Exploring the mode-locking in diode-pumped continuous-wave and Q-switched lasers: short versus long linear cavities
|摘要:||本論文主題是利用線性共振腔的架構下探討鎖模雷射在連續波與Q開關雷射之間的表現。鎖模雷射在最近這幾年蓬勃的發展，且均能產生相當高的功率輸出。在自鎖模雷射的系統中，如何有效的提升鎖模的品質是一門值得研究的課題。本文設計兩種共振腔來達成自鎖模雷射與被動式Q開關雷射。首先利用簡單的線性共振腔找出自鎖模雷射最佳化的腔長與反射率。接著把自鎖模雷射的波長1064 nm推廣到1342 nm 的波段並透過共振腔長度和晶體長度之間的比例關係，將脈衝重複率提升至32.1 GHz。由於摻釹釩酸釔(Nd:YVO4)晶體具有雙折射的特性，根據這樣的特性搭配輸出耦合鏡的旋轉角度就可產生雙偏振且相互垂直的自鎖模雷射輸出。我們知道雷射共振腔長度超過75mm不易有自鎖模雷射的輸出，因此利用耦合共振腔的技術再搭配具有良好偏振特性的釔鋁石榴石(Nd:YAG)晶體，輸出具有雙偏振的諧波自鎖模雷射。在被動式Q開關雷射中我們使用由三個元件組成的線性共振腔，並搭配飽和吸收體(Cr4+:YAG)發展出具有鎖模現象的Q開關雷射。我們知道半導體飽和吸收體來實現被動式鎖模雷射，是這一、二十年來發展的技術。因此研究半導體飽和吸收體上調制深度的大小在不同腔長下對於鎖模雷射的影響。最後，使用摻釹釩酸釔(Nd:YVO4)與摻釹釩酸釓(Nd:GdVO4)結合的晶體以及半導體飽和吸收體產生雙波長的被動式鎖模雷射。|
This theses presents several optical experiments to explore mode locking in continuous-wave and Q-switched operation by diode-pump Nd:YVO4 lasers. There are two resonator designs are applied in our experiments. First of all, we demonstrate a simple two-mirror linear cavity to systematically explore the performances of self-mode-locked Nd:YVO4 laser with different cavity length and different reflectivity of output coupler. Furthermore, we develop a practical approach to achieve harmonically self-mode-locked (SML) operation in a Nd:YVO4 1342 nm laser. We exploit a specially coated gain medium with an appropriate tilt angle to introduce effective spectrum modulation for mode selection. We also numerically demonstrate that the coated gain medium can effectively modify the mode spacing of the laser cavity to be the harmonics of the free spectral range of the gain medium when the optical cavity length is adjusted to be commensurate with the optical length of the gain medium. We further employ a coated Nd:YVO4 crystal to realize harmonically modelocked operation with a highest repetition rate of up to 32.1 GHz. Finally, we exploit the natural birefringence of a wedge Nd:YVO4 crystal and the alignment sensitivity of an optical resonator to achieve the orthogonally polarized dual-comb SML Nd:YVO4 laser at 1342 nm. By precisely, tuning the orientation of output coupler, the laser can mode lock either π-polarization or σ-polarization, or simultaneously with orthogonal polarizations. In the linear three-mirror cavity case, we have employed a linear three-mirror cavity to achieve the simultaneous harmonically self-mode-locking of two orthogonally polarized states in a Nd:YAG laser. The beat frequency fc between two orthogonally polarized mode-locked components have been observed and measured precisely. We report that the Cr4+: YAG crystal has been exploiting as the saturable absorber to realize the Q-switched mode-locker (QML) operation. Experimental reveal that the complete QML can be obtain with short cavity length of 609 mm. However, when we increase the cavity length to be 750 mm, we can obviously see that the mode-locked pulse train is characterized by the multi-pulse behavior. With finely adjusting the position of the output coupler for tuning the cavity length, the shape of the Q-switched pulse was unchanged but the single-pulse mode-locked operation could be experimentally achieved with a repetition rate of approximately 1.0 GHz corresponding three times of the free spectral range of the optical cavity length. Besides the Q-switched mode locking, we also use a linear three-mirror resonator to make a comparative study between the SESAMs composed one and two InGaAs quantum wells (QWs) in the passively mode-locked performance. Our experimental results reveal that the SESAMs with two QWs only can be CW mode-locked at longer cavity length. On the other hands, the SESAMs with one QWs can be CW mode-locked at shorter cavity length. At the cavity length of 240mm, the pulse durations are found to be 12.8 ps and 21.2 ps for two QWs and one QWs SESAMs, respectively. A new type of diffusion-bonded Nd:YVO4/Nd:GdVO4 hetero-composite crystal is originally designed and applied to diode-end-pumped laser for achieving an efficient dual-wavelength picosecond operation with passively mode-locked laser.
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