@cmanoopckr
Just a suggestion. Based on your lens arrangement, you could create an image of the fiber core at the delivery plane. The spatial light distribution will be the same as that of the fiber output, i.e. probably not uniform, but if you are OK with it, keep reading.
As a first order approximation (and starting point), you have a core with a diameter of 1000um that you want to bring down to 50um. This is a magnification of 50/1000 = 0.05X. The magnification of your two lens system is given by the ratio of the collimator lens focal length over the focusing lens focal length. So, if you choose a 10-mm focal length collimator, and a 200-mm focal length focusing length, this will be a magnification of 10/200 = 0.05X. In this arrangement, the distance between the two lenses doesn’t matter much (the magnification will remain 0.05X, vignetting and telecentricity might be affected).
I hope this helps.
Take care,
David