METHOD AND APPARATUS FOR SPECTRAL-BEAM COMBINING OF FIBER-AMPLIFIED LASER BEAMS USING HIGH-EFFICIENCY DIELECTRIC DIFFRACTIVE GRATINGS

• US 20070229939A1
• Filed: 05/22/2007
• Published: 10/04/2007
• Est. Priority Date: 01/26/2005
• Status: Active Grant

• Current Assignee: Lockheed Martin Aculight Corp. (Martin Marietta Corporation)
• Original Assignee: Aculight Corporation (Martin Marietta Corporation)
• Inventors: Brown, Andrew, Liu, Anping, Mead, Roy, Lemaire, Charles, Honea, Eric, Hamilton, Charles, Loftus, Thomas
• Granted Patent: US 7,535,631 B2
• Time in Patent Office: Days
• US Class Current: 359/341.100
• CPC Class Codes: G02B 27/1006 for splitting or combining different wavelengths G02B27/1086, G02B27/141 take precedence
• CPC Class Codes: G02B 27/108 for sampling a portion of a beam or combining a small beam in a larger one, e.g. wherein the area ratio or power ratio of the divided beams significantly differs from unity, without spectral selectivity
• CPC Class Codes: G02B 27/1086 operating by diffraction only
• CPC Class Codes: G02B 27/123 The splitting element being a lens or a system of lenses, including arrays and surfaces with refractive power
• CPC Class Codes: G02B 27/144 using partially transparent surfaces without spectral selectivity G02B27/147 takes precedence
• CPC Class Codes: G02B 27/145 having sequential partially reflecting surfaces

Abstract

Apparatus and method for spectral-beam combining light from a plurality of high-power fiber lasers that, in some embodiments, use two substantially identical diffraction gratings in a parallel, mutually compensating configuration to combine a plurality of separate parallel input beams each having a slightly different successively higher wavelength into a single output beam of high quality. In other embodiments, a single diffraction grating is used to combine a plurality of different wavelengths, wherein the input laser beams are obtained from very narrow linewidth sources to reduce chromatic dispersion. In some embodiments, diagnostics and adjustments of wavelengths and/or positions and angles are made dynamically in real time to maintain the combination of the plurality input beams into a single high-quality output beam.

Claims

1. A method comprising:

providing a plurality of fiber gain media configured to emit laser beams including a first fiber gain medium that emits a first laser beam at a first characteristic wavelength having a full-width half maximum (FWHM) linewidth of at most 1 nm and a second fiber gain medium that emits a second laser beam at a second characteristic wavelength having an FWHM linewidth of at most 1 nm;

providing one or more high-efficiency dielectric diffractive gratings optically coupled to receive the first and second laser beams; and

spectrally combining the first and second laser beams into a single output beam using the one or more high-efficiency dielectric diffractive gratings.

2. The method of claim 1, wherein the spectrally combining outputs the single output beam at a power/area of about 10 W/cm² or more.

3. The method of claim 1, wherein the providing a plurality of fiber gain media further includes

providing a plurality of Yb-doped multiple-clad optical fibers including a first fiber and a second fiber;

pumping an inner cladding of each of the plurality of optical fibers with pump light from one or more laser diodes;

amplifying, with the first fiber, a first seed laser signal to generate the first laser beam at the first characteristic wavelength; and

amplifying, with the second fiber, a second seed laser signal to generate the second laser beam at the second characteristic wavelength.

4. The method of claim 3, further comprising:

pulsing the first seed laser signal and the second seed laser signal to a pulse length of about ten nanoseconds or less and sufficiently short to substantially prevent SBS buildup in the amplifying of the first seed laser signal and the second seed laser signal.

5. The method of claim 1, wherein the providing of the plurality of fiber gain media includes providing polarization-maintaining large-mode area fibers, and wherein the method further comprises amplifying a polarized seed signal.

6. The method of claim 1, wherein the one or more high-efficiency dielectric diffraction gratings exhibit a lowest amount of heat absorption when an incident laser beam is at a Littrow angle, and wherein the method further comprises configuring the incident laser beam substantially at the Littrow angle.

7. The method of claim 1, wherein the providing of the high-efficiency dielectric diffractive gratings further includes

providing a first plurality of half-wave-thick dielectric layers;

and

providing a plurality of grating peaks located on a top layer of the first plurality of half-wave-thick dielectric layers, wherein each one of the plurality of grating peaks includes a second plurality of half-wave-thick dielectric layers, and wherein each one of the plurality of grating peaks include a first face and a second face, and wherein the first face and second face are oriented at respective blaze angles for concentrating spectral energy within a limited angular region.

8. The method of claim 1, wherein the single output beam has a power level of at least 80% of the sum of the power levels of all the laser beams from the plurality of fiber gain media combined.

