Optical gain fiber having segments of differing core sizes and associated method

• US 8,199,399 B1
• Filed: 06/14/2011
• Issued: 06/12/2012
• Est. Priority Date: 11/30/2006
• Status: Active Grant

• Current Assignee: Lockheed Martin Aculight Corp. (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Inventors: Savage-Leuchs, Matthias P.
• Primary Examiner(s): Hellner, Mark
• Application Number:
  US13/160,458
• Time in Patent Office: 364 Days
• Field of Search: 359/333, 359/341.1, 359/337.4, 372/6
• US Class Current: 359/341.1
• CPC Class Codes: B29C 48/10 flexible, e.g. blown foils
• CPC Class Codes: B29C 48/71 for layer multiplication extrusion of multi-component articles B29C48/16
• CPC Class Codes: H01S 2301/02 ASE (amplified spontaneous emission), noise; Reduction thereof
• CPC Class Codes: H01S 3/06716 Fibre compositions per se C03C13/04or doping with active elements lasing materials in general H01S3/14
• CPC Class Codes: H01S 3/06729 Peculiar transverse fibre profile
• CPC Class Codes: H01S 3/06733 Fibre having more than one cladding
• CPC Class Codes: H01S 3/06741 Photonic crystal fibre, i.e. the fibre having a photonic bandgap
• CPC Class Codes: H01S 3/06745 Tapering of the fibre, core or active region
• CPC Class Codes: H01S 3/0675 Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
• CPC Class Codes: H01S 3/06754 Fibre amplifiers H01S3/06708 takes precedence
• CPC Class Codes: H01S 3/06758 Tandem amplifiers
• CPC Class Codes: H01S 3/094007 Cladding pumping, i.e. pump light propagating in a clad surrounding the active core
• CPC Class Codes: H01S 3/094019 Side pumped fibre, whereby pump light is coupled laterally into the fibre via an optical component like a prism, or a grating, or via V-groove coupling
• CPC Class Codes: H01S 3/094053 Fibre coupled pump, e.g. delivering pump light using a fibre or a fibre bundle
• CPC Class Codes: H01S 3/09415 the pumping beam being parallel to the lasing mode of the pumped medium, e.g. end-pumping

Abstract

Apparatus and method for amplifying laser signals using segments of fibers of differing core diameters and/or differing cladding diameters to suppress amplified spontaneous emission and non-linear effects such as four-wave mixing (FWM), self-phase modulation, and stimulated Brillouin and/or Raman scattering (SBS/SRS). In some embodiments, different core sizes have different sideband spacings (spacing between the desired signal and wavelength-shifted lobes). Changing core sizes and providing phase mismatches prevent buildup of non-linear effects. Some embodiments further include a bandpass filter to remove signal other than the desired signal wavelength and/or a time gate to remove signal at times other than during the desired signal pulse. Some embodiments include photonic-crystal structures to define the core for the signal and/or the inner cladding for the pump. Some embodiments include an inner glass cladding to confine the signal in the core and an outer glass cladding to confine pump light in the inner cladding.

Claims

1. A method comprising:

providing a plurality of amplifying fiber segments including a first amplifying segment and a second amplifying segment, wherein the first amplifying segment includes a first core that amplifies light of a signal-light wavelength and that is surrounded by a first inner cladding, wherein the first inner cladding is surrounded by a first outer cladding, wherein the first inner cladding includes a first defined absorbing structure that is doped with a species that absorbs light of the signal-light wavelength, wherein the second segment includes a second core that amplifies light of a signal-light wavelength and that is surrounded by a second inner cladding, wherein the second inner cladding is surrounded by a second outer cladding;

coupling pump light into the first core from the first inner cladding along a length of the first core;

coupling pump light into the second core from the second inner cladding along a length of the second core;

coupling signal light of a signal-light wavelength into the first core;

amplifying the signal light in the first amplifying segment to generate first-amplified signal light;

coupling the first-amplified signal light from the first core into the second core;

further amplifying the first-amplified signal light in the second amplifying segment to generate second-amplified signal light, wherein at least some light having the signal-light wavelength travels in the second amplifying segment as backwards-traveling second-segment signal-wavelength light;

coupling at least some of the backwards-traveling second-segment signal-wavelength light into the first inner cladding from the second core; and

absorbing, in the first defined absorbing structure, at least some of the backwards-traveling second-segment signal-wavelength light that was coupled into the first inner cladding from the second core.

