Guide to Fiber Optic Connectors

Optical fiber connector is a device for detachable (movable) connection between optical fibers,It precisely connects the two end faces of the optical fiber to maximize the coupling of the optical energy into the receiving optical fiber and minimize the impact of the optical link on the system.

Fiber optic connectors include plugs that feature a ferrule that holds the fiber in place and an associated adapter for aligning and mating fibers to form a connection point.

In the 1970s, the ferrule was composed of metal or hard plastic, after decades of development, ceramic ferrules were introduced to improve alignment and provide lower insertion loss performance, lower cost, higher density, and easy to terminate.

The optical fiber connector is composed of three main components: the ferrule, the connector body and the coupling mechanism.

Ferrule: it is a thin structure that actually fixes the glass fiber. It has a hollowed-out center that can hold the fiber tightly. the ferrules is usually made of ceramic, metal, or high-quality plastic.

Connector Body: It is a plastic or metal structure used to fix the ferrule and attaches to the jacket and strengthens the fiber cable itself.

Coupling Mechanism: It is a part of the connector body which fixes the connector in place when connected to another device, It may be a latch clip, a bayonet-style nut, or similar device.

Optical performance is the most important characteristic of optical fiber connectors,In addition, interchangeability, repeatability, tensile strength, temperature, and plug-in times should also be considered.

Optical properties: Includes insertion loss and Return loss

Insertion Loss(IL): It is the connection loss, which refers to the loss of the effective optical power of the link. The smaller the insertion loss, the better, and the value should generally not exceed  0.5dB.

Return Loss (RL): refers to the ability of the connector to suppress the reflection of link optical power, and its typical value should not be less than 25dB. In practical application, the surface of pin is specially polished, which can make the return loss bigger, generally not less than 45dB.

Interchangeability & Repeatability

Optical fiber connectors are universal passive components. Generally, the same type of optical fiber connectors can be used in any combination and can be used repeatedly, and the loss is generally less than 0.2dB.

Tensile Strength

The fiber optic connector generally requires a tensile strength not lower than 90N.

Temperature

The optical fiber connector must be able to be used normally under the temperature of -40°C~+75°C.

Pluggable times

Basically, optical fiber connectors can be plugged and unplugged more than 1,000 times.

Optical Performance

Environmental and Mechanical Performance (FC, SC, ST, LC, MU)

While there are many different types of fiber connectors,which can be classified according to different standards such as Fiber Count, Connection methods, Transmission media, and Polishing types.

Fiber Count: Simplex vs Duplex Fiber vs Multiple fiber Connectors

According to the ITU-T definition, a simplex fiber is a fiber in which signals can only flow in one direction at a time. One side is the transmitter and the other side is the receiver, which is irreversible.

On the other hand, a full duplex system uses two optical fibers for communication. This allows one fiber to transmit from point A to point B, and another fiber to transmit from B to A. Therefore, there are transmitters and receivers at both ends of a full duplex system.

Simplex connectors usually connect a glass or plastic optical fiber, while duplex optical fiber connectors need to connect two optical fibers.

ST/SC/FC/LC/MU/E2000/SMA/DIN/BICONIC connectors often use single fiber connectors,MT-RJ/VF-45/Optci-Jack/FDDI/ESCON are use duplex fiber connectors, MTP/MPO connector is a little bit different, they use Multiple fibers , as shown in the picture below.

Connection methods

The commonly used are FC, SC, ST, LC,MTP/MPO connectors and MT-RJ, MU, DIN,E2000,Biconic,FDDI,Deutsh 1000,SMA,Volition Opti-Jack fiber optic connectors are less popular now, which will be described in detail in the following.

Transmission media: Single Mode vs Multimode Fiber

Single-mode optical fiber has a small core diameter through which only one mode will propagate. while multimode means the fiber can propagate multiple light modes.

Single mode fiber uses a 9/125 connector, which refers to the core and cladding diameter of the optical fiber (i.e. the core is 9 µm and the cladding is 125 µm) that provides higher transmission speeds and longer distances compared to multimode fiber.

Multimode fiber requires a 50/125 µm (OM2/OM3/OM4) or 62.5/125 (OM1) connector.

Optical fiber connectors such as SC, LC and FC are compatible with both single mode and multimode fiber cables.

Ferrule Diameter

FC, SC, ST fiber connectors are 2.5mm Ferrule Diameter, on the other hand, LC, MU, E2000 are 1.25mm Ferrule Diameter as shown below.

Polish type: APC/PC/UPC Fiber Optic Connectors

Optical fiber cable connectors can be divided into three types: PC, UPC, and APC connectors. Each polished type exhibits a different level of back reflection, back reflection is a measure of the light reflected off the end of a fiber connector, this light is measured in decibels. Regarding the difference between PC, UPC, and APC connectors, we will discuss later.

