Evolving fiberoptic standardization efforts for harsh-environment avionics now address parts, methods, practices, training, and testing.
By Douglas Parker and Phil Nguyen
With stepped-up usage of fiberoptics in a variety of harsh-environment applications, it is becoming increasingly important to standardize on components, inspection, testing, and system issues. Current efforts to establish standards for harsh-environment applications include the U.S. Navy’s next-generation connector, or “NGCon,” development; SAE’s AS3C3 group on jumpers, end-faces, link loss, and inspection; and the Airlines Electronic Engineering Committee (ARINC)’s series of fiberoptic standards for airline usage.
Naval standards
The NGCon development sponsored by the U.S. Navy is an effort to improve upon current Navy standardized interconnects that are now outmoded. Official use of fiberoptic interconnections components in the Navy is limited to certain fibers, connectors, and termini (fiberoptic “contacts”) that are specified in approved military slash sheets. Some of these go back to the early 1980s when they were on the cutting edge of technology, but in a dynamic field such as fiberoptics, they are now outdated. The Navy recognizes this and is seeking better fiberoptic solutions for harsh environments.
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There are commercial off-the-shelf (COTS) designs that have been available for several years with many improved features and a proven track record, but they have not been subjected to a formal qualification test program and they are usually available from single or limited sources (see Fig. 1). The Navy has thrown the challenge to the industry to work together in developing new standards for connectors, termini, and accessory backshells (the transition from the cable to the connector). The effort has been dynamic, well-supported by manufacturers under Navy coordination, and moving surprisingly quickly to establish complete, approved standards.
The primary reason for initiating the NGCon standardization is the need for implementing better interconnection design into connectors for military harsh-environment usage. This goes hand-in-hand with a desire to make improved connector systems available at a reasonable price.
 FIGURE 1. Mil-COTS fiberoptic connectors are not subjected to formal qualification tests and they are often only available from limited sources. |
Goals for fiberoptic NGCon connector standards include higher density, smaller size, and lighter weight. Ideally, standards should specify 36 positions using 1.25-mm rather than 1.59-, 2.00-, or 2.5-mm ferrules. “Genderless” termini are preferred rather than pin-and-socket type. Connectors should have a standard, removable, front-socket insert for cleaning and inspection, and guide pins to align inserts prior to terminus engagement. Desired threaded connector shells are environmentally sealed when mated. The approved standard should also allow royalty-free licenses for multiple government procurement sources.
One reason for launching such an effort is that there have been several instances over the years when Navy programs chose to use other items, rather than the approved Navy-standard items, as better technology evolved. Undoubtedly, more advanced designs were available, particularly those that were designed for fiberoptic use, and not just an electrical connector adapted to fiber use. Rather than continue to see random use of other connector systems, the Navy has taken the lead in coordinating NGCon development to define a completely new connector family to be approved primarily for Navy use, but also for other military use and available on a competitive basis from several sources of supply.
Drafting the standard
After several coordination meetings that began in November 2002, the connector plug, receptacle, terminus, and backshell designs were established. Details of those designs are being engineered by suppliers. Standards will include required test levels so that suppliers can test the built products for approval to the military specifications (see Fig. 2).
The basic specification will be performance-driven with detailed testing and performance levels. The draft is called MIL-PRF-NGCon. Shell sizes 9 through 25 are included in slash sheets. Threaded coupling will be used. Optional keying configurations will prevent mistakenly mating incorrect connectors. Both connector halves will use identical “genderless” termini. The plug connector will have preferred pin protrusion configuration, while the mating receptacle will be the socket connector half with a removable front insert portion. This allows easy access to the termini for cleaning and inspection. Guide pins assure precise alignment of mating inserts.
 FIGURE 2. Thorough test procedures describe the typical type of instruments and cables used to test fiberoptic link loss. |
The MIL-PRF-NGCon standard specifies performance for connectors of various types, including fiberoptic, circular, NGCon, plug- and receptacle-style, and multiple removable genderless termini. The first draft for the General Specification was realized Oct. 3, 2004.
The SAE avionics fiberoptic standard committee, AS3 group, has a recently formed sub committee, AS3C3, for jumpers, endfaces, link loss, and inspection (JELLI). This group was formed with the charter to create a specification that instructs both the supplier and the user in the testing and characterization of initial-build fiberoptic cable assemblies for avionics and aerospace applications. This can be in the plant or in the avionics “box.” It includes specification of jumpers (avionics measurement-quality jumpers), end faces, link-loss requirements, and inspection.
