6. How to Choose Optical Fiber
Single-Mode Fiber Performance Characteristics
The key optical performance parameters for single-mode fibers are attenuation, dispersion, and mode-field diameter.
Optical fiber performance parameters can vary significantly among fibers from different manufacturers in ways that can affect your system's performance. It is important to understand how to specify the fiber that best meets system requirements.
Attenuation is the reduction of signal strength or light power over the length of the light-carrying medium. Fiber attenuation is measured in decibels per kilometer (dB/km).
Optical fiber offers superior performance over other transmission media because it combines high bandwidth with low attenuation. This allows signals to be transmitted over longer distances while using fewer regenerators or amplifiers, thus reducing cost and improving signal reliability.
Attenuation of an optical signal varies as a function of wavelength (see Figure 9). Attenuation is very low, as compared to other transmission media (i.e., copper, coaxial cable, etc.), with a typical value of 0.35 dB/km at 1300 nm for standard single-mode fiber. Attenuation at 1550 nm is even lower, with a typical value of 0.25 dB/km. This gives an optical signal, transmitted through fiber, the ability to travel more than 100 km without regeneration or amplification.
Attenuation is caused by several different factors, but primarily scattering and absorption. The scattering of light from molecular level irregularities in the glass structure leads to the general shape of the attenuation curve (see Figure 9). Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding. It is these water ions that cause the “water peak” region on the attenuation curve, typically around 1383 nm. The removal of water ions is of particular interest to fiber manufacturers as this “water peak” region has a broadening effect and contributes to attenuation loss for nearby wavelengths. Some manufacturers now offer low water peak single-mode fibers, which offer additional bandwidth and flexibility compared with standard single-mode fibers. Light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation.
Dispersion is the time distortion of an optical signal that results from the time of flight differences of different components of that signal, typically resulting in pulse broadening (see Figure 10). In digital transmission, dispersion limits the maximum data rate, the maximum distance, or the information-carrying capacity of a single-mode fiber link. In analog transmission, dispersion can cause a waveform to become significantly distorted and can result in unacceptable levels of composite second-order distortion (CSO).
Single-mode fiber dispersion varies with wavelength and is controlled by fiber design (see Figure 11). The wavelength at which dispersion equals zero is called the zero-dispersion wavelength (? ? ). This is the wavelength at which fiber has its maximum information-carrying capacity. For standard single-mode fibers, this is in the region of 1310 nm. The units for dispersion are also shown in Figure 11.
Chromatic dispersion consists of two kinds of dispersion. Material dispersion refers to the pulse spreading caused by the specific composition of the glass. Waveguide dispersion results from the light traveling in both the core and the inner cladding glasses at the same time but at slightly different speeds. The two types can be balanced to produce a wavelength of zero dispersion anywhere within the 1310 nm to 1650 nm operating window.
Transmission in the 1550 nm Window
Optical fibers also can be manufactured to have low dispersion wavelength in the 1550-nm region, which is also the point where silica-based fibers have inherently minimal attenuation. These fibers are referred to as dispersion-shifted fibers and are used in long-distance applications with high bit rates. For applications utilizing multiple wavelengths, it is undesirable to have the zero dispersion point within the operating wavelength range and fibers known as non-zero dispersion-shifted fiber (NZ-DSF) are most applicable. NZ-DSF fibers with large effective areas are used to obtain greate capacity transmission over longer distance than would be possible with standard single-mode fibers. These fibers are able to take advantage of the optical amplifier technology available in the 1530 to 1600+ nm operating window while mitigating nonlinear effect, as that can be troublesome at higher power levels.
For applications such as the interconnectino of headends, delivery of programming to remote node sites, high-speed communication networks, and regional and metropolitan rings (used primarily for competitive access applications), NZDSF fiber can improve system reliability, increase capacity, and lower system costs.
Mode-field diameter (MFD) describes the size of the light-carrying portion of the fiber. For single-mode fibers, this region includes the fiber core as well as a small portion of the surrounding cladding glass. MFD is an important parameter for determining a fiber’s resistance to bend-induced loss and can affect splice loss as well. MFD, rather than core diameter, is the functional parameter that determines optical performance when a fiber is coupled to a light source, connectorized, spliced, or bent. It is a function of wavelength, core diameter, and the refractive-index difference between the core and the cladding. These last two are fiber design and manufacturing parameters.
Cutoff wavelength is the wavelength above which a single-mode fiber supports and propagates only one mode of light. An optical fiber that is single-moded at a particular wavelength may have two or more modes at wavelengths lower than the cutoff wavelength.
The effective cutoff wavelength of a fiber is dependent on the length of fiber and its deployment and the longer the fiber, the lower the effective cutoff wavelength. Or the smaller the bend radius of a loop of the fiber is, the lower the effective cutoff wavelength will be. If a fiber is bent in a loop, the cutoff is lowered. The cutoff wavelength of a fiber is reduced when it is cabled. The reduction is predictable, enough so that fiber manufacturers can specify a maximum “cable cutoff wavelength” for the fiber.
While cable design and construction play a key role in environmental performance, optimum system performance requires the user to specify fiber that will operate without undue loss from microbending.
Microbends are small-scale perturbations along the fiber axis, the amplitude of which are on the order of microns. These distortions can cause light to leak out of a fiber. Microbending may be induced at very cold temperatures because the glass has a different coefficient of thermal expansion from the coating and cabling materials. At low temperatures, the coating and cable become more rigid on the glass to cause microbends. Coating and cabling materials are selected by manufacturers to minimize loss due to microbending.
Specification Examples of Uncabled Fiber
To ensure that a cabled fiber provides the best performance for a specific application, it is important to work with an optical fiber cable supplier to specify the fiber parameters just reviewed as well as the geometric characteristics that provide the consistency necessary for acceptable splicing and connectorizing.
Splicers and Connectors
As optical fiber moves closer to the customer, where cable lengths are shorter and cables have higher fiber counts, the need for joining fibers becomes greater. Splicing and connectorizing play a critical role both in the cost of installation and in system performance.
The object of splicing and connectorizing is to precisely match the core of one optical fiber with that of another in order to produce a smooth junction through which light signals can continue without alteration or interruption. There are two ways that fibers are joined:
Fusion splicing provides a fast, reliable, low-loss, fiber-to-fiber connection by creating a homogenous joint between the two fiber ends. The fibers are melted or fused together by heating the fiber ends, typically using an electric arc. Fusion splices provide a high-quality joint with the lowest loss (in the range of 0.01 dB to 0.10 dB for single-mode fibers) and are practically nonreflective.
Mechanical splicing is an alternative method of making a permanent connection between fibers. In the past, the disadvantages of mechanical splicing have been slightly higher losses, less-reliable performance, and a cost associated with each splice. However, advances in the technology have significantly improved performance. System operators typically use mechanical splicing for emergency restoration because it is fast, inexpensive, and easy. (Mechanical splice losses typically range from 0.05.0.2 dB for single-mode fiber.)
Connectors are used in applications where flexibility is required in routing an optical signal from lasers to receivers, wherever reconfiguration is necessary, and in terminating cables. These remateable connections simplify system reconfigurations to meet changing customer requirements.