23.1 Introduction

High-capacity digital transmission systems and analog-modulated microwave frequency systems based on fiber-optic (FO) systems have emerged as very competitive to conventional communication systems. A variety of lightwave components have been developed to support these high-speed systems. Most notable among these components are semiconductor lasers, specifically single-frequency distributed feedback (DFB) lasers, and high-speed pin photodetectors.

The design of the communications systems depends heavily on the characteristics of the lightwave signal propagating through the various components of the system. A lightwave signal is an electromagnetic wave in the wavelength range between 200 and 2000 nm, i.e., a frequency range between 150 and 1500 THz. This signal is generated by the laser in the transmission section of the system. It is critical to the success of the system to use high-quality lasers (high power, large modulation bandwidth, low noise, etc.). In addition, as this signal propagates through the system, its characteristics (power, polarization, etc.) change. Accurate methods of measuring the characteristics (power, modulation speed, spectrum, linewidth, state of polarization, etc.) of a lightwave signal or, alternatively, analyzing the lightwave signal are therefore essential for the design of high-capacity fiber-optic transmission systems.

23.2 Light Sources

The term laser stands for light amplification by stimulated emission of radiation. We focus the discussion in this section on semiconductor laser diodes (LDs) which are now being routinely used in FO systems because of their small size, high efficiency and reliability, and excellent control of wavelength, power,...