An optical interferometer includes a beam splitter module and an optical sensor. The beam splitter module includes a lens assembly and a splitter cube. A light incident surface of the splitter cube is substantially orthogonal to an optical axis of the lens assembly. An acute angle is between the light incident surface and a light splitting surface of the splitter cube. A sampling surface of the splitter cube is substantially parallel to the light incident surface. A light reflecting surface of the splitter cube is substantially orthogonal to the light incident surface. The light incident surface is closer to the lens assembly than the sampling surface. A reference arm is defined between a splitter position on the light splitting surface and the light reflecting surface, a sample arm is defined between the splitter position and the sampling surface, and the reference arm is longer than the sample arm.