In order to drill you need to evaporate the material or to eject it with a gas flow. I think that after 20 mm you are melting the material but you don´t get ejected it out of the hole. What is the diameter of the hole?

At first Where is the focal point, inside the material? Are you moving down the focus point along the vertical axis?

Another explanation: If your beam isn't tightly focused and CW (I assume that is just collimated according to the long working distance in the photo) It could be a thermal issue:

Indeed, the temperature gradient could be locally higher at the beginning of the drilling, but as soon as the heat diffuses into the bulk, the local temperature rise could be not high enough for thermal ablation. Short pulse duration lasers (µs range) reduces heat diffusion length, and femto laser ablation is even non-thermal.

Perhaps you should provide more details to get a meaningful response.

It could be either or both reasons - after 20 mm you are out of focus or/and unable to remove material with gas flow. The focus has to be a bit below the surface of material - not the top of the slab but actual = bottom of the hole. Moreover, the narrow channel may act as a diaphragm for laser beam. Sometimes helps the use of the lens with longer focal length.

What is the material you would like to drill?

We had a pipe injecting dry air to remove gases, you can see in the right photo.

We moved down the laser.

The diameter was 6mm.

The target was concrete but the laser operator said me that it happens also when drilling a metal sheet, he recommend to cut, not begin drilling to avoid this effect.

The hole surface was covered with a semi transparent green glass.

I surprised that at the beginning the laser drilled at a very high speed but it slowed down to 0.

The only reason that I found is that the air inside the hole became plasma that shields the electromagnetic light due eddy currents (the laser radiation became a very bright purple, that corresponds to a very hot plasma)

The only reason that I found is that the air inside the hole became plasma that shields the electromagnetic light due eddy currents (the laser radiation became a very bright purple, that corresponds to a very hot plasma)

We are sure the thermal effect is dominant. However we need the constituents of the material (even the impurities in ppm levels) and the spectroscopic data of the emission to explain this phenomena afterwards.

Is your laser CW or pulsed? Is the gas flow coaxial vith the laser? What is the focal length of your lense? Why The hole surface was covered with a semi transparent green glass? We have and old 2 kW CW CO2 laser and we can drill 50 mm of steel. Normally it's necessary a nozzle (like a cutting nozzle) in order to obtain a good quality gas flow. Maybe this reference is useful for you

An experimental and numerical study on laser percussion drilling of thick-section alumina.

Yinzhou Yan, Lingfei Ji, Yong Bao, Yijian Jiang

Journal of Materials Processing Technology. Volume 212, Issue 6, June 2012, Pages 1257–1270

It was a CW laser, the gas flow was not coaxial, perhaps it was the problem. The focal length was very long (almost parallel).

The semi transparent glass was produced during the drilling.

The concrete test sample was not reinforced.

I agree with you Jose Manuel, a good air flow must help to increase the drilling distance, but the efficiency may be lower than using high pressure water cutting with rotating nozzle

You must move the focal point. You can try with O2. If the gas isn't coaxial will not penetrate the hole. The best gas flow is a laminar flow

Drilling stops due to formation of plasma, that is screening target. How to prevent it you will understand after reading our article and cited in it literature (see aur paper - Chumakov A.N. [et al.]. Plasma formation under high-frequency pulse-periodic laser action on metals in air at atmospheric and reduced pressure // J. of Applied Spectroscopy, 2012, Vol. 79, No. 2, p. 279-287).

Thank you very much Alexander, that is just that I was looking for. I requested you through researchgate for such interesting article

Do you think that the frequency of the reflected bright green light could be just the double of the infrared frequency?

It is a pity that I can not repeat the test now.

For drilling it is better to use pulsed periodic laser irradiation with gase flow (better argon).

Thank you for the document.

I have not pdf-file of my paper. It is available electronically on SpringerLink:

http://www.springerlink.com/openurl.asp?genre=article&id=doi:10.1007/s10812-012-9593-3.

After a while you get into a situation where the electron and ion density in the plasma generated by the beam reach the regime of high absorbtion for the laser wavelength. As already said, pulsed lasers are better than CW, and gas flow will try to sweep away the plasma or at least keep it such that it is not absorbing the beam.

 Elemental Identification of Oil Well Structure During Drilling using High Power Multiclad Yb:silica Fiber Laser

P. Parvin, M. Ilchi-Ghazaan, V. Daneshafrouz

EMSLIBS 2011 6th Euro-Mediterranean Symposium on Laser-Induced Breakdown Spectroscopy; 09/2011

Interesting technology, but there are a lot of problems with such method:

- Removal of the dust

- Generation of high conductive glasses that are not affected by the laser

- Smoke screening

- Low efficiency

- Damage in optics

- It can not be use mud to remove debris

- As it is said by Alexander and Jose Manuel, it is created a plasma in front of the  rock that protect it from the laser that must be removed by using laminar high pressure air flow

There is another non conventional high power drilling systems as high pressure water jets and using high power electrical schocks, the last one gives the best relaion of drilling meters per dollar

I found the link for your very useful document:

Conference Paper Elemental Identification of Oil Well Structure During Drilli...