Single-Mode Fiber-Optic Cabling

ISO-11801 recommends a standard core diameter for 1300 nm single-mode glass fiber of 9 m m to 10 m m. Figure 11.24 illustrates the construction of a standard 10/125 m m step-index single-mode glass fiber core.

Figure 11.24. Construction of a step-index glass fiber core (10/125 m m SMF).

At a diameter of 9 m m to 10 m m, any wavelength in the vicinity of 1300 nm or longer will propagate only as a single mode. The absence of additional propagating modes means that single-mode fiber does not suffer from modal dispersion, differential mode delay, modal noise, or mode partition noise.

Single-mode fiber is not used at 850 nm or below. At that shorter wavelength several propagating modes may develop in a 10- m m core. In general it's best to have either one single mode of propagation in a system or thousands. Having just a few modes exposes the system to a multitude of peculiar mode-quantization effects.

11.6.1 Single-Mode Signal Propagation

High-speed signals, once launched in a single-mode fiber, travel 10 to 100 times farther, with less dispersion, than in multimode fiber. Distances in excess of 20 km at 1 Gb/s are attainable. Higher bandwidth and longer lengths are the primary advantages of single-mode fiber.

The prime disadvantage of single-mode fiber is the difficulty of launching signals into it. A single-mode fiber core spans only 10 m m, much less than a multimode fiber core. LED sources generally do not have a narrow enough beam width to couple into such a small core area. The only choice for single-mode sources is a pinpoint source, like a laser-diode. Unfortunately, laser-diodes and their associated output power control circuitry are significantly more expensive than LED sources. The small core diameter of a single-mode fiber and its attendant requirements for mechanical precision in all coupling components also increase the cost of cable connectors, the cost of optical packages, and the amount of time and energy required in the field to install, test, and maintain the optical connections. Single-mode systems cost big bucks.

Single-mode systems with a 10- m m core are found in long-distance telecommunications applications where ultimate performance is the leading criteria.

The most important optical parameters for signal analysis are the operating wavelength, the attenuation in dB/km, and the chromatic dispersion. Single-mode fibers do not suffer from modal dispersion (because there's only one mode of propagation). Likewise, single-mode fiber links do not suffer from differential mode delay, modal noise, or mode partition noise. Laser-diode RIN noise is less of an issue for single-mode fiber than for multimode because of the generally higher quality of single-mode connectors, which admit fewer reflections.

The signal propagation model and optical power budget for single-mode fiber are identical to the multimode model and budget, with the following exceptions:

• The modal dispersion t m is set to zero,
• Differential mode delay, modal noise, and mode partition noise are ignored,
• Single-mode fibers are not used at 850 nm, and
• Single-mode systems are often hard-spliced rather than mated with connectors to improve the attenuation budget.

POINTS TO REMEMBER

• Single-mode fiber does not suffer from modal dispersion, differential mode delay, modal noise, or mode partition noise.
• The most important optical parameters for a single-mode fiber are the operating wavelength, the attenuation in dB/km, and the chromatic dispersion.

11.6.2 Single-Mode Fiber-Optic Noise and Interference

The noise and interference issues for single-mode fiber are identical to multimode fiber, with the exception that single-mode fiber is not subject to modal noise or mode partition noise.

11.6.3 Single-Mode Fiber Safety

Never look into the end of a single-mode fiber. Single-mode fiber communication systems use infrared light. The human eye cannot see it, but it is very, very bright (in some cases orders of magnitude brighter than a multimode source).

Single-mode systems are subject to international rules of safety that prescribe various labels and warnings on transmitters and fibers. Always provide applicable warnings on your equipment [87] , and never look into the end of any fiber.

11.6.4 Single-Mode Fiber-Optic Connectors

The connectors used for single-mode systems look like the connectors used for multimode systems. They have the same external appearance and the same names . Single-mode connectors assume the use of a 125- m m cladding diameter and a 250- m m polymer coating, just like multimode connectors.

What's different about single-mode connectors are the mechanical tolerances inside the connector ferrule. Single-mode connectors are much more precise, and more expensive, than multimode connectors. You can use a single-mode connector to join multimode fiber, but not the other way around.

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