Fiber Optics Cable Assemblies
What are the opportunities?
By Paul Polishuk
Information Gatekeepers Inc.
Introduction:
The field of fiber optics is going through a boom period
which is expected to continue for the next decade. In the late 70s
the telephone companies began installing fiber optics at a feverish
pace because at that time there was a huge growth of businesses in
the cities, and the telephone companies were running out of duct
space for their thick, heavy copper cables. The feasibility of using
fiber optics for telecommunications was first shown by Corning Glass
in 1970. By the late 70s, fiber optics was starting to come out of the
laboratories just in time to meet the growing needs of the telephone
companies. The copper cables could be replaced with thin, lightweight
fiber optic cables with one hundred times the capacity and there was
room to spare. This was the beginning of the fiber optics industry
and the rest is history.
The fiber optics industry went through a rapid growth phase in the
early eighties, mainly driven by the needs of the local telephone
companies and long distance companies like AT&T, Sprint, and MCI.
At the same time, the government was taking steps to introduce competition
into the telecommunications field. It had started way back as 1968 with
the Carterfone Decision that opened the market for customer premise
equipment (CPE), then in 1984 the break-up of AT&T and finally resulting
in the rewriting of the Communications Act in 1996. The culmination of
the government's actions to open the telecommunications field to competition
and the continued development of the optical fiber technology is driving
another growth cycle in the fiber optics business which is much larger
and faster growing than the period when fiber optics was first introduced.
The introduction of competition and new technologies are the engines that
are driving the market today. On the technology side, the two major
developments have been the optical amplifier (OA) and Wavelength
Division Multiplexing (WDM).
In the earlier applications of fiber optics, the original coaxial cables,
copper cables and microwave were replaced by fiber optics on a
point-to-point basis. Although optical fibers have lower loss than
those transmission media it replaced, it still had to have amplifiers
installed if the link was long, such as in the long distance networks.
The amplifiers or repeaters took the optical signal, converted it to electrical,
amplified, reshaped, and retimed the signal to its original state, and then
converted the electrical signal back to optical and sent it on the way to the
next opto-electronic amplifier. This conversion was complicated, decreased
reliability and added cost to the point-to-point links. The optical amplifiers
allows optical signal to be amplified without the conversion required in the
older opto-electronic amplifiers. These optical amplifiers are cheaper, simpler,
and more reliable.
The second development was wavelength division multiplexing (WDM).
With this device it is possible to put many wavelengths onto a single fiber,
thus increasing the capacity of the fiber by a multiple of the number of
wavelengths. With today's technology, up to 60-80 wavelengths are now
possible. There is no reason that hundreds or even thousands of wavelengths
could be sent over the same fiber. When more than four wavelengths are
transmitted, it is called Dense Wavelength Division Multiplexing (DWDM).
O.K., so what does all this mean?
When fiber optics was first introduced, it was the dream of the
optical technologists to be able to develop an all optical network.
Think of today's telephone network where calls are transferred by
switching and transmitting electrons replaced by one which uses light
photons instead. This would be an all optical network! Until the optical
amplifier and WDM came along, this was not possible. Now the all-optical
network is in sight, and this has caused a stampede of existing and new
companies into the fiber optics field.
The optical or fiber optics business is still in its infancy and is spreading
rapidly into all sectors of telecommunications and business, such as
premise wiring (business and home), automobiles, cable television,
fiber-to-the-home, fiber-to-the-desk, and industrial controls, to name a
few. This discussion has only pertained to data. Fiber optics can also be
used for sensors, illumination and signs.
This article describes the different types of fibers and their applications,
sizes the cable assembly markets, and attempts to show how existing
new business opportunities exist to those already in the cable harness business.
Different Types of Optical Fibers
There are two main types of optical fibers – glass and plastic.
Within each group there are further types. The following Table
One shows the main advantages and disadvantages of glass and
plastic optical fibers.
Many of the advantages and disadvantages shown in Table 1 are
relative and are shown for illustrative purpose only.
An optical fiber consists of a solid core, cladding, and a buffer diameter,
shown in Figure 1. The light is transmitted in the core, reflecting back and
forth off the core cladding interface. The buffer is usually a plastic material
which serves to protect the fiber.
One of the key differentiating characteristics of glass and plastic optical
fibers is the fiber diameter. Table 2 shows the three types of glass fibers,
single-mode, graded index multimode, and step index multimode, and
plastic optical fibers. These fibers have the sizes shown. Glass fibers are
much smaller than plastic fibers and hence, are more difficult to handle.
More on fibers
One way of classifying fibers is by material, as described earlier. Glass,
or pure silica, is the superior material for small, efficient fibers, while less
efficient and larger plastic fibers are generally more rugged and economical
in less demanding applications. A third type of fiber has a glass core and
plastic cladding. These plastic-clad silica (PCS) fibers have size and
performance intermediate to glass and plastic fibers.
