Free running and Q-switch infrared Er: YAG laser radiations were compared in the case of hard tissue preparation. The interaction energy of 40 mJ in pulse 200 us long yielding to the interaction intensity 62 kW/cm2, and the energy of 20 mJ in 100 ns long pulse yielding to the interaction intensity 62 MW/cm2 were used for the case of free running, and Q-switch regime, respectively. For the radiation delivery, a waveguide transfer system was used. It consisted of an input lens (40 mm focal length), a cyclic olefin polymer-coated silver hollow glass waveguide (700/850 um diameter), and output lens (55 mm focal length). For the interaction experiment samples of the extracted human teeth cut into longitudinal sections and polished were used. The thickness of the prepared samples ranged from 5 to 7 mm. The methods were compared from the point of prepared cavity shape (SEM), inner surface, and possibility of selective removal of carries. Composite filling material was used to reconstruct the cavities.