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Views: 146 Author: Site Editor Publish Time: 20-01-2021 Origin: Site
Fiber optic connectors provide a critical piece of infrastructure for telecommunications, Datacom processes, precision measurement equipment, and lasers, among other applications. Ferrules are an important component of fiber optic connectors, making precision alignment of fibers possible.
Fiber optic connectors use light to transmit information. Secure, precise connections between fibers are essential to high-quality, reliable transmissions. Ferrules make those connections possible, ensuring that the fibers align properly and that the connection isn’t thrown off by jolts, bumps, or other forms of interference.
In fiber optic connections, two critical factors their users must worry about are insertion loss and reflection loss. Insertion loss refers to a loss of signal power resulting from the way fibers connect. Reflection loss, or return loss, refers to a loss of signal power resulting from a portion of the signal being reflected back to its source.
When choosing fiber optic connectors, users must consider how much insertion and reflection loss is acceptable, and what connector will most effectively and cheaply keep them within this range.
In fiber optic connectors, the fiber end being connected is encased in a 2.5 mm ferule, typically made of ceramic, metal, or a composite material. The tips of the connectors are polished to create a rounded surface. This smooth surface allows fibers being connected to touch only at their cores, creating a better quality connection.
How the connector is polished is important to its quality. Manufacturers often have several grades of polish for connectors. The higher the grade of polish used, the less insertion loss and lower back-reflection the fiber optic connection will have.
In general, there are four types of ferrule polish, including:
PC fiber connector refers to the connector that is polished in the physical contact style. It is the most common polish type that is found on OM1 and OM2 multimode fiber. PC fiber connector was generated to overcome the air gap between two surfaces from small imperfections in the original flat fiber connector. In PC fiber connector design, there is a slightly cylindrical cone head with the aiming to eliminate the air gap, so that the typical return loss in single mode applications is about -40dB, higher than the return loss of the original flat polish style (-14 dB or roughly 4%). Up to now, this polish style has been outdated and the evolution - UPC polish style has come into being.
UPC is the acronym for Ultra Physical Contact. It is an improvement of the PC fiber connector with a better surface finish after an extended polishing and the return loss is better than PC structure, nearly -50dB or higher. Though it has a relatively lower back reflection in comparison with the PC connector, it is not robust enough. Note that repeated connections and disconnections will bring about the degradation of the surface and ultimate performance.
Out of the pursuit of lower back reflection, APC fiber connectors were developed. APC refers to Angled Physical Connect, the ferrule endface radius of which is polished at an 8°angel, minimizing the back reflection as a result. Since adding the angled endface, the reflected light will leak out into the cladding instead of staying in the fiber core. It should be noted that APC connectors should only be mated with other angled-polished connectors rather than non-angled polished connectors, or it will cause very high insertion loss. The optical return loss of APC connectors is -60dB or higher, better than the other connector types.
The major difference in terms of appearance is the fiber endface. PC connectors and UPC connectors are both polished with no angle, though endface of UPC has a slight curvature. Instead, an APC connector is featured with 8° angle endface.
Another obvious difference is the color. It is visible to distinguish which type of polish style the connector features by the color of the connector body. Take an FS LC APC single mode fiber optic patch cable as an example, the connectors on both fiber ends are all green. While for an LC UPC single mode fiber optic patch cable, the connectors are generally blue. It is also a simple way to tell these connectors apart.
What does this difference mean in terms of performance? With UPC connectors, any reflected light is reflected straight back toward the light source. The angled endface of the APC connector causes reflected light to reflect at an angle into the cladding vs. straight back toward the source. This causes some differences in return loss, which is a measurement of reflected light that is expressed as a negative dB value (the higher the value, the better). Industry standards recommend that UPC connector return loss should be -50 dB or greater, while APC connector return loss should be -60 dB or greater.
Since some applications are more sensitive to return loss, APC connectors are preferred in these fields, for instance, in higher wavelength range like those used for RF video signals, especially FTTx applications and for applications such as passive optical networks and other WDM systems using high wavelength via single mode fiber.
