Integrated Photonics: Fundamentals / Edition 1 available in Hardcover
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All integrated optical components and devices make use of "waveguides", where light is confined by total internal reflection. The elements in such "photonic chip" are interconnected through waveguides, and also the integrated optics components themselves are fabricated using waveguide configuration, such as couplers, switches, modulators, multiplexors, amplifiers and lasers, etc. These components are integrated in a single substrate, thus resulting in a compact and robust photonic device, which can be optically connected through optical fibres. With and increase in the number of integrated optical components and devices emerging from the research laboratories to the market place an up-to-date book is essential in collecting, summarizing and presenting the new developed photonic devices. This includes fundamental aspects, technical aspects (such as fabrication techniques and materials) and characterisation and performance. This is an advanced text aimed at specialists in the field of photonics, but who may be new to the field of integrated photonics. The fundamental aspects have been carefully considered, and all the topics covered by the book start at a medium level, making it highly relevant for undergraduate and post-graduate students following this discipline.
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About the Author
Ginés Lifante is the author of Integrated Photonics: Fundamentals, published by Wiley.
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By Gines Lifante
John Wiley & SonsISBN: 0-470-84868-5
Chapter OneINTRODUCTION TO INTEGRATED PHOTONICS
The term "integrated photonics" refers to the fabrication and integration of several photonic components on a common planar substrate. These components include beam splitters, gratings, couplers, polarisers, interferometers, sources and detectors, among others. In turn, these can then be used as building blocks to fabricate more complex planar devices which can perform a wide range of functions with applications in optical communication systems, CATV, instrumentation and sensors. The setting-up of integrated photonic technology can be considered as the confluence of several photonic disciplines (dealing with the control of light by electrons and vice versa) with waveguide technology. In fact, optical waveguides are the key element of integrated photonic devices that perform not only guiding, but also coupling, switching, splitting, multiplexing and demultiplexing of optical signals. In this chapter we will introduce the main characteristics of integrated photonic technology, showing relevant aspects concerning material and fabrication technologies. Also, we will briefly describe some basic components present in integrated photonic devices, emphasising the differences in their design compared to conventional optics. Some examples of integrated photonic devices (passive, functional, active andnon-linear) are given at the end of the chapter to show the elegant solution that this technology proposes for the development of advanced optical devices.
1.1 Integrated Photonics
Optics can be defined as the branch of physical science which deals with the generation and propagation of light and its interaction with matter. Light, the main subject of optics, is electromagnetic (EM) radiation in the wavelength range extending from the vacuum ultraviolet (UV) at about 50 nanometers to the far infrared (IR) at 1 mm. In spite of being a very ancient science, already studied by the founder of the School of Alexandria, Euclid, in his Optics (280 BC), during the last quarter of the past century, the science of optics has suffered a spectacular renaissance, due to various key developments. The first revolutionary event in modern optics was, no doubt, the invention of the laser by T.H. Maiman in 1960 at Hughes Research Laboratories in Malibu, which allowed the availability of coherent light sources with exceptional properties, such as high spatial and temporal coherence and very high brightness. A second major step forward came with the development of semiconductor optical devices for the generation and detection of light, which permitted very efficient and compact optoelectronic devices. The last push was given by the introduction of new fabrication techniques for obtaining very cheap optical fibres, with very low propagation losses, close to the theoretical limits (Figure 1.1).
As a result of these new developments and associated with other technologies, such as electronics, new disciplines have appeared connected with optics: electro-optics, optoelectronics, quantum electronics, waveguide technology, etc. Thus, classical optics, initially dealing with lenses, mirrors, filters, etc., has been forced to describe a new family of much more complex devices such as lasers, semiconductor detectors, light modulators, etc. The operation of these devices must be described in terms of optics as well as of electronics, giving birth to a mixed discipline called photonics. This new discipline emphasises the increasing role that electronics play in optical devices, and also the necessity of treating light in terms of photons rather than waves, in particular in terms of matter-light interactions (optical amplifiers, lasers, semiconductor devices, etc.). If electronics can be considered as the discipline that describes the flow of electrons, the term "photonics" deals with the control of photons. Nevertheless, these two disciplines clearly overlap in many cases, because photons can control the flux of electrons, in the case of detectors, for example, and electrons themselves can determine the properties of light propagation, as in the case of semiconductor lasers or electrooptic modulators.
