產生具圓柱向量特性的皮秒脈衝雷射
研究生:洪必瑾 指導教授:魏明達 博士
國立成功大學光電科學與工程系
摘要
本論文藉由共振腔設計在c-cut的Nd:GdVO4脈衝雷射中產生圓柱向量偏振，利用晶體的雙折射效應，並將共振腔操作在穩定區邊界，使e-ray穩定或o-ray穩定，便可選擇輸出為徑向偏振或方位角偏振，我們使用半導體可飽和吸收輸出耦合鏡(SOC)取代一般的輸出耦合鏡，使輸出為鎖模脈衝雷射，在三面鏡系統，我們在穩定區邊界得到方位角偏振和徑向偏振形式的Q開關鎖模脈衝雷射；在輸入功率9W時，偏振比分別為17.90±4.88及19.71±8.85，重複頻率450MHz及脈衝寬度31ps及37ps。在四面鏡系統中，隨功率增加雷射可經由Q開關鎖模脈衝轉換到連續波鎖模；當輸入功率在6.45W時，我們做出方位角偏振形式其偏振比為14.04±4.57，重複頻率127.5MHz及脈衝寬度90ps。

關鍵字：徑向偏振、方位角偏振、鎖模

Generation of picosecond pulse laser with cylindrical vector polarization

Student: Pi-Chin Hung
Advisor: Dr. Ming-Dar Wei
Department of Photonics, National Cheng Kung University

SUMMARY

In this thesis, we produce passively mode-locking pulses with cylindrical vector polarization in c-cut Nd:GdVO4 laser by using semiconductor saturable absorber output coupler (SOC). Base on the intrinsic characteristic of birefringence of the laser crystal, the optical path between the extraordinary ray (e-ray) and ordinary ray (o-ray) are different. when the cavity is operated around the edge of stable region, we can achieve one ray is stable and other ray is unstable, and so the polarization is radially polarized or azimuthally polarized. In three-element system, we achieve the Q-switched mode locking (QML) pules laser with radially polarized and azimuthally polarized beams nearby the boundary of stable region at pump power 9W, and the polarization ratios are 17.90±4.88 and 19.71±8.85 respectively. The repetition rate are measured to be 450MHz. The pulse width with radially polarized and azimuthally polarized are 37ps and 31ps. We also achieve continuous wave mode locking (CWML) laser with azimuthal polarization in four-element system at pump power 6.45W, and the polarization ratio is 14.04±4.57. The repetition rate and pulse width of CWML are measured to be 127.5MHz and 90ps.

Key words : radially polarized beam、azimuthally polarized beam、mode-locking
Abstract
INTRODUCTION

Due to the spatial-dependent polarization property, the cylindrical vector beams (CV beams) have attracted much interest in the past decade. CV beams include radial polarization and azimuthaul polarization which with axial symmetry are the famous cases because of the practice importance in the various applications of material processing, optical traping, high resolution microscopy, and high resolution microscopy.

Many methods for the active or passive mechanisms have been developed to generate radially or azimuthaully polarized beams. The active method can generate the CV beams directly in the laser cavity by using intra-cavity element, including the birefringent crystal. The birefringence property plays an important role for producing CV beams. Because the birefringence property of the crystal, the optical path between the extraordinary ray (e-ray) and ordinary ray (o-ray) are different. We can achieve one ray is stable and other ray is unstable when the laser cavity is operated near the boundary of stable region, and so the polarization is radially polarized or azimuthally polarized. Most research works on CV beams were almost always operated in CW mode in the past. In this thesis, we use c-cut Nd:GdVO4 and semiconductor saturable absorber output coupler (SOC) to generate passively mode-locking CV beams by three-element and four-element system..

PICOSECOND PULSE LASER WITH CYLINDRICAL VECTOR POLARIZATION

We use three-element system to generate Q-switched mode-locking pulse laser with cylindrical vector polarization. Crystal in cavity is 8mm long Nd:GdVO4, and output coupler is semiconductor saturable absorber output coupler (SOC). A high pass len of 75mm radius for 808nm is put in cavity. Near the boundary of stable region, the cavity length decides extraordinary ray or ordinary ray stable, respectively generate radial polarization and azimuthal polarization. The repetition frequency and the pulse width are recorded by RF spectrum analyzer and autocorrelator, respectively. We also use 300um slit to select part area on output ring, and rotate the slit 15 degree per operated point. We ensure that all the polarization ratio on every part of the ring has the quality of 10.

Besides using three-mirror system to generate Q-switched mode-locking pulse with cylindrical vector polarization, we also use four-element system to generate continuos wave mode-locking pulse laser with azimuthaul polarization. In four-element cavity, we use the same crystal and SOC . High pass lens of 20mm radius and 75mm radius for 808nm are put in cavity. Compared with the three-element cavity, the length of the four-element cavity is longer and the spot size at SOC is smaller. In the four-element system, we find the QML can transfer to CWML by increasing the pump power. At pump power 6.45W, we achieve CWML with azimuthal polarization. At this operating point, we also ensure that all polarization ratio on every part of the ring has the quality of 10.

RESULTS AND DISCUSSION

In three-mirror system, we get QML pulse laser with radial polarization and azimuthal polarization respectively at pump power 9W. In radial polarization region, we get the QML pulse repetition frequency is 7.87±0.57kHz. The optical slope efficiency is 6.66%. The width of QML envelope is about 1.033us. Inside the QML envelope, the repetition frequency and pulse width are 450MHz and 37ps respectively. The polarization ratio and the degree of polarization (DOP) are 17.90±4.88 and 89.02%±3.75%. In azimuthal polarization region, we get the QML pulse repetition frequency is 7.0.4±0.41kHz. The optical slope efficiency is 6.27%. The width of QML envelope is about 0.967us. Inside the QML envelope, the repetition frequency and pulse width are 450MHz and 31ps respectively. The polarization ratio and the degree of polarization (DOP) are 19.71±8.85 and 90.42%±4.61%.

In four-element system, a CWML pulse laser with azimuthally polarization is demonstrated at pump power 6.45W. The repetition frequency and pulse width are 127.5MHz and 90ps respectively. The polarization ratio and the degree of polarization (DOP) were 14.04±4.57 and 85.77%±3.41%.

CONCLUSION

In conclusion, we demonstrate two experiments for generating mode-locking cylindrical vector beam with the same crystal and SOC. One is using three-element system and the other one is using four-element system. We generate QML pulse laser with radial and azimuthal polarization near the boundary of stable region in three-element cavity at pump power 9W. Compare to the three-element cavity, the four-element cavity has the longer cavity length and the smaller spot size at SOC. As increasing the pump power, the laser output change from QML to CWML. Finally, we achieved CWML pulse laser with azimuthal polarization at pump power 6.45W.

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