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Pro Beam EB16
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FIBER OPTIC SIZE 16 EXPANDED BEAM TERMINUS
Soren Grinderslev
TE Connectivity
Aerospace, Defense & Marine
2900 Fulling Mill Road, Middletown, PA. 17057, USA
Abstract
Fiber optics is replacing copper-based interconnects at an increasing rate for many aerospace applications. With
advantages such as superior bandwidth capability, reduced weight, EMI immunity and high density, fiber optics is
1becoming more attractive as a replacement for copper connectors. The MIL-PRF-29504 specification defines the
design and performance of a pair of fiber optic physical contact (PC) termini designed for the MIL-DTL-38999
style cavities, and is one of the first standardizations of fiber optics used in a harsh environment. Today, more than
20 years later, the reliability of the physical contact fiber interface is still an issue when operating in environments
such as those typical for aerospace. The integrity of the pristinely polished glass fibers which are kept in physical
contact at all times via springs is constantly being challenged when exposed to a harsh environment. Dust and
vibration near the optical interface are often the cause of performance degradation and can occasionally lead to
disastrous failures.
To address these performance issues, TE Connectivity has introduced the PRO BEAM EB16 Terminus, a novel
optical non-contact terminus types that uses the expanded beam (EB) concept in order to obtain a repeatable and
reliable performance. The external design conforms to the size 16 termini dimensional envelope of the MIL-PRF29504/4D/5D physical contact specification and incorporates, as an option, a separable lens unit designed to enable
field termination and repair. The rear removable EB16 pin and socket termini can easily be mounted as a
replacement for the physical contact MIL-PRF-29504/4D and /5D as well as for the size 16 copper contacts in a
MIL-DTL-38999 connector or be used in a hybrid combinations.
The EB16 design advantages provides consistent performance in a robust environment and reduces costly
maintenance downtime to a minimum. The new design further satisfies the demand for a reliable and affordable
low maintenance fiber optic solution.
Physical Contact Termini
The Physical Contact (PC) optical connectors were primarily designed for indoor applications in a controlled
environment, such as data and telecom. When it comes to rugged uncontrolled environments, the fiber optic PC
concept often performs poorly and is susceptible to irreversible degradation for various reasons. Unmated
connectors will expose the sensitive fibers to possible contamination of the optical pathway by particulates often
present in a harsh environment. Contamination of the pristine polished fiber endface by dust or other damaging
particles can be detrimental to a system. Once an interface is disconnected for any reason, cleaning and inspection
is required before a re-assembly can be done. The fiber and the ferrule endface, including the inside of alignment
sleeves, require careful cleaning using special cleaning agents and specific procedures. The diligence required in
cleaning and inspecting a PC interface for risk mitigation demands a high level of skill and training. An unqualified
operator can actually add more contaminants to the fiber than what is being removed during cleaning. Even the
consumables themselves must be kept contamination free. If a small particle is caught at the interface between the
spring-loaded fibers, it will initially block part of the light and introduce optical loss to the system. A scenario which
is particularly serious is if the particle is abrasive as it will result in irreversible damage to the fiber. This issue is
further amplified if the operation includes vibration levels such as those typically experienced for aviation
applications. During such an exposure, an abrasive particle may get imbedded into the glass surface and cause
permanent damage. Likewise, if the application is regularly subjected to vibration or shock or requires multiple
separations, wear of the fiber surface will cause optical degradation sooner rather than later. Maintenance may
include frequent re-polishing and inspection under a magnification or even a replacement of the entire harness. All
of these actions require costly downtime and includes verification testing before operation can be re-established. A
desirable alternative to the PC fiber is a dedicated design suitable for use in rugged environments with less
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sensitivity to the failure modes described above and the avoidance of prolonged downtime. One such solution is an
expanded beam design.
