Overview of Optical Overview of Optical Fiber CommunicationsFiber Communications

2Fiber-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.

3 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 thosetechnologies 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.

Optics and Photonics

4What is photonics?

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.

What is electronics?What is electronics?

Photonics is analogous to electronics. Photonics is analogous to electronics.

5What is photonics? PhotonicsPhotonics 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!

6A 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 And many more!!

7 An optical communications system consists of many components.

.

information information

Electricalsignal

Opticaltransmitter

Opticalreceiver

Electricalsignal

Opticalelectrical electrical

Communications Channel

(Opt. fibers)

Photonics for communications

8Enabling photonic components for communications

Optical fibers

Wavelength-Division Multiplexing (WDM) components

Laser diodes

Modulators

Photodetectors

Optical amplifiers

9Laser 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

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Active Optical Cables

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.

Ref. Lasers in Communications, Patricia Daukantas, pp. 28-33, March 2010

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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. 28-33, March 2010

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Optical interconnect technology is motivating the development of the R&D field of silicon photonics.

Optical communications for computing

13N. Savage, IEEE Spectrum, pp. 32- 36 August 2002.

Electrical interconnects (Copper):

Resistance-capacitance (RC) delay

Power consumption Bandwidth limitation (~5 GHz)

Optical interconnects

High bandwidth (> 40 Gb/s) Relatively low power consumptionWavelength-division multiplexing (WDM)

2002

2007

2012

2017+

Optical interconnects

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Enabling components for on-chip optical communications

Source: Intel

Intel optical cables

15Source: Intel Light Peak

16

Photonics for data storage

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

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Nanophotonics in nature

Ref. Optical filters in nature, OPN Optics & Photonics News, pp. 22-27, Feb. 2009

Nature pulls off spectacular optical filters using nanoscale structures:butterflies, moths, beetles, birds, fish, etc.

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Photonics for human-machine interface: pico-projectors

Ref. Scanned laser pico-projectors, OPN Optics & Photonics News, pp. 28-34, May 2009

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Photonics for medicine

Ref. Lasers in ophthalmology, OPN Optics & Photonics News, pp. 28-33, Feb. 2010

Lasers in ophthalmology (laser surgery)

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Photonics for defense

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

Laser weapons (?)

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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.

Motivationsforhighspeedcommunications

LifestylechangesfromtheInternetgrowthanduse Averagephonecalllasts3minutes AverageInternetsessionis20minutes

Moreandmorebandwidthhungryservicesareappearing Websearching,homeshopping,highdefinitioninteractivevideo,

remoteeducation,telemedicineandehealth,highresolutioneditingofhomevideos,blogging,andlargescalehighcapacityescienceandGridcomputing

IncreaseinPCstoragecapacityandprocessingpower 20Gharddriveswerefinearound2000;nowstandardis160G Laptopsranat300MHz;nowthespeedisover3GHz

Thereisanextremelylargechoiceofremotelyaccessibleprogramsandinformationdatabases

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Motivationsforfiberopticcommunications

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.

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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)

Carrier Information Capacity

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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).

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Electromagnetic spectrum

Frequency(Hz) 106 107 108 109 1010 1011

radio microwave

1012 1013 1014 1016 1017

infrared ultraviolet

Wavelength(m) 100 10 1 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9

lightwave

(10-6 m = 1 m; 10-9 m = 1 nm)

visible

700 nm 400 nm

*In optics and photonics, due to conventions, wavelength unit (nm or m) is often adopted.

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wavelength (nm)400 700

UV Near-IR

visible

1000 2000

x

c

= c = 3 108 m/se.g. = 1 m = 1000 nm = 10-6 m, = 3 1014 Hz = 300 1012 Hz = 300 THz

frequency wavelength = speed of lightIn free space (i.e. vacuum or air)

Lightwave spectrum

Optical carrier frequency ~ 100 THz, which is five orders of magnitude largerthan microwave carrier frequency of GHz.

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Optical fiber communications systems use lightwave in the near-infrared.

800 900 1000 1100 1200 1300 1400 1500 1600

(nm)

850 nm1300-nmband

1550-nmband

Early systems (1980s);also modern short-distance networks using polymer optical fibers

From 1990s to present networks(long-haul/metro/access)

Most optical fiber communications systems now use the silica glass fiber lowest-loss window which is around ~ 1550 nm.

