What is the difference between Single Mode and Multi Mode?

What is the difference between Single Mode and Multi Mode? How do Fiber Optic Cables Work? How are Fiber Optic Cables Made? Read the fiber optic training information below to learn more about the different types of Fiber Optic Cables. Or, for pictures of Fiber Optic Cables, visit our Fiber Optic Cables page.

The problem with "glass" or optical fiber is determining how to guide the light through optical fiber that might be hundreds or even thousands of miles in length. The key comes from your school physics lessons. You may remember shining some light through glass blocks, for instance prisms, and seeing how it was bent, or "refracted" when it left the glass. If you sent the light into the glass at an angle greater than the "critical angle," it would be totally reflected within the block and not exit where you expected. It all has to do with refractive indices and is analytically explained by a principal called Snell's Law. Snell's law basically states that the refractive index of a material determines how quickly light can travel through it.

Once you have your optical fiber core of pure silica, you add an extra layer of glass known as "cladding," which has a lower refractive index than the core. This refractive index difference guides the light in the core allowing as little as possible to leave through the sides (according to Snell's Law). On top of the cladding layer are polymer coatings that make it easier to handle the fiber and help prevent damage to the fiber.

The size of the optical fiber core determines how light travels through it. Each optical signal can actually generate many different lightwaves. These lightwaves can all travel through the fiber at the same time. This is allowed to happen in appropriately named 'multimode' fibers, but can cause problems when each wave arrives at the end of the fiber slightly out of sync. Most modern optical networks will use 'singlemode' fiber, which has a much smaller core than multimode. The core size is small enough to ensure that only one lightwave from each optical signal can travel inside the fiber. This ensures there are no problems at the receiving end.

To give you an idea of sizes, a singlemode fiber's core is usually around 10 micrometers in diameter (0.000010m) with cladding over 12 times thicker, or a diameter of 125 micrometers (0.000125m or 0.125mm). Once the polymer coatings are added, the whole package may be around 0.25 millimeters in diameter. Still very small. Even the very pure glass in optical fiber "attenuates" light. This means it causes some of the intensity of the light to be lost within the fiber. These losses can be due to several mechanisms: light losing its energy to atoms in the fiber or absorption (this is the mostprevalent), light scattering because of slight changes in the core's refractive index, and light scattering because the core is not always a perfect cylinder.

Now that you know some of the basics on fiber cables, we'll have a little more fiber optic training on the two types of fiber optic cables, singlemode and multi-mode fiber

Singlemode fiber A mode is one of the various lightwaves transmitted in an optical fiber. Each optical signal generates many different modes but, in singlemode fiber, the desire is to have only one of them transmitted. This is achieved through having a core with a very small diameter (usually around 10 micrometers). Singlemode fiber has a higher bandwidth than Multimode and, for this reason, it is the ideal transmission medium for many applications

Multimode fiber Multimode fiber is an optical fiber in which more than one mode can be transmitted through the core. A multimode fiber core is much larger in diameter than a singlemode fiber core (usually 6-8 times the size). This larger core size generally has greater bandwidth and is easier to couple and interconnect. It allows hundreds of rays of light to propagate through the same fiber simultaneously.

How Fiber Optic Signals are Transmitted Light that travels along the fiber is made up of a binary code that pulses "off" and "on" and determines what information a signal contains. The advantage of fiber is that these on/off pulses can be almost anything: translated video, computer, or voice data depending on the type of transmitter and receiver used.

Advantages of Fiber Optic Cables Over Copper

Speed: Fiber optic networks can operate at speeds of up to 2.5 gigabits-per-second, versus 1.54 megabits per second for copper

Bandwidth: Taken in bulk, it would take approximately 33 tons of copper cable to transmit the same amount of information handled by 1/4 pound of optical fiber.

Resistance: Fiber optic cables have a greater resistance to electromagnetic noise such as motors, radios, or other nearby cables. Because optical fibers carry beams of light, they are free of electrical interference and noise.

