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Lasers

Applied Physics, 0512.11.2014

Negar Khast, Pegah Khast, Andreas Schäfer

Why does mirror in the laser reflect light?

1.Initial explanation

1. What is the most important technical invention and why?

Laser is recognized as one of the top technological achievements of 20th century and there are few areas in technology that are not influenced by it. It plays an important role in, medicine, industry, and entertainment has resulted in fiber-optic communication, CDs, CD-ROMs, and DVDs.Without lasers there would be no supermarket bar code readers, certain life-saving cancer treatments, or precise navigation techniques for commercial aircraft.

2. How the physics is involved this?

Laser science is a part of optics that explain the theory and practice of lasers. It is mainly involved with quantum electronics, population inversion, quantum electronics, laser construction, optical cavity design, the physics of producing a population inversion in laser media, and the temporal evolution of the light field in the laser. It is also concerned with the physics of laser beam propagation, particularly the physics of Gaussian beams, with laser applications, and with associated fields such as nonlinear optics and quantum optics. [1]

3. Our interpretation how it works

Based on our previous knowledge we know that electrons are in a specific energy levels. Electrons can go to higher energy levels after being energized or excited. Although they will eventually fall back to their initial state. Thus they release this extra energy as a light of a specific color when they are falling back to their original energy level.

Also we know in In a laser there are two mirrors one is fully reflective and the other one is partially reflective. I We think these mirrors are used to repeat the process of energizing electrons inside the laser and about the mirror reflecting laser light, I think depending on the lasing material that affects wavelength of the laser beam, reflecting this light with a mirror can be different. Because there different type of lasers ,with some of them you can cut other materials and some of them are just a light which you can use for presentation and other things.

24. Critical analysis of our interpretation

According to our previous knowledge we know that our explanation is correct but it is not well detailed Our explanation was not based on a physical fact and we need to find out more information about construction of lasers so that we can provide a detailed and good answer to this questionlaser construction, optics, population inversion and related physics facts to improve our explanation.We tried to explain this question based on our knowledge about the lasers but we are not confident about our answer.

35. Finding more information

5.1 History of lasers

The name LASER is an acronym for Light Amplification by the Stimulated Emission of Radiation. In 1917, Albert Einstein first theorized about the process which makes lasers possible called "Stimulated Emission."Before the Laser there was the Maser.

In 1954, Charles Townes and Arthur Schawlow invented the maser ( microwave amplification by stimulated emission of radiation), using ammonia gas and microwave radiation - the maser was invented before the (optical) laser. The technology is very close but does not use a visible light.

On March 24, 1959, Charles Townes and Arthur Schawlow were granted a patent for the maser. The maser was used to amplify radio signals and as an ultrasensitive detection for space research.

In 1958, Charles Townes and Arthur Schawlow theorized and published papers about a visible laser, an invention that would use infrared and/or visible spectrum light, however, they did not proceed with any research at the time.

Many different materials can be used as lasers. Some, like the ruby laser, emit short pulses of laser light. Others, like helium-neon gas lasers or liquid dye lasers emit a continuous beam of light. [2]

5.2 3.1 Laser Construction

Lasers are constructed from three main parts which are:

1. an energy source also known as a pump source

2. a gain medium or a laser medium

3. two or more mirrors that form an optical resonator [

1

4. 3]

35.12.1 Pump source

The part of laser system that provides energy is called pump source. Depending on the gain medium of laser the type of pump source that is used can be different which also defines how the energy is transmitted to the medium. Arc lamps, electrical discharges, flash lamps, chemical reactions, light from another laser and explosive devices are some of the examples for pump sources. For example an electrical discharge in the helium-neon (HeNe) gas mixture is used as pump source in a helium-neon laser.

35.12.2 Gain medium

The gain medium is the main part that defines  the wavelength of process and other properties of the laser. In various materials gain media can have linear or wide range. Wide range gain media allows adjusting the frequency of the laser. In order to generate a population inversion, pump source excites the gain medium. Impulsive and stimulated emission of photons occurs in the gain medium which leads to the amplification or optical gain.

35.12.3 Optical resonator

The simplest type of an optical resonator or optical cavity is consist of two parallel mirrors which are located around the gain medium to supply reaction of the light. These mirrors have an optical coverage that specifies their reflective features. One of the mirrors is an excellent reflector while the other is partially reflective which is called the output coupler. The latter mirror produces the laser beam by allowing some of the light to leave the cavity.  The light that is produced by impulsive emission from the medium is reflected back to the medium by the mirrors. In the medium the light may be amplified by stimulated emission. Before leaving the cavity the light may reflect from mirrors and pass over the gain medium many times.

