Interconnect history, design, types, applications, polishing considerations and properties comparisons
Since the 1980s, there have been a myriad of fiber connectors in our industry and consistent improvements of the designs and offerings. Fiber optic connectors, also known as terminations, connect two ends of fiber optic cables. This allows for quickly connecting and disconnecting of fiber optic cables without splicing. The connector features a ferrule, the connector end piece that holds and secures the fiber and aligns it for light to pass through, it is a critical part of dependable fiber optic transmission.
There are fiber optic connector types for differences in applications, properties, and standards. As the connector world evolves, its history is as important to understand as the future innovations to come. Fiber Optic Center intends to offer a timeline, brief explanations and matrix options to see this information here in one place.
Fiber optic cables, used to transmit data using light over large distances, have been traditionally known in the telecommunications and broadband industries but more recently in the growing data center industry. Fibers in lasers are found in the medical, industrial equipment, military, and transportation industries. There are many different connectors for fiber optic cables because of these various applications and end user requirements dictating designs.
Connectors are designed for single-mode or multi-mode fiber optic cables. In fiber-optic communication, a single-mode optical fiber, or SMF, is designed to carry only a single mode of light used over long distances (greater than 500 m) like in interbuilding connections or WANs (wide area networks). In contrast, multi-mode cables are designed for use over shorter distances (less than 500 m) like in LAN (local area network) backbones within individual buildings. Multi-mode comes in two core sizes and five varieties: 62.5-micron OM1, 50-micron OM2, 50-micron OM3, 50-micron OM4 and 50-micron OM5. OM2 to OM5 are backward compatible with optimization to the fiber design to carry 850nm laser signals. Additional connector requirements dictate design include in-socket stability, environmental durability and protection (long vs short boots) and dual (duplex) vs single (simplex) connections. Cabling in data centers have their own specific recommendations and industry standards continue to evolve. With such diversity in the physical cables, transmission speed and distance needs, the connector designs are also diverse. Understanding these differences is critical.
Design of Fiber Optic Connectors
The type of fiber optic connector will dictate a unique design, but all have four main parts:
- ferrule: the core for the alignment of the optic fibers
- connector: the body supports the ferrule
- mechanism: connects the connector body and optical equipment
- boot: protects for fiber optic cable from external damage by providing strain relief
Additional parts of a fiber optic connector, using the SC Connector as an example, have:
- dust cap: protects fiber end of cable when not in use
- connector housing: covers the connector sub-assembly
- connector sub-assembly: keeps the fiber, ferrule and cable in place
- crimp eyelet: secures the connector to the optical fiber cable jacket
To intermate two fiber optic connectors, almost all fiber optic connectors require an adapter. In many cases this adapter has an alignment mechanism to line up the 2 ferrules. In other cases, the alignment mechanism is part of the connector / ferrule design. The main outliers in this case are military style connectors. For these applications the 2 ends are usually referred to as pins/sockets.
Figure 1: SC connector parts: dust cap, connector housing, fiber ferrule, connector sub-assembly, crimp eyelet, boot
Designs of housing for fiber optic connectors usually differ in how they secure two connections together. Various ways to connect include but are not limited to snap together technology, latches, bayonet design, spring-loaded with constant force and plug-ins for mating adapter and screw to hold together. The objective of a fiber optic connector is to provide some force to hold the connection securely in place to reduces the chance of cables accidentally pulling apart during installation and/or regular use, while still allowing fast connecting and disconnecting.
The trend to smaller footprint connectors is fueled by the need for reliable connections and increased density in telecommunications and data center applications.
Fiber Optic Cables
The choice of fiber optic cables is the first question to be answered. There are two basic types of fiber optic cables. You will either be using single-mode or multi-mode. Advances in technology offer fiber connectors compatible with both single-mode and multi-mode fibers, like the FC and LC, but an understanding of the differences is still critical for many.
In all telecommunication applications, the outer fiber (cladding) diameter is standardized at 125um. The fibers have buffer diameters, most commonly 900um (tight buffered) and 250um (loose tube), depending on the application. Some intermediate buffer diameters are used in small diameter duplex fiber cables. The optical fiber itself differs mainly in the core size.
