Laser Fundamentals and its Applications

Photonic NetworkBy Dr. M H Zaidi

Laser Fundamentals and its Applications


LASERLASER is acronym of Light Amplification by Stimulated Emission of Radiation.

http://www.semicon.toshiba.co.jp


Lasers


Outline

Introduction and OverviewTheory of OperationLaser ExpositionProperties of Laser BeamApplications• Optical Communication• Military Applications

Conclusion


Introduction and OverviewLaser Invention Landmark achievement of previous centuryImportant industrial and research tool in fields from medicine to communications engineering

What is a LASER ?Acronym: Light Amplification stimulated emission of radiationsLaser light is special lightOrdinary light --- Spontaneous emissionLaser light --- Stimulated emissionStimulated emission first postulated by Albert Einstein in 1917In 1950’s Charles Townes group invented laserIn 1960 T.H.Maiman invented first laser


Incandescent vs. Laser Light

1. Many wavelengths

2. Multidirectional

3. Incoherent

1. Monochromatic

2. Directional

3. Coherent


Properties of Laser LightMono chromaticity• Light of a single wavelength.

Directionality• A Laser Beam is so directional that it can be easily seen from

the moon or reflected back to the earth and detected here.Brightness• A 1 mw He-Ne Laser is 100 Times brighter than the sun.• Focused Laser beam is so intense that it can initiate nuclear

reactions.Coherence• Coherent light is in phase and has the same wavelength.


Laser applicationOptical communicationsUnguided systems• Deep space communications (Line of sight communications).

Guided systems• Optical fiber communications (claded dielectric waveguide).

Military applications• Laser range finding.• Pinpointing target for bombs and missiles.• Anti sensor weapons• Anti satellite weapons• Anti missile weapons• Battle simulation• Simulating effect of nuclear weapons.


Laser for tactical military applicationsATLIS POD

Air to ground “Automatic Tracking and Laser Illumination System” to deliver LGBs.Pin point accuracy of laser illumination provided to laser guided weapon.High Laser power enables use at distance beyond enemy ground to air weapons.Advance optical system offers several fields of vision and very high magnification.


Laser FundamentalsActive Medium• Collection of atoms, molecules, or ions that emit radiation in

the optical part of the electromagnetic spectrum.Population Inversion• Generated by pumping

Resonant Cavity• Provides optical feedback

Spontaneous Emission• It occurs without any external simulation

Stimulated Emission• It occurs when atom in exited state interacts with an incident

quantum of light energy at the transition frequency.


ENEGY-STATE-TRANSITION DIAGRAM

BEFORE AFTER

STIMULATEDABSORPTION

SPONTANEOUSEMISSION

STIMULATEDEMISSION

(C )

(b)

(a)

E1

E0

E1

E0

E1

E0

E1

E0

E1

E0

E1

E0


Absorption and Emission

spontaneaous emission: isotropic!

ground state

excited state

E

excited state

ground state

E

normal: t~10-8 s metastable: : t~10-3 s


ABSORPTIONAbsorption occurs when a photon collides with a lower state atomOne quantum of energy is removed from the optical electromagnetic field and the lower state atom rises to the state with energy level E2.

Where B = Einstein’s B Coefficient

ABSORPTION RATE = BN1 I


EMISSIONThis process results in atom being stimulated to descend from the upper state to the lower state giving off a photon of proper energy level in the process.The stimulated photon has the same phase, same polarization and travel in the same direction and propagation mode as the incident photon.

STIMULATED EMISSION RATE = BN2 I


SPONTANEOUS EMISSIONThis process is independent of incident field.It represents tendency of all systems, to move towards a lower energy state.Emitted photon, has random phase and direction, and it is thus a source of noise in these devices.

Where A = Einstein A Coefficient

SPONTANEOUS EMISSION RATE = AN2


Stimulated Emission (key to Laser Activity)

Lets E1 and E2 represent the two of the energy levels of an atom.

E2– E1 = h f f = ( E2 - E1 ) / h λ = c / f = h c / ( E2 - E1 )Where h = 6.626 x 10-34 Joules sec (Plank’s constant)Let N2 = # of atoms in the upper state

N1 = # of atoms in the lower stateN1 + N2 = Total Number of atoms of interests


Population InversionLet I(x) be the intensity of light crossing a plane perpendicular to x-

axis at x.

