ECE 455 Lecture 1
What is a Introduction laser?
Three Atomic ECE 455 Optical Electronics Processes
Three Components of Every Laser Tom Galvin Laser Light Characteristics Gary Eden Units of Laser Spectroscopy If changes need to be made to these notes, History please contact Kavita Desai: [email protected] Laser Applications ECE Illinois Contact Information
ECE 455 Lecture 1
What is a Professor: laser? Gary Eden Three Atomic Processes ECEB 2042
Three [email protected] Components of Every Laser
Laser Light Characteristics Teaching Assistant: Units of Laser Spectroscopy Kavita Desai History [email protected] Laser Applications LASER
ECE 455 Lecture 1
What is a laser? Light Three Atomic Processes
Three Components Amplification by of Every Laser
Laser Light Characteristics Stimulated Units of Laser Spectroscopy History Emission of Laser Applications Radiation Atomic Energy Levels
ECE 455 Lecture 1
What is a laser?
Three Atomic Individual atoms Processes have discrete energy Energy Three Components levels of Every Laser Solids will have Laser Light Characteristics energy bands Units of Laser Spectroscopy Light can cause History atoms to transition Laser between energy Applications levels Absorption
ECE 455 Lecture 1
What is a laser?
Three Atomic Processes 2
Three Components of Every Laser When a two-level h = − Laser Light system absorbs a Characteristics photon, it is left in Units of Laser Spectroscopy an excited state
History 1
Laser Applications Spontaneous Emission
ECE 455 Lecture 1 Two-level system spontaneously What is a decays from a higher laser?
Three Atomic energy level to a Processes lower energy level 2 Three Components and emits a photon. of Every Laser The decay rate is Laser Light h = − Characteristics characterized by the Units of Laser spontaneous Spectroscopy emission lifetime τ History 1 Laser Emission is uniform Applications in all 4π steradians Spontaneous emission can only be explained by QED Stimulated Emission
ECE 455 Lecture 1 Photon interacts with two-level What is a laser? system in an excited Three Atomic state Processes 2 Three The system goes to Components of Every Laser a lower energy level Laser Light and a photon with Characteristics the corresponding Units of Laser Spectroscopy energy is emitted History Emitted photon has 1 Laser Applications the same direction, polarization, phase and energy as the incident photon Absorption and Stimulated Emission
ECE 455 Lecture 1
What is a laser?
Three Atomic Processes Absorption and stimulated emission are inverse processes. Three Components The probability for a photon to be absorbed by an atom in of Every Laser its ground state is the same as the probability for the same Laser Light Characteristics photon to induce stimulated emission from an atom in its Units of Laser upper state. Spectroscopy
History
Laser Applications To Have Optical Gain...
ECE 455 In order to have gain, there must be more atoms in the upper Lecture 1 energy state than in the lower energy state. This situation is
What is a known as population inversion. laser?
Three Atomic Processes N2 N2
Three Components of Every Laser
Laser Light Characteristics
Units of Laser Spectroscopy
History N1 N1 Laser Applications Figure: Net loss will occur Figure: Net gain will occur What would happen if the populations of the upper and lower states were equal? Three Components of Every Oscillator
ECE 455 Lecture 1 Power Supply What is a laser?
Three Atomic Processes
Three Components of Every Laser Amplifier Output
Laser Light Characteristics
Units of Laser Spectroscopy
History
Laser Applications
Feedback Three Components of Every Laser
ECE 455 Lecture 1
Power What is a (PUMP) laser?
Three Atomic Processes M M Three Components of Every Laser Optical frequency Laser Light Characteristics amplifier (GAIN MEDIUM) Units of Laser Spectroscopy History L Laser Applications
Feedback (Cavity) Characteristics of Gain Medium
ECE 455 Lecture 1
What is a Provides optical gain laser? Stores energy from the pump Three Atomic Processes Energy levels in atomic media are set by nature. It is up to Three Components the laser designer to figure out how to use them. of Every Laser In semiconductor lasers, designers tweak semiconductor Laser Light Characteristics dimensions and composition to adjust the energy levels. Units of Laser Examples of gain media Spectroscopy
History Titanium atoms doped into sapphire crystal (Ti:Al2O3)
Laser Neon atoms in a plasma Applications Organic dye molecules in a liquid solution Quantum dots in semiconductor lasers Gain Medium
ECE 455 The gain medium by itself is not very exciting. Lecture 1
What is a laser?
