The　further　development　of　＂lab　on　fiber＂　technology　demands　more　powerful　optical　microsystems　and　optical　fibers　with　special　functions　to　create　sensors　with　better　performance.　Whispering　gallery　microcavities　(WGMs)　made　of　different　materials　with　high　quality　factors　show　very　good　sensitivity　to　gases,　nanoparticles,　and　biocomponents.　However,　realizing　the　excitation　and　detection　of　the　resonant　modes　of　WGMs　on　the　facets　of　optical　fibers　is　still　a　great　challenge.　This　work　employs　the　advanced　3D　manufacturing　technology　of　two-photon　lithography　to　fabricate　an　optical　microsystem　on　the　end　facet　of　a　multicore　optical　fiber,　which　enables　excitation　and　detection　of　whispering　gallery　modes　on　the　end　facet　of　an　optical　fiber.　Whispering　gallery　modes　with　high　quality　factors　are　observed,　and　the　sensing　characterization　of　this　device　for　vapors　of　three　types　of　volatile　organic　compounds　is　investigated.　Such　optical　fiber　vapor　sensors　can　be　applied　in　the　fields　of　medical　care,　environmental　monitoring　and　chemical　manufacturing.　Additionally,　the　out-of-plane　light　coupling　strategy　may　be　useful　for　the　design　of　integrated　optical　circuits　on　chips.