Can engraving a mirror, from the front, with a laser diode, damage the laser module?

I use a laser diode(Sculpfun S9) to engrave mirrors from the back side to great effect. The results are stellar. Then I saw a suggestion that engraving a mirror from the front, reflecting side, gives even better results. I tried this, and the results were better, giving a finer, more uniform engraving.

But, it then occurred to me that the reflecting surface of the mirror was reflecting the laser beam back, through the optical system, into the laser diode and this could potentially injure the laser diode.

Hence my question - Can engraving a mirror, from the front reflecting surface, with a laser diode, damage the laser module?

I have not experienced any adverse effects but that might be because I have only done it once.

Keep in mind that you are using a beam that has a specific focal point.

Can’t think of the ‘focused’ beam bouncing back. If it focus point is 30mm from the lens, a reflection would put the lens at 60mm, out of focus and a much low power density.

Tubes, LED lasers are all ‘consumables’, so if you like the results, I’d run with them and see if you see any kind of problems. Some of us turn our machines hard, part of the trade off.

Maybe someone is on this board that has a good explanation. I think the power loss from a diverging reflection is pretty substantial and I doubt it would make any real difference.

Good luck


I’m curious what’s going on physically to account for the difference. Are you focusing on the top of the glass when lasering the front of the mirror or are you accounting for the thickness of the glass and refraction to still aiming to focus at the reflective layer?

I’m wondering if it’s just a difference in focus or possibly a difference in how the material is being vaporized. I could see this being similar to how the backside of a sawcut is more likely to get tearout than the supported front side but not sure if that’s the right way to think about this.

I took into account the thickness of the glass and focussed on the reflective layer.

I am at a loss to account for the difference.

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Thanks, that makes a lot of sense.

The key to good laser work is understanding how the laser is damaging the material…

Maybe that coating is heating up the glass and shattering the bottom surface…?

A combination of the glass and reflective surface…?

What does the mirror look like from the back side. ?


The coating is removed

Is it visually different than when you lase from the back.? Something is different…

Do you have a microscope?


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