Optical fiber connectors are available with differently polished endfaces, which will affect level of return loss (back reflection) of the connector.

When the connecotr is installed on the end of the fiber,the light will reflect from the optical fiber back to the source, thereby disturbing the singal, PC/UPC/APC polish technology is designed to improve the efficiency of the optical fiber.

PC

PC (Physical Contact) polishing method is the most common polish style, usually used for OM1 and OM2 multimode fibers. PC fiber connectors are designed to minimize the air gap between the connector surfaces. In this polish style, the ferrule of the connector is shaped like a slightly cylindrical head, helps to minimize the air gap between the two surfaces of connectors. Through physical contact polishing, the return loss can be minimized down to -40dB – better than  the original flat polish style which is approximately -14dB, Nowadays, the PC polish style is considered to be outdatd, and the UPC style is becoming popular.

UPC

UPC (Ultra Physical Contact) connector using the extended polishing method creates a  finer fiber surface finish, which results in a lower back reflection (ORL) than standard PC connectors, nearly -50dB or higher, allowing it to be used in digital TV, telephony and data systems, However, it is not good enough as the overall performance of the connector degrades due to frequent connections and disconnections.

APC

APC refers to Angled Physical Connect, the ferrule is polished with an 8-degree angle that end-face brings the fibers tighter and reflects light at an angle into the cladding instead of reflecting it directly to the light sources, which causes its Optical Return Loss to be -60dB or higher.

Differences between PC,UPC,APC

The major differences between PC/UPC/APC  in terms of applications, appearance, and performance, as shown in below table:

     Note: Due to the angle of an APC connector, it cannot be mated to a UPC connector. they are not interchangeable, may cause poor performance or damage the connectors.

As we mentioned before, there are many different types of fiber optic connectors, choosing the right type depends on the equipment and type of optical fiber you are installing, these connectors are covered by EIA/TIA-604 Fiber Optic Connector Intermateability Standards (FOCIS).Here is a rundown of the different types of fiber optic connectors that commonly used.

SC  (Standard Connector or Subscriber Connector)

SC is Developed by Nippon Telegraph and Telephone (NTT), covered by the TIA connector intermateability standard FOCIS-3 (TIA-604-3). the SC connector is easily recognized by its square shape, It is a snap-in connector with a 2.5mm ferrule that using a push-pull action with a spring-loaded ferrule.  simplex or duplex available, easy to snap on, it is a popular choice for such equipment like Fiber Media Converters, Industiral Ethernet Switches.

SC Connector use on Fiber Media Converter

FC  (Ferrule Connector)

Unlike the square SC connector, the FC is a round threaded connector and uses a stainless-steel shell around the ceramic ferrule. Covered by TIA-604-4 (FOCIS-4), the FC uses an alignment key and is firmly tightened with a screw-like movement, Once the connector is installed, its position is maintained with total precision, suitable for vibration environments, FC connector is usually used in Video over Fiber Transmission Equipment or ODF(Optical Distribution Frame), simplex or duplex available,It's slowly being replaced by SC and LC.

FC connector use on Video Over Fiber Converter

ST (Straight Tip)

ST connector is comply with TIA-604-2 (FOCIS-2) standard, introduced by AT&T in the late 1980s. It has a bayonet mount and a long cylindrical 2.5 mm ceramic (usually) ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic.available in single mode and multimode. because STs are spring-loaded, you have to make sure they are seated properly.

ST Connector

LC (Lucent Connector)

LC is one of the most popular Small Form Factor (SFF) connector, licensed by Lucent Technologies provides a pull-proof design and small size perfect for high-density applications. Available in simplex or duplex versions. The LC connector is provided with a 1.25mm ferrule.(half the size of SC, ST and FC), use a unique latching mechanism providing stability,low insertion loss performance, and easily terminated with any adhesive, The SFP modules we often see all use LC connetor.

LC connector use for SFP Transceivers

MPO (Multi-fiber Push-On)

MPO fiber optic connector is comply with IEC-61754-7; EIA/TIA-604-5 (FOCIS 5) standards, MPO is a 12 fiber connector for ribbon cable, often called an array connector. the connector ferrule is plastic and contains 12 or 16 fibers in a row.Fiber ferrule alignment is by pins in one ferrule and holes in the other.the most common fiber counts are 8, 12, 16, and 24, 32, 48, it often used for higher speed multimode links (40 or 100Gb/s) that use parallel optics.

MTP  connector stands for "Multi-fiber Termination Push-on", made by US Conec, it is an improvement of the original MPO connector designed by NTT. designed to terminate several fibers (up to 12 strands)in a single ferrule, hold in place by a push-on/pull-off latch, and can also be distinguished by a pair of metal guide pins that protrude from the front of the connector.

