Optical fibers are flexible glass cables, primarily made of silica, through which light signals are transmitted. They consist of a core, a cladding, and are protected by a plastic jacket (Figure 1). Light travels through the core, and the cladding prevents light from escaping the core.
There are two main types of optical fibers: single-mode and multimode. Single-mode fibers have a very small core diameter (8 to 10 µm) and can only transmit one mode of light (beam).
Multimode fibers have core diameters greater than 50 µm, allowing the transmission of multiple modes of light.
Single-mode fibers can transmit more information because they have a higher bandwidth than multimode fibers.
Types of Splices
There are two types of splices: mechanical splices and fusion splices. A mechanical splice consists of aligning the fibers on a support that allows them to be fixed mechanically or with glue.
Mechanical splices are primarily used with multimode fibers, although they can also be used with single-mode fibers if these have low eccentricity levels, assuming that insertion losses can be higher and return losses lower than those of fusion splices.
Fusion splices are made by melting the fiber material using a high-voltage electric arc or another heating method. These splices can be used with any type of fiber: single-mode or multimode, and offer lower insertion losses and higher return losses (reduced reflections).
Fusion Splicing:
A fusion splice, broadly speaking, consists of joining two fibers by melting the material at their ends using a heat source, which is usually composed of two electrodes between which an electric arc is produced when a high-voltage source of 4000 to 5000 volts with controlled current is applied. The heat generated by the electric arc will depend on the current supplied by the high-voltage source at any given time.
Fusion splicing is performed using a machine commonly called a splicer, fusion splicer, or splicing machine. The machine's main functions include fiber approximation, alignment, fusion, and estimated loss calculation (Figure 2). Finally, the machines have an integrated heater that allows the splice protector to be applied.
How to Make a Fusion Splice
: To perform a splice, the fibers must be stripped (the primary coating removed), cleaned with lint-free paper or gauze soaked in alcohol, preferably ethanol, although isopropyl alcohol can also be used. Finally, they must be cut using a precision cleaver that ensures the cutting angle with respect to the perpendicular is less than 1°. The fibers are placed in the machine, taking care not to contaminate them, and the splice is performed simply by pressing a button, as the machine carries out the process automatically. Once the fusion is complete, the machine evaluates the splice losses, and the splice protector can be applied.
Fiber alignment is the factor that most influences optical signal loss in fiber optic splices; it can be of three types: longitudinal (separation), lateral, and angular.
Optical signal loss depends on the error values in these alignments and the characteristics of the fibers being spliced, including core diameters and Numerical Apertures (NA). See Figure 3.
One of the most important factors for achieving low losses when splicing optical fibers is the cleavage angle relative to the perpendicular to the axis; this angle must be less than 1° across the entire cleaved surface. Precision cleavers are used to make these cuts (Figure 4).
Composition of a Fusion Splicer
Fusion splicers are composed of different systems, each with distinct functions, which we could differentiate as follows:
- Primary shielding clamping systems. Their purpose is to prevent the fibers from moving into or out of the machine and to prevent them from rotating during splicing.
- Splice point viewing system. These allow visualization of the area where the fusion will occur. They can consist of a microscope (older or low-cost machines), a video camera and a mirror, or two video cameras. The latter are the fastest, as they can view both alignment axes simultaneously.
- Alignment system. Its main function is to align the fiber ends so that the cores of both fibers coincide as precisely as possible. There are two main types: core alignment and cladding alignment. The former locates and aligns the fiber cores, and the latter does the same with the cladding.
Machines that have the ability to align cores use alignment systems such as LID or PAS.
- The LID (Light Injection & Detection) system works by injecting light through the shielding into one of the fibers via a
bend and detecting the light transferred to the other fiber by extracting it from the shielding using the same injection method. This system has drawbacks: the amount of light injected depends on the opacity and thickness of the shielding, so it doesn't work correctly with some fibers. Furthermore, the amount of light injected and extracted
is often very weak, which can slow down the alignment process.
