Download 7. Lasers

考试时间：2011.1.4
上课时间和教室
Chapter 7.
Lasers
(acronym)
Light Amplification by Stimulated
Emission of Radiation
The History





1916, Einstein predicted the stimulated emission.
1954, Townes and co-workers developed a Microwave
Amplifier by Stimulated Emission of Radiation(maser)
using ammonia, NH3.
1958, Schawlow and Townes showed that the maser
principle could be extended into the visible region .
1960, Maiman built the first laser using ruby as the
active medium.
From then on, laser development was nothing short of
miraculous, giving optics new impetus and wide publicity.
7.1 Stimulated Emission of Radiation
1. Boltzmann Distribution
the transitions that occur between different energy states
 absorption: the upward transition from a lower energy state
to a higher state, E1  E2
 Emission: the downward transition, E2  E1,
 population N : the number of atoms, per unit volume, that
exist in a given state.
given by Boltzmann's equation
N e
 E / T
E : energy lever of the system
 : Boltzmann's constant
T : absolute temperature.
7.1 Stimulated Emission of Radiation
Boltzmann's ratio or relative population : the ratio of the
populations in the two states, N2 / N1.
N2
e  E 2 / T
  E1 / T
N1
e
Or
N 2  N1 e
 ( E2  E1 ) / T
plot the energy in the higher state relative to that in the lower
state, versus the population in these states(E versus N), the result
is an exponential curve known as a Boltzmann distribution.
 When the Boltzmann distribution is normal, it means that the
system is in thermal equilibrium, having more atoms in the
lower state than in the higher state.
7.1 Stimulated Emission of Radiation
2. Einstein's Prediction
Assume first: an ensemble of atoms is in thermal equilibrium and
not subject to an external radiation field.
• At higher temperatures, a certain number of atoms is in the excited
state; on return to the lower state, these atoms will emit radiation, in
the form of quanta h . -- spontaneous emission
 rate of the transition: the number of atoms in the higher state that
make the transition to the lower state, per second.
 lifetime of the transition: the reciprocal of the rate of transition.
 rate of the spontaneous transition:
P21  N 2 A21
A21: constant of proportionality
N2 : number of atoms (per unit volume) in the higher state
7.1 Stimulated Emission of Radiation
Assume next: the system is subject to some external
radiation field.
one of two processes may occur, depending on:
the direction (the phase) of the field with respect
to the phase of the oscillator.
7.1 Stimulated Emission of Radiation
 the two phases coincide:a quantum of the field may cause the
emission of another quantum. -- stimulated emission.
Its rate is
P21  N2 B21 u 
B21 : constant of proportionality
u(): energy density (J m-3), function of frequency .
the two phase is opposite : the impulse transferred counteracts the
oscillation, energy is consumed, and the system is raised to a higher
state -- absorption.
Its rate is
P12  N1 B12 u 
B12 : constant of proportionality.
7.1 Stimulated Emission of Radiation
Transitions between energy states
7.1 Stimulated Emission of Radiation
Einstein's coefficients: A21, B21, B12
Einstein's relations
B21 = B12
A21 8  h  3