Types of Fiber Cables

Now that you know a little more about the two types of fiber optic cables, we'll have a little more fiber optic training on different forms those two types of glass can be combined together with cable jackets and overall jackets. Both Multimode and Singlemode Fiber Optic Cable can come in many different forms. Typically, Fiber Optic Patch Cables will be assembled in Duplex for Multimode Cables and Simplex for Singlemode Cables. A Duplex Multimode cable is one that contains two fiber optic cables, generally in the form of a zip cable. This duplex cable is used in networking applications where there is a transmit and receive signal. Duplex fiber optic zip cable is the orange cable pictured at the bottom of this photograph. Simplex Singlemode Cables are typically yellow and have one fiber optic cable. In most cases the outer coating of singlemode cables will be yellow, and multimode cables will be orange. Trunk Fiber Optic Cables are becoming more and more prevalent. Several examples of trunk fiber optic cables are pictured in the photograph to the left. Cypress Industries specializes in terminating trunk fiber optic cables with multiple strands and multiple connectors. Plastic Optical Fiber (POF) is a newer plastic-based fiber cable which promises performance similar to Single Mode Cable but at a lower cost. POF in general is still in the infancy stage although many companies are noticing its potential.

Thank you for reading our Fiber Optic Training online information. Should you have any other questions on products, or need ordering information, please do not hesistate to contact us. Our staff is very helpful and knowledgeable, and can walk you through the ordering process to determine what type of fiber optic cable you may need.

What is Fiber Optic Splicing

Knowledge of fiber optic splicing methods is vital to any company or fiber optic technician involved in Telecommunications or LAN and networking projects.

Simply put, fiber optic splicing involves joining two fiber optic cables together. The other, more common, method of joining fibers is called termination or connectorization. Fiber splicing typically results in lower light loss and back reflection than termination making it the preferred method when the cable runs are too long for a single length of fiber or when joining two different types of cable together, such as a 48-fiber cable to four 12-fiber cables. Splicing is also used to restore fiber optic cables when a buried cable is accidentally severed.

There are two methods of fiber optic splicing, fusion splicing & mechanical splicing. If you are just beginning to splice fiber, you might want to look at your long-term goals in this field in order to chose which technique best fits your economic and performance objectives.


Mechanical Splicing vs. Fusion Splicing

• Mechanical Splicing:
Mechanical splices are simply alignment devices, designed to
hold the two fiber ends in a precisely aligned position thus enabling light to pass from one fiber into the other. (Typical loss: 0.3 dB)

• Fusion Splicing:
In fusion splicing a machine is used to precisely align the two fiber ends then the glass ends are "fused" or "welded" together using some type of heat or electric arc. This produces a continuous connection between the fibers enabling very low loss light transmission. (Typical loss: 0.1 dB)

• Which method is better?
The typical reason for choosing one method over the other is economics. Mechanical splicing has a low initial investment ($1,000 - $2,000) but costs more per splice ($12-$40 each). While the cost per splice for fusion splicing is lower ($0.50 - $1.50 each), the initial investment is much higher ($15,000 - $50,000 depending on the accuracy and features of the fusion splicing machine being purchased). The more precise you need the alignment (better alignment results in lower loss) the more you pay for the machine.

As for the performance of each splicing method, the decision is often based on what industry you are working in. Fusion splicing produces lower loss and less back reflection than mechanical splicing because the resulting fusion splice points are almost seamless. Fusion splices are used primarily with single mode fiber where as Mechanical splices work with both single and multi mode fiber.

Many Telecommunications and CATV companies invest in fusion splicing for their long haul singlemode networks, but will still use mechanical splicing for shorter, local cable runs. Since analog video signals require minimal reflection for optimal performance, fusion splicing is preferred for this application as well. The LAN industry has the choice of either method, as signal loss and reflection are minor concerns for most LAN applications.


Fusion Splicing Method
As mentioned previously, fusion splicing is a junction of two or more optical fibers that have been permanently affixed by welding them together by an electronic arc.

Four basic steps to completing a proper fusion splice:

Step 1: Preparing the fiber - Strip the protective coatings, jackets, tubes, strength members, etc. leaving only the bare fiber showing. The main concern here is cleanliness.

