Hello, I'm trying to obtain a collimated beam from my 1550 nm light source. I have a fibered mono-mode source and when I analyze its profile with an IR camera at one end of the fiber, it appears clean. But when I put an IR coated microscope objective or an IR coated lens between the end of the fiber and the camera to start to build my collimation system, interference rings appear. I know that, for reflection, when you use a thin glass slide (for example) you obtain interferences but here, I can't explain why I obtain this phenomenon in transmission. I tried to change my fiber, to put the end of the fiber at the focus length of the microscope objective, then closer to the optic or further, to use a lens with small focus length (2.5 mm) and bigger focus length (20 mm) ... I obtain these rings all the time. I'm sure I missed something simple. Does anybody know how to deal with these rings ?