Lasers and the Space Lasers and the Space ElevatorElevator

Adaleena MookerjeeAdaleena Mookerjee

August 16, 2006August 16, 2006

Center for Structures in Extreme Center for Structures in Extreme Environments, Rutgers UniversityEnvironments, Rutgers University

What are Lasers?What are Lasers?

LASER (light amplification by stimulated LASER (light amplification by stimulated emission of radiation)emission of radiation)A device that controls the way energized atoms A device that controls the way energized atoms release photonsrelease photons

Laser HistoryLaser History

Conceptually Conceptually developed by Albert developed by Albert Einstein in 1917Einstein in 1917Actually built by Actually built by Theodore Maiman in Theodore Maiman in the 1960s (Ruby laser)the 1960s (Ruby laser)Previously, was not Previously, was not important at allimportant at allToday, it is all around Today, it is all around us from printers to us from printers to barcode readers at the barcode readers at the bookstore or mallbookstore or mall

Basic Phenomena of LightBasic Phenomena of Light

Wave duality of matter: light is capable of Wave duality of matter: light is capable of behaving like a wave and particlebehaving like a wave and particle

Light – The WaveLight – The Wave

Any electromagnetic Any electromagnetic radiation with a radiation with a wavelength visible to the wavelength visible to the eyeeye3 properties of light:3 properties of light:– Intensity (amplitude) – Intensity (amplitude) –

how bright something how bright something appears to the human appears to the human eyeeye

– Frequency (wavelength) Frequency (wavelength) – color of light produced– color of light produced

– Polarization – angle of Polarization – angle of vibration of lightvibration of light

Light the PhotonLight the Photon

Photons – quanta of Photons – quanta of electromagnetic radiation electromagnetic radiation which can be lightwhich can be lightBohr Model:Bohr Model:– Made up of 3 subatomic Made up of 3 subatomic

particlesparticlesProtons (positively charged Protons (positively charged subatomic particle)subatomic particle)Neutrons (neutrally charged Neutrons (neutrally charged subatomic particle)subatomic particle)Electrons (negatively charged Electrons (negatively charged subatomic particle)subatomic particle)

– Protons and neutrons are Protons and neutrons are located in nucleuslocated in nucleus

– Electrons are located in a Electrons are located in a hypothetical region hypothetical region surrounding the nucleussurrounding the nucleus

electrons neutrons

protons

Producing LightProducing Light

Step 1: Light is emitted and Step 1: Light is emitted and photons are released, photons are released, colliding with the orbiting colliding with the orbiting electronelectron

Step 2: Collision causes change Step 2: Collision causes change in velocity and position, in velocity and position, making the electron absorb making the electron absorb photon’s energyphoton’s energy

Step 3: Electron moves to a Step 3: Electron moves to a higher energy level or higher energy level or position around nucleus position around nucleus (excited!)(excited!)

Step 4: To return, electron Step 4: To return, electron releases energy from photon, releases energy from photon, producing lightproducing light

Wave and Photon CorrelationWave and Photon Correlation

Speed of light (c) = 300,000 km/sSpeed of light (c) = 300,000 km/sRelationship between photon and Relationship between photon and wave:wave:

Where Where λλ = wavelength and f = frequency = wavelength and f = frequency

fc

fc

Terminology – PhotonsTerminology – PhotonsPopulation inversion: when a Population inversion: when a system of either a group of system of either a group of molecules or molecules exist in a molecules or molecules exist in a state where more electrons are in state where more electrons are in an excited state than in the lowest an excited state than in the lowest energy level possibleenergy level possibleAbsorption: photon with a Absorption: photon with a particular frequency hits an atom particular frequency hits an atom at rest & excites it to a higher at rest & excites it to a higher energy level while photon is energy level while photon is absorbedabsorbedSpontaneous emission: atom in an Spontaneous emission: atom in an excited state emits a photon with a excited state emits a photon with a particular frequency & returns to particular frequency & returns to ground stateground stateStimulated emission: photon with Stimulated emission: photon with a certain frequency hits excited a certain frequency hits excited atom & releases two photons of atom & releases two photons of same frequency while electron same frequency while electron returns to ground statereturns to ground state

