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The discovery of coherent laser light in 1960 shifted and expanded the biomedical applications of radiation to the non-ionizing part of the electromagnetic spectrum. As in the case of ionizing radiation, but considering the laser specific features, the effective, safe and ethically acceptable use of biomedical laser technology requires interdisciplinary collaboration between physicists, engineers and physicians. This should extend at the research, preclinical and clinical level, inspiring at this time the dynamic discipline of Medical Physics in new areas. With this work we aim to introduce the interested reader in the need of dosimetry in medical applications of laser radiation, as this field is still unexplored. After some necessary definitions, we give a brief review of the basic biophysical mechanisms of coherent light-matter interactions. The manuscript focuses on biomedical laser applications in diagnosis and therapy (i.e. in Theranostics). From the vast field of laser theranostic applications we have chosen some experimental and theoretical results - examples of quantification of the laser effect, particularly relevant to soft and hard tissue laser ablation, laser induced photodiagnosis and photodynamic therapy of cancer. These topics intend to highlight the important role of Medical Physicists in the optimization of well-established laser based clinical procedures and mainly emerge the necessity of the relevant dosimetry for each application. Finally, we hope that this effort is going to give food for thought and highlight the importance of deep knowledge of the physics behind some everyday medical applications.

Measurement of the transmission laser radiation from Q-switched and free-running Er:YAG lasers operating at 2.94 μm through fluoride glass multimode optical fibers of 448, 465 and 620 μm core diameter were performed. Attenuation measurements were obtained as a function of the laser energy input, of the bending angle and of the radius of curvature. The fibers exhibited low attenuation under straight and bent conditions. The output beam quality was also studied using the appropriate beam profiler. The output beam profiles showed a central multi-spiked energy distribution.

Two different kinds of chalcogenide glass IR fibers were evaluated relative to transmission of pulsed IR radiation produced by several laser sources in the wavelength range from 1 to 10 μm. Fibers composed either from As–Se–Te or from As 2 S 3 glass, of 250, 500, 750 and 1000 μm and 250, 750 and 1000 μm core diameters were studied, respectively. Attenuation measurements were obtained as a function of the laser energy input and as a function of curvature, wherever this was possible. The output beam quality was also studied using a beam profiler. The lasers used were a Q-switched Nd:YAG laser, emitting at 1.06 μm, a free-running or Q-switched Er:YAG laser emitting at 2.94 μm and a tunable pulsed CO 2 laser emitting in the range of 9.3–10.6 μm. The fibers exhibited better behavior when tested with the Er:YAG laser and they were found fragile in pulsed radiation from the Nd:YAG and the CO 2 laser. The output beam profiles generally showed a central multi-spiking energy distribution.

Biomedical laser light dosimetry relies on spatial light distribution measurements in combination with the appropriate laser–tissue interaction model that may be used to determine the tissue optical coefficients and the fluorophore concentration from its fluorescence spectra. Therefore, modelling of light transport in tissue, for optimization of laser dosimetry, requires the development of simple theoretical models and the experimental implementation in tissue-simulating phantoms, with known optical and fluorescence properties. Low cost phantoms were used, based on agar dissolved in water as the transparent matrix. The latter is loaded with various amounts of Intralipid and fluorophores as light scatterer and absorbers, respectively. The optical properties of these phantoms were measured at two different visible wavelengths. By comparing the fluorescence emission spectra of the turbid samples containing the same fluorophore in different concentrations, we can calculate the concentration. In addition, the evaluation of the scattering and absorption coefficients allow us to predict the spatial distribution of the light intensity inside and outside of the phantom. It seems that the Monte Carlo simulation is an effective and relatively simple mathematical approach for tissue optical properties evaluation.

Quasi-simultaneous laser action in the UV (0.337 μm ) and the IR (10.6 μm ) was observed from a pulsed laser with a sliding discharge plasma cathode. The laser operates at atmospheric pressure, with a gas mixture of CO2/N2/He, at a 0.26/0.50/4.0 l min −1 flow rate. Output energies of 30 mJ in the IR and 0.35 mJ in the UV were obtained, from a laser discharge volume of 38.0×1.0×2.8 cm 3 . The optimum gas mixtures have been determined and the temporal behavior of the discharge parameters, the performance characteristics of the laser and the beam spatial distributions were investigated.

A circuit and discharge simulation is presented for a TEA HF laser operating with a He/SF 6 /C 3 H 8 gas mixture. A comparison of the simulated discharge voltage, current, resistance and input power with the corresponding experimental results is presented together with a discussion on the factors affecting the simulation accuracy.

The ablative etching of nitrocellulose membranes with CO 2 ( λ = 10.6 μ m), Nd : YAG ( λ = 1.06 μ m) and XeCl ( λ = 308 nm) lasers has been studied. From etch rate measurements the threshold fluence was estimated and the energy deposition at threshold fluence was calculated for the three wavelengths. The electron microscope examination of the infra-red laser ablated areas reveals a fairly clean etching under CO 2 laser irradiation, confirming that melting of the crater surroundings cannot be considered as a criterion for the thermal mechanism. It is proposed that the CO 2 laser, which also possesses some operational advantages, compared with excimer lasers, can be effectively used for the clean ablation of the nitrocellulose film dielectric.

Intense laser operation is reported by a novel design of TEA laser heads employing double discharge and semiconductive preionisers-electrodes. Their performance, beam profile and electrical characteristics are presented and are compared to the characteristics of semiconductor or spark array preionised laser heads. These novel laser heads of minimum complexity are capable of producing N 2 laser pulses up to 12mJ of energy and efficiencies up to 0.11%, or CA 2 laser pulses up to 800 mJ of energy and efficiencies up to 5.6%, at pulse repetition rates up to 100 Hz. To the best of our knowledge, these figures compare favorably with any published figures, obtained from lasers of similar size.

A high current sliding discharge has been used to pump directly a N 2 :He mixture at an overall atmospheric pressure and produce laser output at 0.3371 μm and 0.3577 μm. In addition to that the sliding discharge has been used, to the best of our knowledge for the first time, to pump directly Ar and Xe and produce laser action at 1.79 μm, 1.27 μm and 0.912 μm from Ar:He mixtures and at 1.73 μm and 0.98 μm from Xe:He:Ar mixtures. The performance parameters of these lasers are given and among them their charging voltage, pressure and SF 6 addition dependence.