Three Atomic Processes
Three Components of Every Laser
Laser Light Characteristics
Units of Laser Spectroscopy
History
Laser Applications
Ground State Atom Excited Atom Photon Characteristics of a Pump Source
ECE 455 Lecture 1
What is a Source of energy for the laser laser?
Three Atomic Moves the lasing species from its ground state to an Processes excited state, though not necessarily the upper level of the Three Components laser transition of Every Laser Examples of pumps Laser Light Characteristics Flashlamp
Units of Laser Another laser Spectroscopy Direct electrical pumping (as in a diode laser) History Collisional pumping (electron collisions in a plasma) Laser Chemical reactions (reactions do not necessarily leave the Applications product molecule in its ground state) Gain Medium and Pump
ECE 455 Lecture 1
What is a laser?
Three Atomic Processes
Three Components of Every Laser
Laser Light Characteristics
Units of Laser Spectroscopy
History
Laser Applications Ground State Atom Excited Atom Photon Gain Medium and Pump
ECE 455 Lecture 1 Photons only pass through a fraction of the gain medium before escaping. What is a The average distance a photon travels through a gain laser?
Three Atomic medium before causing stimulated emission is: Processes
Three 1 Components Lopt = (1) of Every Laser σse (N2 − N1)
Laser Light Characteristics where σse is a constant and N1 and N2 are the population Units of Laser number of the upper and lower states. Spectroscopy
History If Lgain Lopt , there will be negligible stimulated
Laser emission. Applications Excited atoms relax via spontaneous emission Light is emitted equally in all directions If a signal is introduced into one end of this device, the gain medium can be used as an optical amplifier. Characteristics of Optical Cavities
ECE 455 Lecture 1 Provides feedback of the optical field What is a laser? Increases the effective length of the gain medium
Three Atomic Processes Determines the spatial characteristics of the laser beam Three Resonant modes of the cavity determine the exact Components of Every Laser frequency of oscillation Laser Light Characteristics Output power strongly influenced by loss of cavity.
Units of Laser Examples of cavities Spectroscopy A pair of high-reflecting mirrors may be placed around a History gain medium or form its boundaries Laser Applications Edge-emitting semiconductor lasers receive feedback from reflections at the boundaries of the semiconductor Cavities are not required for all lasers “Mirrorless Lasers”) Gain Medium, Pump and Cavity
ECE 455 Lecture 1
What is a laser? Pump
Three Atomic Processes M M Three Components of Every Laser
Laser Light Characteristics
Units of Laser Spectroscopy
History Gain Medium Laser Applications Ground State Atom Excited Atom Photon Gain Medium, Pump and Cavity
ECE 455 Lecture 1 Mirrors force photons to pass through the gain medium several times, increasing the effective length of the gain What is a laser? medium along the axis of the cavity.
Three Atomic If Lgain,eff ≥ Lopt , then on average, each photon causes Processes more than one stimulated emission before it leaves the Three Components gain medium and the number of photons grows. A laser is of Every Laser born! Laser Light Characteristics Note that the effective gain length is only increased on the Units of Laser Spectroscopy cavity’s axis History Therefore strong stimulated emission is only observed Laser Applications along the axis of the cavity As atoms move to lower states via stimulated emission, spontaneous emission is suppressed (though not completely). Gain Medium, Pump and Cavity
ECE 455 Lecture 1
What is a laser? Photons passing through an inverted gain medium
Three Atomic experience positive feedback: Processes Each stimulated emission increases the number of photons Three Components More photons cause more stimulated emission of Every Laser In a laser, the positive feedback is balanced by two factors: Laser Light Characteristics Saturation of the gain medium – increased stimulated Units of Laser emission decreases the number of atoms in their upper Spectroscopy state History Loss of photons from the cavity, both from the laser’s Laser output as well as undesirable absorption and scattering Applications Gain Medium, Pump and Cavity
ECE 455 Lecture 1 The cavity provides the gain medium with a ‘memory’ Without it, all photons emitted at time t = 0 escape by What is a laser? time Lgain Three Atomic t = (2) Processes c Three Components With a cavity, the average photon escapes the cavity in of Every Laser time Laser Light Lgain,eff Characteristics t = (3) c Units of Laser Spectroscopy where Lgain,eff is the effective length of the laser History The cavity “traps” light Laser Applications But before each photon leaves, it creates other photons with the same direction, polarization, phase and energy via stimulated emission. The net result is to stabilize the output of the laser How is Laser Light Different From Ordinary Light?
