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 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 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, 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.
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 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.
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, 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
|
Type
|
Typical loss
|
Maximum loss
|
|
Non contact
|
1.0 dB/pair
|
2.0 dB/pair
|
|
Contact
|
0.3 dB/pair
|
0.75 dB/pair
|
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.
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 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.
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
|
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
|
Figure 4: The
Duplex SC
The SC can be
duplexed, either through use of left and right outer housings 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:
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.
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).
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.
Figure 6: The
Opti-Jack Plug
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.
Figure 7: The
Opti-Jack
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.
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.
Figure 8: TYCO
MT-RJ System
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.
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 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.
Figure 11:
Open Volition Plug
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, 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.
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.
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.
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.
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.
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.
Figure 16:
SC/APC Connector
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-
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.
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.
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
|
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 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
