Electronic states and probabilities of optical transitions in molecules and crystals are determined by the properties of atoms and their spatial arrangement. An electron in an atom possesses a discrete set of states, resulting in a corresponding set of narrow absorption and emission lines. Elementary excitations in an electron subsystem of a crystal, that is, electrons and holes, possess many properties of a gas of free particles. In semiconductors, broad bands of the allowed electron and hole states separated by a forbidden gap give rise to characteristic absorption and emission features completely dissimilar to atomic spectra. It is therefore reasonable to pose a question: What happens on the way from atom to crystal? The answer to this question can be found in the studies of small particles with the number of atoms ranging from a few atoms to several hundreds of thousands atoms. The evolution of the properties of matter from atom to crystal can be described in terms of the two steps: from atom to cluster and from cluster to crystal.

The main distinctive feature of clusters is the discrete set of the number of atoms organized in a cluster. These so-called magic numbers determine unambiguously the spatial configuration, electronic spectra, and optical properties of clusters. Sometimes a transition from a given magic number to the neighboring one results in a drastic change in energy levels and optical transition probabilities.

The dramatic reduction in transmission loss of optical fibers coupled with equally important developments in the area of light sources and detectors have brought about a phenomenal growth of the fiber optic industry during the past two decades. Indeed, the birth of optical fiber communications coincided with the fabrication of low-loss optical fibers and operation of room temperature semiconductor lasers in 1970. Since then, the scientific and technological progress in the field has been so phenomenal that optical fiber communication systems find themselves already in the fifth generation within a span of about 25 years. Broadband optical fiber amplifiers coupled with wavelength division multiplexing techniques and soliton communication systems are some of the very important developments that have taken place in the past few years, which are already revolutionizing the field of fiber optics. Although the major application of optical fibers has been in the area of telecommunications, many new related areas such as fiber optic sensors, fiber optic devices and components, and integrated optics have witnessed considerable growth. In addition, optical fibers allow us to perform many interesting and simple experiments permitting us to understand basic physical principles.

With the all-pervading applications of optical fibers, many educational institutions have started courses on fiber optics. At our Institute, we have a threesemester M.Tech. program on Optoelectronics and Optical Communications (jointly run by the Physics and Electrical Engineering Departments) in which we have an extensive coverage of the theory of optical fibers and optical fiber communications and also many experiments and projects associated with it.

Introduction

The most important and widely exploited application of optical fiber is its use as the transmission medium in an optical communication link. The basic optical fiber communication system consists of a transmitter, an optical fiber, and a receiver. The transmitter has a light source, such as a laser diode, which is modulated by a suitable drive circuit in accordance with the signal to be transmitted. Similarly, the receiver consists of a photodetector, which generates electrical signals in accordance with the incident optical energy. The photodetector is followed by an electronic amplifier and a signal recovery unit.

Among the variety of optical sources, optical fiber communication systems almost always use semiconductor-based light sources such as light-emitting diodes (LEDs) and laser diodes because of the several advantages such sources have over the others. These advantages include compact size, high efficiency, required wavelength of emission, and, above all, the possibility of direct modulation at high speeds.

In this chapter, we discuss the mechanism of light generation, basic device configurations, and relevant output characteristics of the light source. In Section 11.2 we discuss the basic requirements that the source should meet to be suitable for use in an optical fiber communication system. In Section 11.3 we briefly present an elementary account of the principle of operation of a laser. In Section 11.4 we discuss basic semiconductor physics relevant to the operation of a semiconductor laser followed by the device structure and characteristics in Section 11.5. Finally, in Section 11.6 we briefly discuss the characteristics of LEDs that are relevant to a fiber optic communication link.

Introduction

Characterization of optical fibers is very important for a number of reasons. The users of optical fibers need the fiber characteristics to design the optical fiber communication system, whereas the manufacturers need them for optimizing their fabrication processes to obtain fibers with desired characteristics. The fiber characteristics are also necessary for the development and verification of various theoretical models for predicting various performance properties of the fiber. The two most important characteristics of an optical fiber are bandwidth (or pulse dispersion) and loss. In addition, one requires knowledge of various other parameters such as refractive index profile, core diameter, and so forth for predicting losses at joints. Table 19.1 lists the various characteristics of optical fibers along with their effect on system performance.

A large number of techniques have been developed for measuring various fiber characteristics. In this and the following chapter, we briefly discuss some of the standard techniques used in fiber characterization; for more details of the various techniques, readers may consult Pal, Thyagarajan, and Kumar (1988) and Thyagarajan, Pal, and Kumar (1988b).

In Section 19.2 we discuss some general experimental considerations relevant to fiber measurements, and in Section 19.3 we discuss various techniques for the measurement of refractive index profile, spectral attenuation and pulse dispersion, or bandwidth. In Chapter 20 we discuss measurement of characteristics specific to single-mode fibers only – namely, mode field diameter, cutoff wavelength, and birefringence.