Electro-optical effects in materials are used to switch, modulate, detect, amplify, or generate optical radiation in guided-wave devices.

The best known electro-optical effect is probably the amplification of optical radiation by stimulated emission of radiation. In edge-emitting semiconductor lasers, the amplification of the guided wave is obtained via current injection in a forward biased p–n junction. In a laser oscillator, the waveguide is terminated by reflectors (or coupled to a feedback grating) to form a resonant cavity. When amplification exceeds losses in the cavity, oscillation is obtained. When end reflections (or feedback) are absent and when there is net gain, a laser amplifier is obtained. The second well known electro-optical effect is detection of optical radiation by photo-generation of carriers. For optical radiation incident on a semiconductor with photon energy greater than the semiconductor bandgap, electrical carriers are generated by the absorption of incident radiation. In a semiconductor detector, photo-generated carriers in a reverse biased p–i–n junction are collected and transmitted to the external circuit. In waveguide photo-detectors, the optical radiation is incident on to and absorbed in a waveguide so that the absorption can be distributed over a distance, enabling the detector to absorb more effectively the incident optical power over a longer distance while maintaining a large operation bandwidth. Discussion of carrier injection, stimulated emission and carrier transport in semiconductor junctions requires extensive review of semiconductor device physics. There are already many books on lasers and detectors.

Fields of planar guided waves are confined in the depth direction (designated as the x direction in this book) to the vicinity of the high index layer which is the core. The mathematical description of the planar waveguide modes has already been discussed in Sections 1.2.3 and 1.2.4. Since the high index layer is often located near the surface, the guided waves are sometimes called surface waves. As the surface contour of the various layers of the waveguide changes gently, the planar guided-waves will follow the contour. Planar guided waves have three distinct properties.

1. (1) The evanescent field of the guided-wave modes extends into the air (or cladding) above the surface. Thus they can be excited or coupled out of the core from the air (or cladding layer) adjacent to the surface.

2. (2) The scattered radiation of the propagating wave is often also visible in the free space above. It can be used to monitor the propagation of the guided wave.

3. (3) Guided waves are free to propagate in any direction in the transverse plane (designated as the yz plane in this book).

Summation of planar guided waves can form divergent, convergent or diffracted waves in the transverse plane. How to analyze the generalized planar guided waves has already been discussed in Section 1.2.5.

A distinct feature of planar waveguide devices is the utilization of the diffraction, focusing and collimation properties in the transverse plane to achieve focusing, switching, deflection, wavelength filtering or other functions.

Optoelectronic guided-wave devices are used in many optical fiber communication and optoelectronic systems. In these systems optical and electrical signals are transmitted, received, multiplexed and converted by means of a variety of procedures. In guided-wave optoelectronic devices, laser radiation propagates in a waveguide and energy can be coupled effectively to and from single mode optical fibers. The properties of materials used to fabricate the waveguides have a profound effect on the phase, amplitude or directional variations of the optical waves used for the generation, modulation, switching, conversion, multiplexing, and detection of optical signals. The small lateral dimensions of the waveguide structures provide for efficient control of their optical properties by means of electrical voltages or currents. On the other hand, optical signals are converted back into electrical signals via detectors. Therefore, the electrical characteristics of these devices are as important as their optical properties. Devices may potentially be monolithically integrated optically on the same chip. This is called photonic integration. Optical components may also be integrated, monolithically, with electronic devices on the same chip. This is called optoelectronic integration. In earlier times, these were called integrated optical devices, as opposed to integrated electronic devices.

The manner in which different material properties affect the electrical characteristics as well as the propagation of optical signals in optoelectronic devices is of great importance. Also of considerable importance is the process of back and forth conversion of the electrical signals and of the optical signals.

Introduction to optical waveguides

Optical waveguides are made from material structures that have a core region which has a higher index of refraction than the surrounding regions. Guided electromagnetic waves propagate in and around the core. The transverse dimensions of the core are comparable to or smaller than the optical wavelength. Figure 1.1(a) illustrates a typical planar waveguide. Figure 1.1(b) illustrates a typical channel waveguide. For rigorous electromagnetic analysis of such guided-wave structures, Maxwell's vector equations should be used. Many of the theoretical methods used in the analysis of optical guided waves are very similar to those used in microwave analysis. For example, modal analysis is again a powerful mathematical tool for analyzing many devices, applications and systems.

However, there are also important differences between optical and microwave waveguides. In microwaves, we usually have closed waveguides inside metallic boundaries. Metals are considered as perfect conductors at most microwave frequencies. Microwaves propagate within the metallic enclosure. Figure 1.2 illustrates a typical microwave rectangular waveguide. In these closed structures, we have only a discrete set of waveguide modes whose electric fields terminate at the metallic boundary. Microwave radiation in the waveguide may be excited either by an electric field or by a current loop. At optical wavelengths, we avoid the use of metallic boundaries because of their strong absorption of radiation. Ideal optical waveguides, such as those illustrated in Fig. 1.1(a) and (b), are considered to have dielectric boundaries extending to infinity. They are called open waveguides.