Single-Mode Fibers for Communications
5.2 SYSTEM IMPAIRMENTS INFLUENCING FIBER DESIGN
5.2.1 Limitations from Optical Signal-to-Noise Ratio
Two cases are considered in which optical transmission may be limited by the degradation of OSNR. The trivial example is a low-cost nonamplified link that is simply loss limited. A receiver has a specified sensitivity in dBm (logarithmic unit of power), and the power budget specifies the maximum loss allowed to achieve the desired bit error ratio (BER) at the data rate. If we assume a 10-Gbps PIN diode receiver sensitivity of18 dBm and a launch power in the range of a few dBm, then a loss budget of approximately 20 dB is available for fiber, splice, and connector loss. Carriers often budget for a worst-case cabled fiber loss of approximately 0.25 dB/km at 1550 nm.1 As an example, allowing 2 dB for
1 The loss of good-quality modern fiber on spool is typically in the range of 0.19 dB/km at 1550 nm. The loss may change somewhat in cable, depending on cable design and quality or upon installation if the cable is left in a condition of high mechanical stress.
splices and connectors results in a calculated span length of 72 km. For many years, almost a third of the optical transmission window between 1260 and 1625 nm was unusable because of the presence of a large ‘‘water peak’’ at 1383 nm with loss as high as 2 dB/km. Today, zero water peak (ZWP) fibers, described in detail later, open previously unusable spectrum for low-cost coarse wavelength division multiplexing (CWDM).
The more interesting case is that of multiple spans, in which noise accumu-lates in an amplifier chain. An expression that approximates the OSNR for a link comprising Nspanspans of loss Lspanin which each amplifier has noise figure, NF, is given in [1] as
OSNR¼ 58 þ Pch NF Lspan 10 log 10(Nspan), (5:1) where all units are in dB or dBm as appropriate and Pchis the span launch power.
The required OSNR scales inversely as the data rate increases, increasing by 6 dB from 10 to 40 Gbps. This is true because for a fixed received power, the number of photons per bit decreases by the same factor that the bit rate increases. All else being equal, a requirement of 6 dB higher OSNR would decrease the reach of a 40-Gbps system by a factor of four relative to a 10-Gbps link. Equation (5.1) shows that the most effective method for increasing OSNR in an amplified span is to reduce the loss by spacing amplifiers more closely, because the OSNR improves linearly with span loss but only rises logarithmically with the number of ampli-fiers. However, this option is available only to the submarine system designer, in which repeater spacing is flexible. Distributed Raman amplification is a key-enabling technology for 40-Gb transmission, improving OSNR by about 3 dB, as well as forward error correction and advanced modulation formats. Transmis-sion fiber designs that improve the efficiency and cost-effectiveness of Raman amplification are described in the following subsections.
5.2.2 Limitations from Intersymbol Interference
ISI occurs when pulses broaden out of their assigned bit slots because of some form of dispersion in system components. Chromatic dispersion (see Chapter 2) arises from the wavelength dependence of the propagation constant in the fiber and is proportional to fiber length and laser line width. The requirements on chromatic dispersion become very severe for higher bit rates, increasing as the square of bit rate. PMD arises from the small usually randomized birefringence of the fiber and grows with the square root of fiber length. Systems operating at 2.5 Gbps or less permit the use of low-cost directly modulated lasers with wider line widths arising from chirp. Systems running standard NRZ modulation at 10 Gbps over more than 10–20 km of G.652 standard single-mode fiber (SSMF) rely on externally modulated, CW sources with very narrow intrinsic line widths.
As in the case of OSNR, we may distinguish two cases of dispersion limitation:
one applicable to low-cost systems, in which it is desirable to avoid in-line dispersion compensation altogether, and the other relevant to long-reach systems based on concatenated amplified spans.
The simplest ISI limit occurs for uncompensated transmission, in which the dispersion tolerance of a ‘‘receiver’’—often defined as that dispersion in ps/nm resulting in a 1-dB power penalty—limits the reach of link. For SSMF, with a 1550-nm dispersion of approximately 17 ps/nm-km, that limit is usually reached at 60–80 km. At the time of this writing, efforts are underway to write standards for electronically equalized receivers [2] that extend uncompensated reach with G.652 SSMF from 80 to 120 km. However, optical fiber designs with one-fourth the dispersion of SSMF can increase the reach of uncompensated links by a factor of four.
Maintaining tight dispersion tolerances over many channels in an amplified DWDM system comprising many spans once posed a significant challenge for transmission line design. This obstacle was overcome by the milestone develop-ment of the slope-matched DCM, which is capable of tightly compensating dispersion across the entire C- or L-band in one broadband device [3, 4]. For the purpose of this chapter, it is critical to note that transmission fibers can be designed so they co-optimize the design of compensating fiber in the DCM for optimum broadband compensation.
PMD is a special challenge because it is an inherently statistical quantity that depends sensitively on the mechanical state of the fiber. The single number known as the PMD coefficient used to characterize an optical fiber is intended to describe the (normally) maxwellian distribution of differential group delay (DGD) values that a fiber can assume. A single fiber displays its full range of DGD values as its mechanical stress state changes through all possible config-urations, usually varying with temperature cycles or mechanical vibrations (e.g., for a cable laid alongside a railroad track). The link design value (LDV), defined in Chapter 2, is used to identify the worst-case PMD that a link comprising many cabled fiber segments will experience. As shown later in this chapter, there is a small but nonzero probability that DGD in a system will fluctuate through very high values that can halt transmission for short periods each year.
