以稀土離子摻雜及半導體光放大器為增益介質之被動鎖模雷射

Abstract

在本論文中，我們研究兩個主題。第一部份，我們使用一種全新的方法製作不含聚 合物的單壁奈米碳管與氧化石墨烯薄膜，作為可承受高功率光照射的飽和吸收體。利用不同濃度的單壁奈米碳管與氧化石墨烯飽和吸收體，我們研製出操作在1 微米光譜波段的被動鎖模掺釹釩酸釓與掺釹釩酸鎦雷射。經由非線性穿透量測，得知單壁奈米碳管飽和吸收體擁有630 飛秒的快速回復時間以及約3%的調變深度。當泵激功率為15 瓦時，連續波鎖模掺釹釩酸釓雷射可產生3.63 瓦之最大輸出功率，其操作波長為1063 奈米、脈衝寬度約8.4 皮秒、重複頻率為121.8 兆赫茲，對應之脈衝能量為30 奈焦耳、最高脈衝峰值功率為3 千瓦，其消噪比則高達61 分貝。至於氧化石墨烯，我們在非線性穿透量測中分別得到2.54%、4.14%與5.36%的調變深度。將這一系列的氧化石墨烯飽和吸收體置放到掺釹釩酸鎦雷射雷射共振腔中，當泵激功率為15 瓦時，連續波鎖模雷射可產生3.89 瓦的最大輸出功率，其操作波長為1065.7 奈米、脈衝寬度約5.89 皮秒、重複頻率為121.3 兆赫茲，其對應之脈衝能量為32 奈焦耳、最高脈衝峰值功率為5.44 千瓦，消噪比則高達66 分貝。就我們所知，這是分別使用單壁奈米碳管與氧化石墨烯作為連續波鎖模雷射飽和吸收體的報導中，所產生之最高輸出功率。另外，為了能將氧化石墨烯飽和吸收體應用在全光纖雷射中，我們製備了含聚合物的氧化石墨烯薄膜，並且將它分別置放到摻鉺與摻鐿光纖雷射中產生被動鎖模脈衝。這個含聚合物的氧化石墨烯薄膜在1.06 微米和1.55 微米波長處分別提供了2.95%與6%的調變深度。利用同樣一片飽和吸收體，我們在摻鉺和摻鐿光纖雷射中均已獲得穩定且自啟動的鎖模脈衝，也確認了氧化石墨烯薄膜沒有波長選擇性，而且成本低、適合當作1.06 微米與1.55 微米的飽和吸收體。 第二部份，我們利用半導體光放大器當作雷射增益介質，使用八字型共振腔的結構 使其操作在1.06 微米全正常色散的諧頻鎖模狀態。此雷射可以產生不同脈衝重複率的高階諧頻鎖模，當半導體光放大器的操作電流從80 毫安培變化至660 毫安培時，調整共振腔內的偏振控制器的角度，可以讓諧頻鎖模的重複頻率從30 兆赫茲調至12.02 吉赫茲，而且最高重複頻率與操作電流幾乎是線性關係。在半導體光放大器的電流為660 毫安培時，腔內功率為46 毫瓦，此時可以得到的最高重複頻率為12.02 吉赫茲，相當於第1202 階的諧頻鎖模。就我們所知，這個結果是目前為止操作在全正常色散範圍的雷射中，所需要的腔內功率最小、但卻有最高的諧頻鎖模重複頻率。

In the thesis, we investigate two topics. First, we use a new fabrication method for single-walled carbon nanotube (SWCNT) and graphene oxide (GO) saturable absorbers (SAs) to sustain high-power illumination. Using a series of SAs incorporating different amounts of SWCNTs and GOs, we demonstrate mode-locking for Nd:GdVO4 and Nd:LuVO4 lasers in the 1 μm spectral range. Using SWCNT-SAs for Nd:GdVO4 laser, continuous-wave mode-locking (CWML) with a maximum output power of 3.6W at 1063 nm and high noise extinction of 61 dB has been achieved to give the highest pulse peak power of 3.6 kW and pulse energy of 30 nJ under 15-W pumping. For Nd:LuVO4 laser, GOSAs are used and CWML pulses with maximal output power of 3.89 W at 1065.7 nm are obtained under 15-W pumping to give the highest pulse peak power of 5.44 kW and pulse energy of 32 nJ, for which the high noise extinction of 66 dB has been achieved. To our knowledge, those are the highest reported output power for CWML lasers with SWCNT-SAs and GO-SAs, respectively. The measured nonlinear absorption of the SWCNT-SAs shows a modulation depth of ~3% with subpicosecond recovery time of ~630 fs. For GOSAs, the nonlinear absorption shows the modulation depth of about 2.54%, 4.14%, and 5.36% for GO amounts of 0.6, 0.9, and 1.25 mg. On the other hand, in order to insert the GOSAs between two fiber connectors, we fabricate the polymer-based GOSAs to be use in the all fiber laser system. Broadband graphene oxide/PVA films, which are used as saturable absorbers in the mode locking of erbium-doped and ytterbium-doped fiber lasers, have been demonstrated. They provide modulation depths of 2.95% and 6% at 1.06 μm and 1.55 μm, respectively. Stable self-starting mode-locked pulses are obtained for both lasers, confirming that graphene oxide does not have wavelength selectivity and is cost effective for 1.06-μm and 1.55-μm pulse generations. Second, we report the generation of passively harmonic mode-locked pulses using a 1.06 μm semiconductor optical amplifier (SOA) in a figure-eight laser configuration operated in the all-normal-dispersion regime. Different orders of harmonic mode locking can be obtained from 30 MHz to 12.02 GHz by tuning the injection current of the SOA from 80 to 660 mA together with the adjustment of polarization controllers. The highest pulse repetition rate increases almost linearly with the SOA current. As SOA current is set to 660 mA, we obtain the intracavity power of 46 mW at the highest repetition rate of 12.02 GHz, corresponding to the 1202th harmonic of the fundamental mode locking frequency. To our best knowledge, this is the lowest intracavity power to generate the highest repetition rate with a passively mode-locked laser in the all-normal-dispersion regime.