Method for producing high strength and long coiling length single-mode fiber for submarine cable

A method for producing a single-mode fiber for submarine cables comprises the steps of:

• a) washing a preform using a mixed solution comprising nitric acid and hydrofluoric acid with a weight ratio thereof of 1:2-8 and an acid concentration of the mixed solution being 30-80%, and treating the preform using flame polishing to remove 10-100 pm silica from the surface thereof, wherein hydrogen in an oxyhydrogen flame of the flame polishing reacts with silica under high temperature and as reaction products evaporable silicon monoxide and water are produced and taken away by high-speed oxyhydrogen flame current;
• b) fixing the preform on a fiber drawing furnace comprising a graphite heating element and a stainless steel furnace, an ash content of the graphite heating element being less than or equal to 20 ppm and a surface roughness thereof being less than or equal to 6.3, above the graphite heating element disposed an upper, a middle, and a lower gas inlet pipe, and an inert gas which is performed with a secondary filer prior to entering the fiber drawing furnace moving therein in the form of laminar flow;
• drawing the preform at a temperature of 2,000-2,300° C., with a gas flowrate of 10-50 L/min and a speed fluctuation of less than 20 m/min;
• cooling resultant bare fibers, coating each fiber with a first coating layer and curing therein using an UV-curing device, coating with a second coating layer and curing therein using the UV-curing device, sine twisting the fibers using a sine-twisting wheel, and taking up the fibers using a double take-up system, wherein during coating, a high efficiency air filter is employed to ensure that the clean conditions and granules in the air are monitored and that the granule content in the air reaches one hundred-grade purification effect;
• c) collecting fibers produced-by the middle of the preform and selecting a fiber with a screening strain of 2% and a coiling length up to 100 km;
• and
• d) testing the properties of subsections of each fiber using a fiber testing instrument, and calculating averages of the properties of the subsections to represent parameters of the fiber, a zero dispersion wavelength thereof being 1,300-1,324 nm, a zero dispersion slope being less than or equal to 0.092 ps/(nm2km), a cable cutoff wavelength being less than or equal to 1,260 nm, a mode field diameter being 9.2±0.4 μm, an attenuation at 1,310 nm being less than or equal to 0.35 dB/km, and an attenuation at 1,550 nm being less than or equal to 0.21 dB/km.