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eoghan
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Hi! I hope this is the right forum. I know how laser works (population inversion, electronic transitions etc...), but I can't understand why the light emitted is coherent
eoghan said:I know … population inversion …, but I can't understand why the light emitted is coherent
When an electron decays without external influence it is said to be due to "spontaneous emission." The phase associated with the photon that is emitted is random.
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An external electromagnetic field will affect an atom's state - changing the quantum mechanical variables mentioned above. Specifically, the atom will act like a small electric dipole which will oscillate with the external field. One of the consequences of this oscillation is that it encourages electrons to decay to the lowest energy state. When this happens due to the presence of the electromagnetic field from a photon, a photon is released in the same phase and direction as the "stimulating" photon …
conway said:People tend to understate the importance of population inversion in explaining how a laser works. I've seen this in a few previous discussions. If you have a normal, thermal distribution of excited atoms in the cavity, the light is not coherent.
tiny-tim said:As described above, a population inversion is required for laser operation, but cannot be achieved in our theoretical group of atoms with two energy-levels when they are in thermal equilibrium.
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To achieve non-equilibrium conditions, an indirect method of populating the excited state must be used.
But coherence requires more, it requires an external electromagnetic field (ie a photon) which effectively converts the atom into a dipole oscillator resonating in phase (and direction) with the field.
As a result, any electron produced from that atom will also be in phase.
Cthugha said:In fact this is not a case of more requirements, but rather less requirements.
The in-phase emission as seen in stimulated emission defines coherence time.
… are you saying that sunlight is coherent?All kinds of light will have some coherence time. It might be in the fs range (sunlight) or in the second range for lasers or number-squeezed light.
tiny-tim said:… are you saying that sunlight is coherent?
tiny-tim said:What's coherence time?
tiny-tim said:What does it have to do with why the emitted photon is coherent?
In particular …
… are you saying that sunlight is coherent?
jambaugh said:The essential feature of the laser is the process of stimulated emission which is a phenomenon dependent on the photons' Bose-Einstein statistics. Just as fermions satisfy the exclusion principle due to their statistics, bosons follow a sort of "inclusion principle" preferring to manifest in in a common mode and that is the source for the stimulated emission effect.
Cthugha said:If you know its phase …
tiny-tim said:Isn't this begging the question … surely an incoherent beam of photons doesn't have "a" phase?
To put it another way, aren't you talking about the coherence of an individual photon … but this thread is about coherence between two photons?
DrDu said:Usually in the definition of the correlation functions one splits E into positive and negative frequency parts E=E^(+)+E^(-), which are hermitian adjoints. Then E^(-) depends only on the a and E^(+) only on a^+. The expression for the correlation function is normal ordered <E^(+)(x_2)E^(-)(x_1)> so that it has also a nonvanishing expectation value in a Fock state. A characteristic of Fock states is the so called "anti-bunching" which obviously is a correlation of the detection times of the photons. So a Fock state cannot be void of correlations.
DrDu said:What distinguishes a laser is that it is fully coherent in any order in the sense that the correlation function can be factored into equal factors e and e^*. A Fock state is fully coherent only in first order, although all correlation functions up to order n (the particle number) are different from 0.
conway said:This partial explanation does not explain the sign of the stimulated transition, but it does explain the phase/direction coherence.
Coherent light refers to light waves that have a constant phase relationship with each other, meaning they are all in sync. This allows the light to travel in a specific direction with minimal spreading or scattering, resulting in a concentrated and powerful beam of light.
Laser light is different from regular light in several ways. It is monochromatic, meaning it consists of a single wavelength, whereas regular light is a mixture of different wavelengths. Laser light is also coherent and highly directional, while regular light is usually incoherent and spreads in all directions.
The coherence of laser light is due to the stimulated emission process. In a laser, atoms are excited to a higher energy state and then emit photons of light in a specific direction. These emitted photons then stimulate other atoms to emit photons that are in phase and travel in the same direction, resulting in a coherent beam of light.
The coherence of laser light is crucial in many applications of laser technology. It allows for precise and controlled delivery of energy, making it useful in medical procedures, manufacturing, and cutting-edge research. The highly directional and concentrated nature of laser light also makes it ideal for long-distance communication and high-resolution imaging.
Coherence is maintained in a laser system through careful design and alignment of the components. This includes using a gain medium with a narrow emission spectrum, a stable optical cavity, and a method to select and amplify only a single mode of light. Any disturbances or fluctuations in these components can lead to a loss of coherence in the laser beam.