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Reference Guide To Fiber Optics


Topic: OSP Fiber Optic Transmission Systems Table of Contents: The FOA Reference Guide To Fiber Optics


OSP Fiber Optic Transmission Systems

Jump To:
Sources for Fiber Optic Transmitters  
Detectors for Fiber Optic Receivers  
Wavelength Division Multiplexing (WDM)
 
Signal Regernators  
Fiber Optic Network Transmission Bands  
Datalink Performance  



fiber optic link
Fiber Optic Datalink


Fiber optic transmission systems all use data links that work similar to the diagram shown above. Each fiber link consists of a transmitter on one end of a fiber and a receiver on the other end. Most systems operate by transmitting in one direction on one fiber and in the reverse direction on another fiber for full duplex operation. It's possible to transmit both directions on one fiber but it requires couplers to do so and fiber is less expensive than couplers. A FTTH passive optical network (PON) is one of the only systems using bidirectional transmission over a single fiber because its network architecture is based around couplers already.

fiber optic transceiver
Fiber Optic Transceiver



Most systems use a "transceiver" which includes both transmission and receiver in a single module. The transmitter takes an electrical input and converts it to an optical output from a laser diode or LED. The light from the transmitter is coupled into the fiber with a connector and is transmitted through the fiber optic cable plant. The light from the end of the fiber is coupled to a receiver where a detector converts the light into an electrical signal which is then conditioned properly for use by the receiving equipment.

  

Analog or Digital

Analog signals are continuously variable signals where the information in the signal is contained in the amplitude of the signal over time. Digital signals are sampled at regular time intervals and the amplitude converted to digital bytes so the information is a digital number. Analog signals are the natural form of most data, but are subject to degradation by noise in the transmission system. As an analog signal is attenuated in a  cable, the signal to noise ratio becomes worse so the quality of the signal degrades. Digital signals can be transmitted long distances without degradation as the signal is less sensitive to noise.
analog and digital

Fiber optic datalinks can be either analog or digital in nature, although most are digital. Both have some common critical parameters and some major differences. For both, the optical loss margin or power budget is most important. This is determined by connecting the link up with an adjustable attenuator in the cable plant and varying the loss between transmitter and receiver until one can generate the curve shown above. Analog datalinks will be tested for signal to noise ratio to determine link margin, while digital links use bit error rate as a measure of performance. Both links require testing over the full bandwidth specified for operation, but most data links are now specified for a specific network application, like AM CATV or RGB color monitors for analog links and SONET, Ethernet or Fibre Channel for digital links.


Sources for Fiber Optic Transmitters

The sources used for fiber optic transmitters need to meet several criteria: it has to be at the correct wavelength, be able to be modulated fast enough to transmit data and be efficiently coupled into fiber.

Four types of sources are commonly used, LEDs, fabry-perot (FP) lasers, distributed feedback (DFB) lasers and vertical cavity surface-emitting lasers (VCSELs). All convert electrical signals into optical signals, but are otherwise quite different devices. All three are tiny semiconductor devices (chips). LEDs and VCSELs are fabricated on semiconductor wafers such that they emit light from the surface of the chip, while f-p lasers emit from the side of the chip from a laser cavity created in the middle of the chip.  

fiber opitc sources

LEDs have much lower power outputs than lasers and their larger, diverging light output pattern makes them harder to couple into fibers, limiting them to use with multimode fibers. Laser have smaller tighter light outputs and are easily coupled to singlemode fibers, making them ideal for long distance high speed links. LEDs have much less bandwidth than lasers and are limited to systems operating up to about 250 MHz or around 200 Mb/s. Lasers have very high bandwidth capability, most being useful to well over 10 GHz or 10 Gb/s.
Because of their fabrication methods,
LEDs and VCSELs are cheap to make. Lasers are more expensive because creating the laser cavity inside the device is more difficult, the chip must be separated from the semiconductor wafer and each end coated before the laser can even be tested to see if its good.

