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. Monochromatic The light emitted from a laser is monochromatic , that is, it is of one wavelength color.
Directional Lasers emit light that is highly directional. Coherent The light from a laser is said to be coherent, which means the wavelengths of the laser light are in phase in space and time.
Regular light, such as from an incandescent light bulb or from a fire, is incoherent. The light from a fire contains different light waves that are not the same frequency, are not in phase, are not traveling in the same direction, and are not in the same polarization state. Focusing incoherent light, such as by using a glass lens, does not make the light waves have the same frequency, the same phase, the same local direction, or the same polarization.
Therefore, focusing incoherent light does not make it coherent like a laser beam. Focusing light just concentrates the energy of the light into a smaller area. The phenomenon of stimulated emission in a laser is very useful because it produces light that is usually temporally, spectrally, spatially, and polarizationally coherent.
Stimulated emission means that an electron is in an excited state when a bit of light a photon comes along and knocks the electron down to the unexcited state, causing it to emit another bit of light another photon in the process. In the act of knocking the electron down, the original photon causes the new photon to be coherent with it. The process repeats in domino fashion; each time a new photon being added to the beam that is coherent with the original photons.
Stimulated emission is not as exotic as it may sound and in fact happens a lit bit all the time. The hard part in designing a working laser is getting stimulated emission to be the main way an electron de-excites. In a regular chunk of matter, an excited electron most often de-excites by colliding with other electrons or atoms, thereby loosing its energy to heat, or by spontaneously emitting a bit of non-coherent light.
Designing a laser therefore involves making the stimulated emission transition of the excited electron very probable, and making the other transition possibilities less probable. Note that stimulated emission is not the only way to make coherent beams.
For high frequency waves such as visible light, stimulated emission is the most effective way of creating coherent beams. But for low frequency waves such as radio waves, coherent beams are much easier to create simply by driving a sine-wave electrical current into an antenna.
The waves that are created by an antenna that is driven at a single frequency, such as those that carry radio station broadcasts, are coherent. In quantum optics , the term coherence is often used for the state of light-emitting atoms or ions. In that case, coherence refers to a phase relationship between the complex amplitudes corresponding to electronic states.
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By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. If you later retract your consent, we will delete those inputs. As your inputs are first reviewed by the author, they may be published with some delay. See also: coherence time , coherence length , monochromatic light , optical phase , laser beams , beam quality , linewidth , interference , laser speckle , diffusers , supercontinuum generation , coherent beam combining , The Photonics Spotlight , The Photonics Spotlight , The Photonics Spotlight and other articles in the category general optics.
If you like this page, please share the link with your friends and colleagues, e. These sharing buttons are implemented in a privacy-friendly way! Sorry, we don't have an article for that keyword! Figure 1: A prism is inserted into a spatially coherent laser beam, generating an interference pattern on the screen. There are two very different aspects of coherence: Spatial coherence means a strong correlation fixed phase relationship between the electric fields at different locations across the beam profile.
For example, within a cross-section of a beam from a laser with diffraction-limited beam quality , the electric fields at different positions oscillate in a totally correlated way, even if the temporal structure is complicated by a superposition of different frequency components. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. First of all, note that there are two relevant forms of coherence for a laser: temporal where the phase of the field is steady with time, i.
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.
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