Code-multiplexed read-out for ladar systems

Details Code-multiplexed read-out for ladar systems Code-multiplexed read-out for ladar systems

2001-09-05

2003-09-09

Buczinski, Stephen C. (Department: 3662)

Optics: measuring and testing

Range or remote distance finding

With photodetection

C356S141100, C348S302000, C348S332000

Reexamination Certificate

active

06618125

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to laser detection and ranging (ladar) systems; and more particularly, to an incoherent ladar which utilizes code division multiplexing for capturing image data on focal plane detector arrays.
2. Description of the Prior Art
Use of ladar technologies to form images of targets is set forth in U.S. Pat. Nos. 5,608,514, 5,877,851 and a U.S. Provisional Application filed Jun. 29, 2001 entitled “A Ladar System For Detecting Objects” assigned to the same entity, and herein incorporated by reference.
Ladar is basically radar transformed from a system for measuring reflected radio frequency electromagnetic radiation, typically in the 1-25 cm wavelength range to locate remote objects, to a system that operates on laser radiation, typically in the 0.1 to 10.0 &mgr;m range. The resulting combination of radar principles and optics provides a system with inherently enhanced accuracy in the measurement of range, velocity, and angular displacement. Moreover, the high carrier frequency allows ladar systems to be made compact in physical dimension, which is particularly attractive in aircraft, projectile, space and other volume-limited applications.
U.S. Pat. No. 5,608,514, (the '514 patent) issued Mar. 4, 1997 describes a technique for achieving high range resolution for a single pixel ladar by employing frequency modulation (FM) radar ranging principles.
Three-dimensional imaging of a scene is achieved by mechanically scanning the single pixel ladar or by building arrays of such ladars. To perform ranging on a single pixel, the ladar's laser transmitter is amplitude modulated with a radio-frequency subcarrier which itself is linearly frequency modulated. The target-reflected light is incoherently detected with a photodiode and converted into a voltage waveform.
The voltage waveform is then mixed with an undelayed sample of the original laser amplitude modulated waveform. The output of the mixer is processed to remove “self clutter” that is commonly generated in FM ranging systems and obscures the true target signals. The clutter-free mixer output is then Fourier transformed to recover target range.
The '514 patent uses a mechanical scanner to scan the laser output beam and the receiver or detected field of view to form an electronic image of the target. The development of a scannerless ladar generally requires a receiver, which incorporates focal plane array (FPA) technology, such as found in imaging cameras.
As a consequence, another receiver design was conceived and disclosed in U.S. Pat. No. 5,877,851 (the '851 patent) where an electro-optic (EO) light modulator, positioned in the receiver light path, performs demodulation optically and a standard focal plane array performs detection of the demodulated light. Thus, while the laser transmitter is modulated as in the '514 patent, the entire scene or field of view is illuminated.
The method and apparatus of the '851 patent may be applied to both one-dimensional and two-dimensional detector arrays having any number of detectors. Numerous image frames are recorded periodically over the frequency modulation (FM) period. A Fourier transform taken over time for a pixel establishes the range to the target in that pixel. Performing the Fourier transform for all pixels yields a three-dimensional image of objects in the field of view. Using a laser illuminator, a focal plane detector array, electro-optical light modulator, and microwave circuit elements in conjunction with FM-CW radar ranging theory yields a scannerless ladar possessing high range resolution without range ambiguities.
While the ladar system of the '851 patent is scannerless, there are limitations. As an example, the ladar approach of the '851 patent requires a high speed electro-optic modulator such as a quantum-well device (QWEO) commensurate in size with the ladar focal plane array (FPA). For a QWEO modulator that matches the size of a typical focal plane array (4×4 mm), the capacitive load presented by such a device is in the low 1000's of picofarads which is difficult to drive using microwave amplifiers. Another limitation with this approach is that the bandgap of the QWEO modulator and the laser line should remain coincident over temperature variations which adds to system complexity.
With the provisional application, the demodulation and detection functions of the EO light modulator and the FPA, respectively are combined in a FPA comprised of “self-mixing” detectors. Image frames from the focal plane arrays of “self-mixing” detectors are processed analogously to the ‘851’ patent. In both the ‘851’ patent and the provisional patent, the frame-rates required by the ladar system for a variety of applications will often exceed the ability of existing techniques to read data out of the FPA's.
SUMMARY OF THE INVENTION
A ladar system and method for detecting a target, comprises a signal generator for generating a waveform signal, a laser for producing a waveform modulated light signal, a mixing and detecting means coupled to and driven by the signal generator for converting reflected light signals received from the target to electrical signals and for mixing the converted electrical signals with the waveform signal to produce an output electrical signal containing information for detecting the target. The ladar system also comprises a read-out means connected to the mixing and detecting means for removing the output electrical signals from the mixing and detecting means and a signal processor connected to the read-out means for processing the output electrical signals to detect and determine the range to the target. The mixing and detecting means comprises a plurality of detecting elements wherein each detecting element detects and converts reflected light signals received from the target to electrical signals and the read-out means comprises a code generator for generating a different code signal for each of the detecting elements and for combining the code signal for a each detecting element with the output electrical signal produced by the respective detecting element during the waveform period to produce combined output signals for each of the detecting elements. The read-out means further comprises a summing means for summing the combined output signal for each detecting element in a pre-selected group of the detecting elements to produce a summed group output signal. The read-out means further comprises a de-multiplex means for separating the output electrical signal of a detecting element in a the group from the summed group output signal by multiplying the code signal for the detecting element by the summed group output signal to generate a product that contains the output electrical signal of the detecting element and an integrating means for recovering the output electrical signal.
The invention also comprises a method and apparatus for processing light signals received from an object by a focal plane array having a plurality of detecting elements, wherein each detecting element detects and converts a reflected light signal received from the object to an electrical signal, and a read-out circuit connected to the focal plane array for extracting the converted electrical signals from each of the plurality of detecting elements in the array and for combining the extracted signals to form an image of the object.
The above brief description sets forth rather broadly the more important features of the present invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described and which will form the subject matter of the claims appended hereto.
Accordingly, one object of the present invention to provide a ladar system of efficient design that is capable of being used for high speed requirements.
Another object of the present invention is to provide a m

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