SCANNER/OPTICAL SYSTEM FOR THREE-DIMENSIONAL LIDAR IMAGING AND POLARIMETRY

• US 20070279615A1
• Filed: 03/08/2007
• Published: 12/06/2007
• Est. Priority Date: 05/31/2006
• Status: Active Grant

• Current Assignee: Intergraph Corporation (Hexagon AB)
• Original Assignee: Sigma Space Corp. (Hexagon AB)
• Inventors: Degnan, John James, Wells, David Nelson
• Granted Patent: US 8,493,445 B2
• Time in Patent Office: Days
• US Class Current: 356/4.10
• CPC Class Codes: G01S 17/89 for mapping or imaging
• CPC Class Codes: G01S 7/499 using polarisation effects

Abstract

An optical scanner system for contiguous three-dimensional topographic or volumetric imaging of a surface from an aircraft or spacecraft is disclosed. A servo controller synchronizes the rotation rates of a pair of wedge scanners with high precision to the multi-kilohertz laser fire rate producing an infinite variety of well-controlled scan patterns. This causes the beam pattern to be laid down in precisely the same way on each scan cycle, eliminating the need to record the orientations of the wedges accurately on every laser fire, thereby reducing ancillary data storage or transmission requirements by two to three orders of magnitude and greatly simplifying data preprocessing and analysis. The described system also uses a holographic element to split the laser beam into an array that is then scanned in an arbitrary pattern. This provides more uniform signal strength to the various imaging detector channels and reduces the level of optical crosstalk between channels, resulting in a higher fidelity three-dimensional image.

Claims

1. An imaging LIDAR system for use onboard an aircraft or spacecraft for three-dimensional and polarization imaging of topographic surfaces and volumetric scatterers comprising:

a light source that can transmit a beam of light,

an optical dual wedge scanner comprising, a first optical wedge, a second optical wedge, and means for controlling the rotations of the first and the second optical wedge to simultaneously scan both the transit and receive beams of light,

means for detecting receive beams of light reflected from the topographic surfaces and volumetric scatterers and generating signals responsive to the received light,

and a processor system for processing signals generated by the detecting means.

2. The apparatus of claim 1 where the imaging LIDAR system further comprises a laser polarimeter for detecting changes in polarization of the beam of light caused by reflections of the transmitted beam of light from the topographic surfaces and volumetric scatterers.

3. The apparatus of claim 1 where the imaging LIDAR system further comprises a Holographic Optical Element which splits the beam of light transmitted by the light source into an array of quasi-uniform intensity spots in the far field of the LIDAR.

4. The apparatus of claim 3 where an array of quasi-uniform intensity spots is a 10×

10 array.

5. The apparatus of claim 1, where the light source is a laser.

6. The apparatus of claim 5, where the laser is a Q-switched laser.

7. The apparatus of claim 6, where the Q-switched laser is a frequency-doubled Nd:

YAG microchip laser.

8. The apparatus of claim 1 where the imaging LIDAR system further comprises a splitter mirror and a pulse detector where the splitter mirror directs a portion of laser light into the pulse detector.

9. The apparatus of claim 1 where the imaging LIDAR system further comprises an annular Transmit/Receive mirror which passes the beam of light transmitted by the light source with negligible loss of light and reflects the returning light to the receiver.

10. The apparatus of claim 8 where the optical dual wedge scanner further comprises driving motors and a motion control system.

11. The apparatus of claim 10, where the motors directly support the wedges with their shafts in a direct-drive system.

12. The apparatus of claim 11, where the motor shafts are hollow.

13. The apparatus of claim 10, where the motors utilize drive belts to drive the wedges in an indirect-drive system.

14. The apparatus of claim 10, where the motion control system synchronizes the rotation rates of the wedges with high precision to the laser pulse fire times.

15. The apparatus of claim 14 where the precise synchronization of the rotation rates of the wedges is achieved utilizing laser pulses fire time as a clock oscillator time.

16. The apparatus of claim 14 where the laser pulse fire frequency is in the kilohertz range.

17. The apparatus of claim 14 where the laser pulse fire frequency is between 8 kHz and 30 kHz.

18. The apparatus of claim 15 where the precise synchronization of the rotation rates of the wedges is achieved by the motion control system timed by the pulse detector to advance motion of the wedges by predetermined integer number n of encoder counts per laser fire.

19. The apparatus of claim 18 where the predetermined integer number n of encoder counts is between 1 and 72 000.

20. The apparatus of claim 18 where the predetermined integer number n of encoder counts divides into the total number of encoder counts by an integer value.

