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1. Amplified entanglement-assisted communication systems operated in a desert environment and strong atmospheric turbulence regime

   https://doi.org/10.1364/OE.543021  

   Nafria, Vijay; Djordjevic, Ivan B (January 2024, Optics Express)

   In this paper, we are presenting results that go against the common belief that entanglement is destroyed by the amplification using an EDFA. Here we demonstrate the quantum advantage of entanglement-assisted communication at 10Gb/s, employing LDPC-coded BPSK, over classical laser communication even after the amplification of signal photons is performed by the EDFA in order to improve the reliability of entanglement-assisted (EA) communication operating in turbulent 1.5 km terrestrial FSO channels. To make the EA system more robust against various atmospheric effects such as scattering, absorption, and turbulence effects we perform the optical phase-conjugation on idler photons rather than turbulence-affected signal photons and use adaptive optics to make additional improvements in terms of the bit-error rate. 

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2. Low probability of intercept (LPI) free-space optical communication with wavelength diversity in strong atmospheric turbulence regime

   https://doi.org/10.1364/OE.547978  

   Nafria, Vijay; Djordjevic, Ivan_B (December 2024, Optics Express)

   In physical-layer security and cryptography we are concerned with the security of the transmitted data, while in low probability of intercept (LPI) communication with protecting the privacy of the end users. In our recent publications related to LPI communications and radars over free-space optical (FSO) links we proposed to hide the constant-amplitude modulated data, imposed on thermal source beam, in ambient solar radiation to protect the end users privacy and at the same time improve the reliability and security, while reducing the detectability of transmitted signal by the adversary Willie. In order to study both LPI and covert communication concepts we have developed an FSO communication testbed at the University of Arizona campus with a 1.5 km-long FSO link. Here we present results of our FSO experiments, where we conducted both LPI and covert communications at data rates ranging from 125 Mb/s to 10 Gb/s, wherein the information beam is kept completely hidden under the ambient solar radiations as random thermal noise. To improve the system reliability to atmospheric turbulence effects we make use of wavelength diversity method as a low-cost, easy to implement and far more practical alternative to conventional adaptive optics systems. 

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3. Incoherent Light Sources-Based Low Probability of Detection and Covert Radars over Atmospheric Turbulence Channels

   https://doi.org/10.1109/TELSIKS57806.2023.10315967  

   Djordjevic, Ivan B; Nafria, Vijay (October 2023, IEEE)

   In this paper we are concerned with the low probability of detection (LPD) and covert radars employing optical incoherent sources. Key idea of our proposed LPD/covert radar concept is to hide the radar signal in solar radiation by employing the broadband (>30 nm) Erbium-doped fiber amplifier source, modulating such source output beam with a constant amplitude modulation format at high-speed, and detect the presence of the target by the cross-correlation method. To demonstrate the proposed concept we developed an outdoor free-space optical (FSO) testbed at the University of Arizona campus. To improve the tolerance to atmospheric turbulence effects the adaptive optics is used. We demonstrate that the LPD/covert radar concept over strong turbulent FSO channel is feasible in a desert environment. 

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4. Entanglement Assisted Radars Operated over Strong Atmospheric Turbulence Channels

   Djordjevic, Ivan B; Nafira, Vijay (July 2024, ICTON)

   In this invited paper we review our recent research activities on experimental demonstration of entanglement based (EB) radars, operated over strong atmospheric turbulence channels. In conventional EB communications, sensing, and radars the phase-conjugation, required before homodyne detection takes place, is performed on received signal photons. In atmospheric turbulent channels, the signal photons are affected by diffraction, absorption, scattering, and atmospheric turbulence effects so that only limited number of weak target probe returned signal photons reach the receiver side in EB radars. Moreover, it is extremely difficult to perform any phase-conjugation on weak signal photons when the average number of received photons is <<1. To solve this problem, we have recently proposed to perform phase-conjugation on bright idler photons instead. Namely, we perform the wavelength conversion by the PPLN waveguide on bright idler photons, so that the idler photons will have the same wavelength as the signal photons, and after that we use a classical homodyne balanced detector as an entanglement assisted detector. To generate entangled photon pairs, we use C-/L-band tunable laser, EDFA, the PPLN waveguide, and WDM demultiplexers. To demonstrate the high-potential of the proposed EB radar concept, we developed an experimental outdoor free-space optical (FSO) testbed at the University of Arizona campus. Using this FSO testbed we experimentally demonstrate that the proposed EB radar significantly outperforms the corresponding classical counterpart and can operate in strong turbulence regime. To improve the detection probabilities further, we use deformable mirror-based adaptive optics. 

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5. Metalens-Based Miniaturized Optical Systems

   https://doi.org/10.3390/mi10050310  

   Li, Bo; Piyawattanametha, Wibool; Qiu, Zhen (May 2019, Micromachines)

   Metasurfaces have been studied and widely applied to optical systems. A metasurface-based flat lens (metalens) holds promise in wave-front engineering for multiple applications. The metalens has become a breakthrough technology for miniaturized optical system development, due to its outstanding characteristics, such as ultrathinness and cost-effectiveness. Compared to conventional macro- or meso-scale optics manufacturing methods, the micro-machining process for metalenses is relatively straightforward and more suitable for mass production. Due to their remarkable abilities and superior optical performance, metalenses in refractive or diffractive mode could potentially replace traditional optics. In this review, we give a brief overview of the most recent studies on metalenses and their applications with a specific focus on miniaturized optical imaging and sensing systems. We discuss approaches for overcoming technical challenges in the bio-optics field, including a large field of view (FOV), chromatic aberration, and high-resolution imaging. 

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