IC "Telecom-Service" offers services for the design, installation and service support of corporate communications built on the basis of fiber-optic lines. Special offer companies – in an integrated approach to the creation of corporate telecommunications and information systems. In addition to installing optics, we effectively implement the creation of office PBXs and call centers (including those based on VOIP), as well as the creation of data processing centers and storage systems. Attention: equipment is supplied only as part of the project, there is no retail sale.

It is obvious that in an ideal optical information transmission system, the light flux should pass unhindered from the source to the photodetector. Optical fiber- this is nothing more than the same signal propagation route. It is not possible to stretch a single fiber from the source to the receiver. The technological length of the fiber usually does not exceed several kilometers. And if this problem can still be solved by welding light guides, then ensuring the mobility of the local optical subnetwork is achieved only with the use of cross-connect equipment. Problems of transmitting light waves from one piece of fiber to another cannot be avoided. For multiple and easy connection optical links, light guides can be terminated with optical connectors. Considering that modern light guides are micron technologies, terminating fibers with optical connectors is a challenging task.

Losses in optical connectors

Let us describe the problems that arise when a signal passes from one fiber to another. Loss of power or attenuation of the optical wave occurs when the optical fibers are not accurately aligned. In this case, some of the rays simply do not pass into the next light guide, or enter at an angle that is more critical. When the physical contact of the fibers is incomplete, an air gap is formed. In this connection, the effect of return losses arises. When passing through transparent media with different densities, some of the rays are reflected in the opposite direction. When they reach the resonator, they are amplified and cause signal distortion.

Not ideal geometric shape fiber also contributes to power loss. This may be due to the ellipticity of the fiber and the non-centricity of its core. The end of the light guide itself may contain deformations: chips and roughness, which in turn reduces work surface fiber contact

Optical connector tips

Thus, it is necessary to align both light guides accurately and tightly. To ensure the safety of fragile fibers during repeated alignment, their end sections are placed in ceramic, plastic or steel tips. Most tips have cylindrical shape with a diameter of 2.5 mm. Conical designs are available, and LC connectors have a tip with a diameter of 1.25 mm.
Inside the tips there is a channel into which a chemical or mechanically light guide stripped of its sheath. When deleting protective coating can be used as special mechanical tools, and chemically active solutions. Inside the tip, the light guide can be fixed both along the entire length of the channel (more often these are glue-based methods) and at the point where the fiber enters the tip ( mechanical methods). The process of mechanical fixation takes much less time (up to several minutes) and is based on “pressing” the fiber using polymer materials. But it is less reliable and short-lived. Chemical method speaks for itself. Most often, the fixing composition in this technology is epoxy solutions, as they are the most reliable. However, the period of complete thickening of such a composition is very long - up to a day. Therefore, if necessary, more quick installation connectors, other components may be used or special ovens for drying.

After installing the light guide into the connector, it is necessary to grind the end of the tip. Excess fiber protruding is removed special tools. The basic principle is to cut and break off the light guide, after which you can begin to directly polish the surface.
Of particular interest is the shape of the ends of the tips. Their processing is a whole art. The simplest option end - flat shape. It is characterized by large return losses, since the likelihood of an air gap in the vicinity of the fibers is high. There are enough irregularities even in the non-working part of the end surface. Therefore, convex ends are more often used (the radius of rounding is about 10-15 mm). With good centering, tight contact of the light guides is guaranteed, which means there is more likely to be no air gap. An even more advanced solution is to round the end at an angle of several degrees. Rounded ends are less dependent on deformations generated when connecting connectors, so such tips can withstand a larger number of connections (from 100 to 1000).

The material of the tip is also important. The overwhelming majority of connectors are built on the basis of ceramic tips, as they are more durable.
After terminating the light guides with connectors, it is necessary to analyze the quality of the tip surface. Microscopes are most often used for this. Professional devices have a magnification factor of hundreds of times and are equipped with special illumination from various angles. They may also have an interface for connecting to additional measuring equipment.

According to the TIA/EIA 568A standard, the return loss for multimode fiber in optical connectors should not exceed -20 dB, and for single-mode fiber -26 dB. Based on the magnitude of return losses, connectors are divided into classes

Type	Losses	Type	Losses
PC	less than 30 dB	Ultra PC	less than 50 dB
Super PC	less than 40 dB	Angled PC	less than 60 dB

PC is an abbreviation of the English Physical Contact.

Connecting optical connectors

Fundamentally, the connection of two optical connectors cross-connect equipment is built according to the following scheme:
The platform for installing the connectors is the socket. The connectors included in it are fixed in such a way that the axes of their tips are centered, parallel and pressed tightly. Such sockets are usually installed in patch panels or mounting box inserts.

Connector type	Tip	Loss (dB) at 1300 nm
		Multimode	Singlemode
ST	Ceramics	0.25	0.3
S.C.	Ceramics	0.2	0.25
L.C.	Ceramics	0.1	0.1
F.C.	Ceramics	0.2	0.6
FDDI	Ceramics	0.3	0.4

ST connector

Connectors differ not only in the tips used, but also in the type of fixation of the structure in the socket. The most common representative in local optical networks is the ST-type connector (from the English Straight Tip). The ceramic tip has a cylindrical shape with a diameter of 2.5 mm with a rounded end. Fixation is carried out by rotating the frame around the axis of the connector, while there is no rotation of the connector base (theoretically) due to the groove in the socket connector. The guide frames, engaging with the stops of the ST-socket when rotating, press the structure into the socket. The spring element provides the necessary pressure.

The weak point of the ST technology is the rotational movement of the frame when connecting/disconnecting the connector. It requires a large living space for one link, which is important in multiport cable systems. Moreover, there is no rotation of the tip only in theory. Even minimal changes in the position of the latter entail an increase in losses in optical connections. The tip protrudes from the base of the structure by 5-7 mm, which leads to its contamination.

SC connector

Weak sides ST connectors are currently being solved through the use of SC technology (from the English Subscriber Connector). The body section has rectangular shape. The connector is connected/disconnected forward movement along the guides and secured with latches. The ceramic tip also has a cylindrical shape with a diameter of 2.5 mm with a rounded end (some models have a beveled surface). The tip is almost completely covered by the body and is therefore less susceptible to contamination than in the ST design. Absence rotational movements causes more careful pressing of the tips.

In some cases, SC connectors are used in a duplex version. The design may have clamps for pairing connectors, or special brackets may be used to group housings. Connectors with single-mode fiber are usually blue in color, while connectors with multi-mode fiber are gray.

LC connector

LC type connectors are a small-sized version of SC connectors. He also has rectangular section housings. The design is made on a plastic base and is equipped with a latch similar to the latch used in modular connectors of copper cable systems. As a result, the connector is connected in a similar way. The tip is made of ceramic and has a diameter of 1.25 mm.

There are both multimode and single-mode connector options. The niche of these products is multiport optical systems.

FC connector

In single-mode systems, there is another type of connector - FC. They are characterized by excellent geometric characteristics and high tip protection.

FDDI connector

To connect a duplex cable, not only paired SC connectors can be used. FDDI connectors are often used for these purposes. The design is made of plastic and contains two ceramic tips. To avoid incorrect connection of the link, the connector has an asymmetrical profile.
FDDI technology provides four types of ports used: A, B, S and M. The problem of identifying the corresponding links is solved by providing connectors with special inserts, which can differ in color scheme or contain letter indices.
Mostly this type used to connect terminal equipment to optical networks.

MT-RJ connectors

Guaranteed parameters of cable assemblies:

• Direct losses<0.5 дБ (типичное значение - 0.25 дБ для ММ)

Areas of use:

• Wiring in buildings (horizontal and backbone)
• Telecommunication networks

Note: MT-RJ cords are assembled in accordance with MFO 86001-0112 procedures.

Peculiarities:

• Latch size and design similar to RJ-45
• Duplex ferul
• Low cost
• High port density
• ISO/IEC 11801 and TIA/EIA 568A compliant
• Low direct losses:

< 0.22 дБ для ММ
< 0.19 дБ для ОМ

The development of the MT-RJ connector was aimed at solving the following problems: small size, low cost and ease of installation. The use of the MT-RJ connector doubles the port density of standard connectors and makes it ideal for use in fiber-to-the-desk applications. The connector design complies with TIA requirements.

