Optical interference measurement apparatus having optical path lengths set to satisfy a given relation

An optical interference measurement apparatus comprising:

• a light source that outputs measurement light;
• a measurement unit that emits the measurement light and receives return light that is reflected from a first measurement object and a second measurement object;
• a first reference surface;
• a second reference surface;
• a detector that receives an interference signal of the return light and reference light and converts the interference signal into an electric signal;
• and
• a processing unit that obtains first information relating to a distance, a speed, or an oscillation of the first measurement object and second information relating to a distance, a speed, or an oscillation of the second measurement object, based on the electric signal, wherein
• a fiber coupler that branches the measurement light from the light source is provided on an optical path between the light source and the measurement unit,
• the measurement unit comprises a first measurement head to which a first measurement light for obtaining the first information of the first measurement object is branched from the fiber coupler is input via a first optical path, and a second measurement head to which a second measurement light for obtaining the second information of the second measurement object is branched from the fiber coupler is input via a second optical path,
• a first reference light that is reflected from the first reference surface positioned in a downstream direction toward the light source from a leading end of the first measurement head is guided to the first optical path the first reference surface formed at a connection part of the first optical path and the first measurement head,
• a second reference light that is reflected from the second reference surface positioned in the downstream direction from and spaced away from an input end of the second measurement head is guided to the second optical path, the second reference surface being provided at an intermediate portion on the second optical path between the fiber coupler and the input end of the second measurement head,
• a first return light that is received by the first measurement head is guided to the detector via the first optical path and the fiber coupler,
• a second return light that is received by the second measurement head is guided to the detector via the second optical path and the fiber coupler,
• an optical path length D1 from the fiber coupler to the leading end of the first measurement head, an optical path length D2 from the fiber coupler to a leading end of the second measurement head, a maximum optical path length R1 max of the measurement range of the first measurement head, an optical path length S1 of the first reference light from where the measurement light is emitted from the light source to the first reference surface where the first reference light is combined with and interferes with the first return light, and an optical path length S2 of the second reference light from where the measurement light is emitted from the light source to the second reference surface where the second reference light is combined with and interferes with the second return light are set such that the relation
• D1+R1 max−S1<D2−S2
• is satisfied,
• the second optical path is formed by coupling a first optical fiber and a second optical fiber, and
• the second reference surface is an end face on the second optical fiber side of the first optical fiber, which is spaced from the input end of the second measurement head, and
• the processing unit is further configured to
• calculate a first measurement distance of the first measurement object measured by the first measurement head based on a first frequency of a first peak that is present within a first frequency range based on the obtained first information, and
• calculate a second measurement distance of the second measurement object measured by the second measurement head based on a second frequency of a second peak that is present within a second frequency range that does not overlap with the first frequency range based on the obtained second information, and
• output an error if the first peak and the second peak are within the second frequency range.