What Is Coherence Distance in Laser Interferometers?

[poor mystic]

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?

 

[Ranger Mike]

what kind of budget do you have? how much $ do you have to spend on this ?

 

[poor mystic]

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.

 

[johnbbahm]

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.