B 21
c3
(1) the coefficients for both stimulated emission and
absorption are numerically equal
(2) the ratio of the coefficients of spontaneous versus
stimulated emission is proportional to the third power of the
frequency of the transition radiation
explains why it is so difficult to achieve laser emission in
the X-ray range, where  is rather high
7.1 Stimulated Emission of Radiation
3. Population Inversion
 thermal equilibrium system
absorption and spontaneous emission take place side by side
N2 < N1, absorption dominates: an incident quantum is
more likely to be absorbed than to cause emission.
 population inversion condition
a majority of atoms in the higher state, N2 > N1
on return to the ground state, the system will probably lase.
Incandescent vs. Laser Light
Light from bulbs are due to spontaneous emission
1. Many wavelengths
1. Monochromatic
2. Multidirectional
2. Directional
3. Incoherent
3. Coherent
Coherence
Coherent: If the phase of a light wave is well defined at all
times (oscillates in a simple pattern with time and varies in
a smooth wave in space at any instant).
Example: a laser produces highly coherent light. In a laser,
all of the atoms radiate in phase.
Incoherent: the phase of a light wave varies randomly from
point to point, or from moment to moment.
Example: An incandescent or fluorescent light bulb produces
incoherent light. All of the atoms in the phosphor of the
bulb radiate with random phase.
Stimulated vs Spontaneous Emission
Stimulated emission requires the presence of a photon.
An “incoming” photon stimulates a molecule in an excited
state to decay to the ground state by emitting a photon.
The stimulated photons travel in the same direction as the
incoming photon.
Spontaneous emission does not require the presence of a
photon.
Instead a molecule in the excited state can relax to the
ground state by spontaneously emitting a photon.
Spontaneously emitted photons are emitted in all
directions.
two-level system(ex. ammonia maser)
Em, Nm
Em, Nm
En, Nn
En, Nn
Even with very a intense pump source, the best one can
achieve with a two-level system is
excited state population = ground state population
three-level system
 in equilibrium
 normal Boltzmann distribution
 absorptive rather than emissive
 excited
 population inversion
7.2 Practical Realization
1. General Construction
Pumping: an energy source to supply the energy needed for
raising the system to the excited state.
active medium: in which, reaches population inversion and
lases when excited.
may be a solid, liquid, or gas
thousands of materials that have been found to lase
cavity:optional
laser amplifiers: no cavity
laser oscillators:  medium enclosed in a cavity  provides
feedback and additional amplification  cavity formed by two
mirrors: one full reflectance, the other partially transparent
7.2 Practical Realization
Basic components of a laser oscillator
 Energy source
 Medium
 Full reflectance mirrors
 Partially transparent mirrors
 Radiation
Common Components of all Lasers
1. Active Medium
The active medium may be solid crystals such as ruby or Nd:YAG, liquid
dyes, gases like CO2 or Helium/Neon, or semiconductors such as GaAs.
Active mediums contain atoms whose electrons may be excited to a
metastable energy level by an energy source.
2. Excitation Mechanism
Excitation mechanisms pump energy into the active medium by one or
more of three basic methods; optical, electrical or chemical.
3. High Reflectance Mirror
A mirror which reflects essentially 100% of the laser light.
4. Partially Transmissive Mirror
A mirror which reflects less than 100% of the laser light and transmits the
remainder.
7.2 Practical Realization
2. Excitation
optical pumping: ruby laser
 a light source  another laser.
electron excitation: argon laser , helium-neon laser
 direct conversion of electric energy into radiation:
light-emitting diodes(LEDs), semiconductor lasers
thermal excitation : CO2 laser.
chemical pumping: chemical laser
H2 + F2  2HF
7.2 Practical Realization
3. Cavity Configurations
Plane-parallel cavity: very efficient ( good filling), difficult
alignment(low stability)
confocal cavity: poor filling, easier to align
concentric cavity (spherical cavity) : poor filling, easier to
align
hemispherical cavity: poor filling, much easy to align
long-radius cavity: good compromise between the planeparallel and the confocal variety, type of cavity used most often
in today's commercial lasers.
Cavity configurations
7.2 Practical Realization
L:distance between
mirrors
R:radius of curvature
7.2 Practical Realization
4. Mode Structure
Assume: the cavity is limited by two plane-parallel mirrors.
the wavelength possible of the standing-wave pattern
inside the cavity is:
2
λ  L
q
L : length of the cavity
q : number of half-wavelengths, or axial modes
 the resonance condition for axial modes:
c
ν q
2 nL
n: index of medium contained in a laser cavity
7.2 Practical Realization
two consecutive modes (which differ by q = 1), are
separated by a frequency difference ,
ν 
c
2S
different frequencies are closely, and evenly, spaced, lie
within the width of a single emission line.
 the output of the laser consists of a number of lines
separated by c/2S
Mode-locking
Active mode-locking
7.2 Practical Realization
 TEM: transverse electromagnetic, modes
 few in number, easy to see.
 Aim the laser at a distant screen, spread the beam out by a
negative lens.:
 bright patches, separated from one another by intervals
called "nodal lines".
 Within each patch, the phase of the light is the same,
but between patches the phase is reversed.
7.2 Practical Realization
TEM00 :
 lowest possible transverse
mode
 no phase reversal across the
beam, the beam is "uniphase"
 highest possible spatial
coherence, can be focused to
the smallest spot size and reach
the highest power density.
lowest possible axial mode:
laser oscillates in one frequency
highest possible temporal coherence
TEM modes
7.2 Practical Realization
5. Gain
Gain of a laser depends on several factors. Foremost among
them is the separation of the energy levels that provide
laser transition.
The two levels are father apart, the gain is higher because
then the laser transition contains a larger fraction of the
energy compared to the energy in the pump transition
7.2 Practical Realization
Gain is the opposite of absorption
0
   0 e x
--definition
: initial power in the cavity
: power of exit light
absorptivity  positive: for thermal equilibrium where N2 < N1.
absorptivity  negative: in population inversion, where N2 > N1,
laser emission could be considered negative absorption
gain coefficient : the negative of the "absorption coefficient“
    0 e x
  