Typical Fiber Optic Source Specifications
Device TypeWavelength (nm)Power into
Fiber (dBm)
BandwidthFiber Types
LED850, 1300
-30 to -10
<250 MHzMM
Fabry-Perot Laser850,
1310 (1280-1330) 1550 (1480-1650)
0 to +10>10 GHzMM, SM
DFB Laser 1550 (1480-1650)0 to +25>10 GHzSM
VCSEL850-10 to 0>10 GHzMM

LEDs have a limited bandwidth while all types of lasers are very fast. Another big difference between LEDs and both types of lasers is the spectral output. LEDs have a very broad spectral output which causes them to suffer chromatic dispersion in fiber, while lasers have a narrow spectral output that suffers very little chromatic dispersion. DFB lasers, which are used in long distance and DWDM systems, have the narrowest spectral width which minimizes chromatic dispersion on the longest links. DFB lasers are also highly linear (that is the light output directly follows the electrical input) so they can be used as sources in AM CATV systems.

LEDs and lasers



The choice of these devices is determined mainly by speed and fiber compatibility issues.  As many premises systems using multimode fiber have exceeded bit rates of 1 Gb/s, lasers (mostly VCSELs) have replaced LEDs. The output of the LED is very broad but lasers are very focused, and the sources will have very different modal fill in the fibers. The restricted launch of the VCSEL (or any laser) makes the effective bandwidth of the fiber higher, but laser-optimized fiber, usually OM3, is the choice for lasers.

launch into multimode fiber

The electronics for a transmitter are simple. They convert an incoming pulse (voltage) into a precise current pulse to drive the source. Lasers generally are biased with a low DC current and modulated above that bias current to maximize speed.



Detectors for Fiber Optic Receivers

Receivers use semiconductor detectors (photodiodes or photodetectors) to convert optical signals to electrical signals. Silicon photodiodes are used for short wavelength links (650 for POF and 850 for glass MM fiber). Long wavelength systems usually use InGaAs (indium gallium arsenide) detectors as they have lower noise than germanium which allows for more sensitive receivers.


detector sensitivity

Very high speed systems sometimes use avalanche photodiodes (APDs) that are biased at high voltage to create gain in the photodiode. These devices are more expensive and more complicated to use but offer significant gains in performance.

Packaging

Transcivers are usually packaged in industry standard packages like these XFP modules for gigabit datalinks(L) and Xenpak (R). The XFP modules connect to a duplex LC connector on the optical end and a standard electrical interface on the other end. The Xenpak are for 10 gigabit networks but use SC duplex connection. Both are similar to media converters but are powered from the equipment they are built into.

transceiver  
transcivers

Performance

Just as with copper wire or radio transmission, the performance of the fiber optic data link can be determined by how well the reconverted electrical signal out of the receiver matches the input to the transmitter. The discussion of performance on datalinks applies directly to transceivers which supply the optical to electrical conversion.

Every manufacturer of transcivers specifies their product for receiver sensitivity (perhaps a minimum power required) and minimum power coupled into the fiber from the source. Those specifications will end up being the datalink specifications on the final product used in the field.


Wavelength Division Multiplexing (WDM)

Why Is WDM Used?
With the exponential growth in communications, caused mainly by the wide acceptance of the Internet, many carriers are finding that their estimates of fiber needs have been highly underestimated. Although most cables included many spare fibers when installed, this growth has used many of them and new capacity is needed. Three methods exist for expanding capacity: 1) installing more cables, 2) increasing system bitrate to multiplex more signals or 3) wavelength division multiplexing.
Installing more cables will be the preferred method in many cases, especially in metropolitan areas, since fiber has become incredibly inexpensive and installation methods more efficient (like mass fusion splicing.) But if conduit space is not available or major construction is necessary, this may not be the most cost effective.
Increasing system bitrate may not prove cost effective either. Many systems are already running at SONET OC-48 rates (2.5 GB/s) and upgrading to OC-192 (10 GB/s) is expensive, requires changing out all the electronics in a network, and adds 4 times the capacity, more than may be necessary.
The third alternative, wavelength division multiplexing (WDM), has proven more cost effective in many instances. It allows using current electronics and current fibers, but simply shares fibers by transmitting different channels at different wavelengths (colors) of light. Systems that already use fiber optic amplifiers as repeaters also do not require upgrading for most WDM systems.