The emergence of novel photonic devices, as well as resulting in the important connection between optics and electronics, has given rise to other sub-disciplines within photonics. These new areas include electro-optics, opto-electronics, quantum optics, quantum electronics and non-linear optics, among others. Electro-optics deals with the study of optical devices in which the electrical interaction plays a relevant role in controlling the flow of light, such as electro-optic modulators, or certain types of lasers. Acousto-optics is the science and technology concerned with optical devices controlled by acoustic waves, driven by piezo-electric transducers. Systems which involve light but are mainly electronic fall under opto-electronics; these systems are in most cases semiconductor devices, such as light-emitting diodes (LEDs), semiconductor lasers and semiconductor-based detectors (photodiodes). The term quantum electronics is used in connection with devices and systems that are based on the interaction of light and matter, such as optical amplifiers and wave-mixing. The quantum nature of light and its coherence properties are studied in quantum optics, and the processes that involve non-linear responses of the optical media are covered by the discipline called non-linear optics. Finally, some applied disciplines emerging from these areas include optical communications, image and display systems, optical computing, optical sensing, etc. In particular, the term waveguide technology is used to describe devices and systems widely used in optical communications as well as in optical computing, optical processing and optical sensors.
A clear example of an emergent branch of optics that combines some of the above disciplines is the field of integrated optics, or more precisely, integrated photonics. We consider integrated photonics to be constituted by the combining of waveguide technology (guided optics) with other disciplines, such as electro-optics, acousto-optics, non-linear optics and opto-electronics (Figure 1.2). The basic idea behind integrated photonics is the use of photons instead of electrons, creating integrated optical circuits similar to those in conventional electronics. The term "integrated optics", first proposed in 1960 by S.E. Miller, was introduced to emphasise the similarity between planar optical circuits technology and the well-established integrated micro-electronic circuits. The solution proposed by Miller was to fabricate integrated optical circuits through a process in which various elements, passive as well as active, were integrated in a single substrate, combining and interconnecting them via small optical transmission lines called waveguides. Clearly, integrating multiple optical functions in a single photonic device is a key step towards lowering the costs of advanced optical systems, including optical communication networks.
The optical elements present in integrated photonic devices should include basic components for the generation, focusing, splitting, junction, coupling, isolation, polarisation control, switching, modulation, filtering and light detection, ideally all of them being integrated in a single optical chip. Channel waveguides are used for the interconnection of the various optical elements. The main goal pursued by integrated photonics is therefore the miniaturisation of optical systems, similar to the way in which integrated electronic circuits have miniaturised electronic devices, and this is possible thanks to the small wavelength of the light, which permits the fabrication of circuits and compact photonic devices with sizes of the order of microns. The integration of multiple functions within a planar optical structure can be achieved by means of planar lithographic production. Although lithographic fabrication of photonic devices requires materials different from those used in microelectronics, the processes are basically the same, and the techniques well established from 40 years of semiconductor production are fully applicable. Indeed, a lithographic system for fabricating photonic components uses virtually the same set of tools as in electronics: exposure tools, masks, photoresists, and all the pattern transfer process from mask to resist and then to device.
1.2 Brief History of Integrated Photonics
For 30 years after the invention of the transistor, the processing and transmission of information were based on electronics that used semiconductor devices for controlling the electron flux. But at the beginning of the 1980s, electronics was slowly supplemented by and even replaced by optics, and photons substituted for electrons as information carriers. Nowadays, photonic and opto-electronic devices based on integrated photonic circuits have grown in such a way that they not only clearly dominate long-distance communications through optical fibres, but have also opened up new fields of application, such as sensor devices, and are also beginning to penetrate in the own field of the information processing technology. In fact, the actual opto-electronic devices may be merely a transition to a future of all-optical computation and communication systems.
