Download The Nobel Prize in Physics 2009

The Nobel Prize in Physics 2009
"for groundbreaking
achievements concerning
the transmission of light
in fibers for optical
communication"
"for the invention of an
imaging semiconductor
circuit – the CCD sensor"
Charles K. Kao
高錕
Willard S. Boyle George E. Smith
1/2 of the prize
1/4 of the prize
•Standard
Telecommunication
Laboratories
Harlow, UK;
Chinese Univ of
Hong Kong
Hong Kong, China
1/4 of the prize
Bell Laboratories
Murray Hill, NJ, USA
b. 1933
b. 1924
b. 1930
(in Shanghai, China) (in Amherst, NS, Canada)
How fiber optics impacts you
This is reason for
development of
global all-optical
communication
networks. This is
your Internet,
Email, …
Orders of
magnitude
more
information
capacity than
is available
through
electrical
systems
The Light Guide: Guiding light from
one location to another
• Guiding light by (a) lenses; (b) mirrors
• Guiding distance is limited because of losses due to partially reflective lenses or absorptive mirrors.
Alternative: the fiber light guide
• Feasibility of transmitting optical radiation through glass fiber
waveguides was first suggested by KC Kao and E Hockham
in 1966.
• Treats the fiber as a dielectric waveguide in which the light is
guided down the fiber by reflections at the surface of the glass.
• “Total internal reflection” (TIR) guides light down the fiber.
Total Internal Reflection (TIR)
•
•
Total internal reflection occurs at the boundary between
two media of different refractive indices
– when light travelling from the medium of higher
refractive index to the lower one strikes the interface
at the critical angle.
– For angles less than this critical angle, the ray is
partially transmitted into the second medium energy
is lost.
– For angles greater than the critical angle the ray is
totally reflected back into the original medium.
In practice, the core is surrounded by a glass cladding
of lower refractive index to protect the core and to
prevent changes in the critical angle due to contamination on the surface.
Typical dimensions are 10 or 50 micrometers for the core
and 125 micrometers for the fiber. In addition, a protecting
plastic “buffer” is placed around the fiber.
Types of optical fibers
Geometry, refractive index profile and typical rays in a
step-index multi-mode fiber (MMF), single-mode fiber
(SMF) and graded index (GRIN) MMF.
• Light propagates in the form of modes, with a distinct
propagation constant and group velocity, maintaining its
transverse spatial distribution and polarization.
• Each mode is described as the sum of the multiple
reflections of a TEM wave bouncing within the guide, in
the direction of an optical ray at a certain bounce angle.
Charles K. Kao and optical fiber
Kao at
work in his
laboratory
at Harlow,
England in
1966.
“[...] a fibre of glassy material constructed in a cladded
structure [...] represents a possible practical optical Waveguide with important potential as a new form of communication medium […] Compared with existing coaxial-cable
and radio systems, this form of waveguide has a larger information capacity and possible advantages in basic material
cost.”
K.C. Kao and G.A. Hockham, “Dielectric-Fibre Surface
Waveguides for optical frequencies”
Proc. IEEE, 113, 1151 (1966).
Attenuation in optical fibers
•
•
•
Of particular importance in the application of fibers is
the amount of light that can be transmitted down the
fibre.
– The first optical fibres had an attenuation of
around 1000 dB/km.
– Fibres are now available with attenuation < 0.1
dB/km.
Several factors contribute to the attenuation of light
through fibers, including absorption, scattering and
bending effects.
– Absorption occurs at impurities in the fiber such
as beads of water which become imbedded in the
glass during manufacture and absorb the light
preferentially at certain wavelengths.
– Electronic and vibrational resonances
Scattering is the change in direction of propagation of
a ray by collision with an inhomogeneity or irregularity
in the glass.
– This effect is known as Rayleigh scattering and is
proportional to λ-4.
– The overall effect is that the total attenuation is
wavelength dependent.
Attenuation (dB)= 10 log(Pin/Pout).
Attenuation in optical fibers
Absorption by
electronic band
silicon-oxygen
(Si-O) bonds
• The variation of attenuation with wavelength of a typical
modern fiber (close to fundamental limit).
• Kao and Hockham pointed out that the attenuation of glass
fiber (in 1966) was largely caused by the presence of impurity
metal ions, such as iron, copper, vanadium, and chromium.
• If a glass could be developed with attenuation of only 20
dB/km, then optical communication could become a reality.
With such a fiber, 1% of the input light reaches the far end of
a 1 km length of fiber.
Rayleigh Scattering
• The scattering of light off of the molecules of the air, or
• from particles up to about a tenth of the wavelength of the light.
• It is Rayleigh scattering off the molecules of the air which gives
us the blue sky.
• In optical fibers, Rayleigh scattering is caused by density
fluctuations or non-uniformity during the manufacturing process,
dominant for 800 nm band.
TAT-8
• In 1988 AT&T laid the first fiber-optic
transatlantic telephony cable 3,148 miles
long
• Connected North America to France
• Repeaters every 40 miles
• 565 Mbps bandwidth
• Used 1300 nm light
• Attenuation 0.4 dB/km
Fiber Amplifier
• Special fiber with Erbium atoms in it
is used to amplify light without
changing it to an electrical signal first
• Uses stimulated emission, the same
principle that makes lasers work
Freeway Analogy to
the fiber-based network
• TAT-8 in 1980
– 565 Mbps
– Electro-optical repeaters
• TAT-12/13 in 1996
– 2.5 Gbps
– Optical amplifiers
• 1998
– 20 Gbps
– WDM with 8 wavelengths
Charge-coupled Devices or CCDs
CCD, the electronic image
sensor, converts the optic
image to electronic signals
that are translated into
digital ones and zeros.
The galaxy
cluster Abell
2218. Image:
WFPC2,
Hubble
Space
Telescope,
NASA.
CCD (Charge-Coupled Devices)
Bucket
Brigade
• Integration
• Charge
Shift and
Read-out
•Charge
Amplifier
Charge-coupled Devices or CCDs
Basic Building
Block: the MOS
capacitor
The charge genera- ted
by photons is forced to
move one step at a time
through the application
of voltage pulses on the
electrodes.
color imaging by generally
use a Bayer mask over
the CCD. Each square of
four pixels has one filtered
red, one blue, and two
green (the human eye is
more sensitive to green
than either red or blue).
Charge-coupled Devices or CCDs
Willard
Boyle (left)
and
George
Smith (right)
with the
CCD
Camera.
Original notes from
the Boyle and
Smith’s brainstorm
meeting on
September 8 1969,
when they made the
first sketch of a CCD.
Further Reading
• Web site of Nobel Foundation
http://nobelprize.org/nobel_pri
zes/physics/laureates/2009/in
dex.html (See the Info and
Scientific Background).
• Blog on Nobel Prize by
Physics Today
http://blogs.physicstoday.org/
newspicks/2009/10/nobelprize-in-physics-awarded.html
• Sec. 4.7 and 5.2 of Hecht