An optical interferometer disclosed in Patent Document 1 uses a branching-combining unit made of, for example, silicon to partially reflect incident light on one plane of the branching-combining unit, and transmit the rest of the incident light through the plane, to branch the light into first branched light and second branched light, and combine the first branched light and the second branched light and output as combined light. That is, the optical interferometer commonly uses one plane of the branching-combining unit as the branching surface for branching the incident light into the first branched light and the second branched light and the combining surface for combining the first branched light and the second branched light to form the combined light. Further, in the optical interferometer disclosed in this document, wavelength dispersion occurs when one light of the first branched light and the second branched light reciprocates in the branching-combining unit, and to eliminate the wavelength dispersion, the other light is made to reciprocate in a dispersion compensating member.
An optical interferometer disclosed in Patent Document 2 uses a branching-combining unit made of, for example, silicon to partially reflect incident light on a first principal surface of the branching-combining unit, and transmit the rest of the incident light through the surface, to branch the light into first branched light and second branched light, and combine the first branched light and the second branched light on a second principal surface of the branching-combining unit and output as combined light. That is, the optical interferometer uses different surfaces as the branching surface (first principal surface) for branching the incident light into the first branched light and the second branched light and the combining surface (second principal surface) for combining the first branched light and the second branched light to form the combined light. The optical interferometer disclosed in this document can decrease the wavelength dispersion, because each of the first branched light and the second branched light passes through the branching-combining unit only once.