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More complex lasers have four or more mirrors to form the cavity. Other optical devices, such as spinning mirrors, modulators, filters, and absorbers, may be placed within the optical resonator to produce a variety of effects on the laser output, such as altering the wavelength of operation or the production of pulses of laser light. [13]

5.3 How a Laser works

Electrons are at specific energy levels . We can describe this energy levels as rings or orbits around a nucleus. Electrons that are in outer orbits have higher energy levels in compared with the electrons that are in inner rings. By injecting the energy, electrons can go to a higher energy levels. When an electron goes from a higher energy level to a lower energy level it releases its excess energy as a light. The color and wavelength of this emitted light is depended on the amount of energy that is released. Based on the lasing material that is used certain wavelengths of light are absorbed to excite or energize electrons and when the electrons fall back to their initial level specific wavelengths are emitted.

For example in a ruby laser, a crystal of ruby is formed into a cylinder. A fully reflecting mirror is placed on one end and a partially reflecting mirror on the other. A high-intensity lamp is spiraled around the ruby cylinder to provide a flash of white light that triggers the laser action. The green and blue wavelengths in the flash excite electrons in the chromium atoms to a higher energy level. Upon returning to their normal state, the electrons emit their characteristic ruby-red light. The mirrors reflect some of this light back and forth inside the ruby crystal, stimulating other excited chromium atoms to produce more red light, until the light pulse builds up to high power and drains the energy stored in the crystal.

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The quartz flash tube emits an intense burst of light causing by high voltage electricity which excites some of the atoms in the ruby crystal to higher energy levels. Some of the atoms at a specific energy level emit photons (particles of light). At first the photons are emitted in all directions. Photons from one atom stimulate emission of photons from other atoms and the light intensity is rapidly amplified.Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification.The photons leave through the partially silvered mirror at one end. This is laser light. [4]

5.4 New question:  Why .2 Why mirrors reflect light?

During this assignment another subject that we wanted to find out about was to understand why mirrors reflect light. According to what we found that mirrors reflect mainly because they are electrically conductive. Light is an electromagnetic field, and when it hits a mirror the metal inside of it which is usually aluminum or silver cancels out the electric field parallel to the mirror which causes it to change directions and reflect away. Not all reflective mirrors are conductive though, a certain percentage of light will reflect wherever there is a change between two types of materials like air and water, because the electric field changes as it goes through different materials some of the light will get reflected while the rest will pass through or be absorbed. [45]

46. Summary

During this assignment we found out about laser construction and that by using parallel mirrors we can make a more powerful amplification so that when the light is passing through the gain medium it gets amplified. Then it bounces off the first mirror (on the right side) and then passes to the other mirror and gets amplified. It repeats this process and jumps backwards and forwards so as it passes through the gain medium it becomes more powerful. One of the mirrors is partially reflective because light needs to shine out of the laser so if the mirror is fully reflective light will be trapped inside the cavity permanently. So with a partially reflective mirror on every cycle light loses a little of its energy and that makes laser beam output to have a limited power.

After learning about different parts of laser we wanted to know what happens when two laser beams cross each other. According to what we found lasers are made of light waves so if we point two laser beams at each other we will have  two different sets of waves that are moving in the opposite ways. Light waves add together so when high points of the waves are at the same spot the result is even higher. Conversely when a high point of one of the waves is at the same spot with a low point of the other one they can cancel each other. So there will be some spots that light is stronger and some spots with no light.

References

[1] Wikipedia, (2014). Laser science. [online] Available at: http://en.wikipedia.org/wiki/Laser_science [Accessed 29 October. 2014].

[2]  Worldoflasers.com, (2014). History of Lasers -WorldOfLasers.com. [online] Available at: http://www.worldoflasers.com/laserhistory.htm [Accessed 29 October. 2014].

[3] Wikipedia, (2014). Laser construction. [online] Available at: http://en.wikipedia.org/wiki/Laser_construction [Accessed 5 Nov. 2014].

[24] Bellis Bellis, M. (2014). What Makes Lasers Possible?. [online] About. Available at: http://inventors.about.com/od/lstartinventions/ss/LaserWorks.htm#step-heading [Accessed 29 October. 2014].

[5] Scienceline.ucsb.edu, (2014). UCSB Science Line sqtest. [online] Available at: http://scienceline.ucsb.edu/getkey.php?key=3903 [Accessed 12 Nov. 2014].

[36] ThenakedscientistsBellis, M. com, (20122014). Stronger lasers with mirrors? - The Naked ScientistsWhat Makes Lasers Possible?. [online] About. Available at: http://wwwinventors.thenakedscientistsabout.com/HTMLod/questionslstartinventions/qotw/question/3542/ss/LaserWorks.htm#step-heading [Accessed 5 Nov. 2014].

[47] SciencelineThenakedscientists.ucsb.educom, (20142012). UCSB Science Line sqtestStronger lasers with mirrors? - The Naked Scientists. [online] Available at: http://sciencelinewww.ucsb.edu/getkey.php?key=3903 [Accessed 12 thenakedscientists.com/HTML/questions/qotw/question/3542/ [Accessed 5 Nov. 2014].