faster and moves it closer to cracking.

How to inspect a protective window

Take the window out in its holder and hold it over a dark background. Shine a bright LED torch across the surface at a low angle, not straight through it. Contamination that is invisible face on lights up when the light grazes the surface. You are looking for three things:

• White hazy spots or a dusty film: fine fume and dust sitting on the surface.
• Black or dark brown pits: spatter that hit the glass hot and burned into the coating.
• A yellow or brown tint: a film across the whole face, often from contaminated assist gas.

Check both faces and pay most attention to the centre, where the beam passes. A chip on the outer edge outside the beam path is cosmetic; a pit in the middle is not.

Clean it or replace it

The rule is simple. If the contamination is sitting on the surface, clean it. If it has burned into the surface, replace the window.

• Clean: loose dust, light haze, fingerprints and spatter that has not fused to the glass. Use lens tissue or a clean swab with optical grade alcohol, wipe once in one direction, and use a fresh tissue for each pass.
• Replace: any burn pit, any crack, coating that is lifting or discoloured, and haze that comes back after a careful clean. A burn pit is damage to the coating, and no amount of wiping restores it.

Raytools recommends cleaning the cover glass once a week in its BM115 head manual. On a machine piercing thick plate all day, you will be inspecting it daily and replacing it far more often than that. Keep a running count of how many windows each machine uses per month. That figure tells you how many to stock and shows up gas or nozzle problems when it suddenly rises.

How to change the window without contaminating the head

Most heads hold the window in a drawer or cartridge that slides out of the side of the head. The procedure in the Raytools BM115 manual is typical:

• Move the head to a clean position and switch off the laser.
• Open the buckle and pull out the window holder by pinching its two edges.
• Remove the pressing ring, then the old window.
• Clean the holder and the seal ring. Replace the seal ring if it is flattened, nicked or burned. Do not lever it out by its edge; it tears easily.
• Fit the new window. Raytools states either face can go down on this head.
• Refit the pressing ring, slide the holder back in and close the buckle.

Work in still, clean air, away from grinding and blowing. Wear finger cots or clean nitrile gloves and hold the window by its edge only. Skin oil on the glass absorbs the beam like any other contamination. Leave the new window in its case until the moment you fit it, and cover the open slot in the head if you have to step away. A damaged seal ring lets fume past the window, so keep spares from our laser seal range with the windows.

Finding the right size

A protective window is specified by four things: the head maker's part number, the diameter, the thickness and, on high power heads, the power rating. The cutting head windows in our protective window range run from D18 x 2 mm for small Raytools heads up to D134 x 3 mm for a Trumpf head, with thicknesses from 1.5 to 9 mm. All of them are fused silica, anti reflection coated on both faces for 1064 nm fiber lasers.

Start with the part number. It is printed on the packaging of the window you are replacing and in the parts list for your head. Each of our listings shows the head maker's number next to the SPM number, so you can match it directly. The Raytools 211LCG0069 D50 x 2 mm window, for example, fits eleven Raytools heads, including the AL06K, BF12K and BW20K.

If the number is gone, measure the old window with calipers. Measure the diameter across the widest point and the thickness at the edge, to a tenth of a millimetre. Some sizes differ by fractions: we stock D24.4, D24.9 and D25.4 mm windows, and D37, D38 and D38.1 mm windows.

Same size, different window

Matching diameter and thickness is necessary but not sufficient. Head makers sell several windows in the same dimensions, each rated for a different laser power, and the price rises with the rating. Our D34 x 5 mm windows for BOCI heads show it clearly:

• DP.V.P0049AA, 8 to 10 kW: see current price
• P.S.B0021, 15 kW: see current price
• DP.V.P0004AA, 20 kW: see current price
• DP.V.P0005AA, 30 kW: see current price
• DP.V.P0012AA, 40 kW: see current price

All five fit the same BOCI heads (BLT310, BLT420, BLT421, BLT441, BLT461, BLT481 and BLT641) and look identical on the bench. On a 20 kW source you need at least the 20 kW DP.V.P0004AA. Fitting the cheaper 8 to 10 kW window to save money puts a window rated for half the power in the beam path.

The same pattern appears across brands. In D37 x 7 mm we list the Raytools 211LCG0078 rated for 12 kW at see current price and the 211LCG0125 rated for 30 kW at see current price. Precitec's D37 x 7 mm windows include a 20 kW version, P0595-58601, and a 30 to 60 kW version, P0595-117302. Go by the part number and the rating of your source, not by what fits the holder.

The reverse also holds. D30 x 5 mm windows appear under Raytools 211LCG0068, Trumpf 0766719 and Precitec P0795-1201-00002. The dimensions match, but they are listed against different heads with their own part numbers. Order against your head's number.

Handheld laser welding guns

Handheld fiber welding guns use smaller protective lenses and go through them faster, because the operator holds the gun close to the weld and the spatter comes straight back. The common sizes are 18 x 2 mm for SUP and WSX guns, 20 x 2 mm for Qilin, 20 x 3 mm for Ospri and Relfar, and 20 x 4 mm for Hanwei. We sell them in packs of 10, 20 or 50; the 20 x 3 mm lens for Ospri and Relfar guns is see current price for 10 and see current price for 50. Change the lens at the first dark spot. If penetration drops or the bead goes uneven at the same settings, check the lens before touching power or wire speed.

How many to keep in stock

Keep enough on the shelf to cover your usual monthly use plus the lead time on the next order. Buying in packs brings the unit price down: the Raytools 211LCG0069 is see current price as a single and see current price for a pack of 10, which works out at see current price each. If you run several heads or need a size we have not listed, send the part number through our quote request page.

When you order windows, check the rest of the consumables in the same column. Check the cutting nozzle and the ceramic ring on the same visit. They sit directly below the window in the same column.

Frequently asked questions

How often should a fiber laser protective window be replaced?
There is no fixed interval. Inspect it daily on a busy machine and replace it when it shows burn pits, cracks, coating damage or haze that will not clean off.

Can I clean a protective window instead of replacing it?
Yes, if the contamination is only on the surface: dust, light haze or loose spatter. Clean it with lens tissue and optical grade alcohol. Once a spot has burned into the coating, cleaning will not remove it and the window must be replaced.

Can I use a lower power window if the size is the same?
No. Windows in the same dimensions are rated for different laser powers. Fit a window rated at or above the power of your source, and order against your head's part number.

Are your windows made by the head manufacturer?
No. They are aftermarket windows made to the same dimensions. The head maker's part number is shown on each listing so you can match the window you are replacing.