## 1 Introduction  
The fiber fuse effect, or an optical discharge, is a phenomenon that occurs in fiber-optic lines during the propagation of high-power density radiation. The fiber fuse effect is a multistage process that begins with the optical breakdown of the fiber end face and then continues as a plasma spark moving along the fiber towards the radiation source, which destroys large sections of fiber-optic lines (Figure 1). This effect was first described by [PERSON] and [PERSON] in 1987-1988 [1, 2, 3]. At present, a study of the optical breakdown initiation and plasma spark motion along fibers is one of the urgent tasks in the development field of fiber-optic instruments. Of particular interest in this connection are quasiperiodic structures resulting from the passage of plasma spark. These structures can be used as sensing elements for sensors or radiation scatterers. There are several papers related to the phenomenon of the fuse effect, which describe its physical essence and consequences for optical fibers, as well as methods for its study. In [4, 5, 6], optical discharge in light guides and its interaction with the fiber material are described. The mechanism of the optical breakdown initiation in a fiber light guide is closely related to the nonlinear absorption of quartz glass [7, 8, 6]. If the power density is sufficient to sustain the optical breakdown, the resulting plasma focus flares up and begins to propagate along the fiber core [9; 10]. The works of [PERSON] [7; 8; 9; 11] studied the phenomenon of the fuse effect, leading to the destruction of optical fibers under the influence of high-power laser pulses. The article [10] describes the formation of cavities in the fibers and their dynamics in the propagation process of plasma spark along the fiber. In general, the most significant contribution to the study of the fiber fuse effect at the fundamental level was made by the works of [PERSON] [7; 8; 9; 11] as well as [PERSON] [10; 11; 12; 13]. The latter devoted his research to the experimental confirmation of theoretical developments. The most interesting article of [PERSON] in the context of this study is the work [10], in which the initiation times of the fuse effect were experimentally investigated. The minimum reaction initiation time was 0.18 s, and the minimum power at which it was possible to start the reaction in the SMF-28 fiber was 0.4 W [10]. Articles [11; 12] are devoted to the study of optical breakdown initiation in the presence of an electric discharge or contamination of optical fiber connectors. The cited sources deepen our understanding of the fiber fuse phenomenon and may be useful for developing measures to prevent or manage it. The dimensions of the internal quasiperiodic structure of an optical fiber obtained during the optical discharge passage depend on the radiation power, its mode composition, and the fiber geometry [14; 15]. As mentioned above, these structures can be used as a sensing element for a fiber optic sensor based on a Fabry-Perot interferometer or a fiber optic radiation scatterer [16; 17]. For different sensors and optical radiation scatterers, there are different requirements for the geometrical parameters of the internal structure [18; 19]. Our team is developing technology for creating in-fiber devices such as sensors, filters, and radiation scatterers that exceed current fiber device technologies in cost effectiveness, speed of device creation, and improved durability of the resulting devices. However, different devices require different internal structures formed during fiber fuse. Therefore, it is very important to know how this process works, when the plasma spark arises, what sizes it reaches, and how it depends on external parameters such as the power and wavelength  
Figure 1: Plasma spark moving in an optical fiber. of radiation. Thus, this present study is aimed at creating a possibility to predict the consequences of optical breakdown.