“Hidden Phase” in Two-wavelength Adaptive Optics
Document Type
Article
Publication Date
3-13-2024
Abstract
Two-wavelength adaptive optics (AO), where sensing and correcting (from a beacon) are performed at one wavelength 𝜆𝐵 and compensation and observation (after transmission through the atmosphere) are performed at another 𝜆𝑇 , has historically been analyzed and practiced assuming negligible irradiance fluctuations (i.e., weak scintillation). Under these conditions, the phase corrections measured at 𝜆𝐵 are robust over a relatively large range of wavelengths, resulting in a negligible decrease in AO performance. In weak-to-moderate scintillation conditions, which result from distributed-volume atmospheric aberrations, the pupil-phase function becomes discontinuous, producing what Fried called the “hidden phase” because it is not sensed by traditional least-squares phase reconstructors or unwrappers. Neglecting the hidden phase has a significant negative impact on AO performance even with perfect least-squares phase compensation. To the authors’ knowledge, the hidden phase has not been studied in the context of two-wavelength AO. In particular, how does the hidden phase sensed at 𝜆𝐵 relate to the compensation (or observation) wavelength 𝜆𝑇 ? If the hidden phase is highly correlated across 𝜆𝐵 and 𝜆𝑇 , like the least-squares phase, it is worth sensing and correcting; otherwise, it is not. Through a series of wave optics simulations, we find an approximate expression for the hidden-phase correlation coefficient as a function of 𝜆𝐵 , 𝜆𝑇 , and the scintillation strength. In contrast to the least-squares phase, we determine that the hidden phase (when present) is correlated over a small band of wavelengths centered on 𝜆𝑇 . Over the range 𝜆𝐵,𝜆𝑇∈ [1,3] µm and in weak-to-moderate scintillation conditions (spherical-wave log-amplitude variance 𝜎2𝜒∈ [0.1,0.5] ), we find the average hidden-phase correlation linewidth to be approximately 0.35 µm. Consequently, for |𝜆𝐵−𝜆𝑇| greater than this linewidth, including the hidden phase does not significantly improve AO performance over least-squares phase compensation.
Source Publication
Applied Optics
Recommended Citation
Milo W. Hyde, Jack E. McCrae, Matthew Kalensky, and Mark F. Spencer, "“Hidden phase” in two-wavelength adaptive optics," Appl. Opt. 63, E1-E9 (2024). doi: 10.1364/AO.516039
Comments
The "Link to Full Text" on this page opens the full article at Optica.
Author Mark F. Spencer co-affiliated with Joint Directed Energy Transition Office.