Novel constant-cladding tapered-core ytterbium-doped fiber for high-power fiber laser oscillator

Yun Ye,Jinyan Li,Xiaoming Xi,Baolai Yang,Xiaolin Wang,Xiaojun Xu,Xianfeng Lin,Chen Shi,Hanwei Zhang

doi:10.1017/hpl.2021.9

Abstract

Abstract Power scaling based on traditional ytterbium-doped fibers (YDFs) is limited by optical nonlinear effects and transverse mode instability (TMI) in high-power fiber lasers. Here, we propose a novel long tapered fiber with a constant cladding and tapered core (CCTC) along its axis direction. The tapered-core region of the fiber is designed to enhance the stimulated Raman scattering (SRS) threshold and suppress higher-order mode resonance in the laser cavity. The CCTC YDF was fabricated successfully with a modified chemical vapor deposition (MCVD) method combined with solution doping technology, which has a cladding diameter of 400 μm and a varying core with a diameter of ~24 μm at both ends and ~31 μm in the middle. To test the performance of the CCTC fiber during high-power operation, an all-fiber laser oscillator based on a CCTC YDF was investigated experimentally. As a result, a maximum output power of 3.42 kW was achieved with an optical-to-optical efficiency of 55.2%, although the TMI effect was observed at an output power of ~3.12 kW. The measured beam quality (M2 factor) was ~1.7, and no sign of the Raman component was observed in the spectrum. We believe that CCTC YDF has great potential to simultaneously mitigate the SRS and TMI effects, and further power scaling is promising by optimizing the structure of the YDF.

Highlights

High-power fiber lasers have been broadly applied in various fields, such as industrial processing, national security, and medical sciences, owing to their advantages of high conversion efficiency, good beam quality, easy thermal management and compact structure[1]
We proposed and demonstrated a novel constant-cladding tapered-core (CCTC) ytterbium-doped fiber (YDF) for the first time, which is fabricated using modified chemical vapor deposition (MCVD) in conjunction with a solution doping technique
The core region varying from a large core size to a small core size inevitably reduces the optical efficiency, because a certain proportion of high-order mode (HOM) experience a higher bending loss owing to fiber coiling and leak into the cladding

Summary

Introduction

High-power fiber lasers have been broadly applied in various fields, such as industrial processing, national security, and medical sciences, owing to their advantages of high conversion efficiency, good beam quality, easy thermal management and compact structure[1]. Compared with conventional uniform double-cladding fiber, a long tapered fiber has many advantages, such as increasing the pump absorption, suppressing the nonlinear effects, and maintaining good beam quality[22]. Simultaneously mitigating the SRS and TMI effects, obtaining good beam quality and realizing further power scaling of CW YDF lasers is an urgent issue. The CCTC fiber consists of a constant cladding diameter (~400 μm) and a varying core diameter (~24 μm at both ends and ~31 μm in the middle) Based on this fiber, the laser performances, the TMI and SRS characteristics, are investigated in detail by establishing a high-power all-fiber laser oscillator system. The maximum output power was scaled up to 3.4 kW with an optical efficiency of ~55.2%, and the measured M2 factor degrades from 1.60 to 1.88, with no sign of the SRS effect within the spectrum

Fiber design and fabrication

Experiment setup

Results and discussion

Conclusion