Lecture 1
Overview of Photonics and Optical Fiber Communications
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What is Photonics?
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Moti Motiva vati tion onss for for Ligh Lightw twav avee Com Commu muni nica cati tion onss
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Adva Advant ntag ages es of Opti Optica call Fib Fiber er Comm Commun unic icat atio ions ns
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Optical Spectral Bands
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Decibel Units
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Network In Information Ra Rates
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Wave Wa vele leng ngth th-D -Div ivis isio ion n Mult Multip iple lexi xing ng (W (WDM DM)) Conc Concep epts ts
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Stan Standa dard rdss for for Opti Optica call Fib Fiber er Comm Commun unic icat atiions ons
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Historical Development
Readin Reading: g:
Keiser Keiser 1.1 – 1.8 Seni Senior or 1.1 1.1 – 1.3 1.3 Article: A Snapshot of Optical Communica Communications, tions, OPN Optics & Photonics Photonics News, pp. 24 – 30, Jan. 2010 Part of the lecture materials were adopted from powerpoint slides of Gerd Keiser’s book 2010,
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Fiber-optic communications and modern society • The The rec recen entt awa award rd of of the the Nob Nobel el Pri Prize ze in Phy Physi sics cs 2009 to Prof. Prof. Charles Charles Kao Kao – widely regarded regarded as the “father “father of of fiber-optic fiber-optic communicati communications” ons” – underscores the tremendous changes that optical fiber has brought about in modern society. • Fibe Fiberr opt optic icss has has revo revolu luti tion oniz ized ed the the way way we rece receiv ivee information and communicate with one another, and it has played a major role in ushering in the Information Age. 2
Fiber-optic communications and modern society • The The rec recen entt awa award rd of of the the Nob Nobel el Pri Prize ze in Phy Physi sics cs 2009 to Prof. Prof. Charles Charles Kao Kao – widely regarded regarded as the “father “father of of fiber-optic fiber-optic communicati communications” ons” – underscores the tremendous changes that optical fiber has brought about in modern society. • Fibe Fiberr opt optic icss has has revo revolu luti tion oniz ized ed the the way way we rece receiv ivee information and communicate with one another, and it has played a major role in ushering in the Information Age. 2
Optics and Photonics • Optics – the scienc ncee of light (e.g. physical optics, nonlinear optics, quantum optics, nano-optics) • Photonics Photonics – the technology technology using using light light (“photons”) (“photons”) and electrons 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” “optoelectronics” to differentiate those those technologies using photons and electrons (e.g. light-emitting diodes) from those technologies using using only photons photons (e.g. optical fibers). But this distinction has been falling out of favor in recent years and the term “photoni “photonics” cs” become become commonl commonlyy adopt adopted. ed. 3
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
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!
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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, pico projectors 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 “
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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
Optical transmitter
electrical
information
Communications Channel (Opt. fibers)
Optical
Optical receiver
Electrical signal
electrical
information 7
Enabling photonic components for communications • Laser diodes • Modulators • Optical fibers • Optical amplifiers • Wavelength-Division Multiplexing (WDM) components • Photodetectors 8
Laser modules in communications
These modern laser modules incorporate a wavelength tunable laser with a semiconductor optical amplifier on a III-V semiconductor compound indium phosphide (InP) chip.
Ref. Lasers in Communications, Patricia Daukantas, pp. 28-33, March 2010 9
Active Optical Cables
Ref. Lasers in Communications, Patricia Daukantas, pp. 28-33, 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 10 cables, thus making active optical cables.
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. 11
Optical communications for computing
Optical interconnect technology is motivating the development 12 of the R&D field of “silicon photonics.”
Optical interconnects Electrical interconnects (Copper): Resistance-capacitance (RC) delay Power consumption Bandwidth limitation (~5 GHz)
2002 2007
2012
Optical interconnects
2017+
High bandwidth (> 40 Gb/s)
Relatively low power consumption
Wavelength-division multiplexing
(WDM) 13
Enabling components for on-chip optical communications
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Intel optical cables
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Photonics for data storage
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(Nano) Photonics on CD/DVD/Blu-ray disks
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Nanophotonics in nature
• Nature pulls off spectacular optical filters using nanoscale structures: butterflies, moths, beetles, birds, fish, etc. 18
Photonics for human-machine interface: pico-projectors
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Photonics for medicine Lasers in ophthalmology (laser surgery)
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Photonics for defense Laser weapons (?)