http://perg.phys.ksu.edu/vqm/laserweb/index.html

Population inversion: when a Population inversion: when a system of either a group of system of either a group of molecules or molecules exist in a molecules or molecules exist in a state where more electrons are in state where more electrons are in an excited state than in the lowest an excited state than in the lowest energy level possibleenergy level possibleAbsorption: photon with a Absorption: photon with a particular frequency hits an atom particular frequency hits an atom at rest & excites it to a higher at rest & excites it to a higher energy level while photon is energy level while photon is absorbedabsorbedSpontaneous emission: atom in an Spontaneous emission: atom in an excited state emits a photon with a excited state emits a photon with a particular frequency & returns to particular frequency & returns to ground stateground stateStimulated emission: photon with Stimulated emission: photon with a certain frequency hits excited a certain frequency hits excited atom & releases two photons of atom & releases two photons of same frequency while electron same frequency while electron returns to ground statereturns to ground state

Terminology -- WavesTerminology -- WavesScattering: when atoms of a Scattering: when atoms of a transparent material is not transparent material is not smoothly distributed over smoothly distributed over distances greater than the distances greater than the length of a light wavelength of a light waveReflection: light normally Reflection: light normally collides with the boundary of 2 collides with the boundary of 2 materialsmaterials– Objects contain free Objects contain free

electrons which jump from electrons which jump from one atom to another within itone atom to another within it

– Energized electrons vibrate Energized electrons vibrate sends back of object as sends back of object as light wave with same light wave with same frequency of incoming wavefrequency of incoming wave

– Does not deeply pierce Does not deeply pierce materialmaterial

Refraction: bending of Refraction: bending of light when it passes from light when it passes from one kind of material to one kind of material to anotheranother– Frequency of incoming Frequency of incoming

light matches natural light matches natural frequency in electronsfrequency in electrons

– Penetrates deeply into Penetrates deeply into material causing material causing vibrations in electronsvibrations in electrons

– Waves slow down and Waves slow down and outside it maintains outside it maintains original frequencyoriginal frequency

ExamplesExamples

scattering

reflection

refractionrefraction

Goals of PresentationGoals of Presentation

Discuss what exactly a Discuss what exactly a laser islaser isDiscuss how a laser Discuss how a laser works and how to build works and how to build your ownyour ownDiscuss the types of Discuss the types of lasers available today lasers available today (solid, gas, liquid, (solid, gas, liquid, semiconductor, semiconductor, excimer, free electron)excimer, free electron)Propose the best laser Propose the best laser for the space elevatorfor the space elevator

What is a Laser? (in words)What is a Laser? (in words)

Did you know? Actually, it is an acronym: LASER (light Did you know? Actually, it is an acronym: LASER (light amplification by stimulated emission of radiation)amplification by stimulated emission of radiation)A device that controls the way energized atoms release A device that controls the way energized atoms release photonsphotonsLaser light is very intense, highly directional & pure in Laser light is very intense, highly directional & pure in colorcolorNot very safe to look directly at laser lightNot very safe to look directly at laser light4 types of lasers:4 types of lasers:

1) solid state lasers1) solid state lasers2) gas lasers2) gas lasers3) liquid lasers3) liquid lasers4) semiconductor lasers4) semiconductor lasers

**Other forms also are excimer and free electron lasers, but they don’t fall into any of these categories.

Classified based on gain medium (that will be defined in one slide…) used

Parts of a LaserParts of a LaserEnergy source:

--begins the lasing process

--examples include: electrical discharges, flashlamps, arclamps, lights from another laser, lights from chemical reactions, lights from explosions

Gain Medium (excitation mechanism):

--transfers external energy to beam

--excites particles

--keeps laser at desired wavelength

--absorbs energy in the laser

--maintains the laser

Optical Resonator (feedback): arrangement of optical components allowing beam to circulate

Output Coupler:

--where light is allowed to come out

--semitransparent mirrors

--controls effective output of light produced

Laser Function & ConstructionLaser Function & Construction

As you noticed, light is As you noticed, light is commonly used as the commonly used as the energy source and the energy source and the gain mediumgain mediumEnergy source applied to Energy source applied to systemsystemGain medium transfers Gain medium transfers energy to beam, energy to beam, energizing electronsenergizing electronsElectrons give off light Electrons give off light energy to return back to energy to return back to its original energy levelits original energy levelResonators produce Resonators produce more laser lightmore laser light

