No abstract

A method of speckle suppression without any active device is expected for pico-projectors. The effectiveness of the passive method of speckle reduction using a single multimode fiber and a multimode fiber bundle was actually measured and theoretically analyzed. The dependences of the speckle contrast and speckle suppression coefficient on the parameters of multimode fiber and projection systems were investigated. Our results shown that the efficiency of speckle suppression was limited because only the radial direction of the objective lens aperture was used. An improvement using both of the radial and azimuthal directions of the objective lens aperture is required.

Static laser speckle suppression using multimode fibers has practical limitations as the technique requires an extremely long fiber to achieve an acceptable speckle contrast. An effective method based on liquid light guides was developed in this study to suppress laser speckle. In this study, a speckle simulation model of the liquid light guide was established for numerically calculating the speckle contrast without solving the analytical solution of the photon diffusion equation. The obtained simulation results were compared with the experimental results for the dependence of speckle contrast on the required length and numerical aperture with different liquid core types of liquid light guides. A speckle contrast of 12% and a speckle suppression efficiency of 5 was achieved at the end of a 2.4 m long liquid light guide. For the same fiber length, liquid light guides were found to suppress speckle more efficiently when compared to multimode fibers.

The speckle phenomenon produced by coherent waves interfering with each other is undesirable in laser imaging systems. For each of the laser speckle reduction methods in the literature, it is difficult to reduce speckle to an extremely low level (<3%) and also ensure good image quality. Therefore, a compound speckle reduction method based on the combination of a vibrating multimode fiber and a tracked moving flexible DOE loop is proposed and demonstrated for the first time. We have experimentally demonstrated the effectiveness of the proposed compound method, which can reduce the speckle contrast to 1.96% and obtain good spot quality. The relationship between the time-averaging effect of the speckle patterns from a vibrating multimode fiber and from a tracked moving DOE loop is discussed thoroughly. Our experimental results are in good agreement with Goodman's speckle theory. We expect that the compound speckle reduction method we proposed will have promising potential for applications in laser imaging systems.

Unmanned missions have become more and more popular in recent years. The related technologies of unmanned ground vehicles and unmanned aerial vehicles are growing rapidly, but research on unmanned surface vehicles (USVs) is rare. Water surface object detection algorithms play a crucial role in the field of USVs. However, achieving an object detection algorithm that balances speed and accuracy in the presence of interference is a difficult challenge. We proposed a network, DBCR-YOLO, that improved the detection accuracy while meeting real-time requirements. Based on YOLOv5, we added an additional detection head for detecting tiny objects. Then, we replaced the downsampling in YOLOv5's backbone network with the proposed double sampling mechanism to solve the problem that paying attention to the key features of objects cannot be done in the downsampling process of YOLOv5. Finally, we substituted the proposed BCR neck for YOLOv5's neck, thus improving the fusion of features between different scales based on fewer parameters and fewer calculations. We tested our network on the water surface object detection dataset. Compared with YOLOv5, DBCR-YOLO improved the detection accuracy by 3.4%. At the same time, DBCR-YOLO achieved the highest accuracy in comparison with other networks.