Significance Optical fibers are appealing objects that are both fragile and powerful. However, they are micro-meter-class waveguides that are easily fractured by most external forces. Alternatively, another fact that is surprisingly unnoticed is that they are the core make-up of our society. For example, tons of information at every moment are loaded in the laser blood transmitting in the blood vessels of optical fibers and are used freely and timely worldwide. Many objects are processed, welded, and machined by lasers, scaled up to bulk materials in buildings and vehicles and down to micro- and nano-electronics. Optical fibers and fiber lasers are essential in various industrial areas, including industrial manufacturing, biomedical sensing, smart wearables, or even quantum-encrypted communications. Currently, the smooth running of day-to-day activities follows the safe, stable, and reliable operation of optical fiber systems. Therefore, potential threats to the operation of optical fiber systems cannot be treated with insufficient care.
A fiber fuse is a chain damage effect that propagates in optical fibers transmitting light. It was first reported in 1987 by Raman Kashyap, who at that time worked at a laboratory of British Telecom. Since then, fiber fuse damage effects have been observed in almost all types of optical fibers made from a variety of materials, including silica and organic polymers. It resembles a burning fuse emitting bright light from a moving spot; it happens spontaneously provided suitable conditions and causes irreversible damage to online fiber components it passes through in the inverse direction of the laser light. Therefore, it imposes a serious threat to many important technologies and applications nowadays, such as fiber communication networks and high-power fiber lasers. Studying mechanisms and characteristics of fiber fuse are not only important for controlling the hazards but also beneficial for realizing novel and effective methods for modifying fiber waveguide structures with more intimate aid from the laser inside.
Progress Studies discussing fiber fuse, ranging from former to the recent ones, are reviewed herein. Following authors' experiences with respect to the fiber fuse over the past decade, this study provides an introductory and to-date knowledge on physical mechanisms of fiber fuse, prevention of fiber fuse, monitoring or mitigating its propagation, and applications demonstrated using fiber fuse itself. Thus, directions for future research and major existing problems are also discussed. Regarding the characteristics of fiber fuse, the black-body assumption for obtaining the temperature of fiber fuse can be less rigorous because of the significant omission of other important mechanisms of radiation. Also, oxygen is formed during the fiber fuse and has been left inside the in-fiber bubbles. This implies that physical models that describe fiber fuse should focus more on chemical changes in the materials and their effects on other characteristics of the ongoing phenomenon. The critical temperature and the critical laser power conditions correlated with the initiation of fiber fuse, wherein the mathematical derivation directly suggests that the initiation of fiber fuse is dominated by a chemical process of formation energy around 1 eV, depending on the respective type of optical fiber, which may be attributed to the oxygen diffusion in the silica substances of the fibers. Furthermore, applications of fiber fuse have been developed to an extent wherein highly sensitive fiber sensors of various parameters are made using optical fibers damaged by fiber fuses as raw materials. Moreover, a possible method that uses controlled initiation of fiber fuse without propagating fiber fuse as an effective noninvasive one-step method to fabricate in-fiber microcavities, which are both highly cost-effective and hundreds of times faster than conventional methods, is proposed herein.
Conclusions and Prospects Studies discussing the fiber fuses in the past three decades have yielded many important and useful findings. With such considerable knowledge on the external characteristics of the damaging effect, however, there is room for deeper and further studies. Regarding the propagation characteristics of fiber fuse, the propagation velocity of fiber fuse increases with increasing laser power in fibers but the acceleration rate decreases. Nevertheless, in kilowatt-level high-power fiber lasers, the propagation velocity can be tens of meters per second, which hinders safe operation. Regarding physical mechanisms, oxygen is formed during fiber fuses; the initiation of fiber fuse is dominated by the diffusion of oxygen, which caused critical temperature and power conditions for the initiation; the fiber materials during fiber fuse can be plasma state. A more inclusive and comprehensive physical model for revealing more details and hidden characteristics of fiber fuse is necessary. Future studies on fiber fuses could be extremely beneficial. Groundbreaking changes can root from studies of physical mechanisms. If more physics of fiber fuse is revealed with rigorous theoretical and experimental proof, a wider connection among parameters of design can be built for fiber systems and the stochastic spontaneous initiation of fiber fuse can be avoided with assurance, which will profoundly secure many fields. Moreover, this will benefit the direct application of fiber fuse as a material modification tool that can bring potentially various new in-fiber microstructures into reality.
