Modified Single Trench Fiber With Effective Single-Mode Operation for High-Power Application

In this paper, the modified single trench fiber (MSTF) is proposed. The core of MSTF is successively surrounded by the trench and ring whose refractive index is lower than that of the inner cladding and identical with that of the core, respectively. Performances of MSTF have been numerically investigated by the finite element method at 1-mu m region. Simulations show that the MSTF can provide much larger loss of least lossy high-order-mode than the step-index fiber (SIF) and even larger than the standard single trench fiber, which is beneficial to achieve effective single-mode (ESM) operation and enhance the threshold of transverse-mode instabilities. Furthermore, the MSTF fabricated by using a conventional technique (core NA similar to 0.06) with 25 and 30 mu m core diameter can guarantee the ESM operation, which is much superior than the SIF with the same parameters. The MSTF with the core diameter of 25 mu m is fabricated by using a conventional fabrication method. Excellent high-order-mode suppression and ESM operation based on the MSTF are experimentally verified. Finally, the MSTF is employed in a high-power fiber laser (HPFL), and the ESM operation (M-2-factor < 1.1) is obtained up to 807 W. The MSTF is a practical candidate aiming at industrial HPFLs requiring ESM.