Fiber Optic Connectors Basics, Styles, Trends

:: A Little Fiber Optic History Background

1965 – Charles Kao and George Hockham showed that if optical fiber’s attenuation can be reduced to less than 20dB/km, then it can be used as a transportation media for communication

1970 – Robert Maurer, Donald Keck, Peter Schultz, and Frank Zimar from Corning made a silica glass fiber with an attenuation of 17dB/km.

1977 – General Telephone and Electronics sent the first live telephone traffic through optical fiber, at 6 Mbit/s, in Long Beach, California.

1986 – David Payne and Emmanuel Desurvire invented EDFA (Erbium-Doped Fiber Amplifier) which eliminated the need for O-E-O repeaters, significantly reduced the cost for long distance fiber optic systems.

:: Fiber Optic Connector Market

:: Fiber Optic Connector Standards

• TIA/EIA-4750000-B
  Generic Specification for Fiber Optic Connectors
• TIAEIA-604
  Fiber Optic Connector Intermateability Standards (FOCIS)
• TIA/EIA-568-B.3 /C.0 /C.3
  Commercial Building Fiber Optic Standards

• GR-326
  Generic Requirements for Single Mode Optical Fiber Connectors
• GR-1435
  Generic Requirements for Multi-fiber Optical Connectors

:: Key IEEE Standards and Media for 10 Gigabit Ethernet

• IEEE Ethernet protocol standard 802.3 for Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications
• IEEE standard 802.3ae for 10 Gigabit Ethernet over Optical Fiber (single mode and multimode)
• IEEE standard 802.3aq for 10 Gigabit Ethernet over Installed Multimode Optical Fiber up to 220 meters

:: Key Physical Layer Interfaces (PHY) and Media

• 10GBase-SR (“short range”)
  This standard supports short distances over deployed multimode fiber cabling. It has a range of between 26m to 550m depending on the bandwidth of the glass cores. Uses wavelength at 850nm and typically VCSEL lasers.
• 10GBase-LX4
  Uses WDM (wavelength division multiplexing) to support ranges of between 240m and 300m over deployed multimode cabling. Also supports 10km over single mode fiber. Uses wavelength of 1310nm.
• 10Gbase-LR (“long range”)
  This standard supports distances of up to 10km over single mode fiber using 1310nm wavelength.
• 10GBase-ER (“extended range”)
  This standard supports distances up to 40km over single mode fiber using 1550nm. Recently several manufacturers have introduced 80km range ER pluggable interfaces.
• 10GBase-LRM
  This standard will support distances up to 220m 10Gbit/s on FDDI-grade multimode cable. Various combinations of offset and center launch cables on both ends of the link. Very complex.
• 10GBase-SW, 10GBase-LW and 10GBase-EW
  These varieties use the WAN PHY, designed to interoperate with OC-192/STM-64 SDH/SONET equipment using a light-weight SDH/SONET frame. They correspond at the physical layer to 10GBase-SR, 10GBase-LR and 10GBase-ER respectively, and hence use the same types of fiber and support the same distances. (There is no WAN PHY standard corresponding to 10GBase-LX4).

:: A Little Bit Background Info about Fiber Optic Connectors

• Fiber optic connectors have traditionally been the biggest concern in using fiber optic systems.
• Connectors were once unwieldy and difficult to use.
• Connector manufacturers have standardized and simplified connectors greatly.
• This increasing use-friendliness has contributed to the increase in the use of fiber optic systems.
• The sole purpose of a connector is to mate fiber optic cable with minimal loss of light.
• Connectors are designed for many different applications including telecommunications, local area networks, and harsh environments.

:: The Fiber Optic Connector Body

• Also called the connector housing, the connector body holds the ferrule
• Usually constructed of metal or plastic and includes one or more assembled pieces which hold the fiber in place.
• Details vary among connectors, but bonding and/or crimping is commonly used to attach strength members and cable jackets to the connector body.
• The ferrule extends past the connector body to slip into a coupling device.

:: The Fiber Cable

• The cable is attached to the connector body.
• Typically, a strain-relief boot is added over the junction between the cable and the connector body, providing extra strength to the junction.

