Fiber Optic Coupling -- how to couple a 200μm core fiber into a 50μm core fiber?

[Jamison Lahman]

Is it possible to couple a 200μm core fiber into a 50μm core fiber by using a 4x magnifier/reducer?

 

[Charles Link]

The answer I believe is yes. You can even do it with a single lens with $\frac{m}{b}=\frac{1}{4}$ where b is object distance, and m is image distance. The most difficult part is getting it aligned properly, i.e. centering the focused beam on the 50 um input. The alignment in the z-direction also needs to be quite precise.