兆赫波(THz)係屬於尚在研究發展階段的波段光源，相較於其他波段的光源而言，我們對於兆赫波之物理特性(如:光源是屬於熱量燈光源或者同調光源)了解較為稀少，為了得知其性質，我們參考HBT實驗來量測兆赫波的二階相干函數，以了解兆赫波係屬於古典光源或者量子光源。
因為實驗架設的系統量測得到為兆赫波的電場訊號，為了得到二階相干函數，需先將其電場訊號平方並且相乘。又因擷取訊號的儀器係為鎖相放大器，因此平方相乘之後的訊號會有許多不需要的常數影響量測的結果，我們需要將其用計算消除，最後再和我們的理論值作比較分析。

Terahertz wave is a light source which is still on research and development, comparing to other wave length, we are not familiar with the terahertz wave (such as thermal light or coherent light), and we want to understand the properties about it, we refer to the HBT experiment to measuring the second order correlation function of Terahertz wave, and understanding whether the terahertz wave is classical or quantum light.
Because the electric field of Terahertz wave from experiment setup, to getting the second order correlation function, we need to make the electric filed signal square and multiplying with each other. Because the signal is from the lock-in amplifier, so we need to cancel the constant which we do not need after the calculation from above. Finally, the experiment results is compared to the theoretical prediction

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