Lecture 1 Overview of Photonics and Optical Fiber Communications

Transcription

1 Lecture 1 Overview of Photonics and Optical Fiber Communications What is Photonics? Motivations for Lightwave Communications Advantages of Optical Fiber Communications Optical Spectral Bands Decibel Units Network Information Rates Wavelength-Division Multiplexing (WDM) Concepts Standards for Optical Fiber Communications Historical Development Reading: Keiser Senior Article: A Snapshot of Optical Communications, OPN Optics & Photonics News, pp , Jan Part of the lecture materials were adopted from powerpoint slides of Gerd Keiser s book 2010, Copyright The McGraw-Hill Companies, Inc. 1

2 Fiber-optic communications and modern society The recent award of the Nobel Prize in Physics 2009 to Prof. Charles Kao widely regarded as the father of fiber-optic communications underscores the tremendous changes that optical fiber has brought about in modern society. Fiber optics has revolutionized the way we receive information and communicate with one another, and it has played a major role in ushering in the Information Age. Source: OPN, pp Jan

3 Optics and Photonics Optics the science of light (e.g. physical optics, nonlinear optics, quantum optics, nano-optics) Photonics the technology using light ( photons ) and electrons (e.g. optical fiber communications, light-emitting diodes, laser diodes, photodetectors, photovoltaic devices, optical switches, optical modulators, displays, etc.) In the past, people used the term optoelectronics to differentiate those technologies using photons and electrons (e.g. light-emitting diodes) from those technologies using only photons (e.g. optical fibers). But this distinction has been falling out of favor in recent years and the term photonics become commonly adopted. 3

4 What is photonics? Photonics is analogous to electronics. What is electronics? Electronics is the study of the flow of charge (electron) through various materials and devices such as, semiconductors, resistors, inductors, capacitors, nano-structures, etc. All applications of electronics involve the transmission of power and possibly information. 4

5 What is photonics? Photonics is the technology of generating / controlling / detecting light and other forms of radiant energy whose quantum unit is the photon. (In physics, a quantum is the minimum unit of any physical entity involved in an interaction. The word comes from the Latin quantus for how much. ) The science includes light emission, transmission, deflection, amplification, detection nonlinear optics The importance of photonics often derives from the powerful interplay between optics and electronics! 5

6 A snapshot of photonic technologies Communications --- fiber-optic communications, optical interconnects, optical wireless Computing --- chip-to-chip optical interconnects, on-chip optical interconnect communications Energy ( Green photonics ) --- solid-state lighting, solar cells Human-Machine interface --- CCD/CMOS camera, displays, picoprojectors Medicine --- laser surgery, optical coherence tomography (OCT) Bio --- optical tweezers, laser-based diagnostics of cells/tissues Nano --- integrated photonics, sub-diffraction-limited optical microscopy, optical nanolithography Defense --- laser weapons, bio-aerosols monitoring Sensing --- fiber sensors, bio-sensing, LIDAR Data Storage --- CD/DVD/Blu-ray, holography Manufacturing --- laser-based drilling and cutting Fundamental Science --- femto-/atto-second (10-15 /10-18 s) science Space Science --- adaptive optics, laser-based interferometers between satellites Entertainment --- laser shows 6 And many more!!

7 Photonics for communications An optical communications system consists of many components. ELEC 4620 will provide an overview and the fundamentals of some of the photonic technologies involved. Electrical signal Communications Channel (Opt. fibers) Optical transmitter Optical receiver Electrical signal electrical Optical electrical information information 7

8 Enabling photonic components for communications Laser diodes Modulators Optical fibers Optical amplifiers Wavelength-Division Multiplexing (WDM) components Photodetectors 8

9 Laser modules in communications These modern laser modules incorporate a wavelengthtunable laser with a semiconductor optical amplifier on a III-V semiconductor compound indium phosphide (InP) chip. Ref. Lasers in Communications, Patricia Daukantas, pp , March