The history of integrated photonics is analogous to that of other related technologies: discovery, fast evolution of the devices, and a long waiting time for applications. The first optical waveguides, fabricated at the end of the 1960s, were bidimensional devices on planar substrates. In the mid-1970s the successful operation of tridimensional waveguides was demonstrated in a wide variety of materials, from glasses to crystals and semiconductors. For the fabrication of functional devices in waveguide geometries, lithium niobate (LiNb[O.sub.3]) was rapidly recognised as one of the most promising alternatives. The waveguide fabrication in LiNb[O.sub.3] via titanium in-diffusion was demonstrated at the AT&T Bell Laboratory, and gave rise to the development of channel waveguides with very low losses in a material that possesses valuable electro-optic and acousto-optic effects. In the mid-1980s the viability of waveguide devices based on LiNb[O.sub.3], such as integrated intensity modulators of up to 40 GHz, and with integration levels of up to 50 switches in a single photonic chip had already been demonstrated in laboratory experiments. A few years later, the standard packaging required in telecommunication systems was obtained, and so the devices were ready to enter the market. The rapid boom of monomode optical fibre systems which started in the 1980s was the perfect niche market for these advanced integrated photonic devices that were waiting in the research laboratories. Indeed, the demand for increased transmission capacity (bandwidth) calls urgently for new integrated photonic chips that permit the control and processing of such huge data transfer, in particular with the introduction of technology to transmit light in multiple wavelengths (WDM, wavelength division multiplexing).
Because of the parallel development of other materials, both dielectrics such as polymers, glasses or silica on silicon (Si[O.sub.2]/Si), and semiconductors such as indium phosphide (InP), gallium arsenide (GaAs) or even silicon (Si), a wide variety of novel and advanced integrated photonic devices was ready to emerge on the market. During the last two decades of the twentieth century we have moved from the development of the new concept of integrated optical devices to a huge demand for such novel devices to implement sophisticated functions, mainly in the optical communication technology market. In fact, at the beginning of the twenty-first century the data transfer created by computer-based business processes and by Internet applications is growing exponentially, which translates into a demand for increasing transmission capacity at lower cost, which can only be met by increased use of optical fibre and associated advanced photonic technologies (Figure 1.3). Today fibres are typically used to transmit bit-rates up to 10 Gbit/s, which is, however, far below the intrinsic bandwidth of an optical fibre. Wavelength Division Multiplexing (WDM) (the transmission of several signals through a single fibre using several wavelengths) paves the way to transmit information over an optical fibre in a much more efficient way, by combining several 10 Gbit/s signals on a single fibre. Today there are commercial WDM systems available with bit-rates in the range of 40 to 400 Gbit/s, obtained by combining a large number of 2.5 and 10 Gbit/s signal, and using up to 32 different wavelengths. The next frontier in data transfer capacity points to the Terabit transmission, which can be achieved by using Time Domain Multiplexing (TDM), an obvious multiplexing technique for digital signals. An equivalent of TDM in the optical domain (OTDM) is also being developed with the purpose of reaching much higher bit-rates which will require the generation and transmission of very short pulses, in the order of picoseconds, and digital processing in the optical domain. Clearly, all these technologies will require highly advanced optical components, and integrated photonic devices based on planar lightwave circuits are the right choice to meet the high performance levels required, which allow the integration of multiple functions in a single substrate (Table 1.1).
1.3 Characteristics of the Integrated Photonic Components
The basic idea behind the use of photons rather than electrons to create integrated photonic circuits is the high frequency of light (200 THz), which allows a very large bandwidth for transporting and managing a huge amount of information. The replacement of electronic by photonic means is forced by fundamental physical reasons that limit the information transmission rate using purely electronic means: as the frequency of an electrical signal propagating through a conductor increases, the impedance of the conductor also increases, thus the propagation characteristics of the electrical cable become less favourable. That is the reason why electrical signals with frequencies above 10 MHz must be carried by specially designed conductors, called coaxial cables, in order to minimise the effect of a high attenuation. Figure 1.4 shows the attenuation in a typical coaxial cable as a function of the frequency.
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Table of Contents
About the Author.
Introduction to Integrated Photonics.
Review of the Electromagnetic Theory of Light.
Theory of Integrated Optic Waveguides.
Coupled Mode Theory: Waveguide Gratings.
Light Propagation in Waveguides: The Beam Propagation Method.
Appendix 1: Complex Notation of the Electric and Magnetic Fields.
Appendix 2: Phase Shifts for TE and TM Incidence.
Appendix 3: Marcatili's Method for Solving Guided Modes in Rectangular Channel Waveguides.
Appendix 4: Demonstration of Formula (4.3).
Appendix 5: Derivation of Formula (4.4).
Appendix 6: Fast Fourier Algorithm.
Appendix 7: Implementation of the Crank-Nicolson Propagation Scheme.
Appendix 8: List of Abbreviations.
Appendix 9: Some Useful Physical Constants.