2. The method of claim 1, wherein the second inner cladding includes a second absorbing structure that is doped with a species that absorbs light of the signal-light wavelength, wherein the plurality of amplifying fiber segments further includes a third amplifying segment that is joined to the second segment, wherein the third amplifying segment includes a third core that is surrounded by a third inner cladding, wherein the third inner cladding is surrounded by a third outer cladding, the method further comprising:

coupling pump light into the third core from the third inner cladding along the length of the third core;

coupling the second-amplified signal light from the second core into the third core;

further amplifying the second-amplified signal light in the third amplifying segment to generate third-amplified signal light, wherein at least some light of the signal-light wavelength travels in the third amplifying segment as backwards-traveling third-segment signal-wavelength light;

coupling at least some of the backwards-traveling third-segment signal-wavelength light into the second inner cladding from the third core; and

absorbing, in the second absorbing structure, at least some of the backwards-traveling third-segment signal-wavelength light that was coupled into the second inner cladding from the third core.

3. The method of claim 1, wherein the first amplifying segment and the second amplifying segment are joined to one another at a first junction, wherein an outer diameter of the first inner cladding of the first amplifying segment at the first junction is smaller than an outer diameter of the second inner cladding of the second amplifying segment at the first junction, and wherein the coupling of pump light into the second core further includes coupling pump light into an end of the second inner cladding at the first junction from outside the outer diameter of the first inner cladding of the first segment.

4. The method of claim 1, wherein the first amplifying segment and the second amplifying segment are joined to one another, the method further comprising tapering a core diameter from a larger first-segment core diameter to a smaller second-segment core diameter along a first transition length between the first amplifying segment and the second amplifying segment.

5. The method of claim 1, further comprising a fourth segment, wherein the fourth segment includes a fourth core that is surrounded by a fourth inner cladding, wherein the fourth inner cladding is surrounded by a fourth outer cladding, wherein the first amplifying segment and the fourth segment are joined to one another, wherein the fourth segment and the second amplifying segment are joined to one another, the method further comprising:

tapering a signal-waveguide diameter from a larger first-segment core diameter to a smaller fourth-segment core diameter along a first transition length between the first amplifying segment and the fourth segment; and

tapering the signal-waveguide diameter from the smaller fourth-segment core diameter to a larger second-segment core diameter along a second transition length, and wherein the first transition length is longer than the second transition length.

6. The method of claim 1, wherein the first-amplified signal light has a desired first signal wavelength, the method further comprising bandpass-wavelength filtering the first-amplified signal light in order to remove light having wavelengths other than the desired first signal wavelength.

7. The method of claim 1, further comprising gating light transmission between the first amplifying segment and second amplifying segment such that more light of the signal-light wavelength is transmitted during a signal-pulse time as compared to other times.

14. The apparatus of claim 1, further comprising a light gate that increases transmission of light of the signal-light wavelength between the first gain-fiber segment and second gain-fiber segment during a signal-pulse time as compared to other times.

8. An apparatus comprising:

an optical-fiber amplifier having a plurality of optically coupled gain-fiber segments including;

a first gain-fiber segment, wherein the first gain-fiber segment has a first core that is configured to amplify light of a signal-light wavelength, and that is surrounded by a first inner cladding, wherein the first inner cladding is surrounded by a first outer cladding, and wherein the first inner cladding includes a first defined absorbing structure that is doped with a species that absorbs light of the signal-light wavelength, and

a second gain-fiber segment that is optically coupled to the first segment, wherein the second segment has a second core that is surrounded by a second inner cladding, wherein the second inner cladding is surrounded by a second outer cladding,

wherein pump light is coupled into the first core from the first inner cladding along a length of the first core, wherein pump light is coupled into the second core from the second inner cladding along a length of the second core, wherein signal light of a signal-light wavelength is coupled into the first core, wherein the signal light is amplified in the first gain-fiber segment to generate first-amplified signal light, wherein the first-amplified signal light is coupled from the first core into the second core, wherein the first-amplified signal light is further amplified in the second segment to generate second-amplified signal light, wherein at least some light having the signal-light wavelength travels in the second gain-fiber segment as backwards-traveling second-segment signal-wavelength light, and wherein at least some of the backwards-traveling second-segment signal-wavelength light is coupled into the first inner cladding from the second core and absorbed in the first defined absorbing structure.

9. The apparatus of claim 8, wherein the second inner cladding includes a second absorbing structure that is contiguous along a length of the second gain-fiber segment and doped with a species that absorbs light of the signal-light wavelength, the apparatus further comprising:

a third gain-fiber segment that is joined to the second gain-fiber segment, wherein the third gain-fiber segment has a third core that is surrounded by a third inner cladding, wherein the third inner cladding is surrounded by a third outer cladding, wherein pump light is coupled into the third core from the third inner cladding along a length of the third core, wherein second-amplified signal light is coupled from the second core into the third core, wherein the second-amplified signal light is further amplified to generate third-amplified signal light, wherein at least some light of the signal-light wavelength travels in the third gain-fiber segment as backwards-traveling third-segment signal-wavelength light, and wherein at least some of the backwards-traveling third-segment signal-wavelength light is coupled into the second inner cladding from the third core and absorbed in the second absorbing structure.

