The modal holographic wavefront sensor offers a promising alternative to established wavefront sensing techniques. Since the strengths of individual aberration modes are directly measured, there is no need for time-consuming signal processing and wavefront reconstruction. Bandwidths up to three orders of magnitude higher than those of commercial wavefront sensors can in principle be achieved. However, in practice the accuracy of measurements is compromised by intermodal crosstalk, which arises when the wavefront exhibits additional aberrations to those encoded in the modal wavefront sensor. This issue is particularly prominent when measuring wavefronts disturbed by atmospheric turbulence. To mitigate the effects of intermodal crosstalk, a procedure to optimize the sensor design for prevailing atmospheric turbulence conditions has been proposed. In this paper, we experimentally investigate the effectiveness of this method. We describe the fabrication of a holographic wavefront sensor consisting of a thin phase transmission holographic grating. In an optical testbed, defined wavefront deformations are generated using a spatial light modulator, and the wavefront sensor is used to measure these disturbances. The measurement error is determined for different sensor designs.
Bandwidth requirements on adaptive optics systems to be deployed with a view to improving the performance of communications and energy delivery to, or from, fast-moving airborne platforms are much higher than for stationary systems used e.g. in astronomy. This means that both the wavefront sensor and the deformable mirror or phaser shifter must operate at significantly higher frame rates that are currently possible with standard devices. In this presentation, focus is placed on novel wavefront sensors which can fulfill these high bandwidth requirements.
The effectiveness of free-space laser communications is limited due to wavefront deformations caused by atmospheric optical turbulence. To determine these deformations, we propose a wavefront sensor that utilizes the angular selectivity of an optical transmission filter to measure the first derivative of the wavefront.
The transmission filter converts the gradients of the incident wavefront into an intensity distribution. For each direction (x and y) this distribution is captured twice, with different angles between filter and optical axis for each measurement. The contrast of both measurements (calculated for each pixel of the used detector) and the local gradients of the wavefront (averaged across each pixel) has a nearly linear relation. To reconstruct the wavefront from the obtained local gradients, algorithms developed for the Shack-Hartmann wavefront sensor are used. Simulations demonstrate the applicability of the sensor in atmospheric turbulence. For the experimental proof of concept, we have designed and fabricated volume Bragg gratings (VBG) as angular selective filters. The VBGs were implemented in an optical testbed to evaluate the sensor response to wavefront tilts.
The fabrication of an analog holographic wavefront sensor, capable of detecting the low order defocus aberration, was achieved in an acrylamide-based photopolymer. While other implementations of holographic wavefront sensors have been carried out digitally, this process utilises a recording setup consisting only of conventional refractive elements so the cost and complexity of holographic optical element (HOE) production could be much reduced. A pair of diffraction spots, corresponding to a maximum and minimum amount of defocus, were spatially separated in the detector plane by multiplexing two HOEs with different carrier spatial frequencies. For each wavefront with a known aberration that was introduced during playback of the hologram, the resulting intensity ratio was measured in the expected pair of diffracted spots. A number of HOEs were produced with the diffraction efficiency of the multiplexed elements equalized, for a range of diffraction efficiency strengths, some as low as <5%. These HOEs were used to successfully classify four amounts of the defocus aberration through the observed intensity ratio.
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