When tissue is irradiated with a laser, a certain proportion of the light is absorbed, depending on the type of tissue and wavelength. This mechanism is used to make therapeutic use of the laser energy introduced. The absorption coefficient describes the probability with which irradiated light is absorbed, and its reciprocal value is the mean free path length that a photon travels in the tissue until it is absorbed.

A distinction is made between various thermal laser applications:

• Laser coagulation: Starting at approximately 60°C the tissue is coagulated. The most well-known field of application is the sclerotherapy of blood vessels.
• Vaporization: CO₂ lasers or diode lasers generate temperatures of over 300°C, at which tissue evaporates. This effect is used in surgery to cut soft tissue.
• Thermomechanical ablation: Pulsed lasers with a wavelength at which a high absorption in water occurs are used for this process. The fluid in the tissue is suddenly heated. At about 100°C the water wants to evaporate and the rapid increase in pressure in the tissue leads to explosive erosion. The thermal damage to the tissue is considerably less than during vaporization. Efficient and precise ablation of hard tissue, bones, teeth, and bladder and kidney stones is achieved, especially with the Er:YAG laser.
• Ablation: Excimer lasers with UV wavelengths, short pulses, and high intensity are used for this technology. The energy of the photons is so high that even individual atoms can be removed. This procedure is especially used in ophthalmology for the correction of corneal curvature.
• Photodisruption: With high power density, ultra-short pulse durations in the nano, pico, or even femtosecond range, the laser generates a microplasma that expands extremely quickly and generates an acoustic shock wave. The most well-known application is the LASIK procedure, which is used to correct defective vision.