 1.25 and 1.03 um. As an alternative, manufacturers may specify the cutoff wavelength of an uncabled fiber, Acf, which lies between 1.29 and 1.13 um. You should oper- ate above the cutoff wavelength to prevent multimode behavior in a single- mode fiber, causing modal noise and pulse broadening; hence, you want to have Acc as low as possible. 2.5.8 Mode Field Diameter (MFD) (Single-Mode Fiber) The nominal MFD for conventional single-mode fibers is between 8.7 and 10 um. For dispersion-shifted single-mode fibers, the nominal should be between 7 and 8.7 um. However, these values depend on the measurement technique used. Optics Primer 2.6 Fiber-Optic Transmitters Optical transmitters convert digital electrical signals into pulsed optical sig- nals; furthermore, they couple these signals into the fiber waveguide for transmission. The two major kinds of semiconductor transmitters in use are LEDs and LDs. These transmitters have the necessary design characteris- tics of small size, conversion efficiency, coupling efficiency, speed, electrical amenability, environmental resistance, and reliability. LDs are typically used with single-mode fibers, and LEDs are typically used with multimode fibers (however, other combinations are possible). Figure 2-26 shows the pho- tonic footprint of these and other sources. Efforts have been underway for a number of years to integrate the optics and electronics monolithically in an electro-optical chip in order to provide for higher-bit-rate transmission. 2.6.1 Light-Emitting Diodes (LEDs) LEDs come in two types: surface emitters and edge emitters. The surface- type emission pattern is typically 120°X120°. Light emission occurs at cur- rents of 100 mA. The peak power coupled into multimode fiber is about - 10 to -20 dBm; the power coupled into single-mode fiber is about -27 to -37 LIGHTWAVE SOURCES Figure 2-26 Light sources. Super- Edge- Surface- Laser luminescent emitting emitting diode diode <30° X >30° ~20° X 60° ~120° X 30° 120° X 120° 5um X 0.2um 5um X 0.2um 10um X 0.2um 5um Diam. C