APPARATUS AND METHOD FOR A WAVEGUIDE WITH AN INDEX PROFILE MANIFESTING A CENTRAL DIP FOR BETTER ENERGY EXTRACTION

• US 20070206912A1
• Filed: 11/03/2006
• Published: 09/06/2007
• Est. Priority Date: 11/03/2005
• Status: Active Grant

• Current Assignee: Lockheed Martin Aculight Corp. (Martin Marietta Corporation)
• Original Assignee: Aculight Corporation (Martin Marietta Corporation)
• Inventors: Savage-Leuchs, Matthias, Eisenberg, Eric, Henrie, Jason, Jenson, Barton, Minelly, John
• Granted Patent: US 7,400,807 B2
• Time in Patent Office: Days
• US Class Current: 385/124
• CPC Class Codes: G02B 6/02023 Based on higher order modes, i.e. propagating modes other than the LP01 or HE11 fundamental mode mode converters G02B6/14
• CPC Class Codes: G02B 6/03605 Highest refractive index not on central axis
• CPC Class Codes: G02B 6/03611 Highest index adjacent to central axis region, e.g. annular core, coaxial ring, centreline depression affecting waveguiding
• CPC Class Codes: G02B 6/03633 arranged - -
• CPC Class Codes: G02B 6/03638 having 3 layers only
• CPC Class Codes: G02B 6/03661 having 4 layers only
• CPC Class Codes: G02B 6/03688 having 5 or more layers
• CPC Class Codes: G02B 6/03694 Multiple layers differing in properties other than the refractive index, e.g. attenuation, diffusion, stress properties
• CPC Class Codes: H01S 3/06729 Peculiar transverse fibre profile

Abstract

A method and apparatus is described that use an index-of-refraction profile having a significant central dip in refractive index (or another tailored index profile) within the core of a gain fiber or a gain waveguide on a substrate. The benefits of this central dip (more power with a given mode structure) are apparent when an input beam is akin to that of a Gaussian mode. In some embodiments, the invention provides a fiber or a substrate waveguide having an index profile with a central dip, but wherein the device has no doping. Some embodiments use a central dip surrounded by a higher-index ring in the index of refraction of the core of the fiber, while other embodiments use a trench between an intermediate-index central core portion and the ring, or use a plurality of rings and/or trenches. Some embodiments use an absorber in at least one core ring.

Claims

1. An apparatus comprising:

an optical device configured to operate in a band of wavelengths corresponding to a gain band of one or more active rare-earth species, wherein the device includes a multimode signal waveguide having a central dip in the refractive index profile, the central-dip signal waveguide having a characteristic diameter.

2. The apparatus of claim 1, wherein the optical device is a fiber, and the signal waveguide is a core of the fiber that includes a doping of active stimulated-emission species, the apparatus further comprising:

a source of optical pump radiation suitable to pump the active simulated-emission species to an excited state.

3. The apparatus of claim 2, wherein the core includes a co-doping of erbium and ytterbium (an ErYb co-doped fiber).

4. The apparatus of claim 2, wherein the fiber, when excited by a Gaussian beam, preferentially excites an LP02-like mode of the core.

5. The apparatus of claim 4, wherein the fiber is configured such that the LP02-like mode experiences a lower effective nonlinear index than it would in a substantially similar fiber having a corresponding step profile.

6. The apparatus of claim 4, wherein the LP02-like mode experiences a higher dispersion slope than in a substantially similar fiber having a corresponding step profile.

7. The apparatus of claim 4, wherein an effective area of the mode divided by the number of propagating modes is enhanced relative to a corresponding step profile.

8. The apparatus of claim 4, wherein an LP02-like mode of the fiber has a stronger overlap with ions of the active stimulated-emission species located in the wings of the profile relative to the overlap of an LP01 mode in a fiber having a corresponding step profile.

9. The apparatus of claim 4, wherein an LP02-like mode of the fiber experiences less backscatter losses than the LP01 mode of a corresponding step profile.

10. The apparatus of claim 3, wherein an energy transfer from Yb to Er in the fiber having the central-dip core is greater than an energy transfer from Yb to Er in a fiber having a step-index-of-refraction core having an “

equivalent step profile”

in which the diameter of the core of the step-index-of-refraction profile has the same characteristic diameter as the central-dip core but where the numerical aperture of the step-index-of-refraction core is lowered by an amount that provides the step-index core with the same number of modes as the central-dip core.

11. The apparatus of claim 10, wherein an advantage related to the rare-earth dopants “

following”

the index-of-refraction profile enables stronger Yb-Er energy transfer.

12. The apparatus of claim 1, wherein the device includes no doping of active doping species (such as erbium or ytterbium).

13. The fiber amplifier of claim 1, wherein the fiber includes no doping of active doping species (such as erbium or ytterbium).

14. The fiber amplifier of claim 1, wherein an index pedestal is provided in an inner cladding region adjacent to the core

15. The fiber amplifier of claim 14, wherein an LP01 mode produced by the amplifier has a reduced central intensity dip compared with that produced by a similar fiber amplifier without the index pedestal.

16. The fiber amplifier of claim 15, wherein an LP01 mode is approximately Gaussian.

17. The fiber amplifier of claim 14, wherein the core of said structure is mathematically single-moded.

18. The apparatus of claim 1, the amplifying medium of the core includes doping of one or more species selected from the group consisting of Er, Yb, Tm, Nd, Ho, Pr, and Sm.

19. The fiber amplifier of claim 1, wherein the fiber includes Er doping that amplifies in a wavelength range of about 1500 nm to about 1700 nm.

20. The fiber amplifier of claim 1, wherein the fiber includes substantially only Yb doping that amplifies in the wavelength range of about 1020 nm to about 1100 nm.

21. A pulsed fiber amplifier operating in a band of wavelengths from about 1532 nm to about 1620 nm, in which a multimode doped core contains a central dip in the refractive index profile.

22. The fiber amplifier of claim 21, wherein the fiber includes a co-doping of erbium and ytterbium (an ErYb co-doped fiber).

23. The fiber amplifier of claim 22, wherein the fiber is excited by a Gaussian beam that preferentially excites an LP02-like mode of the core.

24. The fiber amplifier of claim 23, wherein the mode experiences a lower effective nonlinear index than it would in a substantially similar fiber having a corresponding step profile.

25. The fiber amplifier of claim 23, wherein the mode experiences a higher dispersion slope than a corresponding step profile.

26. The fiber amplifier of claim 23, wherein the effective area of the mode divided by the number of propagating modes is enhanced relative to a corresponding step profile.

27. The fiber amplifier of claim 23, wherein an LP02-like mode of the fiber has a stronger overlap with the ions located in the wings of the profile relative to the overlap of the LP01 mode of a corresponding step profile.

28. The fiber amplifier of claim 23, wherein an LP02-like mode of the fiber experiences less backscatter losses than the LP01 mode of a corresponding step profile.

29. The fiber amplifier of claim 23, wherein an energy transfer from Yb to Er is greater than that for a step-index fiber having an “

equivalent step profile”

wherein the profile with a dip is compared to an “

equivalent step profile fiber”

in which the diameter of the profile is maintained, but the numerical aperture is reduced until the profile of the “

equivalent step profile fiber”

provides an equal number of modes as the actual profile, such that the advantage related to the rare-earth dopants “

following”

the profile so that the local composition enables strong Yb-Er energy transfer.

30. The fiber amplifier of claim 21, wherein the amplifier operates in a band of wavelengths from about 1532 nm to about 1620 nm.