Optical phasograms for LADAR vibrometry

1. A method of operating a laser detection and ranging (LADAR) system, the method comprising:

transmitting a laser signal towards a target, the laser signal comprising a train of coherent pulses;

receiving a return signal based on a reflection from the target of the transmitted laser signal; and

forming one or more range bins from the return signal, the one or more range bins each comprising a train of coherent pulses formed based upon the transmitted laser signal;

for each range bin:

generating a phasogram associated with the train of coherent pulses, the phasogram generated by determining a relative phase between the return signal and a reference signal; and

generating a vibration spectrum of the return signal based upon the generated phasogram.

2. The method of claim 1 , further comprising:

compensating for at least one of: speckle, fading and motion of the LADAR system.

3. The method of claim 2 , wherein the compensating comprises:

if the return signal is below a threshold, interpolating a value of the return signal based upon one or more recent values of the return signal.

4. The method of claim 1 , further comprising:

generating the reference signal based upon a local oscillator signal of the LADAR system;

for each range bin:

generating a sequence of cross-correlated phase signals, each cross-correlated phase signal having a peak of the relative phase between the return signal and the reference signal;

tracking the peaks of each cross-correlated phase signal; and

based upon the tracked peaks, generating a column of the phasogram as a function of time.

5. The method of claim 4 , further comprising determining a frequency history of the return signal based upon the determined relative phase.

6. The method of claim 5 , wherein generating the vibration spectrum of the return signal based upon the generated phasogram comprises:

determining an unwrapped phase signal based upon the tracked peaks;

determining the frequency history of the return signal by determining a derivative of the unwrapped phase signal; and

performing a Fourier transform on the determined frequency history.

7. The method of claim 6 , wherein the Fourier transform is a fast Fourier transform (FFT).

8. The method of claim 6 , wherein the phasogram is range-resolved.

9. The method of claim 6 , wherein determining the unwrapped phase signal removes one or more phase jumps of the tracked peaks.

10. The method of claim 1 , further comprising:

for each range bin, generating a spectrogram associated with the train of coherent pulses, the spectrogram based upon one or more instantaneous velocity measurements of the return signal.

11. The method of claim 10 , further comprising:

generating a course vibration spectrum based upon the generated spectrogram; and

generating a fine vibration spectrum based upon the generated phasogram.

12. An article comprising a non-transitory machine-readable medium that stores executable instructions, the instructions causing a machine to execute the steps of:

transmitting a laser signal towards a target, the laser signal comprising a train of coherent pulses;

receiving a return signal based on a reflection from the target of the transmitted laser signal; and

forming one or more range bins from the return signal, the one or more range bins each comprising a train of coherent pulses formed based upon the transmitted laser signal;

for each range bin:

generating a phasogram associated with the train of coherent pulses, the phasogram generated by determining a relative phase between the return signal and a reference signal; and

generating a vibration spectrum of the return signal based upon the generated phasogram.

13. An apparatus, comprising:

a transmitter to transmit a laser signal toward a target, the laser signal comprising a train of coherent pulses;

a receiver to receive a return signal based on a reflection from the target of the transmitted laser signal; and

circuitry to:

form one or more range bins from the return signal, the one or more range bins each comprising a train of coherent pulses formed based upon the transmitted laser signal;

for each range bin:

generate a phasogram associated with the train of coherent pulses, the phasogram generated by determining a relative phase between the return signal and a reference signal; and

generate a vibration spectrum of the return signal based upon the generated phasogram.

14. The apparatus of claim 13 , further comprising circuitry to compensate for at least one of: speckle, fading and motion of the LADAR system.

15. The apparatus of claim 14 , wherein if the return signal is below a threshold, the circuitry is configured to interpolate a value of the return signal based upon one or more recent values of the return signal.

16. The apparatus of claim 13 , further comprising circuitry to:

generate the reference signal based upon a local oscillator signal of the LADAR system;

for each range bin:

generate a sequence of cross-correlated phase signals, each cross-correlated phase signal having a peak of the relative phase between the return signal and the reference signal;

track the peaks of each cross-correlated phase signal; and

based upon the tracked peaks, generate a column of the phasogram as a function of time.

17. The apparatus of claim 16 , further comprising circuitry to determine a frequency history of the return signal based upon the determined relative phase.

18. The apparatus of claim 17 , further comprising circuitry to:

determine an unwrapped phase signal based upon the tracked peaks;

determine the frequency history of the return signal by determining a derivative of the unwrapped phase signal; and

perform a Fourier transform on the determined frequency history.

19. The apparatus of claim 13 , further comprising circuitry to, for each range bin, generate a spectrogram associated with the train of coherent pulses, the spectrogram based upon one or more instantaneous velocity measurements of the return signal.

20. The apparatus of claim 19 , further comprising circuitry to:

generate a course vibration spectrum based upon the generated spectrogram; and

generate a fine vibration spectrum based upon the generated phasogram.

21. The apparatus of claim 13 , wherein the receiver comprises a Geiger-mode avalanche photodiode (GMAPD) receiver.