The MT-RJ connector uses an improved version of the industry standard RJ-45 connector. It is the small size and convenience of a latch similar to RJ-45 that determine the advantages of this connector when used in horizontal wiring to the workplace.

A feature of the MT-RJ system from Molex is the use of different PNs for the male (with guide pins protruding from the ferrule) and female (with holes for the pins) connectors. There are two modifications of the adapter, one of which is installed in the socket for a simplex SC adapter.

Quality and characteristics

Materials provided by AESP, a well-known manufacturer of networking and communications equipment, developer of the SygnaMax cable system.

Optical connectors are used for terminating optical fibers and for connecting them to passive or active telecommunications equipment.

As fiber-optic communication lines developed, more than 70 types of optical connectors were developed for various application conditions.

The connection of the station optical fiber to the linear one occurs at the optical cross-connect using optical connectors, which are optical connectors and optical adapters (plugs and sockets, respectively).
The optical adapter is a socket into which connectors are inserted on both sides. In the same way, the patch cord is connected to active VOSP equipment, the front panel of which has an optical adapter corresponding to the type of connector.
Optical detachable connectors (connectors) are designed to ensure the passage of light from one FOTS element to another, for example, from the transmission medium to linear and terminal equipment, with minimal possible losses when exposed to various external factors. Such a connection must be stable and reproducible with repeated use.

There are 2 types of optical adapters:
1) connecting ones, having the same types of connectors on each side for connecting connectors of the same type. The designation of the connecting adapters corresponds to the type of connectors being connected (FC, SC, LC, ST, etc.);
2) adapters, having different types of connectors on each side of the adapter (FC/SC).
The main concept when creating optical adapters is that they transmit the optical signal without any distortion in the connector. From here we can identify the main parameters of the mechanical connection.

Main parameters of optical connectors:
Insertion loss (attenuation caused by the loss of concentricity of the ends) is the difference in the average signal power levels at the input of the optical connector and at the output.
Reflection attenuation (transmitted radiation is partially reflected back into the fiber to the source (laser)). A sufficiently strong return reflection (RL - Return Loss) will lead to disruption of the functioning of the laser and a change in the structure of the transmitted signal. To reduce this phenomenon, several types of polishing have been invented.

FC connector -FC type connectors were developed by NTT and are primarily intended for use in single-mode long-distance lines, specialized systems and cable television networks. A ceramic tip with a diameter of 2.5 mm with a convex end surface with a diameter of 2 mm ensures physical contact of the abutting optical fibers. The tip is manufactured to strict geometric tolerances to ensure low loss and minimal back reflections. The tip radius ensures physical contact between the mating fibers.

FC connector with metal ferrule

To fix the FC connector to the socket, use a union nut with an M8x0.75 thread. In this design, the spring-loaded tip is not rigidly connected to the body and shank, which complicates and increases the cost of the connector, but this addition pays off in increased reliability.
FC type connectors are resistant to vibration and shock, which allows them to be used on appropriate networks, for example, directly on moving objects, as well as on structures located near railways.

Adapter for FC with attenuator

Peculiarities

• Compliant with IEC 61 754-143, TIA/EIA, NTT, Belcore specifications
• Corrosion-resistant housing
• High reliability
• 2.5mm ceramic ferrule
• Resistance to vibration and single impacts

Application area

• Cabling systems, CATV, LAN, WAN
• Medical and test equipment
• Telecommunications and on-board networks

Specifications

ST connector - Recommended for use primarily for multimode applications. The connector tip is not decoupled from the body and cable sheath, which makes the design simpler, more reliable and cheaper, while at the same time this design is fully suitable for multimode applications. The monobloc design of the ST connector is designed for quick termination. The connectors have ceramic tips with a diameter of 2.5 mm.

ST connectors are secured with a bayonet lock

Peculiarities

• Ceramic tipped connectors are used
• Fast, easy assembly, high optical performance
• Operational reliability
• Lucent Technology ST Connector
• Convenient connection using bayonet fasteners
• Fully compatible with IEC 61 754-2

Application area

• LAN systems and equipment
• Optical subsystems of local networks
• Telecommunication networks
• Network processing

Specifications

SC connector - one of the disadvantages of FC and ST type connectors is the need for rotational movement when connecting to the adapter. To eliminate this drawback, which prevents an increase in the installation density on the front panel, SC type connectors have been developed. The SC connector body is rectangular in cross section. The tip is not rigidly connected to the body and shank.

When connecting the SC connector, the tip rotates

SC duplex connector

The SC connector is connected and disconnected linearly (push-pull), which prevents the connector tips from rotating relative to each other when they are fixed in the adapter. The locking mechanism opens only when the connector is pulled out by the housing. The disadvantages of SC connectors include a slightly higher price and lower mechanical strength compared to the previously discussed connectors of the FC and ST types. The force pulling the SC connector out of the adapter is regulated within 40 N, while for the FC series this value can practically be equal to the strength of the minicable. As with ST connectors, this drawback limits the use of SC connectors on moving objects.

Peculiarities

• Low cost,
• Push-pull housing
• Pre-assembly design,
• Compliant with IEC, TIA/EIA-568A TIA/EIA, NTT,
• Low direct losses

Application area

• Cable systems, CATV, LAN, WAN,
• Medical and control equipment,
• Telecommunications

Specifications

LC connector - The popular new generation compact fiber optic connector dominating the telecommunications equipment market is a smaller version of SC connectors. It also has a rectangular body section. The design of the connector is relatively simple: a ceramic core with a diameter of 1.25 mm, not connected to a plastic housing. The fixing mechanism is a latch (similar to RJ-45). As a result, the connector is connected in a similar way. A pair of connectors can easily be combined into a duplex. Using this connector allows you to double the density of active equipment ports, patch panels and wall sockets compared to standard connectors, say, SC, without any compromise on quality.

LC connectors are widely used in the manufacture of optical cords and pigtails, termination of multicore optical cables, and the manufacture of attenuators, splitters, and collimators.

There are connector models specially adapted for installation on 900 µm microcable and cables with outer sheath diameters of 1.6, 2.0, 2.4 and 3 mm. The ferrule in the connector can rotate, sequentially occupying six positions, which makes it possible to achieve direct losses<0,1 дБ.

Peculiarities

• Optimal optical performance due to the use of high-quality ferrules
• Wide selection of ferrules
• Small-sized form
• High concentration when used
• Reconfigurability
• Compatible with Telcordia, ANSI/EIA/TIA, IEC
• Adaptation to 1.6/1.8/2.0mm cable

Application area

• Gigabit Ethernet
• Telecommunication networks
• Basic installations
• Multiport optical systems

Specifications

MU connector - MU fiber connectors represent the trends of the new generation; they are an analogue of the SC connector, approximately halved. Due to the reduced dimensions in connectors of this type, the fixation mechanism may be less reliable.

The tip and centralizer are ceramic, with a diameter of 1.25 mm. The body is made of plastic, parts are made of polymer and metal.

The share of equipment produced with MU type connectors is relatively small, but there are growth prospects, primarily due to a decrease in the share of use of connectors of earlier designs in equipment.

Peculiarities

• Connector with dust plug
• ROHS Compliant
• NTT-MU hardware compatibility
• NTT&JIS compliance
• Push-pull connection
• High leveling accuracy
• Ferrule material – zirconium
• Fully compatible with IEC 61 754-6

Application area

• Telecommunications sector
• Cable TV (CATV)
• LAN (FITL, FTTH and FTTD)
• SONET/SDH
• ATM and WDM applications
• Digital network

Specifications

MT-RJ connector - MT-RJ connectors were developed by a consortium of manufacturers including AMp Hewlett-Packard, Siecor LIN, Fujikura and USConnec. These connectors are manufactured exclusively in the form of duplex pairs and therefore cannot be considered universal. Technologically, they are difficult to produce.

The connector body contains a pair of metal guides into which two optical fibers are pre-installed. The optical fibers of the cable are welded to the pre-installed fibers. After installation, the cable is secured by turning the locking key.

MT-RJ connectors are used in switches, hubs and routers by many leading equipment manufacturers.

Peculiarities

• Latch size and design similar to RJ-45
• Duplex ferul
• Low cost
• High port density
• Compliant with ISO/IEC 67754-18 and TIA/EIA 604-12 standards
• Low direct losses

The use of the MT-RJ connector doubles the port density of standard connectors and makes it ideal for use in fiber-to-the-desk applications. This type of connector allows you to connect duplex optical communication channels using one cord, which saves space when installing communication lines

Application area

• Wiring in buildings (horizontal and backbone)
• Local area networks (LAN) and FTT applications
• Telecommunication networks

Specifications

MPO - connector - MPO (“Multi-fiber Push On”) connector is a small-sized connector designed for MT type ferrules, having the size of a regular simplex SC connector.

