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EYE ON FIBER

 

 

Fiber Optic Connectors: Yesterday Today, and Tomorrow

An Update

 

Eric R. Pearson, CPC, CFOS

Pearson Technologies Inc.

 

Abstract

In this paper, we present an overview of the status of fiber optic connectors. This overview covers: the advantages, status and changes in use of connector types; results of cost analyses of installation methods; and projections of future changes. Significant results include connector type cost comparisons and installation methods cost comparisons. The connector cost comparisons demonstrate a favorable total cost for SC connectors and a favorable cost for the installation methods of epoxy, quick cure and hot melt adhesive.  These two results are the opposite of commonly held views.  The favorable total cost for SC connectors results from a doubling of connector density relative to that of the ST-compatible connectors. The favorable total installed cost for epoxy, quick cure and hot melt adhesive methods results from a cleave and leave connector price premium that is not recovered through reduced labor cost.


Introduction

During the last 25 years, fiber optic connectors have evolved from their original designs to their current designs. Relatively speaking, the original designs had the characteristics of large size, crude design, difficulty of installation, and in some aspects, poor performance. Todays designs have the characteristics of small size, simple design, ease of installation, and in many aspects, excellent performance. 

In this technical session, we present an overview of the state of fiber optic connectors.  We will examine the types in use, the advantages and disadvantages of these types, the methods of installation and their advantages and disadvantages, a comparison of total installed cost, performance concerns, and a review of the recent changes in testing required by TIA/EIA-568 B.

Types and Benefits

In order to simplify the presentation, we organize fiber optic connectors into two groups based on the level of performance required: moderate performance and high performance.  While not always true, moderate performance is required in data communication and CCTV applications; and high performance, in telephony or CATV applications.  This organization is somewhat arbitrary, in that the same style, or type, of connector can be used in either application. This organization ignores special purpose applications, such as underwater and field tactical

Connector Structure

Almost all connectors[1] have the same structural elements: ferrule, retaining nut or latching mechanism, back shell, key and strain relief boot (Figures 1 and 2).  When used on jacketed cables, connectors have a crimp ring or crimp sleeve for attaching the cable strength members to the connector back shell.

 

outer housing inner housing back  crimp    boot

with latch      with ferrule         shell  sleeve

Figure 1: The SC Connector Parts

Figure 2: The ST-Compatible Connector Parts

Ferrules provide the precise alignment of the fiber necessary to provide low power loss. Ferrules can have straight sides, be stepped, as in the 906 SMA, or be conically tapered, as in the biconic connector.  Ferrules can have flat end faces or radius[2] end faces.  Early connector designs have flat end faces; current designs have radius or angled end faces.

Ferrules can be of many different materials, though ceramic,[3] liquid crystal polymer, and stainless steel are the three most common. Early connector ferrule materials included the additional materials of nickel-plated brass, aluminum, zinc, thermoplastic polymer, and thermo set polymer.

Retaining nuts or latching mechanisms allow the connector to be connected to an active device or to a patch panel.  As retaining nuts require space for rotation, use of connectors with nuts increases the cost of the network.  Many early connectors have retaining nuts.[4]  Most current and small form factor (SFF) connectors have latching mechanisms.

The back shell is the location to which the cable is attached to the connector.  The back shell can be rigidly attached to the ferrule or can be separated from the ferrule by a spring. Early connector types had a rigid attachment.  Current and future connector types have separation of ferrule and back shell.

Connectors can be keyed or unkeyed. A key can be attached to the ferrule or be part of the latching mechanism.  Early connector types were unkeyed.  Current and future types are keyed. For most connector types, the key is fixed.  For some, such as the D4, the key is tunable for achievement of the lowest possible loss.

The strain relief boot provides a mechanism for controlling the bend radius of the fiber as it exits the back shell.  This boot improves the reliability of the connector.  Some early connectors had heat shrink tubing instead of the boot.  Almost all connectors sold today have boots.

Four Features Determine Performance

Four connector design features determine the performance and reliability of all connectors: keying; contact of ferrules; isolation of ferrule from axial tension on the cable, known as pull proof  behavior; and isolation of ferrule from lateral pressure on the back shell of the connector, known as wiggle proof behavior.

The first two design characteristics, keying and contact, are common to both the popular styles, which dominate the industry, and the small form factor (SFF) types, the challenging princes and future market leaders.

Keying is a design feature that was absent on the early designs, but mandatory on current and future designs (Figure 3).  The key prevents relative rotation of the ferrules of two mated connectors.  This prevention results in a reduction in the variability of power loss from connection to connection.

