ANSWERS TO QUESTIONS Q1. THROUGH Q28.

A1. Interface circuit, source drive circuit, and an optical source.
A2. The optical source.
A3. Multimode fiber dispersion, the relatively high fiber attenuation, and the LED's relatively low optical output power.
A4. Longer distances and higher bandwidths are possible because fiber material losses and dispersion are significantly reduced at the 1300-nm region.
A5. Light waves that lack a fixed-phase relationship.
A6. LDs.
A7. (1) The angles over which the light is emitted. (2) The size of the source's light-emitting area relative to the fiber core size. (3) The alignment of the source and fiber. (4) The coupling characteristics of the fiber (such as the NA and the refractive index profile).
A8. Silicon and gallium arsenide.
A9. A laser is a device that produces optical radiation using stimulated emission rather than spontaneous emission.
A10. A photon initially produced by spontaneous emission in the active region interacts with the laser material to produce additional photons.
A11. Surface-emitting LEDs (SLEDs), edge-emitting LEDs (ELEDs), and superluminescent diodes (SLDs).
A12. SLEDs and ELEDs.
A13. Cut or polished surfaces at each end of the narrow active region of an ELED.
A14. Threshold current.
A15. The LD's output has a narrow spectral width and small output beam angle.
A16. LED.

A17. Laser.
A18. More.
A19. SLDs have no built-in optical feedback mechanism.
A20. By varying the current through the source.
A21. A current applied in the laser off state just less than the threshold current.
A22. For a laser diode. The laser diode transmitter generally contains output power control circuitry and may contain a TE cooler and some circuitry associated with the TE cooler.
A23. Optical connectors and optical fiber pigtails.
A24. TO can, DIP, butterfly lead microcircuits, circuit cards, and stand-alone optical fiber converters.
A25. LED.
A26. When extremely high transmitter output powers are required.
A27. Laser diode.
A28. LEDs require less complex circuitry than lasers.