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What fiber optic connectors are approved and preferred for military use, specifically for land, sea, and air applications?
The Defense Systems Information Analysis Center (DSIAC) was asked to identify approved fiber optic connectors that are preferred for military use and provide information on organizations that develop standards and lists of preferred parts for military branches. Fiber optic interconnect technologies (MIL-DTL-38999 Type, MIL-DTL-83526 GFOCA Type, MIL-PRF-28876, ARINC 801, and more) deliver high-data-rate and high-bandwidth performance in harsh land, sea, air, space, and C4ISR applications. Reduced size and weight, electromagnetic/spark-arc immunity, enhanced bandwidth over long distances, and reliable security make fiber optic technologies far superior to traditional electrical systems. An overview regarding the fiber optic connectors for military use is provided. Generally, the standards are in place for the MIL-DTL-38999 type connector which has wide use and applications, while emerging fiber optics connectors such as the Next Generation Connector (NGCON, MIL-PRF-64266) are evolving as the state of the art for high-performance solutions for air, sea, and space applications.
The inquirer requested specifics for fiber optic connectors for land, sea, and air applications, including any scenarios where certain connectors are to be used. Information on organizations that develop standards and lists of preferred parts for branches of the military was also requested. The inquirer also asked to confirm if a specific connector, the Next Generation Connector (NGCON) (MIL-STD-64266), has been qualified and if there are any examples of specific applications it has been used for. Finally, the inquirer requested information on any standards regarding the termini in the connectors.
Fiber optic connectors are typically plugs or so-called male connectors with a protruding ferrule that holds the fibers and aligns two fibers for mating. They use a mating adapter to mate the two connectors, which fits the securing mechanism of the connectors (bayonet, screw-on, or snap-in). A primary specification issue for fiber optic connectors is insertion loss, i.e., the amount of light lost in the connection expressed in decibels (dB).
2.0 Fiber Optic Connector Overview
There are many types of fiber optic connectors, but each generally uses either physical contact (PC) or expanded beam (EB) technology.
2.1 PC Connectors
PC connections are characterized by the physical mating of two optical fibers by terminating the optical fiber into a precision ceramic ferrule. Epoxy is used to affix the fiber into the ferrule. The tip of the ceramic ferrule is polished precisely to ensure that light enters and exits at a known trajectory with little scattering or optical loss. Polishing the terminated ceramic ferrule is a critical process in fabricating PC connectors. Since the optical fibers are touching each other using opposing forces (axial springs), light exits one fiber and enters the other with low insertion loss (IL), typically around 0.25 dB. An alignment sleeve positions the two ferrules, ensuring precise axial alignment. This is the prevalent connection method in the global fiber optic industry .
2.2 EB Connectors
EB connectors use two lenses to expand, collimate, and then refocus the light from the transmitting fiber into the receiving fiber. Ball lenses or graded index rod (GRIN) lenses are used in EB connectors and result in reduced signal loss due to contamination at the optic interface. The lens design facilitates cleaning and because the light path travels over an air gap, there is no physical contact. Lack of physical contact potentially eliminates the mechanical wear found in PC connectors, allowing more connector mating cycles .
Since EB connectors expand the light beam across the interconnect point, the impact of debris on the lens surface, such as dirt and dust particles, is minimized when compared with PC connections. However, while PC connections push away moisture and liquids during mating, thus eliminating their impact on performance, EB connections react differently to moisture and liquids. Moisture or liquids on the lenses can degrade performance by scattering the light as it tries to pass through the contaminant.
2.2.2 Ball Lenses
The ball lens has a constant refractive index and is manufactured using commonly available glasses. The light from the source changes direction at the curved boundaries and travels through the inside of the lens in straight lines.
2.2.3 GRIN Lenses
The GRIN lens has a cylindrical form factor, but the refractive index is not constant. Rather, the refractive index distribution varies radially with a parabolic profile, with the maximum index of refraction along the axis of the lens. Unlike the ball lens, the light curves through the inside of the GRIN lens, placing a point light source at the focal point of either lens, which collimates the light.
2.2.4 Military Use
EB technology is less susceptible to contaminants affecting optical performance involving applications with the U.S. Army, Navy, and Marine Corps military tactical deployments.
2.3 Comparison of PC and EB Connectors
In general, EB connectors are more expensive to produce, which has limited their use in telecom and datacom applications. However, EB technology is used in several military and commercial applications where frequent mating and unmating may expose the optical interfaces to contamination. Several recently introduced, lower-cost EB connectors are targeted at commercial medical applications, where reliability and thousands of mating cycles are required. Table 1 summarizes various performance parameters for the two technologies.
