Degree of coherence of light from a red semiconductor laser

In summary, the individual seeking information about the coherence length and spatial coherence of an inexpensive 1 mW red semiconductor laser was advised that this information can only be obtained from the manufacturer or through direct measurements. The coherence lengths of such lasers can vary greatly, even within the same model, and can range from 1 mm to 300 m. Various methods of measuring coherence length and spatial coherence were suggested, including using a Young interferometer and Michaelson interferometer, as well as calculating from manufacturer's specifications.
  • #1
htg
107
0
I would like to have at least a vague idea how coherent is light from an inexpensive 1 mW red semiconductor laser which I have.
I tried to find the manufacturer using Google, but I haven't found it.
I would like to know the coherence length and the degree of spatial coherence.
Is there anyone who may give me some information?
 
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  • #2
It is not possible to get that information anywhere other than either the manufacturer or direct measurements. Even for a single manufacturer with different versions of the same model of laser you can get coherence lengths ranging from 1 mm to 300 m.

http://www.crystalaser.com/new/greenlaser.html
 
  • #3
DaleSpam said:
... you can get coherence lengths ranging from 1 mm to 300 m.

http://www.crystalaser.com/new/greenlaser.html
1mm already is a decent coherence length, because it is >>> λ.
But what about spatial coherence?
 
  • #4
You missed the point. My point was that even minor variations in the model can give you coherence lengths which vary over 5 orders of magnitude.

There is simply no way that someone on an internet forum can answer your question. You either need to measure it yourself, or get the specifications from the manufacturer.
 
  • #5
DaleSpam said:
You missed the point. My point was that even minor variations in the model can give you coherence lengths which vary over 5 orders of magnitude.

There is simply no way that someone on an internet forum can answer your question. You either need to measure it yourself, or get the specifications from the manufacturer.

I think if they call it a laser diode, there should be decent lower limit of coherence length (which should be >> λ ) and there should be significant degree of spatial coherence.

Do you have a suggestion how I can measure spatial coherence and coherence length?
 
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  • #6
htg said:
<snip>
Do you have a suggestion how I can measure spatial coherence and coherence length?

Spatial coherence is usually measured by a Young interferometer, letting the slit spacing vary. Coherence length can be measured with a Michaelson interferometer, letting one arm change in length. The variation of fringe visibility is a measure of the coherence properties.

One way to infer the coherence length is to measure the spectral bandwidth of the output, and IIRC you can infer the coherence area by measuring the size of a single 'speckle'. Finally, both can usually be calculated from manufacturer's specifications: the output spectral FWHM, the beam diameter at the output face, and the beam divergence.
 

What is the degree of coherence of light from a red semiconductor laser?

The degree of coherence of light from a red semiconductor laser refers to the measure of the correlation between two different points on the laser beam. It indicates the extent to which the light waves are in phase with each other.

How is the degree of coherence of light from a red semiconductor laser measured?

The degree of coherence is usually measured using a Michelson interferometer, which splits the laser beam into two paths and then recombines them. The resulting interference pattern can be used to calculate the degree of coherence.

What factors can affect the degree of coherence of light from a red semiconductor laser?

The degree of coherence can be affected by various factors, such as the temperature and current of the laser, the quality of the laser cavity, and any external disturbances or vibrations.

Why is the degree of coherence important in applications of red semiconductor lasers?

The degree of coherence is an important parameter in applications of red semiconductor lasers, as it directly affects the quality and stability of the laser beam. It is particularly crucial in applications that require high precision, such as holography, interferometry, and telecommunications.

Can the degree of coherence of light from a red semiconductor laser be improved?

Yes, the degree of coherence can be improved by using techniques such as optical feedback and active stabilization of the laser cavity. These methods can reduce unwanted noise and fluctuations in the laser beam, resulting in a higher degree of coherence.

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