Vasily M. Bogomolov

Today, the presence of skin pathologies and defects of varying degrees of intensity is considered as a source of serious psychological problems and social consequences for people. Lasers have proven to be an effective tool for the treatment of atrophic and hypertrophic scars, as well as for the correction of other aesthetic imperfections, improving skin turgor, density, texture and pigmentation. One of the most effective laser sources for these purposes today is a non-ablative millisecond laser, which produces radiation at a wavelength of 1550 nm. Its action allows intensive skin remodelling by accelerating the regeneration of characteristic biological markers such as matrix metalloproteinases and interleukins. A major disadvantage of such a source is the thermal damage to the treated tissue, which leads to visual complications (erythema, oedema, etc.) and increased rehabilitation time. The use of laser pulses with a duration shorter than all the characteristic relaxation times in tissues (> 20 ps) makes it possible to eliminate the thermal effects on the tissue and, at the same time, to stimulate the growth of biological markers of remodeling. Therefore, the aim of this work was to study methods of obtaining a compact laser source of simple configuration with a pulse duration not exceeding 20 ps, a radiation spectrum in the range from 1530 to 1580 nm with a variable pulse repetition rate in the range from hundreds of kHz to units of MHz (so as to exceed the ablation threshold) and an average power of not less than 500 mW. The reference oscillator was designed based on a 15.66 m long circular erbium fiber resonator with a total group velocity dispersion (GVD) of 0.005 ps2, producing a stable sequence of pulses in the similariton mode at a repetition rate of 13.3 MHz. The output pulse duration was 2.5 ps, the emission spectrum was 42 nm wide at -5 dB level with λ_center = 1541 nm, and the average emission power was 8.1 mW. The pulses were then stretched in a 48 m passive fiber loop with positive GVD (D = -72 ps/(nm*km) at a wavelength of 1550 nm to a duration of 30 ps, which was necessary to control the peak radiation power in the fibers. The radiation was then amplified in the first stage of the preamplifier, which consisted of 5 meters of active erbium-doped I-25 single-mode fiber with an absorption coefficient of 40 dB/m. This fiber was pumped by continuous emission from a pump diode at a wavelength of 980 nm with a power of 650 mW. The amplified pulse sequence was then thinned by an acousto-optic modulator (AOM) with a division factor ranging from 2 to 20. The second preamplifier stage, consisting of 3 m of I-25 active fiber pumped by continuous emission from a pump diode at a wavelength of 980 nm with a power of 350 mW, compensated for the losses caused by the AOM. The pulse sequence was then amplified in a 7 m loop of a large mode area erbium-doped active fiber (dmod.area = 25 μm) pumped by continuous emission from a pump diode at a wavelength of 980 nm with a power of 30 W. As a result, a sequence of pulses with a duration of 20 ps was obtained at a repetition rate of 0.67...13.3 MHz, and the average radiant power reached 512 mW.

In this article, we report on experimental studies of the influence of several laser radiation parameters, such as the duration of the laser pulse, the radiation wavelength, and the pulse energy, on the efficiency of the destruction of urinary calculi. The study used a laser lithotripter based on a fiber Tm laser generating at a wavelength of 1940 nm with pulses with a duration of about 1800 μs and pulse energy of up to 6 J, as well as a femtosecond solid-state Yb laser generating at a wavelength of 1032 nm with a pulse duration of about 250 fs and pulse energy of up to 400 μJ. A comparative analysis was carried out according to such criteria as the productivity of lasers when removing a unit mass of images and the amount of sample displacement resulting from the retropulsion effect. The results obtained in this work demonstrated that the femtosecond laser loses approximately two times its efficiency in terms of sample material removal. However, this shows the absolute advantage of the photoionization mechanism of femtosecond laser ablation in the study of retropulsion and thermal heating, which were completely absent in this case.