Another, more informative way to classify fibers is by refractive index
profiles and the number of modes supported. The two main types of
index profiles are step and graded. In a step-index fiber, the core has a
uniform refractive index, with a distinct change or step between the indices
of the core and cladding. In a graded index fiber, the core's index is not
uniform; it is highest at the center and decreases until it matches that of the
cladding. There is no sharp break in the refractive index continuum.
Figure 2 shows the paths of light in three types of fibers.
Multimode Step Index
A multimode step-index fiber, the simplest type, has a core diameter
in the 50 micron to more than 200 micron range. The large core allows
many modes of light propagation. Since light reflects differently for
different modes (see figure2), some rays follow longer paths than others.
The lowest-order mode, the axial ray traveling down the center of the fiber
without reflecting, arrives at the other end of the fiber first, before the
higher-order modes that strike the core-to-cladding interface at close to
the critical angle and therefore follow longer paths. Thus, a narrow pulse
of light spreads out as it travels through the fiber. This spreading of a
light pulse is called modal dispersion.
Figure 3: comparison of a Plastics Optical Fiber Data Bus and
Copper Automobile Wiring Harness.
Single-Mode Step Index
One way to limit modal dispersion is to use a fiber with a core diameter
small enough that the fiber propagates only one mode efficiently. a
single-mode fiber, with a core diameter in the order of 2 to 10 microns,
is very efficient and suitable for very-high-speed, long distance applications.
Its small size, however, makes it difficult to work with. Single-mode fibers
find use in telecommunications, CATV, and undersea applications.
Multimode Graded Index
A graded-index fiber also limits modal dispersion. Its core is a series of
concentric rings, each with a lower refractive index. Since light travels faster
in a lower-index medium, light farther from the fiber axis travels faster.
Since high-order modes have a faster average velocity than low-order modes,
all modes tend to arrive any any point at nearly the same time. Rays of light
are not sharply reflected by the core-to-cladding interface; they are refracted
successively by the differing layers of the core. The path of travel appears
nearly sinusoidal.
Plastic Optical Fibers
Plastic optical fibers have been around longer than glass optic fibers,
however, because of their high loss (i.e. high absorption of light) they
were used for light transmission over short distance, mainly for medical
and industrial instruments to observe areas which would be difficult to
reach, such as inside the body or jet engines.
With improvements in the technology mainly lower loss from 1000 to
125 dB/km, it was possible to consider plastic fibers for transmitting
data similar to glass fibers but still only over shorter distances, usually
less than 100 meters (300 feet). Theoretically, plastic optical fiber (POF)
can be made by a continuous process, and should be able to be made cheaply.
With the large diameter, 10-20 times the size of glass, it is possible to use very
cheap sources and detectors. Because of the nature of plastic, it is easy to
terminate and connectorize with simple tools such as a razor blade and clip
on connectors. POF systems which can be built cheaply, say less than
$20 per link can be used for consumer electronics (speakers, high FIs,
DVDs, VCRs, etc.), home wiring (Components could be purchased at stores
like Radio Shack and installed by consumers), and automobiles.
The automotive manufacturers have been using POF for years for lighting and
wiring harnesses have been developed containing both POF and copper wires.
POF has also been researched to death by General Motors as a possible replacement
to copper wires for data transmission within an automobile. Until recently, not much has
happened except for some simple data links in high-end Japanese automobiles.
However, things are changing! With the increased use of electronics in automobiles,
together with the higher speeds for GPS, video anti-skid brakes, collision detection
and audio systems, auto makers are taking a closer look at POF data buses. Last
year Daimler-Benz installed the first POF entertainment data bus in their high-end
models. Work is underway to put all the data functions on a common data bus.
Figure 3 shows a typical automobile wiring harness compared to the entertainment
system shown in the upper left hand corner of the photo.
Table 4 shows some of the reasons why POF is being seriously considered to
replace the existing copper wiring harnesses in automobiles.
Single-mode and Multi-mode Fiber Optics Cable Assemblies
Basic Overview of a Cable Assembly
The most basic cable assembly is comprised of a fiber, or or two connectors,
and a protective jacketing.
All fiber optic assemblies are male to male. Assemblies are interconnected with
female to female adapters. Most connectors are terminated by trapping the bare
fiber within a cylindrical capillary called the ferrule and polishing the end of the
fiber/ferrule combination.
What Affects the Performance of Cable Assemblies?
The performance of a connector interface is determined by a combination of factors
including the fiber size and core centration; the ferrule size, centration, and axiality;
and the methods used in securing and polishing the fiber during termination.