I(x +Δx) = I(x) – B N1 I(x) Δx + B N2 I(x) ΔxwhereI(x) = Inversion of incident beamB N1 I(x) Δx = absorptionB N2 I(x) Δx = stimulated emission

Homogeneous Medium

I (x + Δx) = ?I (x)

x x + Δx


Population Inversion

( I(x+Δx) – I(x) ) / Δx = -B N1 I(x) + B N2 I(x)δI / δx = -B (N1 - N2) I(x)δI / δx = -α I(x)Where α = B( N1 - N2)

The solution to this differential equation is decaying exponential.I(x) = I0 e-αx

Note that

When N1 > N2, α > 0 attenuation

When N1 < N2, α < 0 amplification



Hence, population inversion is a pre-requisite for amplification.Two most common excitation techniques are light and electricity.Practical Lasers involve 3 or 4 energy levelsThree level Lasers are not ideal since ground state is Lower energy levelResonant cavity extracts energy from medium with population inversion.



Highly exited level

LaserTransition


Between these 2 states

Meta Stable State

Ground State

Excitation

Three level system



Highly exited level

ExcitationLaser

Emission

Upper Layer(Meta Stable)

MaturalDepopulation

Population Inversion Between these states

Ground State

Four level system


Laser Exposition

Four General Categories of Lasers.

• Gas Lasers• Solid State Lasers (Doped Insulators)• Semiconductor Lasers• Dye Lasers


Laser expositionGas Lasers

• Most Gas Lasers are pumped by electric discharge.• He-Ne, Argon and CO2 Lasers are quite popular.Salient features

• Visible region operation.• Spectral purity• Coherence• Super beam quality. Uses

• Alignment, Signal and image processing applications


Laser exposition

Electric Power Supply

Electrode

Electric Discharge

Laser Gas

Rear Mirror

Outer Mirror

Laser Beam

Generic Gas Laser


He-Ne-laser

•pump He to metastable state (20.61 eV)•transfer excitation to Ne metastable state (20.66 eV)•laser transition•spontaneous emission (2 times) to deplete lower level (-> low pumping)•not very efficient! (20.6 eV vs 2 eV)


Laser expositionSolid state Lasers

• Most common light emitters are Cr+3, Na+3, Er+3 and HO+3

ions.• Na-YAG Lasers are most popular.Salient features

• High power output.• Spectral purity.• CoherenceUses

• Laser Range finding, Laser designators and industrial applications.


Laser expositionReflective cavity- focuses pump light onto laser rod

Lamp Power Supply

Lamp Light Source

PumpLight

Laser Rod

Output Mirror

Rear Mirror

A Generic Solid-State Laser


Example: solid state laserRuby-laser

τ=0.003 s

Xe

Maiman (1960):

Ruby: Al2O3 + Cr

cavityL =n λ

coherentmonochromatic

collimated


laser ranging experiment

McDonald observatoryApollo 11

D(earth-moon)~385000 km, accuracy: ~3cm!


Laser exposition

Dye Lasers

• Active medium is an organic dye dissolved in a solvent.• Dye called Rhodamine 6G dissolved in methanol is quite

popular,• All Dye lasers are optically pumped.

Salient features

• It has a broad tuning range (570 to 660 nm).

Uses

• Mainly used for scientific research.


Laser expositionSemiconductor Lasers

• Population inversion between conduction band and valence band.

• Forward-biasing the diode provide electrons into the conduction band.

• Pumping is provided by direct current source.• GaAs and AlGaAs are quite popular. Salient features

• Small Size and low cost.• Very efficient• Easy to modulate.Uses

• Optical fiber communication.


semiconductor lasers •photon production by electron hole pair recombination as in LED•above a treshold current, stimulated emission occurs -> lasing

L =n λ

CD player: GaAs, 5mW, 840 nm

Laser printer: AlGaAs, 50mW, 760 nmTelecom: GaInAlP, 20 mW, 1300 nm

compact, cheap, variable wavelength


Tunable lasersAlso known for not having a wavelength range because these lasers can access a wide-variety of wavelengths

Free electron laser ( = tunable): Free electron lasers are the newest and most secretive class of lasers.

These lasers utilize a stream of electrons as the medium and can emit a wavelength of light virtually anywhere in the electromagnetic spectrum. They are considered tunable because the wavelength can be changed, similar to adjusting the wavelength on the dial of a radio

Highly expensive and difficult to use, these lasers, however, can exhibit a clean cutting effect on tooth structure and bone because of the potential for "dialing" an optimal wavelength

Photonic NetworkBy Dr. M H Zaidi http://lw.pennnet.com/Articles/Article_Display.cfm?Section=Articles&SubSection=Display&ARTICLE_ID=94178&VERSION_NUM=1


Future Lasers

X-Ray Lasers

• Nuclear explosion can provide the energy needed to power x-ray Lasers (“Star Wars” program).

Free Electron Laser• Extract energy from a beam of electrons passing through a

wiggler magnet.• It is called wiggler magnet because of its effect on the

electron beam.


