A fiber optic connector is designed to join optical fibers to route the light signal. It features a protruding ferrule to hold the fiber and enables you to connect or disconnect the fiber cables easily. Also, it has a security mechanism, such as a screw, bayonet, latch, push/pull, and snap-in, to align and mate fibers to form a point of attachment.
Since the 1970s, 100 fiber optic connectors have been introduced to the market. As only a few of them represent most of what you may encounter in the telecommunication field, you need not know all their details. First, let us start from the beginning.
In the 1970s, the first ferrules were introduced with a metal or hard plastic material. By the late 70s, the Deutsch 1000, probably the first commercially successful fiber optic connector, was spring-loaded and yielding about 3 dB loss. But since the mid-1980s, with the advent of the ceramic ferrule, the connector design was changed forever because the ceramic ferrule improved alignment and reduced insertion loss. With the introduction of SFF (Small Form Factor) in the late 90s, the LC fiber connector gradually dominates the market.
Return loss VS Insertion loss
Before digging into a more profound understanding of the fiber optic connector, you should know the difference between return and insertion losses. The insertion loss and return loss indicate the performance of the optic fiber connector.
Remember, insertion loss refers to the loss of signal power due to the insertion of a device in fiber optic. Usually measured in decibels(dB), the IEC standard 61753-1 lists five grades for insertion loss, from A (best) to D (worst) and M for multimode. We use insertion loss to determine loss budgets, and the lower insertion loss represents better performance. A good performance of insertion loss should not exceed 0.75 dB.
The Return loss is different than insertion loss. Also referred to as back reflection, it is “a measure in relative terms of the power of the signal reflected by a discontinuity in a transmission line or optical fiber.” This loss of signal power often is the result of an impedance mismatch. Measured in negative dB value, the higher the return loss value is, the lower the reflection, and the better the performance.
The IEC standard 61753-1 gives five grades for return loss from 1 (best) to 5 (worst).
Generally speaking, a good design has these features:
- Low insertion loss: a maximum insertion loss of 0.75 dB
- High return loss: the higher, the better. The minimal return loss is ≥26 dB for singlemode fiber and ≥20 dB for multi-mode fiber.
- Low cost
- High density
- Easy termination
- Low environmental sensitivity
Types of fiber optic connector
As for the types of fiber optic connectors, we could classify them in terms of fiber count, fiber mode, color coding, polish type, ferrule diameter size, utilization with or without adapter panel, termination, and material. Next, we’ll give an outline of these categorizations.
Fiber Count: Simplex VS Duplex
The Simplex means that a light signal could only go a one-way path from start to finish. It can’t turn back from the start. Correspondingly, the Duplex means a two-way channel. A fiber optic connector comes in either simplex or duplex configuration.
Fiber Mode : Singlemode VS Multimode
The singlemode fiber could only admit one light mode to pass through at a time; meanwhile, the multimode fiber could transmit multiple modes simultaneously. The singlemode fiber typically has a 9 µm core diameter, while the multimode fiber core diameter is 50 µm and 62.5 µm traditionally. The former has nearly unlimited bandwidth, while the latter’s maximum bandwidth is 28000MHz*km of OM5 fiber.
The singlemode fiber is suitable for long-distance beyond 550m; the multimode fiber is appropriate for short-distance runs.
Usually, the singlemode fiber costs more than the multimode fiber.
People use different colors to distinguish other fiber optic connectors from their origin. At first, orange, black or gray was multimode and yellow singlemode. But as time went by, the color-coding conventions underwent modifications. According to the ANSI/TIA-568:
- Aqua: 50/125 Multimode (OM3/OM4, compatible OM2)
- Blue: Singlemode UPC
- Green: Singlemode APC
- Beige, variable: 62.5/125 Multimode (OM1)
- Black, variable: 50/125 Multimode OM2
Keep in mind that those mentioned above are only general guidelines. Always first check the specs.
