Past research in adaptive optics (AO) has demonstrated the link between apparent beacon extent and wavefront gradient estimation sensitivity, or optical gain, of a classical Shack-Hartmann (SH) subaperture when using quad-cell detector regions. Pixel diffusion and residual wavefront error broaden the effective subaperture point spread functions as the atmospheric seeing varies in time. Although the AO community has generally shifted toward resolved subapertures to combat these interlinked issues, the quad-cell subaperture design offers efficient light usage for dim beacons, integrating less pixel noise while also reducing sensor readout latency. Particularly for telescopes in poor seeing conditions, in order to reduce beacon magnitude requirements, a quad-cell SH design, coupled with the proposed algorithm, can be an enabling solution. We present research conducted at the Starfire Optical Range over the past 8 years in implementing a robust approach that measures the real-time sensitivity on the site’s natural guidestar and laser beacon AO systems at the 3.5- and 1.5-m telescopes. Emphasis is given toward the practical aspects that must be considered beyond the pure theory, which has been presented in several prior works. A high-signal-to-noise strategy has been implemented that estimates the aperture-averaged subaperture sensitivity (related to beacon size) by exploiting the null space of the least-squares wavefront reconstructor. Careful consideration has gone into the implementation of this estimation method to avoid unintended effects, particularly at low-light levels. Unfortunately, this solution does not in itself address aperture-variant effects, such as sodium beacon elongation for extremely large telescopes.
Polychromatic laser light can reduce speckle contrast in wavefront-sensing and imaging applications that use direct detection schemes. To help quantify the associated reduction in speckle contrast, this study investigates the accuracy and numerical efficiency of three separate wave-optics models that simulate the active illumination of extended objects with polychromatic laser light. The three separate models use spectral slicing, Monte Carlo averaging, and depth slicing, respectively, to simulate the laser-target interaction. The sampling requirements of all three models are discussed. Comparisons to analytical solutions and experimental data are made when possible. In general, the experiments and theory compare favorably with the models.
Digital holography wave-front sensing in the off-axis image plane recording geometry shows distinct potential for directed-energy and remote-sensing applications. For instance, digital holographic detection provides access to the amplitude and wrapped phase associated with an optical field. From the wrapped phase, one can estimate the atmospheric aberrations present and perform adaptive-optics compensation and high-resolution imaging. This paper develops wave-optics simulations which explore the estimation accuracy of digital holography wave-front sensing in the presence of strong atmospheric turbulence and thermal blooming. Specifically, this paper models spherical-wave propagation through varying atmospheric conditions along a horizontal propagation path and formulates the field-estimated Strehl ratio as a function of the image-plane sampling, the coherence diameter, the log-amplitude variance, and the distortion number. Such results will allow one to assess the number of pixels needed in a detector array when using digital holographic detection in the presence of strong atmospheric turbulence and thermal blooming.
Investment in laser technology has led to significant advances in remote sensing, astronomy, industrial processing, and medical technology. To celebrate this rich heritage and promote public awareness in optics and photonics, the SPIE Student Chapter at the Air Force Institute of Technology (AFIT) developed the Laser Propagation Demonstration (LPD). This interactive demonstration serves as one of AFIT’s legacy outreach projects for events involving education in science, technology, engineering, and mathematics (STEM). Initially developed with funding from a LaserFest grant awarded by SPIE in 2010, the goal was to develop a simple hands-on demonstration to highlight the optical effects of diffraction, refraction, and attenuation on laser propagation. Since then, the LPD has undergone several upgrades (thanks to the continued support from a 2012 SPIE Education Outreach Grant) to better highlight these optical phenomena and make it more engaging for a wider range of audiences. This paper celebrates the continued success of the LPD and shares the knowledge gained with an overview of its design and use in STEM-based outreach events.
Adaptive optics is used in applications such as laser communication, remote sensing, and laser weapon systems
to estimate and correct for atmospheric distortions of propagated light in real-time. Within an adaptive optics
system, a reconstruction process interprets the raw wavefront sensor measurements and calculates an estimate
for the unwrapped phase function to be sent through a control law and applied to a wavefront correction
device. This research is focused on adaptive optics using a self-referencing interferometer wavefront sensor,
which directly measures the wrapped wavefront phase. Therefore, its measurements must be reconstructed
for use on a continuous facesheet deformable mirror. In testing and evaluating a novel class of branch-point-
tolerant wavefront reconstructors based on the post-processing congruence operation technique, an increase in
Strehl ratio compared to a traditional least squares reconstructor was noted even in non-scintillated fields.
To investigate this further, this paper uses wave-optics simulations to eliminate many of the variables from a
hardware adaptive optics system, so as to focus on the reconstruction techniques alone. The simulation results
along with a discussion of the physical reasoning for this phenomenon are provided. For any applications using
a self-referencing interferometer wavefront sensor with low signal levels or high localized wavefront gradients,
understanding this phenomena is critical when applying a traditional least squares wavefront reconstructor.
In recent years, optical wave propagation through strong atmospheric turbulence and adaptive optics compensation
thereof has received much attention in literature and technical meetings. At the Air Force Institute of
Technology, recent simulation-based efforts in strong turbulence compensation are expanding into laboratory
experiments utilizing a versatile surrogate turbulence simulator and adaptive optics system. The system can
switch between using two different wavefront sensors, a Shack-Hartmann and a self-referencing interferometer.
Wavefront reconstruction takes place on field programmable gate arrays, operating at kilohertz frame rates. Further,
the system is able to perform reconstruction and control in software for testing of advanced algorithms (at
frame rates below 10 Hz). The entire package is compact enough for transportation to other laboratories and live
test facilities. This paper describes the optical layout, architecture, and initial results of real-time operation.
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