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
|
Method
|
Typical Price
|
Rate/hour
|
|
Epoxy
|
$3.00
|
6-8
|
|
Hot melt adhesive
|
$5.50
|
12-14
|
|
Quick cure adhesive
|
$3.00
|
15
|
|
Cleave and leave
|
$12
|
22-40
|
Table 6: Comparison
of Relative Total Installed Connector Cost vs. Installation Method
|
$/hour
|
epoxy
|
Hot Melt®
|
quick cure
|
cleave & leave
|
|
|
|
|
|
|
|
35
|
0%
|
0%
|
-29%
|
56%
|
|
40
|
0%
|
-3%
|
-31%
|
46%
|
|
45
|
0%
|
-6%
|
-32%
|
37%
|
|
50
|
0%
|
-8%
|
-33%
|
30%
|
|
55
|
0%
|
-11%
|
-34%
|
23%
|
|
60
|
0%
|
-13%
|
-34%
|
17%
|
|
65
|
0%
|
-14%
|
-35%
|
12%
|
|
70
|
0%
|
-16%
|
-36%
|
8%
|
|
75
|
0%
|
-17%
|
-36%
|
4%
|
|
80
|
0%
|
-18%
|
-37%
|
-0%
|
Method of installation
can be related to connector cost.
In general, the connector that enables the fastest installation rate
will have the highest cost (Table 5).
The exception to this statement is the connector with a ceramic ferrule, which can be installed with either epoxy
or quick cure adhesive. Our latest analysis suggests that the cost premium for
connectors with the highest installation rate is larger than the labor savings
from that rate (Table 6).
Installation Methods
Fiber optic connectors
are installed by many methods.
These methods were created and developed to address different concerns,
such as reliability, convenience, and time/cost to install.
Field Installed Connectors
Most of the
development and evolution of installation methods has been for field-installed
connectors. Each solution was designed to reduce one or more factors in the
total installed cost. In most
cases, the effort to reduce the total installed cost resulted in a more
expensive connector.
Epoxy Installation Method
The epoxy,
pot and polish method was the first method used for installation of fiber optic
connectors. This method has four advantages and three disadvantages.
The four
advantages are: high resistance to degradation due to exposure to environmental
conditions; loss stability over a wide temperature range; high installation
process yield; and ability to be used with the lowest cost connectors.
Epoxies are
considered to have the highest resistance to the widest range of environmental
conditions. As such, epoxies can
provide the highest reliability connectors. Other adhesive systems tend to have reduced resistance. As an example, fiber optic connector
epoxies can resist degradation to temperature of 105°C., while the Hot Melt
adhesive system is specified to 85°C.
The loss
stability provided by epoxies is due to a good match of thermal expansion
coefficients of epoxy, fiber and ceramic ferrules. This matching results in
minimal relative movement of the fiber in the ferrule over a wide temperature range. Such limited movement results in stable
power loss.
Use of
epoxies tends to result in a high process yield. This high yield is a result of an epoxy bead on the tip of
the ferrule. This bead supports
the fiber during both hand and machine polishing. This support nearly eliminates damaged and shattered ends,
which we estimate to be 95 % of connector yield losses during installation.
When labor
cost is low, use of epoxies can result in the lowest total installed cost,
since epoxy connectors are lowest in cost (Table 5).
The first two
disadvantages are related: inconvenience of use; and low installation rate,
which can translate to high installation cost. The third disadvantage is the
need for power to heat curing ovens.
The use of
epoxy requires mixing of two parts, transfer of the mixed epoxy into a syringe
or automated injection mechanism, and clean up of excess epoxy. Because of these additional steps, the
rate of installation of epoxy connectors is low, typically 6-8 per man-hour. If
the total loaded hourly labor cost is high, this low connector rate can result
in high total installed cost (Table 6).
Because of
the labor cost factor, epoxy is not the method of choice for all field
installations. During field
installations, the total loaded hourly labor cost tends to be high. In addition, there is reduced labor
utilization, since field installations involve time spent in activities other
than installation. For example,
field installations involve the time factors of: travel to site, set up, pack up,
clean up, and travel to the next installation location within the site. Low
labor utilization increases the cost impact of the low installation rate
possible with epoxy connectors.
