| 摘要: | 本論文結合非線性光學、積體光學與雷射物理之技術將多波長選頻、波長轉換及Q-調制功能整合入鈮酸鋰光子晶片內並置入雷射共振腔，成功實現1047nm與1053nm兩偏振正交雷射之可調式電光選頻、非相位匹配二倍頻、非相位匹配和頻以及其Q-調制之功能，其中這兩波長間距僅僅只有6nm。
藉由鈮酸鋰晶體在施加y方向電場時可改變入射光偏振之特性，本論文使用模擬退火法(Simulated annealing, SA)設計出一款非週期性光學超晶格(Aperiodic Optical Superlattices, AOS)結構，基於此結構藉由微影製程定義圖形做晶疇極化反轉來滿足Nd:YLF雷射系統之1047nm與1053nm作為電光偏振模態旋轉器(Electro-optic polarization mode converter, EOPMC)上的動量守恆匹配條件，並藉由演算法目標函數的設計，使AOS優化出的正負晶疇組合能令雙波長在雷射系統中施加電場時的偏振旋轉程度不同，定義出晶片施加不同電場時雙波長的穿透情形，以此方式來實現Nd:YLF雷射在1047nm與1053nm的正交偏振雙波長主動式電光選頻以及主動式Q-Switching，並透過雷射在鈮酸鋰內部所產生的非相位匹配機制來達成共振腔內二倍頻與和頻產生，實現一款以單晶片主動式電光調制方式來完成1047nm(e-wave)、1053nm(o-wave)、523.5nm(e-wave)、526.5nm(e-wave)以及525nm(e-wave)的多波長正交偏振雷射系統，並且此系統可工作於連續波(Continuous Wave, CW)模式或脈衝(Pulse)模式。
經由演算法之製程結構設計、實際晶片製程，包含黃光微影、薄膜、蝕刻、研磨、拋光、極化反轉……等等到最後量測，得出晶片在連續波(Continuous Wave, CW)表現上，工作電場0V/mm可使雙波長同時輸出，並得到和頻之525nm光譜以及兩波長個別二倍頻之光譜;當晶片在工作電場568V/mm時可單段輸出純1047nm雷射，並得到其二倍頻之523.5nm光譜;當晶片在工作電場725V/mm時可單段輸出純1053nm雷射，並可得到526.5nm之雷射光譜，最後在工作電場154V/mm時可得到雙波長nonlasing的結果，並且我們亦針對上述之結果做光功率量測以及工作電場與工作波長之電光選頻容忍範圍分析。
在Q-調制表現上，本處脈衝重複率皆為1kHz，藉由電路訊號控制High Q與Low Q持續時間，當晶片工作電場在568V/mm與154V/mm作ns等級快速切換時，可產生1047nm以及其二倍頻523.5nm之脈衝雷射，其中1047nm量得之最小脈寬為25.7ns，其脈衝峰值功率為17393瓦，其二倍頻523.5nm之最小脈寬為24.9ns，其脈衝峰值功率為164.26瓦;當工作電場條件更改為725V/mm與154V/mm時，1053nm量得之最小脈寬為32.5ns，其脈衝峰值功率為12215.38瓦，其二倍頻526.5nm之最小脈寬為29.7ns，其脈衝峰值功率為135.91瓦;最後當晶片工作電場為0V/mm與154V/mm時，可量測其和頻525nm及兩波長之二倍頻脈衝疊合之三波長最小脈寬為45.97ns，其三波長脈衝峰值功率為40.24瓦，在基頻光之雙波長表現上則為最小脈寬57.81ns，其峰值功率為10482.62W。
未來若能在模擬退火法上改變準相位匹配條件，使極化反轉結構滿足差頻條件，並將晶片實際製作出來，有機會在正交偏振雷射系統中產生THz光源，會是一大突破。;In this work, the multi-wavelength frequency selection, wavelength conversion and Q-Switching functions are integrated into the lithium niobate chip. Put the chip into the laser resonant cavity, it can successfully realize the functions of tunable electro-optical frequency selection, non-phase-matched second harmonic generation (SHG), non-phase-matched sum-frequency generation (SFG) and its Q-Switching of 1047nm and 1053nm cross-polarized lasers, in which the distance between these two wavelengths is only 6nm.
In this work, we used simulated annealing algorithm to design an Aperiodic Optical Superlattices (AOS) structure and this structure can satisfy the electro-optic polarization mode converter (EOPMC) condition of 1047nm and 1053nm. And through the design of the objective function of the algorithm, the combination of positive and negative crystal domains optimized by AOS can make the polarization rotation degree of the dual wavelengths different when the electric field is applied in the laser system, and define the transmittance situation of the dual wavelengths when the chip is applied with different electric fields. So we can use this method to realize the orthogonal polarization dual-wavelength active electro-optical frequency selection and active Q-Switching of Nd:YLF laser at 1047nm and 1053nm and through the non-phase matching mechanism generated by the laser inside the lithium niobate to achieve the double frequency and sum frequency generation in the resonant cavity.
Through the process structure design of the algorithm, the actual chip manufacturing process to the final measurement, it is concluded that the chip is in the continuous wave (CW) performance, the working electric field of 0V/mm can output two wavelengths at the same time, and the 525nm spectrum of the sum frequency can be obtained. ; When the chip is in a working electric field of 568V/mm, it can output a pure 1047nm laser, and obtain its double frequency spectrum of 523.5nm; when the chip is in a working electric field of 725V/mm, it can output a pure 1053nm laser, and obtain its double frequency spectrum of 526.5nm, and finally the nonlasing result can be obtained when the working electric field is 154V/mm.
In terms of Q-Switching performance, when the working electric field of the chip is rapidly switched between 568V/mm and 154V/mm at ns level, the Q factor of the resonant cavity can be quickly switched between High Q and Low Q, it can generate pulsed lasers of 1047nm and its SHG of 523.5nm. The minimum pulse width measured at 1047nm is 25.7ns, and its peak pulse power is 17393 watts. The minimum pulse width of 523.5nm is 24.9ns, and its peak pulse power is 164.26 watt. When electric field is working at 725V/mm and 154V/mm, the minimum pulse width measured at 1053nm is 32.5ns, the peak pulse power is 12215.38 watts, the minimum pulse width of the SHG 526.5nm is 29.7ns, and the peak pulse power is 135.91 watts. Finally, when the working electric field of the chip is 0V/mm and 154V/mm, the minimum pulse width of the SFG 525nm and the superposition of the SHG pulse of the two wavelengths can be measured to be 45.97ns, and the peak power of the three-wavelength pulse is 40.24W, In the dual wavelength performance of fundamental frequency light, the minimum pulse width is 57.81ns, and its peak power is 10482.62W.
In the future, if the quasi-phase matching conditions can be changed in the simulated annealing algorithm, so that the polarization inversion structure can satisfy the difference frequency generation conditions, and the chip can be actually produced, it will be a great breakthrough to have the opportunity to generate a THz light source in an orthogonal polarization laser system. |
