What is Coherent Light? - A Question on Laser Physics

In summary, coherence refers to the consistent phase relationship of light waves at all points in space, and while diffuse laser light may still exhibit this characteristic, it is not suitable for most applications due to its low power and spatial incoherence.
  • #1
billb
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Optical "coherence"

A question on the nature of coherence that came up on Wikipedia...

If we pass the light from a laser through a stack of frosted plates, is the result still "coherent light?" For most optical applications requiring coherent light, this type of heavily diffused laser light is fairly useless. A spatial filter thinks it's incoherent: pass it through a lens with a pinhole at the focus nearly all the light will be blocked. Yet if we compare the phase of this light at various 3D locations throught the wave, we'll find that the EM wave still varies coherently.

Somehow the textbook definition of coherence doesn't seem correct when it declares diffuse laser light to be coherent light. So what does "coherent light" really mean?


Similar question: should we say that a "white paint laser" emits spatially INcoherent light? See http://tinyurl.com/aybjn Laser Action in Very White Paint
 
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  • #2
The definition of coherence is that the light waves have a consistent phase relationship at all points in space over a certain region. The laser light passing through frosted plates will still show this phase relationship, meaning it is still coherent light. However, due to the scattering of the light by the frosted plates, the power at each point in space will be low and will vary randomly from point to point, making it spatially incoherent. So while the light is still coherent, it is not suitable for most optical applications due to its low power and spatial incoherence. A white paint laser would be spatially incoherent, since the light is scattered randomly by the paint particles, and would not have a consistent phase relationship at all points in space.
 
  • #3


Coherent light refers to light waves that are in phase with each other and maintain a fixed phase relationship over time. This means that the peaks and troughs of the light waves are aligned and do not change in their relative positions as the light travels. This results in a smooth and consistent wave pattern.

In the case of a laser, the light is generated by stimulated emission, where photons are emitted in a highly coordinated manner, resulting in a single wavelength and phase. This makes laser light highly coherent and useful for many applications such as precision measurements, holography, and telecommunications.

When light passes through a stack of frosted plates, it becomes heavily diffused and the phase relationship between the light waves is disrupted. This means that the peaks and troughs of the waves are no longer aligned, and the resulting light is no longer considered coherent. However, as the content mentions, the phase of the light can still be measured at different points in space and it will still show a consistent pattern, indicating that the light is still coherent in terms of its phase.

In the case of a "white paint laser," the light emitted is not coherent in terms of its phase, as it is a mixture of different wavelengths and does not maintain a fixed phase relationship. This type of light would not be useful for applications that require coherent light.

In summary, coherence refers to the fixed phase relationship of light waves, and in the case of a laser, this results in a highly organized and consistent light beam. Light that is heavily diffused or a mixture of different wavelengths is not considered coherent.
 

Related to What is Coherent Light? - A Question on Laser Physics

1. What is coherent light?

Coherent light is a type of electromagnetic radiation that has all of its waves in phase with each other. This means that the waves are in sync and have a consistent wavelength and direction of propagation. This results in a consistent and stable pattern of light, making it useful for applications such as laser technology.

2. How is coherent light created?

Coherent light is created through a process called stimulated emission, where a light source such as a laser produces light waves that are in phase with each other. This is achieved by exciting atoms in a material to a higher energy level, causing them to release photons in a specific direction and with the same wavelength and phase as the original photons. This results in a highly concentrated and coherent beam of light.

3. What is the difference between coherent and incoherent light?

The main difference between coherent and incoherent light is the level of synchronization between the light waves. Coherent light has waves that are in phase with each other, while incoherent light has waves that are randomly out of phase. Coherent light is also highly directional and concentrated, while incoherent light is more diffuse and scattered.

4. What are the applications of coherent light?

Coherent light has many practical applications, including in laser technology, holography, and optical communication. It is also commonly used in scientific research, medical procedures, and industrial processes such as cutting and welding. Additionally, coherent light is essential for technologies such as DVD players, barcode scanners, and fiber optic communication.

5. Why is coherent light important in laser physics?

Coherent light is crucial in laser physics because it allows for the creation of highly focused and controlled beams of light. This makes lasers useful in a wide range of applications, from precision cutting and welding to delicate medical procedures. Additionally, the coherence of laser light allows for interference and diffraction effects, making it a valuable tool for studying the properties of light itself.

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