A broadband ytterbium-doped tunable fiber laser for 3He optical pumping at 1083�nm

Abstract

Large amounts of hyperpolarized 3 He gas with high nuclear polarization rates are required for use in neutron spin filters or nuclear magnetic resonance imaging of human lung. Very high efficiency can be obtained by metastability exchange optical pumping using multimode lasers to excite the 2 3 S-2 3 P transition at 1083 nm. Broadband Ytterbium-doped tunable fiber lasers have been designed for that particular application. Different options for the architecture of the fiber oscillator are presented and compared. Emphasis is given to a linear cavity configuration that includes a high reflectivity fiber mirror and a low reflectivity tunable fiber Bragg grating. Optical measurements are performed to finely characterize the spectral behavior of the lasers. Atomic response is also quantitatively probed to assess the optimal design of the oscillator for optical pumping. Multimode operation matching the 2 GHz Doppler-broadened helium resonance line and tunability over more than 200 GHz are demonstrated. Boosting the output of this fiber laser with a Yb-doped fiber power amplifier, allfiber devices are built to provide robust, high power turn-key sources at 1083 nm for improved production of laser polarized 3 He. 42.55.Wd; 32.80.Bx PACS

Fundamentals of metastability exchange optical pumping in helium

“…Given the development of suitable high power fibre lasers [9,10,11], the only drawback of this method is the limited range of operating pressures (of order 0.5-5 mbar [12]) for which a suitable plasma can excite metastable atoms. When a higher final pressure is needed, non-relaxing compression of the gas becomes necessary and this introduces demanding requirements on the gas handling and compressing devices.…”

Optically polarizedHe3

“…(41) for a given pumping intensity. This accounts for the much higher total power absorption for a pump laser with a suitable bandwidth L ≈ D (Tastevin et al , 2004). …”

“…Although the spectrum was actually a series of narrow peaks with substantial jitter the influence of velocity-changing collisions yielded efficient optical pumping (Elbel et al , 1990; Gentile et al , 2003), resulting in a substantial improvement in polarization and pumping rate (Gentile and McKeown, 1993). More recently Nd:LMA has been superseded by Yb fiber lasers (Chernikov et al , 1997; Gentile et al , 2003; Mueller, 2001; Tastevin et al , 2004). A Yb fiber oscillator and amplifier, properly constructed for MEOP, yields a fairly uniform spectrum with a ≈2 GHz spectral width.…”

Magnetic Resonance Imaging: From Spin Physics to Medical Diagnosis

“…When laser light with adequate spectral characteristics is used, MEOP provides very high nuclear polarization (>70%) with good efficiency (>1 polarised nucleus per absorbed photon) [50]. With the recent development of spectrally suitable high power fiber lasers [51], the only drawback of this method is its limited range of operating pressures, of order 0.5-5 mbar, required to have a suitable plasma discharge in which metastable atoms are populated [52,53]. Whenever a higher final pressure is needed, non-relaxing compression of the gas is required.…”