| 摘要: | 在本論文中，透過基因演算法(Genetic algorithm, GA)設計一非週期晶疇極化反轉之鈮酸鋰結構(Aperiodically poled lithium niobite, APPLN)，並套用在鈦擴散波導之結構中，此結構包含產生光子對的自發參量下轉換(Spontaneous parametric down-conversion, SPDC)，以及寬頻之電光偏振模態耦合器(Electro-optical polarization mode converter, EO-PMC)，而產生的光子對為正交偏振(Type-II)，再利用電光偏振模態耦合器將此正交偏振之光子對轉換成互為相反偏振之狀態，藉此消除光子在晶片內部傳遞所造成的時間延遲，進而達成單一結構同時進行兩種非線性轉換過程，在模擬上使用基因演算法來優化結構，其全域之搜索能力佳，自由度較大，再利用半導體製程的方法實際製作出此晶片，隨後進行量測。
首先進行空間模態量測，以及古典之倍頻(Second harmonic generation, SHG)量測與電光偏振模態耦合器量測，本研究中，利用鈦擴散波導上之20mm之非週期極化反轉之結構，在相位匹配溫度為95°C時，倍頻產生之波峰值為1571.2nm，當施加y方向上電壓為60伏特時，電光偏振模態耦合器轉換之波長範圍為1564.1~1574.6nm，在波長重疊之處其轉換效率可達96%，於此證明在古典量測中兩者可重疊;接著進行量子之巧合計數(Coincidence counting)量測以及Hong-Ou-Mandel干涉實驗，並確認自發參量下轉換之信號與閒置頻譜在溫度為107°C時，兩者波峰約為1569nm幾乎疊合後，進行雙光子干涉量測，本實驗得出其可見度為82.3%±1.7%，結果顯示光子有良好的干涉程度，最後使古典量測電光偏振模態耦合器的結果與自發參量下轉換兩者進行整合，將下轉換產生的偏振正交光子對利用電光偏振模態耦合器調製其偏振以消除兩個偏振光因折射率不同而在晶體內部產生的時間延遲，並透過實驗得以驗證。
未來在晶體結構上可做進一步改善，將重新設計波導線寬給定一個範圍，並可配合目前擁有的雷射光源，使自發參量下轉換與電光偏振模態耦合器可在波長約為1569nm處重疊，達到利用單一晶片即能產生一對偏振糾纏之光子對。;In this paper, the aperiodic domain inversion structure in lithium niobite is designed by genetic algorithm (GA), and applied to the structure of titanium diffusion waveguide. This structure includes spontaneous parametric down-conversion (SPDC) to generate photon pairs, and a broadband electro-optical polarization mode converter (EO-PMC). The generated photon pairs are orthogonal polarization, and the EO-PMC is used to convert the orthogonal polarization photon pairs into the polarizatoin states which are opposite to each other. Thereby eliminating the time delay caused by the transmission of photons inside the chip. Therefore, achieve a single structure and performing two nonlinear conversion processes at the same time. In the simulation, the genetic algorithm is used to optimize the structure, which has better search ability in the global and a large degree of freedom. Then, the semiconductor process method is used to actually produce the chip, and then the measurement.
First, we measured the spatial mode, as well as the classical second harmonic generation (SHG) measurement and the EO-PMC measurement. In this study, a 20mm aperiodically poled lithium niobite(APPLN) structure on a Ti-diffused waveguide was used. When the phase matching temperature is 95°C, the peak value of the wave generated by SHG is 1571.2nm. When the applied voltage in the y direction is 60V, the wavelength range of EO-PMC conversion is 1564.1~1574.6 nm, and the conversion efficiency can reach 96% at the overlapping wavelengths. This proves that SHG and EO-PMC can overlap in classical measurements. Then, we do the coincidence counting measurement and Hong-Ou-Mandel interference experiment of quantum mechanics. Confirm the spectra of signal and idler of SPDC at 107°C, the two peaks are about 1569nm and almost overlap. Begin of measured two-photon interference, and this experiment found that its visibility was 82.3%±1.7%. The results show that photons have good interference performance. Finally, the results of classical measurements of EO-PMC and SPDC. The orthogonally polarization photon pairs of down-converted is modulated by EO-PMC to eliminate the time delay caused by the difference in refractive index between the two polarized lights inside the crystal and verified by experiments.
Further improvements can be made in the crystal structure in the future. To redesign the waveguide line width to give a range, and can compatible with our laser light sources. Enables SPDC and EO-PMC to overlap at wavelengths around 1569 nm. That can achieve a pair of polarization-entangled photon pairs were generated using a single chip. |
