Motion compensated multi-wavelength digital holography

• US 9,581,967 B1
• Filed: 01/07/2015
• Issued: 02/28/2017
• Est. Priority Date: 01/07/2015
• Status: Active Grant

• Current Assignee: Lockheed Martin Coherent Technologies, Inc. (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Coherent Technologies, Inc. (Martin Marietta Corporation)
• Inventors: Krause, Brian
• Primary Examiner(s): Li, Tracy Y
• Application Number:
  US14/591,879
• Time in Patent Office: 783 Days
• Field of Search: None
• US Class Current: 1/1
• CPC Class Codes: G01S 17/90 using synthetic aperture techniques
• CPC Class Codes: G03H 1/0443 Digital holography, i.e. recording holograms with digital recording means holobject computation G03H1/0866
• CPC Class Codes: G03H 1/0465 Particular recording light; Beam shape or geometry G03H1/06 takes precedence
• CPC Class Codes: G03H 1/0866 Digital holographic imaging, i.e. synthesizing holobjects from holograms
• CPC Class Codes: G03H 1/14 Temporal modulation, e.g. extending depth of field or phase compensation for object motion
• CPC Class Codes: G03H 1/32 Systems for obtaining speckle elimination
• CPC Class Codes: G03H 2001/0445 Off-axis recording arrangement G03H2001/0456 takes precedence
• CPC Class Codes: G03H 2001/0452 arranged to record an image of the object
• CPC Class Codes: G03H 2001/0463 Frequency heterodyne, i.e. one beam is frequency shifted
• CPC Class Codes: G03H 2001/0467 Gated recording using pulsed or low coherence light source, e.g. light in flight, first arriving light
• CPC Class Codes: G03H 2001/0469 Object light being reflected by the object
• CPC Class Codes: G03H 2001/0875 Solving phase ambiguity, e.g. phase unwrapping
• CPC Class Codes: G03H 2222/13 Multi-wavelengths wave with discontinuous wavelength ranges G03H2222/18 takes precedence
• CPC Class Codes: G03H 2222/34 Multiple light sources

Abstract

A holography imaging system includes a first laser, a second laser, a transmitter optical system, a receiver optical system, and a detector array. The first laser has a constant frequency, and the second laser has a non-constant frequency. The transmitter optical system can illuminate a target simultaneously using portions of the first and second laser signals. The receiver optical system can focus a returned light onto the detector array. A first and second illumination point sources can direct portions of the first and second laser signals onto the detector array. The first and second illumination point sources are located in-plane with a pupil of the receiver optical system. The system can detect simultaneously holograms formed on the detector array based on the returned light and the portions of the first and second laser signals directed by the first and second illumination point sources.

Claims

1. A holography imaging system, comprising:

a first laser configured to generate an output having a constant frequency;

a second laser configured to generate an output having a non-constant frequency;

a transmitter optical system configured to illuminate a target simultaneously using a first portion of the output of the first laser and a first portion of the output of the second laser;

a detector array;

a receiver optical system configured to receive a returned light from the illuminated target and focus the returned light onto the detector array;

a first illumination point source configured to radiate a second portion of the output of the first laser, wherein a location of the first illumination point source is in-plane with a pupil of the receiver optical system, and the first illumination point source is configured to direct the second portion of the output of the first laser onto the detector array; and

a second illumination point source configured to radiate a second portion of the output of the second laser, wherein a location of the second illumination point source is in-plane with the pupil of the receiver optical system, and the second illumination point source is configured to direct the second portion of the output of the second laser onto the detector array,

wherein the holography imaging system is configured to detect simultaneously holograms formed on the detector array based on the returned light, the second portion of the output of the first laser, and the second portion of the output of the second laser.

