Method of interferometry with modulated optical path-length difference and interferometer

A method of operating an interferometer, said method comprising:

• generating a beam of laser light having a wavelength λ;
• splitting said beam of laser light into a probing sub-beam and a reference sub-beam;
• directing said probing and reference sub-beams through different optical paths having an optical path-length difference, the path of said probing beam being perturbed causing a perturbation in said optical path-length difference;
• varying said optical path-length difference periodically in addition to said perturbation to cause an optical path length difference periodically passing through (2m+1)λ/4, m being an integer;
• and
• detecting an interference signal of said sub-beams including an indicator of optical interference near a (2m+1)λ/4 optical path-length difference.

A method of measuring a displacement of an object vibrating at a first frequency, said method comprising:

• generating a beam of laser light having a wavelength λ;
• splitting said beam of laser light into a probing sub-beam and a reference sub-beam;
• directing said probing and reference sub-beams through different optical paths having an optical path-length difference, said probing beam being reflected from said vibrating object thus causing a perturbation of said optical path-length difference;
• varying said optical path-length difference by at least λ/2 periodically in addition to said perturbation at a second frequency smaller than said first frequency;
• detecting an interference signal of said sub-beams;
• determining from said interference signal a maximum intensity change in said interference signal caused by said perturbation near a λ/4 optical path-length difference and a maximum intensity change in said interference signal caused by varying said optical path-length difference by λ/2;
• and
• determining the maximum displacement of said object based on said maximum intensity changes.

An interferometer comprising:

• source means for generation of a first beam and a second beam of a laser light having a wavelength λ;
• optical means for directing said first and second beams through two different optical paths having a path-length difference;
• detecting means for detection of an interference signal of said first and second beams;
• and
• modulation means for varying said path-length difference periodically to allow detection of said interference signal caused by said perturbation near a λ/4 optical path-length difference and a maximum intensity change in said interference signal caused by varying said optical path-length difference by λ/2.