FBT machines operate on the principle of controlled fiber fusion and tapering: Fusion Stage: Two or more bare fibers are aligned in parallel and fused under precise hydrogen/oxygen flame heating (typically at 1,400–1,600°C). This effect can lead to the rupture of the fibre or to the fibre fuse. Fused Bionical Taper (FBT) technology remains a cornerstone in passive optical network (PON) component manufacturing, particularly for fiber optic couplers, splitters, and WDM devices. At the heart of this process lies the FBT machine—a precision instrument combining thermal engineering, mechanical. This paper investigates the thermal effects in fused-tapered passive optical fibers under near-infrared absorption. The thermal effect is primarily caused by impurities, such as OH-, which absorb incident light and generate heat. The fabrication process and the performance parameters of these devices are reviewed.