Step 2: Cleave the fiber - Using a good fiber cleaver here is essential to a successful fusion splice. The cleaved end must be mirror-smooth and perpendicular to the fiber axis to obtain a proper splice. NOTE: The cleaver does not cut the fiber! It merely nicks the fiber and then pulls or flexes it to cause a clean break. The goal is to produce a cleaved end that is as perfectly perpendicular as possible. That is why a good cleaver for fusion splicing can often cost $1,000 to $3,000. These cleavers can consistently produce a cleave angle of 0.5 degree or less.

Step 3: Fuse the fiber - There are two steps within this step, alignment and heating. Alignment can be manual or automatic depending on what equipment you have. The higher priced equipment you use, the more accurate the alignment becomes. Once properly aligned the fusion splicer unit then uses an electrical arc to melt the fibers, permanently welding the two fiber ends together.

Step 4: Protect the fiber - Protecting the fiber from bending and tensile forces will ensure the splice not break during normal handling. A typical fusion splice has a tensile strength between 0.5 and 1.5 lbs and will not break during normal handling but it still requires protection from excessive bending and pulling forces. Using heat shrink tubing, silicone gel and/or mechanical crimp protectors will keep the splice protected from outside elements and breakage.


Mechanical Splicing Method
Mechanical splicing is an optical junction where the fibers are precisely aligned and held in place by a self-contained assembly, not a permanent bond. This method aligns the two fiber ends to a common centerline, aligning their cores so the light can pass from one fiber to another.

Four steps to performing a mechanical splice:

Step 1: Preparing the fiber - Strip the protective coatings, jackets, tubes, strength members, etc. leaving only the bare fiber showing. The main concern here is cleanliness.

Step 2: Cleave the fiber - The process is identical to the cleaving for fusion splicing but the cleave precision is not as critical.

Step 3: Mechanically join the fibers - There is no heat used in this method. Simply position the fiber ends together inside the mechanical splice unit. The index matching gel inside the mechanical splice apparatus will help couple the light from one fiber end to the other. Older apparatus will have an epoxy rather than the index matching gel holding the cores together.

Step 4: Protect the fiber - the completed mechanical splice provides its own protection for the splice.


Tips for Better Splices:

1. Thoroughly and frequently clean your splicing tools. When working with fiber, keep in mind that particles not visible to the naked eye could cause tremendous problems when working with fiber optics. "Excessive" cleaning of your fiber and tools will save you time and money down the road.

2. Properly maintain and operate your cleaver. The cleaver is your most valuable tool in fiber splicing. Within mechanical splicing you need the proper angle to insure proper end faces or too much light escaping into the air gaps between the two fibers will occur. The index matching gel will eliminate most of the light escape but cannot overcome a low quality cleave. You should expect to spend around $200 to $1,000 for a good quality cleaver suitable for mechanical splicing.

For Fusion splicing, you need an even more precise cleaver to achieve the exceptional low loss (0.05 dB and less). If you have a poor cleave the fiber ends might not melt together properly causing light loss and high reflection problems. Expect to pay $1,000 to $4,000 for a good cleaver to handle the precision required for fusion splicing. Maintaining your cleaver by following manufacturer instructions for cleaning as well as using the tool properly will provide you with a long lasting piece of equipment and ensuring the job is done right the first time.

3. Fusion parameters must be adjusted minimally and methodically (fusion splicing only). If you start changing the fusion parameters on the splicer as soon as there is a hint of a problem you might lose your desired setting. Dirty equipment should be your first check and them continue with the parameters. Fusion time and fusion current are the two key factors for splicing. Different variables of these two factors can produce the same splice results. High time and low current result in the same outcome as high current and low time. Make sure to change one variable at a time and keep checking until you have found the right fusion parameters for your fiber type.

Basic Principles of Fiber Optics

Introduction - Basic Principles - Applied Principles - Optical Fiber Parameters - Fiber Manufacturing - Crossword Puzzle

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What is Fiber Optics?
Fiber Benefits
Key Points in Fiber History
Check Your Understanding
Bibliography

Since its invention in the early 1970s, the use and demand of optical fiber has grown tremendously. The uses of optical fiber today are quite numerous. The most common are telecommunications, medicine, military, automotive, and industrial.

