Optical-fiber connectors are not known for ruggedness. Successful implementation in harsh environments requires attention to design, packaging, installation, testing, and a detailed maintenance plan.
BY Dennis Horwitz
Over the last several years, many military, shipboard, and aerospace networks have adopted fiberoptics in the designing or upgrading of systems and platforms. The evolutionary shift from copper and coax to glass takes advantage of the most salient features of fiberoptics—higher bandwidth, lighter weight, EMI/RFI resistance, and immunity to electronic countermeasures. Feeding this fiber trend is the incorporation of commercial off-the-shelf (COTS) fiberoptic technology, the cost of which has reached parity with copper as the industry has accepted Gigabit Ethernet (GbE), Fibre Channel (FC), and wavelength-division multiplexing (WDM).
 FIGURE 1. Below deck, the USS Ronald Reagan runs on 200 miles of fiberoptic cabling driving a glass Fast Ethernet network. Next year, it will be upgraded to GbE. |
Although the military has embraced COTS technology, deploying optical Ethernet networks on its latest ships (see Fig. 1) and Fibre Channel networks under the hood of its latest fighter jets (see Fig. 2), the harsh reality is that the optical components of the commercial telecom and datacom world are generally qualified to more benign environments. Despite its name, the outside plant (OSP) environment of the commercial telecom world is a relatively sheltered, "fair weather" environment versus the rugged, outwardly exposed harsh environment of portable ground stations, tanks, ships, aircraft, and space.
Several things make these harsh applications different, including: direct exposure to the weather—sun, rain, sleet, and snow—versus the sheltered world of a telecom cabinet or controlled-environment vault; constant levels of sustained vibration and turbulence that avionics and ground vehicles must endure; short but severe levels of mechanical shock and vibration exemplified by rocket launch, take-off/landings of aircraft; and gunfire. Equipment on aircraft and spacecraft must endure constant cycling between abrupt temperature extremes unlike the "gentle" 24-hour day/night/day cycle of land-based equipment. Shipboard and marine environments present high humidity and damaging sea water/salt spray. Exposure to damaging, corrosive fluids such as jet fuel, hydraulic fluid, solvents, and detergents are found in and around aircraft.
Successful implementation of fiberoptic links in these harsh environments requires thorough design analysis, proper selection of components, proven ruggedized packaging techniques, installation by trained personnel, certified post-installation acceptance testing, and a deliberately thought-out repair/replacement/maintenance philosophy. More than just selecting the components, a robust design also depends on a well-conceived and detailed plan for fielding and supporting the product through its lifecycle—from cradle to grave.
Priority one
The first priority for successful fiberoptic links is, quite literally, making the right connections. In the design of any system, we essentially have two main design/packaging issues: the enclosures (modules, boxes, or cabinets) that house the electronic hardware and the cable harnesses that interconnect the various system boxes.
The first consideration is what goes inside the box. In these days of COTS-based designs, the typical commercial GbE and FC optical transceivers on the market range from the limited "datacom"-rated devices (0°C to 60°C) to the wider "telecom"-rated components (-40°C to +85°C). Furthermore, these COTS devices feature optical interfaces based on commercial-grade datacom/telecom connectors such as the FC, LC, MT-RJ, ST, and SC.
Designers strive to isolate internal components, including the optical transceivers, from the harsh environmental conditions that exceed the comparably benign commercial telecom/datacom industry standards. The design tool kit includes techniques for providing some form of internal shock and vibration isolation. Proven methods of internal heating, cooling, or ventilation can maintain a controlled temperature environment within the box. Environmentally sealed enclosures and panel-mount connectors can keep humidity and contaminants out.
However, the weakest link in the reliability chain is quite frankly the commercial telecom-grade optical connections to the COTS optical transceivers. In the worst case, direct connections to the COTS transceivers are secured with RTV, epoxy, or other adhesive—or simply left unmodified. The harsh environment interconnect industry is trying to develop a slightly more ruggedized version of the popular LC connector to mitigate these issues.
So while the internal environment of the box can be designed to mitigate the environment, the connections to the outside world offer no design compromises. From the external box connections, through bulkheads, and eventual connection to the opposite end of the fiberoptic link, both the connectors and cable making up the harness must be selected to meet the harsh environment conditions of the outside world. Usually, a short jumper terminated with a COTS LC connector on one end and a MIL-STD or MIL-COTS (a COTS product that meets MIL-STD harsh environment conditions) optical termini on the other comprises the gap between the controlled internal environment to the uncontrolled outside environment.
An optical interconnect at the box or electronics rack level can have either a fixed, manually coupled connection or a blind-mate/rack-and-panel connection. For fixed connections, circular styles are the most popular, including MIL-C-28876 (for shipboard applications), MIL-C-38999 (popular in aerospace), or any of the several MIL-COTS fiber connectors (Deutsch RSC/MC3/MC5, ITT Cannon PHD, or Radiall LuxCis series). Largely available as MIL-COTS products, rectangular connectors offer higher contact densities and modularity than circular designs. In rack-and-panel or blind-mate interconnects, options include the ARINC 600 series as well as MIL-COTS variants of the same family (available from Deutsch, ITT Cannon, Radiall, Sabritec, and Tyco).
