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2 1 1 nm 1 µm 1 mm Microwaves 30 kHz 30 MHz 3 GHz 30 GHz THz Gap Photonics 3 THz 4 Optics and photonics is essentially concerned with the study of light and its useful application. Hence the aim of this course is to enhance your understanding of optical phenomena, materials and devices and to see how they are applied in engineering. The basic questions we will seek to ask are: 1. What is light? (And by extension, what is optics and photonics?) 2. How does light interact with matter? 3. How do we generate light? 5. How do we detect light? 6. How can we use light? 3 PHz Frequency 3 4. How do we manipulate or modulate the properties of light? X-ray Far-infrared Optics Electronic techniques • HMY 645 • Lecture 01 • Spring Semester 2015 10-8 10-9 Ultraviolet 10-4 10-5 10-6 10-7 Visible 10-2 10-3 Mid-infrared 1 cm 10-1 Sub-Mm-wave 1 Microwave 10 Extremely ultrashortwave Longwave 102 Ultrashortwave Stavros Iezekiel Department of Electrical and Computer Engineering University of Cyprus [email protected] Wavelength (m) 103 Shortwave 104 Mediumwave ECE 645 – Optics and Photonics Lecture 01 - Introduction Mm-wave 1 km • In this course we will consider the part of the electromagnetic spectrum between about 3 THz and 300 THz, which covers the domain of optics and photonics: HOW CAN WE USE LIGHT? 5 Applications of Photonics 6 Displays and lighting Photonics is now an important part of the world economy, with multiple applications, including: Data Storage 7 Biomedical 8 Manufacturing 9 10 Telecommunications Energy Probably the biggest application for photonics is telecommunications, and more specifically the internet, which would not be possible without fibre-optic technology: 11 12 IT departments in financial capital markets are facing huge growth in the volume of market data, while they are also under pressure to improve trade execution times. In these environments, one extra millisecond of trade execution latency can mean as much as $100 million in lost trades per year. [Source: Mellanox.] 14 http://www.arthitectural.com/wp-content/uploads/2013/04/02-TABLE-OF-OPTICKS-SIR-ISAAC-NEWTON-1704.jpg 13 Facebook invests in 55-terabit intra-Asia submarine cable system Facebook has joined a consortium that will build by far the fastest intra-Asia submarine fiber optic network, the Asia Pacific Gateway (APG). Facebook is the only American company involved with the venture, which will see 10,000km (6,000 miles) of prime fiber laid between Malaysia and Japan (pictured above), with branches landing in almost every country along the way (Singapore, Thailand, Vietnam, China, Taiwan, and South Korea). When the cable goes online in 2014, it is slated to use 40Gbps channels, for a total capacity of 55 terabits per second, or a transfer speed of 6.9 terabytes (138 Blu-ray discs) per second. When the various routers and repeaters are upgraded to 100Gbps-per-channel, the cable will have a total capacity of well over 100Tbps. The members of the consortium have put forward a total of $450 million so far, which makes it one of the most expensive submarine cable systems in the world. WHAT IS OPTICS? 15 Optics is one of the oldest branches of science. It is concerned with the generation, propagation, manipulation and detection of light. For many centuries, the development of optical sources and optical detectors was very slow, hence progress was strongest in studies of light propagation and light manipulation, e.g.: 16 By the late 19th century, the theoretical work of Maxwell and the experiments of Hertz had resulted in the electromagnetic view of light, in which it holds that light consists of coupled time-varying electric and magnetic fields that satisfy a wave equation (which itself can be derived from Maxwell’s equations): c = speed of light = 2.998 × 10-8 ms-1 in vacuo c = fλ Solution is a travelling-wave: Refraction (ray optics) Interference (wave optics) k= Polarisation (electromagnetic optics) 2π λ ω= 2π T 18 17 Light as photons (“particles”): The ultraviolet catastrophe However, the development of modern physics (and especially the work of Planck and Einstein) led to the photon view of light. Radiation modes in a hot cavity provide a test of quantum theory. In classical physics, probability of modes being occupied is equal, which predicts increasing radiated energy as frequency is increased. This leads to the “ultraviolet catastrophe”. Energy of a photon: E = hf = Planck showed that by assuming the modes were quantised in energy, quantum theory would match measured results. hc λ h = Planck’s constant = 6.626 × 10-34 J·s Hyperphysics 19 Light as photons: Photoelectric effect 20 Light as photons The Compton Effect • Increasing the intensity of the light increases the number of photoelectrons, but not their maximum kinetic energy. • Red light will not cause the ejection of electrons, no matter what the intensity. • Weak violet light will eject only a few electrons, but their maximum kinetic energies are greater than those for intense light of longer wavelengths! Explained by Planck relationship: E = hf = hc λ • Incoming photon gives part of its energy to electron. • The scattered photon then has less energy, which according to the Planck relationship means it will have a lower frequency (i.e. longer wavelength). • Experimental observations (using X-rays and a carbon target) helped to convince people of the photon nature of light. 