Synthetic aperture ptychography for super-resolution imaging

The resolution of a conventional imaging system is limited by the size of the lens. Physically enlarging the lens size leads to a bulky, expensive, and highly specialized optical design. Alternatively, synthetic aperture imaging mitigates the direct coupling between image resolution and lens size. In the radio frequency regime, synthetic aperture radar improves imaging resolution by capturing many images of a static object using a mobile recording platform such as an airplane or satellite. The resolution is no longer limited by the size of the lens antenna. Instead, it is determined by the synthetic aperture size of the mobile recording platform.

 

Aperture synthesizing in the radio frequency regime is possible because the full complex field with both intensity and phase can be directly measured by the lens antenna with picosecond timing resolution. However, to perform the same trick with visible light would need a detector to record information at femtosecond temporal resolution, a requirement that is well beyond the capabilities of modern detectors. As a result, it is challenging to perform synthetic aperture imaging using intensity-only measurements in the visible regime.

 

To address this problem, the research group led by Prof. Guoan Zheng from the University of Connecticut reported a super-resolution lensless imaging modality termed synthetic aperture ptychography (SAP). This new modality can widen the imaging field of view and expand the spatial-frequency bandwidth at the same time. Its lensless nature further allows it to be implemented with a variety of coherent imaging setups with radiation sources from infrared, visible, extreme ultraviolet, X-ray, to electrons.

 

The research results are published in Photonics Research, Volume. 10, Issue. 7, 2022 (Pengming Song, Shaowei Jiang, Tianbo Wang, Chengfei Guo, Ruihai Wang, Terrance Zhang, Guoan Zheng. Synthetic aperture ptychography: coded sensor translation for joint spatial-Fourier bandwidth expansion[J]. Photonics Research, 2022, 10(7): 1624).

 

The SAP approach is developed using a coherent diffraction imaging technique termed ptychography. First envisioned for solving crystalline structures, ptychography was proposed to address the missing phase problem in crystallography. In a typical implementation, a specimen is laterally translated through a spatially confined probe beam and the coherent diffraction patterns are recorded at the far-field for image reconstruction. Specimen translation in ptychography essentially synthesizes a large field of view of the object and recovers the full complex field with both intensity and phase.

 

Instead of translating the specimen over a confined probe beam, the SAP approach illuminates the entire stationary object using an extended plane wave. A coded image sensor is then translated at the far-field aperture plane for diffraction data acquisition, as shown in Fig. 1. The coded mask attached on the sensor modulates the diffractive light waves from the object and serves as an effective unconfined probe for complex field recovery.

 

Figure 1 Schematic of the synthetic aperture ptychography (SAP) approach. SAP translates a coded image sensor at the aperture plane for data acquisition (left panel). In the reconstruction process, SAP synthesizes a large complex-valued wavefront at the aperture plane (right panel).

 

"Coded sensor translation in SAP synthesizes a large complex-valued wavefront at the intermediate aperture plane," Zheng said. "By propagating this wavefront back to the object plane, we can widen the imaging field of view in the real space and expand the spatial-frequency bandwidth in the reciprocal space at the same time." Compared to the conventional ptychography, the resolution of SAP is no longer limited by the spanning angle of the detector. "Its resolution is only limited by how far one can translate the coded sensor at the aperture plane," Zheng said.

 

Zheng's team validated the SAP approach with both transmission targets and reflection silicon microchips. A 20-mm aperture was synthesized using a 5-mm sensor, achieving a 4-fold gain in resolution and a 16-fold gain in field of view for complex object recovery. In addition, one can digitally propagate the recovered complex exit wave to any axial position for post-acquisition refocusing. The SAP modality does not require lens elements or reference waves for aperture synthesizing, offering a turnkey solution for super-resolution long-range imaging.