LIDAR device and method for clear and degraded environmental viewing conditions

This application claims the benefit of U.S. Provisional Patent Application No. 61/837,839, filed on Jun. 21, 2013 entitled “LIDAR Device and Method for Clear and Degraded Environmental Viewing Conditions” pursuant to 35 USC 119, which application is incorporated fully herein by reference.

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1. Field of the Invention

The invention relates generally to the field of electronic imaging and LIDAR (Laser, Imaging, Detecting and Ranging) systems. More specifically, the invention relates to a LIDAR device and method having a fine resolution mode and a course resolution mode for clear air and degraded environmental viewing conditions.

2. Description of the Related Art

In general, existing time of flight LIDAR imaging systems include a laser imaging source, appropriate optics in conjunction with a two-dimensional detector array such as a focal plane array or “FPA”, processing circuitry suitable for processing the detector array output into a usable form and post-processing circuitry and software capable of taking the processed detector array output and converting it into a usable format such as a three-dimensional voxel image on an electronic display.

In the operation of a typical prior art time of flight LIDAR system, a set of laser pulses are directed toward and illuminate a desired target or scene of interest. The laser reflections or “echoes” from the entire field of view from the scene are received and imaged upon substantially all of the detectors in a two-dimensional set of detector array pixels using appropriate optics.

Because the time of flight of the returning laser echoes will vary proportionally to the distance from the detector array image plane and the target surface features from which the echoes are received, a three-dimensional image can be calculated based upon the relative echo delays.

The laser target transmission and return energy in a LIDAR system is greatly affected by the medium in which the imaging beam travels and dust or smoke in the atmosphere in which the target of interest is located will affect the quality of the image of a LIDAR system.

What is needed is a LIDAR device and method that can accommodate both a clear atmosphere and be adaptable to environments in which smoke, dust or other particulates (i.e., a degraded environment) exist in the atmosphere around the target

No solution to the variable target environments mentioned is known. These and other aspects, embodiments and features of the invention will be better understood in the context of the accompanying drawings and the following detailed descriptions of preferred embodiments.

FIG. 1 depicts laser illumination scan of the detector field of view for both the narrow field of view and wide field of view modes of operation.

FIG. 2 is a schematic of an exemplar focal plane array with Read-out Integrated Circuit (ROIC).

FIG. 3 is a schematic exemplar of the ROIC for a single pixel (unit cell).

The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims.

It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below.

Turning now to FIGS. 1-3 wherein like references define like elements among the several views, Applicant discloses a LIDAR device and method suitable for imaging in a clear and degraded target environment.

The LIDAR system of the invention is configured to operate with two fields of regard: 1) a clear view mode (i.e., wide field of regard) and, 2) a degraded view mode (i.e., narrow field of regard).

The narrow field permits concentrating the system laser pulse on fewer pixels in order to increase penetration of degraded visual conditions and/or extend range in good visual conditions.

The wide field desirably permits a comparable or matching scene overlay with an optionally-provided system LWIR or SWIR camera.

FIG. 1 depicts how the illumination scan is projected onto the focal plane in the two different modes.

Only the illumination beam of the system is mechanically scanned; the receiver's 256×256 detectors stare into their field of view and are electrically read out as appropriate. It is expressly noted the LIDAR system of the invention is not limited to a 256×256 detector array and that any detector pixel size suitable for the user may be incorporated in the invention.

In the wide field of regard or clear air mode of FIG. 1 , the illumination beam is preferably formed into a 30×3.75 degree rectangle that covers about ⅛ th of the focal plane. A single pulse is fired at position 1 in the illustration of FIG. 1 .

The illuminated portion of the focal plane is read out prior to the illumination beam being directed to position 2 of the clear air mode of FIG. 1 .

In the degraded mode of operation of FIG. 1 , the illumination beam is reduced by a factor of two in each dimension. In the degraded mode example of FIG. 1 , the 15×1.875 degree beam thus covers only 1/16 th of the focal plane. The illuminated pixels are read out prior to the reflected illumination beam being stepped into position 2 of the degraded mode of FIG. 1 and so on.

A small galvo, which can be precisely programmed for each mode, is desirably used to control the illumination scan. In the illustrated embodiment, the degraded mode of operation delivers about four times the laser power to each pixel, extending the range by a factor of two over the wide field of view mode or for increasing the penetration of the laser in degraded conditions to meet variable atmospheric conditions.

A preferred embodiment of the optics of the system may comprise a receiver with a six cm aperture and 72 mm focal length, optimized with 1.5 micron anti-reflection coatings and 150 um pixel size, and deviates only slightly from readily available COTS full-frame interchangeable camera lenses.

The invention comprises a receiver focal plane or FPA which may be comprised a two dimensional array of 256×256 active pixels in a preferred embodiment. Each pixel may be 150×150 microns. The use of a 150×150 micron unit cell and the 130 nm IBM BiCMOS GeSi fabrication process assure adequate bandwidth and real estate for a compact design implementation.

An exemplar structure of the focal plane array of the device is shown in FIG. 2 . The four components of the focal plane may comprise two-side buttable imager/ROIC stacks. This permits the ROIC and detector arrays to be manufactured within reticle limits without stitching. Each array may comprise 128×128 pixels in a physical format of 20×20 mm. The detector array may be bump-bonded onto the ROIC. The resulting detector array may comprise dimensions of about 19.2×19.2 mm.

An exemplar ROIC unit cell circuit is shown in FIG. 3 . The detector array is preferably designed with a common cathode. The bump-bonded anodes are connected to a source follower buffer.

The reflected laser pulse energy received from the target surface is integrated onto the detector's parasitic capacitor. The capacitor is initially charged by a reset switch and then released at T zero . The reflected energy discharges the parasitic capacitor. A high pass filter differentiates the integrated signal and transforms it back into a pulse.

The amplifier of the system boosts the signal above the comparator's offset voltage. The comparator trips states whenever the reflected pulse signal transitions its threshold voltage.

The comparator's transition latches the timing circuit's state. The timing circuit comprises a ring oscillator that is held in reset until the T zero pulse, and a counter circuit.

When the T zero is released, the ring oscillator (inverters with an odd number of stages) begins to toggle “ones” and “zeros” at speeds that can be tailored by design. The ring oscillator's stages necessarily have propagation delay which is used to refine the reflection time of arrival measurement (the entire delay through the ring may be about two nsec).

A counter is placed at the end of the ring oscillator that accumulates the number of “ones” that pass through. When the comparator trips, both the state of the counter and the ring oscillator are captured in a series of latches.

The ring oscillator and counter continue until the end of the range gate. If a second pulse arrives within the range gate, the states are saved in a second set of latches; up to four latches per unit cell may be used if desired. This last latch can be overwritten by a subsequent return pulse, thus assuring that the last reflected pulse-per-pixel is captured. The counter may be synchronized with a master 500 MHz clock to eliminate time drifts.

Each counter latch will be 16 bits long in the illustrated embodiment. Each ring oscillator latch may be four or six bits long. Thus, with a 500 MHz master clock, the counter will allow for about a 35 cm range resolution and the ring oscillator will refine this measurement by a factor of about four or about 6.0-7.5 cm or less.

The total range gate is defined by the 16 bit counter and 500 MHz clock (2.3 Km). The length of the range gate always remains the same, but it can be positioned anywhere in space, for example to search for high reflectivity targets beyond two kilometers by holding the ROIC in reset for a specific period after the T zero .