Dispersion is always an issue that designers need to deal with when sending signals over optical connections. But, while easy to manage at lower data rates, dispersion becomes a huge challenge when trying to establish 10-Gbit/s connections over optical links.
Fortunately, electronic dispersion compensation (EDC) can now provide real throughput and distance improvements in both single-mode and multi-mode fiber applications. At the same time, the technology has evolved to a point where it can be combined with other circuitry in 10-Gbit receiver IC designs.
In this article, we'll detail the technical benefits of using EDC for both single- and multi-mode 10-Gbit fiber applications. We'll then show how this technology can easily be embedded in a 10-Gbit optical receiver design.
Dispersion: What It Is and Why It Matters
The term dispersion refers to the phenomenon whereby different components of a signal travel through a transmission medium at different speeds, and therefore arrive at the receiver at different times. In fiber optic communications, the components can be different spectral components (resulting in chromatic dispersion), or different transmission modes (resulting in modal dispersion). Another form of dispersion is polarization mode dispersion, which results from differences in transmission speed for light components with different polarizations.
Dispersion causes pulse broadening and inter-symbol interference. In other words, some of the energy transmitted for one bit overlaps, at the receiver, with that for other (typically adjacent) bits. If the electrical signal within the receiver is represented using an eye diagram, the dispersion can be seen to lead to eye closure. Figure 1 shows how a transmitted pulse broadens as it travels along a fiber, with the corresponding eye diagrams becoming increasingly closed.
Figure 1: The transmitted pulse as it travels along a fiber, with corresponding eye diagrams.
For illustrative purposes, the eye diagrams shown in Figure 1 are noise free. Noise is of course inevitable in real transmission systems, the dominant noise source typically being the front end of the receiver.
Optical communication systems are designed to deliver an end-to-end bit error rate (BER) that does not exceed a desired value, such as 10-12. As eye closure increases, a higher ratio of received signal-to-noise ratio (SNR) is needed to achieve the desired BER. The extra SNR required to counteract the effects of eye closure is known as the dispersion penalty.
Dispersion penalty is one of many penalties that must be considered in the power budget when designing a fiber optic communication system. Other penalties include connector insertion loss and fiber attenuation. Dispersion penalty increases with transmission distance and with data rate. The dispersion penalty is an important factor in determining the distance over which a particular transmitter and fiber combination can be deployed at a particular data rate.
Compensating for Dispersion
Electronic filtering (also known as equalization) can be included in a communications channel to compensate for signal degradation caused by the medium. The use of filtering to compensate for dispersion in an optical communications link is known as electronic dispersion compensation or EDC.
EDC is typically implemented with a transversal filter, the output of which is the weighted sum of a number of time-delayed inputs. A generic transversal filter is shown in Figure 2.
Figure 2: Diagram of a generic transversal filter.
The optimum settings of the weights in the filter of Figure 2 depend upon the nature of the dispersion. However, the dispersion varies greatly from one link to another depending on the fiber, the launch conditions, the link length and the optical spectrum. Therefore, an important aspect of an EDC solution is the ability to automatically adjust the filter weights according to the characteristics of the received signal. This is known as adaptation.
It is also necessary to consider dispersion variations over time in a given link. Temperature variation and component aging give rise to gradual changes, whilst cable movement and vibration can cause rapid changes in the dispersion characteristics. It is therefore important that an EDC solution adapts continuously, responding quickly to changes in the characteristics of the received signal.
Benefits in Single-Mode Fiber Setups
In single-mode fiber of lengths used within metropolitan regional networks, chromatic dispersion is the dominant form of dispersion. For analysis purposes, the phenomenon can be modelled using a single dispersion coefficient parameter, describing variation in propagation velocity with wavelength.
At 10 Gbit/s, chromatic dispersion has cost implications for even the shortest lengths of single mode fiber. This is because the spectrum of the transmitted signal depends upon the transmitter technology used.
The least expensive transmitters are directly modulated lasers (DMLs), which introduce wavelength transients as the output power is switched. These produce a wide spectrum, resulting in severe pulse broadening due to chromatic dispersion. More expensive transmitter technologies such as electro-absorption (EA) modulators and Mach-Zehnder modulators produce narrower transmitted spectra, resulting in less pulse broadening.
By compensating for the effects of dispersion, EDC allows low-cost direct modulated lasers to be used in many applications which otherwise require more expensive modulators. At the longest distances, EDC can allow EA modulators to replace Mach-Zehnder modulators, which also represents a cost saving.
Telecom system designers typically use a dispersion penalty of 2 dB to determine the maximum distance for which a particular transmitter can be deployed. Figure 3 shows dispersion penalties for directly modulated and EA modulated transmitters, with and without EDC.