We maximize the use of the components by measuring the fiber diameter,
selecting the appropriate connector size, using a high quality epoxy, and
carefully monitoring our polishing procedures.
Performance of fiber optic cables can be characterized using several tests. The
industry accepted test methods are visual inspection (>200 x), insertion loss,
and return loss. In addition, we check for polish undercut and centration with
an interferometer.
Insertion Loss
Insertion loss is the amount of optical power lost due to the interface of two
connectors. The loss is measured in the forward direction (away from the source).
Insertion is mostly caused by lateral and angular misalignment, as well as mode
field mismatch. Insertion loss is expressed in dB, and is an important consideration
in determining system power budget requirements.
Return Loss
Backreflection or return loss is the amount of light reflected back towards the source
relative to the forward signal. The primary cause of backreflections is a discontinuity
in the index of refraction of the fiber (Fresnel reflections). This effect can be
controlled by changing the connector endface.
Backreflection measurements are determined by the ratio of back reflected power
to forward power and is expressed in dB.
Backreflection = 10 log
(P back / P forward)
In most systems, backreflection is not a serious problem. However, reflection
back into a laser cavity can cause result in output fluctuations, mode hopping,
and a damaged laser. This effect is especially detrimental in high speed and
analog applications.
Connector Polishes
Currently, connectors are available with flat, convex (PC, SPC, UPC), and
Angle-Polished-Convex (APC) polishes. In general, flat and angle-polished
connectors exhibit higher insertion loss than PC, Super PC (SPC), and Ultra
PC (UPC) polishes. the biggest difference between endface types is seen in
backreflection performance.
Flat polished connectors exhibit high backreflection because of the glass-to-air
junction. All "PC" (Physical Contact) polished connectors have a radiused
endface eliminating the glass-to-air junction. The UPC is the highest quality
optical polish currently available followed by the SPC and PC respectively.
APC connectors create an interface in which most of the reflected light is
decoupled from the fiber core. Low backreflection is inherent in the APC
design, but at the sacrifice of slightly higher insertion loss.
Connectors Used in Fiber Optic Cable Assemblies
A wide ranges of connectors are required for fiber optic cable assemblies
depending on the type of system, age of the system and customer preference.
The fiber optic connector technology continues to improve with the trend to
smaller footprint connectors and multi-fiber connectors. Figures 3 and 4
show some of the commonly used connectors. Some are designed for both
single and multi-mode use and some are designed only for one or the other.
The most popular single mode connectors are the SC, ST, FC, and LC and
multi-mode connectors the SC, LC, ST and MT-RJ (currently introduced
optical fiber connector similar in size and function to the RJ series of electrical
connectors). The LC connector introduced by Lucent Technologies is showing
rapid acceptance for both single and multi-mode applications.
Typical connector specifications are show in Table 5.
The Market for Fiber Optic Cable Assemblies
The market for fiber optic cable assemblies can be divided into two major
segments: single mode and multi-mode as shown in Table 6. The multi-mode
market has been growing approximately 10 percent per year and will continue
at this pace. As also shown in the figure, the multi-mode cable market represents
close to 10 percent of the total cable market. Multi-mode applications, as was
pointed out earlier, require lower cost, and lower quality cable assemblies. However,
multimode systems require more interconnections and a larger number of cable
assemblies. Long distance systems use single mode fiber and have fewer inter-
connections in comparison, but require high quality and hence the cost is higher.
The single-mode market, however, has been growing at twice the rate (20 percent)
of the multimode market.
The fiber optics assembly market is still in the early stages of growth. Even though
there are over 200 U.S. firms manufacturing fiber optic cable assemblies, which
range from very small to large companies, only a few have revenues above
$100 million. The market can be segmented by high, medium and low levels of
quality, each offering opportunity for those companies with established technical,
manufacturing and marketing expertise. The present products are labor intensive.
As a result, off shore suppliers are attracted to the market with lower labor rates.
Automation is badly needed to reduce labor costs, increase throughput and
turnaround time, and improve quality. These are areas where the copper cable
wire harness industry has developed a great deal of knowledge and expertise
over the years.
Cable harnessing firms that have not considered the fiber optics field may
want to take another look, as the market is expected to grow and expand well
into this century.
References
1. Optical Networks/Fiber Optics Yellow Pages, published by
Information Gatekeepers Inc., 214 Harvard Ave., Boston, MA 02134.
Tel.: (617) 232-3111, Fax: (617) 734-8562. E-mail: info@igigroup.com;
Visit Internet: www.igigroup.com.
2. Wave Optics Catalog. 1999.
3. N. Kashima, "Passive Optical Components for Optical Fiber Transmission,"
Artcell House, Norwood, Massachusetts.
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