ECE 445 Optical Fiber Communications Lecture 01 - Introduction

University of Cyprus Πανεπιστήμιο Κύπρου 1 ECE 445 Optical Fiber Communications Lecture 01 - Introduction Stavros Iezekiel Department of Electrical and Computer Engineering University of Cyprus iezekiel@ucy.ac.cy HMY 445 Lecture 01 Fall Semester 2014

University of Cyprus Πανεπιστήμιο Κύπρου 2 The globe is spanned by multiple optical fibre cables

University of Cyprus Πανεπιστήμιο Κύπρου 3 It is no exaggeration to say that, along with computers and wireless systems, optical fibre technology has changed the world in ways we could have only dreamt of only a few decades ago.

University of Cyprus Πανεπιστήμιο Κύπρου 4 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.

University of Cyprus Πανεπιστήμιο Κύπρου 5 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.]

University of Cyprus Πανεπιστήμιο Κύπρου 6

University of Cyprus Πανεπιστήμιο Κύπρου 7 https://www.youtube.com/watch?v=ig3ic9v7cwg

University of Cyprus Πανεπιστήμιο Κύπρου 8 http://support.frontrange.com/common/files/content-explosion/index.html#intro

University of Cyprus Πανεπιστήμιο Κύπρου 9 CISCO predicts that by 2015 we will be in the zettabyte era (i.e. global IP traffic to exceed 1 zettabyte per year).

University of Cyprus Πανεπιστήμιο Κύπρου 10 Gilder s law has successfully predicted increases in bandwidth (bit rates) over optical fiber networks:

University of Cyprus Πανεπιστήμιο Κύπρου 11 The aim of this course is to study optical fibre technology and its application to optical communication links and systems The basic questions we will seek to answer in the first few lectures include: 1. What are the advantages of using photonics for communications? 2. What is optical fibre? 3. What is an optical communications link? (Basic architecture) 4. How does light propagate in an optical fibre?

http://www.arthitectural.com/wp-content/uploads/2013/04/02-table-of-opticks-sir-isaac-newton-1704.jpg University of Cyprus Πανεπιστήμιο Κύπρου 12 WHAT IS LIGHT?

University of Cyprus Πανεπιστήμιο Κύπρου 13 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.: Refraction (ray optics) Interference (wave optics) Polarisation (electromagnetic optics)

University of Cyprus Πανεπιστήμιο Κύπρου 14 By the late 19 th 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): Solution is a travelling-wave: c = speed of light = 2.998 10-8 ms -1 in vacuo c f k 2 2 T

University of Cyprus Πανεπιστήμιο Κύπρου 15 However, the development of modern physics (and especially the work of Planck and Einstein) led to the photon view of light. Energy of a photon: E hf hc h = Planck s constant = 6.626 10-34 J s

University of Cyprus Πανεπιστήμιο Κύπρου 16 Light as photons: Photoelectric effect Increasing the intensity of the light increases the number of photoelectrons, but not maximum kinetic energy. their 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

University of Cyprus Πανεπιστήμιο Κύπρου 17

Longwave Mediumwave Shortwave Ultrashortwave Extremely ultrashortwave Microwave Mm-wave Sub-Mm-wave Far-infrared Mid-infrared Visible Ultraviolet X-ray 1 km 1 cm 1 mm 1 m 1 nm University of Cyprus Πανεπιστήμιο Κύπρου 18 Nowadays, most electronic communication (e.g. wireless) is in the microwave region. Typically there is a three orders of magnitude difference between microwaves and photonics Wavelength (m) 10 4 10 3 10 2 10 1 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 Electronic techniques Optics Microwaves THz Gap Photonics 30 khz 30 MHz 3 GHz 30 GHz 3 THz 3 PHz Frequency

University of Cyprus Πανεπιστήμιο Κύπρου 19 OPTICAL COMMUNICATIONS

University of Cyprus Πανεπιστήμιο Κύπρου 20 The most basic optical communication link: Optical source Modulation Channel Optical detector

University of Cyprus Πανεπιστήμιο Κύπρου 21 Optical communications has a long history, having been used by many civilizations. One example is the friktories of ancient Greece: This was a very early example of digital optical communications.

http://www.ec-lyon.fr/tourisme/chappe/ University of Cyprus Πανεπιστήμιο Κύπρου 22 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

University of Cyprus Πανεπιστήμιο Κύπρου 23 Bell s photophone 1880 - Analogue optical link Transmitter Light modulated by vibrating mirror (i.e. opto-mechanical) Receiver Light is photodetected using selenium (resistance decreases with increasing light intensity) First example of optoelectronic receiver Alexander Graham Bell Foundation

University of Cyprus Πανεπιστήμιο Κύπρου 24 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. 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.

