Advanced Fiber Optic Connector Planning

Advanced Fiber Optic Connector Planning
An Exercise in Tradeoffs: Strategies for Successful Designs in Optical Fiber Connector Systems The purpose of this presentation is to discuss some of the issues confronting and options available to designers of subsea fiber optic systems. As with any other engineering effort, planning a system that includes fiber optics is an exercise in tradeoffs. Knowing what these tradeoffs are and how the optical portion of the system interacts with the rest of the system will make your effort a success.

Getting it right the first time Subsea connector systems, whether electrical, optical, or hybrid, are undeniably expensive. “Do-overs” cost money. Which is worse in your situation: Under-designing the system or over designing it? How will adding more capability now, or replacing components later, affect other systems in the vehicle? Unlike their terrestrial counterparts, neither connectors nor cables for subsea use are commodity items. Tearing out an old or under-designed system to upgrade it with a newer, more capable system is not an inexpensive proposition. “Do-overs” cost money. Conversely, tossing in tremendous amounts of spare capacity may not be cheap either. The designer must decide where the balance is between procurement costs today and replacement costs tomorrow. You must decide what your business can withstand. Is the expense of replacement in the future more or less painful than the cost of adding capability for the future now? Adding more capability to the fiber optic portion of the system, either now or as an upgrade in the future, will have an effect on other parts of the system. You will need to understand these interactions as part of your engineering evaluation. A little added cost to a non-fiber optic part of the system now could save a lot later on.

Getting it right the first time Start the discussions with your vendors early An early start allows you to encourage your vendors to suggest improvements with an eye toward future capability The later you start, the more pressure there will be to “Just build it for what we need to do right now” Get your cable and connector vendors involved in the design process as early as possible. Adding fiber optic connectivity to a subsea system is more than just adding a milestone to a Gantt chart! An early start will free all parties to consider enhancements to the system; particularly those that allow future improvements. Schedule compression will force your team to build only for your current application making later improvements more difficult and more expensive.

Grab the low hanging fruit If adding future capability is easy/inexpensive, do it! Remember that your assessment of “easy” and “inexpensive” must include the impact on other systems Often, adding capability to a system, or to a component within that system, is easy and/or inexpensive to achieve. For example, let’s say that you are purchasing an electro-optical tow for an application that requires 12 fibers in a stainless steel tube. Your vendor may have picked the tube not for its fiber capacity, but rather to satisfy the cable’s geometry requirements; there may be room for more fiber. It doesn’t cost anything to ask your vendor how many more fibers can be placed in the tube. If more fibers can be added, and the cost of the additional fiber is negligible as compared to the total cost of the cable, then it makes sense to add the fibers; not only as spares for the current application, but as “dark fibers” that allow the cable to be re-purposed in the future. But bear in mind, adding features to the system with an eye toward future capabilities will affect other parts of the system. Your assessment of “easy” and “inexpensive” must include the impact of your changes to other parts of the overall system; both now and in the future.

Identify your critical needs Long distance/High bandwidth Short distance or low bandwidth Bend sensitivity EMI sensitivity Adaptability Your distance ● bandwidth product will determine the fiber type; singlemode or multimode. Long distance/high bandwidth applications will likely require singlemode fiber. Short haul, or low bandwidth, applications can be accomplished more easily with multimode fiber. The terminations are easier to do, the components are often less expensive, and the fiber is more forgiving of tight bends. (But bear in mind that your potential upgrade paths may be toward singlemode applications. It’s thus a good idea to be biased toward using singlemode fiber even when your current application can get by with multimode.) If you are forced to route the fiber through tight bends, this may bias you toward using multimode fiber. MM fiber is less bend sensitive than SM. Fiber optic transceivers and/or the instruments they service may be EMI sensitive. This will determine whether or not it’s practical to route high voltage lines through the same hybrid connector that houses the fiber. Also consider whether or not your system must be easily reconfigurable; either in the future or the near term. This will determine whether purpose-built cable assemblies are preferred to simpler cables connecting through junction boxes or pressure bottles. In some cases, your system can be made reconfigurable by using a “one size fits most” connector. For example, a hybrid connector could service both an instrument that returns data or supply power to a tool that does not. In the latter case, a common cable and connector can be used if the tool’s connector includes dummy optical ferrules.

The cards in our hand Connector type All optical Singlemode Multimode Mixed All electrical Hybrid Subsea connector systems can be all: Optical All electrical or hybrid electro-optical connectors There are advantages to each and by no means does one size fit all. Furthermore, the optical fibers can be singlemode, multimode, or even a mix of the two. The fiber type will be determined by the length of the fiber optic link, the required bandwidth, and/or the fiber routing requirements. High bandwidth and/or long lengths are better served with singlemode fiber. Tight quarters, and thus tight bends, are best served with multimode fiber.

The cards in our hand Connections Route through or into junction boxes/bottles Furcated cables There are options on how to route the optical and electrical signals about the vehicle. Connectorized cables can be fitted to junction boxes or pressure bottles; or, the cables themselves can be furcated. (These are sometimes called Y- or H-cables.) Be aware that singlemode fiber is less forgiving of tight bends than is multimode. Fiber routing, especially inside confined J-boxes or pressure bottles, must take this added bend sensitivity into account.