Polish Type: PC VS UPC VS APC
Do you wonder what’s the -PC, -UPC, or -APC suffix onto the name of fiber optic connectors? For example, an SC -APC connector. And what’s the difference between PC, UPC, and APC?
In fact, these words refer to the polish types of the fiber optic connector’s endface. To keep light that reflects from the interface from spreading back up the fiber towards the light source, many manufacturers use a physical contact polish technology to lead the reflected light out into the cladding. Given the difference in the angle polished, there are three kinds of polish types, i.e., PC, UPC, and APC.
PC Fiber Connector
The PC stands for physical contact, which indicates that the connector is polished in the physical contact style. It was designed to overcome the air gap generated from the contact surface of the initially flat polish fiber connectors. Due to minor design imperfection, the flat polish fiber connector had a -14 dB or roughly 4% return loss. So physical contact polish style was introduced. As a result, the PC fiber connector significantly improved the return loss up to -40dB. This polish style once was commonly found on OM1, OM2 multimode fiber. The PC polish style was outdated as technology advanced, and its next-generation UPC was developed.
The color code of the PC is black.
UPC Fiber Connector
The UPC stands for Ultra Physical Contact. Compared to the PC fiber connector, it has a better surface texture, and its return loss is -50dB or higher. But this unique polishing technique produces a significant drawback. Namely, the glass which is altered becomes not much sturdy as before. If you frequently connect or disconnect it, it is more susceptible to damage than a PC or APC equivalent. It was known as a wear-and-tear issue.
Mostly installed, its color code is blue.
Apart from the marginally curved endface of UPC, PC and UPC are both ground with no angle.
APC Fiber Connector
The APC is an acronym for Angled Physical Contact. Because the endface of the ferrule is polished at an 8°angel, the air gap is profoundly reduced. The APC fiber connector generally had a higher insertion loss than the UPC fiber connector. However, the technique solved this issue. Nowadays, the APC connector has more precise angles, so its insertion loss is closer to that of the UPC connector.
As for return loss, APC has the lowest back reflection among all mentioned fiber connectors. As stated by the industry standards, the return loss of PC, UPC, and APC connectors is about -40dB, -50dB, and -60dB or higher, respectively.
Some applications are more sensitive to return loss, so they demand the APC fiber connector.
Ferrule diameter size: 2.5mm VS 1.25mm VS Multifiber
There are three fiber optic connectors in their ferrule diameter size, viz. 2.5mm, SFF, and Multifiber.
Common 2.5mm Connectors
The SC, FC, ST, E2000, and now obsolete ESCON, FDDI, Opti-Jack fiber optic connectors feature a 2.5mm ferrule size. Therefore they can be mated to each other using special hybrid adapters.
SFF: 1.25mm Connectors
To fulfill the demand for a smaller footprint of the fiber optic connector, the SFF connector came into being to support high-density environments. Due to its half the size of SC, FC, and ST fiber connectors and satisfactory performance, the LC fiber connector has progressively been the most popular. Besides LC, the Senko CS, SN, MU, 1053 HDTV, Luxcis, Fibergate, and LX-5 feature a 1.25mm ferrule.
The MPO, which refers to “Multifiber Push-on,” has been increasingly used in data centers today. It is larger than other connectors, for it combines fibers 8,12, or 24 fibers in a single rectangular ferrule. As a result, it can support higher-speed multifiber applications, for instance, 8-fiber 40 and 100 Gig applications that transmit and receive at 10 or 25 Gbps over 4 fibers simultaneously. It’s often used in 40G and 100G high-bandwidth parallel optical connections.
Utilization with or without adapter panel
Most fiber optic connector types, such as LC, SC, MTP/MPO, and ST fiber optic connectors, need an adapter panel. They’re often deployed in data centers, enterprise networks, SANs, MANs, LANs, FTTx (fiber to the x), etc.
Contrariwise, the Rosenberger Q-RMC and NEX10 connector feature a push/pull motion with a latch, enabling quicker connection without using an adapter panel. They’re commonly used in harsh environments.
