Integrated Optics

Frontmatter

1. Introduction

Abstract

The transmission and processing of signals carried by optical beams rather than by electrical currents or radio waves has been a topic of great interest ever since the early 1960s, when the development of the laser first provided a stable source of coherent light for such applications. Laser beams can be transmitted through the air, but atmospheric variations cause undesirable changes in the optical characteristics of the path from day to day, and even from instant to instant. Laser beams also can be manipulated for signal processing, but that requires optical components such as prisms, lenses, mirrors, electro-optic modulators and detectors. All of this equipment would typically occupy a laboratory bench tens of feet on a side, which must be suspended on a vibration-proof mount. Such a system is tolerable for laboratory experiments, but is not very useful in practical applications. Thus, in the late 1960s, the concept of “integrated optics” emerged, in which wires and radio links are replaced by light-waveguiding optical fibers rather than by through-the-air optical paths, and conventional electrical integrated circuits are replaced by miniaturized optical integrated circuits (OIC’s).

Robert G. Hunsperger

2. Optical Waveguide Modes

Abstract

The optical waveguide is the fundamental element that interconnects the various devices of an optical integrated circuit, just as a metallic strip does in an electrical integrated circuit. However, unlike electrical current that flows through a metal strip according to Ohm’s law, optical waves travel in the waveguide in distinct optical modes. A mode, in this sense, is a spatial distribution of optical energy in one or more dimensions. In this chapter, the concept of optical modes in a waveguiding structure is discussed qualitatively, and key results of waveguide theory are presented with minimal proof to give the reader a general understanding of the nature of light propagation in an optical waveguide. Then, in Chap. 3, a mathematically sound development of waveguide theory is given.

Robert G. Hunsperger

3. Theory of Optical Waveguides

Abstract

Chapter 2 has reviewed the key results of waveguide theory, particularly with respect to the various optical modes that can exist in the waveguide. A comparison has been made between the physical-optic approach and the ray-optic approach in describing light propagation in a waveguide. In this chapter, the electromagnetic wave theory of the physical-optic approach is developed in detail. Emphasis is placed on the two basic waveguide geometries that are used most often in optical integrated circuits, the planar waveguide and the rectangular waveguide.

Robert G. Hunsperger

4. Waveguide Fabrication Techniques

Abstract

In Chap. 3, the theoretical considerations relevant to various types of waveguides were discussed. In every case, waveguiding depended on the difference in the index of refraction between the waveguiding region and the surrounding media. A great many techniques have been devised for producing that required index difference. Each method has particular advantages and disadvantages, and no single method can be said to be clearly superior. The choice of a specific technique of waveguide fabrication depends on the desired application, and on the facilities available. In this Chapter, various methods of waveguide fabrication are reviewed, and their inherent features are discussed.

Robert G. Hunsperger

5. Losses in Optical Waveguides

Abstract

Chapters 2 and 3 have explained cutoff conditions in waveguides and described the various optical modes which can be supported. Following the question as to which modes propagate, the next most important characteristic of a waveguide is the attenuation, or loss, that a light wave experiences as it travels through the guide. This loss is generally attributable to three different mechanisms: scattering, absorption and radiation. Scattering loss usually predominates in glass or dielectric waveguides, while absorption loss is most important in semiconductors and other crystalline materials. Radiation losses become significant when waveguides are bent through a curve.

Robert G. Hunsperger

6. Waveguide Input and Output Couplers

Abstract

Some of the methods of coupling optical energy into or out of a waveguide were mentioned briefly in Chap. 5. In this chapter, we shall consider in more detail the various coupling techniques that can be used. The methods that are employed for coupling an optical beam between two waveguides are different from those used for coupling an optical beam in free space to a waveguide. Also, some couplers selectively couple energy to a given waveguide mode, while others are multimode. Each type of coupler has its attendant set of advantages and disadvantages; none is clearly best for all applications. Hence, a knowledge of coupler characteristics is necessary for the OIC user, as well as for the designer.

Robert G. Hunsperger

7. Coupling Between Waveguides

Abstract

The phenomenon of optical tunneling can be used not only to couple energy from a fiber or a beam to a waveguide, as described in Chap. 6, but also to couple one waveguide to another. Couplers of this type are usually called directional couplers because the energy is transferred in a coherent fashion so that the direction of propagation is maintained. Directional couplers have been fabricated in two basic geometries: multilayer planar structures, and dual side-by-side channel waveguides. In this chapter the different types of waveguide to waveguide couplers are described and a concise theory of operation is developed. For a thorough mathematical treatment of these devices the reader is referred to the work of Burns and Milton [7.1].

Robert G. Hunsperger

8. Electro-Optic Modulators

Abstract

This chapter begins the discussion of optical-signal modulation and switching In many cases, the same device can function as either a modulator or a switch depending on the strength of the interaction between the optical waves and the controlling electrical signal, as well as on the arrangement of input and output ports. The device is considered to be a modulator if its primary function is to impress information on a light wave by temporaly varying one of its properties. A switch, on the other hand, changes the spatial position of the light, or else turns it off and on. Many of the same factors must be considered in designing or evaluating both modulators and switches. Hence, it is logical to discuss them together.

Robert G. Hunsperger

9. Acousto-Optic Modulators

Abstract

In the preceding chapter, we have shown that modulators and switches can be made by using the electro-optic effect to produce a grating-shaped variation of the index of refraction within the waveguide. This grating structure causes diffraction of the guided optical waves, resulting in modulation or switching.