The wildcards Fluid-filled versus 1-atmosphere enclosures Mechanical terminations Optical connectors and receptacles that mate to oil-filled junction boxes and enclosures are necessarily different from those that mate to 1-atmosphere vessels. A cable plug and receptacle designed for use in a fluid filled enclosure may be incompatible with those designed for use with a 1-atmosphere pressure vessel. There are several options to prevent connections across types. You can use incompatible keying options from the vendor. This will force your operators to change cables when reconfiguring your vehicle between missions. You can also deliberately specify cable plugs and receptacles for fluid filled enclosures for all of your applications; even those that mate to 1-atmosphere vessels. This will increase the length of the cable plugs’ backshells, but it will make your cable sets compatible across all of your instrument configurations. A mechanical termination can be as simple as a Kellems grip (Chinese finger trap) or it can be something more complex like a socketed design that captures the cable’s armor wires or fibrous strength member. In either case, we are forced to define a distance from the mechanical termination to the connector or to a cable furcation. This set distance from the mechanical termination can create situations where a “do or die” termination must be done to meet the length specification. We can avoid this by adding extra fibers to the system. Should one fiber termination have a problem, we can simply switch to one of the spares rather than finding ourselves remaking the mechanical termination. It is also possible to integrate the mechanical termination into the connector itself, making it load bearing. Again, spare fibers will help us to avoid completely rebuilding the termination over a single bad or high loss fiber.

Hybrid versus dedicated optical and electrical connectors Hybrids tend to be larger and more complex Hybrids reduce the number of hull penetrations Hybrids reduce the chances for mis-wiring Hybrid connector designs require the manufacturer to co-locate wire and fiber in the connector’s backshell space. This complicates the assembly process and care must be taken to avoid damage to the fiber caused by the much stiffer wire. As a result, hybrid connectors are usually larger to accommodate the added materials. But, the result is a single, clean solution with only one hull penetration. And since there is only one connector, it’s impossible to swap multiple, dedicated connectors. For example, let’s say that instead of a single hybrid, two optical and two electrical cables are used in a system. Since this raises the possibility of interchanging cables, incompatible keying would have to be specified for each connection. This will necessarily increase costs.

Hybrid versus dedicated optical and electrical connectors Dedicated optical and electrical connectors are smaller and simpler Cable furcation is necessary, but signals can be routed outside the vehicle Often, there simply isn’t enough room for a single large connector. (Or that single connector is too heavy.) In these cases, multiple dedicated connectors are the better choice. These are smaller, simpler connectors that are easier to produce. co-locating optical and electrical connections can sometimes lead to problems; especially in a compact package. Separating them eases problems such as excessive bending of the fibers. It does become necessary to furcate the cable to separate the optical and electrical signals, but this too can be an advantage. It isn’t necessarily true that you need the optical fibers and the conductors to route to the same location. By using dedicated connectors on a furcated cable, you can do some of your signal routing outside the vehicle where space is at less of a premium.

Hybrid versus dedicated optical and electrical connectors While the fibers are EMI insensitive, the components they connector may not be. Separating the optical and electrical may be desirable. Furthermore, while the fibers themselves are not sensitive to electromagnetic interference, the transceiver modules and other equipment they connect to may be, By breaking out the electrical conductors, you reduce the EMI that could affect those components. (This is especially true where high voltage supplies must be run near COTS components that were intended to use in 120VAC environments.)

Hybrid versus dedicated optical and electrical connectors When a mechanical termination is involved, a single, hybrid is preferable to co-locating a cable furcation With dedicated connectors, the resulting sub-cables are often more pliable than a hybrid cable and thus service loops become more practical When dedicated optical and electrical connectors are used, there must be a furcation of the cable to separate the optical fibers from the electrical conductors. When this split, which will require a water-blocking overmold, is near a mechanical termination, the system designer will find both advantages and disadvantages. The assembly may prove ungainly and make servicing the vehicle, especially on a rolling deck, more difficult. In this case, a single, easy to service (albeit larger) hybrid connector may be more desirable. On the other side of the coin, once the furcation has been done, the resulting sub-cables are smaller and more pliable than their hybrid “mother” cable. These smaller cables are easier to coil into service loops. And having service loops reduces the need for tight tolerances on terminated cable lengths. This, in turn, makes the job of terminating the cables easier. This is particularly important for the optical connector(s) where the terminated fiber leads must be the same length. And though the service loops must be stowed on the vehicle, the added length allows for easier servicing of the connectors.

Preferred A longer, more loosely toleranced pigtail from a mechanical termination is preferable to a short, tightly toleranced pigtail. This allows for a service loop and makes servicing the cable, or updating the connector at a later date, an easier task.

Preferred If a service loop is impractical, then terminating to a larger, hybrid connector is preferable to a furcation point near the mechanical termination.

Simpler cables Easily re-configured Connectors will require incompatible keying Easier to service/repair cables Single, clean assembly Not easily re-configured Connectors may not require incompatible keying Harder to service/repair Dedicated cables and purpose-built cable assemblies each have their advantages and disadvantages. Dedicated cables connected via J-boxes are simpler to produce. They can be more easily reconfigured from one mission to the next. The cables are easier to service or repair. However, multiple cables do present the possibility of mis-wiring during system integration, thus incompatible keying will be required. Furcated cable assemblies are more complex to manufacture, but the finished product is a cleaner, more elegant solution. While it cannot be easily reconfigured from one mission to the next, it’s harder to mis-wire a single cable assembly versus several dedicated cables. The assembly is more difficult to service or repair, but in the example here, a single spare would be required where 3 spares would be needed for the dedicated cables.

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Advanced Fiber Optic Connector Planning

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