Telecommunications applications are widespread, ranging from global networks to local telephone exchanges to subscribers' homes to desktop computers. These involve the transmission of voice, data, or video over distances of less than a meter to hundreds of kilometers, using one of a few standard fiber designs in one of several cable designs.

Companies such as AT&T, MCI, and U.S. Sprint use optical fiber cable to carry plain old telephone service (POTS) across their nationwide networks. Local telephone service providers use fiber to carry this same service between central office switches at more local levels, and sometimes as far as the neighborhood or individual home.

Optical fiber is also used extensively for transmission of data signals. Private networks are owned by firms such as IBM, Rockwell, Honeywell, banks, universities, Wall Street firms, and more. These firms have a need for secure, reliable systems to transfer computer and monetary information between buildings to the desktop terminal or computer, and around the world. The security inherent in optical fiber systems is a major benefit.

Cable television or community antenna television (CATV) companies also find fiber useful for video services. The high information-carrying capacity, or bandwidth, of fiber makes it the perfect choice for transmitting signals to subscribers.

Finally, one of the fastest growing markets for fiber optics is intelligent transportation systems, smart highways with intelligent traffic lights, automated toll booths, and changeable message signs to give motorists information about delays and emergencies.

These are only a few of the many applications possible with the use of optical fiber. Other telecommunications benefits will be emphasized in more detail throughout this text. website focuses primarily on telecommunications uses of optical fiber. To understand these applications, it is important to define fiber optics.

In its simplest terms, fiber optics is a medium for carrying information from one point to another in the form of light. Unlike the copper form of transmission, fiber optics is not electrical in nature.

A basic fiber optic system consists of a transmitting device, which generates the light signal; an optical fiber cable, which carries the light; and a receiver, which accepts the light signal transmitted. The fiber itself is passive and does not contain any active, generative properties.

Corning Cable Systems manufactures and sells those components considered to be part of the passive fiber transmission subsystem; i.e., not active electronic components.

Optical fiber systems have many advantages over metallic-based communication systems. These advantages include:

Long Distance Signal Transmission
The low attenuation and superior signal integrity found in optical systems allow much longer intervals of signal transmission than metallic-based systems. While single-line, voice-grade copper systems longer than a couple of kilometers (1.2 miles) require in-line signal repeaters for satisfactory performance, it is not unusual for optical systems to go over 100 kilometers (km), or about 62 miles, with no active or passive processing. Emerging technologies promise even greater distances in the future.

The optical fiber cable in the foreground has the equivalent information-carrying capacity of the copper cable in the background.

Large Bandwidth, Light Weight, and Small Diameter
While today's applications require an ever-increasing amount of bandwidth, it is important to consider the space constraints of many end-users. It is commonplace to install new cabling within existing duct systems. The relatively small diameter and light weight of optical cables makes such installations easy and practical, and saves valuable conduit space in these environments.

Long Lengths
Long, continuous lengths also provide advantages for installers and end-users. Small diameters make it practical to manufacture and install much longer lengths than for metallic cables: twelve-kilometer (12 km) continuous optical cable lengths are common. Corning Cable Systems manufactures continuous single-mode cable lengths up to 12 km, with a 96-inch reel size being the primary limiting factor.

Multimode cable lengths can be 4 km or more, although most standards require a maximum length of 2 km or less. Multimode cable lengths are based on industry demand. (Single-mode and multimode fibers will be covered in detail later in this text.)

Easy Installation and Upgrades
Long lengths make optical cable installation much easier and less expensive. Optical fiber cables can be installed with the same equipment that is used to install copper and coaxial cables, with some modifications due to the small size and limited pull tension and bend radius of optical cables.

Optical cables can typically be installed in duct systems in spans of 6000 meters or more depending on the duct's condition, layout of the duct system, and installation technique. The longer cables can be coiled at an intermediate point and pulled farther into the duct system as necessary.

System designers typically plan optical systems that will meet growth needs for a 15- to 20-year span. Although sometimes difficult to predict, growth can be accommodated by installing spare fibers for future requirements. Installation of spare fibers today is more economical than installing additional cables later.

The dielectric nature of optical fiber can eliminate the dangers found in areas of high lightning-strike incidence.

