Fantasies of the dreadful death ray led to the discovery...
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1 1 Fantasies of the dreadful death ray led to the discovery of a device that helps people in a million ways … 2
Transcript of Fantasies of the dreadful death ray led to the discovery...
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Fantasies of the dreadful death ray led to the discovery of a device
that helps people in a million ways …
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Lasers
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Follow the Dreams
In the “War of Worlds” (1898) H. G. Wells imagined Martial invaders scattering human armies with devastating heat rays.
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Stimulated Emission
A method for amplifying light had its origins in an idea Einstein developed in 1916. Looking deeply into the new theory of quantum physics, he predicted that rays could stimulate atoms to emit more rays of the same wavelength. But engineers had little notion how to manipulate atoms, and for decades the idea seemed a theoretical curiosity of no practical interest.
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Follow the Money
When the Soviet Union launched the first satellite Sputnik in 1957, the US government redoubled its funding for research.
Might scientists invent a ray device to shoot down enemy missiles?
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Maser: First Step to the Laser
• In 1953, Charles Townes, James Gordon and Herbert Zeigerproduced the first MASER Incapable of continuous output.
• Nikolay Basov and AleksanderProkhorov solved the problem of continuous-output systems by using more than two energy levels.
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Who Invented the Laser
First page of the notebook wherein Gordon Gould coined the LASER acronym, and described the technologic elements for constructing the device.
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Who Invented the Laser
Part of a September 1957 page from Charles Townes’s notebook showing his idea for “A maser at optical frequencies.” He sketched a box lined with mirrors, with little holes where rays could leak out.
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The Patent War
• In 1957 Townes talked over some ideas about pumping light-energy into atoms with Gordon Gould.
• Worried that he might be scooped, Gould wrote down his ideas for the record and in April 1959 he filed patent applications.
• Nine months earlier Schawlow and Townes had applied for a patent.
• When the patent application of Townes et al. was granted, Gouldsued, claiming he was first to conceive the device.
• In 1987 Gould and his backers began to win settlements. One of the greatest patent wars in history was over.
The historical question of how to assign credit for inventing the laser remains controversial!!
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The First Laser
• Theodore Maiman developed the first working laser at Hughes Research Lab in 1960.
• Theodore Maiman used a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light, at 694 nanometers wavelength.
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The First Laser
Picture taken during CLEO 2010, San Jose, CA, USA.13
Laser Applications
Astronomy
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Telescope Welding Laser sight
Lasik
Dentistry
Atmospheric sensing
Biosensing
DNA sequencing
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Course Instructor
Dr. Muhammad Anisuzzaman TalukderProfessorDepartment of EEE, BUET
Contact InfoEmail: [email protected]: 55167100 Ext. 6582Office: ECE 527
Websitehttps://anis.buet.ac.bd/
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LASER THEORY on WEB
Please check
https://anis.buet.ac.bd/Laser_Theory.html
for class notes, home work, grades, and other updates.
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Class Email List
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• To get included in the list, email to [email protected] with
─ Subject: EEE 6503
─ Body: “Your Name, Student Number” <email address>
• It will help me to keep you posted on the course updates.
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Text Book
LASERSAnthony SiegmanUniversity Science Books
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Course Overview
• An introduction to lasers
• Stimulated transitions: The classical oscillator model
• Electric dipole transitions in real atoms
• Rate equations
• Laser amplification
• Oscillation dynamics and oscillation threshold
• Q-switching and mode-locking
• Principles of operation of different lasers.
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Grading
• Home work / assignment / mid-term: 20% ~ 30%
• Project: 30% ~ 40%
• Final examination: 40% ~ 60%
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Stay Tuned …
We may have a changed grading policy!
What is a Laser
• Light Amplification by Stimulated Emission of Radiation
• Coherent radiation All photons have the same phase.
• Monochromatic All photons have the same frequency.
• Directional All photons have the same wave vector.
• Lasers cover frequency from X-rays to Far Infrared.