ECE 455 Lecture 1
What is a Speckle Demo laser? 1 Shine laser pointer on Three Atomic Laser light is: Processes screen
Three Intense 2 Components Observe pattern of spots of Every Laser Quasi-monochromatic 3 Place finger ∼30 cm in Laser Light Characteristics Temporally coherent front of face and focus eye Units of Laser Spatially coherent on it Spectroscopy 4 History Single or few mode Speckle pattern in
Laser background does not go Applications out of focus What is Coherence?
ECE 455 Roughly speaking... Lecture 1 Coherence is the ability to form interference fringes, as
What is a shown below. laser? Coherence length is the characteristic length Lc over Three Atomic Processes which fringes will disappear as ∆L is scanned in a
Three Michelson interferometer. Components of Every Laser Coherence area is the square of the characteristic Laser Light distance (LA) over which fringes disappear in a Young’s Characteristics Double Slit experiment. Units of Laser Spectroscopy
History
Laser Applications Temporal Coherence: Michelson Interferometer
ECE 455 Lecture 1
What is a laser?
Three Atomic Processes Three Components of Every Laser
Laser Light Characteristics Units of Laser Spectroscopy
History
Laser Applications
Spatial Coherence: Young’s Double Slit Experiment
ECE 455 Lecture 1
What is a laser?
Three Atomic Processes
Three Components of Every Laser Laser Light Characteristics
Units of Laser Spectroscopy History
Laser Applications Colors
ECE 455 Lecture 1 Human vision extends from 400-700 nm. Knowing the correspondence between color and What is a laser? wavelength can be an important safety issue. Three Atomic Processes Memorize the chart below:
Three Wavelength (nm) Color Components of Every Laser Below 400 Ultraviolet (UV) Laser Light 400-440 Violet Characteristics 440-490 Blue Units of Laser Spectroscopy 490-560 Green History 560-590 Yellow Laser Applications 590-620 Orange 620-700 Red Above 700 Near Infrared (NIR) The exact boundaries are not standardized. Invisible Light I
ECE 455 Lecture 1 Electromagnetic waves with wavelengths longer than What is a laser? 700 nm are referred to as ‘infrared’ (IR). Infrared is Three Atomic invisible because it cannot be detected by your retina. Processes
Three Electromagnetic waves with wavelengths shorter than Components of Every Laser 400 nm are referred to as ‘ultraviolet’ (UV). Ultraviolet Laser Light light is invisible because it absorbed by your cornea. Characteristics
Units of Laser Your eyes do not have a perfectly sharp cutoff. You still Spectroscopy may be able to see wavelengths longer than 700 nm or History shorter than 400 nm. Laser Applications In the literature both IR and UV will be referred to as ‘light’ and as part of the field of ‘optics.’ Invisible Light II
ECE 455 Lecture 1
What is a Even though it is invisible, IR and UV radiation from lasers laser? may still be dangerous. Three Atomic Processes Although IR light is invisible, it can still focus on and burn Three the retina. Components of Every Laser UV light causes cataracts. Laser Light With sufficiently high peak intensities (such as in a Characteristics
Units of Laser regeneratively amplified femtosecond laser), a laser beam Spectroscopy may stimulate 2-photon transitions and appear blue. History A UV laser will typically appear white or blue upon Laser Applications striking a surface due to fluorescence. Laser Safety
ECE 455 Lecture 1 Your eye is designed to image light onto your retina.
What is a Laser light can cause damage at significantly lower powers laser? than other sources because laser beams focus tightly. Three Atomic Processes Retinal damage (permanent blind spots) may occur from a Three Components 100 ms exposure to a laser with power as low as 10 mW of Every Laser (a typical laser pointer is 5 mW) Laser Light Characteristics Damage thresholds for pulsed lasers are more complicated Units of Laser Spectroscopy Beams should be contained to eliminate stray reflections History Laser safety glasses are your last line of defense Laser Applications THIS SLIDE DOES NOT CONSTITUTE LASER SAFETY TRAINING! See [1] for more information. Wavelength
ECE 455 Lecture 1 All wavelength measurements denoted in terms of the meter. What is a laser? The traditional symbol representing wavelength is λ.
