Engraved Glasses

How is laser engraved 3-D glass cubes made?
I understand the laser technology used to make glass cubes D-3. What's Not know is how.
The Lazer uses coordinates that are shot inside the cube (the absorption of several photons) these vaccines cause a small crack multiplied tens of thousands of times form the image you see with out damaging outside of the cube. The cube is made of optical glass.
Glass Engraving – Lesley Pyke – Tudor Crystal Tankard – LION