How Does WDM Work?
It is easy to understand WDM. Consider the fact that you can see many different colors of light - reg, green, yellow, blue, etc. all at once. The colors are transmitted through the air together and may mix, but they can be easily separated using a simple device like a prism, just like we separate the "white" light from the sun into a spectrum of colors with the prism.

 

Separating a beam of light into its colors

This technique was first demonstrated with optical fiber in the early 80s when telco fiber optic links still used multimode fiber. Light at 850 nm and 1300 nm was injected into the fiber at one end using a simple fused coupler. At the far end of the fiber, another coupler split the light into two fibers, one sent to a silicon detector more sensitive to 850 nm and one to a germanium or InGaAs detector more sensitive to 1300 nm. Filters removed the unwanted wavelengths, so each detector then was able to receive only the signal intended for it.

WDM with couplers and filters

By the late 80s, all telecom links were singlemode fiber, and coupler manufactures learned how to make fused couplers that could separate 1300nm and 1550 nm signals adequately to allow WDM with simple, inexpensive components. However, these had limited usefulness, as fiber was designed differently for 1300nm and 1550 nm, due to the dispersion characteristics of glass. Fiber optimized at 1300 nm was used for local loop links, while long haul and submarine cables used dispersion-shifted fiber optimized for performance at 1550 nm. This simple version of WDM is widely used in fiber to the home (FTTH) applications. Signals are sent downstream to the subscriber at 1490 nm (and 1550 for analog CATV if used) and upstream at 1310 n. 
With the advent of fiber optic amplifiers for repeaters in the late 80s (see below), emphasis shifted to the 1550 nm transmission band. WDM only made sense if the multiplexed wavelengths were in the region of the fiber amplifiers operating range of 1520 to 1560 nm. It was not long before WDM equipment was able to put 4 signals into this band, with wavelengths about 10 nm apart.
The input end of a WDM system is really quite simple. It is a simple coupler that combines all the inputs into one output fiber. These have been available for many years, offering 2, 4, 8, 16, 32 or even 64 inputs. It is the demultiplexer that is the difficult component to make.

WDM demultiplexer

The demultiplexer takes the input fiber and collimates the light into a narrow, parallel beam of light. It shines on a grating (a mirror like device that works like a prism, similar to the data side of a CD) which separates the light into the different wavelengths by sending them off at different angles. Optics capture each wavelength and focuses it into a fiber, creating separate outputs for each separate wavelength of light.

DWDM
Current systems offer from 4 to 32 channels of wavelengths. The higher numbers of wavelengths has lead to the name Dense Wavelength Division Multiplexing or DWDM. The technical requirement is only that the lasers be of very specific wavelengths and the wavelengths are very stable, and the DWDM demultiplexers capable of distinguishing each wavelength without crosstalk.

CWDM
Coarse wavelength-division multiplexing is another variant of WDM. Generally CWDM refers to using lasers spaced 20 nm apart over the full range of 1260 to 1670 nm. It only works on low water peak fibers, where the high water absorption bands have been eliminated in the manufacture of the fiber.

Advantages of WDM
A WDM system has some features that make them very useable. Each wavelength can be from a normal link, for example a OC-48 link, so you do not obsolete most of your current equipment. You merely need laser transmitterss chosen for wavelengths that match the WDM demultiplexer to make sure each channel is properly decoded at the receiving end.
If you use an OC-48 SONET input, you can have 4X2.5 GB/s = 10 GB/s up to 32 X 2.5 GB/s = 80 GB/s. While 32 channels are the maximum today, future enhancements are expected to offer 80-128 channels!
And you are not limited to SONET, you can use Gigabit Ethernet for example, or you can mix and match SONET and Gigabit Ethernet or any other digital signals! You can even mix in analog channels like CATV, as is done with  BPON FTTH systems.


