Assuming you're thinking of temporal coherence, perhaps try thinking about this classically and starting with a related question:. Why is the light that exits a clear piece of glass coherent same frequency, phase, and direction with the light entering? One way to approach this is to imagine the light field as a time-varying perturbation on the atoms with which it is interacting.
Classically, the E-field sinusoidally accelerates the electrons, getting absorbed into their motion. When those accelerating charges re-emit the radiation, it will be with the same phase, frequency, and polarization as the exciting field because that's just how the charges happen to be moving--they were driven that way by the field. Thus the light emerges the same color and phase it had going in, and no one is surprised. Stimulated emission is a similar situation. While obviously more complex in certain ways, again one can add a sinusoidally-varying field to the electron Hamiltonian.
In a population-inverted gain medium, this will have the effect of causing the electrons to evolve to a lower energy state, emitting light. What color and phase will the light have? Well, the same as the excitation light since that's just how the electrons happen to be moving.
There is simply no other phase and frequency for them to have, because there is no intermediary between them and the driving field; they are directly driven. All of this is to say that any process where the emission is directly driven by an external field will preserve the phase information. These are the coherent processes, including, for example, nonlinear second-harmonic or difference-frequency generation. These are in contrast with incoherent processes, such as photoluminescence, which are basically forms of spontaneous emission.
Incidentally as well, preserving the frequency information of the exciting field is a matter of conservation of energy, since photon energy equals frequency times Planck's constant. So this just describes stimulated emission. Kaminskii, H. Eichler, B. Liu, P. Esterowitz, R. Allen, M. Kruer, M. Bartoli, L. Goldberg, H. Jenssen, A. Linz, V. Sutherland, P. French, J. Taylor, B. Knowles, Z.
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Azamatov, P. Arsenyev, M. Jenssen, D. Castleberry, D. Gabbe, A. Voronko, A. Kaminskii, V. Osiko, A. Smart, D. Hanna, A. Tropper, S. Davey, S. Carter, D. Pask, A. Tropper, D. Skip to main content. Toggle menu Go to search page. Search Field. You are here Home » Laser Safety » Laser light. Definition and Properties of Laser Light. It is actually an acronym for: L ight A mplification by the S timulated E mission of R adiation Properties First, let's discuss the properties of laser light and then we will go into how is is created.
In fact, having a light beam that is very close to single-wavelength called "monochromatic" is one of the main reason lasers are so useful. Using monochromatic light can allow us to measure or trigger a very specific response in a material e. The waves have to be in phase spectral coherence The phase of a wave describes what part of a sine wave's cycle exists at a certain reference point. Therefore, even if two waves have the same wavelength, if one wave is shifted forward a bit relative to the other wave, their peaks will not be aligned.
The phase of the various waves in a beam must be the same in order for their peaks to be aligned and for the beam to be spectrally coherent. The stable phase of a coherent beam can be very useful. The phase of a wave tends to shift when it interacts with a material, so using a beam with a stable phase allows us to measure the phase shift due to the material, and therefore learn something about the material e.
The waves have to be locally traveling in the same direction spatial coherence If you take one wave traveling north and another wave traveling north-east, then their peaks can not be lined up. Only if a beam has its waves at each point traveling in the same direction can the peaks line up. Note that some people restate this principle as "all the rays of light are parallel". Such a statement is over-simplified to the point of being wrong. If a coherent beam of light such as a laser beam consisted of completely parallel beams, then such beams would not spread out as they travel.
In reality, beams always spread away from straight line motion as they travel through space we call it "diffraction". You may not notice the divergence of a laser beam with your naked eye, but it is there.
Rather than saying all the rays of light in a coherent beam are parallel, a more accurate statement would be that the wave components at a certain point in space are parallel in a single coherent beam, but are not parallel from point to point.
Also, if two waves are traveling in different directions, but otherwise meet all the other criteria for being jointly coherent, we treat the two waves as completely separate beams, and their combination leads to interference patterns. For a coherent beam that has a very large beam-width compared to its wavelength, the diffraction is very small, so that all of the waves at different locations are very close to parallel.
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