中紅外脈衝光源與奈米材料之非線性研究

Abstract

高功率、短脈衝、2 m波段之中紅外摻銩光纖雷射有許多應用，例如分子光譜、生 物醫學診斷、遙測及非線性光學等。而高功率中紅外超連續光譜亦可應用於仿黑體輻 射、化學感測、光學同調斷層掃描、與光學組織剝除等。因此，摻銩光纖雷射和相關的 光源產生技術是很重要且值得研究發展的。傳統上用在光纖雷射中的被動鎖模機制是半 導體飽和吸收反射鏡或非線性偏振演化。然而，可用於中紅外波長的半導體飽和吸收反 射鏡很少，不易取得，而採用非線性偏振演化架構的共振腔則較不穩定。石墨烯具有很 快的載子回復時間及很大的飽和吸收、超寬的飽和吸收範圍(從可見光至兆赫波範圍)， 且容易製備，因此廣泛被應用在雷射的被動鎖模上。氧化石墨烯則是石墨烯片的前驅 物，比石墨烯更容易製作，且亦擁有很快的熱載子回復時間及很大的飽和吸收，因而在 雷射鎖模應用上具有潛力。但是石墨烯與氧化石墨烯應用於摻銩光纖雷射鎖模上的研究 仍很少見；而這兩種奈米材料的色散和2 m波段的非線性光學係數資料亦很缺乏。因 此，相關的研究深具學術和應用價值。 一個封閉的迴路中串接重新整形及重新放大的組件後，將可形成自脈動光源，它提 供了一種產生超短脈衝的方法。自脈動光源有下列幾種特性：非週期性、幾乎和偏振無 關、帶寬大、以及脈衝平均重複率很容易調整等。若在自脈動光源中加入石墨烯或氧化 石墨烯飽和吸收體，將可觀察光源是否會在連續波、自脈動、鎖模、以及混沌等四種操 作狀態之間演化，進而研究其非線性動力學行為。自脈動光源提供了一些獨特的應用， 但是目前只有在1.55 m波長被研究，而在2 m的波長也是極具開發價值。 本研究團隊已具有超快光纖雷射研究、超連續光譜產生、石墨烯及氧化石墨烯飽和 吸收體製備以及奈米材料量測技術等經驗，在本三年計畫中，我們將從事下列五個研究 課題：一、利用不同層數的石墨稀獲得鎖模雷射脈衝，並且計算石墨稀之色散及對雷射 鎖模的機制；二、利用石墨烯及氧化石墨烯來產生穩定自啟動之短脈衝摻銩全光纖雷 射；三、架設一級放大系統並且使用Z軸掃描技術量測石墨烯與氧化石墨烯之非線性光 學特性；四、建立以石墨烯或氧化石墨烯為基礎之狀態可調2 m自脈動光源；五、建立 短脈衝摻銩全光纖之主振式功率放大器系統，並且利用此雷射注入非線性光纖中產生中 紅外線之超連續光譜。

High power, ultrafast lasers based on thulium (Tm) doped fibers, operating in the ∼2 m, mid-infrared (mid-IR) region, are necessary for a variety of applications, such as molecular spectroscopy, biomedical diagnostics, remote sensing, and nonlinear optics. High power mid-IR supercontinuum (SC) light sources have a wide variety of applications in numerous fields such as, black body radiation emulation, chemical sensing, optical coherence tomography (OCT), and optical tissue ablation. Thus, it is important to investigate the high power all fiber mid-IR SC generation in a nonlinear fiber pumped by a 2-m all-fiber high power pulsed laser. Traditionally, the saturable absorber and nonlinear polarization evolution (NPE) were often used to generate passively mode-locked pulses. However, relatively few semiconductor saturable absorbers (SESAMs) have been made in the mid-IR region. Graphene based materials have been widely researched for laser mode locking because they possess fast recovery time, large saturable absorption, and ease of fabrication. In addition, graphene has very broadband saturable absorption spectrum from visible to THz region. Graphene oxide (GO) is the precursor for graphene flakes, and it is even easier to be fabricated than graphene. Different from graphene, GO not only has strongly hydrophilic and water solubility but also has fast energy relaxation of hot carriers and strong saturable absorption comparable to those of graphene. These properties make GO as potential saturable absorber (SA) material for pulsed lasers. However, there are relatively few researches on mode-locked Tm-doped fiber lasers in use of graphene and GO as the SAs. A regenerative self-pulsating (SP) sources have been generated as cascaded reshaped and reamplification regenerators in closed-loop, which offer an alternative to make pulsed lasers based on SAs. It is not a phase-locked laser but has the properties of aperiodicity, low polarization sensitivity, broad spectral, and femtosecond pulse via nonlinear compression. The regenerative SP sources provide better spatial resolution for OCT than the mode-locked lasers due to the shorter coherent time; and the pulse number in the cavity can be changed by varying the bandwidth or the central frequency of the intracavity filter. However, the regenerative SP sources only have been demonstrated in 1.55 m, thus, it is important to investigate the regenerative SP sources at 2 m. Recently, we successfully fabricated single walled carbon nanotube (SWCNTs), graphene, and GO SAs and demonstrated passively mode-locking solid-state and fiber lasers in 1 m and 1.55 m region and clarified the underlying mechanism through investigating their optical nonlinearities by the Z-scan method and the transient absorption by the pump-probe technique. Base on the experiences above, in this three-year project, we plan to purchase and setup a high power EDFA, spectrometers, high spped digital oscilloscope, two channel power meter, large core fiber fusion splicers, upgraded the auto-correlator, four center wavelength of 790 nm, 10 W diode laser, (4+1) x 1 pump combiner, cleaver for large diameter fiber, and nonlinear fiber, to explore the following subjects: 1. Using the different numbers of layer graphene to study the dispersion of graphene on mode locking fiber lasers. 2. Constructing the self-starting, stable passively mode-locking Tm-doped fiber laser by using graphene and GO as the saturable absorbers. 3. Constructing a preamplifier system, and measure the optical nonlinearities of graphene and GO via Z-scan method. 4. Constructing regenerative self-pulsating (SP) sources with tunable operating states at 2 m wavelength by inserting graphene and GO saturable absorbers. 5. Constructing a 2-m master oscillator power amplifier to get the output power larger than 10 W for generating the high power mid-IR supercontinuum.