System and method for coherent beam combination

1. A laser system comprising:

a master oscillator for generating a primary laser signal;

a beam splitter array for splitting the primary laser signal into a sample reference signal and a plurality of secondary laser signals;

an optical frequency shifter for shifting the frequency of the sample reference laser signal to provide a frequency-shifted reference beam;

a beam expander for expanding the frequency-shifted reference beam to provide an expanded frequency-shifted reference beam;

a plurality of amplifier arms that each receive a respective secondary laser signal of the plurality of secondary laser signals, each amplifier arm comprising a path length adjuster for adjusting a path length of the amplifier arm and an amplifier for amplifying the secondary laser signal to provide an amplified output signal;

a beam sampler that interferes the light of the amplified output signal of the plurality of amplifier arms with the expanded frequency-shifted reference beam to provide a plurality of optical heterodyne detected (OHD) beat signals;

a plurality of photodetectors that each receive a respective OHD beat signal to provide a plurality of OHD electrical beat signals, each OHD electrical beat signal having a maximum amplitude that corresponds to a minimum path length mismatch of a respective amplifier arm; and

a path length controller responsive to the plurality of OHD electrical beat signals for providing a plurality of feedback signals to adjust the path length adjusters to control the path length of each of the plurality of amplifier arms to within a coherence length of the primary laser signal.

2. The system of claim 1 , further comprising an electrical oscillator that generates a fixed frequency electrical signal, and wherein said optical frequency shifter responds to the fixed frequency electrical signal for shifting the frequency of the sample reference signal by the fixed frequency electrical signal.

3. The system of claim 1 , wherein the path length controller is configured scan the actuation range of each of a given path length adjuster while monitoring the amplitude of the respective OHD electrical beat signal to determine a maximum amplitude and set an initial path length delay setpoint of the given path length adjuster.

4. The system of claim 3 , wherein the path length controller is configured to set an initial path length dither range about the initial path length delay setpoint of the given path length adjuster, to measure the amplitude of a given OHD electrical beat signal at a minimum path length and maximum path length of the dither range and to reset the path length delay setpoint to the minimum path length and maximum path length of the dither range that has the greater amplitude.

5. The system of claim 1 , wherein each amplifier is a fiber amplifier.

6. The system of claim 1 , wherein each amplifier arm further comprises a phase adjuster for adjusting the phase of a respective one of the plurality of secondary laser signals and further comprising a phase controller for controlling the phase adjustments of each of the phase adjusters.

7. The system of claim 6 , wherein each of the OHD electrical beat signals has a phase that corresponds to the phase offset of a respective amplified output signal relative to the expanded frequency-shifted reference beam, the phase controller responsive to the OHD electrical beat signals for providing a plurality of feedback signals to adjust the phase adjusters to control the phase of each of the plurality of secondary signals to within an arbitrary multiple of one wavelength of the expanded frequency-shifted reference beam.

8. The system of claim 1 , wherein at least one of the path length adjusters is an actuator configured to control an optical propagation path length of the respective amplifier arm.

9. The system of claim 8 , wherein the actuator is a spiral waveguide.

10. A laser system comprising:

means for providing a plurality of secondary laser signals from a primary laser signal;

a plurality of means for amplifying the plurality of secondary signals to provide a plurality of amplified output signals;

means for providing a frequency-shifted reference beam from the primary laser signal;

means for generating a plurality of optical heterodyne detected (OHD) beat signals from the plurality of amplified output signals and the frequency-shifted reference beam;

means for generating a plurality of feedback signals from the plurality of OHD beat signals;

a plurality of means for adjusting a path length associated with respective means for amplifying for each of the plurality of means for amplifying; and

means for controlling the plurality of means for adjusting a path length based on respective amplitudes of the plurality of OHD beat signals to control the path length of each of the plurality of means for amplifying to within a coherence length of the primary laser signal.

11. The system of claim 10 , wherein the means for providing a frequency-shifted reference beam from the primary laser signal comprises:

means for generating a fixed frequency electrical signal;

means for frequency shifting the primary laser signal by the fixed frequency electrical signal to provide a frequency-shifted reference beam; and

means for expanding the frequency-shifted reference beam into an expanded fixed frequency reference beam.

12. The system of claim 10 , further comprising a plurality of means for generating respective OHD electrical beat signals from respective OHD beat signals.

13. The system of claim 12 , wherein the means responsive to control the plurality of means for adjusting a path length is configured to scan the range of each of a given means for adjusting a path length while monitoring the amplitude of the respective OHD electrical beat signal to determine a maximum amplitude and to set an initial path length delay setpoint of the given means for adjusting a path length.

14. The system of claim 13 , wherein the means for controlling the plurality of means for adjusting a path length is configured to set an initial path length dither range about the initial path length delay setpoint of the given means for adjusting path lengths, to measure the amplitude of a given OHD electrical beat signal at a minimum path length and maximum path length of the dither range and to reset the path length delay setpoint to the minimum path length and maximum path length of the dither range that has the greater amplitude.

15. The system of claim 10 , further comprising a plurality of means for adjusting the phase of the plurality of secondary signals and further comprising means for controlling the phase adjustments of each of the plurality of means for adjusting the phase of the plurality of phase adjustments based on a phase offset of a respective amplified output signal relative to the frequency-shifted reference beam.

16. A method for coherent beam combination, the method comprising:

providing a plurality of secondary laser signals from a primary laser signal;

amplifying the plurality of secondary signals along respective amplifier arms to provide a plurality of amplified output signals;

providing a frequency-shifted reference beam from the primary laser signal;

generating a plurality of optically heterodyne detected (OHD) beat signals based on combining respective amplified output signals of the plurality of amplified output signals with the frequency-shifted reference beam; and

adjusting path lengths of respective amplifier arms based on respective amplitudes of the plurality of OHD beat signals to control the path length of respective amplifier arms to within a coherence length of the primary laser signal.

17. The method of claim 16 , further comprising converting the plurality of OHD beat signals into a plurality of OHD electrical beat signals.

18. The method of claim 17 , wherein the adjusting path lengths comprises scanning the range of each of a given path length adjuster associated with a given amplifier arm while monitoring the amplitude of a respective OHD electrical beat signal to determine a maximum amplitude and setting an initial path length delay setpoint of the given path length adjuster based on the determined maximum amplitude.

19. The method of claim 18 , wherein the adjusting path lengths further comprises setting an initial path length dither range about the initial path length delay setpoint of the given path length adjuster, measuring the amplitude of the OHD electrical beat signal at a minimum path length and maximum path length of the dither range and resetting the path length delay setpoint to the minimum path length or maximum path length of the dither range base on the path length that has the greater amplitude.

20. The method of claim 16 , further comprising adjusting the phase of the plurality of secondary signals based on a phase offset of a respective amplified output signal relative to the frequency-shifted reference beam.