Optical Communication Systems

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Nov 1, 2013 (4 years and 2 months ago)

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Optoelectronics

Spring 2012

Vinod Menon


M/W 10:50 am


12:05 pm

Office Hours: W 2:30 pm
-

4:30 pm

E
-
mail:

vmenon@qc.cuny.edu

Phone: 718
-
997
-
3147


Syllabus

Spring 2012



Historical survey of optical communication



Electromagnetic waves



Waveguides (planar and fiber optics)



Photonic crystals



Microcavities



Mechanism of light emission and absorption in semiconductors



Lasers



Photodetectors

and solar cells



Nonlinear optics



Optical communication systems



Applications





This course will assume your knowledge of the following:

Quantum mechanics: Schrödinger equation (time independent)
-

solve particle in a box.

Electromagnetism: Maxwell’s equations



There is no prescribed text book for the course. I will be posting notes on the course
website:
http://www.physics.qc.edu/pages/vmenon/optoelectronics


Syllabus contd.

Spring 2012

Material for the course will be loosely based on the following books:


Physics of optoelectronic devices

by
Shn

Lien Chuang, Wiley (1995)

Fundamentals of Photonics

by
Saleh

and
Teich
, Wiley
-
Interscience
; 2
edition (2007)

Nanophotonics

by
Paras

N. Prasad, Wiley (2004)

Fiber
-
Optic communication systems

by G. P.
Agrawal
, Wiley (1997)



I will be posting the power point slides as well as lecture notes prior to the
lecture.



Wednesday sessions will be mostly dedicated to problem solving.


Grading Policy:

Homework: 30%

Final Project: 10%

Mid
-
term Exam: 30%

Final Exam: 30%

Introduction to Optoelectronics

Spring 2012



What is a communication system?




Electromagnetic waves


carrier of information




Frequencies ranging from few MHz to several 100 THz.




Optical communication systems use ~ 100 THz ( visible and near IR).




Microwave communication systems operate in the GHz range.




Fiber optic communication systems have been deployed since 1980


advent of “
information age
”.


Historical Perspective

Spring 2012



Smoke and Fire signals


earliest known version of optical
communication!




End of 18
th

century


similar idea using lamps and flags




Most of these were to communicate one piece of information such as
victory in a war.




Claude
Chappe

(1792) transmitted mechanically coded signals over
longer distances (100 km) using intermediate relay stations.




The bit rate (if one wants to call it that way) B was < 1b/s!!!


Historical Perspective

Spring 2012

Electrical communication systems




Telegraph
(1830s) replaced use of light by electricity


era of electrical
communication



B ~ 10b/s (using More code)



Using intermediate relay stations ~ 1000 km



Transatlantic telegraph in 1866



Note


telegraph used a binary format!


dots and dashes




Telephone

(1876)


Analog form of data transmission using continuously
varying current.



Analog electrical communication dominated almost the next 100 years!



Coaxial cables instead of wire pairs increased the bandwidth



First Coax cable system operated at 3Mhz and carried 300 voice
channels.



Single TV channels.



Bandwidth limited by frequency dependent losses.



Losses rapidly for frequency above 10 MHz

Historical Perspective

Spring 2012

Microwave Communication Systems





An EM carrier wave with frequency ranging from 1


10 GHz is used to
transmit the signal



Suitable modulation technique is used.



1948


4GHz deployed



Both coax and microwave systems have evolved to operate at ~ 100 Mb/s



High speed coaxial systems need smaller repeater spacing (~ 1km)



Microwave communication systems can deal with larger repeater
spacing.




Commonly used figure of merit:
Bit rate


distance product (BL).


Advent of
Lightwave

Communication Systems



BL
could be increased using light as carrier waves: Middle of twentieth
century.



What was needed was a coherent optical source and a medium that can
transmit light over long distances.



1960


LASER
-

Coherent Source



1966


Fiber optic cables


medium to guide light



Historical Perspective

Spring 2012

Fiber Optic Communications by G.P.
Agrawal

Historical Perspective

Spring 2012

Lightwave

Communication Systems
(
contd
)





Early optical fibers had huge
transmission losses (1000 dB/km)



1970


breakthrough in loss
reduction around 1
μ
m wavelength
(20dB/km)



Reduction of impurity in the glass
fiber was key to reducing losses



Nobel Prize in 2009:
Charles
Kuen

Kao

"for groundbreaking achievements
concerning the transmission of light in
fibers for optical communication"
,




GaAs

semiconductor lasers
operating at room temperature
(1970)


compact coherent source.


Fiber Optic Communications by G.P.
Agrawal

Historical Perspective

Spring 2012

Generations of
Lightwave

Communication Systems





1
st

generation:
λ

= 0.8
μ
m, 45 Mb/s, repeater spacing 10 km



2
nd

generation:
λ

= 1.3
μ
m, 100 Mb/s




Dispersion of multimode fibers was an issue




Single mode fibers pushed the bandwidth to 2 GB/s over ~ 50 km




Loss at 1.3
μ
m ~ 0.5dB/km


limited the repeater spacing




Loss at 1.55
μ
m ~ 0.2dB/km


but large fiber dispersion


pulse

spreading.




