Coherence delay augmented laser beam homogenizer

• US 5,224,200 A
• Filed: 11/27/1991
• Issued: 06/29/1993
• Est. Priority Date: 11/27/1991
• Status: Expired due to Fees

• Current Assignee: United States Of America As Represented By The Department Of Energy
• Original Assignee: United States Of America As Represented By The Department Of Energy
• Inventors: Bernhardt, Anthony, Rasmussen, Paul
• Primary Examiner(s): Epps, Georgia Y.
• Application Number:
  US07/799,524
• Time in Patent Office: 580 Days
• Field of Search: 372/29, 372/107, 372/6, 372/99, 372/93, 372/9, 385/146
• US Class Current: 385/146
• CPC Class Codes: G02B 27/09 Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for adapting the beam shape of a laser diode G02B19/0052; adapting the beam shape of an LED G02B19/0061; coupling into light guides using intermediate optical elements G02B6/4204; beam shaping specially adapted for lasers H01S3/005
• CPC Class Codes: G02B 27/0927 Systems for changing the beam intensity distribution, e.g. Gaussian to top-hat
• CPC Class Codes: G02B 27/0977 Reflective elements
• CPC Class Codes: G02B 27/0994 Fibers, light pipes optical fibers per se G02B6/02
• CPC Class Codes: G02B 27/48 Laser speckle optics
• CPC Class Codes: G02B 5/005 Diaphragms for cameras G03B9/02
• CPC Class Codes: G02B 6/00 Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
• CPC Class Codes: G02B 6/2861 using fibre optic delay lines and optical elements associated with them, e.g. for use in signal processing, e.g. filtering delay lines in general H03H, H01P9/00; optical computing devices G06E
• CPC Class Codes: G03F 7/70058 Mask illumination systems
• CPC Class Codes: G03F 7/70583 Speckle reduction, e.g. coherence control or amplitude/wavefront splitting
• CPC Class Codes: H01S 3/005 Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping shaping laser beam for working metal or other materials B23K26/06; optical elements, systems or apparatus in general G02B

Abstract

The geometrical restrictions on a laser beam homogenizer are relaxed by ug a coherence delay line to separate a coherent input beam into several components each having a path length difference equal to a multiple of the coherence length with respect to the other components. The components recombine incoherently at the output of the homogenizer, and the resultant beam has a more uniform spatial intensity suitable for microlithography and laser pantogography. Also disclosed is a variable aperture homogenizer, and a liquid filled homogenizer.

Claims

1. An optical system comprising a light beam, a coherence delay line, and a homogenizer, wherein:

said coherence delay line comprises a first partially reflecting mirror and a second totally reflecting mirror, and;

said homogenizer comprises a light tunnel with transparent ends and reflective sidewalls, wherein;

at least a portion of said light beam is incident upon said partially reflecting mirror at a first position where a portion is transmitted into said homogenizer, and a portion is reflected into said totally reflecting mirror.

2. The system of claim 1 wherein said light beam has a coherence length greater than 3 millimeters, and said first and second mirrors are separated by an integral multiple of half of said coherence length.

3. The system of claim 1 wherein the portion of said light beam that is reflected into said totally reflecting mirror is reflected back into said partially reflecting mirror at a second position different from the first position, where a portion is transmitted into said homogenizer, and a portion is reflected back into said totally reflecting mirror.

4. The system of claim 3 wherein the reflection and transmission process is repeated until a portion of said light beam is reflected directly from said totally reflecting mirror into said homogenizer.

5. The system of claim 4 wherein all portions of said light beam that are transmitted through said partially reflecting mirror are substantially equal in intensity.

6. The system of claim 5 wherein the thickness of the partially reflective coating on said partially reflective mirror decreases at each successive position of incidence by said beam.

7. The system of claim 1 wherein said homogenizer comprises a light tunnel with a variable cross section.

8. The system of claim 7 wherein said light tunnel is a polygonal cavity formed by four rods with rectangular cross sections that are contiguous with, and staggered with respect to their neighbors, and said rods are coated with a reflective coating on at least the sides forming said light tunnel sidewalls.

9. The system of claim 1 wherein said light tunnel is a polygonal cavity filled with a liquid having a greater index of refraction than the material forming said light tunnel sidewalls.

10. A coherence delay line comprising a laser beam, a first partially reflective mirror, and a second totally reflective mirror, wherein:

said first and second mirrors have planar surfaces facing each other, and;

said laser beam is incident upon said first mirror at a first position where a portion is transmitted, and a portion is reflected towards, and then reflected from said second mirror, wherein;

the portion reflected from said second mirror is parallel to said incident laser beam.

11. The delay line of claim 10 wherein the portion reflected from said second mirror strikes said first mirror at a position different from said first position, where again a portion is transmitted, and a portion is reflected towards, and then reflected from said second mirror.

12. The delay line of claim 11 wherein the beam is successively reflected back and forth between said first and second mirrors, and each time the beam strikes said first mirror a portion is transmitted, and a portion is reflected towards, and then reflected from said second mirror.

13. The delay line of claim 12 wherein the transmitted portions all have substantially equal intensities.

14. The delay line of claim 10 wherein the reflective coatings on said mirrors are made from quarter wavelength thin films.

15. A homogenizer comprising a polygonal cavity either filled with a gas, and having a reflective coating on the cavity walls, or;

filled with a liquid having an index of refraction greater than the material of the cavity walls.

16. The homogenizer of claim 15 comprising a plurality of polygonal rods that are contiguous with, and staggered with respect to their neighbors to form a polygonal cavity with adjustable dimensions.

17. The homogenizer of claim 16 wherein the cross section of said polygonal cavity is rectangular.