7.2 Practical Realization
As the wave is reflected back and forth between the mirrors, it
will lose some of its energy, mainly because of the limited
reflectivity of one of mirrors.
If the two mirrors have reflectivities r1 and r2,
    0 r1 r2
r1 r2  e

--each round trip
: loss per round trip
    0 e (    ) x --For the system
 > : system will lase, threshold condition necessary to
sustain laser emission.
7.3 Types of Lasers
1. Solid-state Lasers
ruby laser
Ruby is synthetic aluminum oxide, Al2O3, with 0.03 to
0.05% of chromium oxide, Cr2O3, added to it. The
Cr3+ ions are the active ingredient; the aluminum and
oxygen atoms are inert.
The ruby crystal is made into a cylindrical rod, several
centimeters long and several millimeters in diameter,
with the ends polished flat to act as cavity mirrors.
Pumping is by light from a xenon flash tube.
7.3 Types of Lasers
E3: fairly wide and has a short
lifetime; the excited Cr3+ ions
rapidly relax and drop to the next
lower state, E2. This transition is
nonradiative.
Three-level energy diagram
typical of ruby
E2: metastable and has a lifetime
longer than that of E3, and the
Cr3+ ions remain that much
longer in E2 before they drop to
the ground state, E1.
The E2  E1 transition is radiative; it produces the spontaneous,
incoherent red fluorescence typical of ruby, with a peak near 694 nm.
As the pumping energy is increased above a critical threshold,
population inversion occurs in E2 with respect to E1 and the system
lases, with a sharp peak at 694.3 nm.
Lasing Action Diagram
Excited State
Energy
Introduction
Metastable State
Spontaneous
Energy
Emission
Stimulated
Emission of
Radiation
Ground State
Requirements for Laser Action
fast
slow relaxation
efficient pumping
Metastable state
slow
Population
inversion
Fast relaxation
7.3 Types of Lasers
neodymium: YAG laser
The active ingredient is trivalent neodymium, Nd3+, added to an
yttrium aluminum garnet, YAG, Y3Al5O12.
It has four energy levels. The laser transition begins at the
metastable state and ends at an additional level somewhat above
the ground state.
2. Gas Lasers
7.3 Types of Lasers
Gas lasers consist of a gas filled tube placed in the laser cavity. A
voltage (the external pump source) is applied to the tube to excite
the atoms in the gas to a population inversion. The light emitted
from this type of laser is normally continuous wave (CW).
helium-neon laser
Typically, it consists of a tube about 30 cm long and 2 mm in
diameter, with two electrodes on the side and fused silica windows at
both ends. The tube contains a mixture of 5 parts helium and 1 part
neon, kept at a pressure of 133 Pa.
7.3 Types of Lasers
argon laser
It
generates
a
strong
turquoise-blue line at 488 nm
and a green line at 514.5 nm,
in either pulsed or c. w.
operation.
helium-cadmium
It emits a brilliant blue at
441.6 nm.
7.3 Types of Lasers
carbon dioxide laser
 high power: the first CO2 lasers had a continuous output of a
few milliwatts. Today we have powers of some 200 kW, more
than enough to cut through steel plates several centimeters thick
in a matter of seconds.
 High efficient: the efficiency in converting electrical energy
into radiation is better (more than 10%) than that of any other
laser.(TEA CO2 laser)
 Relatively simple in construction and operation are.
Tunable in a small range
Emission is at 10.6 m.
7.3 Types of Lasers
Excimer lasers
contain rare-gas halides such as XeCl, KrF, or others.
These molecules are unstable in the ground state but bound