MPO (Multiple-Fibre Push-On/Pull-off) is a multi-fiber optical connector installed into the adapter without rotation, by direct insertion. MPO is the name of the first version of the 12-fiber connector, which was later improved and renamed MTP, although these connectors remained compatible with each other.

The MPO connector combines strips containing 4, 8 or 12 optical fibers. Laying and connecting fiber optic cables with MPO connectors installed by the manufacturer does not require the use of special tools and the involvement of qualified personnel, since there is no need to terminate the cable. This ensures high connection characteristics.

The advantage of this connector (MPO) is the combination of 12 fibers in one connector and connection with a compact ribbon fiber, which significantly saves space in patch panels and cross-connect cabinets.

A standard MPO connector terminates 12 fibers. Recent developments have increased the number of fibers in a connector with this interface to 72. Thus, the MPO system provides the highest installation density.

MPO simplified plug-and-play technology for connecting backbone fiber optic cables (“connected and ready”) represents an ideal turnkey solution to the installation problem for small projects when connecting several buildings and implementing vertical wiring. The ability to make multiple connections by having multiple fibers in one connector significantly speeds up the installation process.

Using an MPO connector saves time and reduces the likelihood of damage to fragile optical connectors. The MPO system also reduces the risk of dirt getting into the fibers of the adapters.

Peculiarities

• Combining 12 fibers in one connector and connecting to compact ribbon fiber
• Adapted to VSR interface
• Low losses
• Providing significant space and cost savings

Application area

• Relationship with OE modules
• Gigabit Ethernet
• Multimedia
• Telecommunication networks and systems

Specifications

MTP connector - an improved design of a 12-fiber connector, originally designated MPO (Multiple-Fibre Push-On/Pull-off - a multi-fiber optical connector installed into the feedthrough without rotation, by direct insertion). Improvements have affected the design of the connector (collapsible housing, improved tip) and the composition of the material used to manufacture the connectors.

As a result, MTP connectors have significantly higher transmission performance than their predecessors, although they are still compatible with each other.

Attention: MTP connectors are divided into male and female types!

The MTP type is mainly used indoors, for example, in computer centers in corporate networks, where distribution cabinets and parallel optics devices are used. MTP connectors are also widely used in new technologies such as flexible optical input/output multiplexers (ROADM), i.e. where high connection density is extremely important. The ability to make multiple connections by having multiple fibers in one connector significantly speeds up the installation process.

Peculiarities

• Combines up to 72 fibers in one connector and connects to compact ribbon fiber
• Strongest interconnection of MT ferrules with multifiber, increased installation density
• Adapted to VSR interface
• Compliant with Telcordia's GR-326-Core, IEC standards
• Low losses
• Optimal combination of compactness and reliability

Application area

• Local LAN networks (including FTTH and FTTD)
• Gigabit Ethernet
• Active equipment / transceiver interface
• Multimedia

Specifications

SMA - connector - fiber optic connectors and SMA optical products are widely used in medicine, industry, where the use of various sensors, transducers is necessary, as well as in fiber optic test applications. The SMA fiber optic connector has a compact size, high durability and reliability.

SMA fiber connectors can be with a ceramic tip or a stainless steel ferrule. SMA has two versions, SMA 905, SMA 906. The difference is that the SMA 905 fiber connector has a regular (straight) ferrule, and the SMA 906 fiber optic connector "step" tip is used to achieve lower insertion loss. The standard SMA fiber optic connector uses a 3.175 mm ferrule.

Peculiarities

• Metal or ceramic ferrule
• High temperature stability
• High wear resistance
• TIA/IEC Compliant
• ROHS Compliance

Application area

• Telecommunication networks and data transmission systems
• Local networks
• Laser systems
• Medicine/surgery
• Spectrometers

Specifications

E-2000 - connector - Fiber optic connector and E2000 products are becoming increasingly common in the communications field.

E-2000 connectors feature one of the most complex designs.

The connector is connected and disconnected linearly (push-pull). The locking mechanism opens only when the connector is pulled out by the body using a special key insert. Accidentally turning off such a connector without using a key is almost impossible (that is, a load is required to destroy the latch of the connector body).

Connector E-2000 – plastic connector with top lock. Typically used in single-mode networks. E-2000/ARS is more widespread due to the large amount of equipment for television systems where APC polishing is necessary. The peculiarity of the connection of this connector with the adapter prevents dust from entering the surface of the optical elements. It also ensures sufficient mounting rigidity, resistance to vibration loads, and a high degree of precision in aligning the optical fibers. The body cross-section is square, which makes it easy to implement duplex connectors.

Peculiarities

• Secure transmission of high-speed protocols
• Multilayer zirconium ferrule with a diameter of 2.5 mm
• Automatic plastic curtains (spring loaded shutter), which act as plugs when the adapter is turned off and open when turned on
• Push-pull locking type design (push-pull with top lock)
• Compliant with European (EN 186270) and international (IEC 61754-151) standards, TIA/EIA 604-16

Application area

• Local LAN networks
• Modern high power DWDM applications
• Cable TV CATV
• Metrology
• Railways
• Industry

Specifications
DIN connector - DIN connectors are used in test equipment and telecommunications equipment, cable television, LAN, WAN, MAN, as well as in industry, medicine and laser systems.

This unique connector provides superior performance due to its design.

The standard 2.5mm diameter ceramic core protrudes well beyond the body. The plastic case is equipped with a key that prevents the core from rotating around its axis when screwed into the adapter.

Peculiarities

• Compatible with DIN47256
• Special design of ceramic ferrule of free-floating type (free floating)
• Corrosion-resistant housing
• Compact design
• Low direct loss and return reflection

Specifications

Biconic - connector - With polymer tip provides maximum performance for multi-mode and single-mode applications. This "first generation" fiber optic connector is often used when refurbishing legacy installed fiber optic equipment. Fiber size 126 microns.
Consists of a tapered polymer collar that helps align the fibers when connecting it to the interface.
The robust and reliable design allows the use of connectors of this type in military structures and medical institutions.

Specifications

ESCON - connector - (Enterprise Systems Connection) fiber channel interface that provides information exchange between the IBM zSeries server and peripheral devices (or another server). First used in ESA/390 architecture servers. First announced by IBM in 1990. ESCON implements half-duplex transmission mode using request-response protocols.
Physically, the ESCON channel consists of two fiber-optic cables, each of which is designed to transmit information in one direction.
To connect a peripheral device, a point-to-point connection is used (single or via an ESCON switch).


Specifications

Basic data on fiber optic lines for the design of telecommunications systems

Optical fiber allows you to organize communications without regenerators (signal repeaters) up to 120 km for single-mode cables and up to 5 km for multimode cables.

The signals in optical cables are not electrical impulses, but modes (light fluxes). The walls of the central core are dielectric and have the reflective properties of glass, due to which light fluxes spread inside the cable.

Singlemode and multimode fibers

It is customary to divide optical fibers (cables and patch cords) into two types:

Single Mode, abbreviated as SM;

Multimode (Multi Mode), abbreviated: MM.

Moreover, both types have their own advantages and disadvantages, which means each of them can be used to achieve different goals.

Single-mode optical fibers (SM)

8/125, 9/125, 10/125 are markings for single-mode fiber optic patch cords. The first number in the marking is the diameter of the central core, and the second is the diameter of the sheath. It is worth noting that the diameters of FOCL (fiber-optic transmission line) are measured in microns (micrometers).

A single-mode cable uses a focused, narrow laser beam with a light wavelength range of 1,310-1,550 microns (1310-1550 nm).

Due to the fact that the diameter of the central core is quite small, light modes move in it almost parallel to the central axis. Therefore, there is virtually no signal distortion in the fiber, and low attenuation makes it possible to transmit an optical pulse over distances of up to 120 km without regeneration at speeds of up to 100 Gbit/s and higher.

There are single-mode optical fibers:

With unbiased dispersion (standard, SMF);

Dispersion Shifted (DSF);

And with non-zero biased variance (NZDSF).

