Method and system for hybrid coherent and incoherent diffractive beam combining

• US 7,436,588 B2
• Filed: 10/05/2006
• Issued: 10/14/2008
• Est. Priority Date: 10/05/2006
• Status: Active Grant

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Northrop Grumman Corporation
• Inventors: Rice, Robert R., Wickham, Michael G., Rothenberg, Joshua E.
• Primary Examiner(s): BOLDA, ERIC L
• Application Number:
  US11/543,596
• Publication Number:
  US 20080084605A1
• Time in Patent Office: 740 Days
• Field of Search: 359/349
• US Class Current: 359/349
• CPC Class Codes: B23K 26/0604 by a combination of beams
• CPC Class Codes: B23K 26/067 Dividing the beam into multiple beams, e.g. multifocusing
• CPC Class Codes: B23K 26/705 Beam measuring device
• CPC Class Codes: H01S 3/005 Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping shaping laser beam for working metal or other materials B23K26/06; optical elements, systems or apparatus in general G02B
• CPC Class Codes: H01S 3/1307 Stabilisation of the phase
• CPC Class Codes: H01S 3/2391 emitting at different wavelengths

Abstract

A hybrid beam combining system or method combines a plurality of coherent and incoherent light beams into a composite high power diffraction limited beam. N oscillators each transmit light at one of N different wavelengths and each wavelength is split into M constituent beams. M beams in each of N groups are phase locked by a phase modulator using phase correction signals. The phase locked beams are amplified and coupled into an M×N fiber array. Beams emerging from the array are collimated and incident on a diffractive optical element operating as a beam combiner combining the M outputs at each N wavelength into a single beam. The N single beams are incident and spectrally combined on a grating which outputs a composite beam at a nominal 100% fill factor. A low power sample beam, taken from the N beams emerging from the diffractive optical element, is measured for phase deviations from which the phase correction signals are derived and fed back to the phase modulators. The diffractive optical element may include a weak periodic grating for diffracting the low power sample. The diffractive optical element may also be combined with the spectral combining grating into a single optical element.

Claims

1. A system for combining light beams of different and identical wavelengths into a single composite beam of substantially identical shape as the individual beams, comprising:

N oscillators, each transmitting a light beam at a different one of N wavelengths;

one or more beam splitters for splitting each one of the N beams into M beams of common wavelength;

a phase modulation stage for locking phases of the M beams at each of the N wavelengths according to phase correction signals;

a diffractive optical element combining the M×

N phase-locked light beams into N incoherent beams, each incoherent beam a coherent combination of M beams at one of the N wavelengths;

a phase detection means for detecting the phases of the M beams comprising each incoherent beam;

a means for deriving the phase correction signals from the detected phases; and

a grating for spectrally combining the N incoherent beams into a single composite beam;

wherein M and N are integers.

2. The system of claim 1 wherein at least one coherent combination of M beams has a maximum intensity along the 0^th diffraction order of the diffractive optical element.

3. The system of claim 1 further comprising an array of fibers for directing the M×

N light beams from the phase modulation stage to the diffractive optical element.

4. The system of claim 1 wherein the phase detection means includes an optic for extracting a low power sample of the M×

N light beams combined by the diffractive optical element.

5. The system of claim 1 wherein the phase detection means receives a low power sample of the M×

N light beams diffracted from the diffractive optical element.

6. The system of claim 1 wherein the phase detection means receives a low power sample of the M×

N light beams diffracted from the grating.

7. The system of claim 6 wherein the low power sample comprises spurious beams diffracted at the 0^th diffraction order of the grating.

8. The system of claim 5 wherein the diffractive optical element comprises a sampling grating diffracting the low power sample.

9. A system for combining light beams of different and identical wavelengths into a single composite beam of substantially identical shape as the individual beams, comprising:

N oscillators, each transmitting a light beam at a different one of N wavelengths;

one or more beam splitters for splitting each one of the N beams into M beams of common wavelength;

a phase modulation stage for locking phases of the M beams at each N wavelength according to phase correction signals;

an optical element having both an SBC grating and a diffractive element, whereon incident M×

N light beams are combined into a single composite beam comprising N incoherent beams spectrally combined by the grating, each incoherent beam a combination of M beams combined by the diffraction grating at one of the N wavelengths;

a phase detection means for detecting the phases of the M beams at each N wavelength; and

a means for deriving the phase correction signals from the detected phases;

wherein M and N are integers.

10. The system of claim 9 wherein the diffractive element is orthogonal to the SBC grating.

11. The system of claim 10 wherein the optical element provides a sample beam to the phase detection means.

12. A method for combining light beams of different and identical wavelengths into a single composite beam of substantially identical shape as the individual beams, comprising:

transmitting light beams at N different wavelengths;

splitting each one of the N beams into M beams of common wavelength;

synchronizing phases of the M beams at each N wavelength according to phase correction signals;

combining M×

N light beams into N incoherent beams, each incoherent beam a coherent combination of M beams at one of the N wavelengths;

extracting a sample beam from each of the combined incoherent beams;

detecting in the sample beam the phases of the M beams at each of N wavelengths;

deriving the phase correction signals from the detected phases; and

spectrally combining the N incoherent beams into a single composite beam;

wherein M and N are integers.

13. The method of claim 12 further comprising, prior to the combining step, directing the light beams through an M×

N fiber array to a diffractive optical element.

14. The method of claim 12 wherein each of the N incoherent beams has a maximum intensity along a single diffraction order of a diffractive optical element.

15. The method of claim 12 wherein the extracting step further comprises diffracting a low power sample of the combined M×

N light beams by means of a diffractive optical element.

16. The method of claim 15 wherein the combining step and extracting step are effected by the same diffractive optical element.

17. The method of claim 12 wherein the extracting step further comprises diffracting a low power sample of the combined M×

N light beams by means of an SBC grating.

18. The method of claim 17 wherein the low power sample comprises spurious beams diffracted at the 0^th diffraction order of the SBC grating.

19. The method of claim 12 wherein the extracting step further comprises diffracting a sample of the combined M×

N light beams, the sample having total power less than about one percent of total power in the combined M×

N light beams.

20. The method of claim 12 wherein the combining, extracting, and spectrally combining steps are effected by a single optical element.