Fiber-to-the-home (FTTH) deployments are gaining momentum around the world, and network builders are always looking for ways to reduce the time and expense of fiber optic cable branching to the home, the most expensive part of installation. Typical passive optical network (PON) builders have several options for delivering service to households using drop cables. These options include field patching of pre-connected pigtails and splitters, mechanical splices, field-connecting fiber optic cables at network interface devices (NIDs), and pre-connected fiber optic cables. It is important for network builders to carefully consider both performance and installation cost options.
FTTH networks typically include voice, data and video "triple" services. In many cases, revenue from each of these services is necessary for network deployment to be economically viable. It's worth noting that video systems are very sensitive to back reflections. This sensitivity makes mechanical joints and field connections less attractive. Field connections are a possible option for systems that do not provide video capabilities. For field deployment in exposed environments, most network operators in the United States choose mechanical joints only for remediation purposes.
One of the biggest costs of FTTH deployment is the manpower required to install the branch from the very end of the PON network to the residential branch. Deploying corner-polished pre-connected fiber optic cables is considered an option to reduce labor costs. These cables are usually purchased in a series of predetermined lengths, typically in increments of 50 to 100 feet.
While simply connecting pre-connected fiber optic cables saves mating time, there is no overall cost savings because pre-connect requires more expensive raw materials (including connectors, packaging, and waste from on-site handling of raw materials). In addition, the time savings is offset by the additional labor required to deal with additional fiber optic cables created by fiber length mismatches found in the field.
Pre-connected fiber optic cables are generally ordered in "standard" lengths, rather than being cut to specific lengths that match the distance from the splitter to the household. The resulting extra cable length must be coiled in the field or handled in some other way. The additional encapsulation required to protect the connector in a duct or overhead installation creates a significant additional cost. If the fiber optic cables are pre-connected at the splitter end as well as the household end, the attached splitter cover is much more expensive than standard splitter modules and covers. Taken together, the labor and raw material costs for pre-connecting fiber optic cables are generally much higher than for fusion splicing.
Fiber Optic Patching and Termination
Alternative method: fusion
For performance, reliability, and economic reasons, fusion splicing is usually a better method for making taps and household connections. However, in order to fully realize the benefits of fusion splicing, the fusion splicer must be very reliable so that it can make a quality joint every time. Since the extra work assignments to fix a badly installed branch can cost hundreds of dollars in follow-up labor, joints must be done right the first time.
A fusion splicer capable of active core alignment is obviously the preferred method. This fusion splicer is so simple and fault-tolerant that a relatively unskilled operator can successfully make every splice. The most typical active collimation fusion splicers on the market today reliably make extremely low-loss splices every time, with appropriate error messages to prevent poor quality splices.
Investments in fusion splicers should also be carefully considered. Of course, low cost is attractive, and it is tempting to use a fusion splicer with fewer features. Low-cost, fixed V-groove fusion splicers can also be very effective in the hands of skilled skilled workers. However, network builders should keep in mind that the cost of the fusion splicer itself is very small in terms of the overall cost based on the user base relative to the labor required to make the connection.
Because the cost of a fusion splicer will be amortized over years of operation and potentially thousands of splices, the associated labor costs amortized by the extra work caused by improper splices far outweigh the increase in the cost of the fusion splicer. Human costs must be borne once incurred.
Which feature of a fusion splicer is most important in order to ensure a quality splice every time?
Fusion splicers that actively align fiber cores are the most reliable. Splices that use less expensive, fixed V-groove-aligned fibers (rather than active alignment) introduce a greater risk of poor quality splices and additional work assignments, especially if testing is not performed to verify splice losses.
Another feature that should be considered in a fusion splicer is its ability to view both axes of the fiber. Observation in both axes can help ensure proper split angle and more accurate loss estimation, and has the ability to alert the user when the fiber core is shifted too much to eliminate the possibility of poor quality splices. Automatic environmental compensation is necessary to ensure proper arc parameters to minimize splice losses.
As mentioned earlier, the video service portion of a typical FTTH network is very sensitive to back reflections. Although most network designers take this factor into consideration when selecting connector types, back reflections from taps in PON networks still need to be addressed.
The business model of FTTH is often that customers are connected to the network when they subscribe to the service, rather than providing connectivity to everyone during the initial network deployment. This means that the taps in the network are often unterminated. These unterminated taps may generate high levels of back reflections and should be handled to ensure that there are no network problems.
The most common way to deal with this back reflection is to crush the fiber end face. However, crushing the fiber end face results in a very large change in the performance of backreflection control. Testing by AFL Telecommunications showed that less than 20 percent of the fiber end faces met the -65 dB backreflection specification, regardless of whether they were crushed with pliers or popular brands of fiber strippers. Since this problem is relatively new in reality, there is no industry-accepted specification for unterminated taps. Such performance unacceptable in large-scale FTTH deployments is a reasonable assumption.
Fusion splicers designed for factory use can be used to make low-cost splitter terminations. Such terminations are very low cost and can be easily incorporated into splitters so that all terminations are preterminated. Typical performance is much better than -65 dB with no significant impact on performance over the temperature range of -40°C to +80°C. Pre-encapsulating the terminations on the tap branch ensures that all terminations are protected and there will be no back reflection problems. When technicians are ready to patch fiber optic cables for new customers, they simply remove the terminations and prepare to patch fibers through normal procedures.
FTTH systems present an exciting opportunity in today's telecommunications sector. FTTH network designers have a large number of options available, especially when it comes to branch-to-home. The network designer examines the installation cost components in terms of both labor and raw materials, and it is wise to choose a deployment method that minimizes the overall installed cost while ensuring long-term system performance. Careful planning of deployment methods can help reduce installation costs and ensure long-term reliable network operation.
