Optical design of spectral filtering system in sequentially timed all-optical mapping photography for enhancing temporal resolution.

Ultrafast imaging is crucial for understanding phenomena in the femtosecond to nanosecond time domains. Among ultrafast imaging techniques, sequentially timed all-optical mapping photography utilizing spectral filtering enables single-shot acquisition of ultrafast images with high spatial resolution and high quality. However, conventional designs based on Fourier optics struggle to achieve high temporal resolution while maintaining high pixel resolution, because the spatial-spectral dependence in this configuration makes laser wavelengths within each frame broad, resulting in longer exposure times. Here we propose an optical design that minimizes the bandwidth of laser wavelengths within each frame to achieve high temporal resolution by collimating the beam incident on the diffractive optical element. Numerical analysis showed that increasing the magnification of the imaging system before the diffractive optical element sufficiently narrows the bandwidth within each frame compared to the conventional designs. We experimentally demonstrated the effectiveness of the proposed configuration, achieving a bandwidth of 0.9 nm and a wavelength interval of 2.1 nm. These spectral properties enabled imaging with a 1.4 ps frame interval and an exposure time of 0.8 ps, which is 40% of the exposure time in the conventional Fourier configuration with similar setup parameters. Furthermore, the proposed configuration maintained a high pixel resolution of 480 pixels × 480 pixels for each of the five frames and was successfully applied to visualize laser ablation of glass. This article presents a highly spatiotemporally resolved imaging method for the detailed analysis of ultrafast phenomena such as laser ablation, shockwaves, and electric discharges.