Because of these cost factors, other connector installation methods were
developed.
Hot Melt Installation Method
Because of
the inconvenience and time impact of the epoxy installation method, 3M
developed the Hot Melt installation method. In this method, a hot melt adhesive is preinstalled into the
connector. The installer preheats
the connector to soften the adhesive so that he can install the fiber. The installer installs the fiber and/or
cable into the connector and allows the connector to cool in a heat sink (the
cooling stand). Once cooled, the
installer removes the excess fiber and polishes the end in a one step polishing
procedure. This installation method requires a 3M
polishing film, which is not easily clogged by the hot melt adhesive.
The hot melt process
eliminates the time factors of preparing and injecting the epoxy. In addition, this method eliminates the
mess and inconvenience of epoxy.
These eliminations allow an increased installation rate, 12-14 per hour.
This increased rate can result in a reduced installation cost (Table 5),
However, the
Hot Melt connectors are significantly more expensive than epoxy connectors
(Table 5). In addition, the Hot Melt method requires power for the heating oven
and a proprietary oven, holders and polishing film.
Quick Cure Adhesives
Several
manufacturers (e.g., Lucent Technologies and Automatic Tool and Connector) addressed the disadvantages of these
proprietary parts of the Hot Melt method and the inconvenience of epoxy method
through with quick cure adhesive systems.
These systems, either two part systems (from Lucent Technologies, Automatic
Tool and Connector) or one part (from TYCO), eliminate the need for power,
resulting in an increase in the rate of installation. This increased rate, typically 15/hour, can result in a
reduced installation cost.
A second
advantage of quick cure adhesives is their compatibility with the low cost
connectors with ceramic ferrules. This combination of low cost connectors
and high installation rate can result in a low total installed cost. Installers
can achieve this low installed cost if they achieve a high process yield.
There are
three disadvantages to the use of quick cure adhesives. The first two disadvantages of quick
cure adhesives, premature hardening of the adhesive and minimal support of the
fiber during polishing, contribute to a reduced process yield and increased
cost.
Premature
hardening occurs when the fiber is coated with the hardener or accelerator and
inserted into a connector loaded with adhesive. If the installer inserts the fiber slowly, the adhesive
cures and the fiber locks up before it is fully inserted. In this situation,
there is bare fiber inside the connector.
Such bare
fiber can cause a reduction in the reliability of the connector. For example,
SC connectors allow the fiber inside the
back shell to flex slightly during insertion into a patch panel or a
receptacle. Repeated flexing of
such bare fiber can result in breakage of the fiber. We have observed such failure in as few as 5 cycles.
The minimal
support of the fiber during polishing occurs because the adhesive cannot
produce a large bead on the tip of the ferrule. This inability is due to the nature of the adhesive: quick
cure adhesives are edge-filling adhesives, not true anaerobic adhesives. Unlike the bead on epoxy connectors
(Figure 22), the small bead on the tip of the ferrule can allow the fiber to
break below the surface of the ferrule during removal of the excess fiber or
polishing. Without extreme care during the end finishing steps, the
installation yield can be low. For
example, typical yields during training of first-time novices installing epoxy
and Hot Melt connectors are 80-90 %. Typical yields for these same novices with
quick cure adhesives are 50-60 %.
However, typical yields of quick cure adhesive connectors for
experienced professionals are 95-98 %.
Figure 22:
Large Epoxy Bead Results in High Yield
The third,
and final, disadvantage of the use of quick cure adhesives is reduced
reliability. Some of the quick
cure adhesives exhibit degradation when exposed to a wide temperature range, a
wide humidity range, rapidly changing temperature, or rapidly changing
humidity. While there are
exceptions, quick cure adhesives are recommended for indoor use.
No Adhesive With Polish
Several
manufacturers have offered connectors that use a
mechanical method of gripping the fiber and require polishing. These products are more susceptible to
damage during polishing than are connectors installed with quick cure adhesives. This susceptibility to damage occurs
because there is no bead supporting the fiber.
