Apparatus and method for performing two-frequency interferometry

A method for measuring the optical phase difference resulting from a difference in index of refraction between a sample under investigation and a reference sample, said method comprising the steps of:

• a. generating of first wavelength of electromagnetic radiation and a second wavelength of electromagnetic radiation, the first radiation and the second radiation being substantially collinear, having substantially orthogonal polarization and an initial relative phase, and differing in wavelength by between 1.times.10.sup.-13 nm and 7.times.10.sup.-4 nm;
• b. spatially separating the first wavelength of radiation and the second wavelength of radiation by utilizing the difference in polarization thereof into substantially parallel wavelengths of radiation;
• c. directing the first wavelength of radiation into the sample under investigation;
• d. directing the second wavelength into the reference sample;
• e. spatially combining the first wavelength of radiation after the emergence thereof from the sample under investigation and the second wavelength of radiation after the emergence thereof from the reference sample into substantially collinear wavelengths of radiation by utilizing the difference in polarization thereof;
• f. maintaining the temperature substantially constant for said steps of spatially separating the first wavelength of radiation and the second wavelength of radiation, spatially combining first wavelength of radiation and the second wavelength of radiation, directing the first wavelength of radiation into the sample under investigation, and directing the second wavelength of radiation into the reference sample;
• g. polarizing the reunited first wavelength of radiation and the second wavelength of radiation at a chosen polarization angle to enable the interference thereof;
• h. detecting the interference of the first wavelength of radiation and the second wavelength of radiation and generating an electrical interference signal in response thereto;
• and
• i. measuring the phase of the electrical interference signal relative to the phase difference of the initially generated first wavelength of radiation relative to the second wavelength of radiation.

An apparatus for measuring small differences in optical absorption between a sample under investigation and a reference sample utilizing the thermooptic effect, said apparatus comprising in combination:

• a. a light source for providing a first wavelength of electromagnetic radiation and a second wavelength of electromagnetic radiation, the first radiation and the second radiation being substantially collinear, having substantially orthogonal polarization and a initial relative phase, and differing in wavelength by between 1.times.10.sup.-13 nm and 7.times.10.sup.-4 nm;
• b. radiation separation means responsive to polarization for receiving the first wavelength of radiation and the second wavelength of radiation and for spatially separating the first wavelength of radiation and the second wavelength of radiation into substantially parallel wavelengths of radiation, the first wavelength of radiation intersecting the sample under investigation and the second wavelength of radiation intersecting the reference sample;
• c. radiation combining means similar to said radiation separation means responsive to polarization for receiving the first wavelength of radiation after the emergence thereof from the sample under investigation, and for receiving the second wavelength of radiation after the emergence thereof from the reference sample, and for spatially reuniting the first wavelength of radiation and the second wavelength of radiation into substantially collinear wavelengths of radiation;
• d. first polarization means having a chosen angle of polarization for receiving the reunited first wavelength of radiation and the second wavelength of radiation and for enabling the interference thereof;
• e. electromagnetic radiation detection means for receiving the interfering first wavelength of radiation and the second wavelength of radiation and for generating an electrical interference signal in response thereto;
• and
• f. phase detection means for receiving the electrical interference signal generated by said electromagnetic radiation detection means and for measuring the phase thereof relative to the initial relative phase of the first wavelength of radiation and the second wavelength of radiation.