in the excited state.
exceedingly powerful, with outputs as high as several GW.
emit in the ultraviolet.
7.3 Types of Lasers
3. Semiconductor Lasers
LED: light-emitting diode
main application :
• waveguides
• integrated optics
 emit almost anywhere in the spectrum, from the UV to the IR
 an efficiency much higher than with optical pumping (around
40% versus 3%).
 small ,less than 1 mm in diameter
7.3 Types of Lasers
4. Tunable Lasers
 dye lasers: first tunable lasers
 parametric oscillator:
 more compact
 easier to operate
 less expensive
 tuning range much wider
 Color center lasers: tuned over wide bands in the UV,
the visible, and the IR.
 free-electron laser:
 high powers of the order of megawatts
 very efficient
 tuned through a wide range of wavelengths.
Tunable lasers are most welcome to spectroscopists
WAVELENGTHS OF MOST COMMON LASERS
Laser Type
Wavelength (m)
Argon fluoride (Excimer-UV)
Krypton chloride (Excimer-UV)
Krypton fluoride (Excimer-UV)
Xenon chloride (Excimer-UV)
Xenon fluoride (Excimer-UV)
Helium cadmium (UV)
Nitrogen (UV)
Helium cadmium (violet)
Krypton (blue)
Argon (blue)
Copper vapor (green)
Argon (green)
Krypton (green)
Frequency doubled
Nd YAG (green)
Helium neon (green)
Krypton (yellow)
Copper vapor (yellow)
Key:
UV = ultraviolet (0.200-0.400 µm)
VIS = visible (0.400-0.700 µm)
NIR = near infrared (0.700-1.400 µm)
0.193
0.222
0.248
0.308
0.351
0.325
0.337
0.441
0.476
0.488
0.510
0.514
0.528
0.532
0.543
0.568
0.570
Helium neon (yellow)
Helium neon (orange)
Gold vapor (red)
Helium neon (red)
Krypton (red)
Rohodamine 6G dye (tunable)
Ruby (CrAlO3) (red)
Gallium arsenide (diode-NIR)
Nd:YAG (NIR)
Helium neon (NIR)
Erbium (NIR)
Helium neon (NIR)
Hydrogen fluoride (NIR)
Carbon dioxide (FIR)
Carbon dioxide (FIR)
0.594
0.610
0.627
0.633
0.647
0.570-0.650
0.694
0.840
1.064
1.15
1.504
3.39
2.70
9.6
10.6
Laser Output
Pulsed Output (P)
Energy (Watts)
Energy (Joules)
Continuous Output (CW)
Time
Time
watt (W) - Unit of power or radiant flux (1 watt = 1 joule per second).
Joule (J) - A unit of energy
Energy (Q) The capacity for doing work. Energy content is commonly used to characterize the output
from pulsed lasers and is generally expressed in Joules (J).
Irradiance (E) - Power per unit area, expressed in watts per square centimeter.
7.4 Applications
Compared to radiation from other sources, laser radiation
stands out in several ways:
 highly coherent, both spatially and temporally
 generated in the form of very short pulses, at high powers
7.4 Applications
1. Beam Shape
laser operating in the TEM00 mode
the energy has a Gaussian distribution at a given distance r
from the axis, the irradiance I falls off exponentially
I (r )  I 0 e
 ( 2 r / w) 2
parameter w: the distance from the axis at which I has
dropped to 1/e2 of I0, the irradiance in the center
7.4 Applications
w(z) :beam's radius
 z 

w( z )  w0 1  
2 
  w0 
 : wavelength
2
w0:radius at the waist.
For a confocal cavity, this simplifies to
w0 
L
2
L : distance between the mirrors.
7.4 Applications
far field: farther away from the laser
beam's parameters can be considered linear functions of the
distance
far-field half-angle divergence


 w0
7.4 Applications
place a converging lens in the path of the light:
• beam to contract to a "focus“
• another waist where the beam's wavefronts are plane
• diameter radius of beam at the focus 2r:
4 f 
2r 
 d0
f: the lens a focal length
•radius of beam at the focus r
r  1.22
Rayleigh's criterion :
f 
D
  1.22

