在屏蔽磁場下的多通道鉀金屬向量測磁計

Shielded three-axis vector operation of a multichannel atomic magnetometer

吳紫瑜(Tzu-Yu Wu)

指導教授 ： 石明豐

國立臺灣大學/理學院/物理研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.01179

摘要

摘要
建造精準的原子磁力測量儀可以廣泛的用於物理科學上以及生物磁場的測量,包括磁性材料測量，以及資料儲存，至人類大腦及心臟運作時的微量磁場變化。精度可測得極微小變化磁場(約是1fT/ Hz )的鉀原子測磁儀其原理為共振非線性磁光效應。此效應來自於金屬原子自旋極化於微小磁場下的拉馬旋進。當一道偏振光進入此原子氣後，其偏振面會因此效應而改變，因此只要能偵測微小的偏振態的不同，即可計算出造成此現象的磁場大小[1]。這類型的原子磁力測量儀,基本
的雜訊限制來自於量子雜訊,此雜訊的大小和垂直磁場方向的自旋緩和速度有關[2]。由於在零磁場附近下運作的原子磁力測量儀可以降低自旋緩和的影響,進而降低雜訊至0.3 fT/ Hz[3],因此原子磁力測量儀需引進三軸赫姆霍茲線圈抵銷外在磁場;若需更高階的屏蔽效果,則需引進高磁導率金屬屏蔽[4]。除了磁場屏蔽可
以降低雜訊以外,使用多頻道的二維偵測器測同一道光源可以做為梯度計;各頻道相減後的讀值,除了可以得到二維磁場的空間解析度外,亦減去磁場的共同雜訊。
此論文改善參考文獻[6]之系統,主要改善的項目為: 架設多通道的測磁計並配以鎖相放大器軟體作為磁場梯度計的前置作業,設計加熱系統以及溫度回饋以降低空氣熱對流所產生的光擾動,亦改善溫度不穩不均勻的問題,並架設磁場屏蔽於系統以阻隔環境磁場雜訊等項目。

關鍵字

原子測磁計 ； 光驅動 ； 自旋偏極化 ； 拉馬徑動 ； 不受自旋交換緩和影響 ； 非線性磁光效應

並列摘要

Abstract
One of the most potential applications of non-linear optics is a state-of-art atomicmagnetometer, which finds wide range of applications from material science, information
storage to direct measurement of biological magnetic fields.
The basic principle behind the magnetometer is based on the resonant nonlinear magneto-optical effect, which Larmor precession can be used to measure of spin-polarized atoms in a magnetic field.[1] This precession modifies the optical absorptive and dispersive properties of the atoms and causes the polarization of the probe beam to rotate to a small angle, which can be traced back to characterize the magnetic field. The theoretical sensitivity of atomic magnetometer is limited by the quantum shot noise, related to the transverse relaxation time of the spin
polarized atoms.[2] By operating the atomic magnetometers with high atom density in an environment with a near zero magnetic field, spin exchange collision as a source of
relaxation could be eliminated and the noise limit could be lowered down to 0.3 fT/ Hz.[3] To block the environmental field, the center piece of this fT atomic magnetometer is shielded by high-permeability magnetic shielding and the residual fields are compensated by three-axis Helmholtz coil.[4, 5] For the work presented here, I provide
an improved version of an atomic magnetometer in [6] with noise level below 1 nT / Hz . It has a multichannel-inputs system along with its applicable two-stage
Lock-In software been built up for the purpose of constructing a magnetic gradiometer, a heating system redesigned with introducing a temperature feedback controller and a magnetic shielding assembly set on the system.

並列關鍵字

Atomic Magnetometer ； Optical Pumping ； Spin Polarization ； Larmor Procession ； Spin-exchange-relaxation-free ； nonlinear magneto-optical effect

參考文獻

[21] Z. Wu, T. G. Walker, and W. Happer, Physical Review Letters 54, 1921

[1] I. K. Kominis et al., Nature 422, 596 (2003).

[2] R. Guenther, Modern Optics (1990).

Letters 85, 4804 (2004).

Review Letters 81,5788 (1998).