hapter 2 dBm. The spectral (noise) width of the emission is about 35 to 50 nm for short wavelengths and about 80 to 120 nm for long wavelengths. The edge- emitting diode has a more focused emission pattern, typically 120° X30°, and SO the power coupled into the fiber is up to 10 dB greater than that for surface emitters. Over the band of interest, spectral width is about 30 to 90 nm. The spectral width of the LED source and the dispersion characteristic of the fiber limit the bandwidth that can be used. LED structure is similar to lasers and care must be taken to prevent unwanted laser action. Fig- ure 2-27 depicts edge- and surface-emitting LEDs. The LEDs are operated with a small forward bias, that is, low light emission, to overcome a turn-on delay. Also, because the output optical power varies with DC voltage and p-contact Figure 2-27 LED structures. Dielectric p-InGaAsP 0.2 um p-InP InGaAsP n-InP InP substrate in-contact Light beam (a) Edge-emitting Light beam in-contact InP substrate n-InP p-InGaAsP p-InP p-InGaAsP Dielectric Plated heat (b) Surface-emitting p-contact Optics Primer temperature, compensation of the LED is required. A thermoelectric cooler may also be used to stabilize the LED temperature over a range of temper- ature differences between the LED and the ambient. 2.6.2 Laser Diodes (LDs) Semiconductor LDs have a typical emission pattern of 30° 'X30°; this pro- vides for higher coupling efficiency than that of LEDs, particularly for sin- gle-mode fibers. LDs give peak power of -10 to +10 dBm, typically near 0 dBm, coupled into either multimode or single-mode fibers. The spectra of LDs are much narrower than for LEDs, ranging from 3 to 5 nm for typical multilongitudinal-mode lasers and 1 nm or less for single-longitudinal- mode lasers. The structure of a typical multimode laser is shown in Figure 2-28. The LD relies on mirrors formed by cleaved crystal facets at each end of the optical cavity. In this design, the 3- to 5-nm-wide emitted spectrum is made up of a group of lines with the lines sepa