An update on fusion splicers and optical fiber splicing
Single-fiber, mass and mini fusion splicers all have a place in building and maintaining the fiber-optic network.
Keith Houda Sumitomo Electric Lightwave Corp.
As fiber is deployed deeper into the network, installers are confronted with a number of choices to optimize splice performance using a variety of fusion splicers. These machines can splice a variety of fiber types (e.g., singlemode and multimode fiber) in a number of arrangements, including single-fiber, factory-made ribbon, or field-constructed ribbon using loose fibers.
As with connectors, splices of increasingly higher fiber counts are being made for telephone company, cable-TV, and local area network applications. Economics dictate that these splices be performed quickly, accurately, and with increasingly higher performance (i.gif., low loss).
In addition to the skill of the operator, factors that affect the quality and performance of a splice are the fiber itself and the splicer or, more accurately, the splicing technology used.
Fiber geometry is the deciding factor in the quality of the fiber, and is outside the control of the splicing crew. The chief concern in fiber geometry relates to fiber core concentricity or core offset, and it is a design concern of the fiber manufacturer. "Core offset" is the difference between the core`s actual position relative to the true center of the fiber`s outer diameter. Other concerns that may result from the manufacturing process include variations in fiber diameter, coatings, and fiber curl.
Fortunately, improvements in fiber manufacturing are reducing these concerns and the problems related to them as they affect splice quality. Nevertheless, when such problems do occur, skilled splicing engineers can overcome them by using splicing equipment that can identify and compensate for manufacturing inconsistencies.
Selecting the splicer for the job
There are two basic types of fusion splicing technology in use today: core alignment systems and fixed V-groove alignment systems. "High-end" single-fiber splicers use the core alignment technology, while mass fusion splicers and "mini" (portable) splicers use fixed V-groove technology. Over the past decade, advances in technology have resulted in increasingly higher splicer performance. While each type of splicer can deliver a quality splice, splice loss and return loss dictate which type should be used for specific applications (see Fig. 1). Splicing engineers should know where these splicers can be applied in the network installation and what to expect from each type of splicer.
Engineers should also be aware of the operation and maintenance requirements of splicers; after all, they are precision instruments frequently subjected to rough field environments. Indeed, field practice, notably the degree of cleanliness surrounding the splicing operation, ranks most important--and ahead of fiber geometry concerns--among factors crucial to low splice loss. Tips for successful field practice will be presented later in this article.
High-end single-fiber fusion splicers
High-end single-fiber fusion splicers have long set the standard for high-quality splicing work, and in field conditions are generally used to join long-haul fiber segments. The operator is able to directly view and control the alignment of the fiber core of singlemode fiber on both the X and Y axis at magnifications up to 280¥ (see Fig. 2). Multimode fiber is aligned by its outer diameter, because its large core size makes true core alignment unnecessary.
These high-end splicers can be used with many different fiber types, including singlemode, multimode, dispersion-shifted, dispersion-compensated, cut-off shifted, and erbium-doped.
Users should expect from single-fiber fusion splicers typical splice losses of 0.02 dB for singlemode fiber, 0.01 dB for multimode fiber, and 0.05 dB for dispersion-shifted fiber in laboratory conditions, and 0.01 to 0.02 dB more in ordinary field conditions. These splicers can complete a splice cycle in as fast as 25 sec; they feature a built-in heat cycle that shrinks sleeves in 90 sec.
Today`s high-end single-fiber fusion splicers have several features that improve performance and productivity. For example, they can very accurately screen fiber condition, detecting dirty fibers, bad cleaves, and fiber axis angle, while providing a "go/no-go" warning prior to the actual fusion operation. This ability to screen defects significantly reduces the call-back re-splice ratio and ensures the overall end-to-end performance of the network. These splicers are also able to adjust the fusion arc to compensate for changes in atmospheric conditions, (i.gif., altitude, temperature, and humidity) as well as electrode conditions and different fiber compositions.
