## 5 Discussion  
The fiber fuse effect is a complex process that can be roughly divided into several stages. The first is the optical breakdown initiation in the fiber core, which can be caused by various external factors, for example, fiber heating, electrical discharge, or contamination. The second is the emergence and propagation of a self-sustaining plasma focus (spark) along the fiber core. For a stable observation of the fuse effect, the laser power density at both stages of the process must be sufficient to create and maintain the necessary temperature in the plasma spark, and the size of the plasma spark must be comparable to the diameter of the fiber core. The optical breakdown effect is caused by the nonlinear absorption of optical radiation in the fiber core. It directly depends on the type of fiber and its degree of doping. For example, the breakdown of fibers with pure quartz core requires a power density exceeding 40 MW/cm\({}^{2}\), while fibers with a germanium admixture require a threshold breakdown power density of 1 MW/cm\({}^{2}\). The centers of nonlinear radiation absorption or \"coloring centers\" are point defects in the fiber core or E' centers, which are associated with oxygen holes in the X-Si bonds, where Ge germanium or Si silicon can replace X. It is noted that the concentration of GeE\({}^{\prime}\) centers is preserved when the temperature increases, while the concentration of SiE\({}^{\prime}\) begins to decrease at 573 K to complete disappearance at 973 K [9]. Chemically, the optical breakdown process can be described as follows: in the first step, the compound Ge-O-Si decomposes into molecular oxygen O\({}_{2}\) and the compound Ge-Si under the action of high temperature. Then, the new compound is ionized, forming a positively charged Ge-Si\({}^{\ast}\) ion and an electron e\({}^{-}\). Thus, during thermal ionization, the electrons released by the reaction begin to move in the fiber core and form an avalanche,  
Figure 11: Temperature distribution in the center of the plasma spark for radiation with wavelength \(\lambda_{0}=1550\) nm and power \(P=2\) W in 1 ms from the moment of radiation water. the bonds between the atoms are torn, and plasma is formed. In this case, the concentration of germanium-silicon ions and the released electrons are the same. This work is devoted to the construction of a thermophysical mathematical model of an optical breakdown initiation stage at the fiber end in the presence of an initiator in the form of a metal plate. The optical breakdown of the fiber end SMF-28e pressed to duralumin plate D16 according to GOST RF 4784-97 was studied. The process of thermal source development in the fiber core was investigated, and the conditions leading to the appearance of plasma spark were determined. Based on numerical calculations and available experimental data, the authors have suggested that plasma spark motion starts at the moment when the temperature front of plasma formation reaches the fiber core-shell boundary. In this case, the center of plasma formation has a temperature of more than 5000 K. Estimates were made of the plasma spark growth time to the size of the fiber core. For example, at the radiation power of 0.5 W and wavelengths \(\lambda_{0}\) = 1080 nm and \(\lambda_{0}\) = 1550 nm, this time was 1.7 \(\upmu\)s and 5 \(\upmu\)s, respectively. When the power of the laser radiation causing the optical breakdown increased to 3 W or more, the difference between the times for different wavelengths decreased. It can be seen from the results of the calculations that at radiation powers of 10 W, the times for reaching the fiber core boundary by the plasma focus lie in a narrow interval from 0.09 \(\upmu\)s for \(\lambda_{0}\) = 1080 nm to 0.18 \(\upmu\)s for \(\lambda_{0}\) = 2050 nm. Thanks to the mathematical simulation of the fuse effect, it is possible to better study the mechanisms of its emergence, evolution, and transients between the stages of the process.