Objective The coexistence of rain and fog is a common atmospheric phenomenon in winter. When laser is transmitted in rain and fog weather, the attenuation is not only affected by rain but also by fog. Because of the small rainfall rate, fog attenuation is usually greater than rain. Globally, numerous studies have been conducted on the transmission characteristics of laser in rain and fog individually, but research on laser transmission characteristics in rain and fog coexisting weather is inadequate. To the best of our knowledge, the interaction between raindrops and fog droplets has not been considered. In this study, based on the mechanism of rain clearing fog, we improve the existing models and propose a prediction model of atmospheric attenuation in rain and fog coexisting weather, which dynamically shows the changes of atmospheric attenuation and transmittance with time in rain and fog coexisting weather. We believe that the findings of this study will have reference significance for the estimation and evaluation of atmospheric attenuation in wireless optical communication and related fields.
Methods In rain and fog coexisting weather, the precipitation process has a significant effect on fog removal. As fog is removed by raindrops, the scale distribution of fog will change. In this article, we employ the general dynamic equation considering wet deposition to study the dynamic change of fog with the removal of raindrops. Then, we use the lognormal scale distribution model of raindrops and Gamma distribution model of radiation fog and advection fog to calculate the total attenuation of rain and fog after clearing. Further, we employ Lambert-Beer law to reckon the transmittance of laser after a certain distance. Finally, the numerical results are compared with the Monte-Carlo simulation results to verify the rationality of the proposed model to a certain extent.
Results and Discussions The rainfall intensity positively correlates with the fog removal effect. Since the rainfall in rain and fog coexisting weather is small, the attenuation of fog gradually decreases with the removal of fog by rain (Fig. 1). When the rainfall rate is 1 mm/h, the transmittance of advection fog tends to be stable after 5 h of rainfall, whereas, the radiation of fog takes a longer time (Fig. 2). The water content of advection fog is higher than that of radiation fog, so the albedo of advection fog is higher than that of radiation fog. Owing to the obvious removal of advection fog by rain, the density of advection fog decreases, the proportion of raindrops per unit volume increases, the absorption increases, the scattering weakens, and the albedo of particle swarm increases (Tables 4 and 5). When the transmission distance is 1000 m and transmittance is less than 5%, the transmittance calculated using the Monte-Carlo method is larger than that calculated using the Lambert-Beer law, and vice versa in other cases.
Conclusions In this article, we employed the lognormal scale distribution model of raindrops and Gamma distribution model of radiation fog and advection fog to study dynamic change of fog with raindrop removal using the general dynamic equation considering wet deposition. Based on the basic principle of atmospheric attenuation, the attenuation of laser propagation in the atmosphere changes with the fog scale distribution model. Because most of the radiation fog droplets are medium-sized aerosols, they are difficult to be wet removed by rainfall, so the transmittance increases slowly with time. The droplet size of the advection fog is large and quickly removed in the case of moderate and heavy rain, and then the transmittance is at a fixed value. Through Monte-Carlo simulation analysis of laser transmission in the atmosphere, in the case of small attenuation, the photon moving step is larger, the scattering times in a fixed distance range are less, the particles hardly reach the receiving plane after the collision, and the calculation results of the Monte-Carlo method are minute. With the increase in particle number density and transmission distance, the number of scattering increases, and the transmittance calculated using the Monte-Carlo method is slightly higher than that calculated using the Lambert-Beer law. Over time, owing to the removal of fog by rainfall, the droplet number density decreases, and the numerical difference between the two methods increases.
Significance Humans need to observe various targets, including space, air, ground, and sea targets. Space targets include satellites, space debris, ballistic missiles, and hypersonic vehicles. Air targets include aircraft, airships, and small craft. Ground and sea targets include surface ships and ground vehicles. The past 20 years have seen an average of 12 collisions between space debris and space payloads every year. In addition, foreign ships and aircraft frequently invade our territorial waters and airspace and repeatedly spy on the activities in our important places. Therefore, the detection, identification, early warning, interception, and even striking of these abovementioned targets are an important and urgent research topic presently.
Multidimensional detection based on combined polarization detection, spectrum detection, and other optical technologies can provide the shape, material, location, and other information of the target simultaneously, effectively improving the dimensions and accuracy of space target information. At the same time, with the help of space laser communication, massive information can be quickly and safely transmitted to orbiting satellites and management departments, which can provide the decision-making basis for further disposal in time.
Progress In terms of space target detection, the United States has the largest and highest level of space target detection systems, followed by Russia. Europe starts late, but their system has rapidly developed in recent years. China is the latest to start and mainly performs ground-based observations. However, in recent years, China has conducted space-based observation tests and devised various detection methods, including photoelectric observation, radar monitoring, radio detection, and other detection methods.
In multidimensional detection, polarization detection technology has the advantages of highlighting the target, penetrating smoke, and identifying the truth and falsehood of the target. Spectral detection technology can distinguish the physical characteristics of the target material. Intensity detection technology has high light energy utilization and resolution, but it also has its own weaknesses. The information obtained by intensity detection is less and easily disturbed by the environment. Moreover, loss of the receiving energy and decrease in imaging resolution can be introduced by polarization detection.
The X2000 flight terminal was developed in the United States from the aspect of integrating detection, imaging, and communication. It can realize the functions of bidirectional communication, bidirectional laser ranging, and high-resolution imaging. The United States also proposed the ACLAIM scheme, in which the laser communication antenna and space camera sharing a front telescope and a detector array is employed as the acquisition and tracking system and an imaging receiver. In China, satellite payloads were developed toward the direction of multifunctionality and integration to increase the system function and reduce the volume, mass, and power consumption of the load. This study proposes a new scheme for space target detection and information transmission, which integrates the four functions of laser ranging, spectral polarization imaging, super-resolution imaging, and laser communication into one. The system design and development were performed.