Figure 3: The SC Connector Latch (Top) and Key (Bottom)

This variability, referred to as repeatability is commonly specified as a maximum of 0.2 dB[5] from insertion to insertion. In testing repeatability during installation training programs, we find typical repeatability to be closer to 0.1 dB than to 0.2 dB.

From similar testing on unkeyed legacy connector styles (905 SMA and 906 SMA), we have found typical repeatability ranged from 0.5 dB to as high as 1.0 dB.[6]

This improvement from keying is obvious and significant in two situations: initial network certification and maintenance or troubleshooting. During initial network certification, the lack of high repeatability of unkeyed connectors could result in the need to tune connectors to bring link power loss into compliance with specification.  Subsequent disconnection and reconnection of any connector could result in loss of the preferred ferrule orientation and in link failure. 

During maintenance or troubleshooting, current loss measurements are compared to original measurements.  There is a possibility that improvement in the alignment of unkeyed connectors could mask a degradation of link elements.  With this possibility, interpretation of changes is more difficult with unkeyed connectors than with keyed connectors.

Contact of connector ferrules is a second design feature that was absent on the early connector types, but a consistent feature of current and SFF types.  Contacting ferrules provide the advantage of reduced power loss.  Because fibers have a critical angle within which all light travels,[7] a non-contact connector design allows the beam of light to expand to a size larger than the core of the receiving fiber.  This expansion results in the receiving fiber failing to capture, or couple, all of the light from the transmitting fiber (Figure 4). This change, from non-contact to contact connector design, has resulted in a significant improvement of power loss characteristics (Table 1).

Pull proof behavior results from isolation of the ferrule from motion due to axial tension applied to the back shell.

Table 1: Comparison of Non Contact to Contact Connector Losses[8]

Type

Typical loss

Maximum loss

Non contact

1.0 dB/pair

2.0 dB/pair

Contact

0.3 dB/pair

0.75 dB/pair[9]

 

Figure 4: Non-Contact Connectors Exhibit Increased Loss

This isolation, which can be achieved by inclusion of a spring between the ferrule and the back shell, prevents an increase in power loss when tension is applied to the cable

Wiggle-proof behavior results from isolation of the ferrule from the motion due to lateral pressure applied to the back shell.  Such pressure will result in increased power loss if the ferrule tilts in response to such pressure. In summary, a pull-proof, wiggle-proof connector provides more reliable operation than one that is neither.

Types of Connectors

Moderate Performance Connectors

Moderate performance connectors are those that exhibit moderate power loss.[10]  Moderate performance connectors are used in applications in which there is little or no concern for reflectance.

We define moderate power loss as a maximum of 0.75 dB/pair and a typical of 0.30 dB/pair. These values are appropriate for products used in data communications, CCTV, industrial, and process control applications. These values are typical for both multimode and singlemode connectors that are keyed and contact.

Two Popular Connector Types

The two most popular, or commonly used, connector types are the ST-compatible[11] and the SC.

ST-compatible. First introduced in 1986, the ST-compatible (Figure 2) is keyed, contact, moderate loss connector that is not pull proof or wiggle proof.  Installation of the ST-compatible is significantly easier and faster than installation of predecessor connector types.[12]

SC. First available in 1988, the SC (Figures 1 and 3) was developed by Nippon Telephone and Telegraph (NTT) in Japan. The SC did not begin to become commonly used until it became the connector type required for compliance with the Building Wiring Standard, TIA/EIA-568.

The SC is keyed, contact, moderate loss, pull proof and wiggle proof. Because of these latter two characteristics, the SC is more reliable than the ST-compatible.

This isolation results in a price higher than that of the ST-compatible connector (Table 2). This isolation makes damage of the fiber end face difficult by controlling the force with which the ferrules make contact.  In contrast, the installer can apply excess contact force during the insertion of an ST-compatible connector.

Table 2: Comparison of SC and ST Connector Prices

Manufacturer

SC/ST Price Ratio

Fiber Instrument Sales

1.38

Corning Cable Sys.

1.25

3M, ceramic ferrules

1.40

3M, composite ferrules

1.40

Molex

1.43

TYCO

1.48

A second benefit of this isolation is immunity to damage from pull and snap. An SC connector cannot be pulled and allowed to snap back into an adapter.  An ST-compatible connector can be pulled and snapped.  Usually, such action results in damage to the fiber.

We have observed this improved reliability: during fiber optic connector installation training, we experience a damage frequency for ST-compatible reference leads that is 3-6 times that of SC reference leads. Thus, in spite of the increased price (Table 2), the life cycle cost of the SC connector will tend to be lower than that of the ST-compatible connector.

