Coherent length measurement of a monochromatic laser source i need to learn about how to measure coherent length of a monochromatic laser source. I know that I can use michelson interferometer or spectrometry. For laser which has long coherent length michelson interferometer is hard to use.we need to use spectrometry.But when should I use spectrometry? I mean up to what length it is suitable to use michelson? And which spectrometry should I use for long coherent length? And how can I measure the bandwith? 
 A: A very simple way to check would be a Fizeau interferometer. You'll need:


*

*a laser beam expander

*a clean piece of glass.


Say you want to check if your laser has >= 2 meters of coherence length. Expand your laser beam such that the diameter of the beam is 5-15cm at the 2 meter mark.
Now take the piece of glass and lean it against something, reflecting the expanded beam back towards the laser aperture, but a few degrees off to the side onto a piece of paper.
Like with the Michelson interferometer, the contrast of the fringes will indicate whether the light is still in phase. However, instead of concentric rings, you'll see a zebra stripe pattern.
You can repeat this for 2m, 5m, 10m, and so on to get a ballpark estimation of coherence length.
A: A michaelson interferometer can be arbitrarily long if one of the arms is, for example, in a rolled-up length of optical fiber.  It is easy to get optical fiber many kilometers long, and easy to build an interferometer with a fiber arm.
Spectrometry is, in my opinion, more difficult to use for long coherence length measurement.  Coherence length of 1 km would correspond to wavelength stability of roughly one part in $10^{12}$, which is not easy to measure spectroscopically.
