With the increasing demand for large aperture telescopes in astronomical observations, the wavefront sensor, as an important component of adaptive optics telescope systems, is facing more stringent requirements in terms of spatial sampling rate and weak light detection capability.
In recent years, Pyramid Wavefront Sensor (PWFS) has seen rapid development. Among various wavefront sensors, it stands out due to its high energy utilization efficiency, sensitivity, and spatial sampling rate, enabling precise reconstruction of high-order aberrations.
As a result, it has shown extensive application potential in fields such as astronomical adaptive optics, mirror testing, and microscopy imaging. However, the high sensitivity of PWFS also brings limitations to its dynamic range. Traditional solutions rely on mechanical modulation, which undoubtedly increases the complexity of the sensor system.
Through relentless efforts, a research team at the Nanjing Institute of Astronomical Optics and Telescope (NIAOT) developed a novel non-modulated pyramid wavefront sensing method - TA-PWFS (Truncated Axicon - Pyramid Wavefront Sensor).
This method employs a conical optical element to allocate the incident light beam. By using the light beam from the side of the cone for dynamic range extension, while maintaining high sensitivity wavefront detection using the flat part of the beam, sensitivity and dynamic range are successfully decoupled.
In the application of TA-PWFS within closed-loop adaptive optical systems, not only is the high sensitivity of the pyramid wavefront sensor maintained, but its dynamic range is significantly enhanced without the need for tedious dynamic modulation.
The research team designed the optical system (as shown in Figure 1) and key optical components (the phase distribution of the cone and pyramid as shown in Figure 2(a)(b)).
Using TA-PWFS, they conducted comprehensive tests on 1000 randomly selected input wavefronts with PV values ranging from 1.4λ to 5.1λ.
The test results indicated that, without considering inherent errors of the optical system, the wavefront reconstruction accuracy was high, with residual PV values below 1.5×10^-11λ (as shown in Figure 3). The related research findings, "Calibration approach of non-modulated pyramid wavefront sensors for improving the dynamic range," have been published in the academic journal Applied Optics (https://doi.org/10.1364/AO.538334).
Figure 1 : Schematic diagram of TA-PWFS. (a) Structural schematic diagram of TA-PWFS; (b) Arrangement of PWFS output images on the CCD; (c) Arrangement of TA-PWFS output images on the CCD.
Figure 2: Phase distribution diagram of a cone and a pyramid. (a) Phase of the frustum of a cone; (b) Phase of the pyramid.
Figure 3: displays the residual results from reconstructing 100 random atmospheric phase images with an overall tilt.
At present, research teams have conducted in-depth studies on non-modulated pyramid wavefront sensing method. Further experimental testing and applied research are still ongoing, with the potential to overcome the limitation of small dynamic range in non-modulated pyramid wavefront sensors.
This method aims to achieve high-precision and large-dynamic-range wavefront detection. It holds promising application prospects in the field of wavefront detection for large-aperture telescopes.
This research was supported by the National Key Research and Development Program of China (2022YFA1603001) and the National Natural Science Foundation of China (12173063).
DOI: https://opg.optica.org/ao/abstract.cfm?uri=ao-63-29-7767