Table 1: Comparison of PC and EB Connectors (Source: Simonini and Douthit )
3.0 Types of Standard Fiber Optic Connectors
Sections 2.2.1 through 2.2.3 provide an overview of some of the standard fiber optic connectors along with their applications, including some of the emerging standards.
3.1 D38999 Connectors with M29504/04 and M29504/05 Termini
MIL-DTL-38999 type fiber optic interconnection systems are the recognized standard for all military and commercial aerospace applications that depend on high levels of connector reliability, environmental and mechanical performance, and low dB insertion loss.
Designed specifically for air and space, the MIL-DTL-38999 type tight-tolerance fiber optic connection system has been successfully deployed in applications ranging from the F-35 Joint Strike Fighter and the venerable F-22 to dozens of other fixed-wing and rotary aircraft applications.
Glenair manufactures all components in house, including tight-tolerance M38999 fiber optic connectors, ultra-low dB loss QPL M29504/04 (pin) and /05 (socket) termini, as well as a complete range of assembly tools and other accessory components .
Additionally, Interstate Connecting Components (ICC) manufactures general, military standard MIL-DTL-38999 Series I Connectors. These connectors have a bayonet coupling subminiature configuration with high contact density, ideal for smaller wire-gauge, general-purpose applications produced by ICC . These connectors were designed for military and commercial applications where the prime requirements are lower profile and lighter weight.
In MIL-DTL-38999 Series II connectors, reduction of both size and weight was achieved using thinner shell walls and length restrictions. These design restrictions reduced the radio-frequency interference attenuation characteristics and the “scoop” protection while yielding an excellent general-purpose, lightweight connector. Compared to Series I, Series II connectors achieve up to 20% reduction in mated pair length, up to 39% reduction in outside diameter, and up to 40% reduction in weight (128 pin mated pair) .
3.2 MIL-PRF-64266 NGCON System
An emerging industry standard for a New Mil-Aero/Shipboard Fiber Optic Solution is the Glenair MIL-PRF-64266 NGCON, which combines proven technology from standard MIL-PRF-28876 and MIL-DTL-38999 Series III designs. This system includes new innovations such as rear-release, genderless contacts, high-density packaging, and removable alignment sleeve retainers (ASRs). Glenair is an active member of the NGCON design consortium, which is developing high-performance fiber optic interconnect solutions for air, sea, and space military applications .
The NGCON incorporates best practices learned during the evolution of many military fiber optic programs over the last several decades. New and upgraded military platforms that incorporate harsh-environment fiber optic systems will benefit from the use of the NGCON and its exceptional tight tolerance design .
NGCON features and benefits include the following :
- Rear-release/genderless termini.
- NGCON Military Standard compliant.
- Wall-mount and jam-nut receptacle configurations.
- High-density arrangements.
- Operating temperature range from -55 to +165°C.
- Environmental sealing terminus.
- 125mm diameter ceramic ferrule.
- Removable front ASR for easy access to termini for cleaning and inspection.
- D38999 Series III accessory threads and M28876 coupling threads.
NGCON specifications and catalog notes are as follows :
- Conforms to MIL-PRF-64266 (NGCON) military standard.
- Multimode and single-mode capable.
- 25-mm diameter ceramic ferrules and alignment sleeves.
- Environmental O-ring sealing on terminus.
- Various connector material/finish options available.
- M28876 Double-start ACME mating threads, D38999.
- Series III style rear accessory threads.
- Receptacles compatible with M28876 panel cutouts.
- Antidecoupling (ratchet) mechanism on plug connector.
- Keyed connectors and termini available for single-mode, angled physical contact (APC).
Common tools and processes for termination and cable assembly ensure uniformity across the supplier base. This commonality of parts, processes, and tooling saves money and allows the end-user to maintain a single training regimen and tool set .
In addition to the extended temperature range, high-density packaging, and removable ASR, the NGCON includes a ratcheting plug, a full-mate indicator, a double-start thread, and 12 keying options. The ratcheting plug locking mechanism ensures that the coupling nut will not back off during shock or vibration and that it will maintain full-thread engagement during the harshest environmental conditions. Additionally, with 12 keying options, NGCON provides the most configurable, multichannel, harsh-environment connector with a two-fold increase in key options from both the M28876 and M38999 connector families . Table 2 lists the individual parts of the NGCON MIL-PRF-64266 connection system.
