Narrow band laser source for efficient 3 He polarization
Polarized 3 He is widely in polarized neutron scattering as neutron polarizers or analyzers. Increasing 3 He polarization can improve both neutron polarization and transmission which are important for any polarized neutron experiments. Here at the US, 3 Hee is usually polarized using spin-exchange optical pumping (SEOP), where 3 He gains polarization via spin-exchange collisions with laser pumped alkali metals, typically rubidium (Rb) and potassium (K). Over the last two decades, 3 He polarization has improved from around 50% to around 80% due to the application of spectral narrowed diode lasers which narrows the laser linewidth from 2 nm to between 0.2 nm and 0.3 nm. However, the highest 3 He polarization achieved so far is limited to about 85%. The mechanism behind this is still not well understood, but it is suspected that a laser with an ultra-narrow linewidth and lower broadband background may further improve the 3 He polarization. In addition, if we define the spin-exchange efficiency as the ratio of the rate at which the polarized 3 He is produced to the rate at which the alkali atom polarization is lost through various collisions, K has much higher spin-exchange efficiency than Rb does. In principle, direct pumping of K should improve both the pumping efficiency and the production rate of polarized 3 He. However, because of the small fine structure splitting in K (D1 770 nm, D2 766.7 nm), the laser used to pump K needs to have a ultra-narrow spectrum tuned at 770 nm and minimal broadband background as the pumping of the K D2 transition will reduce the pumping efficiency. Such a laser is not readily available. Therefore, the current practice of polarizing 3 He is through a hybrid SEOP method in which the laser tuned at Rb D1 transition (795 nm) polarizes Rb first and then Rb transfers its polarization to K. This hybrid method takes advantages of the more accessible 795 nm laser and gains some pumping efficiency. But to realize the full potential in SEOP, direct pumping of K would be necessary. The new Optigrate laser provides a tool to investigate the impact on the pumping efficiency and achievable 3 He polarization under the condition of ultra-narrow spectrum and very low background.