Multi-pass optical apparatus

1. A method comprising:

reflecting an optical beam back and forth between reflecting surfaces of a first plurality of reflecting surfaces and reflecting surfaces of a second plurality of reflecting surfaces without passing the optical beam through a gain medium, the reflecting including:

re-imaging the optical beam at a reflecting surface of one of the pluralities after only one reflection from a reflecting surface of the other of the pluralities including re-imaging the optical beam from a first reflecting surface of the first plurality onto a second reflecting surface of the first plurality by reflecting the optical beam from a reflecting surface of the second plurality without medium; and

avoiding overlap of two or more optical beams on each of the reflecting surfaces of both of the pluralities.

2. The method of claim 1 , wherein the second reflecting surface of the first plurality on which the optical beam is re-imaged is adjacent to the first reflecting surface of the first plurality.

3. The method of claim 1 , wherein reflecting the optical beam back and forth between reflecting surfaces of the first plurality of reflecting surfaces and reflecting surfaces of the second plurality of reflecting surfaces includes re-imaging the optical beam from a first reflecting surface of the second plurality onto a second reflecting surface of the second plurality.

4. The method of claim 3 , wherein re-imaging the optical beam from the first reflecting surface of the second plurality onto the second reflecting surface of the second plurality includes reflecting the optical beam from a reflecting surface of the first plurality.

5. The method of claim 1 , wherein re-imaging the optical beam at a reflecting surface of one of the pluralities after only one reflection from a reflecting surface of the other of the pluralities includes forming a beam waist between the first and second pluralities of reflecting surfaces.

6. The method of claim 1 , wherein re-imaging the optical beam at a reflecting surface of one of the pluralities after only one reflection from a reflecting surface of the other of the pluralities includes forming a virtual beam waist outside a region between the first and second pluralities of reflecting surfaces.

7. The method of claim 1 , further comprising directing the optical beam through an opening within the first plurality of reflecting surfaces and toward the second plurality of reflecting surfaces prior to reflecting the optical beam back and forth.

8. The method of claim 1 , further comprising directing the optical beam through an opening within the second plurality of reflecting surfaces and away from the first plurality of reflecting surfaces after to reflecting the optical beam back and forth.

9. An apparatus comprising:

a first plurality of concave reflecting surfaces;

a second plurality of reflecting surfaces facing the first plurality of concave reflecting surfaces such that a region is defined between the first and second pluralities, wherein the region lacks a gain medium; and

an input for an optical beam to enter the region and an output for the optical beam to exit the region;

wherein:

the first and second pluralities of reflecting surfaces are arranged relative to each other so that the optical beam is re-imaged at a reflecting surface of one of the pluralities after only one reflection from a reflecting surface of the other of the pluralities including re-imaging the optical beam from a first reflecting surface of the first plurality onto a second reflecting surface of the first plurality by reflecting the optical beam from a reflecting surface of the second plurality; and

so that overlap of two or more optical beams on each of the reflecting surfaces is avoided.

10. The apparatus of claim 9 , wherein each concave reflecting surface of the first plurality is formed on a distinct substrate.

11. The apparatus of claim 10 , wherein each substrate of the first plurality is separately movable from the other substrates of the first plurality.

12. The apparatus of claim 9 , wherein each reflecting surface of the second plurality is formed on a distinct substrate.

13. The apparatus of claim 12 , wherein each substrate of the second plurality is separately movable from the other substrates of the second plurality.

14. The apparatus of claim 9 , wherein a number of concave reflecting surfaces of the first plurality is even.

15. The apparatus of claim 9 , wherein a number of reflecting surfaces of the second plurality is even.

16. The apparatus of claim 9 , wherein the concave reflecting surfaces of the first plurality are arranged so that centers of the concave reflecting surfaces are on a perimeter of a circle.

17. The apparatus of claim 16 , wherein the reflecting surfaces of the second plurality are arranged so that centers of the reflecting surfaces are on a perimeter of a circle.

18. The apparatus of claim 9 , wherein each of the reflecting surfaces of the second plurality is a concave reflecting surface.

19. The apparatus of claim 18 , wherein the first and second pluralities of reflecting surfaces are arranged relative to each other so that a beam waist of the optical beam is formed inside the region.

20. The apparatus of claim 9 , wherein each of the reflecting surfaces of the second plurality is a convex reflecting surface.

21. The apparatus of claim 20 , wherein the first and second pluralities of reflecting surfaces are arranged relative to each other so that a virtual beam waist of the optical beam is formed outside the region.

22. The apparatus of claim 9 , wherein a curvature of each of the concave reflecting surfaces of the first plurality is selected such that the optical beam is re-imaged after each reflection from a reflecting surface of the second plurality.

23. An optical source comprising:

a master oscillator that outputs an optical beam;

at least one amplifier that receives the optical beam; and

an optical delay apparatus between the master oscillator and the at least one amplifier, the optical delay apparatus comprising:

a first plurality of concave reflecting surfaces;

a second plurality of reflecting surfaces facing the first plurality of concave reflecting surfaces, such that a region is defined between the first and second pluralities; and

an input for the optical beam to enter the region and an output for the optical beam to exit the region;

wherein:

the first and second pluralities of reflecting surfaces are arranged relative to each other so that the optical beam is re-imaged at a reflecting surface of one of the pluralities after only one reflection from a reflecting surface of the other of the pluralities including re-imaging the optical beam from a first reflecting surface of the first plurality onto a second reflecting surface of the first plurality by reflecting the optical beam from a reflecting surface of the second plurality; and

so that overlap of two or more optical beams on each of the concave reflecting surfaces is avoided.

24. The apparatus of claim 9 , wherein the foci of each of the reflecting surfaces of both pluralities are along a plane between the two pluralities.