An　ultralow　insertion　loss　(IL)　and　ultrahigh　polarization　extinction　ratio　(PER)　single-polarization　(SP)　coupler　based　on　dual-core　photonic　bandgap　fiber　(DC-PBF)　with　thin　slab　waveguide　(TSW)　is　proposed.　By　introducing　the　TSW　structure　into　the　DC-PBF,　the　loss　of　the　secondary　eigenstate　of　polarization　(ESOP)　mode　is　significantly　increased　through　the　strong　mode　coupling　with　the　higher-order　TE/TM　mode　of　the　TSW.　Thus,　the　SP　performance　is　achieved　through　the　significantly　high　loss　of　the　secondary　ESOP　mode　in　conjunction　with　the　relatively　low　loss　of　the　primary　ESOP　mode.　In　order　to　design　ultralow　IL　and　ultrahigh　PER　SP　couplers　based　on　DC-PBF　with　TSW,　the　low-loss　SP　DC-PBF　with　TSW　is　established　by　considering　the　impacts　of　structure　parameters　and　wavelength　on　both　loss　and　coupling　performance.　Furthermore,　the　dependences　of　powers,　coupling　ratio,　and　PER　in　x(y)-polarization　in　two　cores　with　different　lengths　are　analyzed　for　the　SP　coupler　based　on　DC-PBF　with　TSW.　Finally,　three　ultralow　IL　and　ultrahigh　PER　3　dB　SP　couplers　based　on　DC-PBF　with　TSW　are　designed　through　theoretical　calculation　of　fabrication　tolerance　and　operation　bandwidth.　These　couplers　exhibit　narrow-band　high　PER　(∼　27.2　dB),　short-length　ultralow　IL　(∼　0.00054　dB),　and　long-length　ultrahigh　PER　(∼　61.88　dB),　respectively.　The　proposed　ultralow　IL　and　ultrahigh　PER　SP　couplers　based　on　DC-PBF　with　TSW　have　potential　applications　in　high-precision　resonant　PBF　optic　gyroscopes　as　well　as　other　PBF　applications.　　©　2009-2012　IEEE.