Light Sources

A light source, perhaps more properly called a photon source, has the fundamental function in a fiber-optic communication system to efficiently convert electrical energy (current) into optical energy (light) in a manner that permits the light output to be effectively launched into the optical fiber. The light signal so generated must also accurately track the input electrical signal so that noise and distortion are minimized.

The two most widely used light sources for fiber-optic communication systems are the light-emitting diode (LED) and the semiconductor laser, sometimes called a laser diode (LD). LEDs and LDs are fabricated from the same basic semiconductor compounds and have similar heterojunction structures. They do differ in the way they emit light and in their performance characteristics.

An LED is a forward-biased p-n junction that emits light through spontaneous emission, a phenomenon referred to as electroluminescence. LDs emit light through stimulated emission. LEDs are less efficient than LDs but are considerably more economical. They also have a longer operational life. The emitted light of an LED is incoherent with a relatively wide spectral line width (from 30 nm to 60 nm) and a relatively large angular spread, about 100°. On the other hand, a semiconductor laser emits a comparatively narrow line width (from <2 nm to 4 nm). Figure 9.29a shows the spectral line for an LED, and Figure 9.29b shows the spectral line for a semiconductor laser (i.e., a laser diode or LD).

What is a spectral line? Many of us imagine that if we were to view a radio carrier (without modulation) on an oscilloscope, it would be a vertical line that appeared to be of infinitely narrow width. This thinking tends to be carried into the world of light in

Figure 9.29 Spectral distribution (line width) of the emission from (a) an LED and (b) a semiconductor laser (LD), where X is the optical wavelength and AX is the spectral or line width.

Figure 9.29 Spectral distribution (line width) of the emission from (a) an LED and (b) a semiconductor laser (LD), where X is the optical wavelength and AX is the spectral or line width.

a fiber-optic light-guide. In neither case is this exactly true. The emission line or light carrier has a finite width, as does a radio carrier. The IEEE (Ref. 6) defines spectral width, full-width half-maximum as "The absolute difference between the wavelengths at which the spectral radiant intensity is 50% of the maximum."

With present technology the LED is capable of launching about 100 ^W (-10 dBm) or less of optical power into the core of a fiber with a numerical aperture of 0.2 or better. A semiconductor laser with the same input power can couple up to 7 mW (+8.5 dBm) into the same cable. The coupling efficiency of an LED is on the order of 2%, whereas the coupling efficiency of an LD (semiconductor laser) is better than 50%.

Methods of coupling a source into an optical fiber vary, as do coupling efficiencies. To avoid ambiguous specifications on source output powers, such powers should be stated at the pigtail. A pigtail is a short piece of optical fiber coupled to the source at the factory and, as such, is an integral part of the source. Of course, the pigtail should be the same type of fiber as that specified for the link.

LED lifetimes are in the order of 100,000 hr mean time between failures (MTBF) with up to a million hours reported in the literature. Many manufacturers guarantee a semiconductor laser for 20,000 hr or more. About 150,000 hr can be expected from semiconductor lasers after stressing and culling of unstable units. Such semiconductors are used in the latest TAT and PTAT series of undersea cables connecting North America and Europe.

Current fiber-optic communication systems operate in the nominal wavelength regions of 820 nm, 1330 nm, and 1550 nm. Figure 9.30 is a plot of attenuation per unit length versus wavelength. Based on this curve, we can expect about 3 dB/km at 820 nm, 0.50 dB/km at 1330 nm, and at 1550 nm some 0.25 dB/km of attenuation. Also take note that at about 1300 nm is a region of zero dispersion. For added expense, fiber is available with the dispersion minimum shifted to the 1550-nm band, where attenuation per unit length is minimum. There is mature technology at all three wavelengths.

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