Three Atomic Processes
Three Components Unit Name Conversion Use of Every Laser A˚ Angstrom 10−10 m X-ray through IR wavelengths Laser Light −9 Characteristics nm nanometer 10 m UV through IR wavelengths −6 Units of Laser µm micron 10 m Visble through IR wavelengths Spectroscopy mm millimeter 10−3 m Beam Size History −2 Laser cm centimeter 10 m Beam Size Applications m meter 1 m Distance km kilometer 1000 m Radio Waves Energy and Frequency
ECE 455 Lecture 1
What is a laser? Frequency, energy, and inverse wavelength are all linearly Three Atomic Processes related and used interchangeably in atomic physics. Three Components of Every Laser Unit Name Use Laser Light Hz Hertz Electromagnetic field frequency Characteristics eV Electron Volts Photon Energy/Electron Energy Levels Units of Laser −1 Spectroscopy cm Wavenumber Photon Energy/Electron Energy Levels History J Joule Pulse Energy Laser Applications Conversions and Formulae
ECE 455 Lecture 1 −1 1 Energy (cm ) = λ(cm) (Note NO 2π) What is a laser? −1 −19 Three Atomic 1 eV = 8065 cm = 1.602× 10 J Processes
Three Components λ (nm) × E (eV) = 1240 eV-nm of Every Laser Laser Light −1 Characteristics 1 cm = 30 GHz Units of Laser Spectroscopy 2 λν = c ⇒ ∆λ = λ ∆ν History c
Laser Applications 1eV = 11600 K
1 Torr = 3.22 × 1016 cm−3 Laser Development Milestones
ECE 455 Lecture 1 1914 - Regenerative amplifier/oscillator circuit demonstrated by Edwin Armstrong What is a 1917 - Einstein predicts stimulated emission while studying laser?
Three Atomic blackbody radiation Processes 1954 - First MASER (NH3) demonstrated by Townes Three Components of Every Laser 1958 - Townes and Schawlow predict ’optical maser’ Laser Light 1960 - First LASER (ruby) demonstrated by Theodore Characteristics Maiman (Hughes, Malibu) Units of Laser Spectroscopy 1960 - First CW laser (He-Ne) demonstrated by Ali Javan History 1962 - Semiconductor lasers created by three independent Laser Applications teams within 1 month Post 1962 - You could write several books on all the developments since 1962 2010 - 50th Anniversary Laser Related Nobel Prizes
ECE 455 Lecture 1 Year Name Description Charles H. Townes Development of the maser What is a 1964 Nikolai G. Basov and laser laser? Aleksandr M. Prokhorov Three Atomic Processes 1971 Dennis Gabor Holography Three 1981 Nicolaas Bloembergen Developments in laser Components of Every Laser Arthur Schawlow spectroscopy Laser Light Characteristics Steven Chu Laser cooling and
Units of Laser 1997 Claude Cohen-Tannoudji trapping of atoms with Spectroscopy William Daniel Phillips laser light History 1999 Ahmed H. Zewail Femtosecond spectroscopy Laser Applications 2005 John L. Hall Frequency combs Theodor W. H¨ansch 2009 Charles K. Kao Light transmission in glass fibers Laser Revenue by Application 2010
ECE 455 Lecture 1
What is a Materials Entertainment laser? processing and displays 35.2% Three Atomic 0.4% Processes Image recording Three Communications 0.6% Components 30.6% of Every Laser Pumps 3.3% Laser Light Characteristics Instrumentation Units of Laser and sensors Spectroscopy Data storage 4.3% 12.6% History Scientific and Laser military Applications Medical and 6.3% aesthetic Total: $6.37 Billion 6.8%
Figure: Source: Laser Focus World 47 1 Death Rays
ECE 455 Lecture 1 Contrary to popular perception, funding for directed energy What is a weapons is only a small fraction laser? of laser research. Three Atomic Processes Clouds and atmospheric Three Components turbulence scatter laser beams. of Every Laser Storing energy for lasers in a Laser Light Characteristics portable format is also Units of Laser Spectroscopy challenging.
