Point-source transient events (PSTEs) – optical events that are both extremely fast and extremely small – pose several challenges to an imaging system. Due to their speed, accurately characterizing such events often requires detectors with very high frame rates. Due to their size, accurately detecting such events requires maintaining coverage over an extended field-of-view, often through the use of imaging focal plane arrays (FPA) with a global shutter readout. Traditional imaging systems that meet these requirements are costly in terms of price, size, weight, power consumption, and data bandwidth, and there is a need for cheaper solutions with adequate temporal and spatial coverage. To address these issues, we develop a novel compressed sensing algorithm adapted to the rolling shutter readout of an imaging system. This approach enables reconstruction of a PSTE signature at the sampling rate of the rolling shutter, offering a 1 to 2 order of magnitude temporal speedup and a proportional reduction in data bandwidth. We present empirical results demonstrating accurate recovery of PSTEs using measurements that are spatially under-sampled by a factor of 25, and our simulations show that, relative to other compressed sensing algorithms, our algorithm is both faster and yields higher quality reconstructions. We also present theoretical results characterizing our algorithm and corroborating simulations. The potential impact of our work includes the development of much faster, cheaper sensor solutions for PSTE detection and characterization.
KEYWORDS: Turbulence, Monte Carlo methods, Signal to noise ratio, Digital holography, Holograms, Sensors, Visibility, Staring arrays, Wavefront sensors, Scintillation
This paper uses wave-optics simulations with weak-to-strong scintillation conditions to model the performance of a digital-holography wavefront sensor (DH-WFS). Via Monte Carlo analysis, these simulations predict the optimal signal and reference strengths of a DH-WFS in the off-axis pupil plane recording geometry (PPRG). Incidentally, the Monte Carlo analysis shows that, despite conventional knowledge, we cannot directly relate the signal-to-noise ratio and the field-estimated Strehl ratio to the hologram fringe visibility. Such results are directly inform ongoing experimental efforts on how to design and build a DH-WFS in the off-axis PPRG to properly handle weak-to-strong scintillation conditions.
This paper makes use of digital-holographic detection in the off-axis image plane recording geometry to determine the Fried parameter of transmissive phase screens. Digital-holographic detection, in practice, provides us with an estimate of the complex-optical field (i.e., both the amplitude and wrapped phase); thus, we can use this estimate for determining the Fried parameter of transmissive phase screens, especially when the resulting aberrations follow Kolmogorov statistics. As such, this paper uses two experimental setups and Lexitek phase plates, which make use of Kolmogorov statistics to create aberrations with a prescribed Fried parameter. In both experimental setups, we place the Lexitek phase plates under test near the single-receiver lens of our digital-holographic system and assume isoplanatic conditions. In the first experimental setup, we uniformly illuminate a chrome-on-glass bar chart backed by Labsphere Spectralon®. We then use digital-holographic detection and an image-sharpening algorithm to indirectly measure the aberrations and determine the Fried parameter. In the second experimental setup, we send a collimated beam through the Lexitek phase plates. We then use digital-holographic detection to directly measure the aberrations and determine the Fried parameter. The results show that the first experimental setup overestimates the prescribed Fried parameter by 20%-60%, whereas the second experimental setup produces less variability with estimates of ±20% of the prescribed Fried parameter.
Digital-holographic detection provides a distinct way forward to combat the low signal-to-noise ratios (SNRs) associated with tactical applications. With that said, past efforts studied the major digital-holographic recording geometries used with tactical applications and derived closed-form expressions for their SNRs. They also used wave-optics simulations to validate the use of these closed-form expressions and found that for real-world scenarios, where we interfere a comparatively weak-signal beam with a strong-reference beam, the associated “weak-strong” expressions are accurate. For a laboratory setting, however, we often set the signal and reference beams to be nearly equal in power. As such, in this paper we derive closed-form expressions for this specific use case. We also use wave-optics simulations to explore the accuracy of these “equal-power” expressions. Overall, the results show that the equal-power expressions are accurate.
The Air Force Research Laboratory (AFRL) Directed Energy Directorate has built a supersonic wind tunnel in order to characterize aero-optical effects. Aero-optics is the study of the effect of aircraft-induced and atmospheric disturbances on the efficiency of optical imaging and laser systems. The Aero-Effects Laboratory (AEL) at AFRL consists of a supersonic wind tunnel with the capability of imaging the turbulent flow with large optical access of its test section. In order to gain access to the area of interest, we must first send a beam of light through the access windows of the wind tunnels test section to measure and visualize the flow. Initial measurements have been made and shockwaves between the mating plates of the test section and nozzle have been observed. This paper describes the optical system designed for resolving these shockwaves with a Schlieren imaging system.
The Air Force Research Laboratory (AFRL) Directed Energy Directorate is completing a supersonic wind tunnel to characterize aero-optics effects in high speed flow fields. Optical characterization is accomplished by transmitting a beam of light transverse to the direction of air flow via access windows, thereby illuminating the flow region, a select volume of which is recorded by a suite of sensors. Quantitative measurements of the flow are made using two wave-front sensors (WFS), a Shack-Hartmann (SH) WFS and a digital holography (DH) WFS. Qualitative measurements are made using a traditional Schlieren imaging system. Parenthetically, in addition to characterization of aero-optics effects, we expect to be able to numerically propagate to different planes in the supersonic flow field to characterize boundary layer effects. This paper reviews our wind tunnel system’s requirements and, in particular, the design of the DH WFS.
The Air Force Research Laboratory Directed Energy Directorate is building a supersonic wind tunnel in order to characterize aero-optical effects. In order to gain access to the test section of interest, we must first send light through the access windows of the wind tunnel’s test section. Thus, one of the initial challenges we face is being able to characterize the aberrations caused by the boundary-layer effects over the look-through windows separate from the aberrations caused by aero-optical effects over the test section of interest. As such, we are designing a dual wave-front sensor (WFS) setup in order to characterize these phenomena. The completed product will use a Shack-Hartmann (SH) WFS, in addition to a digital holography (DH) WFS. For all intents and purposes, the data obtained from the SH WFS will serve as the baseline for all experimentation, since it is the tried and true method for wavefront sensing in a wind tunnel environment. The DH WFS, on the other hand, is a unique method for wavefront sensing in a wind-tunnel environment. By providing us with access to an estimate of the complex-optical field, the DH WFS has the potential to transcend the capabilities of the SH WFS. In particular, we should be able to numerically propagate to various planes of interest in the optical train to characterize boundary layer effects over the look-through windows, separate from the aero-optical effects over the test section of interest. This paper reviews the requirements of our wind-tunnel environment and the design for this proposed dual WFS setup.
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