D
7.4 Applications
2. Power and Power Density
A typical laser pulse contains about 10 J of energy. If this
energy is delivered within a pulse only 0.5 ms long, the
output power is 20 kW.
Q awitching: compressing the energy into a very short period
of time.
7.4 Applications
3. Nonlinear Effects
Linear:the refractive index and the absorptivity of a
material are independent of the intensity of the light that
passes through.
Nonlinear: with very intense light, either the index or the
absorptivity or both may become nonlinear functions of the
intensity.
•change the refractive index, even of completely
transparent material.
•Heat and thermal expansion, as they occur with
absorbing materials, are not involved.
7.4 Applications
 plasma: a mixture of ions and free electrons rarely
found in nature except in the atmosphere of the sun
 self-focusing: a beam of light contracts into thin, short
lived, powerful threads of light that quickly shatter the
material through which they pass.
 Optical bistability arises in saturable systems
 Phase conjugation(wavefront reversal): a molecular
reflection of light. The reflected wavefronts are now
distorted opposite to those in the incident beam,
 Frequency doubling: generation of second harmonics.
7.4 Applications
4. Industrial Applications
 Cutting
 Drilling
 Welding
 Communications
 Optical radar
 precision measurements
Laser Technique
in GIS Data Acquisition
GIS:Globe Information System
Tasks
Acquire Laser mapping equipment for GIS
spatial data acquisition:
– Utility mapping (power poles, water valve, gas
pipe, water pipe, etc)
– Construction (ask for higher accuracy)
– Digital geology mapping (geologic features)
Laser Mapping – Why It is
Needed



Laser + GPS = fast
3D spatial data
acquisition
Mapping the
inaccessible area
More efficient and
cost effective
Technical Basics



Distance measure +
Angle (H & V)
measure
Using NIR / Red Laser
pulse for distance
measure
Using magnetic
compass or encoder
for angle
measurement
Emitter
Distance
Receiver
Laser Instrument
Distance = C x T / 2
C – speed of light
T – time
Technical Assessment Standards


Accuracy
 Distance
 Angle (horizontal &
vertical)
Function
 Support
Reflectorless
 Motorized
 Autotrack (highcaliber)
 Continuous mode
 Remote control
(high-caliber)
Performance
Measurement Time

Measurement Range

Ease of use
GPS integration

External interface

Display

Laser Plummet (highcaliber)

Cost
Company Briefs
Laser Atlanta Optics, Inc.,
Norcross, GA USA, has been
designing and manufacturing
eye-safe laser-based distance
and speed measuring
systems since 1989.
Currently manufacture a
product line based on a core
technology called the
Advantage. All of its
rangefinder products are
based on this proven design.
Measurement Devices Ltd.
(MDL) is a leading designer,
manufacturer and supplier of
laser measurement systems.
MDL is committed to preserving
a quality system throughout all
levels of our business, as well
as providing consistent, high
standards of customer service.
Advantage V.S. LaserAce/ALS
Laser Atlanta
Advantage
Accuracy
Performance
Function
MDL
LaserAce / ALS
Distance
±15.3 cm
Typically 10 cm
Typically 5 cm
Angle
±0.01°(en) both H & V;
H:±1°;V:±0.4° w/o en
H(encoder):0.2°; V: 0.3°;
Better ±1° w/o encoder;
H & V(en):0.02°
Range
2-610m w/o reflector;
2-91800m w/ reflector
300m w/o reflector;
5,000m w/ refelctor
300 (DM)/600(RF) w/o
5km (DM)/10km(RF) w/
Time
0.34 sec
0.3 sec
0.5s(DM)/self-adp(RF)
Reflectorless?
yes
Yes
yes
Motorized?
no
No
yes
Cont. Mode
yes
yes
yes
Ext. Interface
RS232 to PC
RS232 to PC
RS232 to PC
GPS integration
Yes(plug & play)
No
No
Display
HUD+ LCD
LCD
LCD
$4000 -$6000
$4,300 – $5,525
$21,000 - $27,720
Ease of Use
Cost
Advantage From Laser Atlanta
Physicals
•905 nm class I eye safe laser
•11.5Hx21.5Wx19Lcm
•4.5 lbs(2.1kg)
•Handle batteries
LaserAce From MDL
Physical
•GaAs Laser Diode 905nm
Physical
•Class I eye Safe
•Semi-conductor 905 nm
•175x106x55 mm (LxWxH)
•Class I eye safe
•600g
•209 x 243 x 420mm (LxWxH)
•Alignment Telescope: red dot
•9.7kg /10.3 kg (w/ tribrach)
•Compass(option)
•Optical encoder
•Pocket PC (option)
•Laptop/palm/desk top PC
Topcon: Pulse Total Station GPT2000 series








