Chapter 2 - Basic Instrumentation for Optical Measurement

Abstract

This chapter introduces a number of basic optical measurement instruments and functional devices, focusing on the discussion of the basic operating principles, general specifications, unique advantages, and limitations of each measurement tool. An optical spectrum analyzer (OSA) based on diffraction gratings is one of the most useful pieces of instruments in fiber-optic system and device measurement, especially when wavelength division multiplexing is introduced into the systems where different data channels are carried by different wavelengths. In addition to being able to identify the wavelength of an optical signal, an OSA is often used to find optical signal power level at each wavelength channel, evaluate optical signal-to-noise ratio and optical crosstalk, and check the optical bandwidth when an optical carrier is modulated. An ultra-high-resolution OSA can be made using a scanning Fabry–Perot interferometer (FPI). The Mach–Zehnder interferometer (MZI) is one of the oldest optical instruments, in use for more than a century. The basic configuration of an MZI consists of two optical beam splitters (combiners) and two mirrors to alter beam directions. The optical configuration of a Michelson interferometer is similar to that of an MZI except that only one beam splitter is used and the optical signal passes bidirectionally in the interferometer arms. The most popular optical wavelength meter is based on a Michelson interferometer, which is structurally simpler than an OSA and can easily be made into a small package using fiber-optic components. The polarimeter is an instrument that can be used to measure the state of polarization of a lightwave signal; it is useful in many applications, such as optical system monitoring and characterization. Furthermore, this chapter discusses the fundamental principle of coherent detection, which helps in the understanding of linear optical sampling and vectorial optical network analyzers.