為了執行從亞穩態到nD態的原子氦光譜，需要用波長為535至552 nm的激光源激發氦原子池。 因此，此類項目的基本初始步驟是開發激光源。 考慮之後，將由半導體激光二極管泵浦的外腔激光器組裝成Litrow構造。 外部腔二極管激光器用於發射約1070 nm的紅外光束，然後通過倍頻工藝與大量的非線性晶體轉換為535 nm的綠光束。 在我們的實驗中使用了摻Mg的全鈮酸鋰晶體。 第一步，通過執行原子碘的吸收光譜測試綠色激光器

In order to perform the spectrum of atomic Helium from a metastable state to nD states, the Helium atom cell is required to be excited by the laser source with the wavelength from 535 to 552 nm. Therefore, the foundationally initial step for this kind of project is developing a laser source. After consideration, an external cavity laser pumped by a semiconductor laser diode was assembled in Litrow configuration. The External Cavity Diode Laser is used to emit an infrared beam, at around 1070 nm, which then is converted into a green beam, at 535 nm, by the frequency-doubled process with a bulk of the nonlinear crystal. The Mg-doped congruent Lithium Niobate crystal was used in our experiment. As the first step, the green laser is tested by performing the absorption spectrum of the atomic iodine.

Abstract
Acknowledgment
Contents iii
List of figures v
List of tables viii
Chapter 1 Introduction 9
1.1 Motivation 9
1.2 Outline of thesis 11
Chapter 2 Theoretical basic review 12
2.1 Semiconductor laser diode 12
2.1.1 Heterojunction structure of the semiconductor laser diode 13
2.1.2 Effect of temperature and current on the laser diode operation 14
2.2 Fundamental theory of external cavity in Littrow configuration 15
2.2.1 Littrow configuration of the external cavity 15
2.2.2 Linewidth of the output beam 17
2.2.3 Solution for mode-hop free tuning 18
2.2.4 External cavity lasing mode 21
2.3 Nonlinear crystal used in doubling frequency laser 22
2.3.1 Overview 22
2.3.2 Phase-matching techniques 23
Chapter 3 From infrared ECDL to green-laser system 25
3.1 Chamber of the external cavity of ECDL 25
3.1.1 Laser diode and its collimator 25
3.1.2 Choice of grating and mount 28
3.2 Electricity system of the ECDL 30
3.2.1 Laser diode protection circuit and driving current source 30
3.2.2 Peltier cooler and temperature controller for the laser diode 31
3.3 Assemblage of ECDL 35
3.3.1 ECDL assembly progress 35
3.3.2 ECDL feedback alignment 36
3.3.3 Complete product and item catalogue 37
3.4 Second harmonic generation 535 nm laser 39
3.4.1 Single-pass configuration of frequency-doubled green-laser 39
3.4.2 Housing of the nonlinear crystal in birefringence phase matching method 40
3.4.3 Absorption observation of Iodine atomic with green-laser 41
Chapter 4 Experimental results 42
4.1 Characteristics of ECDL 42
4.1.1 Dependence of the output power on injection current and temperature 42
4.1.2 Influence of injection current and temperature on the output wavelength 46
4.1.3 Turn-on characteristics of ECDL versus free-running laser diode 48
4.1.4 Linewidth measurement of 1070 nm ECDL 49
4.2 Green-laser system in single-pass configuration 52
Chapter 5 Summary 54
Bibliography of reference 56

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