Non-Conductivity
Another advantage of optical fibers is their dielectric nature. Since optical fiber has no metallic components, it can be installed in areas with electromagnetic interference (EMI), including radio frequency interference (RFI). Areas with high EMI include utility lines, power-carrying lines, and railroad tracks. All-dielectric cables are also ideal for areas of high lightning-strike incidence.

Security
Unlike metallic-based systems, the dielectric nature of optical fiber makes it impossible to remotely detect the signal being transmitted within the cable. The only way to do so is by actually accessing the optical fiber itself. Accessing the fiber requires intervention that is easily detectable by security surveillance. These circumstances make fiber extremely attractive to governmental bodies, banks, and others with major security concerns.

Designed for Future Applications Needs
Fiber optics is affordable today, as electronics prices fall and optical cable pricing remains low. In many cases, fiber solutions are less costly than copper.

As bandwidth demands increase rapidly with technological advances, fiber will continue to play a vital role in the long-term success of telecommunications.

Most people remember Paul Revere's "one if by land, and two if by sea" from early American history. He used lanterns to communicate information. Although not sophisticated, this was an early example of optical communication.

In 1870, British physicist John Tyndal gave us another example. Tyndal set up a tank of water with a pipe that ran out one side. He allowed the water to flow from the pipe, and then shone a bright light from inside the tank into the water stream. As the water fell, an arc of light followed the water down. This demonstrated total internal reflection, a principle that will be discussed in more detail later.

In 1880, Alexander Graham Bell invented the photophone. Bell considered this a greater discovery than his previous invention, the telephone. With the photophone, Bell would speak into a microphone, which would cause a mirror to vibrate. The sun's light would strike the mirror, and the vibration of the mirror would transmit the light across an open distance of about 200 meters (656 feet). The receiver's mirror would receive the light and cause a selenium crystal to vibrate, and the noise would come out on the other end. (See Figure 1 below.) Although the photophone was successful in allowing conversation over open space, it had a few drawbacks: it did not work well at night, in the rain, or if someone walked between the signal and the receiver. Eventually, Bell gave up on this idea.

Figure 1

It wasn't until the late 1950s that the laser was invented. This device was a finely-controlled beam of light that could transmit information over long distances. Unfortunately, the same drawbacks experienced by Alexander Graham Bell also plagued the laser. Although it could be used at night, it didn't work during rain, fog, or any time a building was erected between the sender and the receiver.

Dr. Robert Maurer, Peter Schultz, and Donald Keck of Corning Incorporated in Corning, New York, came up with the first low loss optical fiber, with less than 20 dB/km (decibels per kilometer) loss. (Today, single-mode, premium grade fiber is sold with specifications of 0.25 dB/km or better.)

In 1977, Corning joined forces with another technological giant, Siemens Corporation, to form Corning Cable Systems. Corning's extensive work with fiber, coupled with Siemens' cabling technology, helped launch a new era in optical fiber cable and associated products. Today, Corning Cable Systems is a world leader in the manufacture of fiber optic cabling system products for voice, data, and video communications applications.