Multimode optical fibers (MM)

Multimode Step Ratio Fiber

Multimode Gradient Ratio Fiber

Multimode fibers are labeled as 50/125 or 62.5/125, for example. This suggests that the diameter of the central core can be 50 or 62.5 microns, and the diameter of the cladding is the same as that of the single-mode type - 125 microns.

A multimode cable uses scattered beams from LEDs or a laser with a light wavelength range of 0.85 µm - 1.310 µm (850-1310 nm).

Because the core diameter of a multimode patch cord is larger than that of a single-mode patch cord, the number of paths for light modes to propagate increases. Several light streams move along different trajectories at once, reflecting from the mirror surface of the central core.

However, multimode fibers with a stepped refractive index have a fairly high inter-mode dispersion (gradual expansion of the optical beam as a result of reflections), which limits the signal transmission distance to 1 km and the transmission speed to 100 - 155 Mbit/s. The operating wavelength is usually 850 nm.

Multimode graded index fibers have lower intermode dispersion due to the smooth change in refractive index in the fiber. This allows you to transmit an optical signal over distances of up to 5 km at speeds of up to 155 Mbit/s. Operating wavelengths are 850 nm and 1310 nm.

Differences between single-mode and multimode optical fibers

In single-mode and multimode optical fibers, signal attenuation plays a fairly important role. This is the reason for the short operating distance of multimode fibers (1-5 km). Despite the fact that it would seem that more light flows move along a multimode cable, the throughput of such cables and patch cords is lower than that of single-mode ones.

A narrowly directed (single-mode) beam in single-mode fibers attenuates several times less than a scattered (multi-mode) beam in multimode fibers, which makes it possible to increase the distance (up to 120 km) and the speed of the transmitted signal.

Optical connectors

An optical connector is an inexpensive and efficient way to connect fiber optic cables. It ensures a reliable connection and integrity of transmitted packets.

Today there are a large number of different types of fiber optic connectors on the market. They all have different parameters and purposes. Docking of two identical or different connectors is done using an optical adapter.

Different types of optical connectors have different shapes and connection technologies. Also, in the production of such connectors, various materials can be used, be it metals or polymers.

Main types of optical connectors (connectors)

SC connectors

SC is the most popular optical connector.

The SC connector housing is made of plastic and has a rectangular cross-section. This connector is connected and disconnected linearly, unlike the FC and SC connectors, in which the connection is rotary. Thanks to this, as well as a special “latch,” a fairly rigid fixation in the optical socket is ensured. SC connectors are used mainly in stationary installations. The price is slightly more expensive than FC and SC connectors.

Single-mode SC connectors are marked in blue, multi-mode connectors in gray, single-mode connectors with an APC polish class (with beveled end) in green.

LC connectors

The LC optical connector is similar in appearance to the SC connector, but smaller in size, making it easy to implement high-density optical cross-connects using LC connectors. Fixation in the optical socket is carried out using a latch.

FC connectors

FC connectors are made of a ceramic core and a metal tip. Fixation in the optical socket occurs due to a threaded connection. FC connectors provide low losses and minimal back reflections, and thanks to reliable fixation, they are used for communication on moving objects, railway communication networks and other critical applications.

ST connectors

ST connectors are characterized by simplicity and reliability in operation, ease of installation and relatively low price. Outwardly they are similar to FC connectors, but, unlike FC, in which fixation in the socket is carried out using a threaded connection, ST connectors belong to the category of BNC connectors (the connection is made using a bayonet connector). ST connectors are sensitive to vibration and are used within these limitations.

ST connectors are used mainly for connecting optical equipment to trunk lines and local area networks.

DIN connectors

The DIN connector is similar to the FC connector, but is smaller. A ceramic core with a diameter of 2.5 mm protrudes beyond the plastic case, which, in turn, has a lock that prevents the core from rotating around itself. DIN connectors are often used in measurement equipment.

Connectors E-2000

E-2000 is one of the most complex optical connectors. Connection and disconnection is carried out linearly (push-pull), and opening is carried out using a special key insert. Therefore, it is practically impossible to remove such a connector by mistake.

E-2000 connectors have special plugs in their design that automatically close the end of the connector when it is disconnected from the optical socket, thereby preventing dust from getting inside.

E-2000 connectors are distinguished by high reliability and installation density. The square cross-section of the connector ensures easy implementation of duplex connections.

High Density Connectors

MT-RJ connectors

MT-RJ connectors are manufactured in duplex pairs.

Connectors VF-45 (SJ)

The connector shank is inclined at approximately an angle from the plane of the fiber connection. The VF-45 (SJ) connector is equipped with a self-latching anti-dust curtain.

MU connectors

Analogous to the SC connector, smaller in size. The centralizer is ceramic, 1.25 mm in diameter, the remaining parts are plastic.

Colors of optical connectors (connectors).

FC and ST - nickel plated brass

SC and LC duplex or simplex multimode - beige or gray

SC and LC duplex or simplex single mode - blue

SC/APC simplex - green

Polishing classes for optical connectors

Perhaps the main characteristics of optical connectors are insertion attenuation and back reflection. Optical attenuation has a stronger effect on signal quality than back reflection.

The return attenuation rate depends primarily on the lateral deflection of the cores of the optical fibers being connected.

Polishing optical connectors ensures that optical fibers are tightly connected to each other and reduces the air gap, which in turn reduces signal back reflection.

There are 4 polish classes: PC, SPC, UPC and APC.

Polishing PC, SPC, UPC:

RS (Physically Contact)

The PC class includes hand-polished connectors, as well as connectors manufactured using adhesive technology. Application speed - up to 1 Gbit/s.

SPC (Super Physically Contact)

Mechanical polishing of the ends of optical connectors. Provides a tighter fit and use in systems with speeds greater than 1.25 Gbps.

UPC (Ultra Physically Contact)

Automatic polishing. The planes of the connected connectors fit even more tightly than in PC and SPC, therefore such connectors are used in information transmission systems with speeds of 2.5 Gbit/s and higher.

APC (Angled Physically Contact) polishing:

The contact surface of these connectors is beveled 8 - 12 degrees from the perpendicular. This grinding method is used to reduce the energy level of the reflected signal (at least 60 dB). APC connectors are used only in conjunction with other APC connectors and cannot be used in connection with other types of connectors (PC, SPC, UPC). They are distinguished by green markings on the plastic tips.

Types of optical patch cords

Simplex (SX) and duplex (DX) patch cords

Optical patch cords can be simplex (for one connection) and duplex (for two connections).

Patchcord SC-SC simplex (SX)		Patchcord SC-SC duplex (DX)

Transitional patch cords

To transition from one type of optical connector to another, adapter optical patch cords are used. The need for their use arises quite often when switching equipment for various purposes and production. To do this, adapter patch cords are terminated with different optical connectors: for example, at one end - LC, at the other end - FC.

Transition patch cords are simplex and duplex.

Patchcord colors

The shell of optical patch cords differs depending on the type of optical fiber and has the following color:

• yellow - for single-mode fiber;
• orange - for multimode fiber with a diameter of 50 microns;
• blue, black - for multimode fiber with a diameter of 62.5 microns.

Differences from generally accepted color markings may occur in the manufacture of duplex patch cords.

Marking of optical patch cords

Typically, the marking of optical patch cords indicates:

• connector type: usually SC, FC, LC, ST, MTRJ;
• fiber type: single mode (SM) or multimode (MM)
• polishing class: PC, SPC, UPC or APC;
• number of fibers: one (simplex, SX) or two (duplex, DX);
• diameter of the light-conducting core and buffer: usually 9/125 for single-mode patchcords and 50/125 or 62.5/125 for multimode patchcords;
• patchcord length.

Optical connectors are used when terminating optical fibers for their connection with passive or active telecommunications equipment.

Today there are a large number of specialized optical connectors on the market. In telecommunications and cable television networks, the most widely used connectors are SC, FC, ST, having standard sizes and miniature LC. Optical connectors can connect one or more fibers.

The optical connector consists of a housing, inside of which there is a tip (ferrule) with a precision longitudinal concentric channel. The channel diameter depends on which optical fiber will be used - single-mode or multimode. For single-mode fiber, the diameter of the ferrule channel is 125.5-127 microns, for multimode fiber it is 127-130 microns. The most common outer diameter of ferrules is 2.5 mm, but small form factor optical connectors use 1.25 mm ferrules. The standard ferrule material used is zirconium dioxide.