With the rising costs of network installation and higher demands on "turn-up" deadlines, manufacturers are implementing design features that help eliminate the problems most frequently experienced in the field. These implementations include on-board self-diagnostics, low-maintenance optics systems, and remote interactive maintenance capabilities. In up to 70% of instances where field-related problems occur, diagnostics and corrective measures can be communicated over a telephone line between the field site and the manufacturer`s service department. The ability to investigate and address problems in the field contributes substantially to reducing installation costs and meeting deadlines. Software upgrades can also be accomplished remotely.
An additional feature on the most modern of high-end single-fiber fusion splicers is their ability to store large amounts information on pcmcia memory cards. The stored information can document data pertaining to the splice point and an image of the splice itself.
Mass fusion splicers
A study undertaken by Pacific Bell proved that mass fusion splicers are fully capable of delivering high-performance splices in consumer broadband networks1. As reported, the mass fusion technique provides average splice losses well within required loss budget limits, making it a viable joining method for the specific architecture chosen by Pacific Bell. The study also showed that no statistically significant difference in splice loss could be observed with either "field-ribbonized" loose tube fiber or factory-made ribbon-configured fiber.
The Pacific Bell study was undertaken to investigate the feasibility of using mass fusion splicing as a viable alternative to single-fiber splicing in its Consumer Broadband Advanced Telecommunications Service architecture. While mass fusion splicing met the criteria, it should be understood that in general, this technology is a trade-off between high accuracy and high productivity. The success of mass fusion splicing in an application depends substantially on the use of high-quality fiber and the use of proper housekeeping at the job site. Moreover, the skill of the splicer operator contributes greatly to a successful and productive installation when mass fusion technology is employed.
Fibers joined in mass fusion splicers can move in the "Z" direction (horizontal plane) only--that is, toward or away from each other (see Fig. 3). Unlike the movable V-grooves of single-fiber fusion splicers, the grooves in mass fusion splicers are fixed. This means that cladding-diameter tolerances, fiber core concentricity, and dust on fiber and/or in V-grooves greatly affect fiber alignment.
A regular maintenance program for fiber cleavers is important to avoid bad cleave angles and variance of fiber cleave lengths (see Fig. 4). Such maintenance includes keeping the highest standards of cleanliness in the V-grooves and on the fiber ribbon. These factors are important because a poor splice on a single fiber in a 12-fiber ribbon splice can adversely affect the entire splice.
Mass fusion splicers can accommodate a variety of fiber holders. These holders are designed to accept from single fibers to as many as 12-fiber ribbons from all fiber manufacturers.
Users should expect splice losses (in identical fibers) of 0.06 dB for singlemode fiber, 0.03 dB for multimode fiber, and 0.07 dB for dispersion-shifted products in laboratory conditions, and 0.01 to 0.02 dB higher in actual field conditions. They should expect a splice cycle time of 95 sec for 12 fiber pairs and completion of a shrink-sleeve operation in less than 150 sec.
As with single-fiber fusion splicers, advances have been made in mass fusion splicers to improve the quality of the end-product. Large, high-resolution, movable liquid crystal display monitors with 25¥ magnification allow operators to view up to 12 fiber pairs and evaluate fiber preparation and splicing results (see Fig. 5). More importantly, given the number of splices per cycle, inspection systems are available that sequentially check the condition of each fiber being spliced and sound an alarm if results indicate that an excessive splice loss condition exists (see Fig. 6). This helps the operator to address the problems immediately, thereby contributing to increased productivity.
Mini single-fiber fusion splicers
Mini single-fiber fusion splicers offer the operator true portability. These units, which weigh less than 7 lb, are designed primarily for short-distance applications where there is a looser tolerance on splice loss. These machines are very popular for taut sheath and aerial splicing operations because of their small footprint, portability, and battery operation. However, they are also ideal for standard cable installation and restoration work. These mini splicers are far superior to mechanical splicing, typically exhibiting a splice loss of 0.06 to 0.07 dB for singlemode fiber and 0.03 to 0.04 dB for multimode fiber in actual field conditions, due to recent fiber manufacturing improvements.
