LASER

1. LASER By: Engr. Rizwan Nasir B.Sc. Chemical Engineering October 20, 2009

2. What is Laser? • L: Light • A: Amplification (by) • S: Stimulated • E: Emission (of) • R: Radiation

3. Introduction • Stimulated emission process was predicted by Einstein in 1916. First laser developed in 1959. • “Photons” and atoms can interact via the following processes. • Absorption: Atom can absorb a photon and become excited. • Spontaneous emission: Atom in excited state will spontaneously emit a photon and occupy a lower energy state. • Stimulated emission: Atom in excited state is stimulated by a photon to emit another photon and occupy a lower energy state. Emitted photon has the same wavelength, phase, and direction as the stimulating photon.

4. Definition • A laser is a device that generates light by a process called STIMULATED EMISSION. • The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation. • Semi conducting lasers are multilayer semiconductor devices that generates a coherent beam of monochromatic light by laser action. A coherent beam resulted which all of the photons are in phase.

5. Laser Operation in a 3 Level System Excited Reservoir 1. Pumping: Excites atoms to highest level. Ground Excited 2. Fast radioactive decay to reservoir creates population inversion between reservoir and ground states. Reservoir Ground Excited 3. Seed photon stimulates emission and light is amplified! Reservoir Laser light Ground

6. Properties of Laser Light • High Power Density: At the focus, lasers can be thousands of times more intense than the sun! • Sunlight ~ 1300 W/m2 • Laser ~ 106 W/m2 • Coherence: All the emitted photons bear a constant phase relationship with each other in both time and space.

7. Cont… • Monochromatic Concentrate in a narrow range of wavelengths (one specific colour). • Coherent All the emitted photons bear a constant phase relationship with each other in both time and phase • Directional A very tight beam which is very strong and concentrated.

8. Types of Lasers • Solid state lasers, gas lasers, dye Lasers, semiconductor (diode) lasers.

9. Typical Application of Laser • The detection of the binary data stored in the form of pits on the compact disc is done with the use of a semiconductor laser. • The laser is focused to a diameter of about 0.8 mm at the bottom of the disc, but is further focused to about 1.7 micrometers as it passes through the clear plastic substrate to strike the reflective layer. • The reflected laser will be detected by a photodiode. • Moral of the story: without optoelectronics there will no CD player!

10. Basic concepts for a laser • Absorption • Spontaneous Emission • Stimulated Emission • Population inversion

11. Absorption • Energy is absorbed by an atom, the electrons are excited into vacant energy shells.

12. Spontaneous Emission • The atom decays from level 2 to level 1 through the emission of a photon. It is a completely random process.

13. Stimulated Emission Atoms in an upper energy level can be stimulated in phase by an incoming photon of a specific energy.

14. Population Inversion When a sizable population of electrons resides in upper levels, this condition is called a "population inversion", and it sets the stage for stimulated emission of multiple photons. This is the precondition for the light amplification which occurs in a LASER and since the emitted photons have a definite time and phase relation to each other, the light has a high degree of coherence.

15. Applications of laser • 1. Scientific a. Spectroscopy b. Lunar laser ranging c. Photochemistry d. Laser cooling e. Nuclear fusion

16. Applications of laser • 2 Military a. Death ray b. Defensive applications c. Strategic defense initiative d. Laser sight e. Illuminator f. Rangefinder g. Target designator

17. Applications of laser • 3. Medical a. Eye surgery b. Cosmetic surgery

18. Applications of laser • 4. Industry & Commercial a. cutting, welding, marking b. CD player, DVD player c. Laser printers, laser pointers d. Laser light display