10. The apparatus of claim 8, wherein the first gain-fiber segment and the second gain-fiber segment are joined to one another at a first junction, wherein an outer diameter of the first inner cladding of the first gain-fiber segment at the first junction is smaller than an outer diameter of the second inner cladding of the second gain-fiber segment at the first junction, and wherein pump light is coupled into an end of the second inner cladding at the first junction from outside the outer diameter of the first inner cladding of the first gain-fiber segment.

11. The apparatus of claim 8, wherein the first gain-fiber segment and the second gain-fiber segment are joined to one another, wherein a diameter of the first core is tapered from a larger first-segment core diameter to a smaller second-segment core diameter along a first transition length between the first gain-fiber segment and the second gain-fiber segment.

12. The apparatus of claim 11, further comprising a fourth segment, wherein the fourth segment includes a fourth core that is surrounded by a fourth inner cladding, wherein the fourth inner cladding is surrounded by a fourth outer cladding, wherein the first amplifying segment and the fourth segment are joined to one another, wherein the fourth segment and the second amplifying segment are joined to one another, wherein a signal-waveguide diameter is tapered from a larger first-segment core diameter to a smaller fourth-segment core diameter along a first transition length between the first amplifying segment and the fourth segment, and wherein the signal-waveguide diameter is tapered from a smaller fourth-segment core diameter to a larger second-segment core diameter along a second transition length, and wherein the first transition length is longer than the second transition length.

13. The apparatus of claim 8, wherein the first-amplified signal light has a desired first signal wavelength, the apparatus further comprising a bandpass-wavelength filter configured to filter the first-amplified signal light in order to remove light having wavelengths other than the desired first signal wavelength.

15. An apparatus comprising:

a plurality of amplifying fiber segments including a first amplifying segment and a second amplifying segment, wherein the first amplifying segment includes a first core and that is surrounded by a first inner cladding, wherein the first inner cladding is surrounded by a first outer cladding, wherein the second amplifying segment includes a second core that is surrounded by a second inner cladding, wherein the second inner cladding is surrounded by a second outer cladding;

means for coupling pump light into the first core from the first inner cladding along a length of the first core;

means for coupling pump light into the second core from the second inner cladding along a length of the second core;

means for coupling signal light of a signal-light wavelength into the first core;

means for amplifying the signal light in the first amplifying segment to generate first-amplified signal light;

means for coupling the first-amplified signal light from the first core into the second core;

means for further amplifying the first-amplified signal light in the second segment to generate second-amplified signal light, wherein at least some light having the signal-light wavelength travels in the second amplifying segment as backwards-traveling second-segment signal-wavelength light;

means for coupling at least some of the backwards-traveling second-segment signal-wavelength light into the first inner cladding from the second core; and

means for absorbing at least some of the backwards-traveling second-segment signal-wavelength light that was coupled into the first inner cladding from the second core.

16. The apparatus of claim 15, wherein the plurality of amplifying fiber segments further includes a third amplifying segment that is joined to the second segment, wherein the third amplifying segment includes a third core that is surrounded by a third inner cladding, wherein the third inner cladding is surrounded by a third outer cladding, the apparatus further comprising:

means for coupling pump light into the third core from the third inner cladding along the length of the third core;

means for coupling the second-amplified signal light from the second core into the third core;

means for further amplifying the second-amplified signal light in the third amplifying segment to generate third-amplified signal light, wherein at least some light of the signal-light wavelength travels in the third amplifying segment as backwards-traveling third-segment signal-wavelength light;

means for coupling at least some of the backwards-traveling third-segment signal-wavelength light into the second inner cladding from the third core; and

means for absorbing at least some of the backwards-traveling third-segment signal-wavelength light that was coupled into the second inner cladding from the third core.

17. The apparatus of claim 15, wherein the first amplifying segment and the second amplifying segment are joined to one another at a first junction, wherein an outer diameter of the first inner cladding of the first amplifying segment at the first junction is smaller than an outer diameter of the second inner cladding of the second amplifying segment at the first junction, and wherein the means for coupling pump light into the second core further includes means for coupling pump light into an end of the second inner cladding at the first junction from outside the outer diameter of the first inner cladding of the first segment.

18. The apparatus of claim 15, wherein the first amplifying segment and the second amplifying segment are joined to one another, wherein a diameter of the first core is tapered from a larger first-segment core diameter to a smaller second-segment core diameter along a first transition length between the first amplifying segment and the second amplifying segment.

19. The apparatus of claim 15, wherein the first-amplified signal light has a desired first signal wavelength, the apparatus further comprising means for wavelength filtering the first-amplified signal light in order to remove light having wavelengths other than the desired first signal wavelength.

20. The apparatus of claim 15, further comprising means for gating light transmission between the first means for amplifying and the second means for amplifying such that more light of the signal-light wavelength is transmitted during a signal-pulse time as compared to other times.