The ferrule is connected to an optical fiber: the fiber without a sheath is inserted into the channel of the tip and fixed, the protruding end of the fiber is cleaved parallel to the surface of the ferrule end, and the ferrule end itself is polished. Next, the ferrule with fiber is combined with the connector body. After connecting the fiber and ferrule, the assembly is tested for defects (using a microscope or interferometer). For single-mode fiber, the fiber alignment accuracy in the ferrule must be higher than 0.5 µm, the angular deviation no more than 5 degrees, and the return loss no less than 40 dB.

There are several commonly used types of connectors, each of which requires a different assembly method. But at least two steps of these methods are common to all types.

1) The fiber is secured in the optical connector using epoxy resin. This process is important from the point of view of ensuring connection reliability. The epoxy resin prevents movement of the optical fiber, allowing uniform polishing of the ends of the ferrule and the optical fiber.

2) The end of the ferrule is polished to ensure the tightest connection between the connectors. This is necessary in order to reduce the attenuation and back reflection introduced into the line at the point of connection of the connectors.

There are several types of polishing

• RS (Physically Contact)
• UPC (Ultra Physically Contact)
• APC (Angled Physically Contact)
• SPS (Super Physically Contact)

In the case of UPC polishing, the plane of the ferrule end is perpendicular to the optical waveguide of the fiber, while in APC it is inclined at an angle of 8°.

In telecommunications, UPC polished optical connectors, designated in blue, are used as standard; less commonly, APC, designated in green. APC polished optical connectors are not compatible with other types of connectors; they are widely used in cable television networks.

The choice of polishing method depends on the tip material. If the tip material is very hard, such as ceramic, then typically the tip is rounded at the end end and is referred to as pre-rounded. Soft tip materials such as composite thermoplastics or glass ceramics can be polished flat. These materials are used intensively because they wear out at about the same rate as fiber optics and maintain high quality physical contact.

The end ends of the fiber are rounded so that the light is not reflected directly back to the source (the angle of reflection is equal to the angle of incidence). In the case of a rounded end, the reflection occurs back at an angle and is scattered, and the fibers come into contact with the most protruding points located in the middle part of the light-carrying core of the fiber. Thus, there is no air gap.

Reflection can be further reduced by using Angled Physical Contact (APC). Angular contact reflects light into the cladding of the fiber rather than into the core.

The return loss of the optical connector must, as already mentioned, be at least 40 dB.

Another important characteristic of an optical connector is the number of connection cycles. It is determined by the number of connections/disconnections, from which the characteristics of the connector will begin to deteriorate. This number, as experience shows, ranges from 200 to 600 connections. At the end of the life cycle, the loss on the connector should not increase by more than 0.2 dB.

Requirements for connectors:

• Low insertion loss
• Small back reflection
• Resistance to external mechanical, climatic and other influences
• High reliability and simplicity of design, slight deterioration of parameters after repeated reconnections

Types of optical connectors

ST connectors were developed in the mid-80s. The successful design of these connectors has led to the appearance on the market of a large number of their analogues. Currently, ST connectors are widely used in optical subsystems of local networks. A ceramic tip with a diameter of 2.5 mm, with a convex end surface with a diameter of 2 mm, ensures physical contact of the joined optical fibers. To protect the fiber end from damage when twisting during installation, a side key is used that fits into the groove of the socket; the plug on the socket is fixed with a bayonet lock.

ST connectors are simple and reliable in operation, easy to install, and relatively inexpensive. However, the simplicity of the design also has negative sides: sensitivity to sudden forces applied to the cable, as well as to significant vibration and shock loads, because the tip is a single unit with the body and shank. This drawback limits the use of this type of connectors on moving objects. ST connector parts are usually made of nickel-plated zinc alloy, less often of plastic.

When assembling the connectors, the aramid threads of the cable's reinforcing braid are laid on the surface of the rear part of the housing, after which the metal sleeve is pushed over and crimped. This design significantly reduces the likelihood of fiber breakage when the connector is pulled out. To further increase the mechanical strength of connecting cords, connectors from a number of manufacturers provide for crimping on the back of the housing not only the aramid threads, but also the outer shell of the minicable.

Currently, the ST connector is being replaced by a more advanced FC connector.

This type of connector is widely used for both single-mode and multimode fiber. The SC connector belongs to the class of general-use connectors and is used both in networks with long sections and in local networks. The device uses a push-pull articulation mechanism.

The basic type SC connector consists of an assembly (plug) containing a ferrule, inserted into the connector body, centering the ferrule. An optical SC connector can be combined into a module consisting of several connectors. In this case, the module can be used for a duplex connection (one fiber of which is used for forward transmission, and the other in the reverse direction). The connector has a key that prevents the fibers from being connected incorrectly.

FC type connectors are mainly intended for use in single-mode long-distance communication lines, specialized systems and cable television networks. A ceramic tip with a diameter of 2.5 mm with a convex end surface with a diameter of 2 mm ensures physical contact of the abutting light guides. The tip is manufactured to strict geometric tolerances to ensure low loss and minimal back reflections. To fix the FC connector to the socket, use a union nut with an M8 x 0.75 thread. In this design, the spring-loaded tip is not rigidly connected to the body and shank, which complicates and increases the cost of the connector, but this addition pays off with increased reliability.

The insertion loss level of the FC type connector is<0,4 дБ. Они имеют средства для настройки. Ключ настройки позволяет настраивать уровень вносимых потерь до нескольких десятых дБ. После того, как позиция минимальных потерь найдена, ключ может быть зафиксирован.

FC type connectors are resistant to vibration and shock, which allows them to be used on appropriate networks, for example, directly on moving objects, as well as on structures located near railways.

Miniature LC connectors are approximately half the size of conventional SC, FC, ST versions with a tip diameter of 1.25 mm instead of the standard 2.5 mm. This allows for high-density patch panel mounting and dense rack-mount layouts.

The connector is fixed using a clamping mechanism that prevents accidental disconnection.

D4 connector

This type of optical connector is especially widely used for single-mode fiber. It is similar in many respects to the FC connector, but has a smaller tip diameter of 2.0 mm.

FDDI connector

The FDDI connector is designed as a dual-channel connector, using two ceramic ferrules and a side-latch mechanism. The durable casing protects the tips from accidental damage, while the floating joint ensures a tight, effortless fit. The insertion loss level is about 0.3 dB for single-mode fiber and about 0.5 dB for multimode. FDDI is a local area network technology used for packet data transmission at a speed of 100 Mbps in accordance with the ANSI standard.

Optical connector E-2000 and F-3000

E-2000 connectors are a rather complex design. A special key is required to disconnect the connector, so the likelihood of accidental disconnection of the E-2000 connector is reduced to zero. After disconnecting the connector, the hole is closed with special curtains. These connectors are distinguished by a large number of connection cycles - up to 2000.

F-3000 optical connectors are an improved version of the E-2000 connector. The difference lies in the diameter of the ferrule - 1.25 mm (for the F-3000) and in the material of the curtains; for the F-3000 they are metal.

There are also a large number of types of optical connectors - HDSC, FJ, SC-Compact, MU, SCDC, SCQC, Mini-MT, MT-RJ, Mini-MPO, Optoclip II, VF-45 and others. These connectors have a narrow application purpose and are not currently widely used.

Optical connectors, which are sometimes called detachable connectors, are designed to provide detachable connection of connecting and termination cords to switching equipment in crossovers, data sockets of workstations and to network equipment.

The list of main functions of the fiber optic connector includes:

• ensuring fiber insertion into the splice point with a given bending radius;
• protection of fiber from external mechanical and climatic influences;
• Fiber fixation in the centering system.

Optical connectors must meet the following basic technical requirements:

• introducing minimal attenuation combined with obtaining high backscatter attenuation;
• ensuring long-term stability and guaranteeing parameters;
• high mechanical strength with minimal dimensions and weight;
• ease of installation on the cable;
• simplicity of the connection and disconnection process;
• the presence of convex end surfaces at the tips;
• preliminary special treatment of tips.

Standard requirements for optical connectors are contained in both major regulatory documents (TIA/EIA 568C and ISO/IEC 11801-2008). The standards normalize only the most general provisions and specify:

• type of connectors allowed for use in fiber optic subsystems of SCS;
• basic transfer parameters of various types of connectors;
• connector durability requirements;
• rules for connecting optical connectors.