Robert G. Hunsperger

10. Basic Principles of Light Emission in Semiconductors

Abstract

This chapter is the first in a sequence of four chapters that describe the light sources used most often in integrated-optic applications. Because of their convenience, gas lasers are frequently used in the laboratory to evaluate waveguides or other integrated-optic devices; however, semiconductor lasers and light-emitting diodes are the only practical light sources for use in optical integrated circuits, due to their small size and compatibility with monolithic (or hybrid) integration. Also, LED’s and laser diodes are widely used in fiberoptic applications, because they can be modulated at high frequencies and can be efficiently coupled to the micrometer-size wave-guiding core of the fiber.

Robert G. Hunsperger

11. Semiconductor Lasers

Abstract

In the preceding chapter, we discussed the basic principles of light emission in a semiconductor. Probably the most significant feature of this light emission is that it is possible to design a light source in such a way that stimulated emission of photons will predominate over both spontaneous emission and absorption. If a resonant reflecting structure such as a pair of plane, parallel end faces is provided, a lasing mode can be established and coherent optical emission will result. In this chapter, we consider several basic semiconductor laser structures and develop the quantitative theory necessary to calculate their expected performance characteristics.

Robert G. Hunsperger

12. Heterostructure, Confined-Field Lasers

Abstract

In Chap. 11, it was demonstrated that confining the optical field to the region of the laser in which the inverted population exists results in a substantial reduction of threshold current density and a corresponding increase in efficiency. As early as 1963, it was proposed that heterojunctions could be used to produce a waveguiding structure with the desired property of optical confinement [12.1, 2], At about the same time, others proposed using a heteroj unction laser structure not for optical field confinement, but to produce higher carrier injection efficiency at the p-n junction, and to confine the carriers to the junction region [12.3, 4]. Actually, all three of these mechanisms are present in a heterostructure laser, and their combined effects result in a device that is vastly superior to the basic p-n homojunction laser.

Robert G. Hunsperger

13. Distributed-Feedback Lasers

Abstract

All of the lasers that have been described so far depend on optical feedback from a pair of reflecting surfaces, which form a Fabry-Perot etalon. In an optical integrated circuit, in which the laser diodes are monolithically integrated within the semiconductor wafer, it is usually very difficult to form such reflecting surfaces. They can be formed by etching or cleaving, as described in Chap. 12. However, the planar surface of the wafer is then disrupted, which leads to difficulties in fabricating electrical connections and heat sinks. An alternative approach, which utilizes distributed-feedback (DFB) from a Bragg-type diffraction grating, provides a number of advantages while still utilizing a planar surface geometry.

Robert G. Hunsperger

14. Direct Modulation of Semiconductor Lasers

Abstract

In Chaps. 8 and 9 techniques were described for modulating the light of a semiconductor laser by using external electro-optic or acousto-optic modulators. However, it is also possible to internally modulate the output of a semiconductor laser by controlling either the current flow through the device or some internal cavity parameter. Such direct modulation of the laster output has the advantages of simplicity and potential for high frequency operation. The topic of direct modulation of injection lasers is considered in this chapter; this follows the discussions of semiconductor laser fundamental principles and operating characteristics in Chaps. 10–12, so that the reader will be better prepared to appreciate the subtleties of the methods involved.

Robert G. Hunsperger

15. Integrated Optical Detectors

Abstract

Detectors for use in integrated-optic applications must have high sensitivity, short response time, large quantum efficiency and low power consumption [15.1]. In this chapter, a number of different detector structures having these performance characteristics are discussed.

Robert G. Hunsperger

16. Quantum-Well Devices

Abstract

In all of the devices discussed in previous chapters the dimensions of device structures were large compared to the wavelength of electrons in the device. When the dimensions of the structure are reduced to the point at which they are approaching the same order of magnitude as the electron wavelength some unique properties are observed. This is the case with a class of devices that have come to be known as “quantum well” devices, which feature very thin epitaxial layers of semiconductor material. This chapter will introduce the basic concepts of quantum wells and will describe some of the novel kinds of devices that can be made by using them. Improved lasers, photodiodes, modulators and switches can all be made by employing quantum well structures. Quantum well devices can be monolithically integrated with other optical and electronic devices to produce optical integrated circuits and opto-electronic integrated circuits.

Robert G. Hunsperger

17. Applications of Integrated Optics and Current Trends

Abstract

In the preceding chapters, the theory and technology of optical integrated circuits have been described. Although this a relatively new field of endeavor, numerous applications of OIC’s to the solution of current engineering problems have already been implemented and some OIC’s are now available as “off-the-shelf” commercial products. Of course, optical fiber waveguides, the companion element of OIC’s in an integrated-optic system, are already well recognized as being very useful consumer products. In this chapter, some of the more recent applications of both fibers and OIC’s are reviewed, and current trends are evaluated. In this review of representative integrated optic applications, specific systems and companies are named in order to illustrate the international character of the field and the types of organizations that are involved in it. Recommendation of any particular company or its products is not intended or implied. Also, the performance data that are quoted have generally been obtained from news articles and other secondary sources. Hence, they should be interpreted as being illustrative rather than definite.

Robert G. Hunsperger

Backmatter