2. The holography imaging system of claim 1,

wherein the returned light comprises a first returned light and a second returned light, wherein the first returned light is based on the first portion of the output of the first laser, wherein the second returned light is based on the first portion of the output of the second laser,

wherein the holograms comprise a first hologram and a second hologram,

wherein the first hologram is based on the first returned light and the second portion of the output of the first laser,

wherein the second hologram is based on the second returned light and the second portion of the output of the second laser,

wherein the first portion of the output of the first laser comprises a first wavelength, wherein the second portion of the output of the first laser comprises the first wavelength, wherein the first returned light comprises the first wavelength, wherein the first hologram comprises the first wavelength,

wherein the first portion of the output of the second laser comprises a second wavelength, wherein the second portion of the output of the second laser comprises the second wavelength, wherein the second returned light comprises the second wavelength, wherein the second hologram comprises the second wavelength,

wherein the first wavelength is different from the second wavelength, and

wherein the holography imaging system is configured to detect the first hologram and the second hologram simultaneously.

3. The holography imaging system of claim 1, wherein the second laser is configured to operate with linear frequency modulation enforced with a frequency control servo.

4. The holography imaging system of claim 1, wherein the second laser is configured to operate with stepped frequency changes with an equal step size.

5. The holography imaging system of claim 1,

wherein the first and second illumination point sources are in an exit pupil plane of the receiver optical system,

wherein the receiver optical system has the exit pupil plane, and a (x,y) coordinate system is defined in the exit pupil plane, wherein an exit pupil is centered at coordinates (0,0),

wherein the first illumination point source is located at coordinates

6. The holography imaging system of claim 5,

wherein the second illumination point source is located at coordinates

7. The holography imaging system of claim 1, wherein the detector array is located in a pupil plane of the receiver optical system.

8. The holography imaging system of claim 1, wherein the detector array is located in an intermediate plane of the receiver optical system.

9. The holography imaging system of claim 1, comprising one or more laser amplifiers configured to amplify at least one of a power of the first laser or a power of the second laser.

10. The holography imaging system of claim 1, wherein at least one of the first laser, the second laser, or one or more laser amplifiers is configured to operate in a continuous wave mode.

11. The holography imaging system of claim 9, wherein at least one of the first laser, the second laser, or the one or more laser amplifiers is configured to operate in a continuous wave mode.

12. The holography imaging system of claim 1, comprising one or more amplitude modulators,

wherein the holography imagining system is configured to operate in a pulse mode,

wherein the first portion of the output of the first laser comprises a first pulse,

wherein the first portion of the output of the second laser comprises a second pulse,

wherein the returned light comprises a returned pulse, wherein the returned pulse is based on the first pulse and the second pulse,

wherein the second portion of the output of the first laser comprises a third pulse,

wherein the second portion of the output of the second laser comprises a fourth pulse,

wherein the holography imaging system is configured to illuminate the target with the first and second pulses simultaneously,

wherein the holography imaging system is configured to detect simultaneously the holograms formed on the detector array based on the returned pulse, the third pulse and the fourth pulse, and

wherein a pulse duration of the returned pulse is shorter than a coherence time of the returned pulse.

13. The holography imaging system of claim 9, comprising one or more amplitude modulators,

wherein the holography imagining system is configured to operate in a pulse mode,

wherein the first portion of the output of the first laser comprises a first pulse,

wherein the first portion of the output of the second laser comprises a second pulse,

wherein the returned light comprises a returned pulse, wherein the returned pulse is based on the first pulse and the second pulse,

wherein the second portion of the output of the first laser comprises a third pulse,

wherein the second portion of the output of the second laser comprises a fourth pulse,

wherein the holography imaging system is configured to illuminate the target with the first and second pulses simultaneously,

wherein the holography imaging system is configured to detect simultaneously the holograms formed on the detector array based on the returned pulse, the third pulse and the fourth pulse, and

wherein a pulse duration of the returned pulse is shorter than a coherence time of the returned pulse.