Need to have knowledge Need to have knowledge in glassblowing, in glassblowing, fabricating small parts, fabricating small parts, operating a vacuumoperating a vacuumUse a solid, liquid or gas Use a solid, liquid or gas medium (best gas is medium (best gas is nitrogen)nitrogen)Two resonating mirrors Two resonating mirrors are used to reflect light are used to reflect light formedformedEnergy source applied to Energy source applied to the systemthe systemEmission of photons will Emission of photons will result in lightresult in light

Solid-State LasersSolid-State Lasers

Uses a gain medium Uses a gain medium which is a solid (not which is a solid (not semiconductor)semiconductor)

Energy source: flashlamp Energy source: flashlamp or arclampor arclamp

Optical resonator: two Optical resonator: two mirrors in parallelmirrors in parallel

Produces power ranging Produces power ranging from milliwatts to from milliwatts to kilowatts kilowatts light lasts for light lasts for short durationsshort durations

The Ruby Laser (a solid state laser)The Ruby Laser (a solid state laser)

11stst laser by Theodore Maiman laser by Theodore MaimanUsed a synthetic ruby and Used a synthetic ruby and made it in shape of cylindermade it in shape of cylinderWrapped it around a high Wrapped it around a high intensity lampintensity lampThe blue and green The blue and green wavelengths from the white wavelengths from the white light triggered an excitement light triggered an excitement within electrons of chromium within electrons of chromium atomatomWhen returned to stable When returned to stable state, they released energy in state, they released energy in form of ruby lightform of ruby lightPhenomena continued until Phenomena continued until critical level reached and critical level reached and pulse releasedpulse released

Implementation on the Space Implementation on the Space ElevatorElevator

High powered solid state High powered solid state lasers may be successful lasers may be successful with providing powerwith providing powerObtaining solids may be a Obtaining solids may be a problemproblemMost acquirable item Most acquirable item today is Nd:YAG today is Nd:YAG (neodymium doped (neodymium doped yttrium aluminum garnet)yttrium aluminum garnet)Produces very limited Produces very limited powerpower

Gas LasersGas Lasers

Active Medium: pure gas, Active Medium: pure gas, mixture of gases, metal mixture of gases, metal vaporvaporEnergy Source: electrical Energy Source: electrical discharge, flashlamp, discharge, flashlamp, arclamp, light from other arclamp, light from other laser, chemical reaction laser, chemical reaction or explosionor explosionOptical Resonator: 2 Optical Resonator: 2 mirrors in parallel to each mirrors in parallel to each otherotherPower Generated: 50 Power Generated: 50 watts to 4 kilowatts of watts to 4 kilowatts of powerpower

COCO22, N, N22 and HeCd Lasers and HeCd LasersCO2 Lasers:

--uses CO2 to begin lasing process

--Active medium: 1 carbon dioxide, 1 nitrogen gas, 1 helium

--Process:

1) nitrogen added, exciting carbon dioxide

2) helium added to remove electrons from lowest energy level (population inversion)

3) tube sealed and voltage added exciting system

N2 Lasers:

--uses N2 as active medium

--high voltage power added to system

--creates electrical discharge & population inversion

--laser acts for short time

--good for scientific research, pumping other lasers

--minimal damages

HeCd Lasers:

--metal is cooked and vaporized

--helium excited by collisions with excited electrons

--pass on to cadmium atoms

--cadmium heated and added to helium gas

--helium fills cavity while cadmium goes to cathode

Implementation on the Space Implementation on the Space ElevatorElevator

Possibly a good ideaPossibly a good ideaCould utilize gases Could utilize gases available on Earth in available on Earth in excess, reducing excess, reducing pollution or its harmful pollution or its harmful effectseffectsNegative effect: too much Negative effect: too much gas or some gases could gas or some gases could corrode elevator, cause corrode elevator, cause possible explosions and possible explosions and potentially damage the potentially damage the EarthEarthModerate amounts of gas Moderate amounts of gas needed for useneeded for use

Liquid LasersLiquid Lasers

Active Medium: liquidActive Medium: liquidEnergy Source: light from Energy Source: light from another laseranother laserOptical Resonator: 2 Optical Resonator: 2 mirrors in parallel with mirrors in parallel with one anotherone anotherPower Generated: few Power Generated: few watts covering radius of watts covering radius of 20 micrometers20 micrometersTunable over a wide Tunable over a wide range and produces a range and produces a broad range of colors in broad range of colors in visible spectrumvisible spectrum

