本論文中，我們報告了亞穩態氦原子束的設計與製作。我們使用了一個分
佈回饋型雷射(DFB) 並將其雷射頻率穩定在一放電管中的氦23S1→23P2 躍遷。
接著我們將此雷射光打入一摻鐿光纖放大器(Ytterbium fiber amplifier) 使雷射
功率可以被放大到至少1瓦。此放大後的雷射光將作為二維雷射冷卻氦原子束的
冷卻雷射光。
我們使用了一自製的外腔二極體雷射(ECDL) 並將其雷射頻率調整到氦
的23S1→23P1,2 躍遷上，以此雷射垂直打向氦原子束，以量測氦原子束的特
性，包括發散角，原子數目及原子束通量都將被討論。

In this thesis, we present the design and construction of a metastable helium atomic beam for precision laser spectroscopy. The laser source is a distributed feedback laser (DFB) at 1083 nm and its frequency is stabilized on the helium 23S1→23P2 transition in a gas cell. Then the laser beam is sent into a ytterbium
fiber amplifier (IPG photonics YAR-10K-1064-LP-SF) which can boost the laser power up to at least 1 W. The amplified DFB laser will be the transverse cooling light for the helium atomic beam.
The atomic beam is then characterized by shinning a home-made external cavity diode laser (ECDL) perpendicular to the atomic beam and the laser frequency is resonant with the helium 23S1→23P1,2 transitions. The divergence angle, number of atom and the atomic beam flux of the atomic beam will be discussed.

Contents v
List of Figures vii
List of Tables ix
1 Introduction 1
1.1 Motivation 1
1.2 Helium transitions at 1083 nm 1
2 Theory 3
2.1 Frequency Stabilization 3
2.1.1 Doppler Broadening Effect 3
2.1.2 Saturated Absorption Spectroscopy 4
2.1.3 Frequency Modulation Spectroscopy 6
2.2 Helium Atomic Beam 9
2.2.1 Atomic and Molecular Beam 9
2.2.2 Doppler laser Cooling 9
2.2.3 Cross Beam Method 13
3 Experiment Setup 14
3.1 Laser Source 15
3.1.1 Distributed Feedback Laser (DFB) 15
3.1.2 External Cavity Diode Laser (ECDL) 18
3.2 Helium Discharge Cell 20
3.3 Electro-Optical Modulator (EOM) 22
3.4 Acousto-Optical Modulator (AOM) 23

3.5 Helium Atomic Beam 24
3.5.1 Vacuum System Design 24
3.5.2 Transverse Cooling Chamber 25
3.5.3 Fluorescence Detection 26
3.6 Optical Layout 27
3.6.1 Saturated Absorption Spectroscopy of Helium 23S1 → 23P1,2
Transition 27
3.6.2 Frequency Modulation Spectroscopy 28
3.6.3 Transverse Cooling of Helium Atomic Beam 29
3.6.4 Atomic Beam Measurement 30

4 Data Analysis and Results 31
4.1 Frequency Stabilization 31
4.1.1 Saturated Absorption Spectroscopy 31
4.1.2 Frequency Modulation Spectroscopy 35
4.2 Characteristics of Atomic Beam 38
4.2.1 Spectroscopy of helium 23S1 → 23P1,2 transitions 38
4.2.2 Divergence angle 41
4.2.3 Atomic beam flux 43
4.3 Results and discussion 44

5 Conclusion and Future Works 45
5.1 Conclusion 45
5.2 Future Works 46

References 47

A Energy Level diagram of 4He 49

B Circuit diagrams 50

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