OpticalSpectralBandsforfiberopticcommunications

Originalband(Oband):1260to1360nm Regionoriginallyusedforfirstsinglemodefibers

Extendedband(Eband):1360to1460nm Operationextendsintothehighlosswaterpeakregion

Shortband(Sband):1460to1530nm(shorterthanCband)

Conventionalband(Cband):1530to1565nm(EDFAregion)

Longband(Lband):1565to1625nm(longerthanCband)

Ultralongband(Uband):1625to1675nm30

O-Band E-Band S-Band C-Band L-Band U-Band

1260 1360 1460 1530 1565 1625 1675

Wavelength (nm)

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Silica optical fiber loss spectrum

The Internetare carried in here.

~0.2 dB/km attenuation

Today: 10% of the light remains after more than 50 km of fiber

DecibelUnits Thedecibel (dB)unitisdefinedby

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DecibelUnits(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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DecibelUnits

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.

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DecibelUnits

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

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NetworkInformationRates

Astandardsignalformatcalledsynchronousopticalnetwork(SONET)isusedinNorthAmerica

Astandardsignalformatcalledsynchronousdigitalhierarchy(SDH)isusedinotherpartsoftheworld

36

37

1500 1600 (nm)1550

Lightwave channel within the fiber low-loss window

fiber low-loss window

Current systems can transmit a single lightwave channel at a data rate of 10 Gb/s or 40 Gb/s

WavelengthDivisionMultiplexingConcepts

Manyindependentinformationbearingsignalsaresentalongafibersimultaneously

Independentsignalsarecarriedondifferentwavelengths Dataratesorformatsoneachwavelengthmaybe

different CoarseWDM(CWDM)anddenseWDM (DWDM) arethe

twomajorwavelengthmultiplexingtechniques Wavelengthroutingandswitchingtechniquesbasedon

lightpaths arebeingdeveloped

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WDM combines or multiplexes multiple optical signals into a single fiberby transmitting each signal on a different wavelength . [analogous to Frequency-Division Multiplexing (FDM) in radio communications]

Wavelength-Division Multiplexing (WDM)

single optical fiber

Telecommunication carriers can potentially multiply the capacity of their fibers by WDM, without the expensive investment of laying extra fibers underground or undersea.

12

N

12

N

40

1500 1600 (nm)1550

n WDM channels

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)

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(Scientific American, Jan 2001)

Lightwave networks combine, amplify, switch, and restoreoptical signals without converting the optical signal to an electronicsignal for processing.

WDM optical links

StandardsThethreebasicclassesforfiberopticsareprimary standards,

componenttestingstandards,andsystem standards.

Primarystandards dealwithphysicalparameters:attenuation,bandwidth,operationalcharacteristicsoffibers,andopticalpowerlevelsandspectralwidths.

Componenttestingstandards definetestsforfiberopticcomponentperformanceandestablishequipmentcalibrationprocedures.

ThemainonesareFiberOpticTestProcedures(FOTP)

Systemstandards refertomeasurementmethodsforopticallinksandnetworks.

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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 carrier.

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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.

The fiber proposal

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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 opticalfiber 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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1980s The first generation of fiber-optic communication systems operated at a bit rate of 45 Mb/s and required signalregeneration 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 wavelength-division-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

The era of commercial lightwave transmission systems

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Low data rate, single channel

High data rate, multiple channels (Optical amplifiers (EDFA) + WDM)

Enabling components for sophisticated reconfigurable optical networks

Optical fibers + semiconductor lasers

80s

90s

00s

70s

10sOptical interconnects for next-generation computercom?

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The bandwidth made possible by optical fiber communications has made the Internet economically feasible.

Current optical fiber communications capabilitiesBit 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)

Lecture 1 Overview of Photonics and Optical Fiber CommunicationsFiber-optic communications and modern societyOptics and PhotonicsWhat is photonics?What is photonics?A snapshot of photonic technologiesPhotonics for communicationsLaser modules in communicationsActive Optical CablesVarious types of optical networksEnabling components for on-chip optical communicationsIntel optical cablesPhotonics for data storage(Nano) Photonics on CD/DVD/Blu-ray disksNanophotonics in naturePhotonics for human-machine interface: pico-projectorsPhotonics for medicinePhotonics for defenseCommunications systemMotivations for high-speed communications Motivations for fiber-optic communications Carrier Information CapacityCommunication systems applications in the electromagnetic spectrumOptical Spectral Bands for fiber-optic communicationsDecibel Units Decibel Units (2) Decibel UnitsDecibel UnitsNetwork Information RatesWavelength-Division Multiplexing ConceptsWDM optical linksStandardsHistorical developmentThe fiber proposalThe beginnings of lightwave technologyThe era of commercial lightwave transmission systems