A differential interferometer with the four-focus imaging system

A new type of differential interferometer was created by placing the ring cavity of a cyclic lateral shearing interferometer between two lenses of a four-focus imaging system. The four-focus system is slightly defocused by adding an offset between the object plane and the front focal plane of the first lens. The shear distance of images varies directly as both the offset and the shear distance of the cyclic interferometer. The spatial carrier frequency of fringes varies directly as the shear distance of the cyclic interferometer, but it is independent of the offset. Thus the dependence between the carrier frequency and the shear distance of images is broken and it's possible to adjust them respectively. Furthermore, this differential interferometer is also very stable, which is important when vibrations can't be avoided in experiments. The interferometer has been applied to diagnostics of plasma with large density gradient.