PRO BEAM EB16 terminus
The concept of an expanded beam connection includes a beam expansion and contraction between the emitting and
receiving fibers and is enabled via lenses positioned near or at the respective fiber endfaces (Fig. 1). The lens on
the launch side of the EB connection, typically a spherical ball lens, is used to expand and collimate the beam
emitted from the launch fiber. The expanded beam remains collimated as it travels through the airspace between
the lenses and across the mechanical interface until it reaches the receiving lens, a similar sized spherical lens,
which focuses the beam onto the endface of the receiving fiber. The fact that there is no physical contact between
the sensitive optical elements during operation makes the concept practically immune to the effects from vibrations
and mechanical shocks for the life of the product.
Furthermore, because the ferrule endfaces are enclosed and protected behind the lenses they will never require
cleaning. Only the exposed outer lens surface can be contaminated, but the optical performance is easily restored
by a simple cleaning procedure which most of the time does not even require an inspection under magnification.
Because the beam size is substantially expanded when it traverses the air space between the lenses, the signal will
not be deteriorated by the same size airborne contamination particles that seriously would affect the performance
of a PC connection.
For example if a 10 micron dust particle is caught between two mated single mode fibers it can block the light
transmission completely. If it is a multimode fiber the particle can cause a signal degradation up to 20%. If the same
particle is passing through the airspace containing the expanded beam it will barely be detectable. The EB16 beam
area from a multimode fiber can be expanded up to 35 times while a single mode fiber beam area is expanded 150
times. Thus, the degrading effect of dust and contaminants is strongly reduced when beam expansion is applied.
The fact that the insertion loss level is higher for the EB connector than for a PC design is by far outweighed by the
reliability of the EB performance. While the PC connection basically starts the optical degradation process when it
leaves the factory, the EB performance is extremely stable for the life of the product because there is no contact
between the optical elements at the interface resulting in elimination of the wear factor.
The EB16 has been designed using the same external small form factor dimensions as defined in the MIL-PRF29504/4D/5D standards. Therefore, the EB16 shares the same common universal installation and removal tools as
other rear-mounted size 16 termini. Standard components are used for incorporation of the expanded beam concept
such as a 1.25 mm diameter ferrule and a 1.25 mm spherical ball lens and an alignment sleeve with an internal
diameter of 1.25 mm (Fig. 2).
The EB16 comprises the excellent mechanical alignment accuracy obtained by mount of a same size ferrule and
ball lens within a precision alignment sleeve. The advantage of using a ball lens is that the sphericity is very accurate.
Therefore, the mechanical and optical axes of the lens are inherently coincident. The center of the spherical lens
surface is always positioned on the co-located mechanical and optical axes as well as on the fiber axes and therefore
provides excellent alignment between the fiber and the lens. This arrangement is applied to both the pin and the
socket termini.
In addition, the socket contains a high precision ceramic mating sleeve which telescopically receives and aligns the
barrels containing the optics of the pin and the socket termini when the connector plug and receptacle are mated.
The proper distance between the fiber and the lens (the lens back focal length) is secured by use of a ‘doughnut’
shaped spacer disc for the case of multimode. Other derivatives of the design maintains a direct physical contact
between the fiber and the lens for use in applications where high return loss is required.
One major advantage of the EB16 design is the patented option for field termination. As shown in Figure 3, both
the pin and socket termini can be separated from their lens units. This allows individual field termination of the
fiber into the pin and socket including the pre-assembly fiber polish. Once the polish has been realized, the lens unit
is simply threaded onto the ferrule for completion of the termination. If the operating environment involves severe
vibration it is recommended that a small amount of thread locking adhesive is applied to the threads. The added
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freedom of field termination allows the user to decide on the length of the fiber cable at the last minute or to reterminate the cable if needed. A further advantage is that the individual lens units can be replaced without disturbing
the cable assembly installation or routing.
