Lecture 1 Overview of Photonics and Optical Fiber Communications
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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 1.1 – 1.8
Senior 1.1 – 1.3
Article: A Snapshot of Optical Communications, OPN Optics & Photonics News,
pp. 24 – 30, Jan. 2010
Part of the lecture materials were adopted from powerpoint slides of Gerd Keiser’s book 2010,
Copyright © The McGraw-Hill Companies, Inc.
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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.
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Source: OPN, pp. 24- 30 Jan. 2010.
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.
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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.
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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
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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
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And many more!!
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
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Enabling photonic components for communications
• Laser diodes
• Modulators
• Optical fibers
• Optical amplifiers
• Wavelength-Division Multiplexing
(WDM) components
• Photodetectors
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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. 28-33, March 2010
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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
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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.
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Ref. Lasers in Communications, Patricia Daukantas, pp. 28-33, March 2010
Optical communications for computing
†
Optical interconnect technology is motivating the development
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of the R&D field of “silicon photonics.”
Optical interconnects
Electrical interconnects (Copper):
¾ Resistance-capacitance (RC)
delay
¾ Power consumption
¾ Bandwidth limitation (~5 GHz)
2002
2007
2012
2017+
Optical interconnects
¾ High bandwidth (> 40 Gb/s)
¾ Relatively low power consumption
¾ Wavelength-division multiplexing
(WDM)
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N. Savage, IEEE Spectrum, pp. 32- 36 August 2002.
Enabling components for on-chip optical communications
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Source: Intel
Intel optical cables
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Source: Intel Light Peak
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.
Ref. Optical filters in nature, OPN Optics & Photonics News, pp. 22-27, Feb. 2009
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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
Lasers in ophthalmology (laser surgery)
Ref. Lasers in ophthalmology, OPN Optics & Photonics News, pp. 28-33, Feb. 2010
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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
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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.
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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
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Motivations for fiber‐optic communications Advantages of optical fibers
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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.
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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)
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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
lightwave
visible
radio
microwave
infrared
Frequency
(Hz)
106 107 108 109 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.
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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.
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• 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.
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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)
• 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
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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
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Silica optical fiber loss spectrum
The Internet
are carried in here.
~0.2 dB/km
attenuation
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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.
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Decibel Units
Power levels differing by many orders of magnitude can be
compared easily when they are in decibel form.
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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
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Lightwave channel within the fiber low-loss window
1500
1550
1600
λ (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
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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
λN
λN
⇒ 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
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 25 × 40 Gb/s)
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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.
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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.
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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
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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.
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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 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
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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?
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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.
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