History There are many military uses for lasers: Laser Real Genius (1985) Applications Laser welding Laser rangefinder Laser-guided bombs The Viper laser confuses heat-seeking missiles Single Cell Biological Lasers [2]
ECE 455 Lecture 1
What is a laser? Cells genetically modified Three Atomic to create green fluorescent Processes
Three protein (GFP) Components of Every Laser A high-Q cavity Laser Light surrounded the cell Characteristics
Units of Laser System was pumped with Spectroscopy 850 pJ of 465 nm light History from an OPO Laser Applications Applications in cell sensing and cytometry Figure: Images of a living cell as it is pumped above (bottom) and below threshold (top). National Ignition Facility
ECE 455 Lecture 1 Built for research into What is a laser? inertial confinement
Three Atomic fusion Processes 192 beams focus onto a Three Components millimeter-sized pellet of Every Laser filled with a mixture of Laser Light Characteristics deuterium and tritium Units of Laser Spectroscopy Expected to produce
History the first exothermic ICF Laser reaction Applications Figure: One of two laser bays at LLNL. 1.8 MJ pulses (Source: http://lasers.LLNL.gov) 500 TW peak power Stimulated Emission in Nature [3, 4]
ECE 455 Lecture 1 Non-thermal emission spectra of What is a laser? CO2 are observed in the upper Three Atomic atmospheres of Venus and Mars Processes around 9.4 µm and 10.6 µm. Three Components of Every Laser The vibrational levels of the CO2
Laser Light molecule are pumped to Characteristics inversion by the Sun. Units of Laser Spectroscopy Photons traverse enormous History atmospheric path lengths. Laser Applications Stimulated emission contributes approximately 4% of the photon flux. Vertical Cavity Surface Emitting Lasers (VCSELS)
ECE 455 Lecture 1 Emission is perpendicular to the wafer, rather than What is a laser? along its edge. Three Atomic Processes Cavity length as short as
Three λ/2 Components of Every Laser Cavity volume can be on Laser Light the order of λ3 Characteristics
Units of Laser Used in research to test Spectroscopy fundamental aspects of History quantum mechanics. (Source: www.photonics.com) Laser Applications Commercial use: laser mice, optical data transmission, autonomous vehicles The Internet
ECE 455 Lecture 1 Fiber optics have been carrying telephone data What is a since the early 1980s. laser?
Three Atomic Data is transmitted by Processes modulating the output of Three Components CW lasers. of Every Laser Signals from one or several Laser Light Characteristics lasers at different NEED PICTURE Units of Laser Spectroscopy wavelengths may be sent
History on the same fiber (WDM). Laser Er-doped fibers pumped by Applications diode lasers amplify signals without having to convert the signal. For history, see [5] Frequency Combs
ECE 455 Lecture 1 Nonlinearities can be exploited to generate lasers with a bandwidth exceeding one octave at optical frequencies.
What is a Spectrum will consist of uniformly-spaced lines. laser? Direct measurement of optical transition frequency Three Atomic Processes Key role in the detection of planets around other stars
Three Clocks more accurate than today’s microwave standards Components of Every Laser
Laser Light Characteristics
Units of Laser Spectroscopy
History
Laser Applications Excimer (“Excited Dimer”)
ECE 455 Lecture 1 Transient molecules, not normally found in nature, created What is a laser? by electron (plasma) or optical Three Atomic Processes pumping Three Molecules split, emit a photon Components of Every Laser Strong absorption and high peak Laser Light Characteristics power make excimers ideal
Units of Laser sources for ablation in laser eye Spectroscopy surgery or materials processing History Short wavelength and high Laser Applications energy make excimers good light (Source: Wikimedia) sources for photolithography Minimum feature size scales λ as: d ∝ NA ECE 455 [1] K. Barat, Ed., Laser Safety: Tools and Training, 2nd ed. Boca Raton: CRC Press, 2013. Lecture 1 [2] M. C. Gather and S. H. Yun, “Single-cell biological lasers,” Nature Photonics, vol. 5, pp. 406–410, 2011.
[3] D. Deming, F. Espenak, D. Jennings, T. Kostiuk, M. Mumma, and D. Zipoy, “Observations of the 10-µm natural laser emission from the mesospheres of mars and venus,” Icarus, vol. 55, no. 3, pp. 347 – What is a 355, 1983. [Online]. Available: http://www.sciencedirect.com/science/article/pii/0019103583901070 laser? [4] D. Deming and M. J. Mumma, “Modeling of the 10-µm natural laser emission from the mesospheres of Three Atomic mars and venus,” Icarus, vol. 55, no. 3, pp. 356 – 368, 1983. [Online]. Available: Processes http://www.sciencedirect.com/science/article/pii/0019103583901082
Three [5] J. Hecht, City of Light. New York, NY: Oxford University Press, 2004. Components of Every Laser
Laser Light Characteristics
Units of Laser Spectroscopy
History
Laser Applications