Wavelength division multiplexing (WDM) is the second major fiber–optic revolution in the field of telecommunications. WDM is a technology which combines many different segments of wavelength range, called different independent optical channels, into the same optical fiber. The best feature of an optical fiber is that it has a wide spectral region which ranges from 1260 nm to 1675 nm. The light source used in high-capacity optical fiber communication systems emits a narrow wavelength band of less than 1 nm, thus enabling simultaneous transmission of many optical channels using the same optical fiber. WDM allows a huge increase in capacity of an optical fiber as compared to point-to-point link that carries only a single optical channel. Another big advantage of WDM is that different transmission formats can be supported by various optical channels. It means that without the need of common signal format, any data rate can be transmitted simultaneously and independently using the common optical fiber.

This chapter focuses on WDM concepts and components used in high-capacity optic–fiber communication networks. The discussion begins with the principle of wavelength division multiplexing which contains an orthogonal set of optical carriers with a suitable guard band, which a single-mode fiber can propagate. This is followed by a brief discussion on WDM system configuration involving a number of optical devices. An account of applications of WDM systems is presented next. The discussion is carried forward by describing various types of WDM components, including transmitters and receivers. Finally, an analysis of system performance issues and WDM soliton systems are covered.

Optical measurements are necessary to verify the operational characteristics of the optical fiber communication link. Various measurement techniques and special-purpose test equipments are employed for determining key performance parameters of the constituent components and devices including the optical fiber. It is quite obvious that optical measurements are needed at different levels of research and design, manufacturing and production of optical components and devices, installation and commissioning of optical fiber communication systems in the field. There is wide variety of optical measurement and test equipments used. These include optical power meter, optical oscilloscope and spectrum analyzer, optical time-domain reflectometer (OTDR), optical waveform analyzer, connector inspection microscope, dispersion analyzer, live fiber detector, talk-set, optical test set (combined source and power meter), etc. All these measurements are wavelength specific. Fiber attenuation and occurrence of faults in the optical fiber link is the main concern in ensuring the desired performance. There are several challenges involved with optical measurements like multiple wavelengths/channels, high optical power levels, need to carry tests remotely along with a high degree of automation.

Optical power and insertion loss measurements are among the easiest yet the most important optical measurements in optical fiber communications. An OTDR has several uses such as loss measurements as well as fault detection. Live fiber detectors and talk-sets are useful portable test equipment for the purpose of installation, maintenance and repair. Software prediction of an OTDR trace is a recent development in optical measurements. This chapter focuses on optical measurements of transmission properties of major constituents of optical fiber communication system such as optical source power output, optical amplifier noise characteristics, modulation response, insertion loss, fiber attenuation, dispersion parameters, and link fault detection.