Cleave and Leave
For all the
installation methods presented, the major cause of yield loss is damaged or
shattered fiber ends. This damage occurs during polishing. In addition, a
significant amount of time is consumed by preparation, injection, and use of
adhesives. The cleave and leave installation method addresses both yield loss
and time consumption by elimination of both adhesives and polishing. Such elimination offers the potential
of reduced installation cost. This increase in installation rate is only a
potential cost reduction, since the cost of these connectors is higher than
that of connectors installed by other methods.
Cleave and
leave is a generic term for epoxy less /adhesive less /polish less connector
products with the trade names LightCrimp Plus (TYCO/AMP), Opti-Crimp (Panduit
Corporation), Unicam (Corning Cable Systems), and others. All these products
require cleaving the end of the fiber, inserting the fiber into a connector,
and crimping or clamping the connector to the fiber. The connector contains a pre-installed fiber stub that has
been polished by the manufacturer.
In essence, this connector has a mechanical splice in its back shell.
The primary
advantage of this installation method is reduced installation time, with the
potential of reduced installation cost. A second, potential, advantage is
reduced training cost.
Field
installation rates can be 22-40/hour.
However, field reports and training results have indicated reduced yield
and increased loss for many of the earlier products. To be fair, some of these low-yield, high-loss reports have
been due to improper installation techniques by inadequately trained
installers.
To be accurate,
however, the cleaving tool offered as part of the tool kit is highly
susceptible to installer misuse.
For our training programs, we have replaced this low cost cleaver with a
higher cost cleaver that is, essentially, immune to installer errors and misuse. This replacement has resulted in both
increased yield and reduced power loss.
The
realization of reduced installation cost depends on five factors. Since the cost of these connectors is
high and these five factors vary widely from installation to installation, the
increased installation rate of this method may or may not result in reduced
installation cost.
These five
factors, total loaded hourly labor rate, labor time utilization, installation
rate, installation yield, and connector cost do not always combine to produce
the lowest total installed cost (Table 6). In spite of the complexity of the
cost analysis, we can develop some rough guidelines.
The higher
the total loaded labor rate, the more likely the cleave and leave method will
produce the lowest total installed cost.
The lower the
number of connectors per location, the lower will be the labor time utilization
and the more likely the cleave and leave method will produce the lowest total
installed cost. Such situations include the desk locations in a fiber to the
desk network (FTTD) but not necessarily a central equipment room in a vertical
back bone or in an FTTD network.
In the last
six months, our preliminary testing has indicated that the cleave and leave
method yields and power losses are close to those of epoxy, Hot Melt and quick
cure methods. These test results are a significant improvement on results from
prior testing conducted during the previous four years.
Pigtails
Pigtails are
relatively short lengths of single fiber cable or tight buffer tube with a
connector on one end.
For reduced cost, pigtails are factory-made and, rarely, field
made. Pigtails can be used to
reduce the total cost of field-installed connectors or to enable reduced
reflectance. Such pigtails are spliced, usually by fusion splicing, to the ends
of main cables.
Fusion splicing of pigtails may, or may not, be recommended.
Fusion splicing of multimode pigtails can be done, but is not advisable due to
the potential for reduced bandwidth.
This reduction can occur because the splicing disrupts the multiple
composition layers of the multimode core.
While there is limited evidence of this reduction, there are no data to
support fusion splicing multimode fibers.
Fusion
splicing of singlemode pigtails does not create this bandwidth reduction and is
an accepted method of achieving low cost and low reflectance. Experienced
installers can make 12-15 pigtail splices /hour with reflectance of –50
dB to –65 dB. Experienced
field installers can install only 5-8 adhesive or epoxy singlemode connectors
per hour with the same reflectance performance.
Field Polishing Machines
Since much of
the time and cost of connector installation is consumed by polishing, the
sophisticated installer might consider use of field polishing machines.
These machines polish a small number of connectors, typically 1 or 2,
with controlled pressure to reduce or eliminate damaged ends.
Current
generation field machines can reduce the number damaged ends, but do not
appreciably reduce labor cost, as the time to polish by hand is only slightly
longer than the time to insert and remove connectors from the machine.
These
machines, at $1000-$2500, can allow installers to achieve low reflectance on
singlemode connectors in the field.