Table 2: NGCON MIL-PRF-64266 Connection System (Source: Glenair ).
3.2.3 Military Use
NGCON has not been qualified for military use as the U.S. Navy has not tested it. Funding is no longer available from the Navy for it to get qualified, so currently, it is stagnant. A product that is widely used instead is MIL-DTL-38999. This system is high performing and is qualified as are other connectors, such as the ARINC 801 Fiber Optic Interconnects, for commercial, military, and shipboard applications [11, 12].
3.2.4 Qualification Data
Glenair laboratory link test and qualification data for harsh-environment optoelectronic solutions are available for the 050 Series . Experior Laboratories is qualified to test fiber optic connectors to the newly released MIL-PRF-64266 specification. Qualification testing requirements set forth in the specification are comprehensive to address applicable requirements of the Department of Defense (DoD) .
The TFOCA-XBT4 (Figure 1) integrates EB technology into the proven TFOCA-II connector form factor. The TFOCA-II physical contact connector is the standard fiber optic connector for the U.S. Army, designed for and proven in the harshest of military and industrial applications.
The TFOCA-XBT4 has the following features :
- EB technology less susceptible to contaminants affecting optical performance.
- Available in multimode and single-mode fiber.
- Hermaphroditic design.
- Noncoating interface allows thousands of mating cycles.
- Two- and four-channel configurations available.
- Cable retention designed to meet 400-lb pull strength while protecting fibers from stress.
- Zinc-nickel plating which meets the new Environmental Protection Agency mandate for eliminating heavy metal plating.
- Easy field maintenance and cleaning.
- Available in stainless steel and brass, allowing it to be used in a variety of applications.
3.3.3 Military Use
The TFOCA-XBT4 can be used for U.S. Army, Navy, and Marine Corps military tactical deployments as well as among the oil, gas, and geoscience industries.
Figure 1: TFOCA-XBT4 (Source: Fiber Systems International ).
4.0 Alternative Connectors
MIL-PRF-29504 (formerly MIL-T-29504) is a performance specification describing the general requirements for removable crimp and epoxy-type fiber optic termini for use in connectors and similar components. These termini are unique for military applications and must operate satisfactorily in systems under demanding conditions of 10 g’s vibration (10 g’s), shock (over 1,000 g’s), and temperature excursions (from -40 °C to +70 °C) .
Formerly MIL-C-28876, MIL-PRF-28876 is a performance specification describing the general requirements for circular-, plug-, and receptacle-style, multiple-removable termini, fiber optic connectors that are for DoD applications and that are compatible with multiple transmission element cables. This specification describes a family of general-purpose, interconnection hardware providing a variety of compatible optical coupling arrangements, and includes connector shells, connector inserts, connector insert retention nuts, connector backshells, and connector dust caps. These connectors are unique for military applications and must operate satisfactorily in systems under demanding conditions of 10 g’s vibration (10 g’s), shock (over 1,000 g’s), and temperature excursions (from -40 °C to +70 °C).
Amphenol Fiber Systems International (AFSI) is a leading producer and supplier of M28876 connectors to the U.S. Navy and allied navies around the world .
Formerly MIL-C-39012, MIL-PRF-39012 is a performance specification describing the general requirements and tests for radio frequency (RF) connectors used with flexible RF cables and certain other types of coaxial transmission lines .
Formerly MIL-C-31031, MIL-PRF-31031 is a performance specification describing the general requirements and tests for RF connectors used with flexible cables and certain other types of coaxial transmission lines .
Formerly MIL-C-49142, the MIL-PRF-49142 is a performance specification describing the general requirements and tests for RF, triaxial connectors. These connectors and fittings are intended for use with biaxial cable and can be used for RF applications when more shielding is required, and they can also be used for serial digital transfer .
A vital element of link loss budgets is the loss affiliated with ferrule-to-ferrule contact between connectors. Connector losses are usually determined by implementing insertion loss and reflection loss tests. The outcomes of these tests are relative to a reference connector that is used to perform the tests.
The Fiber Optic Connector Intermateability Standards (FOCIS) are written for manufacturers to follow to ensure their connector will be intermateable with other manufacturers’ connectors of the same type. It also defines the mechanical properties of numerous connector styles, with the objective of guaranteeing that connectors are made to specification and will reach a common level of performance. However, nearly all these standards only mention an advised range for the radius of curvature for the geometry of the ferrule end face and do not mention other important end-face geometrical parameters, so end-face parameters remain an area of research .