The SC differs from the ST-compatible in two additional aspects: insertion method and ability to be duplexed. The SC insertion method is push on/pull off.  Because of this method, the SC connectors can be more closely spaced in a patch panel than can the ST-compatible, which requires space for rotation. In practice, this reduced spacing results in a connector density of two to four times that possible with ST-compatible connectors. This increased density results in a reduction in the total installed cost of SC connectors by reducing the number, or size, of enclosures required (Table 3).

Table 3: Comparison of Total Cost of 24 Connectors and Enclosure[13]

Connector Source

ST Cost

SC Cost

Savings.

%

1

160.00

104.00

35

2

160.00

116.00

27.5

3

160.00

126.80

20.8

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Figure 4: The Duplex SC

The SC can be duplexed, either through use of left and right outer housings[14] or an external clip (Figure 4). This capability results in increased network reliability because it is difficult to reverse the fibers in a duplexed SC connector.

Thus, use of SC connectors results in three cost reductions: reduced life cycle cost, reduced enclosure cost, and increased reliability. The popularity of the ST-compatible connector, which is in large part a consequence of its reduced cost, is, probably, a result of lack of knowledge of these SC-favorable cost factors.

Small Form Factor Connectors

While the ST-compatible and SC connector types have had long, and essentially successful lives, both have the same, significant drawback: large size. This size results in increased optoelectronic cost.  The large size of the connectors forces optoelectronic manufacturers to space transmit and receiver electronics far apart.  This large spacing results in half as many fiber ports in a fiber hub or switch as in a UTP hub or switch.

The optoelectronic manufacturers requested a reduced size fiber connector that would enable them to reduce the per port cost of hubs and switches.  Their goal was for cost parity of fiber switches with UTP switches. Connector manufacturers answered this request with a series of small form factor (SFF) connectors.

Increased use of SFF connectors is guaranteed by the latest revision to the Building Wiring Standard, TIA/EIA-568 B.  This standard states[15]:

Various connector designs may be used provided that the connector design satisfies the performance requirements specified within annex A. These connector designs shall meet the requirements of the corresponding TIA Fiber Optic Connector Intermateability Standard (FOCIS) document.[16] 

By the removal of the prior, exclusive recommendation for use of SC connectors in its earlier revisions, this standard guaranteed the increased use of SFF connectors. Such increased use has occurred and is expected to continue.

The request for reduced size fiber connectors resulted in five small form factor (SFF) connectors:

Fiber Jack

MT-RJ

LC

Volition (VF-45)

MU

These SFF connectors obviously meet the requirement of increased density (Figure 5).

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Figure 5: Comparison of SC, MU and LC Sizes

Fiber Jack. In January 1997, Panduit Corporation introduced the first duplex SFF plug and jack connector system, generically known as a Fiber Jack (Figures 6 and 7). This plug and jack connector system, trade name Opti-Jack, introduced a new package based on four existing and well proven technologies: 2.5 mm ferrules, quick cure adhesive installation, split alignment sleeves, and an RJ-45 form factor.

The 2.5 mm ferrules are the same as those in the ST-compatible, SC, FC, FDDI, and ESCON connectors. Quick cure adhesives had been in use since 1992.  The split alignment sleeves were commonly used in multimode adapters since 1986. The form was the same as that of the RJ-45, with which network installers and users were familiar.

Panduit use of these four well-proven technologies eliminated the risk of using this new product.  By its appearance, the Opti-Jack is the most rugged of the SFF connectors and is extremely well suited for fiber to the desk (FTTD) applications.

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Figure 6: The Opti-Jack Plug[17]

The Opti-Jack is keyed, contact, moderate loss, pull proof and wiggle proof. It is a duplex connector with one ferrule per fiber.  This structure allows separate alignment of each fiber for minimum power loss.

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Figure 7: The Opti-Jack[18]

MT-RJ. The MT-RJ was the second duplex SFF connector.  It is available in two versions, the plug and jack version from TYCO (Figure 8) and the plug, adapter and plug version from Corning Cable Systems and others (Figure 9).  The MT-RJ is keyed, contact, and pull proof. Optoelectronic manufacturers offer more products with the MT-RJ interface than with any other SFF interface.[19]

Although the MT-RJ plug has a size smaller than that of other SFF, the MT-RJ density in patch panels is the same as that of other SFF connectors.

At this time, MT-RJ optoelectronics are available at bit rates up to 1 Gbps. We are aware of no 10 Gbps optoelectronics that use the MT-RJ interface.