In addition, these machines enable repolishing of previously installed
connectors to restore low reflectance.
Such repolishing is less expensive than replacement.
Factory
Installed Connectors
Factory
installation is performed with epoxy, and, occasionally, quick cure adhesives. The cost disadvantage of the epoxy
method is not significant in factory installation, since labor rates are low
and labor utilization is high.
In addition,
the support of the epoxy bead during polishing by machine enables high yield
and fast bead removal by personnel with minimal training. Bead removal from
connectors installed with quick cure adhesive requires increased training of
personnel.
Finally,
factory installation enables consistent achievement of low reflectance
singlemode connectors at a low cost.
A typical machine polishing process has a cycle time of 3 minutes for 24
connectors per cycle. This time is
significantly less than field polishing times: 1-3 minute polishing time per
multimode connector and 3-5 minute time per singlemode connector.
Performance Issues
Insertion Loss
The insertion
loss, in dB/pair, is the main performance parameter by which connectors are
accepted or rejected. The Building
Wiring Standard, TIA/EIA-568 B, allows a maximum loss of 0.75 dB/pair. The typical loss for ST-compatible, SC
and other connector types with the 2.5 mm ferrule is 0.30 dB/pair.
With the
assumption that epoxy or adhesive is used, these 2.5 mm ferrule loss values are
independent of installation method.
Typical loss for a cleave and leave connector is slightly higher than
that of adhesive connectors, between 0.54 and 0.60 dB/pair.
This increased loss is expected because of the mechanical splice in the
back shell.
This loss is
determined by the geometric tolerances of the ferrule, the geometric tolerances
of the fiber, and the quality of the finished core. Fortunately, the geometric
tolerances of ceramic ferrules and US-made fiber are tight enough to provide
consistent 0.30 dB/pair loss for both multimode and singlemode connectors. Our
testing of multimode, ST-compatible, liquid crystal polymer ferrule connectors
produced essentially the same loss at 0.33 dB/pair. The typical loss for LC connectors is
lower, typically 0.2 dB/pair.
Reflectance
At the
present time, reflectance is of concern for some singlemode connectors. While
the Gigabit Ethernet and Fiber Channel standards place a reflectance
requirement on multimode connectors, there is no field test equipment for such
a test. Because of this lack of equipment, there is little concern for
multimode reflectance measurement.
Reflectance
is related to signal accuracy, or BER in digital fiber systems. The crude rule of thumb is that
reflectance becomes of concern at transmission rates above 400 MHz/Mbps. Such rates are most commonly singlemode
rates.
The
reflectance test standard, FOTP-107, defines reflectance as:
10 log
(reflected power/incident power)
When
delivered against Telcordia standards, connectors must meet a reflectance
requirement between –40 dB and –60 dB and two additional
specifications: apex offset and undercut.
Apex offset is the amount by which the high point (apex) of the fiber
deviates from the center of the ferrule (Figure 23). Undercut is the amount by
which the surface of the fiber is below the projected surface of the ferrule
(Figure 24). Undercut occurs with ceramic ferrules but not with softer ferrule
materials. These two additional specifications may well be redundant to the
reflectance specification.
Figure 23:
Apex Offset
Figure
24:Undercut
Repeatability and Range
Repeatability
is a measure of the maximum difference in loss between two successive tests of
the same connector. This parameter
is of concern to the designer and manufacturer of connectors. This parameter is found on connector data
sheets.
Range is the
maximum difference in loss between two successive tests of a link. Such testing involves disconnection and
reconnection of both ends of a link. From this definition of range, we would be
tempted to calculate the range by doubling the repeatability. However, our range testing indicates
that the range is almost always less than double the repeatability.
Range is
important to the installer, since the installer may perform troubleshooting or
maintenance testing. In such testing, the installer will compare the original
loss to the current loss. In order
to interpret this comparison, the installer needs to know the maximum change
between two successive disconnections of both ends of a link under conditions
of no degradation of the link components (connectors, splices and cable
segments). With knowledge of the range, the installer can interpret an increase
in loss: if the increase is less than the range, there is no indication of
degradation. Conversely, if the
increase in loss is larger than the range, there is indication of degradation.
