Interferometer mirror tilt correcting system

Claim 2

In an interferometer having two optical paths from a monochromatic source to a data detector and actuating means for varying the length of one of said optical paths, a servo system for so automatically adjusting the angular tilt of a reflecting planar surface in one of said paths as to maintain parallel the interfering beams of said two paths at the optical output of said interferometer, said reflecting planar surface having three points R, X and Y spaced apart on mutually perpendicular X and Y axes, said point R being at the intersection of said axes, and said points X and Y being on respective X and Y axes, the combination comprising three detectors near said optical output of said interferometer disposed on said X and Y axes as reflected to said optical output on a plane normal to the nominal beam axis at said optical output, one detector placed to receive light reflected from said point R of said reflecting surface, one of the other two detectors placed to receive light reflected from said point X of said reflecting surface and the third detector placed to receive light reflected from said point Y of said reflecting surface, means for producing a 90° phase shifted signal e r in response to light waves detected by said one detector from said point R, said signal e r , being proportional to the optical path difference of light reflected from said point R, and separate means responsive to separate ones of said other two detectors for producing signals e x and e y proportional to the optical path differences of light reflected from said points X and Y, means responsive to an X error signal for tilting said X axis in said reflecting surface through an angle proportional to the amplitude of said X error signal, means responsive to a Y error signal for tilting said Y axis in said reflecting surface through an angle proportional to the amplitude of said Y error signal, means for multiplying said signal e x by said signal e r to obtain said X error signal, and means for multiplying said signal e y by said signal e r to obtain said X error signal.

Claim 3

A servo system as defined in claim 1 wherein said means for producing a 90° phase shifted signal is comprised of means for modulating the length of said one of said optical paths in response to an AC modulating signal, and phase sensitive demodulating means responsive to said AC modulating signal for demodulating the signal output of said one detector to produce said signal e r , said combination further including lowpass filters for filtering residual AC components in said X and Y error signals resulting from modulation of the length of said one of said optical paths and demodulating only the signal output of said one detector.

Claim 4

The combination of claim 2 wherein said interferometer is of the Michelson-type and said reflecting surface is a plane mirror in an optical path of fixed length.

Claim 5

The combination of claim 2 wherein said interferometer is of the type employing cat's-eye retroreflectors and said one reflecting surface is on a selected one of an input beam splitter and an output beam combiner, said beam splitter being nominally parallel to said output beam combiner.

Claim 6

In an interferometer having two optical paths from a monochromatic source to a data detector and actuating means for varying the length of one of said optical paths, a servo system for so automatically adjusting the angular tilt of a reflecting planar surface in one of said paths as to maintain parallel the interfering beams of said two paths at the optical output of said interferometer, said reflecting planar surface having three points R, X and Y spaced apart on mutually perpendicular X and Y axes, said point R being at the intersection of said axes, and said points X and Y being on respective X and Y axes, the combination comprising three detectors near said optical output of said interferometer disposed on said X and Y axes as reflected to said optical output on a plane normal to the nominal beam axis at said optical output, one detector placed to receive light reflected from said point R of said reflecting surface, one of the other two detectors placed to receive light reflected from said point X of said reflecting surface and the third detector placed to receive light reflected from said point Y of said reflecting surface, means for producing a 90° phase shifted signal in response to light waves detected by said one detector from said point R comprised of means for modulating the length of said one of said optical paths in response to an AC modulating signal, and phase sensitive demodulating means responsive to said AC modulating signal for demodulating the signal output of said one detector to produce a demodulated reference detector signal, separate phase sensitive demodulating means responsive to said AC modulating signal for demodulating the signal outputs of said other two detectors to produce demodulated X and Y detector signals, separate means for multiplying the signal output of said one detector with demodulated X and Y detector signals to form respective product signals M 2x and M 2y , separate means for multiplying the undemodulated signal outputs of said other detectors with said demodulated reference detector signal to form respective product signals M 1x and M 1y , means for forming an X error signal as the difference between said signal M 2x and said signal M 1x , means for forming a Y error signal as the difference between said signal M 2y and said signal M 1y , means responsive to said X error signal for tilting said X axis in said reflecting surface through an angle proportional to the amplitude of said X error signal, and means responsive to said Y error signal for tilting said Y axis in said reflecting surface through an angle proportional to the amplitude of said Y error signal.

Claim 7

In an interferometer having two optical paths from a monochromatic source to a data detector and actuating means for varying the length of one of said optical paths, a servo system for so automatically adjusting the angular tilt of a reflecting planar surface in one of said paths as to maintain parallel the interfering beams of said two paths at the optical output of said interferometer, said reflecting planar surface having three points R, X and Y spaced apart on mutually perpendicular X and Y axes, said point R being at the intersection of said axes, and said points X and Y being on respective X and Y axes, the combination comprising three detectors near said optical output of said interferometer disposed on said X and Y axes as reflected to said optical output on a plane normal to the nominal beam axis at said optical output, one detector placed to receive light reflected from said point R of said reflecting surface, one of the other two detectors placed to receive light reflected from said point X of said reflecting surface and the third detector placed to receive light reflected from said point Y of said reflecting surface, means for producing a 90° phase shifted signal in response to light waves detected by said one detector from said point R comprised of means for modulating the length of said one of said optical paths in response to an AC modulating signal, and phase sensitive demodulating means responsive to said AC modulating signal for demodulating the signal output of said one detector to produce a demodulated reference detector signal, separate phase sensitive demodulating means responsive to said AC modulating signal for demodulating the signal outputs of said other two detectors to produce demodulated X and Y detector signals, separate means for multiplying the signal output of said one detector with demodulated X and Y detector signals to form respective product signals M 2x and M 2y , separate means for multiplying the undemodulated signal outputs of said other detectors with said demodulated reference detector signal to form respective product signals M 1x and M 1y , means for forming an X error signal as the difference between said signal M 2x and said signal M 1x , means for forming a Y error signal as the difference between said signal M 2y and said signal M 1y , means responsive to said X error signal for tilting said X axis in said reflecting surface through an angle proportional to the amplitude of said X error signal, and means responsive to said Y error signal for tilting said Y axis in said reflecting surface through an angle proportional to the amplitude of said Y error signal.

Claim 8

The combination of claim 5 wherein said interferometer is of the Michelson-type and said reflecting surface is a plane mirror in an optical path of fixed length.

Claim 9

The combination of claim 5 wherein said interferometer is of the type employing cat's-eye retroreflectors and said one reflecting surface is on a selected one of an input beam splitter and an output beam combiner, said beam splitter being nominally parallel to said output beam combiner.