4.1 Fusion Splicing Procedures

Fusion Splicing Procedures

In fiber optic communication networks, splicing is the process of permanently joining two optical fibers. Splicing is required when a cable run is longer than the maximum length of a single cable reel, when extending a cable run, when transitioning between outdoor and indoor cable types at the building entrance, or when repairing a damaged or severed cable. Splicing is divided into two primary categories: fusion splicing and mechanical splicing. Among these, fusion splicing is the industry standard for telecommunications, long-haul networks, and high-speed data transmission due to its exceptional reliability, low insertion loss, and high return loss.

Fusion splicing involves using localized heat to melt the silica glass endfaces of two prepared fibers and fuse them together into a single, continuous waveguide. The thermal energy is typically generated by a high-voltage electric arc between two electrodes. To achieve a successful fusion splice with minimal attenuation, the technician must execute a precise, multi-step preparation procedure. Any shortcut in this sequence will inevitably lead to a high-loss splice or physical failure.

Step 1: Pre-Positioning the Splice Sleeve

Before any preparation of the optical fiber begins, the technician must slide a protective heat-shrink sleeve (fusion splice protector) onto one of the fibers. The sleeve must be pushed down the cable jacket out of the way of the working area. Splicing sleeves consist of a heat-shrinkable plastic outer tube, a hot-melt adhesive inner liner, and a rigid stainless steel rod (or strength member) that prevents the delicate fused joint from bending or stressing. If the technician forgets to install this sleeve prior to stripping and cleaving, the splice cannot be protected, and the fibers must be cut and prepared again.

Step 2: Stripping to Bare Glass

Optical fibers typically consist of a 9 µm (singlemode) or 50/62.5 µm (multimode) core, surrounded by a 125 µm cladding, which is protected by a 250 µm primary acrylate buffer coating (and sometimes an additional 900 µm tight buffer or 2.0/3.0 mm outer jacket). To perform a fusion splice, all coatings must be removed to expose the bare 125 µm silica glass cladding.

The technician uses a precision fiber stripping tool, such as a three-hole stripper. The first hole strips the outer jacket, the second removes the 900 µm buffer, and the third removes the 250 µm acrylate coating down to the 125 µm cladding. Stripping should be done in short, firm, and smooth strokes, holding the tool at a 35 to 45-degree angle relative to the fiber. This prevents nicking or scratching the glass, which creates microscopic surface flaws that lead to fiber breakage under tension.

Step 3: Cleaning with 99% Isopropyl Alcohol

Once stripped, the bare cladding is coated with microscopic acrylate adhesive residue. This residue must be chemically cleaned. The technician uses lint-free, specialized wipes saturated with 99% pure reagent-grade isopropyl alcohol (IPA). Wiping the fiber creates a distinct "squeak" sound, indicating the glass is clean.

It is critical to use 99% pure IPA. Standard rubbing alcohol (typically 70% or 91% concentration) contains water and denaturing oils that leave a film or mineral residue on the glass. During the fusion process, the intense heat of the electric arc will burn these contaminants, creating carbon deposits, bubbles, or cloudiness inside the glass joint, which causes high optical loss. The fiber must be cleaned before it is cleaved. Cleaning a cleaved fiber by wiping it will contaminate or damage the pristine, cleaved endface.

Step 4: Precision Cleaving

The cleaving process does not "cut" the fiber like scissors. Instead, a precision fiber cleaver uses a diamond or tungsten carbide blade to lightly score the outer surface of the glass cladding. The cleaver then applies controlled bending tension to the fiber, propagating a clean, flat fracture perpendicular to the fiber's longitudinal axis.

The goal of cleaving is to achieve a perfect 90-degree angle. Most modern fusion splicers require a cleave angle of less than 1.0 degree; a cleave angle of 0.5 degrees or less is ideal. An improper cleave angle prevents the endfaces from aligning or fusing smoothly, resulting in air pockets, glass deformation, and high attenuation. After cleaving, the bare fiber is extremely fragile and must not touch any surface (including the technician's fingers) before being loaded into the fusion splicer.

Step 5: Fiber Alignment Methods

Once loaded into the fusion splicer, the instrument must align the two fiber cores. Splicers utilize one of two primary alignment methods:

1. Active Core Alignment: This method uses high-resolution cameras and light sources to view the fiber cores from two orthogonal angles (X and Y axes). Using the Profile Alignment System (PAS) or Core Detection System (CDS), the splicer's internal micro-positioning motors adjust the fibers in three dimensions ($X, Y, Z$) to align the actual light-carrying cores. Active core alignment is mandatory for singlemode fibers due to their small 9 µm core diameter and is highly effective when splicing mismatched fibers from different manufacturers.
2. Cladding Alignment (Fixed V-Groove): This simpler, passive method relies on placing the fibers into precision-machined V-grooves. The splicer assumes that the fiber core is perfectly concentric with the 125 µm cladding. While faster and less expensive, it does not correct for core-cladding eccentricity. Cladding alignment is widely used for multimode fibers (which have larger 50 µm or 62.5 µm cores and wider tolerances) and in ribbon fiber mass fusion splicers.

Step 6: Electric Arc Fusion

After alignment, the fusion splicer initiates the welding process.

1. Pre-fuse Arc: A brief, low-current electric arc is fired between two tungsten electrodes. This arc burns away any remaining dust particles and warms the fiber tips, rounding the sharp edges of the cleaves slightly to prepare them for welding.
2. Fusion Arc: The main electric arc is fired, melting the glass. Simultaneously, the splicer's motors push the two fibers together slightly (known as feed or overtravel). Surface tension of the molten glass helps pull the fibers into alignment, forming a seamless, continuous silica joint.

Step 7: Tension Test and Splice Protection

Following fusion, the splicer performs a mechanical tensile strength test by applying a force of approximately 200 grams (2 Newtons) to the joint. If the splice passes, the technician carefully transfers the assembly to the built-in heat oven. The protective sleeve is centered over the spliced joint, and the oven is activated. The heat shrinks the outer sleeve and melts the inner adhesive, encapsulating the splice in a rigid, waterproof package. Typical fusion splice losses range from 0.02 dB to 0.05 dB, representing the highest quality permanent joint possible.