Fig. 1. Pictures of the fiber fuse phenomenon. (a) The real-time fiber fuse propagation through a fiber coiled on a cylinder, recorded by a 240-frame-per-second camera,and the numbers in brackets are consecutive frames; (b) ongoing fiber fuse in a 3-kW fiber laser system

Fig. 2. Optical spectra of fiber fuse. (a) Data by D. P. Hand, et al.^[5], which compares their measured spectra with theoretically derived blackbody spectra; (b) data by S. Todoroki^[48] in similar comparison; (c) data by E. M. Dianov, et al.^[50] comparing the measured spectra with theoretical blackbody spectra of 10500 K, 7900 K, and 4700 K; (d) data by F. Domingues, et al.^[54]; (e) the color coordination by F. Domingues, et al.^[54-55], which uses a color gamut to show the temperature of fiber fuse; (f) data by Y. Mizuno, et al.^[44] obtained in polymer optical fibers and comparison with theoretical blackbody spectra of 4600 K, 3600 K, and 2600 K; (g) spectra measured before and during the fiber fuse, by the authors of this paper; (h) the spectrum obtained by using the data in Fig. 2(g)

Fig. 3. Propagation speed of fiber fuse and the optical power (density) in the fibers. (a) Data collected from previous studies, by D. D. Davis, et al.^[24]; (b) velocity measured by E. M. Dianov, et al.^[50], the legend distinguishes two kinds of fibers, and the left-side vertical coordinates show temperature calculated by blackbody theory; (c) data from E. M. Dianov, et al. in a 3-kW-peak-power pulsed fiber laser^[57]; (d) data from S. Jiang, et al., pictures from the left to the right show the results respectively in single-mode Ge-doped fiber, single-mode pure-silica fiber, and double-clad fiber^[61]

Fig. 4. In-fiber microcavities generated by fiber fuse. (a) The schematic evolutionary process of the generation of in-fiber microcavities during fiber fuse^[67]; data from (b) S. Todoroki^[67], (c) R. Kashyap, et al.^[2], (d) Atkins, et al.^[62], (e) Dianov’s team^[57] in a 3-kW-peak-power pulsed fiber laser, (f) Y. Mizuno, et al. in polymer fibers^[44], (g) H. Zhang, et al.^[36], (h) the authors of this paper^[37] under 350, 413, 438, and 541 MW/cm², respectively for the subplot and four pictures in the second row are irregular cavity in optical fiber under 438 MW/cm², and (i) the authors of this paper^[38] in a 3-kW-continuous-wave-power fiber laser; in graded-index fiber (j) the fiber fuse termination point, (k) the inversed bullet-shape microcavities, and (l) the long microcavities; (m) the fusion-splicing point after fiber fuse

Fig. 5. Test results of fiber fused samples. (a) Stimulated Raman spectroscopy results from R. Kashyap^[69], of which the peaks around 1555 cm^-1 manifests existence of oxygen; (b) refractive-index (RI) profile measured in pre- and post-fiber fuse samples, by E. M. Dianov, et al.^[71]; (c) electronic spin resonance (ESR) results, by the authors of this paper^[70], show signs of defects in post-fiber-fuse samples; (d) Raman spectroscopy results^[38], which shows peaks around 1555 cm^-1; (e) RI profile results of pre- and post-fiber-fuse samples^[38]; (f) ESR results showing three new g values 2.0257, 2.0248, and 2.0230^[38]

Fig. 6. Critical conditions for spontaneous initiation of fiber fuse^[70]. (a) Experimental setup by the authors of this paper, we removed the coating layers of the fibers carefully, put the undamaged naked fibers into a tube furnace and heated it uniformly while applying constant input laser powers, and measured the conditions when fiber fuses were triggered; (b) data analysis showing that an energy with a unit of eV, being different for diverse fibers, dominates the conditions for each of the fibers; (c) critical temperatures and critical laser powers measured in four kinds of fibers; (d) micrographs of the positions of the initiations of fiber fuses

Fig. 7. Some results from simulations. Results from (a) D. P. Hand, et al.^[3,5], showing the propagation velocity of a thermal shockwave, induced by rapidly rising absorption (left-sided subplot), and its relation with the laser power and the geometrical features of the fibers (right-sided subplot), (b) Y. Shuto, et al.^{[51,77,82,90]}, showing a high-temperature point (up to 106 K) moving in the simulation grid (left column) and the physical models (right column), (c) S. I. Yakovlenko, et al.^[86], showing the perceived formation of in-fiber voids (left-sided subplot, first row) and the relation between propagation velocity (left-sided subplot, second row) and laser power density (right-sided subplot, second row), and (d) A. Ankiewicz, et al.^[91-92]

Fig. 8. Experimental setups or systems for timely detection and monitoring of fiber fuses. (a) Detection by setting a photodiode by the fiber, the right-sided subplot shows current responses when fiber fuse passed through^[100]; (b) detection by monitoring the variation of in-fiber backward-propagating optical power^[93]; (c) detection and monitoring of fiber fuse using an optical frequency-domain reflectometry (OFDR), the right-sided columns showing signals before, during, and after the fiber fuse^[61]; (d) detection and monitoring using a heterodyne system, right-sided subplots show the variation of velocity of fiber fuse in single-mode and few-mode fibers^[52,99]

Fig. 9. Methods and devices for stopping the propagation of fiber fuse. (a) An optical fuse made from TeO2[102], which can melt down when the in-fiber optical power exceeds certain thresholds, note that there were no fiber fuses being initiated in the original paper; (b) stopping fiber fuse propagation by tapered fibers^[104], in which fiber fuses were found to stop propagating due to the decreased optical power density in the tapered region; effect of stopping fiber fuse propagation by (c) the use of tapered fibers^[105-106], (d) a setup of fusion-splicing with hole-assisted fiber (HAF) ^[108], (e) shutting off the power source^[100], and (f) fusion-splicing with multimode fibers^[110]

Fig. 10. Using fiber fuse as a tool for fabricating in-fiber Fabry-Perot (F-P) microcavity and its sensing applications. The results that apply such methods for (a) strain sensor^[125], (b) high-sensitivity temperature sensor with an over-600-μm-long F-P microcavity^[126], (c) humidity sensor, with illustrating the steps of the whole process^[127], (d) FBG interrogator^[135], and (e) pressure sensor^[128]; (f) a strain sensor^[132] and (g) a curvature sensor^[133], which use fiber fuses in polymer fibers as fabrication steps

Fig. 11. Using the controlled initiation of fiber fuse to fabricate high-quality-factor (high-Q) in-fiber microcavity in a single-step manner. (a) Comparing the three fabrication strategies, that of the conventional methods (1), that in the previous studies which used fiber fuse as a step to obtain craves on fiber facets (2), and that of the authors of this paper, which proposed the one-step manner fabrication method^[136]; (b) experimental setup for the one-step manner fabrication of in-fiber microcavities using fiber fuse^[136]; (c) so-fabricated high-Q in-fiber microcavities observed under optical microscope^[136]; (d) setup for testing the microcavities^[136]; (e) transmission spectra of the microcavities, demonstration of strain sensing application with the Q factors of F-P resonances shown in the subplot^[136]