Measuring Phase Errors in the Presence of Scintillation

TL;DR

This paper introduces a novel Fresnel wavefront sensor that effectively measures phase errors in strong turbulence conditions, outperforming traditional sensors like Shack-Hartmann in scintillation-heavy environments, especially at low light levels.

Contribution

The authors designed and tested a Fresnel sensor capable of robust phase measurement under strong turbulence, demonstrating significant sensitivity improvements over existing wavefront sensors.

Findings

Fresnel WFS maintains phase measurement capability despite scintillation.

Offers 9x sensitivity gain over Shack-Hartmann WFS at high scintillation levels.

Operates effectively at very low SNR conditions (~2-3 per pixel).

Abstract

Strong turbulence conditions create amplitude aberrations through the effects of near-field diffraction. When integrated over long optical path lengths, amplitude aberrations (seen as scintillation) can nullify local areas in the recorded image of a coherent beam, complicating the wavefront reconstruction process. To estimate phase aberrations experienced by a telescope beam control system in the presence of strong turbulence, the wavefront sensor (WFS) of an adaptive optics must be robust to scintillation. We have designed and built a WFS, which we refer to as a "Fresnel sensor," that uses near-field diffraction to measure phase errors under moderate to strong turbulent conditions. Systematic studies of its sensitivity were performed with laboratory experiments using a point source beacon. The results were then compared to a Shack-Hartmann WFS (SHWFS). When the SHWFS experiences irradiance fade in the presence of moderate turbulence, the Fresnel WFS continues to routinely extract phase information. For a scintillation index of $S = 0.55$, we show that the Fresnel WFS offers a factor of $9\times$ gain in sensitivity over the SHWFS. We find that the Fresnel WFS is capable of operating with extremely low light levels, corresponding to a signal-to-noise ratio of only $\mbox{SNR}\approx 2-3$ per pixel. Such a device is well-suited for coherent beam propagation, laser communications, remote sensing, and applications involving long optical path-lengths, site-lines along the horizon, and faint signals.