CMOS-compatible integrated 4-f system for mode-transparent spatial manipulation.

To exploit spatial dimension, on-chip optical modes with various spatial profiles have been utilized in optical interconnects and spatial analog computing. An integrated Fourier optical system is able to perform spatial operations. However, the reported schemes based on a subwavelength structure pose difficulty in fabrication, and the fabrication-friendly structure has been investigated only with a fundamental mode. With the complementary metal-oxide-semiconductor process, we propose an integrated 4-f system with simple geometry and a moderate minimum feature size to manipulate the mode's spatial size and position in a mode-transparent way. A size magnification of 2.5 and center-to-center position offset of 7 µm are experimentally demonstrated. Reasonable insertion loss and low inter-mode crosstalk are measured over a 30 nm bandwidth. The work in this Letter paves the way for an on-chip Fourier optical system with convenient fabrication and broadband operation.