University of Cyprus Πανεπιστήμιο Κύπρου 25 For some applications, such as satelliteto-satellite free space optical links, this is not a problem. But for terrestrial free space optical communications, weather conditions have to be considered:

University of Cyprus Πανεπιστήμιο Κύπρου 26 An efficient way of guiding light is essential to modern optical communications.

University of Cyprus Πανεπιστήμιο Κύπρου 27 Kao and Hockham proposed the use of optical fibres for communications - 1966 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).

University of Cyprus Πανεπιστήμιο Κύπρου 28 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). 10 7 Egyptian 10 6 Venetian Optical Loss (db/km) 10 5 10 4 10 3 Optical glass Optical fibre 10 2 10 1 0.1 3000 BC 1000 AD 1900 1966 1979 1983

University of Cyprus Πανεπιστήμιο Κύπρου 29 Fibre offers a lot of bandwidth!

University of Cyprus Πανεπιστήμιο Κύπρου 30 Optical Fibres: Basic Structure 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 BUFFER COATING Not to scale! A circular core of refractive index n 1 is surrounded by cladding with a slightly lower value of refractive index (n 2 < n 1 ). The fibre is encapsulated by the buffer and additional layers as appropriate. Light is confined to the core of the fibre by total internal reflection TIR at the core-cladding interface.

University of Cyprus Πανεπιστήμιο Κύπρου 31 Advantages of optical fibre 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) A fiber-optic cable (right) containing 144 tiny glass fibers is compared with a cross section of a conventional copper cable.

University of Cyprus Πανεπιστήμιο Κύπρου 32 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.

University of Cyprus Πανεπιστήμιο Κύπρου 33 The basic ingredients of a classical communications link include: coherent oscillator (i.e. laser) mixer (e.g. directly modulated laser) envelope detector (e.g. photodiode) However, there are other components (analogous to electronic components) that are also used in optical communications: amplifiers (Erbium-doped fibre) couplers, combiners and splitters wavelength selective components filters, multiplexers isolators and circulators

University of Cyprus Πανεπιστήμιο Κύπρου 34 Basic architecture of an optical fibre link

MUX DEMUX University of Cyprus Πανεπιστήμιο Κύπρου 35 Laser 1 1 1, 2, 3... N 1 Detector 1 Laser 2 2 Fibre EDFA 2 Detector 2 Laser 3 3 3 Detector 3 Laser N N N Detector N Example WDM link 1 = c/f 1 1530nm 0 1550nm 2 = c/f 2 1570nm f c 5 THz 2 0

Photodiode responsivity (A/W) Fibre attenuation (db/km) University of Cyprus Πανεπιστήμιο Κύπρου 36 10 850 nm 1310 nm 1550 nm Three main wavelength windows 1 700 900 1100 1300 1500 GaAlAs InGaAsP Wavelength (nm) Optical sources Semiconductor optical amplifiers 1.0 0.5 Si PDFA EDFA InGaAs Ge Optical fibre amplifiers 700 900 1100 1300 1500 Wavelength (nm)

G.D. Keiser University of Cyprus Πανεπιστήμιο Κύπρου 37 Most fibre links are digital, and consequently we worry about bit rate distance products and bit error rates:

University of Cyprus Πανεπιστήμιο Κύπρου 38 Figures of merit 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: Bit-rate - repeater spacing product (bits/s - km)

Number of wavelength channels Improving photonics University of Cyprus Πανεπιστήμιο Κύπρου 39 1000 100 10 More wavelengths, higher modulation speeds 1996 Using polarization multiplexing 1995 1998 2001 2003 2001 1998 2003 2008 2006 1 1977 1983 1989 1993 1986 1987 1991 1995 Improving electronics 0.1 0.01 0.1 1 10 100 1000 Data rate per channel (Gb/s) Total capacity

University of Cyprus Πανεπιστήμιο Κύπρου 40 Bit rate distance product improvements

University of Cyprus Πανεπιστήμιο Κύπρου 41 TYPES OF OPTICAL FIBRE

University of Cyprus Πανεπιστήμιο Κύπρου Εγκάρσια τομή οπτικής ίνας Μονότροπη Single-mode Πολύτροπη Multimode n c v Refractive index of material In which speed of light is v Ανθρώπινη τρίχα

University of Cyprus Πανεπιστήμιο Κύπρου Χαρακτηριστικά μεγέθη Γεωμετρικά χαρακτηριστικά a b Ακτίνα πυρήνα Ακτίνα ντύματος Κανονικοποιημένη μεταβολή δείκτη διάθλασης n n 1 n 1 2 Αριθμητικό άνοιγμα NA n n Numerical aperture 2 2 1 2

