Task 2
Applied Physics, 05.11.2014
Negar Khast, Pegah Khast, Andreas Schäfer
Why does mirror in the laser reflect light?
1.Initial explanation
In a laser there are two mirrors one is fully reflective and the other one is partially reflective.I 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.
2.Critical analysis
Our explanation was not based on a physical fact and we need to find more information about construction of lasers so that we can provide a detailed and good answer to this question. We tried to explain this question based on our knowledge about the lasers but we are not confident about our answer.
3.Finding more information
3.1 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 [1]
3.1.1 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.
3.1.2 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.
3.1.3 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.
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. [1]
3.2 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.[4]
4. 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 construction. [online] Available at: http://en.wikipedia.org/wiki/Laser_construction [Accessed 5 Nov. 2014].
[2] Bellis, M. (2014). What Makes Lasers Possible?. [online] About. Available at: http://inventors.about.com/od/lstartinventions/ss/LaserWorks.htm#step-heading [Accessed 5 Nov. 2014].
[3] Thenakedscientists.com, (2012). Stronger lasers with mirrors? - The Naked Scientists. [online] Available at: http://www.thenakedscientists.com/HTML/questions/qotw/question/3542/ [Accessed 5 Nov. 2014].
[4] Scienceline.ucsb.edu, (2014). UCSB Science Line sqtest. [online] Available at: http://scienceline.ucsb.edu/getkey.php?key=3903 [Accessed 12 Nov. 2014].