Large-Mode-Area Multi-Resonant All-Solid Photonic Bandgap Fiber with Low Bending Loss and Robust Single-Mode Operation

In this paper, we have theoretically and experimentally demonstrated a novel large-mode-area multi-resonant all-solid photonic bandgap fiber (AS-PBGF). Based on the combination of the multi-resonant structure and leakage channel, the structural parameters are specifically designed for conventional applications of fiber lasers around 1 μm wavelength. Through the stack and draw technique, the fiber is fabricated successfully and the hexagon core is measured to be 46 μm in the corner-to-corner direction. The transmission spectrum exhibits a broad bandwidth from 970 nm to 1180 nm in the 3rd photonic bandgap (PBG). Bending loss lower than 0.1 dB/m with a 5-m-length fiber is obtained in the case of bending radius R > 25 cm. By employing the spatially and spectrally resolved imaging (S2) method, robust single-mode (SM) operation of the fiber is verified within the whole PBG. In addition, we also report a phenomenon about the abnormal variation of M2 within the low loss region of the 3rd PBG. The evolution of beam quality within the 3rd PBG appears as a U-shaped curve rather than a constant. Based on the measured M2 and S2 results, a preliminary conclusion about the relationship between the beam quality and SM characteristics in AS-PBGF, which is quite different from that in conventional SIFs, is demonstrated experimentally for the first time.