鐿掺雜之脈衝雷射

Abstract

本文旨在使用鐿摻雜增益介質為主體以研究高功率、高重覆率脈衝雷射的技術。首先我們使用30 μm芯蕊孔徑的光纖作為增益介質，我們分別使用Cr4+:YAG晶體以及半導體材料AlGaInAs作為飽和吸收體。在以Cr4+:YAG晶體作為飽和吸收體的實驗中可得到脈衝能量為0.35 mJ以及重覆率為38 kHz的脈衝雷射;在以半導體材料AlGaInAs作為飽和吸收體的實驗中，我們可得脈衝能量為0.45 mJ以及脈衝重覆率為30 kHz的雷射脈衝。此外，我們使用一個被動式Q開關的 Nd:YVO4/ Cr4+:YAG雷射作為種子源雷射以實現光纖放大器的架構。在以相同的光纖作為增益介質下，雷射能量為0.192 mJ、脈衝重覆率為25 kHz以及脈衝寬度為1.6 ns的光纖放大器可被實現。 我們更進一步的在被動式Q開關以及光纖放大器的架構下使用芯蕊孔徑為70 μm的光子晶體光纖來提升輸出的脈衝雷射能量。在以Cr4+:YAG晶體作為飽和吸收體的實驗中，相較於30 μm芯蕊孔徑的光纖，脈衝能量為可放大2倍並且脈衝寬度更可縮短2倍。此外我們以此架構作為基礎下更進一步完成腔內光學參量振盪器，目前可得波長在1515 nm，輸出功率為0.47 W的脈衝雷射。而在以半導體材料AlGaInAs作為飽和吸收體的實驗中，相較於30 μm芯蕊孔徑的光纖，我們可以得到脈衝能量放大2.5倍並且脈衝寬度更可縮短6倍的雷射脈衝。此外我們以此架構作為基礎下更進一步完成腔外光學參量振盪器，目前可得波長可調範圍為1513 nm至1593 nm，輸出功率為0.9 W的脈衝雷射。利用此光子晶體光纖作為增益介質，我們也完成以光子晶體光纖放大器作為基頻光光源的腔外非線性波長轉換，在輸入功率為3.3 W的條件下，可得波長為532 nm 的輸出功率為1.7 W 的二倍頻波長轉換以及波長為355 nm，輸出功率為1.1 W 的三倍頻波長轉換。 微晶片之摻鐿釔鋁石榴石雷射已在被動式Q開關、自鎖模、被動式鎖模等不同模式下呈現。在被動式Q開關的模式下，以Cr4+:YAG晶體作為飽和吸收體。我們使用鑽石散熱片來改善摻鐿釔鋁石榴石雷射的熱效應，實驗上證實無論是在能量以及輸出功率的提升、輸出脈衝的穩定性都可以獲得很好的改善。在自鎖模的操作下，使用相同的晶體與鑽石散熱片我們可以藉由改變腔長與晶體長度之間的比值來達到不同階數的諧波鎖模雷射。此外，將自鎖模雷射架構中之輸出耦合鏡更換為一半導體飽和吸收體，我們可以達到雙波長的鎖模脈衝雷射輸出。經由兩個不同波段的光學拍頻，我們可以得到重覆率高達5兆赫茲的超短雷射脈衝。

Conventional large-mode-area (LMA), Yb3+-ion doped fiber with core diameter of 30 μm and inner cladding of 250 μm has been utilized in development of high power, high-repetition rate fiber lasers and amplifiers. Different saturable absorber (SA) such Cr4+:YAG crystals and AlGaInAs multi-quamtum-wells were used to form a passively Q-switch (PQS) fiber lasers. The results reveal that with a Cr4+:YAG crystal as the SA, the laser could generate a pulse energy of 0.35 mJ at the pulse repetition rate of 38 kHz. By employing the AlGaInAs multi-quamtum-wells (MQWs) as the SA, pulse energy up to 0.45 mJ at the repetition rate of 30 kHz can be attained. Besides, a Nd:YVO4/ Cr4+:YAG laser was used as the seed laser in an master oscillator of power amplifier (MOPA) experiment. The amplifier could emit pulses with energy of 0.192 mJ at the repetition rate of 25 kHz and pulse width down to 1.6 ns. Furthermore, for energy scaling, we use the photonic crystal fiber (PCF) with the core diameter up to 70 μm as the gain medium in either the PQS operation or in the MOPA scheme. The pulse energy was enhanced 1.8 times and the pulse width was 2 times shorter with the Cr4+:YAG crystal as the SA compared to the LMA fiber. An intracavity optical parametric oscillator (OPO) was demonstrated based on this scheme, output power of 0.47 W at 1515 nm was obtained. As for using the AlGaInAs MQWs as the SA, the pulse energy was enhanced 2.5 times and the pulse width was 6 times shorter in comparison with the LMA fiber. An extracavity OPO was performed with this PQS laser as foundation. Output power of 0.9 W can be achieved and the wavelength can be tunable from 1513 nm to 1593 nm. A PCF MOPA was used to pump the extracavity nonlinear wasvelength conversions module, output powers of 1.7 W of the second harmonic generation at 532 nm and 1.1 W of the third harmonic generation at 355 nm were realized at the fundamental pump power of 3.3 W. Microchip Yb:YAG lasers were demonstrated in PQS, self-mode-locked, passively-mode-locked operations. In PQS operation, Yb:YAG laser was Q-switched with Cr4+:YAG crystal as the SA. Energy and output power scaling together with pulse stability improvement were achieved via employing a diamond window as the heat spreader. In self-mode-locked operation, high repetition rate, self-mode-locked Yb:YAG laser was attained using the same gain chip and heat spreader. Various order of harmonic mode locking was obtained by means of tunning the cavity length to match a commensurate ratio of the gain chip length. Furthermore, by replaceing a semiconductor saturable absorber mirror as the output coupler, dual-wavelength mode-locked Yb:YAG laser was demonstrated. As a result of the optical beating of the dual spectral bands, THz modulation frequency was generated with an ultrashort pulse duration.