Figure 3: Dispersion penalty vs. distance for 1550 nm, 10-Gbit/s, 10-dB extinction ratio, standard single-mode fiber.
From these simulation results, designers can see that EA modulators may be replaced by DML transmitters together with EDC enabled receivers for distances up to 23 km. This is important as many metropolitan area optical links are of 10 to 20 km in length. Our results indicate also that the reach achievable with EA modulators is increased from 75 to 110 km when EDC is used.
Benefits in Multi-Mode Fiber Situations
Over the distances for which multi-mode fiber is deployed, attenuation is negligible, so that dispersion is the primary penalty that needs to be taken into account. Modal dispersion is the dominant form of dispersion in multi-mode fibers. Modal dispersion limits transmission distance at all data rates, the distance decreasing as the data rate increases. At 10 Gbit/s, the achievable distance may be insufficient to allow operation over installed local area network (LAN) and storage area network (SAN) fibers.
Analysis of dispersion in multi-mode fiber requires more sophistication than for single-mode fiber. The reason this happens is because the set of modes that dominate the transmission varies from sample to sample, depending on the particular fiber used and upon the precise way in which the light is launched into the fiber. The area of the transmitting surface, its exact position with respect to the fiber and the angle of incidence all play important roles. On the other hand, the quality of the transmitted spectrum does not have the significance that it has with single mode fiber.
Designers of data communications systems typically use a dispersion penalty of 3 to 4 dB to determine the maximum distance. Because the modal dispersion is so variable, it is necessary to consider a population of fiber in order to evaluate the percentage of fiber that comply with the dispersion penalty criterion at a given length.
Figure 4 shows simulated dispersion penalties for 1310-nm transmission over 62.5-¼m multi-mode fiber using a conventional receiver, for a selection of "worst case" links designed to represent the worst 5% of fibers.
Figure 4: Dispersion penalties vs. distance for 1310 nm, 10 Gbit/s over 62.5-¼m multi-mode fiber.
Figure 5 shows the percentage of links which comply with a 3-dB dispersion penalty criterion"plotted versus distance. The left-hand Y scale shows the percentage of the worst case selection which are compliant. The right-hand Y scale, on the other hand, indicates the extrapolated percentage of all links which would be compliant. Results are shown for both a conventional receiver (circular markers) and a receiver with EDC (square markers). We see that for example, 99% of all links are compliant at 125 m without EDC, and at 200 m with EDC"a significant increase in transmission distance.
Figure 5: Percentage of links which meet 3-dB dispersion penalty criterion versus distance.
EDC can be combined with other functions on 10-Gbit/s receiver ICs. For small form factor (SFF) modules such as XFP, the EDC function can be combined with signal amplification, clock and data recovery (CDR) circuitry, and the module serial output driver to form an integrated receiver IC. A similar combination of functions can be integrated with parallel outputs for 300-pin MSA modules.
It is possible to add EDC with only modest increases in IC die size and power dissipation. The benefits of significantly reduced transmitter cost for single-mode fiber systems or increased transmission distance for multi-mode systems will therefore be obtainable at only a small receiver cost penalty.
H. Gnauck, R. M. Jopson. Dispersion Compensation for Optical Fiber Systems. Optical Fiber Telecommunications, Vol. 3a (ed. Kaminov and Koch), pages 162 to 195.
P. Pepeljugoski, J. Schaub, J. Tierno, J. Kash, S. Gowda, B. Wilson, H. Wu, A. Hajimiri. "Improved Performance of 10 Gb/s Multimode Fiber Optic Links Using Equalization". OFC Technical Digest, Vol. 2.
Phyworks and others: Presentations to T11 Technical Committee, June 2003. www.t11.org.
About the Authors
Ben Willcocks is lead systems designer at Phyworks. Ben gained a BSc from the University of Wales in 1985, and has 18 years experience in the electronics industry. He can be reached at firstname.lastname@example.org.
Nick Weiner is the co-counder and CTO of Phyworks. Nick has 20 years of experiece in analog and digital IC design and management. He can be reached at email@example.com.
Ian White is a professor of engineering at the University of Cambridge. Ian has B.A. and Ph.D. degrees from the University of Cambridge. He can be reached at firstname.lastname@example.org.
Richard Penty is a professor of photonics at the University of Cambridge. Richard studied for his doctoral research in the Cambridge University Engineering Department on optical fiber devices for signal processing applications, receiving his Ph.D. He can be reached at email@example.com.
Jonathan Ingham is a research associate at the University of Cambridge. Jonathan received a degree in electrical and electronic engineering from Imperial College. He can be reached at firstname.lastname@example.org.