No abstract

Free-space optical (FSO) communication has attracted significant interest recently. This technology can potentially complement or be an integral part of next-generation networks. FSO links provide several advantages compared to conventional radio frequency links, including higher data rates, license-free spectrum, and power efficiency. Furthermore, they could be used to access users in remote areas where optical fiber communications are unavailable.Here, we present a simulation framework for modeling, designing, and analyzing classical and quantum communication systems over terrestrial and satellite free-space optical links. We address different FSO use cases in terrestrial, ground-tosatellite, satellite-to-ground, and inter-satellite links using direct-and coherent detection schemes. For the FSO channel modeling, we discuss two methods. The first approach considers atmospheric scintillation, pointing errors, the Doppler effect, attenuation due to the beam diffraction, and scintillation-induced divergence. The second method captures the wavefront of the optical beam using the phase screens technique, which provides a more detailed description of the signal propagation. Additionally, we provide essential details about system-level simulations to analyze and optimize the entire link performance. Finally, we discuss the simulation environment for designing quantum-key distribution (QKD) systems as an FSO use case.Using this simulation framework, we investigate the performance of several different FSO application examples: a terrestrial link with spatial diversity receivers, inter-satellite communication from low Earth to geostationary orbit, and a polarization-encoded BB84 with decoy states QKD over a satellite downlink.

The generation of spatial rogue waves, or “hot spots,” is demonstrated experimentally in the passively Q -switched Nd:YAG laser system operating in a low-power regime well below the self-focusing limit. Here, we report the dependence of rogue wave statistics on the number of transverse modes that interact in the laser cavity. Our observations show that spatial rogue waves are most likely to occur when the laser exhibits complex output beam configurations that are formed by a large number of interacting high-order transverse modes. These results confirm the hypothesis that one of the main factors affecting the emergence of spatial rogue waves in solid-state lasers is the number of laser transverse modes.

This paper presents the cavity dumping method by the second harmonic generation in a neodymium laser operating in the Q-switched mode. Theoretical modeling of the laser generation in dynamics is performed. The main characteristics of the output pulses and their dependence on the pump power and coefficient of losses are calculated. The proposed method can be implemented in the laser with a second harmonic crystal inside the cavity and an electro-optical crystal, which is operated by a step voltage pulse. The switching pulse length is defined by the time needed to achieve the maximum giant pulse intensity in the cavity, and is in the order of 0.1–1.0 ms depending on the pump power. Moreover, the voltage pulse jitter should not exceed several nanoseconds. In such case the second harmonic pulses with nanosecond duration and peak intensity of 10 –100 MW/cm2 are generated at the laser output. The output pulses duration is defined only by the cavity length under the conditions of a small response time of the electro-optical crystal compared to the cavity round-trip time. To achieve the maximal peak intensity, one should decrease the coefficient of inactive losses to the possible minimum and increase the pump power.