9. The method of claim 1, wherein the providing of one or more high-efficiency dielectric diffractive gratings further includes:

providing a first diffractive element;

introducing a first chromatic dispersion into the first laser beam with the first diffractive element;

providing a second diffractive element;

introducing a second chromatic dispersion into the second laser beam with the second diffractive element;

providing a third diffractive element; and

introducing a third chromatic dispersion into the first laser beam with the third diffractive element, wherein the third chromatic dispersion includes a compensating dispersion that negates at least a portion of the first chromatic dispersion from the first laser beam; and

introducing a fourth chromatic dispersion into the second laser beam with the third diffractive element, wherein the fourth chromatic dispersion is a compensating dispersion that negates at least a portion of the second chromatic dispersion from the second laser beam.

10. The method of claim 9, further comprising:

positioning of the second diffractive element so a diffractive surface of the second diffractive element is parallel to a corresponding diffractive surface of the first diffractive element; and

positioning of the third diffractive element so a diffractive surface of the third diffractive element is parallel to a corresponding diffractive surface of at least one of the first diffractive element and the second diffractive element, and such that the third diffractive element is at about equal distances from the first and second diffractive elements so as to obtain approximately equal chromatic spreading of the first laser beam in the space between the first grating and the third grating, and of the second laser beam between the second grating and the third grating.

11. An apparatus comprising:

a plurality of fiber gain media configured to emit laser beams including a first fiber gain medium that emits a first laser beam at a first characteristic wavelength having a full-width half maximum (FWHM) linewidth of at most 1 nm and a second fiber gain medium that emits a second laser beam at a second characteristic wavelength having an FWHM linewidth of at most 1 nm; and

one or more high-efficiency dielectric diffractive gratings optically coupled to receive the first and second laser beams;

wherein the first and second laser beams are spectrally combined into a single output beam using the one or more high-efficiency dielectric diffractive gratings.

12. The apparatus of claim 11, wherein the single output beam outputs at a power/area of about 10 W/cm² or more.

13. The apparatus of claim 11, wherein the plurality of fiber gain media further comprises

a plurality of Yb-doped multiple-clad optical fibers including a first fiber and a second fiber;

one or more laser diodes, wherein the one or more laser diodes pump an inner cladding of each of the plurality of optical fibers with pump light;

a source of a first seed laser signal, wherein the first seed laser signal is amplified with the first fiber to generate the first laser beam at the first characteristic wavelength; and

a source of a second seed laser signal, wherein the second seed laser signal is amplified with the second fiber to generate the second laser beam at the first characteristic wavelength.

14. The apparatus of claim 13, wherein the first seed laser signal and the second seed laser signal each include a pulse having a pulse length of about ten nanoseconds or less and sufficiently short to substantially prevent SBS buildup during amplification of the first seed laser signal and the second seed laser signal.

15. The apparatus of claim 11, wherein the plurality of fiber gain media includes polarization-maintaining large-mode area fibers, and wherein the apparatus further comprises a source of a polarized seed signal.

16. The apparatus of claim 11, wherein the one or more high-efficiency dielectric diffraction gratings exhibit a lowest amount of heat absorption when an incident laser beam is at or substantially at a Littrow angle.

17. The apparatus of claim 11, wherein the one or more high-efficiency dielectric diffractive gratings further include

a first plurality of half-wave-thick dielectric layers;

and

a plurality of grating peaks located on a top layer of the first plurality of half-wave-thick dielectric layers, wherein each one of the plurality of grating peaks includes a second plurality of half-wave-thick dielectric layers, and wherein each one of the plurality of grating peaks include a first face and a second face, and wherein the first face and second face are oriented at respective blaze angles for concentrating spectral energy within a limited angular region.

18. The apparatus of claim 11, wherein the single output beam has 80% or more of the power sum of the first and second laser beams.

19. The apparatus of claim 11, wherein the one or more high-efficiency dielectric diffractive gratings further include:

a first diffractive element that introduces a first chromatic dispersion;

a second diffractive element that introduces a second chromatic dispersion;

a third diffractive element that introduces a third chromatic dispersion into the first laser beam and a fourth chromatic dispersion into the second laser beam, wherein the third chromatic dispersion includes a compensating dispersion that negates at least a portion of the first chromatic dispersion from the first laser beam, and wherein the fourth chromatic dispersion is a compensating dispersion that negates at least a portion of the second chromatic dispersion from the second laser beam.

20. An apparatus comprising:

a plurality of fiber gain media configured to emit laser beams including a first fiber gain medium that emits a first laser beam at a first characteristic wavelength having a full-width half maximum (FWHM) linewidth of at most 1 nm and a second fiber gain medium that emits a second laser beam at a second characteristic wavelength having an FWHM linewidth of at most 1 nm; and

means for spectrally combining the first and second laser beams into a single output beam.