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.