University of Cyprus Πανεπιστήμιο Κύπρου Ίνα βηματικού δείκτη διάθλασης Ακτινική μεταβολή δείκτη διάθλασης nr () n1 r a n2 r a Τυπικές τιμές = 1-1% n =1.44-1.46

University of Cyprus Πανεπιστήμιο Κύπρου Ίνα βαθμιαίου δείκτη διάθλασης Ακτινική μεταβολή δείκτη διάθλασης x r n1 1 r a a nr () n2 r a

University of Cyprus Πανεπιστήμιο Κύπρου 46 Διάδοση σε μια ιδανική ίνα με βηματικό δείκτη διάθλασης Air n 0 Διαθλασμένη ακτίνα n 2 Cladding n 1 > n 2 > n 0 Ανακλώμενη ακτίνα 0 Core n 1 n 2 Cladding Δεν διαδίδεται. < κρίσιμη γωνία στη διεπαφή πυρήνας-επένδυσης Δίδεται με συνεχείς ολικές ανακλάσεις (TIR) στην διεπαφή πυρήναςεπένδυσης. 0 είναι η γωνία αποδοχής (acceptance angle).

University of Cyprus Πανεπιστήμιο Κύπρου 47 Αριθμητικό άνοιγμα (Numerical aperture - NA) σε μια ίνα με βηματικό δείκτη διάθλασης Ποια είναι η μέγιστη γωνία αποδοχής; (maximum acceptance angle 0 ) επένδυση n 2 n αέρας n 0 sin 0 1 sin 0 n C n1 sin C n 2 0 C 2 πυρήνας n 1

University of Cyprus Πανεπιστήμιο Κύπρου 48 2 2 2 1 2 1 2 1 2 1 1 1 1 0 0 1 sin 1 cos ) 2 ( sin sin sin n n n n n n n n n n C C C Αυτή η εξίσωση μας δίνει το Αριθμητικό Άνοιγμα (numerical aperture NA) σε μια οπτική ίνα βηματικού δείκτη διάθλασης: C 0 επένδυση n2 2 2 2 1 0 0 sin NA n n n 1 sin n 2 n C 2 C πυρήνας n 1

University of Cyprus Πανεπιστήμιο Κύπρου 49 Refractive index profile n 2 n 1 Fibre cross sections & ray paths Typical dimensions n Step-index single mode 125 μm cladding 8-12 μm core n 2 n 1 n Step-index multimode 125-400 μm cladding 50-200 μm core n 2 n 1 n Graded index multimode 125 140 μm cladding 50-100 μm core

University of Cyprus Πανεπιστήμιο Κύπρου 50 Dispersion in multimode fibres Consider a multimode fibre with a step-index profile. Light lauched into the fibre will propagate on different modes (different ray paths). n 2 n n 1 Each ray has the same speed, but as we go along the fibre we see that different rays cover different distances. Hence some rays will exit the fibre before others, which leads to delays between different modes. We refer to this type of dispersion as intermodal dispersion, although some people also use the term multimode dispersion.

University of Cyprus Πανεπιστήμιο Κύπρου If we look at the following scenario in which a single pulse of light is launched into the fibre, this will excite various modes. These then exit the fibre with a spread of delays: 51 input t + + = t output t Over a sufficiently long length of fibre, we may end up with pulses merging into each other (intersymbol intereference) which can then lead to bit errors:

University of Cyprus Πανεπιστήμιο Κύπρου 52 We can minimise multimode (intermodal) dispersion by using graded-index fibre: O 3 2 1 n 2 n 1 n Step-index MM-fibre All rays have different path lengths, and travel at same speed, leading to transit delay differences. O O ' O '' 3 2 1 2 3 n 2 n 2 n 1 n Graded-index MM-fibre Rays still have different path lengths, but their speed varies as they encounter different refractive index values. The profile is designed to equalise the transit time for the modes so that they exit at roughly the same time.

University of Cyprus Πανεπιστήμιο Κύπρου 53 Dispersion limited B T L product for optical fibre 1000 Single-mode 1.3 μm 100 Graded-index 1.55 μm 10 Step-index 1 0.1 1 10 100 Bit rate B T (Mb/s) 1000 10,000 After Saleh & Teich, Fundamentals of Photonics, Wiley

University of Cyprus Πανεπιστήμιο Κύπρου 54 Closing Remarks Optical fibre communications is one of the key technologies in ECE Over the next few weeks we will concentrate on two key aspects: Optical fibre components Optical fibre systems