Signal Regernators

Repeaters
Another technology that facilitates DWDM is the development of fiber optic amplifiers for use as repeaters. They can amplify numerous wavelengths of light simultaneously, as long as all are in the wavelength range of the FO amplifier. They work best in the range of 1520-1560 nm, so most DWDM systems are designed for that range. Now that fiber has been made with less effect from the OH absorption bands at 1400 nm and 1600 nm, the possible range of DWDM has broadened considerably. Technology needs development for wider range fiber amplifiers to take advantage of the new fibers.

Fiber Amplifiers

While the low loss of optical fiber allows signals to travel hundreds of kilometers, extremely long haul lines and submarine cables require regenerators or repeaters to amplify the signal periodically. In the beginning, repeaters basically consisted of a receiver followed by a transmitter. The incoming signal was converted from a light signal to an electrical signal by a receiver, cleaned up to remove as much noise as possible, then was retransmitted by another laser transmitter.

Electronic Repeater

These repeaters added noise to the signal, consumed much power and were complicated, which means they were a source of failure. They also had to be made for the specific bit rate of transmission and upgrading required replacing all the repeaters, a really difficult task in an undersea cable!
Since the 1960s, researchers knew how to make fiber lasers. Proper doping of the fiber (introducing small amounts of active elements into the glass fiber) allowed it to be pumped with external light sources until stimulated emission occurred. While making fiber amplifiers was hypothesized early in the stages of fiber optic development, it was not until 1987 that working models were realized. Major contributors to the development included Bell Labs and NTT.
The typical fiber amplifier works in the 1550 nm band and consists of a length of fiber doped with Erbium pumped with a laser at 980. The pump laser supplies the energy for the amplifier, while the incoming signal stimulates emission as the pulse passes through the doped fiber. The stimulated emission stimulates more emission, so there is a rapid, exponential growth of photons in the doped fiber. Gains of >40 dB (10,000X) are possible with power outputs >+20 dBm (100 mW).


Basic Fiber Amplifier

 

To date, the most efficient fiber amplifiers have been Erbium-Doped Fiber Amplifiers (EDFAs) operating in the 1550 nm range. Since most systems still work at 1310 nm, considerable research has been done to find materials that would work in this range. Praseodymium-doped fluoride fiber amplifiers (PDFFAs) using fibers made from zirconium fluoride or hafnium fluoride have shown some promise, but have not developed the performance needed for widespread applications.
The basic structure of an EDFA is very simple. The amplifier itself emits light energy in a signal wavelength (usually about 1540nm) using energy supplied to it by photons in a pump wavelength (usually 980nm) when stimulated by incoming photons in the signal - the signal which needs amplification. Just like in a laser, the emitted photons then stimulate other emissions, so there is an exponential growth of photons. Supporting the amplifier is a pump laser, which supplies the amplifier's energy, a coupler, which combines the pump laser beams and the signal laser beam and puts them on a single fiber, and an optical filter, which removes the remaining traces of the pump beam so that it doesn't interfere with reception of the signal.

Alternative Designs
The simple diagram of an EDFA shown above is not the only way EDFAs can be made. Pumping can be done in a forward direction as shown, backward from the output end or in both directions. Optical isolators are commonly used at both ends of the EDFA to prevent pump energy from escaping back down the fiber or unwanted reflections that may affect laser stability. Filters, often Bragg gratings (filters fabricated in fibers), are used to flatten the gain over the broadest wavelength range for use in WDM systems.

Other Applications
Besides being used as repeaters, fiber amplifiers are used to increase signal level for CATV systems, which require high power levels at the receiver to maintain adequate signal to noise performance, allowing longer cable runs or using splitters to "broadcast" a single signal through a coupler to many fibers, saving the cost of additional transmitters. In telephony, they combine with DWDM (dense wavelength division multiplexers) to overcome the inefficiencies of DWDMs for long haul transmission.

