Laser Properties

8/3/2019 Laser Properties

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INTRODUCTION .

• Laser : Device that emits light (electromagneticradiation), through a process of opticalamplification based on the stimulated

emission of photons.

• Laser : An acronym for Light Am plification byStimulated Emission of Radiation .



PROPERTIES.

• Laser has certain unique properties, namely,• High monochromaticity

• Coherence &

• Directionality,compared to ordinary sources of light, thoughboth are electromagnetic radiations.



MONOCHROMATICITY .• Consisting of a single wavelength oflight or other radiation

•All light consists of waves travelingthrough space. The color of the light is

determined by the length of thosewaves



• Each color of visible light has its own characteristic wavelength•(Wavelength : The distance over which the wave repeats itself )

• All common light sources emit light of many different wavelengths• White light contains all, or most, of the colors of the visible spectrum while

Ordinary colored light consists of a broad range of wavelengths covering aparticular portion of the visible-light spectrum

• The beam of a laser consists of an extremely narrow range of wavelengthswithin a single portion of the spectrum. It is said to be nearly

"monochromatic," or nearly "single-colored.

•Near-monochromaticity is a unique property of laser light, meaning that itconsists of light of almost a single wavelength. (Mike Leeming)

•

NO PERFECT MONOCHROMATISM



DIRECTIONALITY.•Figure here shows light being emittedfrom a light bulb in all directions.•All conventional light sources emit lightin this manner.

•Devices such as automobile headlightsand spotlights contain optical systemsthat collimate the emitted light, such thatit leaves the device in a directional beam;

•however, the beam produced alwaysdiverges (spreads) more rapidly than thebeam generated by a laser



• This figure illustrates the

highly directional nature oflight produced by a laser.

•

"Directionality" is thecharacteristic of laser light thatcauses it to travel in a singledirection within a narrow cone ofdivergence.

•But again, perfectly collimatedlight cannot be produced. Alllight beams eventually spread(diverge) as they move through

space.



COHERENCE.• In the parallel beam of light waves froman ordinary source traveling through spaceshown here, none of the waves has anyfixed relationship to any of the other waves

within the beam.

•This light is said to be "incoherent,"meaning that the light beam has nointernal order.

•Whereas in case of the light waves withina highly collimated laser beam, all of theindividual waves are in step, or "in phase,"with one another at every point.



•"Coherence" is the term used to describe the in-phase property of lightwaves within a beam

•Most fundamental property of laser light•Distinguishes it from the light from other source



• To make these three properties occur takes somethingcalled stimulated emission.

•This does not occur in your ordinary flashlight -- in a flashlight, allof the atoms release their photons randomly. In stimulated emission,photon emission is organized.•The photon that any atom releases has a certain wavelength that isdependent on the energy difference between the excited state and theground state.•

If this photon (possessing a certain energy and phase) shouldencounter another atom that has an electron in the same excited state,stimulated emission can occur.•The first photon can stimulate or induce atomic emission such thatthe subsequent emitted photon (from the second atom) vibrates with

the same frequency and direction as the incoming photon.

http://products.howstuffworks.com/flashlight-reviews.htm

http://products.howstuffworks.com/flashlight-reviews.htm



•The other key to a laser is a pair of mirrors, one at each end of thelasing medium.•Photons, with a very specific wavelength and phase, reflect off themirrors to travel back and forth through the lasing medium.

•In the process, they stimulate other electrons to make the downwardenergy jump and can cause the emission of more photons of the samewavelength and phase.•A cascade effect occurs, and soon we have propagated many, many

photons of the same wavelength and phase.•The mirror at one end of the laser is "half-silvered," meaning it reflectssome light and lets some light through. The light that makes it throughis the laser light.



Explanation for monochromaticity.•Laser light consists of essentially one wavelength, having its origin instimulated emission from one set of atomic energy levels.

•

EM wave of frequency n = (E2 - E1) only can be amplified, n has a certainrange which is called line width.

•The generation of laser is such that the laser cavity forms a resonant systemand laser oscillation is sustained only at the resonant frequencies of the

cavity.•This leads to the further narrowing of the laser line width. So laser light isusually very pure in wavelength, we say it has the property ofmonochromaticity.



•The two emitted photons are said to be in phase, which means that the crestor the trough of the wave associated with one photon will occur at the sametime as on the wave associated with the other photon. An avalanche of

similar photons is created and these photons have a fixed phase relationshipwith each other.

•Inside a laser, the stimulated emission occurs in a resonant cavity withmirrors at both ends. Thus by repeating this process of interaction of photon

with excited atoms many times, one can produce a highly coherent beam oflight.•Since a common stimulus triggers the emission events, which provide theamplified light, all the emitted photons are "in step" and have a definitephase relation to each other.

Reason for coherence.



WHY DIRECTIONAL.

•The mirrors placed at opposite ends of a laser cavity enables the beam to travelback and forth in order to gain intensity by the stimulated emission of more

photons at the same wavelength, which results in increased amplification dueto the longer path length through the medium.•The multiple reflections also produce a well-collimated beam.• The high degree of collimation arises from the fact that the cavity of the laserhas very nearly parallel front and back mirrors, which constrain the final

laser beam to a path, which is perpendicular to those mirrors.•Collimation refers to the degree to which the beam remains parallel withdistance. A perfectly collimated beam would have parallel sides and wouldnever expand at all.



1. Laser in its non lasing phase

2. The flash tube fires and injects

light into the rod. The light excites

atoms.



3. Some of these atoms emit

photons

4. Some of these photons run in a

direction parallel to the ruby's axis,

so they bounce back and forth off the mirrors. As they pass through

the crystal, they stimulate emission

in other atoms



5. Monochromatic, single-phase, collimated light leaves through the

half-silvered mirror -- laser light!



Thank you

Laser Properties

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