1. 考试时间： 上课时间和教室

2. Light Amplification by Stimulated Emission of Radiation
Chapter 7. Lasers
(acronym)
Light Amplification by Stimulated Emission of Radiation

3. 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.

5. 7.1 Stimulated Emission of Radiation
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
E : energy lever of the system
: Boltzmann's constant
T : absolute temperature.

6. 7.1 Stimulated Emission of Radiation
Boltzmann's ratio or relative population : the ratio of the populations in the two states, N2 / N1. Or
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. 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:
A21: constant of proportionality
N2 : number of atoms (per unit volume) in the higher state

8. 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.

9. B21 : constant of proportionality
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
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.
B12 : constant of proportionality.

10. Transitions between energy states
7.1 Stimulated Emission of Radiation
Transitions between energy states

11. Einstein's coefficients: A21, B21, B12 Einstein's relations B21 = B12
7.1 Stimulated Emission of Radiation
Einstein's coefficients: A21, B21, B12
Einstein's relations
B21 = B12
(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

12. 3. Population Inversion thermal equilibrium system
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.

13. Incandescent vs. Laser Light Light from bulbs are due to spontaneous emission
Many wavelengths
Multidirectional
Incoherent
Monochromatic
Directional
Coherent

14. 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.

15. 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.

16. two-level system(ex. ammonia maser)
En, Nn
Em, Nm
Even with very a intense pump source, the best one can achieve with a two-level system is
excited state population = ground state population

17. three-level system  in equilibrium  excited
 normal Boltzmann distribution
 absorptive rather than emissive
 excited
 population inversion

18. 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

20. Basic components of a laser oscillator
7.2 Practical Realization
Basic components of a laser oscillator
 Energy source
 Medium
 Full reflectance mirrors
 Partially transparent mirrors
 Radiation

21. Common Components of all Lasers
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.
Excitation Mechanism
Excitation mechanisms pump energy into the active medium by one or more of three basic methods; optical, electrical or chemical.
High Reflectance Mirror
A mirror which reflects essentially 100% of the laser light.
Partially Transmissive Mirror
A mirror which reflects less than 100% of the laser light and transmits the remainder.

22. 2. Excitation optical pumping: ruby laser
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

23. 3. Cavity Configurations
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 plane-parallel and the confocal variety, type of cavity used most often in today's commercial lasers.

24. Cavity configurations
7.2 Practical Realization
Cavity configurations
L:distance between mirrors
R:radius of curvature

25. 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:
L : length of the cavity
q : number of half-wavelengths, or axial modes
the resonance condition for axial modes:
n: index of medium contained in a laser cavity
axial modes

26. 7.2 Practical Realization
two consecutive modes (which differ by q = 1), are separated by a frequency difference ,
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

27. Mode-locking

28. Active mode-locking

29. TEM: transverse electromagnetic, modes few in number, easy to see.
7.2 Practical Realization
transverse modes
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.

30. TEM modes TEM00 : lowest possible transverse mode
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.
TEM modes
lowest possible axial mode:
laser oscillates in one frequency
highest possible temporal coherence

32. 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

33. Gain is the opposite of absorption --definition
7.2 Practical Realization
Gain is the opposite of absorption
--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“

34. If the two mirrors have reflectivities r1 and r2, --each round trip
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,
--each round trip
: loss per round trip
--For the system
 > : system will lase, threshold condition necessary to sustain laser emission.

35. 7.3 Types of Lasers 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.

36. Three-level energy diagram typical of ruby
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.
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.
Three-level energy diagram typical of ruby
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 nm.

37. Lasing Action Diagram Excited State Spontaneous Energy Emission
Energy Introduction
Metastable State
Stimulated Emission of Radiation
Ground State

38. Requirements for Laser Action
efficient pumping
slow relaxation
Metastable state
fast
slow
Population inversion
Fast relaxation

39. 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.

40. 7.3 Types of Lasers
2. Gas 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.

41. argon laser helium-cadmium
7.3 Types of Lasers
argon laser
It generates a strong turquoise-blue line at 488 nm and a green line at nm, in either pulsed or c. w. operation.
helium-cadmium
It emits a brilliant blue at nm.

42. 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.

43. 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.

