Why use fiber optics?

• Immunity to electrical interference, ground loops and short circuits
• Does not cause electrical interference
• Near unlimited inexhaustible transmission capacity
• Much lower transmission loss and higher bandwidth than copper
• Very secure transmission
• Cost effective for longer distances
  

What is an optical fiber?

The fibers consist of a hair thin strand of glass coated with a protective plastic insulation. The actual glass strand is actually made up of two components: a core of between 9 and 62.5um in diameter surrounded by a cladding of lower refractive index glass with an outside diameter of 125um. There are several other types of fiber, both in terms of materials used and dimensions, but the ones mentioned here are by far the most common. Light travels in the core region by "Total Internal Reflection" which means that light rays bounce off the core cladding interface and make their way from the input of the fiber to the other end. A fiber with a 62.5 um core and 125um cladding diameter is referred to as a 62.5/125um fiber.

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How Does an Optical Fiber Transmit Light?

Suppose you want to shine a flashlight beam down a long, straight hallway. Just point the beam straight down the hallway. Light travels in straight lines, so it is no problem. What if the hallway has a bend in it? You could place a mirror at the bend to reflect the light beam around the corner. What if the hallway is very winding with multiple bends? You might line the walls with mirrors and angle the beam so that it bounces from side-to-side all along the hallway. This is exactly what happens in an optical fiber. The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls). Because the cladding does not absorb any light from the core, the light wave can travel great distances.

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What is the difference between Single-mode and Multimode fiber?

The difference between a single-mode and a multimode cable is their core size. The core is the actual region of the cable which transmits the light (data).
Single-mode cables: have a small core, with only about 2-9 microns (a micron is a unit of measurement equal to one millionth of a meter). Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest typically 1310 or 1550nm. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type.
Single-mode fiber is best used for applications where there is a longer wiring distance, or high-speed connections. Single-mode fiber gives you a higher transmission rate which is virtually unlimited and up to 50 times more distance than multimode sometimes over 100 miles.
Single Modem fiber is used in many applications where data is sent at multi-frequency (WDM Wave-Division-Multiplexing) so only one cable is needed - (single-mode on one single fiber)
Multimode cable: has a slighter larger diameter. There are actually 2 different types of multimode cables: 62.5/125 or 50/125. Which one you will need to use will depend on the equipment being used with the cable. Multimode is better used for connections that are a short distance or a low speed.
Multimode fiber gives you high bandwidth at high speeds (10 to 100MBS - Gigabit to 275m to 2km) over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable’s core typically 850 or 1300nm. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs (greater than 3000 feet [914.4 meters), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so designers now call for single mode fiber in new applications using Gigabit and beyond.
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What is the difference between a simplex and duplex cable?

A simplex cable is just a single strand of fiber, whereas a duplex cable is 2 fibers which are side by side, and connected by their jacket.

What is the maximum distance that a fiber optic modem can transmit?

The maximum distance a modem can travel is the difference between receiver sensitivity and transmission power of the fiber optic modem divided by the transmission loss of the fiber used. Typical multi-mode modems will transmit approximately 2 miles, while single-mode can transmit up to 40 miles or more. There are some higher powered products that can exceed 100 miless.

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What transmission loss can be expected when using optical fiber cable?

This depends on the wavelength used to transmit the information as well as the type of fiber used.

• Multimode Optical Fiber @ 850nm : Typically 3.0dB/km
• Multimode Optical Fiber @ 1310nm : Typically 1.0dB/km
• Single-mode Optical Fiber @ 1310nm : Typically 0.4dB/km
• Single-mode Optical Fiber @ 1550nm : Typically 0.2 dB/km
  

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How do I calculate link budgets?

The difference between transmitter power and receiver sensitivity is consumed by:

• Fiber losses
• Connector losses (e.g., at patch panels)
• Splice losses
• Link margin

The link margin can be as low as 2 or 3dB in very well engineered and controlled environments and as much as 10dB in some situations. It is intended to allow for:

• Component aging (eg, some light sources’ lifetimes are specified for when their power drops by halfhalf).
• Temperature effects on both transmitters and receivers (it may be necessary to allow 3dB or so for transmitter power variations with at temperature range extremes).
• Possible damage to cables and consequent extra losses due to repairs (this is usually negligible but in some industrial situations it may be possible for cable damage to occasionally occur).
  

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What is an optical transmitter?

The transmitter receives and directs the optical device to turn the light "on" and "off" in the correct sequence, thereby generating a light signal.
The transmitter is physically close to the optical fiber and may even have a lens to focus the light into the fiber. Lasers have more power than LEDs, but vary more with changes in temperature and are more expensive. The most common wavelengths of light signals are 850 nm, 1,300 nm, and 1,550 nm (infrared, non-visible portions of the spectrum).

