High-resolution spiral phase contrast microscope: From Fourier filtering to match filtering

The wide-field spiral phase contrast microscope (SPCM) is an all-optical edge-enhanced imaging technique that performs Fourier filtering based on a 4f system. It allows large phase gradients to be highlighted through convolution of a phase object with the point spread function (PSF) given by the Fourier transform of the coherent transfer function (CTF). In the case of pupil mismatch between the 4f system and the spiral phase filter (SPF) placed at the Fourier plane, the physical edge diffraction of the SPF is non-negligible and will cause strong sidelobes in the PSF, leading to low image resolution and background noise after convolution. Herein, we propose a scanning SPCM, which uses an Airy spot to sample the phase object laterally point by point and detects the on-axis intensities at the image plane accordingly. This structural change transforms the SPCM from Fourier filtering to match filtering. Match filtering allows the SPCM to optically compute the complex weight that measures the matching degree between the Fourier transform of the local sampled field of the phase object and the joint CTF, which is the convolution of the illumination pupil and the imaging pupil. The edge enhanced image is directly given by the complex weights at the sampling coordinates. The scanning SPCM takes the advantage of the joint CTF, which not only shows a much higher cutoff spatial frequency than the wide-field SPCM, achieving high-resolution imaging, but also shows soft edges, achieving suppressions of physical edge diffraction of the SPF and sidelobes in the joint PSF. The imaging results of phase objects verified that the scanning SPCM achieves twofold resolution improvement. The scanning SPCM breaks through the inherent barrier of low image resolution in the wide-field SPCM by transforming Fourier filtering to match filtering, and it possesses great advantages in all-optical high-resolution edge enhancement.