High Power Fiber Laser System for Polarization of 3He Gas

Polarizing high intensity 3He gas is an extremely important task since polarized 3He is an effective neutron target or filter/analyzer for a broad nuclear physics programs such as some conducted in Spallation Neutron Source (SNS) in Oak Ridge National Lab (ORNL) and Jefferson Lab's Continuous Electron Beam Accelerator facility (CEBAF). There is increasing demand for fast production of polarized 3He from various neutron instruments and high 3He polarization efficiency, which is essential to any polarized neutron scattering experiment. Recently, the research groups in ORNL and National Institute of Standards and Technology (NIST) showed that pumping Potassium (K) vapor is more efficient than pumping traditional Rubidium (Rb) vapor to polarize the 3He gas. The goal of this program is to design and develop the prototype of next generation pumping laser at 770nm through frequency doubling the high power 1540nm fiber laser. Compared with traditional fiber pigtailed diode laser used in 3He polarization, the prototype we develop has unique features that have never been accomplished before: the lasing linewidth is tunable from extremely narrow of sub-GHz to 10s GHz, direct linear or circular polarization output with polarization maintaining fiber delivery, and exceptional diffraction-limited output beam quality. The whole laser system not only can provide the scalable output power but also has the ability to tune and lock the center wavelength to match the exact position of absorption line. During the Phase I effort, we demonstrated on the breadboard a laser system with output of 61W at 1540nm and 28W at 770nm. The output at 770nm is highly linear with almost diffraction-limited beam quality. Both linewidth and lasing wavelength at the output is tunable with tuning range of linewidth from sub-GHz to 10s GHz (0.01nm to 0.2 nm) and tuning range of lasing wavelength of more than 1nm. In the Phase II, we will deliver an integrated turn-key fiber laser system with output power of more than 50W at 770nm. The laser output is going to have excellent beam quality with circular polarization. Both lasing wavelength and linewidth are tunable with the tuning range comparable or even better than what we achieve in Phase I. The lasing wavelength not only tunable but also be able to lock to the K absorption line.