Single-mode fiber optic cables have a smaller central core than multi-mode cables that allow one light mode to pass through the fiber at one time. Single-mode cables can be identified by a jacket color of yellow or blue.
Multi-mode cables have a larger central core than the single-mode to allow for different rays of light or multiple light modes to pass through the fiber. Multi-mode cables can be identified by a jacket color of aqua, orange or green.
Fiber connectors will come in single-mode and multi-mode and though they are also usually also identifiable by color (single-mode fiber connectors a blue or green cover and multi-mode fiber connectors an aqua or beige cover) this is not required for manufacturers to follow so verification would be needed with the supplier before procurement.
Termination Types
Choices for type of connector terminations, quick termination or epoxy and polish termination will depend on the installation requirements.
- Anaerobic Termination: fiber connector with a quick-setting glue for indoor fiber optical applications. The glue is not appropriate for water or high temperatures.
- Pre-Polished Termination: connector with a pre-polished fiber end inside the body. Requires stripping the fiber and cleaving before inserting into the body. It is quick and expensive.
- Epoxy and Polish Termination: ferrule is bonded with epoxy and end is polished with lapping film. This is high quality, low cost, good stability but requires more time to install. Epoxy and polish termination is considered the traditional termination.
All factory terminated fiber optic assemblies use epoxy and polish terminations.
Polish Types
Second step after identifying single-mode vs. multi-mode fiber optic cables types is polish type. The type of polish on a fiber can impact insertion loss and back reflection, both critical to the overall performance of a fiber optic system. Insertion loss is the loss of fiber optic light due to misalignment when connecting equipment. Back reflection, also called reflectance or optical return loss, is the light being reflected to the source from where it came from. Bottom line is the quality of the polish is directly related to the insertion loss and back reflection performance. Light travels down the fiber to the mated connection where there is an air gap between the two surfaces and potential loss of light signal depending on the type of polish used on the fiber connector.
There are three main types of polishes that are currently used: physical contact (PC), ultra physical contact (UPC), and angled physical contact (APC). Insertion loss and back reflection vary between these types. The goal is to obtain the highest reflectance loss and best fiber connection (measured as negative values). Typical reflectance loss values for each connector polish type improves from PC to UPC to APC Polishing:
Figure 2: Reflectance loss values by connector polish type
PC Polish
PC polish, or Physical Contact, is the first improvement on the original flat polish. This is the most basic contact or surface polish on a fiber connector and most common, polish type that is found on OM1 and OM2 multi-mode fiber. It was first developed in the 1980s using a slight cone design and curvature to the end face to reduce air gaps in the original flat fiber connector and achieve better insertion loss (IL) and optical return loss (ORL).
The typical back reflection is -40dB (lower number is better performance) and the typical insertion loss between 0.20 - 0.30 dB (lower number is better performance). PC is most common in telecom fiber optics due to lower demands and impact by ORL and standard used for connectors on all multi-mode fiber and MTRJ Connectors.
UPC Polish
UPC polish, or Ultra Physical Contact, an improvement of PC polish. UPC Connectors are the most common end face polish, covering most of the fiber optic application needs. The UPC design extends the polish of PC Connectors for tighter shape, improving physical contact, to reduce air gaps and lower ORL. The LC-UPC, and SC-UPC Connectors are the most common in today's fiber installations. UPC Connectors can be joined with PC and other flat connectors but not with APC Connectors. It has a similar shape to PC but with lower back reflection. This is achieved by improved final polishing films.
As noted above, the typical back reflection is -55dB and the typical insertion loss between 0.20 - 0.30 dB. UPC is most common in digital TV, digital phone, and data systems. With a better fiber surface finish than PC polish that results in lower optical return loss (ORL) it is the standard polish used for connectors on single-mode fiber. Insufficient cleaving and polishing techniques will cause poor performance in a UPC Connector, not its design. The Fiber Optic Center, Inc. technical team consults with customers on processes for quality cleaving and polishing because back reflection (ORL) in UPC Connectors will depend on that. UPC Connectors are not flat, they have a slight curvature for better core alignment and polished with no angle.
APC Polish
APC polish, or Angled Physical Contact, is the connector polish, found in single-mode fiber, with lowest back reflection and repeated connections. This is achieved by polishing an 8 degrees angle on the end face, reflecting light back to the cladding instead of through the core back to the light source. This design reduced back reflection by great amounts when compared to UPC Connectors. It commonly has the green connector housing to indicate APC (connector housing blue = single-mode UPC Connector and connector housing green = single-mode APC Connector).
