In our case, the fiber is highly protected and its tip is covered with collimator lens which makes it impossible to figure out about its structure. Could anyone suggest a solution for this problem? it is a low power laser and doesn't have a trustable manual.
Project the beam on the wall and bend slightly the fiber. With a single mode fiber you will observe only a uniform intensity variation. With a multimode fiber you will also observe the intensity variations, but each mode will behave differently. So you will see that beam intensity will change non-uniformly.
You may also measure the polarization ratio with a strained fiber- for a SMF it will be much higher than for MMF. It does not preserve the polarisation plane anyway, like PMF, but the polarisation contrast is still quite high. (I suppose that you have a fiber coupled laser diode).
Hope this helps.
Dear Sami,
Regarding your question; you should consider some issues that are related to collimation of the light from a small source. This problem is sometimes relying on the point source. For lab reports, They not mentioned this point source or size of the source even in the mathematical equations.
If you try to concentrate the beam into a small spot, to obtain at least acceptable laser power for your experiment. One way to make the spot size smaller is to use a lens of shorter focal length or expand the beam. For the geometric setup. I guess you could use a focal length of (8-12) mm and the maximal ray of angle 0.06.
For further information, you can refer to NEWPORT Manuals. below link give you a general information to set up the lens
http://www.edmundoptics.com/resources/video/tutorials/understanding-collimation-to-determine-optical-lens-focal-length/
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