This chapter addresses optical wave propagation in isotropic and anisotropic media. This chapter begins with general discussions on the energy flow and power exchange as an optical wave propagates through a medium. The next two sections respectively address the propagation of plane waves in isotropic and anisotropic homogeneous media. The polarization normal modes of propagation are defined for a birefringent crystal, which can be uniaxial with only one optical axis or biaxial with two optical axes. The concepts and characteristics of phase velocity, group velocity, and various types of dispersion are then discussed.

The coupled-wave theory is used in the analysis of the interactions among optical waves of different frequencies. In the analysis of the coupling of waveguide modes, coupled-mode theory has to be used. In general, both the interaction among different optical frequencies and the characteristics of the waveguide modes have to be considered for a nonlinear optical interaction in an optical waveguide. In the first section, a combination of coupled-wave and coupled-mode theories is formulated for the analysis of nonlinear optical interaction in a waveguide. In the second section, the coupled equations for a parametric nonlinear interaction in a waveguide are formulated by using three-frequency parametric interaction, second-harmonic generation, and the optical Kerr effect as three examples. In the third section, the coupled equations for a nonparametric nonlinear interaction in a waveguide are formulated by using stimulated Raman scattering and two-photon absorption as two examples.

Photon transduction is a fundamental process of any optical detector or image sensor where the basic task is to estimate an average quantity of photons versus time and/or space. We start from basic physical phenomena of the optical transduction considering photon flux as an average quantity, disregarding the quantum mechanics characteristics of a single photon. Then, we investigate the role of noise in the transduction process to better assess design rules in electronic design of interfaces. As in the other transduction chapters, we treat only a very small part of existing optical sensor implementations to serve as examples of the application of the transduction principle.

This chapter addresses the physics and applications of optical saturation, including optical absorption saturation and optical gain saturation. Optical saturation is a nonlinear optical process that usually cannot be approximated with a perturbation expansion as a second-order or third-order nonlinear process. Instead, a fully nonlinear analysis is required. Following a discussion on the general physics and characteristics of absorption saturation and gain saturation in the first section, the properties and applications of saturable absorbers and saturated amplifiers are discussed in the second and third sections. The last section covers laser oscillation as a consequence of optical gain saturation.

The coupled-wave theory deals with the coupling of waves of different frequencies in nonlinear optical interactions. In the first section, the general coupled-wave equation is derived. Its form under the slowly varying amplitude approximation is then obtained, followed by a form under the transverse approximation. In the second section, the coupled-wave equations for a parametric process are formulated by using three-frequency parametric interaction and second-harmonic generation as two examples. In the third section, the coupled-wave equations for a nonparametric process are formulated by using stimulated Raman scattering and two-photon absorption as two examples.

Most parametric frequency-conversion processes are not automatically phase matched, thus requiring arrangements to achieve phase matching. If a parametric frequency-conversion process is perfectly phase matched, optical power can be efficiently converted from one frequency to another. Otherwise, the conversion efficiency is reduced. The geometric arrangement and the conditions for collinear phase matching and noncollinear phase matching are discussed in the first section. The second section addresses the concept and techniques of birefringent phase matching, which employs the birefringence of a uniaxial or biaxial crystal to accomplish phase matching of a nonlinear optical process. It is the most commonly used method of obtaining collinear phase matching for a second-order frequency-conversion process. The third section covers the concept and techniques of quasi-phase matching, which uses periodic modulation of the nonlinear susceptibility for phase matching. Phase matching in an optical waveguide is discussed in the fourth section.

Understanding the origin of noise is important because it gives hints on how to reduce its effects even from the electronic point of view. This chapter analyzes the physics background of some sources of random processes that are limiting sensing systems referred to as “thermal,” “shot,” and “flicker” noises. It also shows how thermal and shot noises are at the base of other observed electronic effects such as “kTC,” “phase,” and “current” noises. The discussion uses analogies between mechanical and electronic effects of thermal agitation. This is important not only for understanding the process but also to unify the model of noise in microelectromechanical sensor systems so as to use the same analysis framework.

This chapter is focused on the concepts of mechanical and thermal transduction related to the change of conductance and polarization in materials. Therefore, after an introduction on basic concepts, the transduction processes of piezoresistivity, piezoelectricity, and temperature effects on resistance are discussed. Finally, examples of applications of resistance sensors are given, focusing on some techniques to reduce errors due to influence variables.

Optical nonlinearity emerges from nonlinear interaction of light with matter. In this chapter, the basic concept and formulation of light‒matter interaction are discussed through a semiclassical approach with the behavior of the optical field classically described by Maxwell’s equations and the state of the material quantum mechanically described by a wave function governed by the Hamiltonian of the material. An optical field interacts with a material through its interaction with the electrons in the material. A Schrödinger electron is nonrelativistic with a nonzero mass, and a Dirac electron is relativistic with a zero mass. The interaction Hamiltonian can be expressed in terms of the vector and scalar potentials by using the Coulomb gauge. It can be expressed in terms of the electric and magnetic fields through multipole expansion as a series of electric and magnetic multipole interactions, with the first term being the electric dipole interaction. The electric polarization of a material induced by an optical field is obtained through density matrix analysis. The optical susceptibility of the material is then obtained from the electric polarization.

Bistability is a phenomenon that has two stable states under one condition. A bistable device has two possible stable output values for one input condition. The necessary conditions for optical bistability are optical nonlinearity and positive feedback. Depending on whether the optical nonlinearity that is responsible for the bistable function comes from the real or the imaginary part of a nonlinear susceptibility, a bistable optical device can be classified as either dispersive or absorptive. Depending on the type of feedback, a bistable optical device can also be classified as either intrinsic or hybrid. After a general discussion on the condition for optical bistability, this chapter covers dispersive optical bistability, absorptive optical bistability, and hybrid optical bistability of passive optical systems in three sections. The final chapter covers optical bistability in the active optical system of a laser oscillator.

All-optical modulation of an optical wave is accomplished through a nonlinear optical process that involves one or multiple optical waves. A nonlinear optical modulator can be based on either self-modulation or cross-modulation. Such nonlinear optical modulators and switches are also known as all-optical modulators and all-optical switches, respectively. Most all-optical modulators and switches are based on third-order nonlinear optical processes, but some rely on the high-order process of optical saturation, either absorption saturation or gain saturation. There are two fundamentally different types of all-optical modulators and switches: the dispersive type and the absorptive type. All-optical modulation of the dispersive type, which is based on the optical Kerr effect, is discussed in this chapter. In the first four sections, the physics, phenomena, and measurement of the optical Kerr effect are discussed. The last two sections cover all-optical modulators and switches in the bulk form and those in the waveguide form.

This chapter treats two important steps in electronic sensor design. The first is the passage from functional blocks to lumped model electronic circuits. In this approach noise will be no more associated with functional blocks, but with circuit topology and electronic device elements. The second step is to analyze the effects of the readout mode on noise, emphasizing the differences between continuous and discrete-time approaches. Finally, we discuss some tradeoffs related to bandwidth and resolution in acquisition chains.