3
rd

generation:
λ

= 1.55
μ
m, 4Gb/s over 100 km,


-

commercially, 2.5Gb/s over 70 km in 1990


-

dispersion shifted fibers


-

drawback: signal regeneration using electronic repeaters




4
th

generation: Optical amplification using
Er
-
doped fiber amplifiers


-

Wavelength division multiplexing for greater bit rate


-

1996, 11,300 km at 5Gb/s using submarine cables


-

FLAG ( Fiber
-
optic Link Around the Globe).



5
th

generation: dispersion management using optical
solitons



-

All
-
optical networks?

Historical Perspective

Spring 2012

Global fiber optic map


circa 2006

Basic Concepts

Spring 2012

Analog and Digital Signals





Analog: signal current varies
continuously. Examples?



Digital: takes only discrete values



Binary representation of digital: 0 or 1
or bit 0 and bit 1



Bit rate: B = 1/T
B

where T
B

is the time
that each bit lasts.



ASCII code: 0


127 corresponds to
alphabets and other symbols.



Now ASCII extended to 256
characters transmitted through 8 bit
bytes.



Signal bandwidth: range of
frequencies contained within the
signal
-

use Fourier transform

Fiber Optic Communications by G.P.
Agrawal

Basic Concepts

Spring 2012

Analog to Digital Conversion





Sampling Theorem: A bandwidth limited
signal can be fully represented by discrete
samples without any loss of information,
provided that the sampling frequency
satisfies
Nyquist

criterion,





How to Sample?


use the right frequency



the sampled value can be anywhere
between 0 and A
max




Quantization noise that comes about due
to the
discretization
.



Effect can be minimized by choosing the
number of discrete levels

M> A
max

/A
N

(A is the RMS noise of analog
signal)



Number of bits m needed to code each
sample is related to number
ofquantized

signal levels M by M = 2
m



Fiber Optic Communications by G.P.
Agrawal

Bandwidth

Basic Concepts

Spring 2012

Channel Multiplexing





Time division multiplexing



Frequency division multiplexing ( called Wavelength division multiplexing in
fiber
-
optic communication systems)



Synchronous Optical Networks


(SONET) now called Synchronous

digital hierarchy (SDH)


Fiber Optic Communications by G.P.
Agrawal

Basic Concepts

Spring 2012

Modulation Formats





In the optical systems, NRZ and RZ



Advantage of NRZ: bandwidth associated
with bit stream is smaller than that for RZ
format by a factor of 2 because on
-
off
transitions occur fewer times.



NRZ also requires tighter control of pulse
width


dispersion issues









Modulation can be done on any of the
above parameters….AM, FM,. PM



In the digital case


ASK, FSK and PSK ( SK
stands for shift keying)



Simplest technique: on
-
off keying (OOK)

Fiber Optic Communications by G.P.
Agrawal

Polarization

Amplitude

Carrier Freq

Phase

Optical Communication Systems

Spring 2012



Optical carrier frequency ~ 100 THz



Bandwidth of modulated carrier can be a few % of the carrier frequency


-

using 1% that gives ~ 1Tb/s



Guided (fiber optic) versus unguided (free space)



Long haul versus short haul

Optical Transmitter

Communication
Channel

Optical
Reciever

Input

Output

Optical Communication Systems

Spring 2012

Optical Transmitters



Convert electrical signal into optical form and launch it into the
communication channel be it guided or unguided.













Launched power


indicates how much fiber loss can be tolerated.





Power (
dBm
) = 10 log
10

(power/1mW)


1 mw = 0
dBm
, 1
μ
W =
-
30
dBm


LEDs <
-
10
dBm
, Semiconductor Lasers ~ 10dBm




Bit rate of optical transmitter is limited by electronics rather than by
semiconductor laser itself. ~ 20Gb/s

Driver

Electrical

Input

Output

Optical
Source

Modulator

Channel
Coupler

Optical Communication Systems

Spring 2012

Optical Receivers

Electronics

Optical

Input

Electrical

Output

Photodetector

Demodulator

Channel
Coupler



Bit Error Rate (BER): Average probability of incorrect bit identification


BER of 10
-
6

: Average of 1 error per million bits


Most
lightwave

systems, BER ~ 10
-
9




Receiver Sensitivity: Minimum
verage

received optical power when BER is 10
-
9



Depends on signal to noise ratio (SNR)



Quantum noise or shot noise inherent to the detection mechanism


originates
from the particle nature of electrons



Other noise sources such as amplifier noise, thermal noise, noise sources at the
transmitter end from spontaneous emission, chromatic dispersion of fibers, etc
usually dominate over the shot noise.



Also depends on bandwidth


Channel Capacity

Spring 2012


Any optical communication system is limited by the SNR of the received
signal.



Channel Capacity


introduced within the framework of
Information Theory

(
Shanon

1949)



There is a maximum possible bit rate for error free transmission of binary
signal in the presence of Gaussian noise.












Note: Channel capacity cannot be increased indefinitely by increasing
the bandwidth because the shot noise depends linearly on the bandwidth.