_applied_  
_sciences_  
_Article_  
Controlling the Temperature on the Vein Wall Based on the Analysis of the IR Signal during Endovasal Laser Treatment  
[PERSON] 1,2,*(c), [PERSON] 1,3(c) and [PERSON] 1  
1 Faculty of Nanoelectronics, ITMO University, 197101 Saint Petersburg, Russia; [EMAIL_ADDRESS] (D.T.T. ); [EMAIL_ADDRESS] (Y.V.F.) 2 Department of Modern Dental Technologies, The First Pavlov State Medical University of St. Petersburg, 197022 Saint Petersburg, Russia  
3 Faculty of Physical and Chemical Engineering, Le Quy Don Technical University, Ha Noi 122300, Vietnam  
* Correspondence: [EMAIL_ADDRESS]  
**Citation:** [PERSON]; [PERSON]; [PERSON] [PERSON] Controlling the Temperature on the Vein Wall Based on the Analysis of the IR Signal during Endovasal Laser Treatment. _Appl. Sci._**2022**, 12, 4925. [[https://doi.org/10.3390/app12104925](https://doi.org/10.3390/app12104925)]([https://doi.org/10.3390/app12104925](https://doi.org/10.3390/app12104925))  
Academic Editor: [PERSON]  
Received: 18 April 2022  
Accepted: 11 May 2022  
Published: 12 May 2022  
**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. **Cognition:**\(\copyright\) 2022 by the authors. Liconse MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/py/4.0/](https://creativecommons.org/licenses/py/4.0/)]([https://creativecommons.org/licenses/py/4.0/](https://creativecommons.org/licenses/py/4.0/))). **1. Introduction**  
Varicose veins are a common disease [1], which can lead to chronic pathologies and are more common with women [2]. Endovasal laser treatment (EVLT) is currently used to treat this disease, it is a promising minimally invasive procedure that reduces postoperative risks [3]. Typically, solid-state or semiconductor lasers with an average radiation power of up to 30 W are used for EVLT [4]. The radiation of these lasers lies in the near-infrared wavelength range and is transmitted through an optical quartz fiber. During the EVLT, an optical quartz fiber is injected into the vein and then pulled out at a speed of several millimeters per second. At the same time, a carbonized layer is formed at the output end of the fiber under the action of laser radiation. This layer effectively absorbs the energy of laser radiation and can heat up to 1200 \({}^{\circ}\)C [5; 6]. The thickness of carbonized layer is 26 \(\pm\) 6 \(\upmu\)m, the absorption coefficient is 72 \(\pm\) 16 mm\({}^{-1}\), and the scattering coefficient reaches 30 mm\({}^{-1}\)[7]. The high temperature of the carbonized layer leads to the formation of bubbles in the blood surrounding the fiber and to the vein wall heating [8; 9]. Heating the vein wall up to 80 \({}^{\circ}\)C and above leads to the deformation of the collagen contained, coagulation, and collapse of the vein [10]. However, the lifetime of the carbonized layer is short since it is easily destroyed both by laser radiation and mechanically, in contact with biological tissue. In recent studies, to efficiently convert laser radiation into heat, the optothermal fiber converters (OTFC) on the distal end of the optical fiber were proposed to create [11; 12; 13; 14; 15; 16]. OTFC can be used in surgery for effective ablation and coagulation of soft tissues and in photodynamic therapy (PDT) for resonant and non-resonant photoexcitation of photosensitizing drugs [11; 12]. A titanium-containing optothermal fiber converter (TOTFC) has been referred to in [12; 13]. This converter has strong mechanical connection with an optical quartz fiber, can be heated up to 2700 \({}^{\circ}\)C without destruction, and is resistant to laser radiation [12]. TOTFC has the smooth shape similar to a sphere, which eliminates the perforation of the vein wall when moving the optical fiber inside the vein.