The standard requirements for the maximum values ​​of attenuation, reflection losses and durability of SCS optical connectors will be discussed further.

The connector must be marked with symbols in the form of the letters A and B. A plug marked A must always be connected to a socket with the same marking, and vice versa. According to the standard, a double SC connector plug must have different markings for its halves, and if you look at it from the tips so that the keys are on top, then the left plug is always marked with the letter A, and the right one with the letter B. The marking of the pass-through socket has one feature. It has different markings on its different sides. The point of marking the plugs and sockets of the SC connector is that it allows you to determine the direction of “movement” of the fiber optic signal. The plug marked A is always the source, and the socket with the same marking is the receiver, and vice versa. Similarly, on network equipment, the socket marked A is the input of the fiber-optic receiver, and the socket marked B is the output of the fiber-optic transmitter.

Currently, most connectors are designed to connect two optical fibers. There are designs called group (or multi-channel) connectors that allow the simultaneous splicing of two or more pairs of optical fibers. At the same time, the share of such structures in the total volume is growing at a very fast pace. For use in special operating conditions (high humidity, vapors of aggressive materials, etc.), sealed connectors are used. There are also known designs of so-called hybrid connectors, which allow simultaneous splicing of both optical fibers and electrical conductors.

Lens type optical connectors

There are lens and contact versions of optical connectors. Lens-type connectors were widespread in the early stages of the development of fiber-optic communication technology and involve the use of lenses or their equivalents. With the help of this element, the light coming out of the transmitting fiber is first converted into a parallel beam of large diameter, and then, with the help of a second element, is focused onto the core of the receiving fiber. The main advantage of this option is less sensitivity to axial and lateral displacements of the spliced ​​fibers. Contact-type connectors involve connecting fibers end-to-end, and the parallelism of their axes to each other and the minimum possible distance between the ends are additionally controlled. Due to this design, contact-type connectors make it possible to obtain significantly better weight and size parameters and fundamentally lower signal attenuation (there are no losses in lenses and Fresnel reflection). For this reason, the vast majority of modern connector designs implement a contact connection scheme.

Pin type optical connectors

The basis of most contact-type connector designs is the plug tip. This tip is inserted into the alignment element in the form of a sleeve, and the connector itself contains two main components: a plug (connector) and a socket (coupler).

The bulk of connectors produced by industry are implemented according to the so-called symmetrical scheme, that is, both spliced ​​optical fibers are reinforced with identical plugs, which are then inserted on both sides into a connecting socket equipped with a special centralizer. There is also a fairly small group of fiber optic connectors that contain only two elements: a plug and a socket. Such connectors are called asymmetrical.

To fix a plug installed in a socket, a bayonet element (the so-called ST type connector), a latch can be used, and this element can be made either internal (SC type connector) or external lever type (LC, E-2000 connectors), as well as a polygonal or round union nut with a knurled surface (FC and SMA connectors). In a similar way, the terminal active equipment is connected to the fiber optic cable, the interface of which is supplied with the mating part of the fiber optic connector socket.

Connectors are manufactured in both multimode and single-mode versions, the latter being structurally similar to a multimode connector and differing mainly in tighter tolerances on the geometric dimensions of the plug tip and the centering elements of the socket, which make it possible to keep losses when splicing single-mode fibers within acceptable limits. For example, the standard hole diameter of a fork tip for reinforcing single-mode fibers is 126+1/-0 µm, while in fork tips for multimode fibers the value of this parameter is 127+2/-0 µm.

Many multimode connectors have several types of plugs designed to fit fibers with different cladding diameters (125, 140, 280 µm, etc.). Structurally, they differ from each other only in the diameter of the tip hole.

The operating temperature range of most fiber optic connector designs is -40 to +85°C, which is the same as the operating temperature range of most outdoor cable designs.

The principle of operation of the OB connector is quite simple: two fiber optic connectors are combined together inside a special sleeve according to the principle of end joining. Therefore, in order to practically implement the principle of connecting the ends of optical fibers end-to-end, the optical fiber is glued with glue in the center into a cylindrical pin (ferrule) with a very small internal diameter equal to 126-127 µm for a single-mode optical fiber and 127-128 µm for a multimode optical fiber with an external diameter shells 125 microns. In classical technology, epoxy glue (resin) is most often used as an adhesive, which simultaneously performs two important functions. It protects the optical fiber in the connector, cleaned from urethane acrylate sheath, from the effects of ambient temperature and humidity and gives the required flexibility to the optical fiber during the polishing process. The end of the ferrule is then polished until a clean, finely polished, scratch-free surface is achieved.

To obtain an OB detachable connection, two OB connectors are connected end-to-end with pre-polished ends in a centering sleeve. There are many types of OB connectors, however, the standard pin diameter is considered to be 2.5 mm. The ferrules used often differ from each other. So, some manufacturers make them from metal, ceramics or even plastic. It has been experimentally established that the characteristics of pins made of ceramics with zirconium oxide are significantly better than those of metal pins made of nickel-silver alloy or tungsten carbide. Therefore, when choosing an OB connector assembly, you should pay special attention to what the ferrule or pin of the OB connector is made of. The use of pins for optical fiber connectors made of plastic, even of a particularly durable and resistant type, will give an undoubted gain in price, but an obvious loss in technical and operational characteristics.

The main parameters of some types of fiber optic connectors are given in table. 1.

Table 1. Basic parameters of optical connectors
Connector type	Tip material	Retainer	Average attenuation, dB at a wavelength of 1300 nm
			multimode	single-mode
	Ceramics	Union nut
	Ceramics
	Ceramics
		Union nut
	Ceramics	Bayonet
	Cupronickel

Main types of optical connectors SCS

1. SC type connectors

The SC connector (Fig. 4) (from English, subscriber connector - “subscriber connector”, sometimes such an unofficial decoding of this abbreviation as Stick-and-Click is used) was developed in 1986 by the Japanese telecommunications corporation NTT for use in subscriber devices for various purposes. Currently standardized by the international standard IEC-874-13. The current editions of the standards define it as the main type of connector for use in SCS. Can be made in single and double (duplex) versions. The main idea behind its design is to create a device with a plastic housing that protects the tip well and ensures smooth connection and disconnection with a linear movement. The vast majority of SC connector plugs are equipped with ceramic tips; there are also a few examples of these products with tips made of stainless steel. The tip of the SC connector is recessed into the plug body, which protects it from contamination. The linear movement of plugging and unplugging makes this connector especially suitable for use in 19-inch shelves, as it allows for increased port density by bringing the receptacles closer together. The latch only opens when pulled by the housing, which increases operational reliability.

Rice. 4.SC connector

SC connectors provide greater stability of parameters (withstand at least 500 connections and disconnections), which is greatly facilitated by the absence of rotation of the tips relative to each other when turning on and off. As can be seen from Table 1, this connector is one of the best in terms of insertion attenuation. There is a tab-shaped key on the top of the plug body that prevents it from being inserted into the outlet in the wrong position.

To obtain a duplex (double) connector from a simplex (single) connector, two methods are used. The first of them is based on the fact that there are clamps on the fork body that interact with each other when assembled. In the second case, an external fixator is used. It can be made in the form of a cage consisting of two symmetrical halves with sockets for the fork bodies, or it can be an H-shaped part into the side grooves of which the forks are inserted. According to the latter scheme, for example, a type 2A1 latch from Lucent Technologies is implemented, equipped with standard symbolic markings in the form of the letters A and B. The distance between the axes of the plug tips in the double connector is 12.7 mm. The large plastic housing of the SC plug and socket allows, in addition to symbolic marking, effective color marking. Single-mode and multimode versions of the SC connector according to the TIA/EIA-568B standard have, respectively, blue and gray (or beige) body colors. A single-mode SC connector is also available with a green housing and a beveled end tip to reduce back reflection. Individual samples of SC connectors with housings of plugs and sockets of non-standard colors are also widespread.

2. ST type connectors

The optical connector type ST (Fig. 5) (from the English straight tip connector, that is, “connector with direct installation”; sometimes an unofficial decoding of this abbreviation is used - Stick-and-Twist - “insert and twist”) was developed by the Bell Laboratory of the company AT&T (Lucent Technologies) in 1985 to replace the bi-conical connector.