Conclusions and Prospect In summary, we introduce herein the research status of the technology of multidimensional detection and laser communication integration for space objects and summarize the principle, characteristics, and application of the related technologies. The preliminary research results of our team in the related aspects are as follows: 1) for space object multidimensional detection, the detection mechanism is studied, and a large aperture and a wide field-of-view space-based telescope super-resolution imaging optical system is designed; 2) a prototype of simultaneous and time-sharing polarization imaging detection for complex space targets is developed; 3) ground and sea surface tests are conducted. As regards space laser communication, the optical principle of one-point to multipoint simultaneous space laser communication is proposed for the first time by our team at home and abroad. Accordingly, a principal prototype is developed and a demonstration test is performed. For detection and communication integration, the urgent need for this space security technology is expounded, and the system design idea and a specific implementation scheme are given.
Our country should further perform an in-depth research on ultra-high-resolution imaging, full-polarization and hyperspectral multidimensional detection, space- and ground-based combined optical detection, multi-to-multi space laser communication, and integrated laser and microwave network communication to solve the problems of the incomplete detection of low-orbit targets, unclear detection of high-orbit targets, slow response of the dynamic target, and difficulties in numbering space objects, which can provide a technical guarantee for the space security in China.
Significance In the past half century, integrated circuits (ICs) supported by complementary metal-oxide semiconductor (CMOS) technology have developed rapidly, which promotes the continuous progress of modern information technology. As the feature size of transistors continues to decrease, the semiconductor-manufacturing process is gradually approaching its limit, resulting in slow or even stagnant improvement of integration. Meanwhile, the system performance is seriously restricted, mainly due to the electronic bottleneck. In addition, with the increase in the number of microprocessors and computing speed, power consumption and heat dissipation due to parasitic effects are becoming the main limiting factors. To break through the bottleneck of conventional IC technology in the post-Moore era, optical interconnects are considered to gradually replace conventional electrical interconnects. Compared with electrical signals, using light as the carrier for signal transmission has its unique advantages, such as large bandwidth, low loss, strong anti-electromagnetic interference capability, and high-speed parallel transmission without crosstalk. Therefore, optical interconnects will undoubtedly become the enabling technology for high-speed data transfer. Concurrently, at the network nodes, conventional optical-electrical-optical signal processing is still limited by the electronic bottleneck. Processing signals in the optical domain offer an effective strategy to increase speed. Consequently, on-chip optical interconnects and processing are paramount to the development of modern high-speed and large-capacity communication networks.
The photonic integrated circuit (PIC) is paramount to realize on-chip optical interconnects and processing, which achieves rapid development in recent years. Silicon and III-V are both promising materials for the PIC platform. The main advantage of InP and other III-V materials is that they are direct bandgap materials, which can be used to fabricate semiconductor lasers, amplifiers, modulators, detectors, and other active devices. However, the cost is relatively high and size is relatively large, which limit their large-scale commercialization. By contrast, silicon materials have distinct advantages of large reserves in nature, low cost, almost transparent in the near-infrared and even mid-infrared bands, low loss, and large refractive index contrast of silicon on insulator (SOI), making them suitable for large-scale and high-density integration. Importantly, silicon materials are fully compatible with the existing mature CMOS process, which is essential for developing silicon-based PICs. Since silicon material is an indirect bandgap material, it is impossible to produce high-efficiency light sources. Monolithic integration of all active and passive devices on a single material platform is still challenging. The hybrid integration technology provides a possible solution, which enables the integration of discrete active devices, such as lasers and amplifiers, onto silicon-based passive devices through co-packaging, epitaxial bonding, and monolithic growth to realize low-cost and high-performance hybrid PICs.
Although on-chip optical interconnects and processing are the development trends of high-speed communication networks, the sustainable increase of communication capacity is still crucial in the big data era with increasing capacity demand. Notably, photons have multiple physical dimensions, such as frequency/wavelength, polarization, time, complex amplitude, and spatial structure, which can be developed into multiple multiplexing and advanced modulation technologies, making it possible to realize ultra-high-capacity optical communications and interconnects. Wavelength-division multiplexing (WDM), time-division multiplexing (OTDM), polarization-division multiplexing (PDM), space-division multiplexing, and advanced modulation formats have rapidly developed in the past few decades, significantly increasing the transmission capacity of optical communication systems. Therefore, on-chip optical interconnects and processing should also exploit multiple physical dimensions of photons. Particularly, multiple multiplexing technologies and advanced modulation formats can be combined to effectively increase the number of signal channels and aggregate capacity of on-chip optical interconnects and processing systems.
Progress Here, we give a comprehensive review of on-chip integrated multidimensional optical interconnects and processing (
Conclusions and Prospects With the rapid development of cloud computing and data centers, on-chip integrated optical interconnects and processing have become the key technologies to break through the conventional electronic bottleneck with their unique advantages in integration, speed, bandwidth, power consumption, and multiple physical dimensions. In this article, we review the key technologies and recent progress of on-chip integrated multidimensional optical interconnects and processing. Looking to the future, one would expect the development trend toward multiple materials (III-V, silicon, silicon nitride, silica, polymer, lithium niobate, and 2D material), integrations (hybrid integration, monolithic integration, and integration of photonics and electronics), physical dimensions (frequency/wavelength, polarization, time, complex amplitude, and spatial structure), frequency bands (O+E+S+C+L+U, visible, mid-infrared, microwave, and terahertz), mediums (chip, fiber, free space, and underwater), functions (multifunction, reconfigurable, programmable, and intelligent), and applications (communications, sensing, measurement, imaging, computing, and quantum) (