10 Active Optical Cables Ref. Lasers in Communications, Patricia Daukantas, pp , March 2010 Datacom companies are making networking even easier for data-center companies by attaching optical transceivers (transmitters + receivers) permanently to the ends of fiber cables, thus making active optical cables. 10

11 Various types of optical networks Access networks have garnered new interest because of the growing demand for fiber-to-the-home and high-definition video. Ref. Lasers in Communications, Patricia Daukantas, pp , March

12 Optical communications for computing Optical interconnect technology is motivating the development of the R&D field of silicon photonics. 12

13 Optical interconnects Electrical interconnects (Copper): Resistance-capacitance (RC) delay Power consumption Bandwidth limitation (~5 GHz) Optical interconnects High bandwidth (> 40 Gb/s) Relatively low power consumption Wavelength-division multiplexing (WDM) N. Savage, IEEE Spectrum, pp August

14 Enabling components for on-chip optical communications Source: Intel 14

15 Intel optical cables Source: Intel Light Peak 15

16 Photonics for data storage 16

17 (Nano) Photonics on CD/DVD/Blu-ray disks 17

18 Nanophotonics in nature Nature pulls off spectacular optical filters using nanoscale structures: butterflies, moths, beetles, birds, fish, etc. Ref. Optical filters in nature, OPN Optics & Photonics News, pp , Feb

19 Photonics for human-machine interface: pico-projectors Ref. Scanned laser pico-projectors, OPN Optics & Photonics News, pp , May

20 Photonics for medicine Lasers in ophthalmology (laser surgery) Ref. Lasers in ophthalmology, OPN Optics & Photonics News, pp , Feb

21 Photonics for defense Laser weapons (?) Ref. A popular history of the laser, Stephen R. Wilk, OPN Optics & Photonics News, pp , March 2010 Ref. Half a century of laser weapons, Jeff Hecht, OPN Optics & Photonics News, pp , Feb

22 Communications system An optical fiber communications system is similar in basic concept to any type of communications system. The basic function is to convey the signal from the information source over the transmission medium to the destination. The communication system consists of a transmitter or modulator linked to the information source, the transmission medium, and a receiver or demodulator at the destination point. 22

23 Motivations for high speed communications Lifestyle changes from the Internet growth and use Average phone call lasts 3 minutes Average Internet session is 20 minutes More and more bandwidth hungry services are appearing Web searching, home shopping, high definition interactive video, remote education, telemedicine and e health, high resolution editing of home videos, blogging, and large scale high capacity e science and Grid computing Increase in PC storage capacity and processing power 20G hard drives were fine around 2000; now standard is 160G Laptops ran at 300 MHz; now the speed is over 3 GHz There is an extremely large choice of remotely accessible programs and information databases 23

24 Motivations for fiber optic communications Advantages of optical fibers Long Distance Transmission: The lower transmission losses in fibers compared to copper wires allow data to be sent over longer distances. Large Information Capacity: Fibers have wider bandwidths than copper wires, so that more information can be sent over a single physical line. Small Size and Low Weight: The low weight and the small dimensions of fibers offer a distinct advantage over heavy, bulky wire cables in crowded underground city ducts or in ceiling-mounted cable trays. Immunity to Electrical Interference: The dielectric nature of optical fibers makes them immune to the electromagnetic interference effects. Enhanced Safety: Optical fibers do not have the problems of ground loops, sparks, and potentially high voltages inherent in copper lines. Increased Signal Security: An signal is well-confined within the fiber and an opaque coating around the fiber absorbs any signal emissions. 24

25 Carrier Information Capacity In communications systems, the data are transferred over the communication channel by superimposing the information onto an electromagnetic wave, known as the carrier. As the amount of information that can be transmitted is directly related to the frequency range of the carrier, increasing the carrier frequency theoretically increases the available transmission bandwidth, and thus provides a larger information capacity. The trend in communications system developments was to employ progressively higher frequencies, which offer corresponding increases in bandwidth or information capacity (from radio frequencies, microwave and millimeter wave frequencies, to optical range) 25

