The installation of dense wavelength-division multiplexing (DWDM) systems allows network operators to maximize the capacity of the fiber in their networks by transmitting data over several optical wavelength channels on each fiber. The spacing of channels is 25 (0.2 nm) to 200 GHz (1.6 nm). High data-rate transmission for long distances, however, can create transmission problems due to dispersion of the signal in the optical fiber. Fortunately, new design and manufacturing techniques have resulted in the availability of optical fiber, such as nonzero-dispersion-shifted fiber (NZDSF), designed specifically to reduce these dispersion problems.
Dispersion is the broadening of a light pulse as it travels through the optical fiber, resulting in a widening of the pulse width. This widening of the pulse width can limit a fiber`s bandwidth as a function of the length of the fiber. The total dispersion in an optical fiber is a combination of several types: modal, chromatic, and polarization. Chromatic-mode dispersion (CMD) and polarization-mode dispersion (PMD) contribute most to signal distortion in single-mode fiber.
Chromatic-mode dispersion and FWM
Chromatic-mode dispersion (CMD) is caused by differences in wave velocity resulting from variations in the refractive indexes in different parts of the fiber. CMD includes two components-material dispersion and waveguide dispersion. Because most optical fibers are composed of similar materials, the material dispersion of dispersion-unshifted and NZDSF fibers are about equivalent. The use of NZDSF reduces CMD in single-mode fiber in the 1550-nm window by making its waveguide dispersion large and negative, which is accomplished by tailoring the refractive index profiles to compensate for the material dispersion (see Fig. 1).
Early single-mode fiber, known as standard single-mode fiber or dispersion-unshifted fiber, has a zero-dispersion point at 1310 nm on its chromatic dispersion graph; however, the fiber attenuation at 1310 nm is greater than at 1550 nm. The first introduction of dispersion-shifted fiber (DSF) was made with a zero-dispersion crossing at 1550 nm (see Fig. 2). This enabled DSF to have both low dispersion and low attenuation at the same wavelength-1550 nm. While dispersion-shifted fiber is attractive for transmitting data with only one laser near 1550 nm, it creates problems when carrying signals from multiple lasers simultaneously.
Since DWDM systems can modulate many lasers close to 1550 nm, zero dispersion causes a signal at one wavelength to mix with a signal at a nearby wavelength. This frequency mixing is called four-wave mixing (FWM), and produces a new wavelength that can occur at a frequency near the original waveforms, resulting in undesirable extra noise that degrades the performance of a DWDM system.
Four-wave mixing can be a serious problem for DWDM systems used on dispersion-shifted fiber. Since chromatic dispersion is the change in velocity per wavelength (ps/(nm*km)), the light around the zero-dispersion wavelength experiences little change in velocity per wavelength. This lack of change implies that four-wave mixing efficiency can be greatest when the wavelengths are close to the zero crossing on the dispersion graph. The frequency of the noise-producing light [f(mix)] may be close to one of the other input frequencies and is equal to: f(mix) = f1+f2+f3.
A higher magnitude of dispersion reduces the wavelength mixing because of the larger change in velocity per difference in frequency. Four-wave mixing cannot be eliminated completely, but it can be greatly reduced by transmitting data in frequencies that are away from the zero-dispersion point on the dispersion graph. Optical fiber has the lowest attenuation at the 1550-nm band while erbium-doped fiber amplifiers (EDFAs) operate between 1530 and 1560 nm.
The way to reduce four-wave mixing, therefore, is to produce a nonzero dispersion-shifted fiber with a zero-dispersion wavelength outside the normal operating range of the EDFA. The NZDSF can have a zero-dispersion crossing at wavelengths less than 1530 nm or greater than 1560 nm. Any attenuation problems are resolved by the EDFA, which can amplify the optical signals from 1530 to 1560 nm by 10 to 40 dB. The output power can be over 100 mW per channel.
Geometrical and stress asymmetries in fiber cause polarization-mode dispersion (PMD), which is defined as the difference in arrival times of optical power between two orthogonal polarity axes. In the early days of fiberoptic technology, PMD was not a concern because data-transmission rates were not high enough to be affected by it. Although typically having very high PMD, older fiber was not checked for it. Today, with higher data-transmission rates, polarization-mode dispersion has become a more critical factor.
Single-mode fiber actually carries two modes with different polarization, the horizontal mode and the vertical mode. Each mode has a different index of refraction, which may cause different velocities through the fiber. Birefringence in an optical fiber occurs when the index of refraction (n) is different for light with different polarizations. The degree of birefringence is B = |ny-nx|; therefore, if nx equals ny, birefringence is zero and PMD is not present.
Reducing birefringence qualities in the optical fiber during the manufacturing process can reduce PMD. By reducing birefringence, PMD can be kept below 0.20 ps/km0.5. Next-generation, 40-Gbit/s digital data-transmission systems will probably require a total PMD of less than 2.5 ps between regenerators (or terminals).
Corning and Lucent fiber
Two major manufacturers of optical fiber, Corning Incorporated (Corning, NY) and Lucent Technologies(Murray Hill, NJ), have developed products to address problems created by the presence of dispersion in optical fiber.
For example, in 1993, Lucent Technologies began manufacturing TrueWave nonzero-dispersion- shifted fiber designed specifically for DWDM systems. TrueWave`s chromatic dispersion is small enough over EDFA operating wavelengths to allow high-speed data rates over long distances and large enough to reduce the effects of four-wave mixing. Lucent`s most recent NZDSF is TrueWave RS (Reduced Slope) fiber, which has a more consistent dispersion with wavelength and allows the fiber to be used over a broad wavelength range.
Corning`s LEAF (Large Effective Area Fiber) optical fiber, introduced in 1998, is a nonzero-dispersion-shifted fiber with a 30% larger effective area than normal fiber. The large effective area carries more optical power and avoids system-degrading nonlinear effects (see table).
Most optical fiber can transmit over the 1310- and the 1550-nm bands. Historically, the 1400-nm band was not used because of hydroxyls, or OH ions, causing an elevated band of attenuation. Lucent`s AllWave single-mode optical fiber is produced using a new manufacturing technique that virtually eliminates OH ions, resulting in a 1400-nm band that has lower attenuation than the 1310-nm band. AllWave advertises over 50% more optical spectrum, with optimum dispersion for 10 Gbit/s, and low-cost operation.
Robert McMahon is a research engineer with The Light Brigade, 7691 S. 180 St., Kent, WA 98032 The Light Brigade develops fiberoptic training material and teaches fiberoptic training courses. For more information, contact www.lightbrigade.com or 425-251-1240.
FIGURE 1. Different methods of manufacturing NZDF optical fibers vary the index of refraction (n) in distinctive ways. Pedestal, left, has the highest n value in the middle of the core. Trenched, center, goes to the inner cladding. Ring, right, has the highest value in the center core and n goes right down to the cladding value; then the outer core has increases in the value of n. An end view of an optical fiber is shown at bottom. (Courtesy Jeff Hecht.)
FIGURE 2. The slope of a fiber on the dispersion graph shows the change in the dispersion value [ps/(nm*km)] as a function of the wavelength (nm). A steep slope indicates large variability in dispersion value as a function of wavelength of light. No slope indicates there will be no dispersion variability-or no change in the value of dispersion as a function of wavelength.