Future Developments
Fiber amplifiers continue to be developed to support Dense Wavelength Division Multiplexing and to expand to the other wavelength bands supported by fiber optics. Now that fiber manufacturers have all but removed the water bands from the spectrum, there is now a range of 1260 to 1610 nm available for use. Fiber amplifiers and diode lasers will probably be developed within this band to completely fill it with useable bandwidth.


Applications
Two obvious applications are already in use, submarine cables and extending the lifetime of cables where all fibers are being used. For submarine cables, DWDM enhances the capacity without adding fibers, which create larger cables and bulkier and more complicated repeaters. Adding service in areas where cables are now full is another good application.
But this technology may also reduce the cost on all land-based long distance communications links and new technology may lead to totally new network architectures.

Further Enhancements
Imagine an all-optical network that uses DWDM, switches signals in the optical domain without converting signals to electronics, and can add or drop signals by inserting or withdrawing wavelengths at will. All this is being researched right now, and given the speed with which optical technology advances, an all-optical network may not be far in the future!


Fiber Optic Network Optical Wavelength Transmission Bands

As fiber optic networks have developed for longer distances, higher speeds and wavelength-division multiplexing (WDM), fibers have been used in new wavelength ranges, now called "bands," where fiber and transmission equipment can operate more efficiently. Singlemode fiber transmission began in the "O-band" just above the cutoff wavelength of the SM fiber developed to take advantage of the lower loss of the glass fiber at longer wavelengths and availablility of 1310 nm diode lasers. (Originally SM fibers were developed for 850 nm lasers where the fiber core was about half what it is for today's conventional SM fiber (5 microns as opposed to 8-9 microns at 1310 nm.)
To take advantage of the lower loss at 1550 nm, fiber was developed for the C-band. As links became longer and fiber amplifiers began being used instead of optical-to-electronic-to-optical repeaters, the C-band became more important. With the advent of DWDM (dense wavelength-division multiplexing) which allowed multiple signals to share a single fiber, use of this band was expanded. Development of new fiber amplifiers (Raman and thullium-doped) promise to expand DWDM upward to the L-band.
Since the fiber manufacturers have been able to reduce the water peaks at 1244 and 1383 nm to very low levels, several low-cost versions of WDM are in use, generally referred to as Coarse WDM or CWDM. Most do not work over long distances so do not require amplification, broading the wavelength choice. The most popular is FTTH PON systems, sending signals downstream to users at 1490 nm and using low cost 1310 nm transmission upstream. Early PON systems also use 1550 downstream for TV, but that is being replaced by IPTV on the downstream digital signal at 1490 nm. Other systems use a combination of S, C and L bands to carry signals because of the lower attenuation of the fiber. Some systems even use lasers at 20 nm spacings over the complete range of 1260 to 1660 nm but only with low water peak fibers.
Manufacturers have been able to make fiber with low-water peaks, opening up a new transmission band (E-band), but it has not yet proven useful except for CWDM. It is probably mostly useful as an extension of the O-band but few applications have been proposed and it is very energy-intensive for manufacture.

Low water peak fiber
Low Water Peak Fiber


Wavelength Bands Used For Fiber Optic Transmission Systems


Band NameWavelengthsDescription
O-band 1260 – 1360 nmOriginal band, PON upstream, low end of CWDM systems
E-band 1360 – 1460 nmWater peak band
S-band 1460 – 1530 nmPON downstream
C-band 1530 – 1565 nmLowest attenuation, original DWDM band, compatible with fiber amplifiers, CATV
L-band 1565 – 1625 nmLow attenuation, expanded DWDM band
U-band 1625 – 1675 nm


Datalink Performance

Just as with copper wire or radio transmission, the performance of the fiber optic data link can be determined by how well the reconverted electrical signal out of the receiver matches the input to the transmitter.