As noted above, the typical back reflection is below -65dB (lower number is better performance) and the typical insertion loss between 0.25 - 0.35 dB. APC is considered the best polish offered and used in high-speed passive optical applications such as DAS, FTTH, CCTV.
APC Connectors can only be mated with other APC Angled-Polished Connectors or there will be significant return loss, and insertion loss causing poor performance and data loss. They cannot be joined or mated with PC, UPC, PC or flat connectors.
APC should be the first consideration applications that require high precision optical fiber signaling. UPC is a good choice for most optical fiber applications that do not require precision optical fiber signaling.
Due to the angle, proper polishing processes are a must to meet the end face geometry requirement for APC Connectors.
Types of Fiber Connectors
There are hundreds of optical fiber connector types partly to ensure options for compatibility in different applications.
Optic Fiber Connector Types
Fiber cable connector types are differentiated by fiber counts, boot lengths, polishing methods, termination types and application needs. Because of those, there are more than one hundred types of optical fiber connectors to choose from. The most common connector types are SC, LC, ST, FC, and MPO/MTO in the fiber optic market.
The SC Connector is the most popular due to it being considered the easiest to install and use with its push-pull latching mechanism. Both SC and LC Connectors have systems designed to accommodate them based on their popularity.
The following is a list of the most common connectors Fiber Optic Center, Inc. works with:
SMA Connectors
SMA, sub-miniature type A, connectors are coaxial RF Connectors known for its threaded, “screw-type” coupling mechanism suitable for multimode, not single-mode fibers. SMAs are key for instruments, products and procedures that use specialty fibers.
Most common applications include medical for laser delivery and other surgeries, spectroscopy, data and video transmission, cutting, welding, and sensing.
Fiber Optic Center, Inc. (FOC), has SMA connectors in over 40 holes sizes from 127 to 2645 um, in volumes of as little as 10 to over 20,000 pieces, depending on design and hole size.
An overview of the SMA Connector history in the fiber optic industry and its future in instruments, products and procedures which use specialty fibers can be found in our article, What Is an SMA Connector and Why Do We Care?
MPO, MTP® Connectors and MT Ferrules
MPO fiber connectors, also known as Multi-Position Connectors or Multi-Position Optical Fiber Connectors or Multi-fiber Push On, are interchangeable with the term MTP® Connectors (Multifiber Termination Push-on). MTP® is the commercial or marketing brand of MPO connectors. MT Ferrules are a type of ferrule used in high-density optical fiber connectors, such as MPO Connector (Multi-fiber Push On) and MTP® (Multifiber Termination Push-on) commonly used in data centers, telecommunications networks, and other high-bandwidth applications. The MT Ferrule offers improved optical performance, higher fiber count, and reduced cabling space requirements.
Assembly of the MPO or MTP® Connector requires specific equipment and processes. Fiber Optic Center provides all the industry recommended MPO and MTP® Connector materials and equipment designed specifically for these assemblies. Integrating our manufacturing knowledge into our customers’ worldwide operations begins with the best practices and selections listed here:
- MMC Connector Assembly using the new TMT Ferrule specific fixturing and tooling
- MPO Connector Assembly specific fixturing and tooling
- MTP® Connector Assembly specific fixturing and tooling
- Connector Assembly using the MT Ferrule specific fixturing and tooling
LC Connectors
LC Connectors or Lucent Connectors have also been called the “Little Connector” or “Local Connector” around the industry. LC Connectors were developed by Lucent Technologies as a SFF (small form factor) connector. Unlike the FC, SC and ST Connectors with a fiber diameter of 2.5mm, the LC Connector has a fiber diameter of 1.25mm. LC Fiber Connectors are favored in data center and enterprise networks and SC Fiber Connectors are favored in telecommunications networks.
Assembly of the LC Connector is similar to the assembly of SC Connector, however due to its 1.25mm fiber diameter, there is specific equipment and processes needed to assemble the LC.