Using pulse laser technology
Support both prism/non-prism mode
High accuracy:
 Millimeter accuracy in distance measurement (5mm+2ppm
xD in non prism mode; 3mm+2ppmxD in prism mode)
 1”/ 5” (H & V) angle measurement accuracy
Fast data acquisition:
 0.3 sec tacking mode
 1.2 second fine mode
Long range:
 Prism: 7,000m
 Non prism: 150m
All weather operation: water /dust proof
Large data storage: 8000 points
Laser plummet
Total Station GTS-800/800A series
from Topcon




Motorized & automatic tracking – high
speed rotation (up to 50º /sec) and
high speed auto-tracking (up to 5º /sec)
Remote control through radio link or
optical remote controller – enables one
man operation
Flexible data management: Huge data
storage – 2Mb memory plus PCMCIA
card, space for data and software
User friendly






Large graphic display
Built-in MS-DOS OS
Compact and light weight
Water / dust resistant
Handheld data collector
TDS Survey Pro software allows more
functions: job classification, stake out,
etc.
Leica TCRA1100 series Total
Station






Motorized, automatic target recognition, reflectorless and
remote control
Accuracy:
 Angle measurement: from 1.5” to 5”
 Distance measurement: 3mm+2ppm w/o reflector;
2mm+2ppm w/ reflector
Range: 200m (w/o reflector) to 7.5 km (w/reflector)
Time
 1sec w/ reflector
 3 sec w/o reflector
Data storage: PCMCIA card or export via RS232
Software supports:
 computations of area, height, tie distance etc.
 stake outs
 Exchange data between instrument and PC
 Create code list
Laser Plummet
GPT2000 series Display
External Interface
The external interface
provides a way for the
instrument to
communicate with a PC,
a laptop, a palm or a
data logger. All the
models here have this
ability
7.4 Applications
5. Medical Applications
Coagulation & photocoagulation.
Photodisruption
Treatment of retinal
Laser refractive surgery

Correcting a refractive error by changing
the shape of the cornea with surgery.
diagram of an eye
Diagram of the Eye
Normal Focus
Short sightedness
(Myopia)
•Distance vision blurry, near usually OK.
Short-sighted
focus
Short-sighted
correction
Long-sightedness
(Hyperopia)
•Difficulty seeing clearly and comfortably
up close.
Long-sighted focus
Long-sighted correction
Astigmatism
Irregular curvature of the eye (shaped more
like a football than a basketball)
Light in different planes focuses at
different points
A
90
B
180
Shaping the cornea
Flattening the cornea, decreases myopia
 Steepening the cornea decreases hyperopia
 Making the cornea more spherical decreases
astigmatism
 All are possible in most refractive surgeries

Suitability for refractive surgery

Stable refractive error
– must not have changed for at least two years

Healthy eyes
– no underlying corneal abnormalities

Contact lens wear
– needs to be ceased at least three weeks prior to surgery
Refractive surgery does not mean
you will never wear glasses
again

Most people over 40-50 years will need glasses
for reading
– this may not be fixed by laser surgery

Sometimes the corneal shape can gradually
change
– (although this is minimised)