Single-Polarization Coupler Based on Dual-Core Photonic Bandgap Fiber With Thin Slab Waveguide

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.