44. 3. Semiconductor Lasers LED: light-emitting diode main application :
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

45.  dye lasers: first tunable lasers  parametric oscillator:
7.3 Types of Lasers
4. Tunable Lasers
 dye lasers: first tunable lasers
 parametric oscillator:
 more compact  less expensive
 easier to operate  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

46. WAVELENGTHS OF MOST COMMON LASERS
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)
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)
      
Key:      UV   =   ultraviolet ( µm)              VIS   =   visible ( µm)              NIR   =   near infrared ( µm)
WAVELENGTHS OF MOST COMMON LASERS
Laser Type
Wavelength (mm)
                                              

47. Continuous Output (CW)
Laser Output
Continuous Output (CW)
Pulsed Output (P)
                       
Energy (Watts)
Time
Energy (Joules)
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.

48. 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

49. 1. Beam Shape laser operating in the TEM00 mode
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
parameter w: the distance from the axis at which I has dropped to 1/e2 of I0, the irradiance in the center

50.  : wavelength w0:radius at the waist.
7.4 Applications
w(z) :beam's radius
 : wavelength w0:radius at the waist.
For a confocal cavity, this simplifies to
L : distance between the mirrors.

51. far field: farther away from the laser
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

52. place a converging lens in the path of the light:
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:
f: the lens a focal length
radius of beam at the focus r
Rayleigh's criterion :

53. 2. Power and Power Density
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.

54. 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.

55.  Optical bistability arises in saturable systems
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.

56. 4. Industrial Applications
Cutting
Drilling
Welding
Communications
Optical radar
precision measurements

58. Laser Technique in GIS Data Acquisition
GIS:Globe Information System

59. 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)

60. Laser Mapping – Why It is Needed
Laser + GPS = fast 3D spatial data acquisition
Mapping the inaccessible area
More efficient and cost effective

61. 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
Receiver
Distance
Target
Laser Instrument
Distance = C x T / 2
C – speed of light
T – time

62. Technical Assessment Standards
Accuracy
Distance
Angle (horizontal & vertical)
Function
Support Reflectorless
Motorized
Autotrack (high-caliber)
Continuous mode
Remote control (high-caliber)
Performance
Measurement Time
Measurement Range
Ease of use
GPS integration
External interface
Display
Laser Plummet (high-caliber)
Cost

63. Company Briefs
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.
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.

64. Advantage V.S. LaserAce/ALS
Laser Atlanta
Advantage
MDL
LaserAce / ALS
Accuracy
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°
Performance
Range
2-610m w/o reflector;
m 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)
Function
Reflectorless?
yes
Yes
Motorized? no No
Cont. Mode
Ease of Use
Ext. Interface
RS232 to PC
GPS integration
Yes(plug & play)
Display
HUD+ LCD
LCD
Cost
$4000 -$6000
$4,300 – $5,525
$21,000 - $27,720

65. Advantage From Laser Atlanta
Physicals
905 nm class I eye safe laser
11.5Hx21.5Wx19Lcm
4.5 lbs(2.1kg)
Handle batteries

66. LaserAce From MDL Physical GaAs Laser Diode 905nm Class I eye Safe
175x106x55 mm (LxWxH)
600g
Alignment Telescope: red dot
Compass(option)
Pocket PC (option)
Physical
Semi-conductor 905 nm
Class I eye safe
209 x 243 x 420mm (LxWxH)
9.7kg /10.3 kg (w/ tribrach)
Optical encoder
Laptop/palm/desk top PC

67. Topcon: Pulse Total Station GPT-2000 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

68. 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.

69. 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

70. Laser Plummet

71. GPT2000 series Display

72. 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

73. 5. Medical Applications Coagulation & photocoagulation.
Photodisruption
Treatment of retinal

74. Laser refractive surgery
Correcting a refractive error by changing the shape of the cornea with surgery.

75. diagram of an eye
Diagram of the Eye

76. Normal Focus

77. Short sightedness (Myopia) Distance vision blurry, near usually OK.
correction
focus

78. Long-sightedness (Hyperopia)
Difficulty seeing clearly and comfortably up close.
Long-sighted focus
Long-sighted correction

79. Astigmatism
Irregular curvature of the eye (shaped more like a football than a basketball)
Light in different planes focuses at different points A B 90
180

80. 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

81. 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

82. Refractive surgery does not mean you will never wear glasses again
Most people over 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)