For additional Information on LC Connectors, refer to our resource article: LC Connectors Explained
SC Connectors
SC Connectors, or standard connectors, can also be called a square connector or subscriber connector or Sam Charlie Connector. SC Connectors were developed by NTT (Nippon Telegraph and Telephone), a Japanese telecom company, under the original name “Subscriber Connector”. Known for its low cost and push-pull mechanism, SC has become the popular choice for PONs (Passive Optical Networks), converters and telecommunications over ST Connectors. Most of these applications use SC Connectors with APC Ferrules.
Assembly of the SC Connector is similar to the assembly of LC Connector, however due to its ferrule diameter, there is specific equipment needed to assemble the SC.
For additional Information on SC Connectors, refer to our resource article: SC Connectors Explained
FC Connectors
FC Connectors, or Ferrule Core Connectors, can also be called a “Fiber Channel” or “Ferrule Connector” or “Frank Charlie” Connectors at assembly houses and in the fiber industry. FC Connectors were developed by NTT (Nippon Telegraph and Telephone), a Japanese telecom company, under the original name “Field-Assembly Connector”. FCs are known for being the first built with a ceramic ferrule and can be attached with most other fiber connector types, like SC and ST. Most FCs have stainless steel bodies, but some have plastic bodies. It is ideal for single mode fibers due to the alignment key and screw-design motion. Assembly of the FC Connector requires specific equipment and processes.
ST Connectors
ST Connectors, also known as the "Straight Tip" Connector, created and licensed by AT&T, shortly after the arrival of the FC type, were one of the first fiber connector types. STs have seen widespread adoption in fiber optic networks globally. It features a bayonet-style lock and a 2.5mm ferrule, making it easy to connect and disconnect.
ST, LC and SC are the three main types of fiber optic connectors. The ST Connector is a good choice for applications where durability and reliability are important, such as in industrial environments. Assembly of the ST Connector requires specific equipment and processes.
E2000 Connectors
The E-2000 Connector series is a type of fiber optic connector with a 1.25 mm ferrule. E-2000® is an optical fiber plug connector, available for both single mode and multi-mode applications, featuring a spring-loaded shutter and dust-cap. Installing an E-2000 Connector requires a fiber optic cleaver and a fiber optic terminator tool.
Application uses include data centers, telecom, medical, military, and aerospace in addition to traditional WANs, LANs, CATV and general telco systems.
MU Connectors
Like a miniature SC with a 1.25mm ferrule. Featuring a simple push-pull design and compact miniature body, the MU Fiber Optic Connector is used for compact multiple optical connectors and a self-retentive mechanism for backplane applications.
The MU, or Miniature Unit Connector has a 1.25mm ferrule diameter like the LC Connector although one can’t help but notice it is similar in size and design to the SC Connector but with a smaller footprint allowing for more connectors to be installed in a given space. Developed by NTT in the late 1990s as a small form factor connector for telecommunications and data communication systems, the MU Connector is unique for its ability to fit two channels in the same footprint as one SC Connector.
MTRJ Connectors / MT-RJ Connector
MTRJ (Mechanical Transfer-Registered Jack) Fiber Optic Connectors are duplex connectors developed originally by AMP/Tyco and Corning. MTRJ Connectors were the first with multiple fibers inside the same ferrule.
MTRJ / MT-RJs are designed to support high-density connections similar to the standard RJ45 Connector used in Ethernet. They are known for their compact size and high-performance and are typically used in data centers and high-bandwidth applications.
DIN Connector
DIN Fiber Optic Connectors, also known as LSA Connectors, are made of precision screw machined nickel plated brass for consistent performance and durability. It’s pins are arranged in a circular pattern and feature a spring loaded floating zirconia ferrule.
DIN Connectors are widely used in telecommunications, CATV, LAN, WAN, medical and sensors with Telecordia-GR-326-CORE Compliance.
D4 Connectors
The D4 Connector is an older generation single-mode fiber optic connector that is spring loaded and keyed with a 2.0mm precision zirconia ferrule. The screw-on mechanism is a threaded nickel-plated brass body. The key prevents opposing ferrules from touching while mating.
D4 Connectors are available with a APC (Angled Physical) ferrule endface or a PC (Physical Contact) pre-dome ferrule endface. There is also a crimp free or crimp style version.
