Real-time diffraction interferometer

• US 4,624,569 A
• Filed: 07/18/1983
• Issued: 11/25/1986
• Est. Priority Date: 07/18/1983
• Status: Expired due to Fees

• Current Assignee: Lockheed Missiles and Space Co Inc (Martin Marietta Corporation)
• Original Assignee: Lockheed Missiles and Space Co Inc (Martin Marietta Corporation)
• Inventors: Kwon, Osuk Y.
• Primary Examiner(s): Willis, Davis L.
• Assistant Examiner(s): Koren, Matthew W.
• Application Number:
  US06/514,875
• Time in Patent Office: 1,226 Days
• Field of Search: 356/345, 356/353, 356/354, 356/359, 356/360
• US Class Current: 356/521
• CPC Class Codes: G01J 2009/0223 Common path interferometry; Point diffraction interferometry
• CPC Class Codes: G01J 2009/0234 Measurement of the fringe pattern
• CPC Class Codes: G01J 9/02 by interferometric methods using interferometers for measuring optically the linear dimensions of objects G01B9/02

Abstract

A real-time diffraction interferometer for analyzing an optical beam comprises converging means (13) for bringing the beam to a focus at focal point (14), and an apertured grating structure (20) positionable adjacent the focal point (14). The apertured grating structure (20) comprises a transparent substrate (10'"'"'), an obverse surface of which is coated with a translucent coating (11) except for a pinhole-sized spot (12) that is left uncoated so as to function as an aperture in the coating (11). A reverse surface of the substrate (10'"'"') has a lenticulate surface configuration, which functions as a diffraction grating. The beam incident upon the apertured grating structure (20) is separated into a major portion, which is transmitted with attenuated intensity through the translucent coating (11), and a minor portion, which is transmitted with undiminished intensity through the pinhole aperture (12). The major portion of the beam is diffracted into spatially separated diffraction components, and the minor portion of the beam is diffracted by the pinhole aperture (12) so as to acquire a spherical wavefront. Interference patterns produced by interference of the spherical wavefront with each of the wavefronts of the zeroth order and the positive and negative first-order diffraction components of the intensity-attenuated beam transmitted by the coating (11) are separately imaged on conventional solid-state photodetectors (21, 22 and 23).

Claims

1. Apparatus for monitoring quality of an optical beam wavefront, said apparatus comprising:

(a) converging means for bringing said beam to a focus;

(b) separating means positionable in said beam downstream of said converging means, said separating means being only partially transparent to said beam except for a pinhole-apertured portion of said separating means that is substantially transparent to said beam, said separating means comprising homogeneous diffraction grating means of uniform grating period for causing a first portion of said beam to be separated into a plurality of components, said components of said first portion of said beam being of different phases with respect to each other, said pinhole-apertured portion of said separating means defining an aperture that is dimensioned to function as a point diffraction means for causing a second portion of said beam to be transmitted with a substantially spherical wavefront, said second portion of said beam undergoing interference with each of said components of said first portion of said beam; and

(c) detecting means positionable downstream of said separating means, said detecting means separately indicating the interference of each of said components of said first portion of said beam with said second portion of said beam.

2. The apparatus of claim 1 wherein said aperture has a maximum dimension smaller than said grating period.

3. The apparatus of claim 1 wherein said separating means comprises a substrate that is translucent to said beam, said substrate supporting said homogeneous diffraction grating means of uniform grating period, said aperture being positioned on said substrate with respect to said diffraction grating so as to determine the phase difference between any two of said components of said first portion of said beam.

4. The apparatus of claim 3 wherein a surface of said substrate is lenticulate to define said diffraction grating.

5. The apparatus of claim 3 wherein said substrate comprises a material that is substantially transparent to said beam, an area on one side of said substrate being coated with a translucent coating except for a portion of said one side within said coated area that is left uncoated, said coating providing intensity attenuation of said first portion of said beam, said uncoated portion of said one side of said substrate functioning as said aperture.

6. The apparatus of claim 5 wherein said translucent coating attenuates the intensity of said first portion of said beam so that each of said components of said first portion of said beam produced by said diffraction grating is approximately equal in intensity to said second portion of said beam transmitted through said aperture.

7. The apparatus of claim 5 wherein said translucent coating is provided on an obverse side of said substrate, and wherein a reverse side of said substrate is configured as said diffraction grating.

8. The apparatus of claim 7 wherein said reverse side of said substrate is lenticulate.

9. The apparatus of claim 5 wherein said translucent coating is provided on said surface of said substrate that is configured as said diffraction grating.

10. The apparatus of claim 9 wherein said surface configured as said diffraction grating is lenticulate.

11. The apparatus of claim 1 wherein said detecting means comprises a plurality of photodetectors positioned so that each photodetector indicates the interference of a corresponding one of said components of said first portion of said beam with said second portion of said beam.

12. An apertured grating structure for separating an optical beam into first and second portions, said grating structure having a homogeneous diffraction grating of uniform grating period for causing said first portion of said beam to be diffracted into a plurality of components corresponding to different diffraction orders, said components of said first portion of said beam produced by said diffraction grating being of different phases with respect to each other, said grating structure being only partially transparent to said beam except for an apertured portion of said grating structure that is substantially transparent to said beam, said apertured portion of said grating structure defining a pinhole aperture that is dimensioned to cause said second portion of said beam to have a substantially spherical wavefront, said second portion of said beam interfering with each of said components of said first portion of said beam so as to produce a plurality of separate interference patterns of different phases with respect to each other, said aperture being positioned on said grating structure with respect to said diffraction grating so as to enable a selected phase difference between any two of said components of said first portion of said beam to be obtained.

13. The apertured grating structure of claim 12 wherein said grating structure has a surface configured as said diffraction grating.

14. The apertured grating structure of claim 13 comprising a substrate that is substantially transparent to said beam, an area on one side of said substrate being coated with a translucent coating except for a portion of said one side within said coated area that is left uncoated, said coating causing attenuation in intensity of said optical beam so that each of said plurality of components of said first portion of said beam has an intensity approximately equal to the intensity of said second portion of said beam, said uncoated portion of said one side of said substrate defining said aperture.

15. The apertured grating structure of claim 14 wherein said translucent coating is provided on an obverse side of said substrate, and wherein said surface configured as said diffraction grating is provided on a reverse side of said substrate.

16. The apertured grating structure of claim 15 wherein said reverse side of said substrate is lenticulate.

17. The apertured grating structure of claim 17 wherein said translucent coating is provided on said surface that is configured as said diffraction grating.

18. The apertured grating structure of claim 17 wherein said surface configured as said diffraction grating is lenticulate.