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Electromagnetic Spectrum
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Lasers
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Components of a Laser
• Laser medium: The medium typically contains atoms, molecules, ions, or semiconducting crystals.
• Pumping process: Excite atoms into higher quantum-mechanical energy levels.
• Optical feedback: Mirrors
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Light-Medium Interaction
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Absorption Spectroscopy
E1
E2
E3
BA
C
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Pho
tocu
rren
t
Wavelength
BA
C
Absorption Spectroscopy
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Emission of Light from Atoms
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Emission spectrum for Hydrogen
Emission spectrum for Iron
1220 Å 6558 ÅExcitation
Diffraction grating
Hydrogen gas Light
Hydrogen Energy Levels
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Allowed Energy Levels
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Photons
• As an example let’s look at the transition from 2p state to the 1s state. If the electron is initially in the 2p state and goes to the 1s state it loses 10.2 eV of energy! This 10.2 eV becomes the energy of a photon. The emitted light has a frequency such that h = 10.2 eV, where h is Planck’s constant. So, in MKS units we find
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6.626068 10 m kg/s 10.2 1.6 10
10.2 1.6 102.26 10 Hz
6.626068 10
3 101.22 10 m 1220 Å.
2.26 10
h
c
• Note that this electro-magnetic radiation is in the extreme ultra-violet range.
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Excitation and Emission
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Spontaneous Transitions
Quantum mechanically, the atom would stay in the 4p state. However, we see emission from the Hydrogen atom even if there is no external electromagnetic radiation (once we get to the 4p level). Atoms spontaneously decay through the same electric dipole interaction to a lower state (l = 1). This spontaneous transition can only occur in emission.
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Incoherent or noise-like, emerging randomly in all directions.
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Decay Rates
An excited state will decay until it gets to the ground state. This decay is random and is like the familiar nuclear decay. If the number of atoms per unit volume in a given level N is, we find
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1, .
t
t
dNN N N e
dt
N N e
: State lifetime
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Decay
• In Hydrogen atom, the spontaneous decay is radiative.
• In general, the decay may be both radiative and non-radiative. For example, the energy decay may be through phonon coupling. This is non-radiative decay.
Non-radiative
Radiative
h
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Stimulated Transitions
• Absorption/emission occurs if the energy of the photon matches the energy difference between the levels.
• The transition rate is proportional to the intensity of the incident electromagnetic field.
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Two Coupled Levels
Spontaneous Emission
21N2
E2
E1
Stimulated Absorption
KnN1
Stimulated Emission
KnN2
N2
N1
21: Total spontaneous decay rate
n: Photon density (per unit volume)
K: Proportionality constant strength of stimulated response
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Rate Equations
221 2
Spontaneous
( )dN
N tdt
Spontaneous Emission
21N2
E2
E1
Stimulated Absorption
KnN1
Stimulated Emission
KnN2
N2
N1
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Stimulatedabsorption
( ) ( )dN
Kn t N tdt
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Stimulatedemission
( ) ( )dN
Kn t N tdt
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Total Rate Equation
Spontaneous Emission
21N2
E2
E1
Stimulated Absorption
KnN1
Stimulated Emission
KnN2
N2
N1
2 2 2 2
Stimulated StimulatedTotal Spontaneousabsorption emission
1 2 21 2
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Total
( ) ( ) ( ) ( )
dN dN dN dN
dt dt dt dt
Kn t N t N t N t
dN
dt
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Condition for Gain
sig
1 2( ) ( ) ( )adUdU
Kn t N t N tdt dt
• Energy transfer rate to the atoms
sig
1 2
( )
( )( ) ( ) ( )
U n t
dn tK N t N t n t
dt
• n(t) may either decay or grow with time, depending on the sign of
1 2( ) ( ) ( ).N t N t N t
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2 1Population Inversion ( ) ( )N t N t
• Energy transfer rate to the signal
N2
N1
N2
N1
