High Efficiency Butt-Coupling of a 405-nm Laser Diode to Hollow Core Fiber

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We present a compact and efficient method for directly coupling a near-ultraviolet laser diode with high beam ellipticity to a hollow core fiber (HCF) using an all-fiber mode field diameter (MFD) converter. This MFD converter, approximately 1 mm in length and composed of graded-index fiber (GIF) and coreless fiber (CSF), effectively addresses the MFD mismatch between the laser diode and HCF through image magnification. As a proof of concept, we tested a 405 nm laser diode with a beam ellipticity of 2.37. Simulations predicted a coupling efficiency of ~80%, and we achieved 67% experimentally. Additionally, we introduced an elliptic GIF to further enhance coupling efficiency for highly elliptical beams, leading to a significant performance improvement.