於本論文中，提出一個理論模型評估於電光調變器(EO Modulator)施加不同電壓下，外調式光通訊之光源同調程度(Degree of Coherence)的改變。於研究中使用三維波動方程式(Maxwell’s Equation)可推導出一組耦合方程式，藉由耦合方程式解出電光調變器輸出光之解析解並觀察於電壓區間0~10 V下之模擬結果。
當輸入光源以高斯-謝爾形式(Gaussian-Schell model form)輸入電光調變器時，因電光調變器內部晶體受外加電場影響，使晶體內部之折射率(Refractive Index)改變，進而使之形成如同光柵(Grating)之結構。當光源經過該晶體時，以自相關函數(Autocorellation)計算輸出光之同調程度
，透過對LiNbO3晶體施加直流電0 V至10 V進行比較，輸出光於此電壓區間中，觀察O光(Ordinary Light)其同調程度無明顯之變化，觀察E光(Extraordinary Light)其同調程度隨著電壓上升而逐漸下降。

In this thesis, a theoretical model is proposed to evaluate the property of the spatial coherence of an electro-optic modulator. Staring from the Maxwell’s equation, the evolution and interaction of a lightwave inside the eltro-optic modulator can be properly described. In order to investigate the degree of spatial coherence of light during the electro-optic interaction, a set of coupled equations of the ordinary and extraordinary light can be derived from the Maxwell’s equations. The couple equations can be solved analytically and the evolution of the spatial coherence of light can be observed to get a general solution and simulation in applied voltage in 0~10 V.
The lightwave is assumed in the Gaussian-Schell model form and incidents on the E-O modulator and the crystal of the E-O modulator is modulated by an external electric field, therefore the refractive index is formed as a grating type. When the lightwave through the crystal, the degree of coherence can be calculated by the autocorrelation function.
We find that, under the applied voltage 0~10 V on LiNbO3 crystal, there is no significant change of the degree of coherence of the ordinary light, while a significant change of and the extraordinary light can be observed.

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