Because of the high number of fiber strands available in a small connection, MPO/MTP are used for backbone, cross-connect, and breakout applications.

Difference Between MPO and MTP: MPO is a fiber connector type while MTP is a registered trademark of an MPO connector manufactured by US Conec. All MTPs are MPOs but not all MPOs are MTPs.

MT-RJ

MT-RJ stands for Mechanical Transfer-Registered Jack, is another popular SFF connector,covered in the TIA connector intermateability standard FOCIS-12 (TIA-604-12).

MT-RJ is a multimode duplex connector that contains both fibers in a single polymer ferrule. It uses pins for alignment and is available in male and female versions,field terminated only by prepolished/splice method.

MT-RJ Connector

SMA Connector

SMA means "Subminiature A",microwave connector,the SMA 905 and 906 fiber optic connectors are available with stainless alloy or zirconia ferrules. The stainless alloy ferrule can be drilled from 240μm up to 1550μm to accept various fiber sizes,this connector was eventually replaced by the ST, and later by SC connectors, but still in use on some military and industrial systems.

SMA Connector

MU Connector

MU fiber optic connectors covered by the TIA connector intermateability standard FOCIS-17 (TIA-604-17), it looks a miniature SC with a 1.25 mm ferrule. It's more popular in Japan and the far east.

The connector is square and employs a push-pull mating mechanism,designed for high-density connections, the single-fiber connector has a high level of performance, providing more than double the packaging density of the SC connector.

MU Connector

E2000 Connector

The E2000 connector is a latched push-pull locking connector, trademarked by Diamond SA based in Losone Switzerland. it contains an integrated spring loaded shutter that protects the ferrule from dirt, dust and scratches. As this connector uses a monobloc ceramic ferrule, problems associated with different co-efficient of expansion are eliminated.

The E2000 connector supports color keying and its return loss of 0.1 db is one of the lowest in the industry. This connector is widely used in broadband applications, telecommunication networks, Fiber CATV, Fiber-to-the-Home [FTTH] and data networks.

E2000 Connector

BICONIC Connector

Biconic connector is covered in the TIA connector intermateability standard FOCIS-1 (TIA-604-1).Developed by a team led by Jack Cook at Bell Labs in Murray Hill, NJ, the colour of Single mode is yellow and Multi-mode is usually black. the Biconic was molded from a glass-filled plastic that was almost as hard as ceramic.

Biconic started with the fiber being molded into the ferrule until the company could get a 125 micron/5mil pin insert into the plastic mold, at which point the fiber was glued into the ferule with epoxy. When singlemode versions first appeared, the ferrules were ground to center the fiber core in the ferrule to reduce loss. Since it was not keyed and could rotate in the mating adapters, it had an airgap between the ferrules when mated, meaning loss was never less than 0.3 dB due to fresnel reflection. Usually MM Biconics had losses of 0.5-1 dB and SM 0.7 dB or higher.

BICONIC Connector

FDDI Connector

FDDI stands for Fiber Distributed Data Interface, and it actually refers to a local area network standard such as Ethernet or Token Ring. It contains two ferrules in a large, bulky plastic housing that uses a squeeze-tab retention mechanism.the connector contains a 2.5mm ferrule that can mate onto ST and SC connectors by making use of adapters.

FDDI Connector

Volition Connector

Volition connector is covered by the TIA connector intermateability standard FOCIS-7 (TIA-604-7).it is a slick, inexpensive duplex connector that uses no ferrule at all. It aligns fibers in a V-groove like a splice. Plug and jack versions, but field terminate jacks only.

Volition Connector

Opti-Jack Connector

The Panduit Opti-Jack is covered by the TIA connector intermateability standard FOCIS-6 (TIA-604-6).it is a neat, rugged duplex connector resembles the universal RJ-45 connector, contains two ST-type ferrules, and it is available in both male and female versions.

Opti-Jack Connector

Deutsch 1000 Connector

Deutsch 1000 was maybe the first commercially successful fiber optic connector. It was actually a "pin vise" holding a stripped fiber. The nose piece is spring loaded and is pushed back when the connector was inserted into a mating adapter. The fiber stuck out into a drop of index matching fluid on a plastic lens. it was popular in the late 70s,  the insertion loss is about 3 dB.

Deutsch 1000 Connector

D4 Connector

The NEC D4 was probably the first connector to use ceramic or hybrid ceramic/stainless steel ferrules. It uses a smaller ferrule than SC or FC connector. It was widely used in telecommunication networks in the 80s to early 90s and some may still be in use.

D4 Connector