Two powerful laser beams are precisely focused on a minute spot inside the crystal, setting up an extremely high temperature where the beams meet which vaporises a microscopic section of glass. The rest of the glass through which the beams pass remains unaffected. Then the system's computer refocuses the laser beams to burn another spot corresponding with the previous one, as the computer controlled turn-table slowly revolves. This is repeated many thousands of times, to create the 3D image inside the glass, at a rate of 4,000 to 6,000 points a minute. The resultant 3D image appears like a sculpture in light and, like a sculpture, can be viewed from any angle including top and bottom.
The technology was perfected by the Russians during the Cold War and was used for burning the cross-hairs in lenses on tank gun sights. With the collapse of the USSR, the laser technicians developed this further to create incredibly detailed images. These are now also being produced in the West, in fact you can buy a laser engraving machine yourself here, but the Russians are still well ahead in this field and the finest laser art is still coming from Russia.
My Russian laser sculpture, the Jester, is set in a large piece of perfect lens quality crystal glass weighing 0.9 Kg (2 lbs) and measuring 60 x 60 x 100 mm (2.4 x 2.4 x 3.9 inches).
You may find my musings on light and glass here.