Durability
Durability is
a measure of the wear of ferrules during repeated insertion. Such wear results
in increased loss due to degraded core alignment.
Durability is
addressed through choice of ferrule material. The harder ferrule materials, such as ceramic and stainless
steel, produce long lifetimes through hard surfaces. Such materials can produce durability ratings of 100-2000
cycles. Relatively soft ferrule materials, such as liquid crystal polymers, can
withstand 250-500 cycles. Our experience with ceramic connectors used in
training programs indicates no noticeable wear at 1000-2000 cycles. With this
level of performance, durability seems to be, justifiably, of low concern.
Testing Methods
Historical Methods
Historically,
there are two groups of connector loss tests, those used to qualify the
connector as a product and those used to test field installed connector and
cable links. The former group is used by manufacturers to quality their
products and compare the performance of their products to those of other
manufacturers.
Of more interest are the latter methods. Installers perform two
types of testing on installed links: insertion loss testing and OTDR testing.
Insertion
loss testing is performed to ensure that the end-to-end power loss is less than
the maximum loss allowed by the electronics. In addition, this testing provides a crude inference that
the link is properly installed.
Unfortunately, this testing does not provide legal proof that each
element in a link is properly and reliably installed.
Prior to TIA/EIA-568 B, multimode insertion loss testing was
performed according to Method B of TIA/EIA-526-14. Method B of TIA/EIA-526-14 requires setting a reference with
one reference lead
(Figure 25) and measuring the loss with two reference leads (Figure 26).
Figure 25:
Original Method B Reference
OTDR testing
provides loss measurements on almost every element of a link. These measurements can be used to indicate
proper installation and reliable installation. In short, OTDR testing indicates reliability. OTDR testing
is advisable for most networks, but is not required by the Building Wiring
Standard.
Figure 26:
Original Method B Test
Current Testing Methods
With the
issuance of TIA/EIA-568 B, the details of multimode insertion loss testing
changed in two ways: use of a Category 1 source and use of a mandrel on the
source reference lead. This
testing is still performed by Method B, but the reference is now TIA/EIA-526-14
A.
TIA/EIA-568 B
adds a requirement that the source used in testing be a Category 1 source
(Clause 7.1 of TIA/EIA-568 B.3).
The category 1 source overfills the core diameter and the NA. As such, this source will tend to
maximize connector loss and fiber attenuation.
The second change is that the source reference lead must
be wrapped around a mandrel five times.
The mandrel diameter is defined in Table 11-15 of TIA/EIA-568 B.1. This mandrel
will tend to strip out optical power near the core-cladding boundary and at the
angular limits defined by the NA.
As such, the mandrel will tend in reduce connector loss and fiber
attenuation. At this time, the
combined changes of a Category 1 source and a mandrel wrap have not been
defined.
The objective of the addition of the Category 1 requirement and
use of the mandrel is to normalize the test procedure. With such normalization, it is expected
that test results will exhibit a reduced variation when performed with
different test sets.
As of this writing, Category 1 sources are available from only a
few sources. Many
existing test sets, which are Category 2 and 3, cannot be used for compliance
with TIA/EIA-568 B.
Singlemode testing standards have not changed. Insertion loss
testing is performed by TIA/EIA-526-7.
Status of Standards
Use
Prior to
issuance of TIA/EIA-568 B, the dominant connector types were the ST-compatible
and SC. Those who were more
concerned with cost used the ST-compatible. Those who were concerned with
increased reliability and/or compliance with TIA/EIA-568 A used the SC type.
Because TIA
/EIA-568 B removed the exclusive requirement for use of the SC type, this
standard opened the door to use of other connector types, specifically the SFF
types. As a result, the volume and
market share of MT-RJ, LC, Volition, and Opti-Jack have increased
significantly.
Testing
Testing seems
to be behind the standards in that few installers are using the Category 1
sources and mandrel wrapping. Some installers are using the mandrel, but not Category
1 sources.
Trends
Connector Types
Reduced
ST-compatible use: two factors lead us to expect reduced volume and market
share for ST-compatible connectors- reduced difference in cost of SC and
ST-compatible connectors; and increased awareness of the density and
reliability advantages of SC and SFF connectors.
