Rapid single-pixel autofocus using a single Fourier modulation pattern

Conventional autofocus (AF) algorithms rely on sharpness metrics derived from multi-pixel image sensors. Their performance degrades in low-illumination, low-contrast, or cost-sensitive scenarios. We report a non-imaging, single-pixel AF technique that employs a single Fourier-Basis modulation (SFBM) pattern to probe the optical system. Because a single spatial frequency is extremely sensitive to defocus, the optimal focal plane can be identified directly from the detector response without image reconstruction. Only a static modulation pattern and a single-pixel detector are required, which markedly simplifies the hardware and computation. Experiments demonstrate a focusing rate of 500 kHz, surpassing the digital micromirror device (DMD) switching limit and enabling operation under low-light, broadband, and even terahertz conditions. The proposed SFBM-AF scheme therefore provides a compact, high-speed alternative to traditional image-based autofocus and is suitable for a wide range of optical applications.