:: The Ferrule

• The fiber is mounted in a long, thin cylinder, the ferrule, which acts as a fiber alignment mechanism.
• The ferrule is bored through the center at a diameter that is slightly larger than the diameter of the fiber cladding.
• The end of the fiber is located at the end of the ferrule.
• Ferrules are typically made of metal or ceramic, but they may also be constructed of plastic.
• The most distinct differentiations between connector types are the diameter of the ferrule, 2.5mm or 1.25mm, and the type of polish.

:: The Fiber Connector Coupling Device – Mating Sleeves and Adapters

• Most fiber optic connectors do not use the male-female configuration common to electronic connectors.
• Instead, a coupling device such as an alignment sleeve is used to mate the connectors.
• Similar devices may be installed in fiber optic transmitters and receivers to allow these devices to be mated via a connector.
• These devices are also known as feed-through bulkhead adapters.

:: Fiber Optic Connector Performance Definitions

• Insertion Loss (IL)
  Insertion loss is the amount of optical power lost as a result of a connection. Expressed in decibels, it is the ratio of measured optical power before and after the connector. It always is tested because it is the most important connector parameter.
• Return Loss (RL)
  Return loss is a term applied to the light reflection in the connector’s interface that return to the source. The greater the absolute value, the better. Such as –60dB return loss is better than –35dB return loss.

  

• Back Reflection
  Back reflection represents the total accumulated light reflected back to the source along a link. This return of the light is due to different physical phenomena such as multiple connector back-reflections, bad splicing, etc. High back reflection can cause bad or harmful consequences such as light source wavelength fluctuations, output power fluctuations, or even damage the light source permenantly.

:: Fiber Connector Coupling Loss

• Connector loss is caused by a number of factors.
• Loss is minimized when the two fiber cores are identical and perfectly aligned, the connectors are properly finished and no dirt is present.
• Only the light that is coupled into the receiving fiber’s core will propagate, so all the rest of the light becomes the connector loss.

:: Types of Connection Polishing

• The polish on a fiber connector determines the amount of back reflection.
• Back reflection is a measure of the light reflected off the polished end of a fiber connector measured in negative dB.
• The Physical Contact (PC) polish is a flat finish of the connecting area.
• The Angled Physical Contact (APC) is at an 8° angle.
• An APC greatly reduces back reflections caused by the physical interface.

:: Fiber Optic Connector Termination Types

• Anaerobic Adhesive
  Anaerobic adhesive connectors use a quick setting adhesive. They work well if your technique is repeatable, but often they do not have the wide temperature range of epoxies, so they are only used indoors. Thus, generally used for factory terminations only.
• Epoxy/Polish
  These connectors are the simple “epoxy/polish” type where the fiber is glued into the connector with epoxy and the end polished with special polishing film. These provide a very reliable connection with low losses. They can be factory or field installed.
• Crimp/Polish
  Rather than glue the fiber in the connector, these connectors use a crimp on the fiber to hold it in. Early types offered “iffy” performance, but today they are pretty good, if you practice a lot. Expect to trade higher losses for the faster termination speed. And they are more costly than epoxy polish types.
• Pre-Polished
  Many manufacturers offer connectors that have a short stub fiber already epoxied into the ferrule and polished perfectly, so you just cleave a fiber and insert it like a splice. While it sounds like a great idea, it has several downsides. First it is very costly, 2 to 3 times as much as an epoxy polish type. Second, you have to make a good cleave to make them low loss.

:: Fiber Optic Connector Types

>> BICONIC Connector (FOCIS 1)

• The Biconic connector was developed by AT&T and became the de facto standard for long haul telecommunications.
• The Biconic connector features a cone-shaped tip, which holds a single fiber.
• It is non-metallic, using polymer and epoxy in its construction.
• Telcos have long since adopted other connectors, mainly the SC due to the drawbacks of the Biconic such as its large size and the fact that it is mated by screwing into its coupling.
• Screw coupling makes its performance sensitive to rotational changes.