Rice. 5. ST connector

Before the advent of the SC connector, it was most common in optical subsystems of SCS and local networks. The connector design is currently defined by the international standard IEC 874-10, which requires a 2.5 mm diameter ceramic tip with a convex end surface. The plug is secured to the socket using a spring-loaded bayonet element that rotates 1/4 turn. Therefore, the ST connector is sometimes called a BFOC connector (from the English bayonet fiber optic connector).

There are several design options for ST connectors, differing mainly in the shape and material of the bayonet lock, as well as the principle of attaching the plug body to the buffer shells and protective coatings of the light guide.

Lucent Technologies has developed three plug versions of this connector: ST, ST11 and ST11+, which are fully compatible with each other in terms of slots in the socket and have minor design differences that improve their performance properties as they move to a more advanced model. Thus, in particular, the bayonet lock nut of the ST fork has an axially open slot, whereas in both later versions this slot is closed by a bridge. An important feature of Lucent Technologies plugs is that there is no need to use a crimping tool when reinforcing fibers in a buffer coating with a diameter of 900 microns.

The metal design of the body of the plug and socket of the ST connector provides high mechanical strength, but significantly complicates its coding and identification. Sometimes the letters SM and MM are embossed on the socket housings for single-mode and multimode options, respectively. Some companies offer ST forks with plastic shanks of different colors; various rings, sleeves and other similar products that are not standard marking elements are also often used in practice.

The design of the ST connector does not provide the ability to form a duplex plug. Accordingly, its socket is produced by the majority of manufacturers in a single version. Only Nexans Cabling Solutions offers dual ST sockets in one housing.

The advantages of the ST connector include low cost combined with ease of installation and connection, while the disadvantages include the following:

• a strongly protruding tip increases the likelihood of contamination;
• the absence of a double option increases the complexity of connecting double cords and the likelihood of errors during switching;
• the absence of color or other factory markings makes their identification difficult;
• the turning force during connection causes friction on the fork tips, which leads to damage to their polishing and, ultimately, to an increase in the insertion attenuation after repeated connections and disconnections;
• The principle of fixation based on a bayonet nut does not provide the stability of parameters required for some applications under vibration influences.

To partially protect the tips from friction during connection, the designs of ST connector plugs provide a special protrusion that is inserted into the groove of the socket.

Other types of optical connectors

1. FC type connectors

FC type connectors (Fig. 6) are defined by the international standard IEC 874-7 and are mainly aimed at use in single-mode technology. They are most widespread in various telecommunication systems for public communication networks. To ensure low attenuation and minimal back reflection, the connector tip is manufactured with a rounded tip (with very tight dimensional tolerances). The very first version of the connector plug had a tip with a flat end, which did not allow for good performance parameters. After switching to a tip with a rounded end, which ensures physical contact between the spliced ​​fibers, the connector received the name FC-PC (PC - Physical Contact), allowing it to be distinguished from earlier designs. Flat tip FC connectors are not currently manufactured, so the names FC and FC-PC are equivalent.

Rice. 6. FC connector

The design of the connector provides reliable protection of the ceramic tip from contamination, and the use of a union nut for fixing provides greater tightness of the connection area and reliability of the connection when exposed to vibrations. The main disadvantage of the design, along with the large dimensions, is the inconvenience of operation due to the need to perform several turns of the fastening nut during switching on/off.

The element for protecting the connector tip from rotation is made in the form of a cylinder with a diameter of 2 mm. Some companies additionally use other values ​​of this parameter (in particular, Molex produces plugs with a diameter of this element of 2 mm) to solve the problem of mechanical locking against incorrect connection.

Optical connectors of this type are produced mainly for telecommunications equipment operating with SDH, ATM and similar transmission technologies.

The FC connector socket is available in two versions: type SF with a square flange and fastening with two M2 screws and type RF with a round flange and fastening with a nut.

Small form factor (SFF) optical connectors. Designs of optical connectors with tips of reduced diameter.

1. LC type connectors

The most famous representative of the first direction of improving connectors with increased installation density as of 2005-2006. is an LC type connector (Fig. 7) (from English, link control, the decoding of this abbreviation is also very common as Lucent Connector), which was developed by the American company Lucent Technologies in 1997. (according to other sources, in 1996). The connector can be produced in both single-mode and multimode versions. Its design is based on the use of a ceramic tip with a diameter reduced to 1.25 mm and a plastic body with an external lever-type latch for fixation in the socket of the connecting socket. The connector allows both simplex and duplex use.

Rice. 7.LC connector

The developers of this type of fiber optic connector, in accordance with current and future editions of SCS standards, guarantee up to 500 on-off cycles without deteriorating loss characteristics. This, along with the use of a ceramic tip, is facilitated by the principle of linear insertion of the plug into the socket (push-pull).

Standard epoxy sealing procedures are used to install the LC fork. The design of the plug allows its installation both on fiber in a 0.9 mm buffer coating and on connecting cords with a 2.4 mm hose. At the same time, installation on 900 micron fiber can be done in the field, while gluing onto a cable in a 2.4 mm hose during the production of connecting cords, due to its small dimensions, is carried out only in production.

The main technical characteristics of LC type connectors are given in table. 2.

Table 2. Main technical characteristics of connectors with reduced diameter lugs

Table 2. Main technical characteristics of connectors with reduced diameter lugs
Parameter/Connector
Average loss, dB
Standard deviation of losses, dB
Reflection coefficient, dB
Change in losses after 500 connection-disconnection cycles, dB, no more
Change in losses in the temperature range -40...+75 °C, dB, no more
Tip material		Ceramics

2. MU type connectors

The second representative of the design of the variety under consideration is the MU connector (Fig. 8) from the Japanese telecommunications corporation NTT. This product can be considered as a small-sized version of the SC connector, which is emphasized in some publications by the designation “mini-SC”. Similar to its predecessor, this type of connector contains a housing with an internal latch (push-pull principle), and due to the smaller tip diameter and miniaturization of other design elements, it has approximately half the dimensions.

Rice. 6.MU connector

On the commercial equipment market you can find both simplex and duplex versions of the connector of this type. The duplex version of the MU connector is known in two varieties. The first of them is implemented on the basis of a common non-separable holder for two forks with a distance between the centers of the tips of 4.5 mm. The value of this parameter for the second, collapsible version is 6.5 mm.

3. Connectors type F-3000

The F-3000 type connector (Fig. 7) is an improved version of the E-2000 type connector described below. It retains the main design features of the prototype and differs from it in the use of a ceramic tip with an outer diameter of 1.25 mm and a metal protective cover instead of a plastic one. The latest innovation guarantees eye protection for operating personnel when working with equipment equipped with powerful laser emitters. According to the developers, the plug of the F-3000 connector can be freely inserted into the socket of the LC connector.

Rice. 7. Connector F-3000

Small form factor (SFF) optical connectors. Small-sized connectors with 2.5 mm diameter tips

The second type of approach is based on retaining the main element of previously used designs in the connector - a tip with a diameter of 2.5 mm. Improvement in weight and size indicators is achieved through a more dense layout and, possibly, miniaturization of individual elements of the body. The most famous developments in this area are connectors of the E-2000, SC-Compact and FJ types.

1. E-2000 type connector

The E-2000 type connector (Fig. 8) (Europe, 2000) was created by Diamond and has become widespread in some European countries (Switzerland, Germany, etc.). Known in two main design options, completely corresponding to each other in terms of seating. According to the first of them, promoted by the developer, the Diamond company, the tip is made according to a composite design in the form of a cupronickel cylinder, onto which a centering ceramic sleeve is tightly placed. In the E-2000 connector from Huber+Suhner, the tip is made using classical technology in the form of a ceramic cylinder. The plug is secured in the socket using an external lever-type latch.

Rice. 8. E-2000 connector

The connector can be used in both simplex and duplex versions. The duplex connector is known in regular (duplex, distance between the axes of the tips 12.7 mm), compact (compact duplex, distance between the axes 6.4 mm) and vertical (low profile duplex, plugs are located one above the other with a 180° turn) . To obtain one duplex plug from two single ones, a special locking latch is used; the duplex socket is compatible in its seats with a standard modular connector socket only for the compact version. The E-2000 type connector differs from earlier designs in the ability to use effective color coding (currently the standard includes 8 colors) and mechanical locking when using a replaceable socket frame, as well as the presence of a protective cover integrated into the design. The latter, when installed in a socket, opens automatically and reliably protects the tip from contamination.