In GbE, FC, and WDM networks, the MIL-COTS alternative products offer better optical performance and significant fiber-friendly features (see sidebar, p. 15). By putting telecom-proven, low-loss, physical contact technology into a ruggedized 38999-style circular package, the MIL-COTS connectors (such as the Deutsch RSC, MC3 and MC5 series) achieve consistent, attenuation performance of 0.2 dB (0.5 dB maximum, singlemode or multimode) and exceptionally high return loss of over 45 dB (singlemode) over the full range of MIL-STD environmental conditions. In addressing the maintenance concerns raised by optical skeptics, the optical contacts are easily cleanable from the front via a removable socket insert (see Fig. 3). Furthermore, support and repair are simplified by a single, hermaphroditic contact design, unlike the separate pin-and-socket termini (two parts to stock instead of one) of the MIL-STD connectors. Finally, the MIL-COTS products offer greater reliability and lower overall lifecycle cost than their MIL-STD cousins.
Priority two
Concurrent with the system design effort should be the development of a detailed and thorough methodology for installing and supporting the physical layer of the system. Typically, the physical layer is the weakest link of the system (of any kind—whether copper, coax, or optical), usually the subject of last-minute decisions because connectors and cabling are considered mundane and trivial.
In formulating the lifecycle support methodology for the physical layer of the system, installation is a critical issue in most harsh environments. When designing a new platform from scratch, provisions can be built-in for proper fiber installation, routing, and access. Newer aircraft like F/A-22 and F-35 and new generations of ships such as DD(X) and LCS can be designed for ease of fiber installation and support from the very beginning. However, installing fiber as part of a system upgrade into existing platforms for which fiber was not initially considered represents a real challenge. Following best practices and methodologies are absolutely critical.
 FIGURE 4. Testing and certifying an optical link in an aircraft requires several standard tools and methods. |
Once physically installed, the next critical objective is to certify the installation (see Fig. 4). Overly tight bends, overzealous clamping, damaged terminations, broken fibers, or even a bad harness are problems in installation. The first step is a qualitative check with a visual fault finder to verify optical continuity. The second step is to measure end-to-end link loss with an optical test set to verify that the installed link falls within the optical loss budget and (where applicable) optical return loss requirements of the system. A final step is to use a high-resolution, optical time-domain reflectometer (OTDR) to obtain a Link Trace and Feature Chart for the link, which individually identifies all connections (both connectors and splices) and cable segments for ease of reference in future troubleshooting.
At some time, the physical layer may get damaged and suitable restoration methods should be in place. When the fault is isolated, do you repair or replace? Repairing and re-terminating can be the most effective recourse given sufficient access, service loop, and adequate workspace (aircraft and shipboard space is always at a premium). One replacement option includes incorporating spare fibers within the cable, which can be swapped into the defective signal path (or channel). Another option is repair of a damaged line via splicing. These restoration options have pros and cons that must be evaluated with their environment in mind.
All of this planning and process development can be for naught if personnel are not provided proper certifiable training. As much as industry treats fiberoptics just like its copper counterpart, the unique nature of fiberoptic connectors, termini, backshells, installation, cleaning, inspection, and certification requires personnel who been formally trained on proper handling and maintenance practices. "Basic Fiberoptics" training (Level 1) should be defined and extended to all personnel working near or around a fiberoptic-based system. "Fiberoptic Technician" training (Level 2) should be defined and extended to all personnel directly involved in installation, certification, maintenance, and support. A qualified trainer should provide formal fiberoptic training in a formal training environment.
Fiberoptics, harsh environments, COTS, and standards are no strangers to each other. Designers have access to a multitude of COTS and MIL-COTS optical solutions, prior project successes, lessons learned, and industry standards to aid the development and deployment of high-performance system upgrades and platform designs. When approached with the right component selection and proper support planning, the reality is neither as harsh nor as difficult as first imagined.
DENNIS HORWITZ is business director, Tempo Connection Systems, 80 Wood Road, Suite 202, Camarillo, CA 93010. Tel: (805) 384-1835; email: dhorwitz@ tempo.textron.com.
Why the shift to MIL-COTS?
Driven by optical systems requiring better loss performance, the venerable MIL-C-38999 series III electrical/hybrid connector is losing ground to MIL-COTS fiber-specific products that have appeared over the last several years. In practice, MIL-C-38999 has the lesser optical performance. To best understand this shift to MIL-COTS, let's evaluate the strengths (+) and weaknesses (-) of using MIL-C-38999 Series III hybrid connectors in optical configurations:
(+) MIL-C-38999 Series III was originally designed as an electrical connector. With use of MIL-T-29504/4 (socket) and /5 (pin) optical termini, D38999 is the major hybrid circular interconnect.
(+) D38999 shells are offered in a wide range of shell size and insert configurations.
(+) D38999 is available by a number of QPL vendors, although optical termini are offered by far fewer manufacturers. Multiple vendors ensure a reasonable cost.
(-) D38999 connector, cavity, and fairly loose dimensional tolerances do not support consistent, low-loss, or low-reflectance (high return loss) performance. While the MIL-T-29504 data sheet specifies 0.75-dB maximum insertion loss, users report losses as high as 4 to 5 dB in practice.
(-) The M29504/4 socket interface does not allow for effective cleaning without complete disassembly of the harness. That contributes to high lifecycle costs versus initial low component cost.
