Lasers
Applied Physics, 12.11.2014
Negar Khast, Pegah Khast, Andreas Schäfer, Jesse Jesse Luttinen
1. What is the most important technical invention and why?
Laser 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. Laser is acronym of Light Amplification by Stimulated Emission of Radiation Laser is acronym of Light Amplification by Stimulated Emission of Radiation. Laser is a source of light but it is different from other light sources. Laser makes a high intensity and extremely directional beam which has a narrow frequency range. Lasers are more used as a strong electromagnetic beam than a light beam.
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 a laser there are two mirrors one is fully reflective and the other one is partially reflective. 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.
4. Critical analysis of our interpretation
According to our previous knowledge we know that our explanation is correct but it is not well detailed and we need to find out more information about laser 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.
5. 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.
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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 Laser Construction
Lasers are constructed from three main parts which are:
an energy source also known as a pump source
a gain medium or a laser medium
two or more mirrors that form an optical resonator [3]
5.2.1 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.
5.2.2 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.
5.2.3 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.
Figure 1. Laser components
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. [3]
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.
Figure 2. components of the first ruby laser
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 Difference between laser and ordinary light beam
Laser beam is extremely monochromatic while ordinary light is not monochromatic and light spectrum from a source may extend over a wide range of wavelength. Laser beam is a very concentrated parallel beam along a specific direction but an ordinary light spreads in all directions. Another difference between laser beam and an ordinary light is that ordinary light is incoherent but laser beam is highly coherent, waves are in a phase with each other and in the same state of polarization.
5.5 Interaction of light and matter
Light is a form of radiant energy. There are various hypotheses about the nature of light. Huygen considered light is made up of waves while Newton made up a particle model to explain light behaviors. Maxwell determined that light belongs to the group of electromagnetic waves that spread at a c speed a c speed in a vacuum. The wavelength and the frequency of the light wave are related to c through to c through the equation: c c=v
In this conception it is implied that light waves are of infinite extension and continuous and also could carry any ideal amounts of energy.
Figure 3.Reflection, transmission, absorption and scattering
When a light interacts with a matter, depending on the nature of substance, reflection, transmission, absorption and scattering of light to varying degrees may occur. When a light wave hits a transparent solid, it enforces periodic oscillations of orbital electrons with its own frequency. These oscillating electrons behave as point source and emit waves in all directions. In the case that the medium is homogeneous, the secondary waves devastate each other in all directions except in the direction of diffusion of the incident wave. The produced beam has maximum intensity in the direction of propagation of the incident wave and establishes the transmitted beam.
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When a light beam hits obstacles with smaller sizes than a wavelength, it turn into different directions. This is light that is scattered by the obstacle. Scattering also generates attenuation of energy because some of the light energy is missing from the original beam.
5.6 How does mirror work?
What you see when you stand in front of a mirror is actually the conservation of energy. Light is a form of energy that travels at high speed. When light hits an object depending on the object three things can happen. If the object is transparent light passes through, it can be reflected in the case that the object is shiny and reflective and it can also disappear and sink in if the object is dark colored and opaque. In all of these cases there is same amount of energy before and after light hits the object and conservation of energy is work but some of this light energy may be change into other forms.
Mirrors Mirrors reflect back an image of the object in front of it. This image is created from photons ( particles of light ). When a photon of light hits a mirror, electrons of the atom start to vibrate and emit an identical photon of light. Metals have plenty electrons that are shared by all of the metal atoms but in the glass, atoms do not share their electrons like metals do so metals work better as mirrors. Mirrors surface should be smooth and optically plane so that light reflects from a mirror at the same angle it arrives instead of being scattered in different directions and result in the image to be blurred and mostly distorted.
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Mirrors usually have three layers. Bottom layer of the mirror is a dark and protective layer, the middle layer which is the most important part of the mirror is a metal and the top layer is glass because it is clear and it also protects the middle layer which is usually aluminium.
5.7 Why does mirror reflect light?
Light is an electromagnetic field and mirrors are electrically conductive. When light hits a mirror, the metal layer of the mirror cancels the electric field that is parallel to the mirror so it changes it direction and reflect away. Light Light with a given frequency causes the electrons to bounce with that same frequency. In the case of low frequency lights such as visible and infrared light, the electrons can match the speed and reflect the light. However, in the case of UV light, the frequency is too high, and the electrons are not able to bounce fast enough to match the frequency of the light. For this reason, metals are actually transparent to UV radiation.
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
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[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 Laser construction. [online] Available at: http://en.wikipedia.org/wiki/Laser_construction [Accessed 5 Nov. 2014].
[4] 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].
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[7] Thenakedscientists.com, (2012). Stronger Stronger lasers with mirrors? - The Naked Scientists. [online] Available at: http://www.thenakedscientists.com/HTML/questions/qotw/question/3542/ [Accessed 5 Nov. 2014].
[8] Biello, D. (2014). How How do mirrors reflect photons?. [online] Scientificamerican.com. Available at: http://www.scientificamerican.com/article/how-do-mirrors-reflect-ph/ [Accessed 26 Nov. 2014].
[9] Explainthatstuff.com, (2014). How How mirrors work - Explain that Stuff!. [online] Available at: http://www.explainthatstuff.com/howmirrorswork.html [Accessed 26 Nov. 2014].
[10] Flinn, G. (2014). Mirror Mirror Physics - HowStuffWorks. [online] HowStuffWorks. Available at: http://science.howstuffworks.com/innovation/everyday-innovations/mirror2.htm [Accessed 26 Nov. 2014].
[11] Google Books, (2014). An An Introduction To Lasers Theory And Applications. [online] Available at: http://books.google.fi/books?id=NyXU3KBCdMcC&pg=PA4&lpg=PA4&dq=newton%27s+law+lasers&source=bl&ots=KW0_aTjygJ&sig=04u49zA93u15eFzbjMr8UJQ31oY&hl=en&sa=X&ei=PR97VO2SMI7PaPKIgKAB&ved=0CCQQ6AEwATgK#v=onepage&q=newton's%20law%20lasers&f=true [Accessed 26 Nov. 2014].