Reduced SC
market share: two factors lead us to expect reduced market share for the SC
connector type: competition from SFF connectors; and increased use of the SFF
in optoelectronics.
Increased use of SFF connectors: the optoelectronic cost and
density advantages of the SFF connectors will lead to increased volume and
market share. For two SFFs, MT-RJ
and VF-45, the cost/fiber is lower than that for the ST-compatible and SC
types. The MT-RJ and LC will have
the greatest growth. Both types
will experience growth because of the significant support from optoelectronic
manufacturers. An additional
factor favoring LC growth is competitive pricing from multiple
manufacturers. The MT-RJ market
has less competition due to the dominance of two manufacturers. Neither would
benefit from a price war.
Increased MPT
volume: telephone companies are implementing DWDM. DWDM can increase fiber port count in patch panels 32-64
times. In order to avoid moving connection facilities to a new location or
knocking down walls to provide the additional space required, telephone
companies are using the MPT/MPO connectors (Figure 21) that have 24 fibers in
the same volume as 1-2 SC connectors.
This represents a volume reduction of roughly 92 %.
Installation Methods
We expect
reduced use of cleave and leave.
This expectation is based on constant prices or small reductions in
prices. Should there be significant reductions in pricing, there could be a
stable market share for this method.
Fiber vs. UTP
In June 2003,
the Fiber Optic LAN Section
of the TIA completed ten comparisons of the cost of fiber to the desk network
to that of a standard network. This
cost model, which is free for use, demonstrates a lower initial installed cost
for many new build and retrofit FTTD networks than for typical horizontal UTP
and vertical fiber networks. This is the first time that a generic study has
shown significant cost reductions from the use of fiber. In addition, this
study demonstrates a significant cost reduction through use of
fiber-to-the-zone (FTTZ) architecture.
This study,
an update of a 2001 study, clearly demonstrates these cost reductions. These reductions occur when all the
cost factors in both types of networks are included in the comparison.
Prior
studies, which have not supported FTTD, neglected the cost factors related to
telecommunication rooms. These
factors include the cost of the room, the cost of environmental control and
electronic support, and the cost of power. When the FOLS included realistic
costs for these factors,
the initial first cost for FTTD configurations was less than the cost for a
typical network.
Based on the
conclusions from these comparisons, we expect increased installation of
FTTD. Such use will result in
increased use of fiber connectors, larger increases in the use of duplex
connectors and in
increased use of connector installation methods that provide for reduced total
cost.
Summary
From unkeyed,
non-contact, high loss, high reflectance, highly variable, single fiber,
installed exclusively with inconvenient and time consuming methods, connectors
have evolved to keyed, contact, low to moderate loss, low to moderate reflectance,
highly repeatable, one to 24 fiber connectors installed with at least five
different methods. Each of the methods addresses different installation
conditions, with different methods favored under different conditions.
As is to be
expected from typical product life cycles, older products, such as the
ST-compatible and SC, are experiencing reduced market share, while the newer
designs, such as the SFF and MPT/MPO, are experiencing growth in both volumes
and market share.
Changes in
standards have resulted in changes in test procedures and in the equipment
required. These changes are yet to be implemented by most installers.
Changes in
products and reductions in price have resulted in increased use of fiber and
more specifically, of FTTD. This
trend is expected to accelerate as more designers become aware of the cost
advantages of FTTD.
Mr. Eric R. Pearson is President of
Pearson Technologies Incorporated, a Certified Professional Consultant, a
Certified Fiber Optic Specialist, the Director of Certification for the Fiber
Optic Association, an editorial advisor to Fiberoptic Product News, and
a 26-year veteran of the fiber optic industry. Pearson Technologies provides consulting on technical,
marketing and sales issues, training to installers, designers and sales
personnel, expert witness services in patent, installation and damage legal
cases and network design services.
Respectfully submitted for your consideration,
Eric
R. Pearson, CPC, CFOS
President
Pearson
Technologies Inc.
For
Mr. Pearson contact information, click
here.
© Pearson Technologies
Inc.