14. The holography imaging system of claim 1, comprising:

one or more memories comprising instructions stored therein; and

a processor configured to execute instructions to;

record the holograms formed on the detector array,

transform the recorded holograms using two-dimensional Fourier transformation into transformed data in a Fourier domain;

perform data cropping on the transformed data in a first portion of the Fourier domain to segregate first pupil plane optical field data associated with the first laser, and perform inverse two-dimensional Fourier transformation on the first pupil plane optical field data to obtain a first set of complex-valued coherent images associated with the first laser;

perform data cropping on the transformed data in a second portion of the Fourier domain to segregate second pupil plane optical field data associated with the second laser, and perform inverse two-dimensional Fourier transformation on the second pupil plane optical field data to obtain a second set of complex-valued coherent images associated with the second laser, wherein the second portion is different from the first portion; and

perform conjugate product combination of each frame of the first set of complex-valued coherent images with each corresponding frame of the second set of complex-valued coherent images to produce a set of conjugate product images.

15. The holography imaging system of claim 14, wherein the instructions to perform conjugate product combination comprises removing target motion induced phase errors between the frames, of the first set of complex-valued coherent images and the second set of complex-valued coherent images, on a pixel-by-pixel basis by shifting a phase in each pixel of the frames of the second set of complex-valued coherent images using a phase measured in a corresponding pixel of the frames of the first set of complex-valued coherent images.

16. The holography imaging system of claim 14, wherein the processor is configured to execute instructions to average together frames of the set of conjugate product images, wherein each frame in the set of conjugate product images to be averaged is collected at a common offset frequency between the first and second lasers, to produce a second set of conjugate product images.

17. The holography imaging system of claim 14, wherein the processor is configured to execute instructions to perform a three-dimensional image formation data processing operation, wherein the three-dimensional image formation data processing operation comprises:

a global phase compensation operation to remove global phase errors between images of the set of conjugate product images to produce a third set of conjugate product images;

a one-dimensional Fourier transformation operation performed on each lateral element of the third set of conjugate product images to produce a 3D image;

a peak estimation operation to compute a range for each lateral element in the 3D image, comprising Fourier domain zero-padding to compute interpolated range points and fitting a parabola to a peak interpolated point along with two adjacent points, wherein a summit of the parabola is a range report;

a peak selection operation wherein (a) a lateral element is selected for processing, (b) a neighborhood of lateral element range reports is defined surrounding the selected lateral element, (c) a median of the neighborhood range reports is computed, (d) a standard deviation of the neighborhood range reports is computed, and (e) the range report of the selected lateral element is compared to the median, wherein if a difference between the range report and the median is less than or equal to the standard deviation, accepting the range report, and if not, rejecting the range report and repeating the above test of (b) through (e) for the next highest peak in the selected lateral element, and then repeating the above process of (a) through (e) for each lateral element in the 3D image.

18. The holography imaging system of claim 14, wherein the holography imaging system is configured to operate on a moving platform in a translational synthetic aperture imaging configuration, the holography imaging system is configured to observe a rotating target in an inverse synthetic aperture imaging configuration, wherein target rotation provides a diversity of look angles, or the holography imaging system is configured to operate using a combination of both of the configurations.

19. The holography imaging system of claim 14, wherein the processor is configured to execute instructions to perform a synthetic aperture ladar image formation data processing operation, wherein the synthetic aperture ladar image formation data processing operation comprises:

for each lateral element of the set of conjugate product images, arranging data in the each lateral element into a two dimensional space, based on a value of an offset frequency and an along track position, wherein the value of an offset frequency is determined based on a difference between the first and second laser frequencies, wherein a value of the along track position is determined based on a relative position of a platform along the platform'"'"'s trajectory and the target;

applying an autofocus operation on at least some of the data in the two dimensional space;

for each lateral element of the set of conjugate product images, applying two-dimensional Fourier transformation on the data in the two dimensional space corresponding to the lateral element, to produce a two-dimensional synthetic aperture ladar (2D SAL) image corresponding to each lateral element of the set of conjugate product images;

incoherently averaging the 2D SAL images, wherein the 2D SAL images are multi-look images; and

stitching together all of the 2D SAL images into a larger wide area SAL image.

20. The holography imaging system of claim 19,

wherein the incoherently averaging the 2D SAL images comprises averaging together the 2D SAL images, wherein each of the 2D SAL images to be averaged is collected with independent speckle realizations.