How can i achieve efficient light coupling from multimode to single mode fibers?

This real issue is that the speckle in the MM fiber results in random energy into the SM fiber. Makes a very sensitive vibrometer! You will not get good coupling from an energy point of view due to the etendue issue, The only thing you can use is a non-imaging parabolic concentrator like Edmund Optics sell for solar energy and detector concentrators. This will not help much with the efficiency (SM fiber has a small acceptance angle), but the radiation will be homogenized so the speckle goes away.