Measuring phase errors in the presence of scintillation.

Justin R Crepp,Joshua H Follansbee,Stanimir O Letchev,Nicholas T Tusay,Sam J Potier

doi:10.1364/oe.408825

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 × 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 SNR≈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.

Highlights

As a coherent beam of light passes through a turbulent medium, near-field diffraction effects transform phase variations into amplitude variations
For a scintillation index of S = 0.55, we show that the Fresnel wavefront sensor (WFS) offers a factor of 9× gain in sensitivity over the Shack-Hartmann WFS (SHWFS)
We show that the Fresnel WFS can accommodate amplitude variations by measuring near-field diffraction effects along the propagation path

Summary

Introduction

As a coherent beam of light passes through a turbulent medium, near-field diffraction effects transform phase variations into amplitude variations (and vice-versa). As amplitude aberrations increase from moderate levels to strong levels, The phase and amplitude of a propagating beam of light may be retrieved by measuring the intensity of diffracted light at several locations along the optical axis (Fig. 1). Developed for applications in astronomy, a WFS that relies on multiple camera images (generally three or more) recorded on either side of the optical system pupil would offer a large capture range and excellent sensitivity (Guyon 2010). Such a device requires an order of magnitude fewer photons than the SHWFS to reach the same level of wave-front error (Crass et al 2014; Mateen 2015). Named a “non-linear curvature WFS” in the literature, we refer to the device as a “Fresnel WFS”

Methods

Results

Conclusion