Undersea Fiber Optic Connectors Pt 1
In this article Matt Christiansen takes us through an overview of Fiber Optic connectors and their use undersea.
Why use Fiber Optics?
The introduction of fiber optics into subsea cables has allowed the realization of much greater rates of data communications with higher signal to noise ratios than was possible with electrical cables. Fiber optics in subsea cable systems have seen greater usage over the past few years and continues to increase. There are many applications of combined electrical and optical cable systems in use and in development today. SEACON has developed and supplied connector systems combining electrical, optical and mechanical terminations (E/O/M) to many applications with unique requirements. Among those are:
a) Communication umbilicals from shore-side facilities to offshore instrumentation, and for
platform to platform communications offshore.
b) SEACON has supplied terminations for diverse applications to commercial and
c) 2) ROV umbilicals of many different styles have been terminated by SEACON and
are in use in the subsea market place.
d) 3) Downhole geophysical instruments are now turning to fiber optics for data handling.
e) SEACON has supplied E/O/M cable terminations and receptacle connectors capable
of operating at 20000 psi and 200°C with fail-safe open-face capability of the receptacle
for guaranteed protection of the expensive instrumentation operating in these hostile
f) Towed array systems deployed from ships and planes are using hybrid fiber optic
connection systems to communicate with multiple sensor arrays. SEACON supplied
connectors used in military systems have been tested to withstand severe environmental
extremes including underwater explosive shock.
What About Connectors?
In conveying power and information from control locations to subsea systems, it is most often convenient to have a means of connecting and disconnecting cables from equipment. This greatly simplifies handling, installation and maintenance tasks as well as test functions and operations.a connection must be quick and reliable, provide a high level of performance and be cost effective. Connectors for optical and electro-optical cables have a few special design considerations to be aware of:
a) Features of the cable construction greatly influence the design simplicity of the cable head termination. Ideally, cable and connector design should be integrated as a system for best value in performance, cost and reliability as a subsea communications conduit.
b) Each application and operating environment will define the requirements of the connector system. However, a good understanding of operating environments and the definition of performance requirements is necessary in order to design an efficient cable and connector system.
Connector Design The Connector Optical Interface
The fiber optic interface is critical, termination of optical fibers into contacts requires great care in the dimensioning and tolerancing of connector hardware. Costs can be reduced by using standard commercial optical components. Standard termination techniques can be adapted to easily and reliably produce quality connection systems.
a) Fibers – Many types of fibers are in use in subsea cable systems. Most can be handled with similar techniques due to their common 125 micron diameter of the clad glass. Commercial fiber optic technologies support very high quality components for 125 micron fiber. Other types of fibers can be handled with specialized components using the same basic principles of operation.
b) Optical Components – The most basic component to the mainstream fiber optic connection today is the 2.5mm diameter ferrule and alignment sleeve. Both are available with high quality and low cost. Improvements in these components and fiber itself have made today’s low loss connections possible. Zirconia ceramic ferrules are most widely used while other materials and variations of technique serve special applications.The ferrule serves to locate the end of the fiber in a precision component of convenient size for polishing and subsequent alignment to a mating fiber/ferrule.
c) Polishing – The fibers are polished flush to the end face of the ferrules, then the ferrules are aligned to their outside diameters in an alignment sleeve. The accuracies of these components are capable of aligning two fibers with core diameters of only nine microns such that the resultant optical loss across the junction is reliably less than -0.5 dB.
d) Alignment – Issues to be dealt with are physical contact of the fiber faces, concentricity and the angular alignment of the mating components. The ferrule and alignment sleeve junction has been well studied and their use understood. Expanded beam lenses have been used in subsea connection systems. The two mating lenses do not have to touch face to face to achieve a good optical couple, this eases the accurate control of axial tolerance stack-ups in the connector manufacturing process. Unfortunately, expanded beam lenses are susceptible to high losses with angular misalignments and have the added complication of accurately positioning the end of the fiber in the focal point of the lenses. Newer fiber alignment technologies are emerging and SEACON is testing and evaluating them for the advantages they offer. These developments will allow designers to achieve greater fiber channel densities through connector systems.
Shells and Shell Design
The optical contacts are fitted to molded or machined connector inserts which in turn are fitted into proven connector shell designs for the keying and locating the mating contact pairs.Working in the subsea environment involves the use of seals to exclude water intrusion into the connector internals. O-rings have always been a very reliable method to seal connector shells and can be used to seal individual optical contacts as well. Optical contacts molded into nonresilient inserts do not allow for the fine alignment necessary to achieve the highest optical performance. Contacts sitting in resilient seals retain greater compliancy and achieve better alignment.
For each application the extremes of pressure, temperature, tensile load and other environmental factors need to be considered. The composition of the cable and materials of its construction dictate the means of attaching and sealing the cable to the connector shell. Operational considerations may call for the possibility of termination in field settings. Instrument specifications may call for increased levels of insulation protection in electro-optic cables. Requirements for integral armor termination or oil-filled and pressure-balanced systems will also affect the design of the cable entry.
Subsea cables generally handle fibers in a number of ways. The fibers may be loosely bundled together in a common stainless steel buffer tube, commonly know as a “K-Tube”, often with a gel suspension to support the fibers. A variation of this is for the fibers to be embedded in a common plastic matrix helixed around a central ‘king-wire’ strength member. These elements are usually(but not always) located in the center of the cable core. Alternatively, the fibers may be individually buffered and armored with a Kevlar strength member and distributed throughout the cable core. Each of these methods lend themselves to different techniques for “breaking out” the fibers from the core of the cable. The handling of fibers in this area of the cable termination is crucial to efficient low-loss optical connections. Additionally, subsea cables often contain copper electrical conductors within the core. and may contain steel or Kevlar armoring around the cable core