>> ST Connector (FOCIS 2)

• ST stands for Straight Tip – a quick release style connector developed by AT&T. ST’s were the predominant connector in the late 80s and early 90s.
• ST connectors are among the most commonly used fiber optic connectors in networking applications. They are cylindrical with twist lock coupling, 2.5mm keyed ferrule.
• The ST connector has a bayonet mount and a long cylindrical ferrule to hold the fiber. Because they are spring-loaded, you have to make sure they are seated properly. If you experience high loss, try reconnecting.

>> SC Connector (FOCIS 3)

• The SC (Subscriber Connector) was developed by NTT specifically as a telecom connector.
• It features push-pull coupling which eliminates rotation which can damage fiber end-faces. This design also allows higher packaging density.
• An important element of the design is an isolated ferrule, which protects the ferrule and fiber from cable stresses.
• The SC is available in the usual simple configuration and with duplex adapters as well.
• For maximum density, quad and “six-pack” configurations are available.

>> FC Connector (FOCIS 4)

• FC stands for Ferrule Connector or Fixed Connection.
• The FC connector was developed by NTT as a single mode telecom connector.
• It uses a combination of thread (screw-on) and keyed design to provide high repeatability and good fiber end-face protection.

>> MTP/MPO Connector (FOCIS 5)

• The MPO connector family is defined by two different standards. International the MPO is defined by IEC-61754-7. In the USA, the MPO is defined by TIA-604-5 (FOCIS 5).
• The MTP multi-fiber connector is US Conec’s trademarked name for their MPO connector.
• The MTP connector is fully compliant with both FOCIS 5 and IEC-61754-7; therefore it is an MPO connector.
• The MTP connector design is distinctly different than the MPO.
• The MTP connector is a high performance MPO!
• The MTP/MPO is a connector manufactured specifically for a multi-fiber ribbon fiber.
• MPO = Multi-fiber Push On

The Comparison between MTP and MPO Connector.

>> LC Connector (FOCIS 10)

• LC is a small form factor (SFF) fiber optic connector.
• The LC connector uses a 1.25mm ferrule, half the size of the ST. Otherwise, it is a standard ceramic ferrule connector.
• The LC has good performance and is highly favored for single mode and LO multimode and has been gaining the preference of equipment manufacturers because of its compact size and performance.

>> MTRJ Connector (FOCIS 12)

• MTRJ stands for Mechanical Transfer Registered Jack. MTRJ connector is a small form factor (SFF) duplex connector with both fibers in a single polymer ferrule.
• MTRJ Connector uses pins for alignment and has male and female versions.
• MTJR connector is multimode only.
• The MTRJ connector resembles the RJ-45 connector used in Ethernet networks. The MTRJ was designed by AMP, but was later standardized as FOCIS 12 (Fiber Optic Connector Intermateability Standards) in EIA/TIA-604-12.

>> MU Connector (FOCIS 17)

• MU is a small form factor SC.
• MU has the same push/pull style, but can fit 2 channels in the same footprint of a single SC.
• MU was developed by NTT.
• The MU connector looks like a miniature SC with a 1.25mm ferrule.
• Currently it is a popular connector type in Japan.

>> Other Fiber Optic Connector Types

• SMA, D4, Mini-BNC, FDDI, ESCON, SCDC (Corning), Opti-Jack(Panduit), VF-45 (3M Volition), E2000/LX.5 …
• Proprietary – No license available.
• Old / Never adopted by equipment manufacturers.
• No wide spread acceptance in the market.

:: Fiber Optic Connector Applications

>> Private Networks (Enterprise)

• Small to medium networks
  ST, SC are predominant
• Large Networks
  ST, SC with LC growing rapidly
• Data Centers
  LC and MTP dominate

>> Public Networks (Service Providers)

• Telcos
  SC with LC growing due to density
• CATV
  FC, SC

>> Future Fiber Optic Connector Trends

• Higher performance
  10 gig, 40 gig, 100 gig
• Grater density (data centers)
• OSP capable (FTTH networks)
• FTTx Advancements
  — Fiber to the Home
  — Fiber in the Home
  — Fiber to the Wall Plate
  — Fiber to the Desk

Demand for ease of use, greater durability and repeatable performance over time will drive connector technology for decades to come!