Dye LasersDye Lasers

Uses organic liquid Uses organic liquid dyes as active dyes as active mediummedium1 cm long quartz 1 cm long quartz glass tubeglass tubeDye cell: inside of the Dye cell: inside of the tube which consists of tube which consists of partially reflective partially reflective mirrors on the front & mirrors on the front & diffraction grating on diffraction grating on rearrear

Laser Action:Laser Action:– Energy added by a light Energy added by a light

source such as source such as flashlamp/laserflashlamp/laser

– Dye absorbs wavelength of Dye absorbs wavelength of light shorter than what it light shorter than what it emits and input energy in emits and input energy in forms of energy & heatforms of energy & heat

– Absorbed energy creates a Absorbed energy creates a population inversion population inversion (electrons excited)(electrons excited)

– Vibrational energy loss Vibrational energy loss causes dye molecules to causes dye molecules to go into lowest energy statego into lowest energy state

– Emission occurs when Emission occurs when vibrational levels reach vibrational levels reach ground state.ground state.

Implementation on the Space Implementation on the Space Elevator Elevator

Not practicalNot practicalProvides too little powerProvides too little powerNot feasible to use organic Not feasible to use organic dye (harder to maintain)dye (harder to maintain)Dye could get stale over time Dye could get stale over time (requires replacement every (requires replacement every few days)few days)Could cause malfunction of Could cause malfunction of the laser if dye is not the laser if dye is not replacedreplacedDependent on another laser Dependent on another laser for starting, so two lasers for starting, so two lasers would be necessarywould be necessary

Semiconductor LasersSemiconductor Lasers

Active Medium: Active Medium: semiconductor solid semiconductor solid (solid which conducts (solid which conducts electricity) – needs to electricity) – needs to confine carrier & take up confine carrier & take up small volumesmall volumeEnergy Source: Energy Source: electrical impulseelectrical impulse11stst semiconductor: 1962 semiconductor: 1962– Coherent Coherent

electromagnetic electromagnetic radiation produced by radiation produced by a p-n junction using a p-n junction using GaAsGaAs

Semiconductor TerminologySemiconductor Terminology

p-type semiconductor: p-type semiconductor: semiconductor in which semiconductor in which electrical conduction is electrical conduction is due chiefly to the due chiefly to the movement of positive movement of positive holesholesn-type semiconductor: n-type semiconductor: electrical conduction due electrical conduction due chiefly to the movement chiefly to the movement of electronsof electronsp-n junction: where p-type p-n junction: where p-type semiconductor is semiconductor is adjoined with the n-type adjoined with the n-type semiconductorsemiconductor

Valence band: where highest Valence band: where highest energy level has full number energy level has full number of electronsof electronsConduction band: where Conduction band: where lowest energy level has no lowest energy level has no electronselectronsBand gap (energy gap): space Band gap (energy gap): space between valence band and between valence band and conduction bandconduction bandMinority carrier: contains few Minority carrier: contains few mobile electrons in p-type mobile electrons in p-type semiconductor region & free semiconductor region & free holes in n-type semiconductor holes in n-type semiconductor regionregionMajority carrier: free holes in Majority carrier: free holes in p-type region & electrons in n-p-type region & electrons in n-type regiontype region

Semiconductor LasingSemiconductor Lasing

Population Inversion: charge Population Inversion: charge carriers (electrons) cross p-n carriers (electrons) cross p-n junctions junctions minority carriers minority carriers

Minority carriers mix with Minority carriers mix with majority carriersmajority carriers

Photon is absorbed by Photon is absorbed by electrons (gives energy to electrons (gives energy to jump from valence to jump from valence to conduction)conduction)

Leads to stimulated emission, Leads to stimulated emission, releasing a photonreleasing a photon

Optical resonator reflects the Optical resonator reflects the light out & sometimes back light out & sometimes back into the solidinto the solid

Implementation on the Space Implementation on the Space ElevatorElevator

Power Generated: Power Generated: few milliwattsfew milliwatts

Will not produce Will not produce enough powerenough power

Small in size & won’t Small in size & won’t create enough lightcreate enough light

Option: use a system Option: use a system of semiconductor of semiconductor lasers lasers pretty costly pretty costly

Excimer LasersExcimer Lasers

Active Medium: noble gas Active Medium: noble gas (argon, krypton, xenon) + (argon, krypton, xenon) + halogen (fluorine, halogen (fluorine, chlorine, bromine, iodine)chlorine, bromine, iodine)– Exists for 10 nanoseconds Exists for 10 nanoseconds

during excited stateduring excited state– In ground state, exists as In ground state, exists as

separate atomsseparate atoms

Energy Source: UV lightEnergy Source: UV light

Optical Resonator: 2 Optical Resonator: 2 mirrors in parallel to each mirrors in parallel to each otherother

