ET optical cables typically enclose 250μm single-mode or multi-mode fibers in loose tubes made of high-modulus materials. This loose tube material has good hydrolysis resistance and high strength, which can provide reliable mechanical protection for the fibers and help reduce the impact of the external environment on the fibers. The loose tube is filled with non-toxic and harmless waterproof compounds, such as special ointments. This not only prevents moisture from entering the fibers but also buffers external pressure to a certain extent, providing key protection for the fibers.