利用双缝衍射干涉原理，波长微小变化引起折射率变化从而导致两衍射缝之间产生明显的位相差，进而使衍射零级条纹偏离光轴。通过其偏移量的变化可以实时监测波长的微小波动。实验表明，激光波长变化2 pm 可引起衍射零级条纹位移约2.6 μm。由于探测的是衍射零级条纹的位移，这样就避免光强变化带来的影响，极大地提高了波长监测分辨率。

Using the double slit diffraction and interference principle, small fluctuation of wavelength leads to refractive index change and generates a obvious phase difference between two diffraction slits. Thus zero level of the diffraction stripe deviates from the optical axis. Small fluctuation of wavelength is monitored in realtime through the change of offset. Experimental results show that 2 pm change of the laser wavelengh can cause displacement of diffraction stripe for about 2.6 μm. Because the displacement of zero level diffraction stripe is detected, effects of light intensity change are avoided which greatly improves the wavelength monitoring resolution.