被動式Q 開關光纖雷射之研究

Abstract

本文使用摻鐿的雙包層光纖作為主要的增益介質並使用被動式Q開關技術來研究高功率和高重複率的脈衝光源。首先我們使用孔徑30 μm 的雙包層摻鐿光纖並分別利用半導體材料AlGaInAs 和Cr4+:YAG 晶體作為飽和吸收體，在24W 幫浦功率下，使用AlGaInAs 飽和吸收體可得到脈衝能量0.45 mJ 且重複率為30 kHz 的脈衝雷射，而使用Cr4+:YAG 晶體則可得到脈衝能量0.35 mJ 且重複率為38 kHz的脈衝雷射，同時並將兩者結果做一比較。 為進一步提升脈衝能量，我們改使用光纖孔徑達70 μm 的光子晶體光纖作為增益介質，在使用AlGaInAs 作為飽和吸收體時，脈衝能量大幅提升2.4 倍至1.1 mJ，且由於較短的腔長，使得脈衝寬度從60 ns 減小至10 ns，因此輸出尖峰功率從7.5 kW 大幅提升至110 kW，另外我們以此架構建立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、脈衝能量138 μJ 且重複率6.5 kHz 的脈衝雷射輸出。而使用Cr4+:YAG 飽和吸收體時，脈衝能量也有接近2倍的提升至0.63mJ，而脈衝寬度也從70 ns 降至36 ns，也以此架構建構腔內光學參量振盪器，目前可得波長在1515 nm 且輸出功率為0.47 W 的脈衝雷射。 為了再減小脈衝寬以使脈衝功率能大幅提升，我們採用了MOPA 放大器架構，利用被動式Q開關的Nd:YVO4/ Cr4+:YAG 雷射作為種子源和30 μm 孔徑的雙包層摻鐿光纖作為放大器，我們得到脈衝寬度為1.6 ns、能量為0.192 mJ 且脈衝重覆率為25 kHz 之輸出。我們另外完成將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 且輸出功率為1.7 W的二倍頻波長轉換，以及波長為355 nm且輸出功率為1.1 W 的三倍頻波長轉換。

The double-cladding Yb-doped fiber as the gain medium and the passively Q-switching technique have been utilized in study of high power and high-repetition rate fiber lasers. First we use the double-cladding fiber with core diameter of 30 μm, and the AlGaInAs semiconductor material and the Cr4+:YAG crystal are used as the saturable absorbers (SA) respectively. By using the AlGaInAs as the SA, pulse energy up to 0.45 mJ with the repetition rate of 30 kHz in the pump power of 24 W can be attained. With the Cr4+:YAG crystal as the SA, we can have the laser output with a pulse energy of 0.35 mJ at the pulse repetition rate of 38 kHz in the same pump power. We also have comparative studies between these two SAs. For scaling energy further, the photonic crystal fiber (PCF) with the core diameter of up to 70 μm is used as the gain medium. With the AlGaInAs as the SA, the pulse energy is significantly increased 2.4 times to 1.1 mJ, and the shorter cavity length results in the pulse width reducing from 60 ns to 10ns. As a sequence, the peak power is boosted up from 7.5 kW to 110 kW. This scheme is also used for pumping the extracavity optical parametric oscillator (OPO), and the output energy of 138 μJ with the repetition rate of 6.5 kHz at the wavelength which can be tunable from 1513 nm to 1593 nm is obtained. By employing the Cr4+:YAG SA, the pulse energy also has almost 2-times enhancement to 0.63 mJ and the pulse width decreases from 70 ns to 36 ns. An intracavity OPO was demonstrated based on this scheme, output power of 0.47 W at 1515 nm was obtained. To achieve higher peak power, it is necessary to have smaller pulse width, so we adopt the configuration of master oscillator fiber amplifier (MOFA). By using a Nd:YVO4/ Cr4+:YAG passively Q-switching laser as the seed laser and a 30-μm-core double-cladding fiber as the amplifier, the amplifier can generate pules with energy of 0.192 mJ at the repetition rate of 25 kHz and the pulse width is down to 1.6 ns. In addition, a PCF MOFA was used to pump the extracavity nonlinear wavelength 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.