Illustrated Fiber Optic Glossary

A

Absorption: That portion of optical attenuation in optical fiber resulting from the conversion of optical power to heat. Caused by impurities in the fiber such as hydroxyl ions.
A/B Switch: A device that accepts inputs (optical or electrical) from a primary path and a secondary path to provide automatic or manual switching in the event that the primary path signal is broken or otherwise disrupted. In optical A/B switches, optical signal power thresholds dictate whether the primary path is functioning and signals a switch to the secondary path until optical power is restored to the primary path.
AC: Abbreviation for alternating current. An electric current that reverses its direction at regularly recurring intervals.
Acceptance Angle: The half-angle of the cone (a) within which incident light is totally internally reflected by the fiber core. It is equal to sin-1(NA).
Active Device: A device that requires a source of energy for its operation and has an output that is a function of present and past input signals. Examples include controlled power supplies, transistors, LEDs, amplifiers, and transmitters.
A/D or ADC: Abbreviation for analog-to-digital converter. A device used to convert analog signals to digital signals.
Add/Drop Multiplexing: a multiplexing function offered in connection with SONET that allows lower level signals to be added or dropped from a high-speed optical carrier in a wire center. The connection to the add/drop multiplexer is via a channel to a central office port at a specific digital speed (DS3, DS1, etc.)
ADM: Abbreviation for add-drop multiplexer. A device which adds or drops signals from a communications network.
ADSL: Abbreviation for asynchronous digital subscriber line. See DSL.
Aerial Plant: Cable that is suspended in the air on telephone or electric utility poles.
AGC: Abbreviation for automatic gain control. A process or means by which gain is automatically adjusted in a specified manner as a function of input level or another specified parameter.
AM: Abbreviation for amplitude modulation. A transmission technique in which the amplitude of the carrier varies in accordance with the signal.
Amplified Spontaneous Emission (ASE): A background noise mechanism common to all types of erbium-doped fiber amplifiers (EDFAs). It contributes to the noise figure of the EDFA which causes loss of signal-to-noise ratio (SNR).
Amplifier: A device, inserted within a transmission path, that boosts the strength of an electronic or optical signal. Amplifiers may be placed just after the transmitter (power booster), at a distance between the transmitter and the receiver (in-line amplifier), or just before the receiver (preamplifier).
Analog: A continuously variable signal. Opposite of digital.
Angular Misalignment: Loss at a connector due to fiber end face angles being misaligned.
ANSI: Abbreviation for American National Standards Institute. An organization that administers and coordinates the U.S. voluntary standardization and conformity assessment system.	http://www.ansi.org/
APC: Abbreviation for angled physical contact. A style of fiber optic connector with a 5°-15° angle on the connector tip for the minimum possible backreflection.
APD: See avalanche photodiode.
APL: Abbreviation for average picture level. A video quality parameter.
AR Coating: Antireflection coating. A thin, dielectric or metallic film applied to an optical surface to reduce its reflectance and thereby increase its transmittance.
Armor: A protective layer, usually metal, wrapped around a cable.
ASCII: Abbreviation for American standard code for information interchange. An encoding scheme used to interface between data processing systems, data communication systems, and associated equipment.
ASIC: Abbreviation for application-specific integrated circuit. A custom-designed integrated circuit.
ASTM: Abbreviation for American Society for Testing and Materials. An organization that provides a forum for the development and publication of voluntary consensus standards for materials, products, systems, and services that serve as a basis for manufacturing, procurement, and regulatory activities.	http://www.astm.org/
Asynchronous: Data that is transmitted without an associated clock signal. The time spacing between data characters or blocks may be of arbitrary duration. Opposite of synchronous.
Asynchronous Transfer Mode (ATM): A transmission standard widely used by the telecom industry. A digital transmission switching format with cells containing 5 bytes of header information followed by 48 data bytes. Part of the B-ISDN standard.
ATE: Abbreviation for automatic test equipment. Test equipment computer programmed to perform a number of test measurements on a device without the need for changing the test setup. Especially useful in testing components and PCB assemblies.
ATSC: Abbreviation for Advanced Television Systems Committee. Formed to establish technical standards for advanced television systems, including digital high definition television (HDTV).	http://www.atsc.org
Attenuation: The decrease in signal strength along a fiber optic waveguide caused by absorption and scattering. Attenuation is usually expressed in dB/km.
Attenuation-Limited Operation: The condition in a fiber optic link when operation is limited by the power of the received signal (rather than by bandwidth or distortion).
Attenuator: 1) In electrical systems, a usually passive network for reducing the amplitude of a signal without appreciably distorting the waveform. 2) In optical systems, a passive device for reducing the amplitude of a signal without appreciably distorting the waveform.
Avalanche Photodiode (APD): A photodiode that exhibits internal amplification of photocurrent through avalanche multiplication of carriers in the junction region.
Average Power: The average level of power in a signal that varies with time.
AWG (Arrayed Waveguide Grating): A device, built with silicon planar lightwave circuits (PLC), that allows multiple wavelengths to be combined and separated in a dense wavelength-division multiplexing (DWDM) system.
Axial Propagation Constant: For an optical fiber, the propagation constant evaluated along the axis of a fiber in the direction of transmission.
Axis: The center of an optical fiber.

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