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Figure 8: TYCO MT-RJ System

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Figure 9: Corning Cable Systems MT-RJ System

LC. The LC is a simplex SFF connector that can be converted to a duplex form with a clip.  The LC  (Figure 10) was developed by Lucent Technologies, but is available from at least six manufacturers.  The LC was developed as a telephone connector designed to enable telephone companies to increase the density of installed connectors. Dense wavelength division multiplexing (DWDM) is one of the technologies creating the need for increased density.

With DWDM, it is possible to launch up to 200 wavelengths onto the same singlemode fiber.  With this level of multiplexing, the connector count, and patch panel space requirements at a telco location could rise 200-fold.  The SFF was an obvious solution to telephone space requirements.

The LC is a keyed, contact, moderate loss, pull proof and wiggle proof design. The LC has a 1.25 mm ferrule, which is half the diameter of the ferrules used in ST-compatible, SC, FDDI and ESCON connectors. This small size can cause problems for installers, until they realize that half the diameter means one quarter of the usual polishing pressure.

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Figure 10: LC Connector and Barrel

Many optoelectronic manufacturers provide products with the LC interface. These optoelectronics transmit at up to 10 Gbps. It appears that the LC interface is supported by a large number of manufacturers.

Volition. The Volition duplex connector from 3M[20] is a plug and jack system (Figures 11, a plug, cover open with fiber visible, and 12, a jack open with darkened fibers in grooves). The Volition incorporates a design feature that is both evolutionary and revolutionary: V-grooves instead of ferrules. 

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Figure 11: Open Volition Plug

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Figure 12: Open Volition Jacks With Fibers Visible

V-grooves are evolutionary, in that they have been used for alignment of fibers in fusion splicers since the late 1970s.  V-grooves are revolutionary in that they have not been used in connectors prior to the Volition.

The design goal, to produce a fiber optic connector with the same cost as that of a UTP connector, could be achieved by elimination of the single, largest cost in the connector- the ferrule. 3M implemented the use of V-grooves for alignment of the fibers: in the jack, fibers rest in precision, molded, plastic V-grooves (Figure 12).  In the plug, a custom fiber[21], floats in free space (Figure 11). The plug fibers slide into the V-grooves of the jack, with contact of the mating fibers maintained through pressure created by bent fibers.

The Volition plugs and patch cords are factory made.  The jacks are field installable onto a custom Volition cable.  This custom cable is required, since the jack requires fibers with a primary coating of 250 µm. This diameter is smaller than the 900 µm tight tubes of commonly used premises cables. Because of this requirement, it is not possible to retrofit existing fiber optic networks with the Volition system.

In spite of this limitation on retrofitting, the Volition system has proven popular because it provides the lowest cost fiber optic connection system available.  A Volition jack costs less than $6.60.  This price is close to that of a Category 5e jack.  At a minimum of $9.10, duplex SC plug and barrel costs at least 38 % more expensive than the Volition solution.

In addition, the Volition system provides and the lowest cost fiber optoelectronics.  For example, the lowest cost Volition media converters are below $100.  Use of media converters or switch blades with other SFF connectors are $150-$229.

MU. The MU is a simplex SFF connector with the appearance of an SC but with all dimensions reduced by 50 % (Figure 13). Designed and developed in Japan, the MU is a keyed, contact, moderate loss, pull proof and wiggle proof design. It is unique in that it can be assembled to create a duplex, triplex or quadruple fiber connector.

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Figure 13: MU Connector

High Performance Connectors

Introduction

High performance connectors are those that exhibit low to moderate power loss and low reflectance. We define moderate power loss as a maximum of 0.75 dB/pair and a typical of than 0.30 dB/pair. These values represent the values for those products used in telephony and CATV applications. These values are typical for singlemode connectors that are keyed and contact.

Reflectance is a measure of the relative amount of power reflected from a connector back into the light source.[22]  Such power can reflect from the source, back into the fiber and travel to the receiver.  If the reflected power arrives at the receiver with a power level higher than the receiver sensitivity, signal inaccuracy will occur. 

Reflectance occurs whenever light travels through an interface with a significant change in speed.[23]  This reflection is called a Fresnel reflection. Contact connectors can never be perfectly polished. Imperfections in the polished surface create microscopic air gaps in the core. Such gaps create reflectance.

