Highly sensitive double-grating interferometer for direct measurement of a neutral helium gas density in a capillary cell

We developed a highly sensitive double-grating interferometer using four probe beams, which can measure one order of magnitude smaller phase shifts compared with other conventional interferometers. To achieve the unprecedented sensitivity, a highly dynamic 16-bit CCD camera was used and the balanced detection technique with four probe beams was employed in the 2-dimensional (2-D) interferometry for the first time. By using this interferometer, we could directly measure a low helium (He) gas density of <i>n</i> ≃ 1 × 10¹⁷ cm^-3 in a capillary gas cell for the laser-plasma acceleration research, which is almost impossible with other conventional laser interferometers. This interferometer may provide a new tool for applications with extremely small phase shifts in science.