Measuring phase errors in the presence of scintillation

Area of Science:

• Adaptive Optics
• Optical Engineering
• Atmospheric Turbulence

Background:

• Strong turbulence causes amplitude aberrations (scintillation) that hinder wavefront reconstruction in optical systems.
• Traditional wavefront sensors (WFS) struggle with irradiance fade caused by scintillation, complicating phase aberration estimation.
• Robust WFS are crucial for telescope beam control and coherent beam applications facing atmospheric disturbances.

Purpose of the Study:

• To design and build a novel wavefront sensor (WFS) robust to scintillation for measuring phase aberrations.
• To evaluate the performance of the new Fresnel sensor against a Shack-Hartmann WFS (SHWFS) under turbulent conditions.
• To demonstrate the sensor's capability in low light and high scintillation scenarios.

Main Methods:

• Developed a 'Fresnel sensor' utilizing near-field diffraction for phase error measurement.
• Conducted laboratory experiments with a point source beacon to study sensor sensitivity.
• Compared Fresnel sensor performance directly against a Shack-Hartmann WFS (SHWFS).

Main Results:

• The Fresnel WFS successfully extracts phase information even when the SHWFS experiences irradiance fade.
• Demonstrated a 9x gain in sensitivity for the Fresnel WFS over the SHWFS at a scintillation index of S=0.55.
• The Fresnel WFS operates effectively at extremely low light levels (SNR≈2-3 per pixel).

Conclusions:

• The Fresnel sensor offers a significant advancement for phase aberration measurement in moderate to strong turbulence.
• Its robustness to scintillation and low light performance make it suitable for diverse applications.
• Ideal for coherent beam propagation, laser communications, remote sensing, and long optical path-lengths.