For those applications where return loss is not paid much attention to, UPC or PC comes into play. PC connectors are generally seen in telecom operators' equipment, while UPC connectors are popular in digital TV, telephony and data systems.
One thing that should be noted is that APC and UPC connectors cannot and should not be mated. Not only does mating cause poor performance because the fiber cores will not touch, but it can also destroy both connectors. The last thing you want to do is cause permanent transmitter damage—especially with higher-cost singlemode equipment.
Another important point: Cable loss is typically the smallest contributor to system attenuation. The largest loss comes from the connector. Higher-performing connectors offer dB savings that are more significant, measurable and consistent.
Fiber optic ferrules are typically made of ceramic, metal, or composite materials. Each material has its benefits and drawbacks. Ferules are typically the most costly component of a fiber optic adapter, often accounting for about 80 percent of its full cost.
Ceramic ferrule sleeves are typically easier for manufacturers to precisely mold and align to the fiber, making them best suited for single-mode cable connections. Because manufacturers are able to create a closer fit, these ferrules provide a much lower optical loss than ferrules made from other materials. Ceramic ferrule sleeves are used for the most vital network connections, like high-security networks or connections in wiring closets.
Ferrules made of metal, bronze for instance, are more durable than ceramic ferrules, but they often do not offer the same level of precision alignment as their ceramic cousins. Drilling an accurate hole through the metal ferrule sleeve can be difficult, and that can result in less accurate fiber alignment. The use of watch-jeweled centering improves alignment, but overall, metal ferrule sleeves are better suited for multimode fiber applications where absolute alignment isn’t crucial.
Composite ferrules act as a great compromise material. Less expensive than ceramic ferrules, these ferrules also provide a greater level of precision than metal ferrules.
The LC connectors are highly popular within single-mode networks. It is known for its good performance and small size. LC connectors have a 1.25mm ferrule, approximately half the size of SC connectors. It’s also commonly referred to as the “little connector”.
The SC is a snap-in connector that also features a 2.5mm ferrule much like the ST connector and is known for its excellent performance. The connector is simple, rugged, and low cost.
It’s simple push on/pull off operations makes it a popular choice.
The FC connector was widely popular within fiber optic networks however its use has been dwindling in recent times replaced with SC and LC. The connector uses a threaded container and aligned key. Once positioned it can remain in place with perfect precision.
The ST connector remains one of the most widely used connectors especially for multimode networks such as college campuses and most buildings. The connector is very easy to use due to its spring-loaded, keyed, and “push in and twist” mechanism within its design. The ST connector features a bayonet mount and a long cylindrical 2.5mm ceramic or polymer ferrule to hold the fiber.
The MT- RJ is a duplex connector where both fibers are in a single polymer ferrule. It utilizes pins for alignment and has male, female, and plug and jack formats. The MT-RJ is a duplex that is often difficult to test, as most test sets do not allow direct adaption to the connector.
MU connectors resemble a miniature version of SC with a 1.25mm ferrule. Its small size allows the MU connector to have a reduced footprint and are used in dense applications. The connector is square and uses a push-pull mechanism to lock. This type of connector is more popular in countries such as Japan.
The E2000 connector is mainly used in modern telecommunication networks. The connector features a unique spring-loaded shutter that protects the ferrule from dirt, dust, and scratches. Since the connector uses a monobloc ceramic ferrule, problems related to different co-efficient of expansion are nonexistent. The E2000 utilizes a push-pull locking connector. The E2000’s return loss is one of the lowest in the industry at just 0.1 dB.
MPO terminated cables are widely used in high-density cabling environments like data centers. The traditional, tight-buffered multi-fiber cable needs to have each fiber individually terminated by a skilled technician. MPO cable which carries multiple fibers comes pre-terminated. Factory terminated MPO connectors commonly have either 12 fiber or 24 fiber arrays. MPO stands for Multi-Fiber Push On. This connector is commonly used to terminate multi-fiber ribbon connections in indoor environments.
The MPO connector can be either male or female. You can tell the male connector by the two alignment pins protruding from the end of the ferrule. MPO female connectors will have holes in the ferrule to accept the alignment pins from the male connector.
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