Fusion splicing - core alignment vs cladding alignment

The growth in worldwide fibre-optic communications has led to an increase in demand for economical and convenient fusion-splicing systems for a wide range of deployments including telco backhaul, access networks, FTTH, LAN, sensing and submarine installations.

The construction of efficient fibre-optic networks is dependent upon the ability to make pristine connections between fibre-optic cables. Fusion splicing is the process by which two optical fibres are joined together to create one continuous optical path. The goal is to fuse the two fibres together so accurately and securely that light may travel through the fibres without loss.

Prior to splicing, the cable jacket and other protective materials are removed from the fibres, and each fibre is cleaned. The fibre is then cleaved using the score-and-snap process. The cleaver’s precision blade exposes a clean fibre end face that is perfectly flat and perpendicular to the axis of the fibre. The closer to 90 degrees the cleave angle is, the lower optical loss the splice will yield. The two fibre tips are then heated by an electric arc and pushed together to create a continuous link. Once the fibres are fused, the splicer will provide a loss estimate and perform a proof test. The proof test applies a force to the fibre in opposing directions to check the mechanical integrity of the splicing joint. A splice protector sleeve is placed over the joint and, with heat applied, shrunk down over the fibre to provide additional mechanical protection.

Currently, commercially available fusion splicers fall into two categories according to how they align the fibres for splicing.

Cladding alignment

Cladding alignment is a passive alignment that relies on the accurate pre-alignment of fibre V-grooves that grip the outer surface or cladding of the fibre. Fibres are adjusted inwards and outwards. The advantage of this method is that the technology required is low cost and fast, so it is still utilised on low-cost field fusion splicers and ribbon splicers. Fibre position, core-cladding eccentricity and mode field diameter (MFD) influence the effectiveness of cladding alignment and subsequent splice performance. Fibre position is influenced by contamination on the fibre or V-grooves. While the operator controls this parameter, core-cladding eccentricity and MFD are fibre manufacture parameters, and typically come into consideration when splicing new to old fibres and also dissimilar fibres such as singlemode G652.D to G657.A.

Core alignment

Active alignment of the cores, or core alignment fusion splicing, is currently the most commonly used fusion splicing technology. Although more expensive and complex technology, it is also more powerful and flexible, and less sensitive to variations in the cable. Core alignment splicers use a combination of image and light detection systems that ‘see’ the fibre cores to measure and monitor core position during the alignment process. Fibres are located in V-grooves and are adjusted horizontally, vertically and in/out to allow core-to-core alignment. The ability to control core location on fibre results in improved splice performance, as the splicer can compensate against influences such as fibre offset due to contamination or core-cladding concentricity mismatches.

As always, best practice extends further than having the right piece of equipment. Maintain a clean and contamination-free work area by regularly cleaning stripping tools, cleaver blades and pads, V-grooves, camera lenses, mirror and splicer travel case. Observe safety precautions such as disposing of fibre offcuts in sharps containers. Regular operator maintenance of equipment is also vital - this includes arc calibration, electrode stabilisation and battery cycling, as well as monitoring cleaver performance and adjusting cleaver blade position when required. Using manufacturer’s original consumables such as electrodes and cleaver blades will enhance your equipment’s performance. Using endorsed services centres ensures access to manufacturer-trained and certified technicians, and manufacturer original parts and firmware.