Laser SystemsLaser Systems

Fredrick BrownFredrick Brown

ELEC 6750ELEC 6750

Spring 2004 Spring 2004

QuestionsQuestions

What is a laser?What is a laser?

What are the primary parts of a gas laser What are the primary parts of a gas laser system?system?

OverviewOverview

Primary Parts of a laserPrimary Parts of a laserPopulation InversionPopulation InversionGases Used in lasersGases Used in lasersSummary Summary Answers to questionsAnswers to questions

Primary PartsPrimary Parts

A laser consists of two primary A laser consists of two primary parts: a cavity consisting of at parts: a cavity consisting of at least two mirrors and a gas least two mirrors and a gas discharge tube with a means of discharge tube with a means of establishing a population establishing a population inversion.inversion.

The mirrors in a gas laser are The mirrors in a gas laser are highly specialized and will be used highly specialized and will be used to form the ends of the laser to form the ends of the laser cavity. They are coated with thin cavity. They are coated with thin films that make them highly films that make them highly reflective for the exact wavelength. reflective for the exact wavelength. One of the mirrors is 100% One of the mirrors is 100% reflective while the other is about reflective while the other is about 99%. This will allow a thin beam to 99%. This will allow a thin beam to be reflected from the difference in be reflected from the difference in the 1%.the 1%.

http://vcs.abdn.ac.uk/ENGINEERING/lasers/gas.html

Figure 1. Diagram of a laser beam reflecting between mirrors in tube.

Population InversionPopulation Inversion A population inversion is when A population inversion is when

molecules or gas particles are excited molecules or gas particles are excited to a higher energy state larger than to a higher energy state larger than that of a lower energy state. that of a lower energy state.

The gas inside of the cavity will amplify The gas inside of the cavity will amplify the number of photons that are put in, the number of photons that are put in, for example, for every photon which for example, for every photon which passes through the substance, two passes through the substance, two others leave it creating an extremely others leave it creating an extremely coherent beam of light all in phase. coherent beam of light all in phase.

The process of exciting molecules to The process of exciting molecules to create the population inversion is call create the population inversion is call “pumping”. Electrons in a gas “pumping”. Electrons in a gas discharge do the job of exciting discharge do the job of exciting molecules from low energy states to molecules from low energy states to high energy states.high energy states.

http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lasgas.html

Figure 2. Example of population inversion of gas particles.

GasesGases

Helium-NeonHelium-NeonArgon-IonArgon-IonCarbon DioxideCarbon Dioxide

Helium-NeonHelium-Neon The most common and The most common and

inexpensive gas laser is inexpensive gas laser is the HeNe. It is usually the HeNe. It is usually constructed to operate in constructed to operate in the red wavelength at the red wavelength at 632.8nm. It can also 632.8nm. It can also produce laser action in produce laser action in the green at 543.5nm and the green at 543.5nm and in the infrared at 1523nm. in the infrared at 1523nm. Also, it contains about Also, it contains about 85% helium and 15% 85% helium and 15% neon gas and about neon gas and about 1/300 of atmospheric 1/300 of atmospheric pressure.pressure.

http://hyperphysics.phy-astr.gsu.edu/hbase/optmod/lasgas.html

HeNe (Cont.)HeNe (Cont.)

The lighter Helium atoms The lighter Helium atoms are excited by collisions are excited by collisions with electrons in the with electrons in the discharge. Next, the discharge. Next, the Helium atoms collide with Helium atoms collide with the heavier Neon atoms the heavier Neon atoms which excites them to which excites them to their metastable state their metastable state where population where population inversion builds up.inversion builds up.

Bridges, W.B. and Chester, A.N. "Ionized Gas Lasers," in handbook of lasers. Presley, R.J. editor. Chemical Rubber Co., 1971

Argon-Ion Argon-Ion The Argon-Ion laser is The Argon-Ion laser is

capable of producing capable of producing approximately 10 approximately 10 wavelengths in the ultraviolet wavelengths in the ultraviolet and up to 25 in the visible and up to 25 in the visible region, ranging from 275-region, ranging from 275-363.8nm and 408.9-363.8nm and 408.9-686.1nm.686.1nm.

The typical Argon laser has The typical Argon laser has a high melting point and a high melting point and allows the laser to operate at allows the laser to operate at a higher power level with a higher power level with longer tube life than the longer tube life than the HeNe.HeNe.

http://vcs.abdn.ac.uk/ENGINEERING/lasers/gas.html

Argon-Ion (Cont.)Argon-Ion (Cont.)

An unfortunate side effect of the high An unfortunate side effect of the high discharge current and the low gas discharge current and the low gas pressure employed by the Argon-Ion laser pressure employed by the Argon-Ion laser is an extremely high plasma electron is an extremely high plasma electron temperature, which generates a significant temperature, which generates a significant amount of heat. Therefore, this lasers amount of heat. Therefore, this lasers system are water cooled through an system are water cooled through an external chiller or an efficient fan.external chiller or an efficient fan.

Carbon DioxideCarbon Dioxide CO2 laser emits an invisible CO2 laser emits an invisible

infrared beam of a single infrared beam of a single wavelength in the form of a wavelength in the form of a small, intense beam. small, intense beam. Specifically, CO2 emits Specifically, CO2 emits photons at 10.6 and 9.6um. photons at 10.6 and 9.6um.

The beam produces a very The beam produces a very high temperature that may be high temperature that may be used for engraving, cutting, used for engraving, cutting, drilling, marking, welding and drilling, marking, welding and in various medical applications in various medical applications such as dermatology and as a such as dermatology and as a medical tool for producing medical tool for producing clean cuts with little bleeding.clean cuts with little bleeding.

http://www.repairfaq.org/sam/lasercc2.htm

CO2 (Cont.)CO2 (Cont.)

http://www.repairfaq.org/sam/lasercc2.htm

SummarySummary

In conclusion, energy in the form of heat, light, In conclusion, energy in the form of heat, light, and chemical reaction can be used to excite and chemical reaction can be used to excite electrons within atoms and push them to higher electrons within atoms and push them to higher energy states within a laser system to create a energy states within a laser system to create a population inversion. The relaxation of electrons population inversion. The relaxation of electrons to lower energy states, ultimately, to their ground to lower energy states, ultimately, to their ground state using the correct primary parts can state using the correct primary parts can produce ultraviolet light, visible light, and produce ultraviolet light, visible light, and infrared radiation in a laser system.infrared radiation in a laser system.

AnswersAnswers

What is a laser?What is a laser? Light Amplification by Light Amplification by Stimulated Emission of RadiationStimulated Emission of Radiation..

What are the primary parts of a gas laser What are the primary parts of a gas laser system?system? A cavity consisting of at least 2 A cavity consisting of at least 2 mirrors and a gas discharge tubemirrors and a gas discharge tube..