# Ex Vivo Exposure to Soft Biological Tissues by the 2-um All-Fiber Ultrafast Holmium Laser System
## 1 Introduction  
In the last decade, there has been particular interest in fiber sources of the short-wave infrared spectral range (SWIR, 2-3.5 \(\upmu\)m) due to a wide area of potential applications, including laser surgery and biomedicine [1]. Such lasers can be based on silicate, fluoride, chalcogenide, or tellurite fibers doped with rare-earth ions [2, 3]. Fibers based on silicate glasses are the most successful and widely used, allowing the attainment of compact, all-fiber laser schemes with simple standard splices. At present, silica-based fibers doped with thulium (Tm\({}^{3+}\)) or holmium (Ho\({}^{3+}\)) ions have been extensively developed and used for various types of fiber laser systems in the 2-\(\upmu\)m spectral range [4, 5, 6, 7]. However, there is a limitation of the lasing wavelength in silicate glass fiber at about 2.2 \(\upmu\)m, because the propagation losses at longer wavelengths are extremely high due to the value of the phonon energy (1100 cm\({}^{-1}\)) [8]. In [9], [PERSON] et al. have realized a Ho-doped fiber laser with a tunable lasing wavelength from 2.025 to 2.2 \(\upmu\)m and with a maximum slope efficiency of58% and 27%, respectively. These fibers, together with another glass matrix or crystalline one, are used to operate at longer lasing wavelengths [10]. The presence of strong water absorption makes 2-um laser sources promising for medical applications [11], because a decrease in the depth of radiation penetration into water-saturated tissue would be expected, leading to their precision processing with less tissue heating. Some medical procedures, such as minimally invasive surgery, laser enucleation, skin treatment, etc. have the potential to be more accurate and reliable using SWIR sources [12; 13]. Considering that one of the water absorption peaks is located at a wavelength of about 1.94 um, the use of a holmium laser with its emission wavelength shifted into the infrared range allows one to vary the penetration depth of laser radiation [14; 15]. It is worth noting that 2-um laser radiation is eye-safe, which is also an important advantage for a number of applications [16]. The field of laser application in medicine is wide and is constantly expanding, from diagnostics and therapy to surgery, while having non-destructive and destructive effects on biological tissues [17; 18]. Therefore, laser radiation parameters should be chosen based on the desired result in each specific case. It should be taken into account that the effect of laser radiation depends on the wavelength, power density, operation mode (continuous-wave orpulsed), duration of exposure and pulse repetition rate, as well as on the optical properties of biological tissues [19; 20]. Frequently, medical laser systems of the 2 um spectral range operate in continuous-wave (CW) and nanosecond or longer pulsed modes [21; 22; 23]. These laser systems are mainly based on linear absorption of laser radiation by tissues, due to the relatively low peak intensity of the radiation, which is insufficient to induce a noticeable nonlinear interaction at initial average powers. In this regard, photodamage resulting from exposure to such lasers is strongly wavelength-dependent and thermal in nature [24]. On one hand, this can have a selective effect on the tissue, but on the other hand, it can lead to a non-deterministic cutting effect in heterogeneous tissue and limit the efficiency in transparent or weakly absorbing tissues [25]. Moreover, in this case, the heat diffusion from the laser spot increases the collateral thermal damage to surrounding tissues, contributing to scarring. This fact limits the precision of laser radiation effects inside thick specimens. The development of ultrafast pulsed laser sources and amplifiers producing short picosecond or femtosecond pulses [26; 27] has made it possible to improve surgical precision beyond the optical diffraction limit through new, mostly non-thermal, modes of tissue photodamage.