非線性光譜干涉(Nonlinear spectral interferometry, NLSI)是一種量測准相位匹配光柵(quasi-phase matched gratings, QPM)的複數相位匹配頻譜相位成分(phase-matching spectral phase)的技術。它的理論建立在光譜干涉技術(spectral interferometry)上，配合寬頻光源與一薄非線性參考晶體，以及一非線性待測晶體，量測干涉儀兩臂的二倍頻干涉頻譜(interferogram)。由於薄非線性參考晶體的相位可近似於常數，干涉儀兩臂的光頻譜相位差可近似於待測晶體的頻譜相位。最後，藉著量測出的頻譜相位與其功率頻譜結合，此技術甚至能大致還原出准相位匹配光柵在空間上的週期分佈。
此技術的好處是，它提供了一種快速、靈敏、準確、非破壞性的方法。它具有不受限於光源本身的啁啾(chirp)程度，以及可以量測任何種類准相位匹配光柵的優點。不同於一般利用顯微鏡拍照的技術，此種方法量測到的並非是晶體表面的狀況，而是晶體內部真正的情形，也不會有顯微拍照技術中，因為照片銜接問題產生的隨機誤差。據我們所知，這是第一個以實驗方法，量測出准相位匹配光柵頻譜相位，以及正確還原出晶體在空間上週期分佈的研究成果。

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