26 Communication systems applications in the electromagnetic spectrum Freq. (khz) The increase in carrier frequency led to the development of radio, TV, radar, and microwave links (now in 2-5 GHz frequency). 26

27 Frequency (Hz) Electromagnetic spectrum radio microwave infrared lightwave visible ultraviolet Wavelength (m) (10-6 m = 1 μm; 10-9 m = 1 nm) 700 nm 400 nm *In optics and photonics, due to conventions, wavelength unit (nm or μm) is often adopted. 27

28 Lightwave spectrum visible λ c UV Near-IR x wavelength (nm) frequency wavelength = speed of light In free space (i.e. vacuum or air) υλ = c = m/s e.g. λ = 1 μm = 1000 nm = 10-6 m, υ = Hz = Hz = 300 THz Optical carrier frequency ~ 100 THz, which is five orders of magnitude larger than microwave carrier frequency of GHz. 28

29 Optical fiber communications systems use lightwave in the near-infrared. 850 nm Early systems (1980 s); also modern short-distance networks using polymer optical fibers From 1990 s to present networks (long-haul/metro/access) 1300-nm band 1550-nm band λ(nm) Most optical fiber communications systems now use the silica glass fiber lowestloss window which is around ~ 1550 nm. 29

30 Optical Spectral Bands for fiberoptic communications O-Band E-Band S-Band C-Band L-Band U-Band Wavelength (nm) Original band (O band): 1260 to 1360 nm Region originally used for first single mode fibers Extended band (E band): 1360 to 1460 nm Operation extends into the high loss water peak region Short band (S band): 1460 to 1530 nm (shorter than C band) Conventional band (C band): 1530 to 1565 nm (EDFA region) Long band (L band): 1565 to 1625 nm (longer than C band) Ultra long band (U band): 1625 to 1675 nm 30

31 Silica optical fiber loss spectrum The Internet are carried in here. ~0.2 db/km attenuation Today: 10% of the light remains after more than 50 km of fiber 31

32 Decibel Units The decibel (db) unit is defined by 32

33 Decibel Units (2) The decibel is used to refer to ratios or relative units. It gives no indication of the absolute power level. A derived unit called the dbm can be used for this purpose. This unit expresses the power level P as a logarithmic ratio of P referred to 1 mw. The power in dbm is an absolute value defined by 33

34 Decibel Units A rule-of-thumb relationship to remember for optical fiber communications is 0 dbm = 1 mw. Therefore, positive values of dbm are greater than 1 mw and negative values are less than 1 mw. 34

35 Decibel Units Power levels differing by many orders of magnitude can be compared easily when they are in decibel form. 35

36 Network Information Rates A standard signal format called synchronous optical network (SONET) is used in North America A standard signal format called synchronous digital hierarchy (SDH) is used in other parts of the world 36

37 Lightwave channel within the fiber low-loss window λ (nm) 1550 fiber low-loss window Current systems can transmit a single lightwave channel at a data rate of 10 Gb/s or 40 Gb/s 37

38 Wavelength Division Multiplexing Concepts Many independent information bearing signals are sent along a fiber simultaneously Independent signals are carried on different wavelengths Data rates or formats on each wavelength may be different Coarse WDM (CWDM) and dense WDM (DWDM) are the two major wavelength multiplexing techniques Wavelength routing and switching techniques based on lightpaths are being developed 38

39 Wavelength-Division Multiplexing (WDM) WDMcombines or multiplexes multiple optical signals into a single fiber by transmitting each signal on a different wavelength λ. [analogous to Frequency-Division Multiplexing (FDM) in radio communications] λ 1 λ 1 λ 2 λ 2 single optical fiber λ N λ N Telecommunication carriers can potentially multiply the capacity of their fibers by WDM, without the expensive investment of laying extra fibers underground or undersea. 39

