selection guide to fiber optic connectors

8/13/2019 Selection Guide to Fiber Optic Connectors

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SELECTION GUIDE TO FIBER-OPTIC CONNECTORS

BYDennis Horwitz, Vice President

RIFOCS CORPORATION

One of the most critical components of a fiber-optic systemis the connector. A fiber-optic connector joins two fiberstogether such that light passes from one fiber to the other. Themain requirements are to minimize signal loss, and reflections,and to provide a connection that is stable both mechanically andoptically. Achievable losses for various connector types are in

the range of 0.25 to 1.5dB.

A fiber-optic connection consists of three components: twoof which are the male connector plugs which are joined togetherthrough a coupling (or inline adapter). Unlike electricalconnectors, all simplex fiber-optic connectors are only availableas male plugs (there is no female version). The connector plugconsists of a precision ferrule, cable or fiber strain reliefsupport and housing. The coupling device allows two connectors tointermate through a bulkhead or panel. It offers femalereceptacles on each end and incorporates a precision sleeve, orbushing, to align two mated connectors.

When choosing a connector and coupling for a particularapplication, the designer must consider the fiber type, requiredoptical performance, operating environment and installation andmaintenance of the system as well as cost. If an existing systemis being modified or expanded, intermateability with existingcomponents must also be considered. The selection process shouldaddress the following topics:

1. Contact Type (i.e., NC, PC, SPC, APC)2. Connector Type (i.e., SMA, Biconic, ST, FC, SC, DIN, etc.)3. Termination Type (i.e., Epoxy/Polish, Epoxyless, etc.)4. Material Types (i.e. Ferrule and sleeve composition)

Contact Types

While there are a multitude of connector styles, there arethree variations in the way a connector ferrule can be prepared:non-contacting (NC), physical contact (PC) and angled physicalcontact (APC). The selection of one of these options based uponperformance requirements is the most crucial first step in


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connector selection.

The NC finish is either a flat or concave polish whichresults in an air gap between two mating connectors. This was theoriginal termination method for fiber-optic connectors. Overallinsertion loss (IL) and return loss (RL) is substantially higher

than either the PC or APC types. Insertion loss range is 0.5-2dB;typical return loss is 15-25dB. Not recommended for laserapplications.

The improved optical performance of PC connectors rose fromthe development of keyed connector designs combined with convexpolishing techniques. Non-keyed connectors were subject to largevariations in insertion loss performance because the fiber rotatedinto a different position during each mating. Keyed connectorscan only be inserted into the bulkhead in one orientation andexhibit substantially improved repeatability over non-keyed types.A small dome or radius at the end of the ferrule allows aphysical, or point, contact between connectors with an internalspring usually controlling fiber contacting pressure. The PC

connector offers the substantially lower insertion loss (0.2-1dB)versus its older NC counterpart. Regarding return lossperformance, a standard PC connector offers 30-35dB return loss; ahigh performance SPC (Super PC) version offers RL>40dB.

Applying an 8 degree angled convex polish produces an APCconnector. The minimum 60dB return loss is required for reliableoperation of high speed digital, analog, CATV and microwave-on-fiber applications. APC technology can only be incorporated intokeyed connector designs and requires special couplings due to itscritical alignment tolerances.

Connector Types

The SMA connector was the first "standard" fiber-opticconnector in the industry. This multimode connector was developedby Amphenol as an adaptation of the popular microwave connector tofiber-optics. It is an unkeyed, threaded connector in whichmating connectors do not make contact (flat polish). There aretwo types: the straight ferrule 905 style, and the stepped ferrule906 style. The stepped ferrule allows for more precise alignmentof connectors within a special ferrule sleeve, and is thepreferred type for use in bulkheads. The newer FSMA version is asomewhat more precision version of the standard SMA. It is stilla popular multimode connector in LAN and military systems.

The BICONIC connector was originally developed by AT&T forthe telecommunication industry. With both connectors fullythreaded into the bulkhead, springs control the fiber contactingpressure (important in all contacting connectors). Because of theconical shape, taper length must be precisely controlled duringthe termination process. Unfortunately, this aspect of thetermination process is the step most often ignored and violated byinstallers. Insertion loss variance can be quite substantial inthe range of 0.3-2dB; return loss range is 15-30dB.


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The ST-PC connector was developed by AT&T as a higherperformance alternative successor to the Biconic connector. Itintroduced the familiar bayonet coupling with keying to ensureconsistent connector orientation within the bulkhead. It isspring loaded, allowing two connectors in the bulkhead to maintainface contact with the same pressure. This eliminated the need to

manufacture to a precise length, unlike the sensitive Biconicdesign. Inside the bulkhead the connectors are aligned within astraight ferrule sleeve. However, these sleeves can be made ofdifferent materials (polymer, ceramic, phosphor bronze, copper,tungsten carbide), with widely differing effects on overallconnector performance. A low cost connector, the ST connector canbe quite sensitive to vibration and susceptible to accidentaloptical disconnect.

The FC-PC connector is an improved FC or D3 connector,originally developed by NTT in Japan. The outer collar crews ontothe bulkhead with little mechanical coupling to the inner fiberand ferrule. This connector was one of the first keyed designsand offers excellent repeatability. An internal spring controls

the fiber contacting pressure. It offers excellent overallperformance and is one of the most robust designs in the industryin use in a wide range of applications. An FC-APC version isavailable for applications requiring high return isolation.