This four-part specification has developing sections for each of the major topics in the title: jumpers, end faces, link loss, and inspection. Measurement-quality jumpers are built to high standards and periodically checked to assure a high level of performance. They are used in testing the product both at the manufacturer’s inspection and at the customer’s inspection. Cable harnesses for avionics applications represent a wide variety of configurations. Consequently, aerospace-certified measurement-quality jumpers (AMQJs) interfacing with test equipment can be very simple or more complex.
A detailed definition of acceptable fiberoptic terminus end faces is established in the end-face section. End-face geometry, including protrusion/recess, spherical radius, and apex offset, will be specified for avionics fiber sizes. The purpose of this section of the JELLI specification is to define physical attributes of end faces of fiberoptic termini for aerospace applications. This will specifically address harsh-environment fiberoptic applications beyond the end-face configuration specified in Telcordia GR-326-CORE.
 FIGURE 3. Thorough test procedures describe the typical type of instruments and cables used to test fiberoptic link loss. |
Loss levels will be checked at manufacturer shipping and at customer receiving. The goal is to provide a single source to specify avionics and military fiberoptic link-loss measurement. Product-acceptance test procedures are defined, including equipment required (sources, power meters), cable and connector attenuation/insertion loss, and connector reflection/return loss. The standard also defines a reference-and-measurement procedure (see Fig. 3).
Inspection
Surface defects can adversely contribute to performance by affecting both insertion loss and back reflection, especially within the fiber-core zone. However, good manufacturing practices should minimize defects and measure them consistently. Key definitions in this section include scratches, pits, contaminants, and epoxy rings. Inspection zones are defined.
The JELLI standardization group formed in April 2004, and met in June, September, and November 2004. A draft specification is anticipated by the end of first-quarter 2005, prior to AS3 meetings in April 2005.
The ARINC series pertains to fiberoptic avionics standards. The ARINC 80X series is a comprehensive effort to establish fiber-optic standards for airline use of connectors, cables, system design guidelines, active device specification, test, installation, and maintenance procedures.
The objective of the ARINC 801 document for fiberoptic connectors is to provide standardization of a fiberoptic interconnect system for the air-transport industry (Draft 4 was completed Nov. 5, 2004). The goal is to avoid the proliferation of different designs of connectors. This specification defines generic fiberoptic connectors needed for commercial aircraft. Connectors include circular and rectangular designs based around a standard terminus. The terminus accommodates different cable types. The termini may be used for singlemode or multimode applications. The ARINC 801 series includes designs for butt-joint and expanded-beam lens connectors.
The ARINC 802 series is intended to provide standardization of fiberoptic cables to be used in harsh-environment avionics applications, both within the cabin and outside of the pressurized cabin, and within the airframe including external download cables (Draft 4, Aug. 27, 2004).
Fiberoptic system design guidelines under ARINC 803 provide design and implementation guidelines for the system-design engineer. The purpose is to assist in the identification and implementation of design requirements for aerospace fiberoptic-based systems. The specification includes recommendations for good practice and system testing. The ARINC 803 provides guidance on system definition, system design, component selection, and installation (Draft 5, Oct. 12, 2004).
The objective of ARINC 804 is to aid aircraft fiberoptic network designers in specifying and selecting active devices for avionics applications. Specific guidance is provided for designers of ARINC 664-compliant equipment, in accordance with ARINC 803. This document is intended as a basis for developing procurement specifications for active devices in avionics fiberoptic networks (Draft 4, Oct. 8, 2004).
The ARINC 805 document defines general practices for testing a fiberoptic system. This document outlines proven practices for engineers and technicians in testing and supporting fiberoptic systems in aircraft. This section has nine chapters on fiberoptic test procedures and related issues (Draft 2, Nov. 8, 2004). It contains recommended optical test procedures. Included are product acceptance of fiberoptic cable assemblies, certification of installed links, and basic system-level testing. A detailed section on safety emphasizes proper technician training and certification. Tooling and equipment is covered, including a definition of AQMJs for testing, probes, visual fault locators, power meters, and stabilized light sources, including launch conditions. It addresses optical-loss test sets, high-resolution OTDRs, inspection devices, and visual-inspection calibration artifacts (see Fig. 4). It also presents connector end-face examination, cleaning criteria, and product acceptance for many specific configurations.
Finally, ARINC 806 is a comprehensive collection of avionics fiberoptic installation and maintenance procedures. It defines general practices for maintenance and restoration, as well as practices and general standards of workmanship for technicians. The standard contains information on methods and practices for technicians engaged in fiberoptic maintenance, including safety, termination, restoration, training, certification levels, equipment requirements, and quality assurance (Draft 4, Oct. 13, 2004).
DOUGLAS PARKER is program manager, and PHIL NGUYEN is business manager at Tempo Research, 80 Wood Road #202, Camarillo, CA 93010. Tel: (805) 384-1834; email: dparker@tempo.textron.com.