Excimer LasersExcimer Lasers

Chemical Composition:Chemical Composition:– 0.1-0.2% halogen0.1-0.2% halogen– Little noble gasLittle noble gas– 90% of helium or neon90% of helium or neon

Laser Action:Laser Action:– When electrical discharge or energy is added to noble When electrical discharge or energy is added to noble

gas, can bind to halogen (excited)gas, can bind to halogen (excited)– Gives up additional energy through stimulated Gives up additional energy through stimulated

emission, forming ground state moleculeemission, forming ground state molecule– Within picoseconds, can separate into 2 atoms Within picoseconds, can separate into 2 atoms

population inversionpopulation inversion

Implementation on the Space Implementation on the Space ElevatorElevator

FACT: sun is composed FACT: sun is composed of hydrogen, helium, of hydrogen, helium, oxygen, carbon, iron, oxygen, carbon, iron, neon, nitrogen, silicon, neon, nitrogen, silicon, magnesium & sulfurmagnesium & sulfurUse solar radiation as Use solar radiation as energy source for the energy source for the laser (excess sunlight)laser (excess sunlight)Lasing process is for few Lasing process is for few nanoseconds, but power nanoseconds, but power generated: few watts to generated: few watts to few hundreds of wattsfew hundreds of wattsRadiation exposure will Radiation exposure will be minimal, but effective be minimal, but effective for the space elevatorfor the space elevator

Free Electron LasersFree Electron Lasers

Best laser according to Best laser according to Edwards & WestlingEdwards & WestlingDevice which emits high Device which emits high powered electromagnetic powered electromagnetic radiation at any radiation at any wavelengthwavelengthContains an array of Contains an array of magnets in magnetic field magnets in magnetic field to excite free unbound to excite free unbound electronselectronsTunable over broad range Tunable over broad range of wavelengthsof wavelengthsClass IV lasers: capable Class IV lasers: capable of starting fires, burn flesh of starting fires, burn flesh and cause eye damageand cause eye damage

Free Electron LasersFree Electron LasersBeam of electrons Beam of electrons accelerated to relativistic accelerated to relativistic speeds (electron speeds (electron accelerator)accelerator)Electrons pass through Electrons pass through periodic, transverse periodic, transverse magnetic fieldmagnetic fieldMagnetic field causes Magnetic field causes electrons to travel at a electrons to travel at a sinusoidal pathsinusoidal pathElectrons move at higher Electrons move at higher speeds, releasing photonsspeeds, releasing photonsOptical mirrors lengthen Optical mirrors lengthen processprocess

Implementation on the Space Implementation on the Space ElevatorElevator

Efficiency of 65%Efficiency of 65%Can emit radiation at Can emit radiation at any wavelength any wavelength (tunable)(tunable)Accelerated electrons Accelerated electrons release x-rays at release x-rays at hazardous levelshazardous levelsProduces high quantities Produces high quantities of powerof powerElectron accelerator is Electron accelerator is bigbigVery expensiveVery expensive

ConclusionsConclusions

Best Laser: gas or excimer laserBest Laser: gas or excimer laserWhy? – can use gases in the solar system or Why? – can use gases in the solar system or atmosphereatmosphereOther lasers would require further researchOther lasers would require further researchOther Factors to Study: Other Factors to Study: – Threshold of maximum gas neededThreshold of maximum gas needed– Overall harmful effects of lasers for necessary Overall harmful effects of lasers for necessary

protectionprotection– Some type of radiation shieldingSome type of radiation shielding– What should be done if gas runs out?What should be done if gas runs out?– Process of the laserProcess of the laser– Strength to endure effects of natureStrength to endure effects of nature

AcknowledgementsAcknowledgements

At this time, I would like to thank: At this time, I would like to thank: Professor BenaroyaProfessor Benaroya for giving me this for giving me this opportunity to learn about lasersopportunity to learn about lasersYuriy GulakYuriy Gulak for setting me up here and for setting me up here and familiarizing me with the technology familiarizing me with the technology available hereavailable hereEveryone of youEveryone of you for teaching me about for teaching me about your research & making me feel your research & making me feel comfortable herecomfortable here