Improved design and engineering efforts make the interface a strong asset, not a weak link.
by Tom Huegerich, ADC
As the industry deploys 40-Gbit/sec optical networks and moves ahead to 100-Gbit/sec data rates, service providers and data center managers may question whether the multi-fiber push-on (MPO) connector can perform reliably at those speeds. They saw the first-generation MPO as a “weak link” in optical networking, primarily because of its higher optical loss, lack of durability mechanically in more-demanding applications and limited environmental performance.
However, vendors have tackled those issues by redesigning and re-engineering their MPO products. ADC has gone one step further by combining its radically improved MPO connector with a cable assembly containing a small (3-mm) cable with reduced bend radius fiber (RBRF). The result is a next-generation MPO which, in both laboratory tests and in the field, proves it can satisfy--and in a number of cases exceed--the Telcordia GR-326 requirements for single-fiber connectors.
Early problems on three fronts
Service providers and data center managers generally perceived the first-generation MPO as having both high insertion loss and a poor-value return loss, relative to single-fiber connectors. As networks accelerate to 40-Gbit/sec data rates and higher, carriers and data center operators clearly cannot afford that kind of negative impact on their link-loss budgets.
ADC’s plug-and-play MPO cable assemblies are round 12-fiber optical cables preterminated with a high-density MPO connector on both ends.
On the mechanical front, early versions of the MPO also fell short of performance requirements, sometimes lacking strength and durability. When a technician pulled on a 12-fiber MPO connector or wiggled it, it often unmated on one side or the other, with obviously negative implications on network reliability.
Environmentally, the performance of the first-generation MPO at temperature extremes was poor, thus limiting the areas in which service providers could deploy it. The MPO often failed at low temperatures and, if common outside-plant factors such as dirt and dust blocked light from going through, the connector sometimes would actually decouple.
Enter the revamped MPO
Recognizing those shortcomings, connector vendors basically went back to the drawing board. ADC, for example, redesigned the back end of the connector and then combined the connector with a new high-density, small-form-factor cable containing bend-insensitive fiber. Targeting better optical performance, greater durability in the field and improved assembly quality, the design changes include:
- replacing the flat cable assembly (for example, 0.178 inches x 0.08 inches) and its SMF-28 glass with round 3-mm cable and advanced bend-insensitive fiber;
- replacing a round fan-out with a flat fan-out; and
- replacing the 8-fiber ferrule with a 12-fiber ferrule, with fibers 1, 2, 11 and 12 not polished.
By using a 12-fiber, rather than an 8-fiber, ferrule, vendors can incorporate standard components in their MPO connectors and thus mitigate supply-chain issues. Routing problems prompted the shift to small form factor cable as technicians can route round, 3-mm cable easily with much less congestion. For a more durable solution than standard single fiber (G-652) provides, the new MPOs come standard with RBRF (reduced bend radius fiber–G-657). Finally, the new flat fan-out assembly not only makes it possible for technicians to apply an even layer of epoxy inside, free of air bubbles, but also allows the fiber matrix to separate gradually into individual fibers.
Tests verify performance
While the first-generation multi-fiber MPO often failed to meet GR-326 specifications for single-fiber connectors, the redesigned, next-generation MPO has demonstrated that it can meet or even exceed those requirements. During both mechanical stress testing and environmental aging test cycles, the new MPO cable assembly showed significant improvements in terms of insertion- and return-loss performance. In tests conducted over four operating wavelengths (1310 nm to 1625 nm), the new MPO connector had a maximum insertion loss of 0.45 decibels (dB) over an operating temperature range of -40C to +70C; typical insertion loss was less than 0.30dB with return-loss performance over the same operating temperature range typically better than -70dB.
Better performance in faster data centers
Technologies such as 40- and 100-Gigabit Ethernet, Fibre Channel over Ethernet, IP convergence and server virtualization are moving rapidly into data centers, and savvy managers are moving even faster to ensure their infrastructures can support them. For example, 40- and 100-GbE over multimode fiber will require very precise performance, six to 12 times more fiber and next-generation MPO connectors.
