With a traditional dry laser, the machining is either carried out completely dry or under a gas atmosphere. Under such dry conditions, heat can build in the work piece, thermal expansion can result in distortions, brittle materials may crack and a large heat affected zone (HAZ) and oxidation may also result. A gas may give a better atmosphere but it does not have sufficient thermal capacity for adequate cooling. Last but not least, the cut produced for a dry, focused laser beam is "V" shaped -the kerf is wider at the entry side of the cut than at the laser's focal point at the bottom of the cut.
A wet laser is very different. It works by first focusing the laser onto the aperture of a water jet nozzle. The water jet is typically 50 microns in diameter and up to 50mm in lenght before it turns to droplets. The laser pulses travel down this liquid optical "fibre" by total internal reflection. The laser-water-jet cuts the workpiece by repeatedly scanning the required cutting pattern so deepening the trench. As a result, the cut produced is parallel sided rather than "V" shaped and it is constantly cooled and cleaned of debris by the 300 bar water jet.