 Simonini, E., and E. Douthit. “Expanded Beam and Physical Fiber Optic Connectors.” White Papers, http://www.fibersystems.com/pdf/whitepapers/Expanded-Beam-White-Paper.pdf, 15 July 2019.
 Glenair. “Military Standard Part Numbers Available from Glenair.” https://cdn.glenair.com/ mil-spec/pdf/military-standard-part-numbers-and-full-spectrum-no-gap-product-lines.pdf, 15 July 2019.
 Interstate Connecting Components. “MIL-DTL-38999 Series I Connectors.” https://www.connecticc.com/products/mil-dtl-38999si.php, 15 July 2019.
 Interstate Connecting Components. “MIL-DTL-38999 Series II Connectors.” https://www.connecticc.com/products/suppliers/conesys/aeroelectric/mil-dtl-38999sii.html, 15 July 2019.
 Glenair. “Next Generation (NGCON): Emerging Industry Standard for a New Mil-Aero/Shipboard Fiber Optic Solution.” https://www.glenair.com/fiberoptics/next-generation-ngcon-fiber-optics.htm, 15 July 2019.
 Fiber Systems. “The Next Generation Connector.” White Papers, http://www.fibersystems.com/pdf/whitepapers/NGCon-Final-Copy-08092011_2_.pdf, 15 July 2019.
 Amphenol Fiber Systems International, Inc. “NGCON Fiber Optic Connector.” http://www.fibersystems.com/products/ngcon-fiber-optic-connector/, 15 July 2019.
 Glenair. “Next Generation High-Density (NGCON): Sea and Air.” https://cdn.glenair.com/datasheets/pdf/fiber-optic-connection-system-next-generation-high-density.pdf, 16 July 2017.
 LeClair, L., and D. Frattin. Personal communication. MTEQ Incorporated, Lorton, VA, 2019.
 Glenair. “Next Generation Fiber Optic Connection System MIL-PRF-64266 (NGCON) General Information.” https://cdn.glenair.com/fiberoptics/pdf/h/general-information.pdf, 2012.
 ITT Canon. “ARINC 801-Compatible Fiber Optic Solutions.” https://www.ittcannon.com/ campaigns/arinc-801-fiber-optic/, 15 July 2019.
 Glenair. Personal communication. Glendale, CA, 2019.
 Glenair. “Small Form-Factor: Opto-Electronic Interconnect Solutions for Harsh-Environment Ethernet, Video, High-Speed Data, and Signal Aggregation.” https://cdn.glenair.com/catalogs/opto-electronic-interconnect-solutions-capability-guide.pdf, May 2014.
 Experior Laboratories. “Fiber Optic Testing.” https://experiorlabs.com/fiber-optic-testing/, 15 July 2019.
 Amphenol Fiber Systems International, Inc. “TFOCA-XBT4.” http://www.fibersystems.com/images/product-pdfs/expanded-beam-connectors/TFOCA-XBT4_Data_Sheet.pdf, 15 July 2019.
 Fiber Systems International. “TFOCA-XBT4.” http://www.fibersystems.com/images/product-pdfs/expanded-beam-connectors/TFOCA-XBT4_Data_Sheet.pdf, accessed July 2019.
 Wikipedia. “U.S. Military Connector Specifications.” https://en.wikipedia.org/wiki/ U.S._Military_connector_specifications#cite_ref-42, 15 July 2019.
 Amphenol Fiber Systems International. “M28876 Fiber Optic Connectors.” https://www.connecticc.com/pdf/m28876%20catalog%208.5×11-2.pdf, 15 July 2016.
 The Fiber Optic Association, Inc. “Guide to Fiber Optics and Premises Cabling.” https://www.thefoa.org/tech/focis.html, 15 July 2019.
Amphenol Aerospace. “MIL-DTL-38999 Connectors: For Military, Aerospace and Harsh Environments.” http://www.amphenol-aerospace.com/pdf/catalogs/Amphenol-38999-2016.pdf, 27 April 2018.
U.S. Department of Defense. “MIL-PRF-64266, Performance Specification: Connectors, Fiber Optic, Circular, Plug and Receptacle Style, Multiple Removable Genderless Termini, Environment Resisting, General Specification.” http://everyspec.com/MIL-PRF/MIL-PRF-030000-79999/MIL-PRF-64266_37947/, 25 November 2008.
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