Predetermined lengths of multi-fiber cables with preterminated MPO connectors now are readily available, and vendors offer several next-generation MPO solutions, including trunk cables, array cables and plug-and-play cassettes. With fiber panels and high-density optical distribution frames (ODFs) easily accommodating advanced MPO connectors at interconnects and crossconnects in both the main and equipment distribution areas, managers can choose from a variety of cabling configurations. Because these redesigned, re-engineered MPO connectors are factory-terminated and tested in a clean environment, they ensure superior performance for 40- and 100-GbE. Further, they reduce total cost of ownership because, compared with field termination or splicing, they significantly reduce labor costs and are quick and easy to install.
Better performance opens MDU market
The vastly improved optical, mechanical and environmental performance, plus integration with a smaller, round cable and bend-insensitive fiber, also creates a whole new use for the MPO connector--the high volume multi-dwelling unit (MDU) market. Service providers are looking for solutions that not only enable them to build fiber-to-the-premises networks in high-rise buildings quickly and cost-effectively, but also reliably support the higher data rates and multiple termination points that characterize MDU applications.
One network, two strategies
Compare splicing and connectorization approaches in a large high-rise MDU with 23 floors and 15 units per floor. In a stub-pull configuration, a 432-fiber indoor fiber distribution hub (FDH) with three 144-fiber stubs is located on the lower level. On each floor above, a fiber distribution terminal (FDT) routes 12 or 24 fibers down to the indoor FDH where a technician splices them in. In this example, the technician splices 432 fibers between the FDH and the FDTs. From the FDT on each floor, an individual fiber drop is routed to an optical network terminal (ONT) at each residential unit that delivers service. Here, 345 individual drop cables would run from the FDTs to the ONTs, creating another potential splice point because it is not possible to predict the exact length of each drop.
With a 432-fiber indoor FDH on the lower level, installers would pull several 72-fiber (or larger) distribution cables between the FDH and the FDTs on higher floors. On each floor, a technician routes one of the cables through the FDT, opens it, opens two of the 12-fiber ribbons and routes individual fibers to the splice tray in the FDT. In this example, the technician would splice 345 fibers between the FDH and the FDTs. Again, 345 individual drop cables would run from the FDTs to the ONTs, creating a potential splice point.
In the second approach, the same MDU fiber infrastructure can be more easily and quickly completed by using next-generation MPO connectors between the indoor FDH and the FDTs and centers on an indoor FDH with built-in 12-fiber MPO connectors and an MPO connector mounted on the stub of each FDT. Technicians plug each connection from every floor into the FDH. Installing fiber is now a simple matter of mounting the enclosures and making plug-and-play connections with the cables. With no need to set up, strip and clean fibers, align a splice, fuse the fibers or apply a splice protector or sleeve, installers only have to clean and plug in the connectors.
A connectorized system includes a built-in fiber spool on the FDT to accommodate the varying distances between each FDT and the FDH. Holding up to 500 feet of fiber cable, the spool allows installers to pay out the required length of cable to the FDH and simply plug it in. They do not need to cut cables to length, have slack storage or deal with cable storage during the installation. The spool stores its own slack, up to 200 feet, and the only splice required is to connect the feeder cable to the FDH.
Capturing a share of the large MDU market is crucial to service providers’ efforts to satisfy the growing demand for fiber-based bandwidth and thereby improve their competitive positions and strengthen their margins. Yet the MDU market segment, with its variety of architectures and its installation time and cost pressures, presents service providers with challenges.
By adopting a connectorization strategy based on next-generation MPOs in their MDU network buildouts, they can reduce installation-planning time; minimize the number of splices and technicians required; reduce overall installation costs to achieve a higher, faster return on investment; and accelerate the installation process and begin delivering revenue-generating services more quickly.
MPO connectors now deliver the optical, mechanical and environmental performance that data center managers and service providers need.
Tom Huegerich is vice president, global fiber engineering with ADC (www.adc.com).
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