Fiber optic splicing

　　(1) Fiber optic splicing. The principles to be followed in fiber optic splicing are as follows: When the number of fibers is equal, corresponding fibers of the same color within the same cable bundle should be spliced together; when the number of fibers differs, first splice the fibers with the larger count, then proceed to splice those with the smaller count in sequence. (2) There are three methods for fiber optic splicing: fusion splicing, active connectors, and mechanical connectors. In most engineering applications, fusion splicing is the preferred method. This fusion splicing method results in low splice loss, high back reflection loss, and high reliability. (3) The process and steps for fiber optic splicing:

　　① Strip the optical cable and secure it inside the splice closure. Be careful not to damage the fiber bundle. The stripping length should be about 1 meter. Use tissue paper to thoroughly wipe away any grease or lubricant. Insert the optical cable into the splice closure, and make sure to firmly tighten the steel wires during installation—there must be no looseness whatsoever. Otherwise, the cable could roll around and cause the fiber core to break.

　　② Splice the fibers by threading them through heat-shrink tubing. Separate fibers from different bundles and of different colors, then thread them through the heat-shrink tubing. The fibers with their coating removed are quite fragile; using heat-shrink tubing helps protect the fiber splices.

　　③ Turn on the power of the Furukawa S176 fusion splicer and perform splicing using one of the 42 pre-set programs. During and after use, promptly remove dust from the splicer, paying special attention to removing dust and fiber debris from the fixtures, all optical surfaces, and V-grooves. The optical fibers used in CATV systems include standard single-mode fibers and dispersion-shifted single-mode fibers, with operating wavelengths of either 1310 nm or 1550 nm. Therefore, before splicing, you must select an appropriate splicing program based on the type of fiber and operating wavelength used in your system. Unless there are special circumstances, the automatic splicing program is generally recommended.

　　④ Prepare the fiber optic end faces. The quality of the fiber optic end faces directly affects the splicing quality; therefore, it is essential to ensure that the end faces are properly prepared before fusion splicing. Use a specialized wire stripper to remove the coating, then gently wipe the bare fiber several times with an alcohol-soaked cleaning cotton swab, applying moderate pressure. Next, use a precision fiber cleaver to cut the fiber. For 0.25 mm (outer coating) fibers, the cutting length should be between 8 mm and 16 mm; for 0.9 mm (outer coating) fibers, the cutting length must be exactly 16 mm.

　　⑤ Place the optical fiber. Put the optical fiber into the V-groove of the fusion splicer, and carefully press down the fiber pressing plate and fiber clamp. Adjust the position of the fiber on the pressing plate according to the length of the fiber cut. Close the wind shield, and the fusion splicing will be completed automatically in just 11 seconds.

　　⑥ Use a heating oven to heat the heat-shrink tubing for fiber optic cable splicing. Open the wind shield, remove the fiber optic cable from the fusion splicer, place the heat-shrink tubing centered over the bare fiber, and then put it into the heating oven for heating. The heater can accommodate 20-mm micro heat-shrink sleeves as well as standard 40-mm and 60-mm heat-shrink sleeves. A 20-mm heat-shrink sleeve requires 40 seconds, while a 60-mm heat-shrink sleeve requires 85 seconds.

　　⑦ Fiber winding and fixing. After splicing the optical fibers, wind them onto the fiber storage tray. During fiber winding, the larger the radius of the coil and the greater the curvature, the lower the overall transmission loss of the entire line. Therefore, it is essential to maintain a certain minimum radius to ensure that the laser light travels efficiently within the fiber core without incurring unnecessary losses.

　　⑧ Sealing and Hanging. The outdoor splice closure must be sealed tightly to prevent water ingress. Once water enters the fusion splice closure, prolonged immersion of the optical fibers and fiber splices in water may initially lead to an increase in attenuation in certain fiber segments. Attach a stainless steel hook to the closure and hang it on the suspension cable. At this point, the fiber optic splicing is complete. 2. Fiber Optic Testing After the fiber optic cables have been installed and splicing completed, the next step is testing. The primary instrument used for testing is the OTDR tester—the portable, Chinese-language, color-touchscreen OTDR tester FTB-100B manufactured by Canada’s EXFO (with dynamic ranges of 32/31, 37.5/35, 40/38, and 45/43 dB). This instrument can accurately determine the location of fiber breaks, measure the total loss along the entire fiber optic link, analyze the distribution of loss along the fiber length, and assess the connector loss at each splice point. To ensure accurate testing, the pulse size and width of the OTDR tester must be appropriately selected and set according to the refractive index n value provided by the manufacturer. When identifying fault locations, if the exact length of the optical cable is unknown beforehand, you can first use the automatic mode of the OTDR to roughly locate the fault. Then, switch to the advanced mode of the OTDR, select a smaller pulse size and width that corresponds to the cable length, and reduce the blind zone until it aligns with the coordinate line. The narrower the pulse width, the more precise the measurement—but be careful not to make the pulse too small, as this could introduce noise into the curve. Additionally, adding a fiber-optic tracer disc helps prevent undetected blind zones in close proximity. Regarding fault location identification: if the break is not located at the splice closure, open the nearest splice closure, connect the OTDR tester, and precisely measure the distance from the test point to the fault location. By using the meter markings on the cable, locating the fault becomes remarkably straightforward. When using the meter markings to find the fault, keep in mind that for stranded optical cables, there’s a twist ratio issue: the actual length of the fiber is approximately 1.005 times the length of the cable itself. By applying the methods described above, you can successfully identify multiple break points and high-loss areas.