21. The apparatus of claim 1, where the means for collecting the receive beams of light comprise a shared afocal telescope.

22. The apparatus of claim 21, where the shared afocal telescope has an aperture less than 50 cm in diameter.

23. The apparatus of claim 21, where the shared afocal telescope has an aperture less than 10 cm in diameter.

24. The apparatus of claim 21, where the shared afocal telescope has an aperture less than 5 cm in diameter.

25. The apparatus of claim 1, where the means for detecting the received beams of light comprise a photon counting array detector.

26. The apparatus of claim 1, where the means for detecting received beams of light comprise a multi-anode photomultiplier and a multichannel range receiver.

27. The apparatus of claim 26, where the multi-anode photomultiplier is a segmented anode michrocannel plate photomultiplier.

28. A method of utilization of imaging LIDAR system onboard an aircraft or spacecraft for three-dimensional and polarization imaging of topographic surfaces and volumetric scatterers comprising steps:

transmitting a beam of light,

scanning the beam of light using an optical dual wedge scanner comprising, a first optical wedge, a second optical wedge, and a means for controlled rotating the first and the second optical wedge to simultaneously scan both the transit and receive beams of light,

detecting received beams of light reflected from the topographic surfaces and volumetric scatterers and generating signals responsive to the received light,

processing signals responsive to the received light.

29. The method of claim 28, where topographic surfaces include objects and combination of objects chosen from a set of objects consisting of land, ice, water surfaces and basins, man-made objects, solid and liquid surfaces of planets, satellites, comets, asteroids, and other celestial bodies.

30. The method of claim 28, where volumetric scatterers include objects and combination of objects chosen from a set of objects consisting of vegetation, tree canopies, crops, biomass, clouds, and planetary boundary layers.

31. The method of claim 28, where the step of transmitting beam of light comprises generation of a multikilohertz train of short light pulses, transmitting the light beam through a splitter mirror which redirects a fraction of light to a laser pulse start detector, expanding the light beam by a laser expander, portioning the laser beam into an array of quasi-uniform far field spots by a Holographic Optic Element, and transmitting the laser beam array through an opening on an annular Transmit/Receive mirror and a shared afocal telescope.

32. The method of claim 28, where the step of detecting beams of light comprises passing the returning photons through the optical dual wedge scanner and the shared afocal telescope, reflecting the majority of the returning photons by the annular Transmit/Receive mirror, separating the returning photons into imaging and polarimetry channels, restricting the noise background using spectral and spatial filters, and imaging the array of quasi-uniform far field spots onto corresponding segmented anodes of a microchannel plate photomultiplier using a telephoto lens.

33. The method of claim 28, where the step of detecting the receive beam of light further comprises separating of the returning photons of polarimetry channel into two fractions based on polarization using a polarizer and detecting the polarization signals by focusing the polarized fractions of returning photons on separate detectors.

34. The method of claim 28, where the step of scanning the beam of light further comprises:

locating accurately the home position for each wedge motion axis,

stabilizing the laser pulse fire frequency,

directing the wedges to move in unison in precision-locked motion controlled by the motion controller,

initiating slow rotations of the wedges synchronized to the laser pulse fire frequency,

gradually accelerating the rotations of the wedges to the point where each consecutive laser pulse commands the wedges to advance the angular displacement by additional predetermined integer n number of encoder counts such that n divides into the total number of encoder counts by an integer value,

preserving the synchronization of the rotations of the wedges with the laser pulse fire frequency such that every consecutive n^th laser pulse is transmitted at the practically identical exit angle as the corresponding pulse in the prior scan cycle.

35. The method of claim 28, where the rotations of the first and the second wedge is controlled to produce counter-rotating wedges with different angular velocities, resulting in a linear pattern of scanning points that rotates at a rate that is the difference between the two wedge angular velocities.

36. The method of claim 28, where the rotations of the first and the second wedge are controlled to produce co-rotating wedges rotating at the same speed and resulting in a conical scanning pattern with the deviation angle being a simple function of the relative phase angle between the wedges.

37. The method of claim 28, where the rotations of the first and the second wedge is controlled to produce co-rotating wedges with different rotation rates resulting in a spiral scanning pattern which periodically oscillates between a point and maximum deviation angle.

38. The method of claim 28, where the rotations of the first and the second wedge are controlled to produce counter-rotating wedges with the same angular velocities, resulting in a linear pattern of scanning points whose orientation is dependent on the relative phase of rotation.