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標題: | 高強度全光纖奈秒掺鐿光纖雷射主從式放大器系統 High intensity mirror-free nanosecond ytterbium fiber laser system in master oscillator power amplification |
作者: | Chun-Lin Chang 張俊霖 |
指導教授: | 黃升龍(Sheng-Lung Huang) |
關鍵字: | 高功率光纖雷射與放大器,?鐿光纖,主從式放大器,寄生受激放大,多通光纖放大器,峰值功率/脈衝能量提升,光纖非線性光學,超連續頻譜產生, High power fiber laser and amplifier,ytterbium fiber,master oscillator power amplification,parasitic stimulated amplification,multi-pass fiber amplification,peak power/pulse energy scaling,fiber nonlinear optics,supercontinuum generation, |
出版年 : | 2013 |
學位: | 博士 |
摘要: | 掺稀土族元素光纖雷射與放大器可相對容易地在一個小型又可靠穩定且免校準的光路架構下,有效產生高穩定與高光束品質的雷射光。尤其鐿離子的掺雜濃度高又不會造成雷射淬熄且量子缺陷低。因此,高功率光纖雷射系統近年來已取代了傳統固態雷射系統,被廣泛地應用在學術研究、工業製造與軍事用途上。主從式雷射放大器系統通常用來有效提升雷射的脈衝能量、峰值功率或平均功率,其架構定義包含了可高度控制的種子源、高增益的預放大器與高效率的功率放大器。其中,以直接電流調變方式來產生單橫模半導體奈秒雷射可簡便地作為一輕薄短小且可高度控制的種子源,使得輸出參數如重覆頻率與脈衝寬度,甚至脈衝波形的調變更具有彈性。然而,為了高強度雷射的應用,在提升峰值功率時,以半導體雷射作為種子源且結合光纖雷射放大器的多功能主從式雷射放大器系統,到目前為止尚未能夠完全發揮本身相較於傳統固態雷射系統的優勢。由於高調控性的種子源在單橫模下輸出能量太低,導致每級脈衝放大器都無足夠強的輸入訊號,使得同時在光纖波導中傳播的放大自發輻射很容易主導整個放大過程,造成具傷害性的自激發雷射或脈衝效應,與定量模擬比較的困難度。基於上述,本論文開發了一套高效率與高強度的掺鐿奈秒光纖雷射系統,其高穩定性與高光束品質可適用在許多高強度雷射的應用。藉由最優化高功率光纖的接合技術,此全極化保持光纖的系統架構可有大於12分貝的消光比輸出。
在半導體雷射作為種子源的主從式光纖雷射放大器系統架構下,主要是藉由除去寄生受激放大的影響何使用雙通架構的光纖預放大器,以達到有效率的能量擷取來提高峰值功率。此寬頻寄生受激放大在實驗上被發現,並且經研究後與相對應的模擬結果可達到定量上的吻合。實驗上可觀察到波長1064 奈米且直接電流調變型式的Fabry-Perot半導體種子奈秒雷射,會伴隨著輸出一波長從1045奈米延伸到1063奈米的微弱放大自發輻射脈衝。縱使輸入的雜訊脈衝低於整體能量的5%,它仍然能在放大過程中變成暫態下顯著的突波訊號。當量子井種子雷射由高峰值電流驅動時,此藍移的脈衝雜訊來自於帶填補效應。其有助於適應性波形調變技術的發展來進一步有效率地提升峰值功率或脈衝能量。另一方面,此寬頻突波的半高寬線寬約為8.8 奈米,可分離出來作為新種子源,經放大後能產生次奈秒等級的高功率脈衝輸出。 由於種子雷射的脈衝能量低,故高增益預放大器的設計極為關鍵。使用單橫模纖芯激發的光纖預放大器不僅能不經過光纖纏繞效應就能改善基頻模態的能量對比度,而且能顯著地抑制光纖非線性效應。在缺乏可獲得的種子能量與纖芯激發幫浦功率的狀況下,經實驗與模擬的研究顯示,使用雙通架構可提升此預放大器的能量擷取效率。以至於可成功地在中端的纖衣激發功率放大器得到峰值功率超過30千瓦且脈衝能量超過0.23毫焦耳的輸出,且光束品質為M2 ~1.1;據我們所知,這是至今文獻上在此條件下能達到的最大輸出紀錄。再經過功率放大器後,在重覆頻率為居中的20千赫茲下,系統整體的光轉換效率可大幅地提升至超過56%以上,此時對應的整體能量增益超過63分貝,且光束品質為M2 <1.5。此時輸出的脈衝能量大過1.1毫焦耳且脈衝時寬約在6.1奈秒,所得到的峰值功率可達116千瓦以上;其峰值功率已被光纖損壞效應所限制,導致無法安全地長時間運作。此輸出條件在應用端可得到的光強度已能大於60 GW/cm2,足以達到運用雷射產生錫電漿源的需求。相關模擬結果顯示雙通架構的功率放大器可進一步地簡化放大器鏈的架構。 如此高強度的纖衣激發功率放大器能進一步地取代非線性未掺雜光纖,轉變成可運作在全常色散範圍的非線性光纖放大器。將雷射放大與非線性波長轉換的功能結合在一起,可以用來克服在未掺雜光纖中傳播時,提供非線性轉換的功率的顯著損耗,進而提升非線性轉換後的功率。因此,可成功地產生一高強度的超連續頻譜的高功率奈秒光源,其頻譜擴張範圍可從波長980奈米到大於1600奈米,光轉換效率可達65%以上,據我們所知,這是關於奈秒超連續頻譜光源在文獻上所紀錄的最高輸出峰值功率。因為主從式放大器鏈的優點,也可觀察到此非線性光纖放大器的輸入參數對於高功率超連續頻譜產生的影響。各級間各種光纖非線性效應的產生與交互作用可藉由比較而被釐清。如此獨特的高強度與線性極化光源,包含了一高強度且波長1064奈米的幫浦光源以及寬頻的邊帶種子光源,非常適合用來有效地產生寬頻且可調頻的光參數放大器,並且可簡化架構與免除時序顫動。 Rare-earth-doped fiber lasers and amplifiers are relatively easy to efficiently produce a stable and high quality laser beam in a compact, robust, and alignment-free configuration. Recently, high power fiber laser systems have facilitated wide spread applications in academics, industries, and militaries in replacement of bulk solid-state laser systems. The master oscillator power amplifier (MOPA) composed of a highly-controlled seed, high-gain preamplifiers, and high-efficiency power amplifiers are typically utilized to scale up the pulse energy, peak power, or average power. Furthermore, a direct-current-modulated nanosecond diode laser in single transverse mode can simply provide a compact and highly-controlled seed to result in the flexible output parameters, such as repetition rate, pulse duration, and even temporal pulse shape. However, when scaling up the peak power for high intensity applications, such a versatile diode-seeded nanosecond MOPA laser system using rare-earth-doped fibers is unable to completely save its own advantages compared to bulk laser systems. Without a strong seeding among the amplifiers, the guided amplified spontaneous amplification is easy to become dominant during the amplification, leading to the harmful self-lasing or pulsing effects, and the difficulty of the quantitative numerical