Heterodyne interferometer arrangement

A heterodyne interferometric metering method comprising the steps of:

• producing a reference signal representing a reference phase of light beams having first and second frequencies, respectively, at an input of an interferometer unit;
• dividing the light beams in the interferometer unit and transmitting them through two paths, each having an optical path length in the interferometer unit, such that one path contains light beams having the first frequency and the other path contains light beams having the second frequency;
• combining the light beams after they have traversed the two paths and producing an optical measuring beam having a phase which is proportional to any change in the optical path length occurring between the two paths, said change representing a change in a displacement to be measured by the interferometer unit;
• producing a measuring signal representing the phase of the optical measuring beam;
• and
• comparing the phase of the reference signal and of the measuring signal so as to produce a phase difference which depends on the displacement to be measured;
• wherein the step of producing a measuring signal comprises the production of first and second measuring signals representing light components which are orthogonal to each other;
• and
• wherein the step of comparing the phases comprises the production of first and second phase differences between one of the first and second measuring signals and the reference signal and the generation of a mean value of the two phase differences as a measure of the displacement to be measured.

A heterodyne interferometer arrangement comprising:

• a light source producing light beams having first and second frequencies, respectively;
• an interferometer unit having a non-linearity error and coupled to receive the light beams, said interferometer unit further including a first splitter for dividing the light beams and sending them along two paths, each having a respective optical path length, one path for transmitting a portion of the light beams having the first frequency and the other path for transmitting a portion of the light beams having the second frequency;
• the interferometer unit so constructed and arranged that the light beams after traveling through the two paths are recombined to produce an optical measuring beam having a phase which is proportional to any change in optical path length occurring between the two paths, said change representing a change in a displacement to be measured by the interferometer unit;
• a second splitter coupled to receive and divide the optical measuring beam into two orthogonal light beams each having components which are orthogonal to one another;
• first and second measuring transducer units, which are arranged to receive respectively the two orthogonal light beams and respondto produce first and second measuring signals;
• and
• a phase difference circuit coupled to receive the first and second measuring signals for producing a phase difference signal, which is representative of a phase difference between the first and second measuring signals and which is proportional to the non-linearity error of the interferometer unit.

A heterodyne interferometric metering method comprising the steps of:

• generating light beams having first and second frequencies;
• applying said light beams in an interferometer unit having a non-linearity error;
• dividing the light beams in the interferometer unit and transmitting them through two paths, each having an optical path length in the interferometer unit, such that one path contains light beams having the first frequency and the other path contains light beams having the second frequency;
• combining the light beams after they have traversed the two paths and producing an optical measuring beam having a phase which is proportional to any change in the optical path length occurring between the two paths, said change representing a change in a displacement to be measured by the interferometer unit;
• splitting the optical measuring beam into two orthogonal light beams each having components that are orthogonal to one another;
• producing first and second measuring signals representative of the two orthogonal light beams;
• and
• producing a phase difference signal, which is representative of any phase difference between the first and second measuring signals and which is proportional to the non-linearity error of the interferometer unit.