2. SC-Compact connector

The SC-Compact connector from the Swiss company Reichle & De Massari is a successful example of a deep modernization of a well-proven product in mass production in order to obtain new properties. The prototype of the connector is the well-known SC, however, by eliminating external fastening elements and developing a new fixing mandrel, Reichle & De Massari engineers were able to reduce the distance between the axes of the tips from the usual 12.7 mm to 7.5 mm and thereby fit the socket into Modular connector socket seats. Note that the so-called vertical version of the duplex plug SC connector of the Japanese company Honda Tsushin Kogyo has a distance between the axes of the tips of 8.5 mm. The socket of this plug is close in seating to the socket of the modular connector, however, it is not interchangeable with respect to it.

3. High Density SC Connector

Another representative of connectors that use a similar idea is the High Density SC Connector product from ZM. This connector differs from the standard density connector in that it has the overall dimensions of the plug, reduced in cross section to 6.0x7.2 mm, versus 7.4x9.0 mm for the prototype. This development provides the greatest advantage when using a quad socket for connection. With this design, the distance between the centers of the sockets is approximately 7 mm, that is, this connector provides a port density approximately equal to that of its electrical counterparts, however, without maintaining backward compatibility.

4. FJ type connector

Back in 1996, the Panduit company proposed an FJ (fiber jack) or Opti-Jack type connector (Fig. 9). This product is intended for use in a PAN-NET structured cabling system and is only available in duplex version. The basis of the connector is also a ceramic tip with a diameter of 2.5 mm, however, due to a more dense layout and, in particular, reducing the distance between the axes of the tips to 6.4 mm (0.25 inches), the dimensions of the socket are reduced to the size of the socket of an electrical modular connector. The plug is secured in the socket using a lever-type latch. To improve operating conditions, the latch lever is covered with a dome-shaped shank cover. The design allows for field assembly, for which an original adhesive technology has been developed using two-component anaerobic glue. Cleaning the end surfaces of the tips from contamination, the need for which may arise during routine operation, is ensured through the use of a collapsible socket design: its individual parts are attached to each other with latches.

Rice. 9. FJ connector (Opti-Jack)

The FJ type connector differs from other designs in that its socket is not a separate structural element, but is always combined with one of the plugs. Only in 1998 did a classic socket for connectors of this type appear, but it is intended exclusively for use for measuring purposes.

The FJ connector was originally released only in a multimode version with a beige housing. In 1998, its single-mode version with a blue body appeared.

Fiber optic connectors of group type

The third type of approach is represented by a fairly large group of developments of multi-channel or group connectors. The most advanced products in this group allow you to simultaneously splice up to 18 optical fibers, that is, they exceed electrical modular connectors in packaging density by nine times. Quite often these products are designed as a smaller or simplified version of a “large” group connector designed for use in telecommunications applications. A common distinguishing feature that unites all the designs discussed below is the use of a linear principle of installation in a socket (push-pull principle) without the use of threaded or bayonet fasteners.

1. SCDC and SCQC type connectors

SCDC and SCQC connectors are promoted by a consortium that includes Siecor, Siemens and IBM, and are distinguished by the fact that they use the outer housing of the traditional simplex SC connector to reduce development time and partially unify with existing products. What is new is the use of a centering element, very similar to a conventional tip and having two (SCDC) or four (SCQC) channels for fixing spliced ​​fibers in them.

2. Mini-MT and MT-RJ type connectors

The principle of partial unification is also used in the Mini-MT connectors (the abbreviation “MT” means Mass Termination) developed by Siecor and MT-RJ (Fig. 10) by a consortium of AMP, Siecor, Hewlett Packard, USConec and Fujikura. These products use the same centering element with a shape close to rectangular in cross-section, designed for two or four light guides. The difference between these connector options is that in MT-RJ the element for fixing the plug in the socket has the appearance familiar to SCS users and is similar to the latch of the lever type plug of an electrical modular connector. Note that the MT-RJ connector is one of the main elements of the Solarum fiber optic cable system from AMP.

Rice. 10.MT-RJ connector

3. MPO and Mini-MPO type connector

MPO (Multofiber Push-On) group connectors are actively used for connecting fiber optic ribbon cables. The largest share among the promising types of fiber optic connectors for SCS is occupied by the Mini-MPO optical connector from Berg Electronics, which allows splicing up to 18 fibers simultaneously. It is expected that connectors of this type have great prospects for installation in data storage centers (SAN) where high connection density is required. As such, we can expect widespread adoption of MPO group connectors for 24- or 48-fiber cables in the coming years.

Fiber optic connector designs without centering tip

The centering tip of a fiber optic connector plug is an expensive precision part (according to some estimates, the tip's share in the plug design reaches 40% of its cost), and the process of reinforcing the optical fiber with it is a rather complex and time-consuming procedure. The desire to eliminate these shortcomings has led to the emergence of two designs in which there are no tips, and the process of centering the fibers during the joining process is carried out by other means.

The common distinguishing features of the connectors of this group are:

• a fiber protruding several millimeters from the holder, the end of which is chipped and prepared for splicing during the installation of the connector plug on a special technological device;
• the mandatory presence of a spring-loaded cover that covers the fibers when not in use;
• the ability to install a plug or socket only using a set of proprietary technological equipment.

1. Optoclip II type connector

The Optoclip II type connector (Fig. 11) from the Swiss company Huber+Suhner (according to other sources, the developer of the connector is the French company Compagnie Deutsch) is implemented according to the most common symmetrical scheme and is based on the use of a single plug, which, if necessary, can be connected to another plug to obtain a duplex option.

Rice. 11. Optoclip II connector

Preliminary alignment of the fibers when connecting them is carried out using a cone-shaped guide, final alignment is carried out using a system of three balls shifted relative to each other by 120°, one of which is movable in the vertical direction.

2. VF-45 type connector

In contrast, the fiber optic connector VF-45 (Fig. 12) (sometimes the name VG-45 may be used) from the ZM company is implemented on the basis of a V-shaped groove and is designed to reinforce two ribbon cable fibers simultaneously with one plug. To ensure the possibility of clear insertion of the light guides into the guide grooves and to obtain physical contact between the end surfaces of the spliced ​​fibers when the plug is installed, the end section of the light guides in the socket is fixed at an angle of 45°, which additionally slightly reduces the total length of the connector. As an interesting technical feature of the connector plug, we note that when it is installed in a socket, the protective cover, unlike the vast majority of other designs, moves sideways and does not rise up.

The VF-45 connector solves in a rather original way the problem of cleaning the end surface of spliced ​​fibers, which is a very difficult task for any product without a centering tip. A special washing device cleans the fibers by pumping a large amount of cleaning liquid through the connector socket. To obtain the required level of back reflection, the end surface of the fiber is beveled at an angle of 9° when processed in a cleaver during connector installation.

Note also that these connectors solve the color coding problem differently. The Optoclip II version uses a conventional housing made of plastic in different colors, while in the VF-45 the multimode and single-mode versions are coded using only a protective door of different colors.

The list of considered types of promising fiber-optic connectors used by some manufacturers is presented in Table. 3.

Table 3. Some types of promising fiber optic connectors supported by various SCS manufacturers
ADC Telecommunications, USA
	NetConnect (Solarum)
BTR Telecom, Germany
Corning, USA IBM, USA	Corning Cable Systems
Lucent Technologies, USA
	Molex Premise Networks
Ortronics, USA
RiT Technologies, Israel
	Siemon Cabling System

At technical seminars on optical fiber solutions in SCS, I repeatedly heard from course students that optical fiber connectors from one or another manufacturer could not cope with the functions assigned to them. This concerned both the mechanical characteristics of the OF connectors and the characteristics of insertion attenuation and reflection losses.

It should be noted that the amount of insertion attenuation mainly depends on the following main factors:

Radial displacement of the OB,
- end clearance,
- angular displacement of the OB,
- an air gap formed due to excessive polishing of the ends using the RS method (physical contact).

Modern OB connectors used in LAN networks have a typical attenuation of about 0.2 dB or better.

In addition to the above factors, additional attenuation introduced into the optical fiber connector can be introduced by various designs of optical fiber connectors with large tolerances on their parts. Thus, the recent flooding of the market with cheap OB connector assemblies with no manufacturer’s brand (noname) from Southeast Asia sometimes in practice leads to a complete loss of functionality of the OB channel. Choosing RF solutions from well-established and time-tested manufacturers of RF equipment will result in a definite win.

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