Long time aperture optical tapped delay line
An optical tapped delay line device in accordance with the present invention is a method for and device for spatially resolving the spectral components of an optical signal, i.e., channelizing or spectrum analyzing the wavelength content of an optical signal. The device is based on a tapped optical delay line and enables numerous related optical signal processing functions.
1. A device comprising:
an optical cell having a cavity and an output plane and configured to direct a collimated input beam on a beam path comprising multiple round trips through the cavity, the collimated input beam having a diameter, the beam path establishing at the output plane multiple output taps that are spatially distributed and temporally separated,
a beam divergence controller located within the cavity that controls beam divergence in the cell by re-imaging the beam on each round trip through the cavity, and
a near-retroreflector, configured such that a given incident ray path associated with the near-retroreflector is not parallel to a corresponding outgoing ray path associated with the near-retroreflector, forming an end of the cavity opposite the output plane,
wherein each of the multiple output taps has a diameter substantially not greater than the diameter of the input beam.
2. The device of claim 1 wherein the beam divergence controller comprises a lens through which the beam travels at least one time.
3. The device of claim 2 wherein the near-retroreflector comprises a dihedral minor pair and an angle between mirrors of the dihedral mirror pair is 90 degrees plus or minus δθ wherein
δθ=Δ/2 f
where
Δ is a spacing of the taps; and
f is a focal length of the lens.
4. The device of claim 3 wherein
Δ≧πω o
where π o is a radius of an input Gaussian beam waist.
5. The device of claim 1 wherein the beam divergence controller comprises a lens through which the beam travels multiple times.
6. The device of claim 1 wherein the beam divergence controller is configured to control beam divergence so that each of the output taps has substantially the same diameter.
7. The device of claim 1 wherein the beam divergence controller is configured to control beam divergence so that each of the output taps has a diameter substantially equal to the diameter of the input beam.
8. The device of claim 1 wherein the beam divergence controller is configured to control beam divergence so that the multiple output taps are mutually parallel.
9. The device of claim 1 wherein the beam divergence controller is configured to control beam divergence so that each of the multiple output taps has substantially the same waist size and each of the waists are in a common plane.
10. The device of claim 1 wherein the beam path is not less than one centimeter.
11. The device of claim 1 wherein the beam path is not less than one meter.
12. The device of claim 1 wherein the beam path is not less than twenty meters.
13. The device of claim 1 wherein the beam divergence controller is configured to control beam divergence so that the device has a free spectral range lower than about 10 GHz.
14. The device of claim 1 wherein the beam divergence controller is configured to control beam divergence so that the device has a free spectral range between about 5 GHz and about 10 GHz.
15. The device of claim 1 wherein the beam divergence controller is configured to control beam divergence so that the device has a spectral resolution .rho. of about 100 kHz, a time-bandwidth product (FSR/ρ) of about 100,000 and a time aperture (1/ρ) of about 10 microseconds, where FSR represents free spectral range.
16. The device of claim 1 wherein the beam divergence controller is configured to control beam divergence so that each of the output taps has a diameter not greater than 1.1 times the diameter of the input beam.
17. The device of claim 1 wherein the beam divergence controller is configured to control beam divergence so that each of the output taps has a diameter not greater than 1.5 times the diameter of the input beam.
18. The device of claim 1 wherein the near-retroreflector comprises a dihedral minor pair.
19. The device of claim 18 wherein an angle between mirrors of the dihedral minor pair is 90 degrees plus or minus δθ where δθ is a nonzero deviation angle.
20. A method comprising:
providing an optical cell having a cavity, an output plane, and a near-retroreflector forming an end of the cavity opposite the output plane, the near-retroreflector comprising a dihedral minor pair having an oblique angle between first and second minors of the mirror pair that differs from ninety degrees by a small deviation angle, δΓ;
directing a collimated input beam on a beam path comprising multiple round trips through the cavity, the collimated input beam having a diameter, the beam path establishing at the output plane multiple output taps that are spatially distributed and temporally separated; and
controlling beam divergence in the cell by re-imaging the beam on each round trip through the cavity, such that each of the multiple output taps has a diameter substantially not greater than the diameter of the input beam.
21. The method of claim 20 wherein the step of controlling beam divergence comprises providing a lens through which the beam travels at least one time.
22. The method of claim 21 wherein
δθΔ/2 f
where
Δ is a spacing of the taps; and
ƒ is a focal length of the lens.
23. The method of claim 22 wherein
Δ≧πω o
where ω o is a radius of an input Gaussian beam waist.
24. The method of claim 20 wherein the step of controlling beam divergence comprises providing a lens through which the beam travels multiple times.
25. The method of claim 20 wherein the step of controlling beam divergence comprises controlling beam divergence so that each of the output taps has substantially the same diameter.
26. The method of claim 20 wherein the step of controlling beam divergence comprises controlling beam divergence so that each of the output taps has a diameter substantially equal to the diameter of the input beam.
27. The method of claim 20 wherein the step of controlling beam divergence comprises controlling beam divergence so that the multiple output taps are mutually parallel.
28. The method of claim 20 wherein the step of controlling beam divergence comprises controlling beam divergence so that each of the multiple output taps has substantially the same waist size and each of the waists are in a common plane.
29. The method of claim 20 wherein the beam path is not less than one centimeter.
30. The method of claim 20 wherein the beam path is not less than one meter.
31. The method of claim 20 wherein the beam path is not less than twenty meters.
32. The method of claim 20 wherein the step of controlling beam divergence comprises controlling beam divergence so that the cell has a free spectral range lower than about 10 GHz.
33. The method of claim 20 wherein the step of controlling beam divergence comprises controlling beam divergence so that the cell has a free spectral range between about 5 GHz and about 10 GHz.
34. The method of claim 20 wherein the step of controlling beam divergence comprises controlling beam divergence so that the cell has a spectral resolution .rho. of about 100 kHz, a time-bandwidth product (FSR/ρ) of about 100,000 and a time aperture (1/ρ) of about 10 microseconds, where FSR represents free spectral range.
35. The method of claim 20 wherein the step of controlling beam divergence comprises controlling beam divergence so that each of the output taps has a diameter not greater than 1.1 times the diameter of the input beam.
36. The method of claim 20 wherein the step of controlling beam divergence comprises controlling beam divergence so that each of the output taps has a diameter not greater than 1.5 times the diameter of the input beam.
37. A device comprising:
a planar coupler minor positioned at a first end of an optical cell, the planar coupler having a first surface and a second surface;
a dihedral mirror pair, comprising first and second mirrors, positioned at a second end of the optical cell, the two mirrors being positioned with edges at a common vertex to form a first, reflex angle and a second, oblique angle, with the second angle being smaller than the first angle, the first and second mirrors being oriented such that the surfaces of the mirrors forming the second angle faces the first surface of the planar coupler minor across the optical cell; and
a lens located within a cavity of the optical cell between the planar coupler minor and the dihedral minor pair and configured to control beam divergence in the optical cell.
38. The device of claim 37 , wherein each of the planar coupler mirror and the dihedral minor pair are configured to direct a collimated input beam on a beam path comprising multiple round trips through the cavity, the collimated input beam having a diameter, the beam path establishing at the second surface of the planar coupler minor multiple output taps that are spatially distributed and temporally separated, each of the multiple output taps having a diameter substantially not greater than the diameter of the input beam.
39. The device of claim 37 , wherein the lens has an associated focal length, and the lens is positioned such that each of a vertex of the dihedral mirror pair and the first surface of the planar coupler mirror is separated from the lens by a distance substantially equal to the focal length of the lens.
40. The device of claim 37 , wherein the beam path is not less than twenty meters.