Termination: on-site termination vs. Factory Pre-termination
On-site termination, also known as field termination, refers to the termination of the end of the fiber in the field. This operation requires professionals and a specific kit to strip, prep, polish, connect, inspect, and test. Although the termination cost is high, it admits flexibility and easy cable routing.
On the contrary, a factory pre-terminated fiber connector spares the above-mentioned cumbersome procedure and high technical skills required, and supplies the customer with factory-level precision and quality assurance. However, While the termination cost is low, the pre-measured lengths of the fiber cables must be exact.
After this overview of fiber optic connectors from a broader perspective, let us individually introduce the most common fiber optic connectors. Some newly introduced fiber optic connectors will also be briefly discussed.
Invented by Lucent Technologies, the LC has been the most popular choice of fiber connectors. The LC is the acronym for Lucent Connector. Initially designed to satisfy the need for a smaller, low-loss connector, the LC features a 1.25mm ferrule, half the size of SC, FC, and ST. Therefore is also known as the “little connector.”
Due to its tiny footprint, faster and easier to operate, low insertion loss, overall reliability, and push/pull with a latch design, the LC gradually dominates both singlemode and multimode deployments. Yet its introduction was not booming, as Lucent Technologies charged high license fees. The LC is widely used in high-density networks, such as datacoms, SFP and SFP+ transceivers, XFP transceivers, etc.
Three different types of LC connectors are available:
- Singlemode LC APC
- Singlemode LC UPC
- Multimode LC UPC
The LC is covered in the FOCIS 10 Fiber Optic Connector Intermateability Standard TIA-604-10
Once the most popular fiber connector, the SC remains the second-best choice of fiber connector for simplex applications, duplex applications, and polarization-maintaining applications.
Designed by JP NTT (Nippon Telegraph and Telephone), the SC was among the first connectors to enter the market after the advent of ceramic ferrules. The SC refers to “the subscriber connector,” also known as a square connector, featuring a snap-in coupling mechanism, a square shape, and a spring-loaded 2.5mm ferrule. It was the first connector standardized in the telecommunications specification TIA-568-standard in 1991.
Owing to its initial high price, twice as expensive as an ST, the SC at first didn’t make a hit. Thanks to its increasing popularity in the market, its manufacturing cost decreased. Due to good performance and fast deployment, it was the best connector until the 1990s. It has been widely used in singlemode fiber optic cable, analog CATV, GPON, GBIC, EPON ONU’s, Fiber Media Converters, and datacoms and telecom applications like point-to-point and PON (passive optical network).
The SC is covered in the FOCIS 3 Fiber Optic Connector Intermateability Standard TIA-604-3.
The FC used to be one of the most popular singlemode connectors for a long time, but nowadays, it has been phasing out in favor of LC and SC.
The FC stands for ferrule connector. Also invented by JP NTT, the FC first introduced a 2.5mm ceramic ferrule into the market. It uses a threaded connection and an aligned key that admits proper insertion, with a stainless-steel housing around the ferrule. It has a round screw-type fitment made from nickel-plated or stainless steel. Tightened firmly by screwing using a threaded collet, it is ideally suitable for high vibration environments and applied for single-mode optic fiber and polarization-maintaining optic fiber.
However, due to its higher cost and time-consuming to unplug, its share in the market has been significantly dwindling. As its design is efficiently resistant to vibration, the FC still sees some uses, especially in the singlemode cables in high vibration environments.
The FC is covered in the FOCIS 4 Fiber Optic Connector Intermateability Standard TIA-604-4.
Designed by AT&T in the late 1980s, soon after the coming of the FC, ST was arguably the most popular connector for multimode networks till around 2005.
The ST is the acronym for Straight Tip. It resembles the FC and uses a long cylindrical 2.5 mm ceramic ferrule, although some ferrules are made of metal or plastic. But the ST is keyed and spring-loaded. It utilizes a bayonet mount to replace a threaded screw, enabling fast deployment and low cost, making it popular for the campus, buildings, enterprise, and military multimode networks.
