Julian Gamboa, Northwestern University; Xi Shen, Northwestern University; Tabassom Hamidfar, Northwestern University; Selim M. Shahriar, Northwestern University
Keywords: Image Recognition, Surveillance, Shift Invariance, Scale Invariance, Rotation Invariance, Digital Holography, Hybrid Opto-electronic Correlator, Polar Mellin Transform.
Abstract:
For some aspects of space situational awareness (SAS), it is necessary to monitor ground sites involved in controlling space vehicles and activities, using unmanned aerial vehicles or space platforms. For this type of surveillance, there is a need to recognize objects in a speedy and robust manner. It is well known that rapid image recognition can be carried out using holographic techniques. However, these techniques have not become of practical utility due largely to lack of suitable materials for dynamic holography. Recently, we showed how to overcome this constraint by resorting to digital holography. In particular, we proposed the Hybrid Opto-electronic Correlator (HOC), which achieves the same functionality as that of a holographic correlator, but uses only Focal Plane Arrays (FPAs), spatial light modulators (SLM), phase stabilization circuits, and VLSI chips [J. Opt. Soc. Am. A 31, 41-47, 2014]. In the HOC, the amplitude and phase information are recorded via FPAs through interference with plane waves. The HOC is able to detect objects in a shift invariant manner. We also showed later that the HOC could be augmented via incorporation of the polar Mellin transform (PMT), thus making it possible to achieve shift, rotation and scale invariance simultaneously [J. Opt. Soc. Am. A 31 No. 6, 1259, 2014]. Recently, we demonstrated experimentally the basic functionality of the HOC [Applied Optics 56, Issue 10, 2754, 2017]. This was followed by our demonstration of the ability of the PMT-augmented HOC to detect images in a shift, rotation and scale invariant (SRSI) manner [Optics Express 27, 16507, 2019].
To recall briefly, the PMT augmented HOC works as follows. First, a laser beam is split into two paths using a beam-splitter (BS). One path leads to a Mach-Zehnder Interferometer (MZI), redirected by a mirror mounted on a piezo-electric transducer, and is used as a phase-stabilization and scanning circuit. This allows us to control the relative phase of the plane waves that then interfere with the image beams. The second path is split into the reference and query arms. Each of these two beams reflects off an SLM and is then directed towards a lens, which produces the Fourier transform of the image at its focal plane. The image beam as well as the reference beam interferes with its corresponding plane wave prior to being detected by an FPA. The signals from the FPAs are then processed in the digital domain, and sent to another SLM for carrying out the final correlation.
For a practical system employing the HOC, it would be necessary to store a large set of PMT images as a database. During field operation, an image captured by a camera will be converted to the PMT version thereof, and used as the query image, while an image retrieved from the database will serve as the reference image. As such, it would be necessary to retrieve many imaged from the database rapidly until a match is found. If the images are stored digitally, this retrieval process would be slow due to the high latency of digital memory. Furthermore, the retrieved images have to be converted to the optical domain, via transfer of the data to an SLM using a serial bus, thus slowing down the process even further. These limits represent bottle-necks in the overall operating speed of the HOC. To circumvent this problem, what is necessary is a holographic memory disc (HMD) from which the images can be retrieved directly in the optical domain, at the speed of light. Here, we report the realization of such an HMD, and demonstration of the operation of the HOC in an SRSI manner using PMT images retrieved rapidly from the HMD.
To realize the HMD, it is necessary to make use of a thick (few mms) substrate made from a material that can produce a permanent volume grating corresponding to the intensity resulting from the interference between a plane wave and an image. No such material is currently available commercially. As such, we have developed such a medium in our laboratory. Briefly, this material consists of PMMA (poly-methyl meth-acrylate), doped with a dye called phenanthrene-quinone (PQ). First, we carried out an extensive series of experimental studies to realize a repeatable process for large and thick discs of this material without any imperfections, such as bubbles or surface roughness. Second, we developed a model for the PQ-PMMA medium as an effective two level system, determined how the refractive index depends on the frequency of the read laser, and demonstrated close agreement between the theoretical model and the experimental result [Optics Materials Express, Vol. 11, No. 11, 3627, 2021]. Third, we developed the algorithm for recording multiple, angle-multiplexed gratings in a single location, written at one wavelength and read-out at a different wavelength [Applied Optics, Vol. 60, No. 28, 8851 (2021)]. Finally, we demonstrated storage and retrieval of PMT images in a PQ-PMMA based HMD, with high fidelity, and preliminary demonstration of its use in an HOC [Optics Express, Vol. 29, No. 24, 40194, 2021].
We have incorporated a high-capacity HMD into the HOC architecture, as follows. First, a set of reference images were converted to PMT images. Second, these images were divided into sub-groups, and each group was recorded in one of many locations on the HMD, using angular multiplexing. The HMD was mounted on a high-speed rotation and translation stage to access each spatial location. An acousto-optic deflector was then used to access sequentially the angle-multiplexed images at each location. The images retrieved from the HMD were then used as reference images during the operation of the HOC in a scale, rotation and shift invariant (SRSI) manner. To expedite the operation of the HOC further, we used a field programmable gate array (FPGA) for PMT conversion of the query image in real time. The incorporation of the FPGA and the HMD into the HOC has paved the way for developing an ultra-fast and robust image recognition system for surveillance relevant to space situational awareness. This work has been supported by AFOSR Grant No. FA9550-18-01-0359.
Date of Conference: September 27-20, 2022
Track: Optical Systems & Instrumentation