Laser Fundamentals and its Applications. Photonic Network By Dr. M H Zaidi
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1 Laser Fundamentals and its Applications
2 LASER LASER is acronym of Light Amplification by Stimulated Emission of Radiation.
3 Lasers
4 Outline Introduction and Overview Theory of Operation Laser Exposition Properties of Laser Beam Applications Optical Communication Military Applications Conclusion
5 Introduction and Overview Laser Invention Landmark achievement of previous century Important industrial and research tool in fields from medicine to communications engineering What is a LASER? Acronym: Light Amplification stimulated emission of radiations Laser light is special light Ordinary light --- Spontaneous emission Laser light --- Stimulated emission Stimulated emission first postulated by Albert Einstein in 1917 In 1950 s Charles Townes group invented laser In 1960 T.H.Maiman invented first laser
6 Incandescent vs. Laser Light 1. Many wavelengths 2. Multidirectional 3. Incoherent 1. Monochromatic 2. Directional 3. Coherent
7 Properties of Laser Light Mono 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.
8 Laser application Optical communications Unguided 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.
9 Laser for tactical military applications ATLIS 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.
10 Laser Fundamentals Active 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.
11 ENEGY-STATE-TRANSITION DIAGRAM BEFORE AFTER E 1 E 0 STIMULATED ABSORPTION (a) E 1 E 0 E 1 E 0 E 1 E 0 SPONTANEOUS EMISSION (b) STIMULATED EMISSION (C ) E 1 E 0 E 1 E 0
12 Absorption and Emission excited state ground state spontaneaous emission: isotropic! excited state normal: t~10-8 s metastable: : t~10-3 s E ground state E
13 ABSORPTION Absorption occurs when a photon collides with a lower state atom One quantum of energy is removed from the optical electromagnetic field and the lower state atom rises to the state with energy level E 2. ABSORPTION RATE = BN 1 I Where B = Einstein s B Coefficient
14 EMISSION This 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 = BN 2 I
15 SPONTANEOUS EMISSION This 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. SPONTANEOUS EMISSION RATE = AN 2 Where A = Einstein A Coefficient
16 Stimulated Emission (key to Laser Activity) Lets E1 and E2 represent the two of the energy levels of an atom. E 2 E 1 = h f f = ( E 2 -E 1 )/ h λ = c / f = h c / ( E 2 -E 1 ) Where h = x Joules sec (Plank s constant) Let N 2 = # of atoms in the upper state N 1 = # of atoms in the lower state N 1 + N 2 = Total Number of atoms of interests
17 Population Inversion Let I(x) be the intensity of light crossing a plane perpendicular to x- axis at x. I (x) I (x + Δx) =? x x + Δx Homogeneous Medium I(x +Δx) = I(x) B N 1 I(x) Δx + B N 2 I(x) Δx where I(x) = Inversion of incident beam B N 1 I(x) Δx = absorption B N 2 I(x) Δx = stimulated emission
18 Population Inversion ( I(x+Δx) I(x) ) / Δx= -B N 1 I(x) + B N 2 I(x) δi / δx = -B (N 1 -N 2 ) I(x) δi / δx= -α I(x) Where α = B( N 1 -N 2 ) The solution to this differential equation is decaying exponential. I(x) = I 0 e -αx Note that When N 1 > N 2, α > 0 attenuation When N 1 < N 2, α < 0 amplification
19 Population Inversion 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 levels Three level Lasers are not ideal since ground state is Lower energy level Resonant cavity extracts energy from medium with population inversion.
20 Population Inversion Excitation Laser Transition Highly exited level Meta Stable State Population Inversion Between these 2 states Ground State Three level system
21 Population Inversion Highly exited level Upper Layer (Meta Stable) Excitation Laser Emission Population Inversion Between these states Matural Depopulation Ground State Four level system
22 Laser Exposition Four General Categories of Lasers. Gas Lasers Solid State Lasers (Doped Insulators) Semiconductor Lasers Dye Lasers
23 Laser exposition Gas Lasers Most Gas Lasers are pumped by electric discharge. He-Ne, Argon and CO 2 Lasers are quite popular. Salient features Visible region operation. Spectral purity Coherence Super beam quality. Uses Alignment, Signal and image processing applications
24 Laser exposition Laser Gas Laser Beam Rear Mirror Electric Discharge Electrode Outer Mirror Electric Power Supply Generic Gas Laser
25 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)
26 Laser exposition Solid 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. Coherence Uses Laser Range finding, Laser designators and industrial applications.
27 Laser exposition Reflective cavity- focuses pump light onto laser rod Rear Mirror Laser Rod Pump Light Lamp Light Source Output Mirror Lamp Power Supply A Generic Solid-State Laser
28 Example: solid state laserruby-laser Maiman (1960): cavity L =n λ Ruby: Al 2 O 3 + Cr Xe coherent monochromatic collimated τ=0.003 s
29 laser ranging experiment Apollo 11 McDonald observatory D(earth-moon)~ km, accuracy: ~3cm!
30 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.
31 Laser exposition Semiconductor 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.
32 semiconductor lasers photon production by electron hole pair recombination as in LED above a treshold current, stimulated emission occurs -> lasing CD player: GaAs, 5mW, 840 nm L =n λ Laser printer: AlGaAs, 50mW, 760 nm Telecom: GaInAlP, 20 mw, 1300 nm compact, cheap, variable wavelength
33 Tunable lasers Also 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
34
35 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.
36 Laser Classification The 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.
37 Types of Laser Hazards
38 Important Laser Types & Wavelengths Type Krypton-Fluoride Excimer 249 Xenon-Chloride Excimer 308 Nitrogen Gas (N 2 ) 337 Organic Dye (in solution) Krypton Ion Argon Ion Wavelength, nm (tunable) (488 & strongest) Helium Neon 543, 632.8, 1150 Semiconductor (GaInP family) Ruby 694 Semiconductor (GaAlAs family) Neodymium YAG 1064 Semiconductor (InGaAsP family) Hydrogen-Fluoride Chemical Carbon Dioxide (main line 10,600)
39 Efficiency Laser Efficiency Ar % Rh. Dye 0.005% HeNe 0.05% Ruby 0.1% Ti:Sapphire 0.01% Nd:glass 1% Nd:YAG 0.5% CO 2 10% Semiconductor ~50%
40 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
41 Economical Impact Current 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
42 Economical Impact Nortel 980nm Pump Laser. Unit: PC Price(USD): $ Unit Price for 10pcs or above: $ mW. Fitel 1480nm Pump Laser Unit: PC Price(USD): $ Unit Price for 10pcs or above: $
43 Conclusion TOPICS COVERED IN THIS PRESENTATION: How laser works? Laser Structure Laser Types Laser Applications
44 Laser 1060 nm (5 000 CHFR) 1 USD = 1.5 CHF 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 backfacet monitor is InGaAs. Optical and electrical characteristics: Threshold Current ma Forward voltage V Optical Power mw Peak wavelength nm Spectral Width 4 nm Rise Time <0,5 ns Fall Time <0,5 ns Thermistor R Kohm TEC current % A Pigtail fiber SM fiber 9/125 microns Fiber length ~50cm End fiber Connector FC/PC
45 Laser 660 nm (5 000 CHFR) 1 USD = 1.5 CHF 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 lensed fiber 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 ma Forward voltage V Optical Power mw Peak wavelength nm 20 nm Spectral Width 4 nm Rise Time <0,5 ns Fall Time <0,5 ns Thermistor R Kohm TEC current % A Pigtail fiber MM fiber 62.5/125 microns Fiber length ~50cm End fiber Connector FC/PC
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