One of the most common connectors used to house the termini is the circular MIL-DTL-38999 series III (shown in
Fig. 2). This connector was originally designed for electrical copper contacts where generous tolerances are
acceptable in satisfying the dimensional needs of the copper contacts. The M38999 standards specifies an allowed
alignment error between the plug and receptacle cavities of up to 0.50 mm [.020 inches]. Such a large cavity offset
is a challenge for the fiber optics which require a higher precision. Therefore, if fiber optic termini are mounted in
connectors having a large cavity offset, it will result in angular tilting forces applied to the optical pin and socket
connection with added losses as a result. Therefore, it is sometimes seen that 38999’s from different manufacturers
yield different results for the same set of termini.
A comprehensive collection of testing has been performed on a total of 78 termini pairs distributed between 8
different 38999 series III connectors of different sizes. The samples were divided and tested in two groups
(mechanical and environmental) to simulate a harsh environment. The insertion loss of all channels was measured
pre and post testing and is showed graphically in Fig. 10 for comparison. The result shows that no statistical insertion
loss degradation occurred throughout the execution of all the mechanical and environmental testing.
Results and graphs of the product testing are shown in Figures 4-12.
Figure 11 shows a version of the EB16 termini designed for the EN4165 (European Norm) connector commonly
used for low leg seat-to-seat disconnects on commercial aircraft. For these termini the pin is unchanged but the
socket must be a springless terminus with a reduced length due to the limited space available in the EN4165
connector half.
Finally, Figure 12 shows some of the test results obtained for an EB16 single mode version configuration. Notably,
these EB16 cable assemblies were successfully tested in a 38999 connector exposed to a wide temperature cycling
range between -55°C and +165°C [-67°F to +329°F] with a rapid ramp rate of 3°C/min.
Conclusion
The PRO BEAM EB16 terminus has been introduced.
The design satisfies the demand for a reliable transfer of high data rates and provides a fiber optic alternative in
applications formerly reserved for copper. Commercial air, Military air, Unmanned Aerial Systems, shelter and
vehicle modifications are just a few examples of platforms and applications that are embracing a distributed
architecture approach demanding higher capacities at lower weight and power consumption than what is offered by
a copper solution.
In summary, the EB16 termini has all the advantages offered by the expanded beam concept. Installed in a harsh
environment, it requires little or no maintenance and provides consistent optical and mechanical performance
throughout the life of the product. It delivers a performance with low sensitivity to vibration, shock and
contamination. It comes either as a kit for field terminations or as a factory built harness which offers an easy
replacement of copper contacts or allows the flexibility of a hybrid supplement in concert with either copper size
16 contacts or optical MIL-29504 physical contact termini. Affordability with a price approaching that of the optical
MIL-29504 PC termini version makes the EB16 an appealing design option.
The TE Connectivity PRO BEAM EB16 design was recently announced as the selection for the ARINC-FOS 845
industrial standard for size 16 expanded beam termini. The design is currently also being considered as one of the
candidates for the expanded beam standardization for the SAE AS3 Photonics Committee (AS6250).
References
1.
2.
3.
Grinderslev, Soren, “Fiber Optic Connectors: Expanded Beam versus Physical Contact,” Connector Supplier, March 17, 2009.
Grinderslev, Soren, “Single-Channel Expanded Beam Connector,” US Patent 7,775,725, August 17, 2010.
Grinderslev, Soren, “Field-Installable Expanded Beam Connector System”, US Patent 8,827,567, September 9, 2014.
Pro Beam EB16
FIG 1. Expanded Beam concept
FIG 2. EB16 Concept
FIG 3. EB16 Field termination version
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FIG 4. Vibration
FIG 5. Mechanical Shock
FIG 6. Mating Durability, 1,000 X without cleaning. Performance within +/- 0.1dB change.
Pro Beam EB16
FIG 7. Temperature Life
FIG 8. Temperature Cycling
FIG 9. Thermal Shock
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Pro Beam EB16
FIG 10. Before/After Insertion Loss Statistics
FIG 11. The EB16 concept can be used in other connector types – here: EN4165
FIG 12. EB16 Testing of Single Mode Termini in 38999 series III
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