22 Source: EPSRC 21 WHAT IS PHOTONICS? 23 Photonics as a word has similar roots to the word electronics. http://www.coltecnica.com/images/transistor.jpg Electronics: is concerned with the behaviour and control of electrons in materials (e.g. semiconductors) and devices (e.g. transistors) and circuits (e.g. microchips). It is commonly divided into analogue and digital applications in areas such as communications and computing. (ICT – Information and Communication Technology.) The versatility of the transistor and the ability to integrate large numbers of transistors into a single chip is responsible for the electronics revolution. The transistor supports many functions such as amplification, oscillation, signal mixing, active filtering and switching. 24 Moore’s law has successfully predicted both the increasing number of transistors per chip and the reduced cost per transistor: 25 26 As a consequence, electronics has led to a computing revolution: The term photonics was coined in 1967 by Pierre Aigrain, a French scientist, who gave the following definition: 'Photonics is the science of the harnessing of light. Photonics encompasses the generation of light, the detection of light, the management of light through guidance, manipulation, and amplification, and most importantly, its utilisation for the benefit of mankind.‘ Notice that no explicit mention is made of generation or control of photons. This is because in some areas of photonics we use the wave model of light instead of the photon model. http://spie.org/x39920.xml Unlike electronics, there is no single device which performs many different functions. 27 Gilder’s law has successfully predicted increases in bandwidth (bit rates) over optical fiber networks: However, the development of the laser is credited as having started the modern field of photonics. This in turn encouraged the development of optical fiber and other components for use in optical communications. 28 CISCO predicts that by 2015 we will be in the zettabyte era (i.e. global IP traffic to exceed 1 zettabyte per year). 30 29 DEFINITIONS 31 One of the problems with this field is that different words are used to describe it, for example: • Electro-optics: used for optical devices in which electrical effects play a role (e.g. lasers and modulators). • Optoelectronics: mainly used for devices that are essentially electronic in nature but involve optical effects (e.g. photodiodes, light emitting diodes). • Quantum electronics: mostly used to describe devices that rely on the interaction between light and matter (e.g. lasers) • Lightwave technology: Used to describe devices and systems used for optical communications (e.g. lasers, optical fibre, photodetectors). 32 ELECTRONICS OPTICS ELECTRO-OPTICS OPTOELECTRONICS LIGHTWAVE TECHNOLOGY QUANTUM ELECTRONICS PHOTONICS All of the above involve the interaction between the two core technologies of: • Optics –implies the use of exclusively optical components and techniques • Electronics – implies the exclusive use of electrical effects • Some people regard photonics as exclusively optical, while others also include electronic interactions. We will take this second point of view. • Although photonics has many applications, the main use to date has been for optical communications, and this is what has driven many advances in photonics devices and systems. 33 34 The key elements of this course (ECE 645) Optics Photonics • Ray optics • Optical fibre • Laser resonators • Wave optics • Electromagnetic optics • Polarization • Wave-guiding • • • • Optical fibre Lasers Photodiodes Modulators OPTICAL COMMUNICATIONS 35 The most basic optical communication link: Optical source Modulation 36 Optical communications has a long history, having been used by many civilizations. One example is the friktories of ancient Greece: Channel Optical detector This was a very early example of digital optical communications. 