- The PAS (Profile Alignment System) performs alignment using the profile of the fiber cores in images obtained by one or two cameras when the fibers are illuminated by a collimated light source (parallel beams). It is the most widely used system because it is the fastest (splices in 9 seconds) and most versatile.
- Fusion system. This consists of the electrodes and the high-voltage power source, and its function is to achieve proper fusion of the fiber optic material.
- Heating system. Its function is to heat the heat- shrink tubing of the splices.
- Control system. This is responsible for controlling the different systems to automate the machine's operation and perform the splicing in the shortest possible time and with the highest quality.
The precision and quality of these systems significantly influence the results of the splices and the speed of work execution, which will translate into greater profitability. (Figure 5).
Types of Fusion Splicers
There is a wide variety of fusion splicers, which can be divided into two main types: field splicers, the most commonly used because they are used in the installation of fiber optic cables, and factory splicers, which are used to splice optical fibers in the manufacture of electro-optical components (amplifiers, compensators, etc.) or in research laboratories (Figure 6).
Field splicers can join multimode fibers: G651, EIA-492, or ISO/IEC 793, and single-mode fibers: G652, G653, G654, G655, G656, and G657.
Figure 7. Field fusion splicers for ribbon and single fibers.
Factory fusion splicers can also be used to splice the types of fibers used in field splicers and fibers with cladding diameters greater than 150 µm. More complex factory fusion splicers are needed for fibers that maintain polarity (Panda, Bow Tie, etc.), as these require rotating the fibers to maintain polarization.
Optical fiber cables are manufactured with either fibers bundled in ribbons or loose fibers inserted into tubes or sheaths; therefore, field fusion splicers can be divided into ribbon fusion splicers and single-fiber fusion splicers (Figure 7).
To join ribbon-bundled fibers, a machine capable of splicing all the fibers in the ribbon (12, 10, 8, 4, etc.) simultaneously is required.
Splicing cables composed of loose fibers is performed with single-fiber fusion splicers.
There are two main types of single-fiber field fusion splicers, which differ in the type of alignment they use to join the fibers: core-alignment splicers and cladding-alignment splicers.
Core-alignment splicers offer the best quality because light travels through the fiber cores, so aligning them minimizes optical signal loss at the splice. For this reason, they are primarily used for splicing single-mode fibers, although they can also splice multimode fibers. Cladding-alignment splicers are well-suited for joining multimode fibers, but they can also be used for single-mode fiber splices if the centers of the cores align with the centers of the claddings (low eccentricity levels).
Core-alignment splicers consist of two V-shaped slots (where the fibers are placed) equipped with lateral and vertical movement mechanisms. These machines are more expensive than cladding alignment machines because they have more motors and higher-quality cameras and lenses.
Cladding alignment machines have a V-shaped groove on which the fibers to be joined are placed, thus aligning the fiber cladding. Single-mode fibers used in cables for several years have very low eccentricity levels. Therefore, cladding alignment machines can also be used to work with single-mode fiber cables less than 15 years old.
Which fusion splicer should you buy?
To select the type of machine that best suits all your needs, you should analyze the following aspects:
- Alignment type. Given that core alignment machines offer better characteristics than cladding alignment machines, we must consider the following: for single-mode fibers with minimal losses (telecommunications sector), a core alignment machine is the most recommended option, while for splicing single-mode fibers with reasonable losses (FTTx, CATV, LAN, etc.) or multimode fibers (LAN, industrial control, etc.), any type of fusion splicer can be used.
- Costs. Core alignment machines are more expensive (² +35%) than cladding alignment machines.
- Environmental conditions. It is advisable to select from the different options on the market the machine that can withstand the temperature, humidity, and altitude conditions at which it will be used, as well as resistance to vibrations during transport, impacts, dirt, etc. For indoor work, the requirements are very basic, but for outdoor work, greater margins are needed.
- After-sales service. The machines are high-precision instruments that require periodic readjustments and quick repairs, so it is advisable to have a good after-sales technical service at a national level.
Author:
Pedro Notario, Technical Director TELECOM-UNITRONICS