comparison. In this dissertation, we study a high-efficiency and intense nanosecond ytterbium fiber MOPA system with good beam quality and stability for high intensity applications. The all-PM-fiber structure is achieved with the output extinction ratio of >12 dB by optimizing the interconnection of high power optical fibers. The diode-seeded MOPA configuration without parasitic stimulated amplification (PAS) is implemented using the double-pass scheme to extract energy efficiently for scaling peak power. The broadband PAS was studied experimentally, which matches well with our numerical simulation. The 1064-nm nanosecond seed was a direct-current-modulated Fabry-Perot diode laser associated with a weak and pulsed noise spanning from 1045 to 1063 nm. Even though the contribution of input noise pulse is only <5%, it becomes a significant transient spike during amplification. The blue-shifted pulsed noise may be caused by band filling effect for quantum-well seed laser driven by high peak current. The study helps the development of adaptive pulse shaping for scaling peak power or energy at high efficiency. On the other hand, the broadband spike with a 3-dB bandwidth of 8.8 nm can support pulses to seed the amplifier for sub-nanosecond giant pulse generation. Because of the very weak seed laser, the design of high-gain preamplifier becomes critical. The utilization of single-mode core-pumped fiber preamplifier can not only improve the mode contrast without fiber coiling effect but also significantly suppress the fiber nonlinearity. The double-pass scheme was therefore studied both numerically and experimentally to improve energy extraction efficiency for the lack of attainable seed and core-pumped power. As a result, a record-high peak power of > 30 kW and energy of > 0.23 mJ was successfully achieved to the best of our knowledge from the output of clad-pumped power amplifier with a beam quality of M2 ~1.1 in a diode-seeded 15-μm-core fiber MOPA system. After the power amplifier, the MOPA conversion efficiency can be dramatically improved to >56% for an energy gain of >63 dB at a moderate repetition rate of 20 kHz with a beam quality of M2 <1.5. The output energy of >1.1 mJ with a pulse duration of ~6.1 ns can result in a peak power up to >116 kW which is limited by fiber fuse in long-term operation. Such a condition able to generate the on-target laser intensity of > 60 GW/cm2 for applications is qualified to preliminarily create a laser-plasma light source. Moreover, the related simulation results also reveal the double-passed power amplifier can further simplify MOPA. Such an intense clad-pumped power amplifier can further become a nonlinear fiber amplifier in all-normal dispersion instead of a nonlinear passive fiber. The combination of laser amplification and nonlinear conversion together can therefore overcome the significant pump depletion during the propagation along the passive fiber for power scaling. As a result, an intense spectrum spanning from 980 to 1600 nm as a high-power nanosecond supercontinuum source can be successfully generated with a conversion efficiency of >65% and a record-high peak power of >116 kW to the best of our knowledge. Because of MOPA structure, the influence of input parameters of nonlinear fiber amplifier on supercontinuum parameters can also be studied. The onset and interplay of fiber nonlinearities can be revealed stage by stage. Such an unique and linearly-polarized light source composed of an intense pump and broad sideband seed is beneficial for efficiently driving the broadband tunable optical parametric amplification free from the bulkiness and timing jitter. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/58891 |
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