40 n WDM channels λ (nm) If each channel has a capacity or data rate of 10 Gb/s (40 Gb/s), then the capacity of an n-channel WDM system has a capacity n 10 Gb/s (n 40 Gb/s)!! WDM systems have n: 4, 8, 16, 32, 64 or more (1 Tb/s accumulated system capacity can be achieved by Gb/s) 40

41 WDM optical links (Scientific American, Jan 2001) Lightwave networks combine, amplify, switch, and restore optical signals without converting the optical signal to an electronic signal for processing. 41

42 Standards The three basic classes for fiber optics are primary standards, component testing standards, and system standards. Primary standards deal with physical parameters: attenuation, bandwidth, operational characteristics of fibers, and optical power levels and spectral widths. Component testing standards define tests for fiber optic component performance and establish equipment calibration procedures. The main ones are Fiber Optic Test Procedures (FOTP) System standards refer to measurement methods for optical links and networks. 42

43 Historical development A renewed interest in optical communications was stimulated in the early 1960s with the invention of the laser in Laser provides a coherent light source and the possibility of modulation at high frequency. The low beam divergence of the laser made free-space optical transmission a possibility. However, the light transmission constraints in the atmosphere still restrict such systems to short-distance applications. Some modest free-space optical communication links have been implemented for applications such as the linking of a television camera to a base vehicle and for data links of a few hundred meters between buildings. The invention of the laser stimulated a tremendous research effort into the study of optical components to attain reliable information transfer using a lightwave carrier. 43

44 The fiber proposal The proposal for optical communications via dielectric waveguides or optical fibers fabricated from glass to avoid degradation of the optical signal by the atmosphere was made in 1966 by Kao and Hockham (Kao and Hockham, Dielectric fiber surface waveguides for optical frequencies, Proc. IEE, 113(7), , 1966.) Such systems were viewed as a replacement for coaxial cable transmission systems. Initially the optical fibers exhibited very high attenuation (1000 db km -1 or 1 db m -1 ). The coaxial cables loss was 5 10 db km -1. Within 10 years optical fiber losses were reduced to below 5 db km

45 The beginnings of lightwave technology 1960 T. Maiman: Invention of Ruby laser, the 1 st working laser, nm, pulsed mode operation 1966 Kao: Identifying the key problem (glass attenuation) for optical fiber communications 1970 Corning pulled the first low-loss glass fiber that satisfied the required fiber attenuation 1970 Demonstration of room-temperature operation of semiconductor lasers 45

46 The era of commercial lightwave transmission systems 1980s The first generation of fiber-optic communication systems operated at a bit rate of 45 Mb/s and required signal regeneration every ~10 km. 1990s Bit rate increased to 10 Gb/s, allowed regeneration after ~80 km Development and commercialization of erbium-doped fiber amplifiers (EDFA), fiber Bragg gratings, and wavelengthdivision-multiplexed (WDM) lightwave systems 2000s Capacity of commercial terrestrial systems exceeded 1.6 Tb/s A single transpacific system bit rate exceeded 1 Tb/s over a distance of 10,000 km without any signal regeneration 46

47 70 s Optical fibers + semiconductor lasers 80 s Low data rate, single channel 90 s High data rate, multiple channels (Optical amplifiers (EDFA) + WDM) 00 s Enabling components for sophisticated reconfigurable optical networks 10 s Optical interconnects for next-generation computercom? 47

48 Current optical fiber communications capabilities Bit rate: single channel 10 Gbit/s (many upgraded to 40 Gbit/s); system bit rate exceeding 1 Tb/s Distance: ~80 km without amplification Transmission medium: silica singlemode fiber Operation wavelengths: 1550 nm/1310 nm windows Optical sources: semiconductor laser diodes / light emitting diodes Optical amplification: fiber-based optical amplifiers (erbium-doped fiber amplifiers, Raman fiber amplifiers) The bandwidth made possible by optical fiber communications has made the Internet economically feasible. 48