Optically modified fiber tips are under investigation or already in use clinically for the recanalization of totally occluded arteries. It has not been determined to what extent their mechanism of action is optical, thermal and mechanical. We studied a 2.2 mm diameter, rounded Sapphire Contact Probe (SLT, MTR 1.5) and a 1.5 mm diameter ball-shaped fiber (ACS) coupled to a continuous wave Nd-YAG laser using 1 second pulses. The probes were positioned perpendicular to homogeneous porcine fatty tissue samples in plasma using preset axial forces. Penetration depth per pulse and temperature of the collar of the probes were measured in relation to the force. Starting with new, clean probes no tissue penetration was achieved using forces up to the equivalent of 105 gr and powers up to 25 W for 10s. On purpose, the probes where exposed to high powers in plasma until a coagulum was formed on the tip. After cleaning, a ring of carbonized particles deposited on the surface of the probe bordered the exit window of the beam on the tip. The power absorbed by the probes increased from 5 to 32%. Tissue penetration with 'dirty' probes was force dependent. For sapphire contact probes it was 1.2 - 2.9 mm/pulse (1 mm diameter spot, 15 W, 1 s) in the force range of 23 - 105 gr. For the ball shaped fibers it was 2.7 - 10 mm/pulse (0.4 mm diameter spot, 10 W, 1 s) in force range of 23 - 35 gr. The optically modified fiber tips studied did not penetrate tissue by absorption of the Nd-YAG beam by the tissue only. Tissue penetration started when part of the laser beam was absorbed by pollution on the surface of the probe creating a 'hot tip'. Tissue penetration was force dependent due to the smaller diameter of the ablative beam in comparison to the diameter of the probe. Thus, the recanalization mechanism of both probes is partly mechanical.