Applications for the D4 Connector include telecom networks, WANs (Wide area networks), and LANs (Local area networks).
Industrial and Broadcast Connectors
Industrial and Broadcast harsh environment fiber optic connectors, termini and cable assemblies have rugged construction that can handle the most severe environments and provide dependable performance.
Plastic Fiber Connectors
Plastic Fiber Connectors are fiber connectors. They use a simple latching and non-latching, that work on a snap-together concept removing the need for crimping.
Plastic optical fiber (POF) or polymer optical fiber is simply an optical fiber made from polymer. Fiber optics is the core competency of Fiber Optic Center but POF is not only similar to glass optical fiber, transmitting light through its core for illumination or data, it has advantages for some unique applications that need greater strength when bending and stretching then glass can provide.
OptiTap
OptiTap Connectors are designed for outdoor applications and environmental protection including extreme temperatures, harsh conditions, humidity, and moisture. As hardened connectors for outside plant (OSP) applications, they are most often used to deliver fiber from fiber-to-the-premises (FTTx) to fiber-to-the-home (FTTH) and meet Telcordia standards. Fiber Optic Center offers both an Opti-tap Compatible SM SC/APC Ruggedized Connector 125.5um and can offer technical information and source OptiTap® and OptiTip® for our customers.
Telcordia United States standards are critical for successful deployment in outdoor applications. Extreme environmental conditions from wide temperature ranges (-40°C /-40°F to +70°C / +158°F, and environmental factors like dust, water penetration, extreme humidity, vibration, and hurricane resistance dictate new designs and testing criteria.
OptiTap® and OptiTip®, are connectors from Corning, designed for drop cable portions of a network. We often are asked about the differences because their names and branding are so similar.
Both OptiTap® and OptiTip®, are used for outdoor applications, designed for environmental protection including extreme temperatures, harsh conditions, humidity, and moisture. As hardened connectors for outside plant (OSP) applications, they are most often used to deliver fiber from fiber-to-the-premises (FTTx) to fiber-to-the-home (FTTH) and meet Telcordia standards.
OptiTap® is a hardened connector based on the SC inner housing; 2.5mm ferrule.
- Telcordia GR-3120-CORE for single-fiber OptiTap® Connectors
OptiTip® is a hardened connector based on the MT ferrule (same as in MPO/MTP).
- Telcordia GR-3120 GR-3152-CORE for multifiber OptiTip® Connectors
Telcordia United States standards are critical for successful deployment in outdoor applications. Extreme environmental conditions from wide temperatures ranges (-40°C /-40°F to +70°C / +158°F, and environmental factors like dust, water penetration, extreme humidity, vibration, and hurricane resistance now define the new designs in order to pass the testing criteria.
Quick Termination Connectors
Quick Termination Fiber Connectors are factory polished connectors that can be terminated in the field without any epoxy or polishing.
Boots, Dust Caps, Ferrules, Housing
A connector boot is a protective covering used to provide strain relief, maintain proper bend radius and shield the connector and cable from wear and fraying of the cable caused over time. Jacketing Sizes range from 2mm, 3mm and 900um to adjustable. The color of the connector boot can indicate type of fiber or help indicate network path.
Fiber connector dust caps prevent environmental contamination and play an important role in network stability. Protective dust caps protect from contamination, by dirt and other environment sources, that damage fiber optic connectors.
Ferrules, the tip of the connector secures the fiber and provides a stable surface for polishing. The ferrule is used with mating adapter or device receptacles to precisely align and mate two connectors together or terminate into a device.
A connector housing is typically made from molded plastic or metal and provides the mechanism to secure connections and protect the internal parts of the connector. Housing color can indicate fiber type and if the connector has an angled end-face. Some housings are keyed to ensure proper orientation when mating with other connectors.
Fiber optics are critical in several applications including but limited to Communication Systems, Telecommunications, Internet Systems, Cable Television, Inspection Devices, Medical Applications, Lighting, Security Systems, Public Utility Networks, Sensors Infrastructure, Industrial Networking, Automotive, Military Networks, Aerospace Applications and Sensor Infrastructures. Understanding the correct material, equipment and process for assembly in each of these applications is critical. The connectors in these networks are the quintessential link enabling reliable transmission of data and signals. Connectors are vital to most industries and current global necessities.
