Imaging through single multimode fiber prevails its counterpart using single mode fiber bundle on spatial resolution limit and minimum radius. Current multimode fiber imaging suffers from fussy calibration, which can be reduced by recent developed compressive sensing scheme [ [L. V. Amitonova, Opt. Lett. 43, 5427 (2018)]. Experiments demonstrate improvement on depth of field by more than three orders of magnitude, together with robustness against macro fiber bending, which is vital to endoscopic applications.

When the sampling data of ghost imaging are recorded with less bits, i.e., experiencing quantization, a decline in image quality is observed. The fewer bits that are used, the worse the image one gets. Dithering, which adds suitable random noise to the raw data before quantization, is proved to be capable of compensating image quality decline effectively, even for the extreme binary sampling case. A brief explanation and parameter optimization of dithering are given.

Fiber bend is a major challenge of multimode fiber (MMF) imaging. More robustness against fiber bend is demonstrated in compressive MMF imaging using mean speckle patterns captured at multiple potential bending configurations beforehand, rather than sticking to single patterns at initial configuration. Experiments demonstrate an overall quality improvement on recovered images than previous work, which is important for robust endoscopic application.

Long-distance radio frequency (RF) synchronization through fiber link is attracting more attention in recent years. The repeater becomes increasingly important with the increase of the transmission distance. In this article, we analyze the three main relay methods principally including single-span, signal relay, and cascade connection frequency transmission and compare their frequency instability. The proof-of-concept experiments are also conducted along a 3009.8 km fiber link in the laboratory environment. The system used for comparison is a combination of phase conjugation and phase-locked loop (PLL) designed for long-distance frequency transmission. The best transfer preference can be obtained by using the cascade connection, whose frequency instabilities of $8.8 \times {10^{ - 14}}$ @ 1 s and $8.4 \times {10^{ - 17}}$ @ 10 000 s have been measured for phase-stable RF signal. Our study can be useful for the calibration and comparison of ultra-long-distance atomic clocks.

Rather than the commonly used sparsity constraint, a new assumption taking advantage of regularity between rows or columns of a two-dimensional image is introduced to ghost imaging using compressive sensing, namely, low-rank constraint. Both simulation and experiment suggest explicit improvement on image quality of ghost imaging under low-rank constraint over ghost imaging under sparsity constraint (GISC), especially for under-sampling cases, and being in particular advantageous on image smoothness assessed by equivalent numbers of looks. Robustness of low-rank parameter setting is demonstrated. Low-rank constraint is also shown to be a powerful reference-less image quality enhancement tool for images restored by GISC.