37 38 Bell’s photophone 1880 - Analogue optical link http://www.ec-lyon.fr/tourisme/Chappe/ Digital optics, 1793-1852: • • • • • Claude Chappe’s Optical Telegraph (France) Based on a semaphore system Repeater spacing ≈ 6 miles Message could cover 100 miles in 30 minutes Bit rate < 1 bit/s Transmitter Receiver • Light modulated by vibrating mirror • Light is photodetected using selenium (i.e. opto-mechanical) (resistance decreases with increasing light intensity) • First example of optoelectronic receiver © Alexander Graham Bell Foundation 39 40 One of the problems with these early systems was the fact that there was no guided channel between the transmitter and receiver, in other words the channel was free-space optics. For some applications, such as satelliteto-satellite free space optical links, this is not a problem. “The ordinary man will find difficulty in comprehending how sunbeams are to be used. Does Prof. Bell intend to connect Boston and Cambridge with a line of sunbeams hung on telegraph posts?” New York Times, 30 Aug. 1880. But for terrestrial free space optical communications, weather conditions have to be considered: 41 42 Kao and Hockham proposed the use of optical fibres for communications 1966 An efficient way of guiding light is essential to modern optical communications…. At this stage, however, losses are way too high (1000 dB/km for glass, as opposed to tens of dB/km at most for coaxial cable). 43 However, work at Corning in the early 1970’s led to fiber losses of 20 dB/km, and over time these have been reduced to as low as 0.2 dB/km (at 1550 nm). 107 Egyptian 106 Venetian 105 Optical Loss (dB/km) 104 Optical fibre 103 Optical glass 102 10 1 0.1 3000 BC 1000 AD 1900 1966 1979 1983 44 Fibre offers a lot of bandwidth! 45 46 Optical Fibres: Basic Structure • • Advantages of optical fibre dielectric waveguides that operate at optical wavelengths; mostly made from silica glass, but plastic versions (for multimode) also available • confine electromagnetic energy in the form of light within core and guide the light parallel to the longitudinal axis: CORE CLADDING • • • • Very wide bandwidth compared to metallic transmission lines, i.e. potentially thousands of GHz Very low loss (as low as 0.2 dB/km) Can achieve low dispersion (depends on wavelength of source and fibre type) Small size and weight Electrical isolation (glass and plastic) BUFFER COATING Not to scale! •A circular core of refractive index n1 is surrounded by cladding with a slightly lower value of refractive index (n2 < n1). The fibre is encapsulated by the buffer and additional layers as appropriate. A fiber-optic cable (right) containing 144 tiny glass fibers is compared with a cross section of a conventional copper cable. • Light is confined to the core of the fibre by total internal reflection – TIR at the core-cladding interface. 48 47 However, even though fibre itself is small in cross-section, in some applications the overall cable is not so small or light: A lot of optical fibre is installed in undersea (submarine) systems, and must be well protected. The basic ingredients of optical communications include: • coherent oscillator (i.e. laser) • mixer (e.g. directly modulated laser) • envelope detector (e.g. photodiode) However, there are other components (analogous to microwave components) that are also used: • amplifiers (Erbium-doped fibre) • couplers, combiners and splitters • wavelength selective components – filters, multiplexers • isolators and circulators 49 50 Basic architecture of an optical fibre link λ1 λ1 Laser 1 Fibre Detector 2 EDFA MUX λ3 Laser 3 λ2 DEMUX λ2 Laser 2 λ3 Detector 3 λN λN Laser N Example WDM link Detector 1 λ1, λ2, λ3 ..... λN λ1 = c/f1 λ0 λ2 = c/f2 ≈1550nm ≈1530nm ≈1570nm Detector N ∆f ≈ c ∆λ = 5 THz 51 1310 nm Fibre attenuation (dB/km) 850 nm 1550 nm 10 Three main wavelength windows λ20 52 Most fibre links are digital, and consequently we worry about bit rate – distance products and bit error rates: 1 700 900 1100 GaAlAs 1300 1500 InGaAsP Wavelength (nm) Optical sources Semiconductor optical amplifiers Optical fibre amplifiers EDFA 1.0 © G.D. Keiser Photodiode responsivity (A/W) PDFA InGaAs Si 0.5 Ge 700 900 1100 1300 1500 Wavelength (nm) 53 54 More wavelengths, higher modulation speeds Figures of merit 1000 • Using polarization multiplexing 2008 Bit-rate - repeater spacing product (bits/s - km) 2001 2003 2001 1998 100 10 Improving photonics • • The designers of a long distance high-bit rate fibre link have a number of objectives. One is to achieve as high a bit rate as possible. However, it is also important to maximise the distance between optical amplifiers or repeaters (i.e. the repeater spacing). The two figures are multiplied to give a key figure of merit used to assess link performance: Number of wavelength channels • 1998 2006 2003 1996 1995 1993 1989 1977 1995 1 1983 1986 1987 1991 Improving electronics 0.1 0.01 0.1 10 1 Data rate per channel (Gb/s) 100 1000 Total capacity 56 55 Bit rate – distance product improvements Closing Remarks • Optics is one of the oldest sciences • Photonics is some ways the successor of optics, and is a very fast moving field with many applications. • So far, the highest impact applications of photonics have been in the ICT (information communications technology) sector • Without photonics, we would have no internet • Without photonics, computing itself will start to run into problems