Today`s mini fusion splicers allow the operator to select from up to six fusion splicing settings for different fibers, including dispersion-shifted fibers. Mirror-free designs decrease maintenance, while dual light-emitting diode (led) observation systems allow fast X and Y axis imaging with a 50x magnification. An arc test function that automatically determines the optimum arc settings based on ambient environmental conditions is also incorporated. Like the full-featured machines, the mini splicers will inspect and measure the quality of the splice and display the image and results (see Fig. 7), as well as store up to 100 splice data points.
Portability is enhanced by built-in batteries with the capacity to perform more than 30 splices, including operation of the splice protection sleeve heater. Options include backup batteries as well as an ac/dc power converter unit to enable 110-V AC operation.
Splicing components
Fusion splicers--single, mass or mini--are capable of delivering high-quality fiber splices. Nonetheless, a quality splice begins with good housekeeping practices, proper fiber preparation, and efficient use of precision tools. Precision jacket removers are used to strip single fiber coatings in one pass without damaging the fiber. These are available to accommodate fiber of varying diameters.
Fiber ribbon stripping is handled by thermal strippers that grip, heat-soften, and remove the jacket in one pass. Thermal strippers could also be a good solution for fiber coatings that are hard to remove with mechanical stripping tools. Fiber arrangement tools let the operator prepare loose fibers for mass splicing in a quick, simple step that can accommodate up to 12 fibers. Strippers and fiber arrangement tools accommodate interchangeable fiber holders that grip 1- to 12-fiber ribbons securely during the stripping and cleaving operations.
Cleavers are designed specifically for single-fiber or ribbon applications and should produce an endface angle within 0.5 of perpendicular to assure a high-quality splice. Cleavers for ribbon applications also minimize variances in fiber cut length.
Equipment care and maintenance
As suggested above, all fusion splicers are precision instruments requiring care and maintenance to deliver a quality splice, reduce downtime, and eliminate the need to "reopen the hole." When outdoor work is performed in unprotected areas and windblown dust reaches the splicer, cleaning maintenance is critical, starting with the cleaver.
If problems such as fiber set error and high splice loss are experienced, begin by cleaning the equipment (e.g., V-grooves, fiber clamps, mirror, and lens) using recommended procedures and equipment.
Isopropyl alcohol is an important tool for splicing operators. Applied with a soft brush or cotton swab, it helps loosen small particles of dirt that can make the difference between a clean, vertical cut and something less (see Fig. 8). If the cleaver is clean, but still produces bad cleaves, it`s time to adjust the cleaver blade.
Cleanliness is crucial to the splicer itself. Clamps and V-grooves must be cleaned thoroughly using recommended procedures and equipment. Optical components are key elements used to view the fibers and also must be kept clean because dirt on mirrors, lenses, or leds can result in false information. The inability to clearly inspect the fibers may result in rejection of a splice that otherwise meets pre-set parameters.
Finally, checking the electrodes` condition is also important. If you start having problems with high splice loss, perform an "arc test." The arc temperature may have changed due to dust deposits around the electrode tips. If you don`t find any improvement in splice loss, it is time to change electrodes.
Electrodes require ongoing inspection and maintenance to ensure that they provide even heat at the proper temperature. Dirty electrodes display gray or white discoloration at the tips, or sizzle or hiss when the arc is fired.
The shape and condition of the electrode tip is very important, so cleaning electrodes is not recommended; damage to the electrode tip may occur that causes problems such as curved arc. In an emergency situation, cleaning electrodes with steel wool may temporarily improve their performance and enable the operator to complete the job.
Summary
Continued improvements in fusion splicing equipment allow skilled splicing engineers to deploy this fiber-joining technology deeper into the network with assurance that splice performance will meet preselected criteria. This overview suggests where high-end single, mass, and mini fusion splicers can be used to capitalize on their specific features, describes how these features contribute to low splice loss, and explains why proper field practice is the most important contributor to splice performance. u
Reference
1. Arvind Mallya et al., "Mass fusion splicing attenuation performance and its use in a consumer broadband network." Proc. National Fiber Optic Engineering Conf., Boston (June 1995), p. 131.
Keith Houda is marketing manager of fusion splicing products for Sumitomo Electric Lightwave Corp., Research Triangle Park, NC. He can be contacted at khouda@ sel-rtp.com or (800) 358-7378.