BER

The ability of any fiber optic system to transmit data ultimately depends on the optical power at the receiver as shown above, which shows the data link bit error rate as a function of optical power at the receiver. (BER is the inverse of signal-to-noise ratio, e.g. high BER means poor signal to noise ratio.)  Either too little or too much power will cause high bit error rates. Too much power, and the receiver amplifier saturates, too little and noise becomes a problem as it interferes with the signal. This receiver power depends on two basic factors: how much power is launched into the fiber by the transmitter and how much is lost by attenuation in the optical fiber cable plant that connects the transmitter and receiver.


The optical power budget of the link is determined by two factors, the sensitivity of the receiver, which is determined in the bit error rate curve above and the output power of the transmitter into the fiber. The minimum power level that produces an acceptable bit error rate determines the sensitivity the receiver. The power from the transmitter coupled into the optical fiber determines the transmitted power. The difference between these two power levels determines the loss margin (power budget) of the link.


Power Budget


High speed links like gigabit or 10gigabit Ethernet LANs on multimode fiber have derating factors for the bandwidth of fiber caused by dispersion spreading out the data pulse. Older 62.5/125 OM1 fiber will generally operate only on shorter links while links on 50/125 OM3 laser-optimized fiber will go the longest distance. Even long distance singlemode fiber links may have limitations caused by chromatic or polarization-mode dispersion.

If the link is designed to operate at differing bit rates, it is necessary to generate the performance curve for each bit-rate. Since the total power in the signal is a function of pulse width and pulse width will vary with bit-rate (higher bit-rates means shorter pulses), the receiver sensitivity will degrade at higher bit-rates.

Every manufacturer of datalinks components and systems specifies their link for receiver sensitivity (perhaps a minimum power required) and minimum power coupled into the fiber from the source. Typical values for these parameters are shown in the table below. In order for a manufacturer or system designer to test them properly, it is necessary to know the test conditions. For data link components, that includes input data frequency or bitrate and duty cycle, power supply voltages and the type of fiber coupled to the source. For systems, it will be the diagnostic software needed by the system.

Typical Fiber optic link/system performance parameters
Link type Source/Fiber Type

Wave-

length (nm)

Transmit Power (dBm) Receiver Sen- sitivity (dBm) Margin (dB)
Telecom laser/SM 1300/1550 +3 to -6 -30 to -45 30 to 40
DWDM 1550+20 to -30 to -45 40 to 50
Datacom LED/VCSEL 850 -3 to -15 -15 to -30 3 to 25
LED/laser 1300 -0 to -20 -15 to -30 10 to 25
CATV(AM) laser/SM 1300/1550 +10 to 0 0 to -10 10 to 20

 

Within the world of datacommunications links and networks, there are many vendor-specific fiber optic systems, but there are also a number of industry standard networks such as Ethernet which have fiber optic standards. These networks have agreed upon specifications common to all manufacturers' products to insure interoperability. This page in FOA Tech Topics shows a summary of specifications for many of these systems.

Long Distance Considerations
Fiber optic links can now be hundreds or even thousands of kilometers long and operate at 10 gigabits per second or more – certainly lots more in the future – and at many wavelengths over a single fiber. Problems that are ignorable on short or metropolitan links may become extremely important on long links. The  section on optical fiber will cover topics like chromatic dispersion, polarization mode dispersion and spectral attenuation that can be important in these long links.


Summary

Datalinks must have proper receiver power, neither too little nor too much, for proper operation.

The link margin can be measured with a power meter and variable attenuator.


Test Your Comprehension


References
The FOA Reference Guide to Outside Plant Fiber Optics  

The FOA Online Reference Guide to Fiber Optics  
FOA Reference Guide to Outside Plant Fiber Optics
You can buy the printed version of the 
The FOA Reference Guide to Outside Plant Fiber Optics from the FOA eStore or Amazon.





Table of Contents: The FOA Reference Guide To Fiber Optics




 


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