Due to its incompatibility with APC, thus its uses in telco, FTTx singlemode deployments are limited. Its use has been considerably dwindling in favor of SC and LC, though less severe than FC. Despite this, today the ST is still commonly seen in multimode applications.
The ST is covered in the FOCIS-2 Fiber Optic Connector Intermateability Standard TIA-604-2.
The MPO, short for Multiple Fiber Push-On, is a multifiber connector, while the MTP is a brand name of the MPO connector type registered by US Conec. The MTP is the acronym for “Multifiber Termination Push-on.” It’s like an advanced version of the MPO, fully compliant with the latter’s standards, and features high mechanical and optical specs, some even covered by patents. That’s almost no difference in their appearances.
The MPO Connectors could carry up to 72 fibers into one ferrule and are typically available with 8, 12, or 24 fiber arrays. They are suitable for high-density networks, such as data centers and LANs in singlemode and multimode deployments. It is often used in 40G and 100G high-bandwidth parallel optical connections. MPO could satisfy the needs for fast deployment of aggregate fibers, high-density patching, or crucial smaller ODFs and nodes.
Besides, they’re also used for breakout cables. The MPO connector is on one end, and other fiber connectors such as LC or SC connectors are opposite. As a result, the deployment is time effective using the only necessary number of cables.
The MPO connector utilizes pins to align and has the plug-and-jack format, i.e., with or without pins. So, the MPO connector needs to be positioned appropriately to minimize insertion loss. The MPO connector will typically produce about 0.25 dB insertion loss in singlemode and about 0.20 dB insertion loss in multimode.
The MPO cable currently comes factory preterminated. Each fiber is preterminated individually by a skilled technician. Given its complexity, it’s not appropriate for field termination, as it’s mainly applied for preterminated cable assemblies and cabling systems. Compared to a single ceramic ferrule connector, it has a higher attenuation loss.
The MPO is covered in the FOCIS 5 Fiber Optic Connector Intermateability Standard TIA-604-5.
The MT-RJ stands for Mechanical Transfer Registered Jack. Developed by AMP, the MT-RJ is a duplex connector with polymer ferrules. Like the security design of the MPO connector, it also uses pins for alignment and has plug-and-jack formats. It’s easy to plug and unplug, so low termination cost. However, compared to the LC, it has a less good mating quality, a higher insertion loss, and little support from active equipment manufacturers. Furthermore, it is difficult to clean and evaluate, as most tests kits do not allow direct adaption to the connector.
MT-RJ is covered in the FOCIS 12 Fiber Optic Connector Intermateability Standard TIA/EIA-604-12.
The MU stands for “Miniature Unit,” developed by JP NTT to fulfill the need for high-density packaged systems. The MU resembles a miniature SC and features a simple push/pull design and a 1.25mm ferrule. It is square, mainly used in dense applications. It is more prevalent in Japan and the far east.
Three different variations of the MU connectors are available:
- Singlemode MU APC
- Singlemode MU UPC
- Multimode MU UPC
The MU is covered in the FOCIS 17 Fiber Optic Connector Intermateability Standard TIA-604-17.
The E2000 connector, also known as LSH (laser shock hardening), uses a push/pull coupling mechanism with a unique integrated spring-loaded metal shutter to protect the ferrule from dirt, dust, and scratches. It has a custom diameter ferrule for multifiber technology, varies from 1.25 mm to 2.5 mm. it has the unique insertion loss at 0.1 dB, color keying, and Ultra-low ferrule O.D. tolerances. It’s easy to clean and inspect, and its optical performance makes it an ideal choice for high-performance applications such as test and measurement equipment.
It is suitable for high safety and high-power applications. And is used in broadband applications, telecommunication networks, LANs, CATV, FTTx, and data networks.
The E2000 is trademarked by Diamond SA, based in Losone, Switzerland.