Laser ClassificationThe following criteria are used to classify lasers:

Wavelength: based on the most hazardous wavelength. For continuous wave (CW) or repetitively pulsed lasers the average power output(Watts) and limiting exposure time inherent in the design are considered. For pulsed lasers the total energy per pulse(Joule), pulse duration, pulse repetition frequency and emergent beam radiant exposure are considered.


Types of Laser Hazards


Important Laser Types & Wavelengths

Type Wavelength, nmKrypton-Fluoride Excimer 249Xenon-Chloride Excimer 308Nitrogen Gas (N2) 337Organic Dye (in solution) 300 - 1000 (tunable)Krypton Ion 335 - 800

Argon Ion 450 - 530 (488 & 514.5 strongest)

Helium Neon 543, 632.8, 1150Semiconductor (GaInP family) 670 - 680Ruby 694Semiconductor (GaAlAsfamily) 750 - 900

Neodymium YAG 1064Semiconductor (InGaAsPfamily) 1300 - 1600

Hydrogen-Fluoride Chemical 2600 - 3000Carbon Dioxide 9000 - 1100 (main line 10,600)

http://www.coseti.org/9010-011.htm


Efficiency

Laser Efficiency

Semiconductor ~50%

Ar+ 0.05%Rh. Dye 0.005%HeNe 0.05%Ruby 0.1%Ti:Sapphire 0.01%Nd:glass 1%Nd:YAG 0.5%CO2 10%


Applications

cut precise patterns in glass and metal reshape corneas to correct poor vision to provide intense heat in controlled fusion experiments supermarket checkout lines CD players


Economical ImpactCurrent Market Trends

Today as opto electronics become more commercial, this market generate about $ 15 billion a year

Future Market Projection

the world market for tunable lasers by 2007 should be about $ 2.4 billion a year


Nortel 980nm Pump Laser.

Unit: PCPrice(USD): $230.00 Unit Price for 10pcs or above: $195.00

Fitel 1480nm Pump Laser 160mW.

Unit: PCPrice(USD): $250.00 Unit Price for 10pcs or above: $225.00

http://www.go4fiber.com/

Economical Impact

http://www.go4fiber.com/bargain/products_detail.aspx?product_id=1306183&main_id=31&sub_id=103






ConclusionTOPICS COVERED IN THIS PRESENTATION:

How laser works?Laser StructureLaser TypesLaser Applications


Laser 1060 nm (5’000 CHFR)

Product code: LD-1060

Description: LD-1060s are thermoelectrically cooled single-mode-fiber pigtailed 1060 nm laser with advanced stainer layer multiple quantum well. A lensed fiber insure low tracking error, while the laser’s low threshold current results in long-term reliability. A backfacetmonitor is InGaAs.Optical and electrical characteristics:Threshold Current 10...40 mAForward voltage 1.2...2.5 VOptical Power 1...2 mWPeak wavelength 1060 +- 10 nmSpectral Width 4 nmRise Time <0,5 nsFall Time <0,5 nsThermistor R 10 +- 0.2 KohmTEC current 0.8 +- 20% APigtail fiber SM fiber 9/125 micronsFiber length ~50cm End fiber Connector FC/PC

http://72.14.203.104/search?q=cache:Ae9xk76606sJ:rd48.web.cern.ch/RD48/rosemail/000706-laser.doc+laser+prices+optical+transmitter&hl=en&gl=pk&ct=clnk&cd=17

1 USD = 1.5 CHF


http://72.14.203.104/search?q=cache:Ae9xk76606sJ:rd48.web.cern.ch/RD48/rosemail/000706-laser.doc+laser+prices+optical+transmitter&hl=en&gl=pk&ct=clnk&cd=17

Laser 660 nm (5’000 CHFR)

Product code: LD-660

Description: LD-660s are thermoelectrically cooled multi-mode-fiber pigtailed 660 nm laser with advanced stainer layer multiple quantum well. A lensedfiber insure low tracking error, while the laser’s low threshold current results in long-term reliability. A backfacet monitor is InGaAs.Optical and electrical characteristics:Threshold Current 20...60 mAForward voltage 1.4...2.5 VOptical Power 0.5...1.5 mWPeak wavelength 660 + 50 nm –20 nmSpectral Width 4 nmRise Time <0,5 nsFall Time <0,5 nsThermistor R 10 +- 0.2 KohmTEC current 0.8 +- 20% APigtail fiber MM fiber 62.5/125 micronsFiber length ~50cm End fiber Connector FC/PC

1 USD = 1.5 CHF

Laser Fundamentals and its Applications

Documents

Transcript of Laser Fundamentals and its Applications