Early connector types, such as the biconic, SMA 905, SMA 906, had flat end faces and deliberate air gaps, which resulted in high reflectance, –14 to -18 dB (Figure 14, top). In order to achieve both low reflectance and low loss, the connector industry changed the end face from flat and non-contact to radius, or domed, and contact (Figure 14, bottom).  This change results in a reduction of both reflectance and loss. This change eliminated the need to achieve perfect perpendicularity (Figure 14, top).  This change resulted in reflectance being determined by the smoothness of the surface of the fiber. The polishing process determines this smoothness.

 

Figure 14: Comparison of End Faces of Early to Current Generation Connectors

Historically, reflectance has been a singlemode concern at bit rates or bandwidths above 400 Mbps or 400 MHz.  Because of the use of multimode gigabit Ethernet and 10 Gigabit Ethernet, multimode connector reflectance is now a concern.[24] 

While we define low reflectance of singlemode connectors as less than – 40 dB, reflectance requirements for high performance connectors range from –40 dB to –65 dB.[25]

Reflectance is qualitatively described by the terms PC, UPC and APC. Commonly, PC (physical contact) refers to reflectance of less than –40 dB; UPC (ultra physical contact), to less than –50 dB; and APC (angled physical contact), to less than –55 dB.

Figure 15: Ideal (Top) and Real Fiber End Faces in Contact Connectors

The radius ferrule end face can be machine polished to achieve –55 dB consistently.  For reflectance below this value, the APC version is specified.  The APC connector has a beveled end face at an angle to the fiber axis (Figures 16 and 17).  For North American connectors, the angle is 8°; for some European connectors, the angle is 9°.

This bevel reflects the light backwards outside of the critical angle, or NA, of the fiber. Thus, no power is reflected back into the source.  By convention, the APC connectors are green, to distinguish them from the PC and UPC products, which are blue.[26]

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Figure 16: SC/APC Connector

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Figure 17: The FC/APC Connector

High performance connectors are available in the following types:

SC

SC/APC

FC

FC/APC

LC

LC/APC

LX.5

LX.5/APC

MTP/MPO

High Performance Connectors

SC. We presented the SC connector earlier.  The SC/APC (Figure 16) has a beveled end face and the same characteristics as the SC.

FC. Originally developed in Japan, the FC connector (Figure 18) has some of the characteristics of both the ST-compatible and the SC connector types.  Like the ST-

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Figure 18: The FC Connector

compatible, the FC has a rotational insertion method, which requires a relatively large spacing in a patch panel.  Like the SC, the FC is keyed, contact, moderate loss, pull proof and wiggle proof. The FC/APC (Figure 17) has a beveled end face and the same characteristics as the FC.

LC. The LC, presented earlier, has an APC version for extremely low reflectance.

LX.5. ADC Telecommunications developed the LX.5 (Figure 19), a product similar to the LC, for use in telephone networks. The LX.5, available in radius and APC versions, has a unique feature: a built in dust cover (Figure 19, bottom).  This cover lifts as the connector is installed into a patch panel.

Similarly, the adapter for the LX.5 has a built in dust cover (Figure 20). The dust covers on the connector and in the adapter increase the eye safety of the product. With multiple wavelengths on singlemode fibers in DWDM networks, the total power level at the connector can be as high as 0.2 watts.

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Figure 19: The LX.5 Connector

Figure 20: The LX.5 Adapter

MTP/MPO. MTP/MPO connectors (Figure 21) have 6-24 fibers in a single, molded polymer ferrule. These connectors reduce the sizes of enclosures and patch panels.

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Figure 21: 12 Fiber MTP Connector

2.5 Cost Comparisons

We present four types of cost comparisons: cost of connectors by type; cost of connectors and enclosure; cost of connectors by installation method; and total installed cost.

Connector costs differ by type: the more commonly used connectors, the ST-compatible and SC, cost less than other types.  We present typical relative connector costs in Table 4.

 

Table 4: Comparison of Relative Costs of Multimode Connectors by Type[27]

Type

Relative Cost/fiber

ST-compatible

1.0

SC

1.3

LC

1.5

MT-RJ

0.8

VF-45

0.7

Connector and enclosure cost provides a different picture from that in Table 4.  The ST-compatible and FC types require increase space for rotation during insertion into patch panels.  This space requirement limits the number of connectors to 12 per 1U, 19 inch wide patch panel.  The SC connector allows for doubling[28] this number to 24.  With this doubling of density, use of SC connectors, which are slightly more expensive than ST-compatible, costs less than use of ST-compatible (Table 3).

SFF connectors allow for a doubling of the standard SC density, to 48 fibers in a 1U, 19 inch wide enclosure.  This second doubling of density can, in some cases, result in a reduced hardware cost, in spite of increased connector cost.

Table 5: Comparison of Costs of Connectors Installed by Different Methods