The D4-PC connector was originally developed by NEC (Japan)and is somewhat similar to the FC-PC connector but features a2.0mm OD ferrule. It does not offer the mechanical isolation fromthe cable as does the FC. Therefore, it can offer variableperformance due to accidental cable pull and tension. It iscommonly seen only in telecommunication applications, particularlycentral office equipment manufactured by NEC.

The DIN-PC (DIN 47256) connector is a threaded couplingdesign originated by SIEMENS for the German telecommunications

industry. Originally known as Deutsche Industry Norm (DIN) 47256,this connector is an IEC standard and is popular in many parts ofEurope and Asia. The DIN connector is also the smallest overallconnector with optical performance and reliability equivalent tothe FC-PC connector. Unlike the other connector types mentionedin this article, the extended DIN "family" include blind-mating(back plane) versions as well as a DIN/AVIONICS model forcommercial/military avionics and high vibration applications.

The SC-PC connector is a more recent addition to theconnector market by NTT. Unlike the previous connectors, the SChas a rectangular cross section and push-pull instead of threadedcoupling. It is primarily plastic, occupies very little space,and can be packed very densely into a panel. It is also keyed,with an internal spring to snap positively in and out of place andmaintain contact pressure. It has recently been recognizedinternationally by ISO and IEC as the proposed standard for allfuture premises wiring in voice and data communicationapplications. Any designer should seriously consider thisconnector for all new applications. An SC-APC version is alsoavailable for critical RL applications.


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Material Options

Once a given style has been selected, the design engineermust also be concerned with the performance variations found in amultitude of material options. Connector performance and

reliability is most directly influenced by the materials used inthe fabrication of the ferrules and alignment sleeves. There isalways a cognizant cost/performance tradeoff in what may seem anunimportant (certainly unexciting) detail.

Ferrule options include molded plastic polymer, brass,stainless steel, ceramic, zirconia and tungsten carbide. Adisposable application may deem a polymer or stainless ferruleacceptable as a compromise of cost and performance. Moredemanding telecommunications applications may dictate ofconnectors with more expensive yet more reliable ferrule designssuch as ceramic (100-200 matings), zirconia (200 matings) ortungsten carbide (over 1000 matings). High wear, precisionapplications should as test and measurement equipment should

possibly consider only the most durable tungsten carbide variety.Choice of couplings are no less important. Inside the

bulkhead the connectors are aligned within a straight ferrulesleeve. These sleeves can also be made of different materials andmanufacturing processes, with widely differing effects on quality.Sleeve materials can include polymer, ceramic, zirconia, phosphorbronze, copper and tungsten carbide. Process variations (andrelated performance) are more evident in medium-priced phosphorbronze and copper sleeves where material can be cold rolled, lasercut or formed.

Termination Options

Most connectors require epoxy/polish termination. After thefiber is stripped to the proper dimension, it is attached withepoxy to the connector. Strength members are then crimped to theconnector body and finally, the fiber/ferrule end is polished to afine finish.

Epoxyless connectors, which have been the object of muchresearch and development over the last fiver years, exhibitperformance generally comparable to standard epoxy/polishconnectors in field applications, but significantly lowerinstallation cost. Various types of epoxyless connectors areeither directly derived from "crimp-and-cleave" or mechanicalsplice technology. While multimode versions are now in common useand very popular, singlemode variations have still yet to be fullyproven and accepted by the telecommunications industry.

A third termination option is splicing, either by fusion ormechanical methods. It is becoming the preferred method of"termination" in the outside plant (OSP) environment oftelecommunications. A high quality, factory-terminated pigtailcan be more efficiently spliced into a system than to mount anepoxy/polish connector in the field which is subject to installer


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variances.

Summary

Connector selection is certainly not a trivial designdecision. Remember that the connector is the most common site of

failure and is potentially the weakest link in the system. Theold caveat - "You get what you pay for" - is as true for fiber-optic connectors as anything else. In general, the choice of moreexpensive, higher quality connectors can actually save money whenoverhead support and failure costs are considered over and abovethe initial price of a lower quality connector.

In general, the system designer would be best served byrestricting his scope to the various PC and APC offerings. Thisautomatically guarantees a reasonable expectation of low insertionloss. SUPER PC is the industry norm in telecommunications whereminimum RL of 40dB is needed for high bit rate digital systems.Critical WDM digital systems as well as analog designs, generallyrequire the higher return loss isolation (RL>>45dB) offered by APC

connector technology.


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MULTIMODE CONNECTORS

TYPE InsertionLoss

ReturnLoss

Repeatability StrainReliefStress

Vibration TemperatureCycling

ContaminationPerformance

SMA -- -- -- + -- -- +

Biconic - - - - o - -

ST + + o - o o o

FC-PC ++ ++ ++ + + + +

D4 o + o o o + o

DIN ++ ++ ++ ++ + + +

DIN/AVIO ++ ++ ++ ++ ++ ++ +

SC ++ ++ ++ + + + o

-- Very Poor - Poor o Neutral + Good ++ Very Good

SINGLEMODE CONNECTORS

TYPE InsertionLoss

ReturnLoss

Repeatability StrainReliefStress

Vibration TemperatureCycling

ContaminationPerformance

Biconic -- -- -- o o - -

ST + + o - o o o

FC-PC ++ ++ ++ + + + +

D4 o + o o o + o

DIN ++ ++ ++ ++ + + +

DIN/AVIO ++ ++ ++ ++ ++ ++ +

SC ++ ++ ++ + + + o

-- Very Poor - Poor o Neutral + Good ++ Very Good


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selection guide to fiber optic connectors

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