What is Coherent Light? - A Question on Laser Physics

[billb]

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

 

[Valerie Park]

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.

 

[Graeme Robinson]

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.