Scanner/optical system for three-dimensional lidar imaging and polarimetry

• US 8,493,445 B2
• Filed: 03/08/2007
• Issued: 07/23/2013
• 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 III, Wells, David Nelson
• Primary Examiner(s): TRUONG, LAN DAI T
• Application Number:
  US11/683,549
• Publication Number:
  US 20070279615A1
• Time in Patent Office: 2,329 Days
• Field of Search: 348/144, 356/5.01, 356/4.01, 356/117, 356/67, 356/146
• US Class Current: 348/144
• 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 three-dimensional and polarization imaging of topographic surfaces and volumetric scatterers comprising:

a common light source arranged for use onboard an aircraft or spacecraft that can transmit a coherent beam of light characterized by at least two different wavelengths;

an optical dual wedge scanner including, a first rotating optical wedge, a second rotating optical wedge, and configured to control the first and the second rotating optical wedge to simultaneously scan both a transit beam of light and a receive beam of light;

a pulse detector configured to detect the receive beam of light, after being redirected from the topographic surfaces and volumetric scatterers arranged for use onboard an aircraft or spacecraft, and a polarizer configured to generate at least one imaging signal and at least one depolarization signal responsive to the redirected receive beam of light; and

a microprocessor configured to process the at least one imaging signal and at least one depolarization signal generated by the means for detecting the receive beam of light;

wherein, the common light source is arranged to generate and to transmit a multitude of distinct pulses of light having the at least two different wavelengths, of which the at least one wavelength had been chosen for generation of the at least one imaging signal and the at least another wavelength had been chosen for generation of the at least one depolarization signal.

2. The imaging LIDAR system of claim 1, wherein the detecting the receive beam of light include a laser polarimeter for detecting changes in polarization of the receive beam of light caused by reflections of the transmitted beam of light from the topographic surfaces and volumetric scatterers.

3. The imaging LIDAR system of claim 1, wherein the common light source incorporates a Holographic Optical Element which splits the coherent beam of light into an array of quasi-uniform intensity spots in a far field of the imaging LIDAR system.

4. The imaging LIDAR system of claim 3, wherein the array of quasi-uniform intensity spots is a 10×

10 array.

5. The imaging LIDAR system of claim 1, wherein the common light source incorporates a Q switched frequency-doubled Nd:

YAG microchip laser.

6. The imaging LIDAR system of claim 5, wherein the Q switched frequency-doubled Nd:

YAG microchip laser have been arranged to transmit a multitude of laser pulses having a laser pulse fire frequency in a kilohertz range.

7. The imaging LIDAR tem of claim 6, wherein the laser pulse fire frequency is between 8 kHz and 30 kHz.

8. The imaging LIDAR system of claim 1 further comprising a splitter mirror and a pulse detector where the splitter mirror directs a portion of the multitude of distinct pulses of light into the pulse detector.

9. The imaging LIDAR system of claim 8, wherein the optical dual wedge scanner further comprises at least one driving motor and a motion control system.

10. The imaging LIDAR system of claim 9, wherein the at least one driving motor directly supports at least one of the first optical wedge and the second optical wedge with at least one shaft in a direct-drive arrangement.

11. The imaging LIDAR system of claim 10 wherein the at least one shaft is hollow.

12. The imaging LIDAR system of claim 9, wherein the at least one driving motor utilizes at least one drive belt to drive the at least one of the first optical wedge and the second optical wedge in an indirect-drive arrangement.

13. The imaging LIDAR system of claim 9, wherein the motion control system has been arranged to synchronize at least one rotation rate of the at least one of the first optical wedge and the second optical wedge to the multitude of distinct pulses of light fire times, as detected by the pulse detector.

14. The imaging LIAR system of claim 13, wherein the synchronization of the at least one rotation rate of the at least one of the first optical wedge and the second optical wedge have been arranged to utilize the multitude of distinct pulses of light fire times as a clock oscillator time.

15. The imaging LIDAR system of claim 14, wherein the motion control system, have been arranged to be timed by the pulse detector in order to advance the at least one rotation of the at least one of the first optical wedge and the second optical wedge by a predetermined integer number n of encoder counts per at least one laser pulse fire.

16. The imaging LIDAR system of claim 15, wherein the predetermined integer number n of encoder counts have been prearranged between 1 and 72 000.

17. The imaging LIDAR system of claim 15, wherein the predetermined integer number n of encoder counts have been prearranged to divide into a total number of encoder counts by an integer value.

18. The imaging LIDAR system of claim 1 further comprising an annular Transmit/Receive mirror which passes the coherent beam of light transmitted by the common light source and reflects the received beam of light.

19. The imaging LIDAR system of claim 1, wherein the detecting the receive beam of light incorporate a shared afocal telescope.

20. The imaging LIDAR system of claim 19, wherein the shared afocal telescope has an aperture less than 50 cm in diameter.

21. The imaging LIDAR system of claim 19, wherein the shared afocal telescope has an aperture less than 10 cm in diameter.

22. The imaging LIDAR system of claim 19 wherein the shared afocal telescope has an aperture less than 5 cm in diameter.

23. The imaging LIDAR system of claim 1, wherein for detecting the received beams of light incorporate a photon counting array detector.

24. The imaging LIDAR system of claim 1, wherein the detecting received beam of light incorporate a multi-anode photomultiplier and a multichannel range receiver.

25. The imaging LIDAR system of claim 24, wherein the multi-anode photomultiplier is a segmented anode microchannel plate photomultiplier.

26. A method for three-dimensional and polarization imaging of topographic surfaces and volumetric scatterers by an imaging LIDAR comprising;

transmitting a coherent beam of light using a common light source arranged for use onboard an aircraft or spacecraft that can transmit the coherent beam of light characterized by at least two different wavelengths;

simultaneously scanning the coherent beam of light using an optical dual wedge scanner including, a first rotating optical wedge, a second rotating optical wedge, and controlling the first and the second rotating optical wedge to simultaneously scan both a transit beam of light and a receive beam of light;

using means for detecting to detect the receive beam of light, after being redirected from the topographic surfaces and volumetric scatterers arranged for use onboard an aircraft or spacecraft, and generating at least one imaging signal and at least one depolarization signal responsive to the redirected receive beam of light, and using a microprocessor for processing the at least one imaging signal and at least one depolarization signal generated by the means for detecting the receive beam of light;

wherein, the common light source is arranged to generate and to transmit a multitude of distinct pulses of light having the at least two different wavelengths, of which the at least one wavelength had been chosen for generation of the at least one imaging signal and the at least another wavelength had been chosen for generation of the at least one depolarization signal.

27. The method of claim 26, where topographic surfaces include objects and combination of objects chosen from a set of objects consisting of tend, ice, water surfaces and basins, man-made objects, solid and liquid surfaces of planets, satellites, comets, asteroids, and other celestial bodies.

28. The method of claim 26, 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.

29. The method of claim 26, where the step of transmitting the coherent 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.

30. The method of claim 26, where the step of using means for detecting received beams of light after being reflected from the topographic surfaces and volumetric scatterers 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.

31. The method of claim 30, where the step of using means for detecting received beams of light after being reflected from the topographic surfaces and volumetric scatterers 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.

32. The method of claim 26, where the step of simultaneously scanning the coherent beam of light using an optical dual wedge scanner 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.

33. The method of claim 26, where the rotations of the first and the second wedge are 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.

34. The method of claim 26, 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.

35. The method of claim 26, 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.

36. The method of claim 26, 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.