Objectives: From this chapter you should learn:
How fiber optic data links and transmission systems work What components are used in transceivers
Types of sources and detectors used in transceivers
Performance parameters of fiber optic transmission systems
Fiber Optic Data Links
Fiber optic transmission systems 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 few systems using bidirectional transmission over a single fiber because its network architecture is based around couplers already. See Appendix B for more information on FTTH.
Most systems use a “transceiver” which includes both transmitter 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.
Fiber optic datalinks can be either analog or digital in nature, although most are digital.
Telephone and computer networks are digital, CATV is currently using analog transmission but will be converting to digital and CCTV may be either.
Both analog and digital links have some common critical parameters and some major differences. For both, the optical loss margin or power budget is most important. 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.
Packaging
Transceivers are usually packaged in industry standard packages to allow multiple sources to be accommodated in transmission equipment. Modules connect to a duplex connector on the optical end and a standard electrical interface on the other end. Transceivers are powered from the equipment they are built into.
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 four 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 and DFB lasers emit from the side of the chip from a laser cavity created in the middle of the chip.
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 and 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.
LEDs have much lower power outputs than lasers and their larger, diverging light output pattern makes them harder to couple the light output into fibers, limiting them to use with multimode fibers.
LED devices have much less bandwidth than lasers and are limited to systems operating up to about 250 MHz or around 200 Mb/s.
Lasers have less diverging light outputs and are easily coupled to singlemode fibers, making them
ideal for long distance high speed links. Lasers have very high bandwidth capability, most being useful to well over 10 GHz or 10 Gb/s.
Typical Fiber Optic Source Specifications
Another big difference between LEDs and lasers is the spectral output. LEDs have a very broad spectral output which causes them to suffer chromatic dispersion in fiber (see Chapter 5 on optical 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 with little distortion) so they can be used as sources in analog (AM) CATV systems.
The choice of these devices is determined mainly by speed and fiber compatibility issues. Short, low speed multimode systems can use LEDs. 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 or OM4, is the choice for lasers.
Singlemode systems use lasers, typically the fabry-perot type for shorter digital links, while DFB lasers are preferred for longer, faster links, especially when wavelength division multiplexing is used.
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.
Very high speed systems sometimes use avalanche photodiodes (APDs) that have higher bandwidth capability than other photodiodes. APDs are biased at high voltage to create gain in the photodiode, increasing sensitivity and frequency capability. These devices are more expensive and more complicated to use but offer significant gains in performance.
Specialty Fiber Optic Transmission Components Wavelength Division Multiplexing
Since light of different wavelengths does not mix within the fiber, it is possible to transmit signals at several different wavelengths on a single fiber simultaneously. While fiber is inexpensive, installing new cables can be expensive, so utilizing installed fibers for carrying additional signals can be very cost effective.
Wavelength division multiplexing (WDM) was first used with multimode fiber in the earliest days of fiber optics, using both 850 and 1310 nm on multimode fiber, sometimes carrying two signals in the same direction and sometimes allowing bidirectional transmission on one fiber. When used in bidirectional transmission on a single fiber, the two different wavelengths prevent crosstalk. When used in the same direction, filters remove one wavelength to prevent crosstalk. Both of these techniques are used in FTTH.
Bidirectional WDM on one fiber
WDM with two wavelengths on one fiber in the same direction.
Singlemode transmission networks can carry signals at 10 Gb/s on 64 or more wavelengths, which is called dense wavelength division multiplexing (DWDM). Coarse wavelength division multiplexing (CWDM) uses new low water peak fibers to transmit signals on lasers in the range of 1260 to 1670 nm. Both systems work by coupling many wavelengths into one fiber using an optical coupler, then separating them at the receiving end with a WDM coupler that aims each wavelength at a separate fiber. Multimode systems using WDM have been less popular, but some standards use coarse WDM to carry multiple signals at greater than 1 Gb/s over laser-optimized multimode fibers to create higher bit rate links.
Repeaters And 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. 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!
The solution to fiber optic repeaters was the fiber amplifier. The typical fiber amplifier works in the 1480-1650 nm band. It consists of a length of fiber doped with Erbium pumped with a laser at 980 or 1480 nm. 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 optical power in the doped fiber. Gains of
>40 dB (10,000X) are possible with power outputs >+26 dBm (400 mW).
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, fiber amplifiers combine with DWDM (dense wavelength division multiplexers) to overcome the inefficiencies of DWDM couplers for long haul transmission.
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.
Several low-cost versions of WDM are in use, generally referred to as Coarse WDM or CWDM which uses broadly separated laser wavelengths. Most do not work over long distances so do not require amplification, broadening 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 spacing over the complete range of 1260 to 1660 nm but only with low water peak fibers.
Manufacturers have been able to make singlemode fiber with low-water peaks, opening up a new transmission band (E-band), which has proven useful 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.
DWDM Band Wavelength Range
Data Link Performance And Link Power Budget Measuring Data Transmission Quality
Just as with copper wire or radio transmission, the performance of the fiber optic data link can be determined by how well it transmits data; how well the reconverted electrical signal out of the receiver matches the input to the transmitter.
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.
Link Power Budget
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.
High speed links like gigabit or 10gigabit Ethernet LANs on multimode fiber or extremely long distance OSP links have derating factors for the bandwidth of fiber caused by the dispersion or spreading out the digital data pulses. Older 62.5/125 OM1 fiber will generally operate only on shorter links while links on 50/125 OM3 or OM4 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 several different 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
Of all the 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 that have fiber specific versions. These networks have agreed upon specifications common to all manufacturers’
products to insure interoperability. FOA Tech Topics includes 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 next chapter on optical fiber will cover topics like chromatic dispersion, polarization mode dispersion and spectral attenuation that can be important in these long links.
Review Questions True/False
Indicate whether the statement is true or false.
____1. Fiber optic links generally use two fibers for full duplex (bidirectional) links.
____2. LEDs have higher output power and bandwidth than lasers.
Multiple Choice
Identify the choice that best completes the statement or answers the question.
____3. Singlemode fiber systems covering long distances and using DWDM use __________
sources.
A. F-P laser B. DFB laser C. VCSEL D. LED
____4. Wavelength division multiplexing uses __________ to transmit multiple signals on a single fiber.
A. The ability of fiber to transmit multiple wavelengths without mixing B. F-P Lasers
C. The wide spectral width of lasers D. The high fiber bandwidth
____5. CWDM systems require __________ to allow transmitting in the E-Band (1260-1360 nm).
A. Low water peak fiber B. DFB lasers
C. 1490 nm pump lasers
____6. Long wavelength singlemode links at wavelengths in the range of 1300-1650 nm links must use __________ detectors in the receiver for the best sensitivity performance.
A. Silicon B. Germanium C. InGaAs
____7. Fiber amplifiers used in DWDM systems work in the __________ wavelength range.
A. 650-850 B. 850-1300 C. 1300-1550 D. 1480-1650 Multiple Response
Identify one or more choices that best complete the statement or answer the question.
____8. Singlemode transceivers use __________ sources for their higher coupled power and bandwidth.
A. LED B. VCSEL C. F-P lasers D. DFB lasers
____9. Multimode transceivers use __________ sources depending on their requirements for coupled power and bandwidth.
A. LED B. VCSEL C. F-P lasers D. DFB lasers
Additional Study And Projects
Review websites of manufacturers of fiber optic sources, detectors and transceivers to see what performance specifications are quoted and what applications supported.
In class or lab, set up a fiber optic link and see how it works. The link can be built from components or from commercially available media converters.