5.2.3 Limitations from Nonlinearity
Nonlinear impairments can broaden the frequency content and temporal evo-lution of a single pulse, cause crosstalk between pulses in adjacent channels, and cause two adjacent channels to produce noise in a third channel. Nonlinear impairments are most pronounced in very long systems in which small non-linear products are able to accumulate. Cross-phase modulation (XPM), as an
interchannel impairment, is usually the dominant nonlinearity in 10-Gbps DWDM systems. Four-wave mixing (FWM) can be essentially eliminated as an impairment between channels in 10-Gbps systems as long as the fiber dispersion across the band is greater than approximately 2 ps/nm-km [5]. Self-phase modulation (SPM), XPM, or FWM between temporally adjacent bits at the same wavelength (i.e., intra-channel nonlinearities) tend to dominate in 40-Gbps systems. The OSNR improves linearly (on a decibel scale) as transmitted power increases; however, the penalty due to nonlinear impairments increases with transmitted power. An optical transmission system is often designed to operate in the sweet spot that maximizes OSNR but avoids significant nonlinear distortions. It will be shown that the same transmission fiber design strategy that allows co-optimization of broadband dispersion compensation also reduces system nonlinear penalties.
5.2.4 Limitations from Amplifier Technology
The advent of the erbium-doped fiber amplifier (EDFA) [6] in the early 1990s revolutionized the economics of optical transmission in several ways. Not only did it make expensive optical-to-electronic-to-optical (OEO) conversion, also known as regeneration, necessary only at the endpoint of a link, but it also favored adding as many WDM wavelengths as practical within the amplifier bandwidth. The EDFA is most efficient in the C-band but has been successfully extended as a practical L-band amplifier as well. Although the C- and L-bands together provide tremendous capacity with DWDM technology at 40 Gbps, it is prudent to consider future expansion needs because optical fiber cable is rated for a lifetime more than 20 years.
Raman amplification not only is an enabling technology for 40 Gbps but can also provide gain where other technologies are not available. The gain spectrum for stimulated Raman scattering is based on the phonon (vibrational) spectrum of the silica glass, not on the electronic spectrum of dopants in the glass.
Essentially the peak of the Raman gain curve will lie approximately 13 THz lower than the pump frequency, which is approximately 100 nm to the red of the pump wavelength in the telecommunications bands. This means, for example, that Raman pumps can be placed at 1410 nm to provide gain for S-band channels near 1505 nm [7].
5.2.5 Can Fiber Design Be Used to Optimize a Transmission System?
The question addressed in this chapter is How can optical fiber design facilitate the mitigation of these impairments to enable higher capacity, more
cost-effective transmission? To answer that question, one must address three issues:
1. The impact of fiber of properties on transmission impairments 2. The fiber properties that can be manipulated
3. The design tradeoffs that must be considered
The first question regarding how fiber properties affect transmission impair-ments is more complex than commonly supposed. The cabled fiber is often referred to as the ‘‘transmission fiber’’ (Tx fiber) to distinguish it from the dispersion compensation fiber (DCF). The Tx fiber is only one part of a trans-mission line, but the Tx fiber properties critically affect the design and perfor-mance of the other components of the transmission line. It is the system OSNR, PMD, and nonlinearity that must be optimized, not the properties of the indi-vidual components. Figure 5.1 illustrates the basic elements of the optical trans-mission line. The loss of the fiber and connectors must be offset by either EDFA, Raman amplification, or a combination of both. For distances greater than approximately 80 km, the chromatic dispersion of the fiber must be compen-sated to eliminate ISI. For older fibers, or even modern fibers manufactured without ‘‘spinning,’’ some form of PMD compensation may be required. The key point here is that the optical properties of the Tx fiber directly impact the requirements for efficiency, linearity, loss, and PMD of the other elements of the transmission line.
The second issue concerns which fiber properties can be effectively manipu-lated by the fiber design. The primary fiber property available for modification is the dispersion curve, which describes how an optical pulse of a given wavelength and spectral width will broaden in time as it propagates down the fiber. Altering the dispersion curve necessarily involves altering the effective area (Aeff) of the fiber as well, which also affects fiber cutoff wavelength.
Finally, it is critical to understand that design tradeoffs are inevitable. The Maxwell equations dictate that key transmission properties of an optical fiber cannot be varied independently. In general, adding waveguide dispersion to the
Optical Amplifier Transmission
Fiber
Dispersion Compensation
Figure 5.1 The basic elements of an optical transmission line, the designs of which are highly interdependent.
fiber to modify the dispersion curve will reduce the mode field diameter (MFD) and Aeff. System design issues dictate that the ideal fiber will have a lower dispersion at 1550 nm than standard fiber (see Chapter 2), as well as a lower dispersion slope (see later discussion). The cartoon in Fig. 5.2 illustrates the fundamental problem. If we assume that the cabled cutoff must be maintained below 1260 nm to permit 1310-nm applications, then manipulation of the dis-persion curve to reduce the disdis-persion and disdis-persion slope must be balanced against the need to maintain a reasonable Aeff while keeping bending losses low.
Aeff must be kept moderate to high to limit nonlinearities. Low macrobending and microbending losses are critical for good cable performance in the field.
Once we choose any two of the attributes in Fig. 5.2, the third is chosen for us by the Maxwell equations. Intelligent decisions on these tradeoffs are the key factor in fiber design and require close coupling of expertise in fiber design and processing with understanding of optical transmission systems.