Lightwave interference measurement apparatus that calculates absolute distance using lightwave interference

A lightwave interference measurement apparatus comprising:

• a wavelength-variable laser configured to periodically perform wavelength scanning between a first reference wavelength λ1 and a second reference wavelength λ2 to emit light beam;
• a wavelength-fixed laser configured to emit light beam having a third reference wavelength λ3;
• a wavelength reference element configured to be able to set a wavelength of the light beam emitted from the wavelength-variable laser to the first reference wavelength λ1 and the second reference wavelength λ2;
• a light beam splitting element configured to split the light beams emitted from the wavelength-variable laser and the wavelength-fixed laser into reference light beam and light beam under test;
• a reference surface configured to reflect the reference light beam;
• a surface under test configured to reflect the light beam under test;
• a phase detector configured to detect a phase based on an interference signal of the reference light beam that is reflected on the reference surface and the light beam under test that is reflected on the surface under test;
• and
• an analyzer configured to sequentially determine an interference order of the third reference wavelength λ3 based on the third reference wavelength λ3, a first synthetic wavelength Λ12, a second synthetic wavelength Λ13, an integer component of a phase change amount in the wavelength scanning, and interference orders of the first synthetic wavelength Λ12 and the second synthetic wavelength Λ13, and calculates an absolute distance between the surface under test and the reference surface, where the first synthetic wavelength Λ12 is represented by λ1·λ2/|λ1−λ2| and the second synthetic wavelength Λ13 is represented by λ1·λ3/|λ1−λ3|.