The E2000 connector is available in the following variations:
- Single Mode APC
- Single Mode UPC
- Multi-Mode UPC
The E2000 is covered in the FOCIS 16 Fiber Optic Connector Intermateability Standard TIA/EIA-604-16.
The MT is the acronym for Mechanically Transferable. The JP NTT started the research on the MT connector in the early 1980s. The technologists aimed for a design of splicing of high-density and high-count cables in the joint cable closures. The repeated connection was also one of the intentions. They named MT connector from one of its applications for fiber ribbons’ high-speed mechanical transfer switching. To cover MT ferrule for intra-office applications, they designed the MPO connector in 1986. Then in 1991, they created APC-MPO to enable low-loss and high-return-loss multifiber connections without index matching materials for the central offices’ applications. 
MT connector could contain up to 72 fibers into a single ferrule. The MPO/MTP mentioned earlier is the application of the MT connector.
The MT is covered in the FOCIS 5 Fiber Optic Connector Intermateability Standard TIA-604-5.
The LX-5 connector resembles the LC connector. It has a one-piece design, uses a 1.25 mm ferrule, and features an automatic metal shutter over the end of the fiber. Due to this, it combines high packing density, reliability, high performance, and safety. It has color coding, and high mechanical and thermal resistance, according to Telcordia GR-326-CORE.
Combining an insertion loss of 0.1 dB with an APC return loss of 85 dB or higher is ideal for installation in global high-performance networks.
The LX-5 Connector is covered in the FOCIS-13 Fiber Optic Connector Intermateability Standard TIA/EIA-604-13.
Mating Dissimilar Connectors
The SC, ST, FC, FDDI, and ESCON connectors share a typical 2.5 mm ferrule size, so they can be mated to each other using hybrid mating adapters, and it’s convenient to evaluate.
Otherwise, as for the breakout cables, you can use the MPO connector on one side and LC or SC on the other side. It’s easy and quick.
Cleaning before connection
It’s essential to first clean the ceramic ferrule before each connection, as this maintaining procedure helps prevent dust, dirt, and scratches, so it will extend the connector’s life expectancy.
Do not mix multimode fiber type with singlemode fiber type
Remember, while you can use a singlemode fiber connector on multimode, but not the opposite way. As they have different core sizes and light modes, you shouldn’t mix them or connect them directly.
UPC/APC can’t intermate with each other
Owing to the angle of an APC connector, it can’t be mated to a UPC connector. Since this will produce poor performance and may destroy both connectors. They’re not interchangeable. If you choose the APC connector, you should keep using the APC connector all the while.
How to choose a fiber connector
Always check your installation requirements first. Check the type of ac
tual fiber cables, the equipment to be plugged into the fiber cable, and your budget. Also, various types of adapters are available, just in case.
Then you could take these points below to consider:
- Which type of fiber connector is required?
- Which type of fiber cable are you using? Singlemode or multimode?
- Singlemode is suitable for long-reach data transmission applications, widely used in carrier networks, MANs and PONs. While multimode is ideal for shorter reach and high-density networks, commonly deployed in the enterprise, data centers, and LANs.
- If singlemode, UPC is the most common type except for applications that are more sensitive to return loss, and in this case, APC is preferable.
- If multimode, which grade? OM2, OM3, or OM4?
Finally, suppose you accidentally made a mistake and find that what you really need is another type of fiber optic connector. In that case, you could consider purchasing a corresponding adapter or a preterminated patch cable.
The LC and SC connectors are the most popular choices of connectors.
With this somewhat detailed introduction of the fiber optic connector, we sincerely hope this exposition will be of any help to you. The cable loss is typically the most negligible factor to attenuation, and the most considerable loss comes from the connector, so choosing the best performance connector is particularly important.
 Toshiaki Satake, Shinji Nagasawa et al. “MPO-type single-mode multi-fiber connector: Low-loss and high-return-loss intermateability of APC-MPO connectors” in Optical Fiber Technology 17 (2011) 17–30, p. 18.