Date of Award
December 2020
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
School of Computing
Committee Member
Liang Dong
Committee Member
John Ballato
Committee Member
Eric Johnson
Committee Member
Lin Zhu
Abstract
Power scaling of rare-earth-doped fiber lasers is vital to many emerging applications. Average output power scaling with Yb doped fiber lasers operating near 1.1 μm was very successful in the 10 years between 2000 and 2010. The discovery and rediscovery of many limiting factors such as nonlinear effects, thermal lensing, optical damage, and so forth showed various bottlenecks in further power scaling of output power from a single fiber. On the other hand, single-mode average output power scaling of fiber lasers operating at ~980 nm and ~1560 nm has lagged much behind their pioneering counterparts at ~1.1 μm. Single-mode high-power fiber lasers at these scarce wavelengths are in great demand for many applications such as pumping ultrafast lasers, nonlinear frequency conversion, lidar, free-space optical communication, etc. However, currently, commercial single-mode laser didoes at ~980 nm have very limited output power of near 1 W. Many different types of Yb-doped fibers for ~980 nm lasers were reported previously but the record output power has only reached to ~15 W from a monolithic single-mode Yb fiber laser. One of the major limiting factors is the amplified spontaneous emission (ASE) at ~1030 nm which has a net positive gain at very low inversion (>5%) while Yb laser operation at ~980 nm needs very high inversion (>50%). In the first part of this dissertation, we propose the utilization of a Yb-doped multiple cladding resonance (MCR) based all-solid photonic bandgap fiber (AS-PBF) for power scaling of single-mode laser operating at ~980 nm. This novel type of AS-PBF provides three major advantages, including large core to cladding ratio, superior HOM suppression, and built-in wavelength filtering to suppress ASE at ~1030 nm. We have experimentally explored the power scaling of ~980 nm Yb fiber laser pumped at 915 nm. An optimized Yb PBF with 24 μm core and 130 μm cladding was fabricated after several fine-tuning processes of fiber dimensions (thus bandgap positions) by carefully characterizing the Yb AS-PBF and analyzing the output diagnostics of the laser oscillator. Eventually, we were able to obtain a near-diffraction-limited laser output with ~150 W from an all-fiber Yb fiber laser operating at ~980 nm, which represents a factor of 15 improvement over the previously reported record. This monolithic fiber laser demonstrates the potential of building a compact and robust commercial high-power single-mode fiber laser operating at ~980 nm. Additionally, it shows the potential of MCR AS-PBF to be incorporated in many other fibers for power scaling of fiber lasers at other wavelengths. In the second part of this dissertation, we explore the power scaling of single-mode fiber lasers operating at ~1.5-1.6 μm using Er/Yb co-doped LMA fibers. Due to its good atmospheric transmission and “eye-safe” nature of single-mode fiber lasers operating at this wavelength range, there is a growing demand for applications in many areas such as pumping Tm fiber lasers, CW coherent lidar, free-space optical communication, remote sensing, etc. Yb-free Er-doped fibers are another option to generate lasers at this wavelength, but further power scaling is limited due to low pump absorption cross section of Er fiber at ~980 nm. On the other hand, co-doping with Yb enables a factor 100 increase in pump absorption (10× from higher absorption cross section and 10× from higher maximum doping level) thus shortens effective fiber length. There are two major limiting factors for power scaling with Er/Yb co-doped fiber laser, including Yb→Er energy transfer bottleneck and excessive heat load due to large quantum defect. The former leads to strong Yb ASE (or parasitic lasing) at ~1.06 μm at certain threshold pump powers and will eventually clamp output power at ~1.6μm when the pumping rate exceeds the energy transfer rate. However, the working principle regarding the ASE threshold and output power clamping was not well understood as conventional models assume that all Yb-ions are equally responsible for Yb→Er energy transfer. The new model proposed in this dissertation is based on two types Yb ions, including coupled Yb ions and isolated Yb ions. The numerical simulations results show perfect agreement with experimental results, and it can predict and explain all the observed behaviors very well. We have also carried out a detailed experimental study on a master oscillator power amplifier (MOPA) using a commercial Er/Yb fiber (LMA-EYDF-25P/300-HE), which was counter pumped by kW-level multi-mode pump diode at 915nm. The achieved record single-mode output power of 302W was not limited by ASE (i.e. energy transfer bottleneck) but by fiber fuse in our case. Further analysis of MOPA output diagnostics and Er/Yb fiber parameters shows that lower Yb to Er ions ratio and pumping at a shorter wavelength (915 nm or 940 nm) played a major role in better ASE suppression, i.e high Yb ASE threshold power. Finally, we conclude that the energy transfer bottleneck will eventually come in with any Er/Yb fiber albeit at a much higher threshold pump power depending on how well the Er/Yb fiber is optimized. In the third part which is also the last part of this dissertation, we report our preliminary experimental results for power scaling of 1064 nm high-power single-frequency Yb fiber laser system based on a 976 nm counter-pumped MOPA configuration. There are two main limiting factors in this case, including SBS and TMI. Utilizing an LMA fiber could mitigate SBS by lowering laser intensity in the core, but the large core also leads to a low TMI threshold due to the excitation of HOMs. We propose to incorporate the previously studied MCR AS-PBF design into a new Yb AS-PBF with ~56 μm core and ~401 μm cladding, which expected to have a high SBS threshold above 1 kW. The preliminary experimental results show that single-frequency output power reaches ~500W before the onset of TMI. Also, the MOPA output does not show any signs of SBS indicated by careful characterizations such as backward power, spectrum, and linewidth measurement.
Recommended Citation
Maitiniyazi, Tuerhong, "Power Scaling of Single-Mode Ytterbium and Erbium High-Power Fiber Lasers" (2020). All Dissertations. 2715.
https://open.clemson.edu/all_dissertations/2715