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May 03, 2010

Holographic Adaptive Optics

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Air Force Office of Scientific Research-supported holographic, adaptive, optics research may help transform software into computer-free, electronics for unmanned aerial vehicles, high energy lasers and free-space optical communications that will enable each to run faster and more efficiently than before

"The current system for UAV imagery, lasers and optics is computer software driven, but the next phase is to replace that with an electronics system called High Altitude Large Optics," he said. "Such a system would be orders of magnitude faster than anything else available, while being much more compact and lightweight."

It is hoped that HALOS will become the standard in adaptive optics of the future. It may also create entirely new markets for sharper telescopes and camera images that will be used for military purposes.
Holographic Adaptive Optics (7 page paper from 2009)

For the last two decades adaptive optics has been used as a technique for correcting imaging applications and directed energy/laser targeting and laser communications systems affected by atmospheric turbulence. Typically these systems are bulky and limited to < 10 kHz due to large computing overhead and limited photon efficiencies. Moreover most use zonal wavefront sensors which cannot easily handle extreme scintillation or unexpected obscuration of a pre-set aperture. Here we present a compact, lightweight adaptive optics system that utilizes a hologram to perform an all-optical wavefront analysis that removes the need for any computer.


The critical design/performance issues of the holographic adaptive optics system can be summarized as follows:
1. Speed: Using photon counting multi-pixel photon counter (MPPC) arrays we can achieve wavefront measurements limited only by the detector response. The closed-loop performance is thus limited only by the deformable mirror bandwidth which for some MEMS-based systems can be over 1kHz.

2. Computational overhead: The key significance of the holographic adaptive optics system is the removal of the need for computations to assess the wavefront phase error. In the closed-loop system shown above, the entire wavefront correction is achieved autonomously. This not only reduces the footprint and mass but greatly improves the ruggedness and usefulness. Furthermore, increasing the number of actuators has no affect on speed as in conventional devices, so this system has many applications in areas of extreme turbulence where D/ro can be very large.

3. Obscuration/Scintillation: Holographic AO utilizes a modal detection method, so unlike conventional zonal phase characterization, the system is virtually insensitive to obscuration or scintillation effects. Dramatic variations in intensity are all factored out when comparing the brightness of two spots because the measurement is based on the relative intensities rather than the absolute intensities.

4. HEL operation: Holographic adaptive optics can be configured for both high power applications and infrared wavelengths. While IR holograms are less commonplace, several photopolymers can provide reasonably high efficiency. Of course, there is still the possibility of sensing at one wavelength (with a visible illuminating laser, say) and correcting for another. The only requirement would be the use of a dichroic beamsplitter in the set-up.

5. Other applications: The high speed and scintillation insensitivity of holographic AO makes it ideal for directed energy weaponry where a laser beam travels through fast-changing, highly turbulent airflows. Spin-off applications include lightweight, compact image correction systems for UAVs, free-space optical communications, eye surgery systems and intra-cavity laser correction.

We have presented a novel holographic adaptive optics system that uses a hologram to deconvolve a wavefront phase in terms of the precise response functions of actuators in the corrective deformable mirror. We have constructed a working closed-loop prototype with a CCD and software control. Ultimately, however, the system will be configured to operate autonomously without any computer in the loop with the appropriate detectors. Beyond the significant speeds achievable the system offers improvements over conventional Shack-Hartmann or curvaturebased AO systems in compactness, simplicity, ruggedness and insensitivity to scintillation.


Fast autonomous holographic adaptive optics (2010)
We present details of a MEMS-based holographic adaptive optics system. The modal wavefront sensing relies on measuring the intensity of focal spots using a multiplexed hologram and multi-pixel photon counter. The basis set for the sensing is a direct recording of the actuator responses in the deformable mirror. This allows us to directly control the wavefront correction in closed loop without need for any calculations or computer. The entire system is compact and lightweight and the limiting speed is set only by the dynamics of the deformable mirror and not the number of actuators
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