Thank you.. is there any advice from about a source which explain to me how to measure spectral bandwith using fabry perot interferometer ? 

can I use this method http://www.ijens.org/Vol_12_I_06/122306-8383-IJECS-IJENS.pdf ? 

And if you would like to use your light source for interferometry application, for example in Michelson-Interferometer, you could measure the temporal coherence length by changing the optical path difference between the 2 arms of the interferometer and monitoring the contrast of the interference pattern.

But in interferometry, you also need to have spatial coherence. And the spatial coherence length = 1.22 *lambda*d/s

where d is the distance between the light source and the plane where you measures the coherence and S is the radius of your light source. plz note that this equation is valid for circular light source.

Best regards

Thank you for your help.I will study these.coherent length is related to temporal coherence am I right ? For the method which I will use in the attachment it  says turnable laser.my laser source is monochromatic and will not be turnable ? Is that make sense ? I am going to present these topic on tuesday thanks to you.I couldnt find the way now I have.thank you again

The method you've posted is actually independent of tuneability of the light source. They mention tuneable lasers because diode lasers are in principle tuneable, but when they are tuned to their operation wavelength than they are monochomatic (= very narrow bandwidth).

Here is a good tutorial for scanning FP interferometers:

http://www.thorlabs.de/tutorials.cfm?tabID=e9d2d96f-1efd-4ef0-bb17-fa2ca60f8a8d

By the way is your laser continous work or pulsed?

Actually I dont know. Im taking a optic lab and this is my final presentation subject.The teacher ask me how to measure coherence length of a monochromatic laser source and he didnt give any extra information.I search on  google and books I found the ways but any source dont include how it can be done using these interferometer.For example I can use mach zhender interferometer also.Now I m confused about that.Should I study the the method I've posted ?

I advice you study first the theoretical foundations of optical coherence and say, temporal coherence, coherence length, spectral bandwith of a source, spatial coherence. I can suggest you the seminal book by L. Manel and E. Wolf. I suggest you, do not use monochromatic laser source as a definition, indeed, there are no strict monochrmatic sources neither in nature nor bu those artificially constructed by humankind. You may better talk in terms of cuasi-monochromatic laser source.

As said by previous colleagues, all is related to the spectral profile of the source and value of the bandwidth, for measuring you need spectrometer such as Michelson. If you want to use Fabry-Perot be sure that the interferometer has enogh temporal resolutioln, otherwise you can get wrong values.

sorry for late the late answer.I will study that book.Am I use mıchelson for monochromatıc laser source? I read that monochromatıc laser sources have too long coherence lenght so we can have troubles and we need to use another interferometer.İs this right ? Thank you so much 

Unfortunately, I cannot answer your question without knowing first what specific laser source. The coherence length dramatically depends in the type of laser.

Michelson interferometer is suitable for laser outputs with line widths less than 1GHz. I couldnt find the what iste linewitdh range for monochromatic laser source.İf it is less than 1 GHz then I can use it 

Thank you again :)

If I understand your question correctly, the answer is, first, coherence length might depend on line width of the source but it also varies  depending on quality of laser. Depending on the laser quality, same line width may show different coherence length (temporal and spatial). 

Michelson interference  technique can be used for  measuring temporal coherence length of any length and any line width. 1/e of roll off in fringe visibility as a function of one arm distance will give the coherence length. For measuring the spacial coherence, double slit experiment can be used. For more details 

http://www.rp-photonics.com/coherence_length.html

http://en.wikipedia.org/wiki/White_light_interferometry

Thank you :)

For narrow linewidth lasers with huge coherence length the delayed self-heterodyne technique is often used. https://www.osapublishing.org/oe/abstract.cfm?uri=oe-22-9-10642

Lcoh = Lambda^2/Delta Lambda

Which Lambda is the wavelength and Delta Lambda is differences between higher and shorter wavelength

You can read and study the contents of this multimedia on coherence of light and related experiments for measuring temporal and spatial coherence:

http://e-ico.org/node/255

@Light coherence, Could you please provide some reference for "temporal and spatial coherence can also depend upon quality of laser other than spectral linewidth"?