What is an optical receiver?

The optical receiver takes the incoming digital light signals, decodes them and sends the electrical signal to the other user’s computer, TV or telephone. The receiver uses a photocell or photodiode to detect the lightt.

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How does digital fiber transmission work?

As in analog-based fiber systems, transmitters in digital systems take in analog or digital video, audio and data signals and send them as light pulses over fiber cable to digital fiber receivers. ?The receivers output these signals in their original format.
In a digital system, the incoming baseband signals immediately run through "analog to digital" converters within the transmitter. This converts the incoming signal or signals to a series of 1s and 0s, called "digital streams." Then, if more than one signal has been processed, the transmitter combines all the resulting digital streams into a single digital stream. This combined stream is used to turn on and off the emitting diode at a very high speed, corresponding to the 1s and 0s to be transmitted.
At the receiving end, the process performed by the transmitter is reversed. The combined digital bit stream is separated into multiple bit streams, representing each of the unique, transmitted signals. These are then run through digital to analog converters, and the receiver outputs video, audio and data in the same, analog or digital format in which the signals originated.
For the fiber optic transmission of video, the following transmission systems are commonly utilized:

• AM
• FM
• Digital
  

What is Amplitude Modulation (AM) fiber optic video transmission?

Amplitude or Intensity Modulation (AM):
the brightness or intensity of a light-emitting diode (LED) within the optical transmitter unit varies linearly with the incoming video level. The amplitude modulated optical signal is transmitted through the fiber to the optical receiver unit, where the signal is converted back to analog baseband video.

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What is Frequency Modulation (FM) fiber optic video transmission?

Frequency (or Pulsed-Frequency) Modulation (FM):
a radio frequency (RF) carrier is frequency modulated linearly by the incoming video signal. This modulated RF carrier in turn is applied the LED or laser emitter in the optical transmitter unit, and the frequency modulated signal is transmitted through the fiber to the optical receiver unit, where the FM signal is converted back to analog baseband video.

What is a Digitally-Encoded fiber optic video transmission?

Digitally-Encoded Video Transmission:
A standard baseband analog CCTV video signal is sampled at a very high rate and converted to digital signal format, and applied to the LED or laser emitter within the video optical transmitter unit. This digitized signal is transmitted through the fiber, and then converted back to an analog baseband video signal within the optical receiver unit.

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What are the advantages of digital processing over traditional analog AM and FM systems?

AM

• AM video transmission equipment is only available at the 850 Nm. multimode operating wavelength, and this limits the maximum usable transmission distance.
• The signal-to-noise ratio with an AM-based system degrades relatively linearly at 2 dB for every 1 dB of optical path loss so acceptable quality video transmission may only be obtainable at relatively short fiber optic cable distances.
• Some manufacturer’s equipment may require receiver gain adjustments during the initial set-up of the equipment, complicating the installation process.
• AM product will not support the RS-250C Medium or Short-Haul video transmission.
• Multiplexing of many video, audio and data signals taxes the limit of the technology.

FM

• The signal-to-noise ratio will degrade at higher levels of optical attenuation and on longer fiber optic cable runs.
• The relationship between signal-to-noise ratio vs. optical attenuation is not linear, so the performance is not fully predictable or consistent.
• RS-250C Short-Haul Transmission quality is very difficult to achieve.
• FM video transmitter and receiver units may be susceptible to external sources of electromagnetic and radio frequency interference (EMI/RFI) from cellular telephones and walkie-talkies, etc.
• Multiplexing of many video, audio and data signals taxes the limit of the technology.

Digital

• In digital transmission, fidelity of the baseband video, audio and data signals are repeatable, predictable and remain constant throughout the system’s entire available optical budget, no matter if transmitting one or multiple signals and no matter over short or long distances (up to the longest distance allowed by the system). This is an attribute that cannot be matched by analog-based AM and FM systems.
• In a digital system, there is no opportunity for distortion to occur in the transmission process or as a result of the method of modulation.
• Do not require linear light sources and have a high immunity to noise.
• In a digital system, all incoming signals- video, audio or data-are converted to the same basic units of 1s and 0s. Therefore, the system processes them all in the same manner, providing a high level of design flexibility. For instance, bits representing an audio signal can be substituted for the bits representing a data signal. And bits can be easily combined either from multiple same-type signals, such as 4 video channels, or from different-type signals, like combined audio and data.
• True broadcast-quality performance that can exceed all of the parameters defined for RS-250C Short-Haul Transmission.