I have an all-solid band-gap fiber with the core diameter about 10 um. How can I excite the core mode only without exciting the cladding modes? I learned that someone splicing a piece of single mode fiber to band-gap fiber to excite the core mode only. In my experiments, I spliced a piece of 9/125 single mode fiber to the band-gap fiber, but I find that most light transmit in the Ge-doped area. What’s wrong? Someone have any good ideas? Thank you so much!