Coherence distance in lasers

In summary, the speaker is looking to build an interferometer and needs a light source with a coherence distance of at least 5mm. They have a limited budget and are open to using a laser diode or gas laser. They mention that coherence distance is not mentioned on the company data sheets for the devices they have looked at. They also mention that in the past, they used a white light source to find "white light fringes" in their interferometer design.
  • #1
poor mystic
128
1
Hi people :)
I want to build an interferometer.
In the course of looking for a suitable light source for my instrument I have been advised to check the 'coherence distance' of the sources; it is clear to me that this distance places practical limits on the design of the machine. However 'coherence distance' is not mentioned on the company data sheets for the devices I have been looking at.
I need to find a light source that I can afford, with as short a wavelngth as possible and a coherence distance of at least 5mm. Can anyone help me please?
 
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  • #2
what kind of budget do you have? how much $ do you have to spend on this ?
 
  • #3
Thanks for your question Ranger Mike :)
I have very little money, being a pensioner.
The design I'm using is one I cooked up for myself in the olden days (ended last week) when I believed that lasers just continuously lased, and did not continually restart as seems to be the case.
 
  • #4
The original interferometers we used in school, used a white light source, and with a smooth hand you could find the "white light fringes". Coherence length is inversely proportional to the bandwidth. I think even a laser diode would have a decent enough coherence to show and measure fringes. Almost any gas laser will work also.
 
  • #5


I understand your concern about coherence distance in lasers and its importance in designing an interferometer. Coherence distance refers to the distance over which a laser beam remains coherent, meaning that all the photons travel in phase with each other. This is crucial for precise interferometry measurements.

I recommend checking the specifications for the laser sources you are considering, as coherence distance is often included in the technical details. If it is not listed, you can contact the manufacturer for this information.

In general, shorter wavelength lasers have shorter coherence distances. So, you may need to balance your requirements for a short wavelength with the desired coherence distance of at least 5mm. Additionally, the cost of the laser source may also increase with shorter wavelengths.

There are some techniques that can increase the coherence distance of a laser, such as using a single longitudinal mode laser or using an external cavity to stabilize the laser. These options may be more expensive, but they could help you achieve your desired coherence distance.

I hope this information helps in your search for a suitable light source for your interferometer. Good luck with your project!
 

What is coherence distance in lasers?

Coherence distance in lasers is the distance over which a laser beam maintains its coherence, or the ability for all the photons to be in phase with each other. In other words, it is the distance at which the laser beam starts to lose its coherence and becomes scattered.

Why is coherence distance important in lasers?

Coherence distance is important because it determines the quality and stability of a laser beam. A longer coherence distance means a more focused and precise laser beam, while a shorter coherence distance can lead to a less focused and less stable beam.

How is coherence distance measured in lasers?

Coherence distance is typically measured by using an interferometer, which is a device that splits the laser beam into two paths and then measures how well the two paths stay in phase with each other. The distance at which the two paths become out of phase is the coherence distance.

What factors affect coherence distance in lasers?

The main factors that affect coherence distance in lasers are the type of laser, the wavelength of the laser, and the environmental conditions such as temperature and vibrations. Higher quality lasers and shorter wavelengths tend to have longer coherence distances.

How can coherence distance be increased in lasers?

Coherence distance in lasers can be increased by using higher quality laser components, controlling the environmental conditions, and using techniques such as optical feedback to enhance the coherence. Additionally, using shorter wavelengths and specialized laser designs can also increase the coherence distance.

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