Measuring the angle of incidence into a fiber

[nekronaut]

Hello everyone, I'm currently working on a little project that has been puzzling me for some time.

If you send a laser beam into a fiber, the ingoing angle is sometimes slightly different than the outgoing angle.

To measure this ratio of angles, I am currently using a setup that looks like this

First, the laser hits a beam expander -> fiber -> collimating lens -> focusing lens.

After the beam exits the focusing lens, It travels through an aperture (4mm diameter) and enters a new fiber at the focal point. This aperture can be moved parallell to the beam propagation direction and it is used to regulate the ingoing angle into the second fiber. It effectively "cuts off" the beam so that its height varies and thus affects the angle.

Depending on what angles I want, I can program this aperture to move to exact locations and thus determining the ingoing angle. (Everything is done automatically).

After the beam has gone through the fiber, it hits a collimating lens and then a new focusing lens with different focal length and travels through a second aperture (same diameter) and then hits a photodiode at its focal point which measures its intensity.

The current algorithm for finding the outgoing angle of the fiber is to measure the maximum intensity when the beam is not cut off at all by the second aperture, and then moving the aperture towards the lens, find out at what position the intensity has dropped by 1/e^2 (Definintion of beam width) and then through basic trigonometry find the angle.

The thing is, I get the same out angle no matter what in angle is being used and I have ripped my hair trying to find out what is wrong.

Am I using the definition of beam width incorrectly?
Diffraction shouldn't be a concern since the wavelength of the laser is 1064nm and the aperture is 4mm in diameter.

If anyone knows what could be wrong, please help.

 

[eljose79]

Hello there,

Thank you for sharing your project with us. It sounds like you have put a lot of thought and effort into your setup. From what you have described, it seems like you have a good understanding of the principles involved.

Without being able to see your setup or data, it is difficult for me to pinpoint exactly what might be causing your issue. However, I can offer some suggestions that may help you troubleshoot the problem.

Firstly, have you checked the alignment of your setup? Even small misalignments can cause significant changes in the angle of the outgoing beam. It may be helpful to double check the alignment of your lenses and fibers to ensure they are all properly aligned.

Another thing to consider is the quality of your fibers. If the fibers are not high quality, they may cause distortions in the beam, leading to inaccurate measurements. It may be worth investing in higher quality fibers to see if that makes a difference.

You mentioned that you are using a collimating lens before the fiber. Have you tried using a different collimating lens? Different lenses may have different properties and could affect the angle of the outgoing beam.

Lastly, have you considered other sources of error, such as thermal effects or vibrations in the setup? These could also affect the angle of the outgoing beam and could be worth investigating.

I hope these suggestions are helpful to you. Good luck with your project, and please let me know if you have any further questions.