Ref. A popular history of the laser, Stephen R. Wilk, OPN Optics & Photonics News, pp. 14-15, March 2010
Ref. Half a century of laser weapons, Jeff Hecht, OPN Optics & Photonics News, pp. 14-21, Feb. 2009 21
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
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 –
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There is an extremely large choice of remotely accessible programs and information databases 23
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.
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Large Information Capacity: Fibers have wider bandwidths than copper wires, so that more information can be sent over a single physical line.
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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.
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Immunity to Electrical Interference: The dielectric nature of optical fibers makes them immune to the electromagnetic interference effects.
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Enhanced Safety: Optical fibers do not have the problems of ground loops, sparks, and potentially high voltages inherent in copper lines.
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Increased Signal Security: An signal is well-confined within the fiber and an opaque coating around the fiber absorbs any signal emissions. 24
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
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
Electromagnetic spectrum lightwave
visible radio
Frequency (Hz)
microwave
106 107
infrared
108 109 1010 1011 1012 1013 1014
Wavelength (m) 100 10
1
ultraviolet
1016 1017
10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 700 nm
400 nm
(10-6 m = 1 μm; 10-9 m = 1 nm) *In optics and photonics, due to conventions, wavelength unit (nm or μm) is often adopted. 27
Lightwave spectrum
λ
visible
c x
Near-IR
UV
wavelength (nm) 400 700 1000
2000
frequency × wavelength = speed of light In free space (i.e. vacuum or air)
υ λ = c = 3 × 108 m/s
e.g. λ = 1 μm = 1000 nm = 10-6 m, υ = 3 × 1014 Hz = 300 × 1012 Hz = 300 THz Optical carrier frequency ~ 100 THz, which is five orders of magnitude larger than microwave carrier frequency of GHz. 28
• Optical fiber communications systems use lightwave in the near-infrared . Early systems (1980’s); also modern short-distance networks using polymer optical fibers
850 nm
From 1990’s to present networks (long-haul/metro/access)
1300-nm band
1550-nm band
λ (nm) 800
900
1000 1100 1200 1300 1400 1500 1600
• Most optical fiber communications systems now use the silica glass fiber lowestloss window which is around ~ 1550 nm. 29
Optical Spectral Bands for fiber‐ optic communications O-Band
1260
E-Band
1360
S-Band
1460
C-Band
1530
L-Band
1565
U-Band
1625
1675
Wavelength (nm)
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Original band (O‐band): 1260 to 1360 nm –
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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
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Short band (S‐band): 1460 to 1530 nm (shorter than C‐band)
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Conventional band (C‐band): 1530 to 1565 nm (EDFA region)
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Long band (L‐band): 1565 to 1625 nm (longer than C‐band)
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Ultra‐long band (U‐band): 1625 to 1675 nm
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Silica optical fiber loss spectrum The Internet are carried in here.
~0.2 dB/km attenuation
31 Today: 10% of the light remains after more than 50 km of fiber
Decibel Units •
The decibel (dB) unit is defined by
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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
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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
Decibel Units
Power levels differing by many orders of magnitude can be compared easily when they are in decibel form. 35
Network Information Rates
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A standard signal format called synchronous optical network (SONET) is used in North America
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A standard signal format called synchronous digital hierarchy (SDH) is used in other parts of the world
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Lightwave channel within the fiber low-loss window
1500
1550
1600
(nm)
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
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 •
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Wavelength-Division Multiplexing (WDM) • WDM combines 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 ]
λ1
λ1
λ2
λ2
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.
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n WDM channels
1500
1550
1600
(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 25 × 40 Gb/s) 40
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
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.
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Component testing standards define tests for fiber‐optic component performance and establish equipment‐calibration procedures. –
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The main ones are Fiber Optic Test Procedures (FOTP)
System standards refer to measurement methods for optical links and networks. 42
Historical development • A renewed interest in optical communications was stimulated in the early 1960s with the invention of the laser in 1960. • 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 43 carrier.
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), 1151-1158, 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-1. 44
The beginnings of lightwave technology 1960
T. Maiman: Invention of Ruby laser, the 1st working laser, 694.3 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
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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
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
