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  1. Optical isolators are an essential component of photonic systems. Current integrated optical isolators have limited bandwidths due to stringent phase-matching conditions, resonant structures, or material absorption. Here, we demonstrate a wideband integrated optical isolator in thin-film lithium niobate photonics. We use dynamic standing-wave modulation in a tandem configuration to break Lorentz reciprocity and achieve isolation. We measure an isolation ratio of 15 dB and insertion loss below 0.5 dB for a continuous wave laser input at 1550 nm. In addition, we experimentally show that this isolator can simultaneously operate at visible and telecom wavelengths with comparable performance. Isolation bandwidths up to ∼100 nm can be achieved simultaneously at both visible and telecom wavelengths, limited only by the modulation bandwidth. Our device’s dual-band isolation, high flexibility, and real-time tunability can enable novel non-reciprocal functionality on integrated photonic platforms.

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  2. Free, publicly-accessible full text available April 1, 2024
  3. We evaluate the burst-error performance of the regular low-density parity-check (LDPC) code and the irregular LDPC code that has been considered for ITU-T’s 50G-PON standard via experimental measurements in FPGA. By using intra codeword interleaving and parity-check matrix rearrangement, we demonstrate that the BER performance can be improved under ∼44-ns-duration burst errors for 50-Gb/s upstream signals. 
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    Free, publicly-accessible full text available January 1, 2024
  4. Quantum cryptography is the study of unconditional information security against an all-powerful eavesdropper in secret key distillation. However, the assumption of an omnipotent eavesdropper is too strict for some realistic implementations. In this paper, we study the realistic application model of secret key distillation over a satellite-to-satellite free-space channel in which we impose a reasonable restriction on the eavesdropper by setting an exclusion zone around the legitimate receiver as a defense strategy. We first study the case where the eavesdropper’s aperture size is unlimited so their power is only restricted by the exclusion zone. Then, we limit Eve’s aperture to a finite size and study the straightforward case when her aperture is in the same plane of Bob’s, investigating how an exclusion zone can help improve security. Correspondingly, we determine the secret key rate lower bounds as well as upper bounds. Furthermore, we also apply our results on specific discrete variable (DV) and continuous variable (CV) protocols for comparison. We show that, by putting reasonable restrictions on the eavesdropper through the realistic assumptions of an inaccessible exclusion zone, we can significantly increase the key rate in comparison to those without and do so with relatively lower transmission frequency. We conclude that this model is suitable for extended analysis in many light-gathering scenarios and for different carrier wavelengths.

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  5. The growth of data-driven technologies, 5G, and the Internet pose enormous pressure on underlying information infrastructure [...] 
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  6. The growing data demands are pushing researchers to pay more attention to spectrally efficient modulation formats. The four-dimensional (4D) signal constellation modulation format has been investigated for metro networks’ applications to achieve better power efficiency. To cope with such modulation formats, the requirement of better digital signal processing (DSP) is also increasing rapidly. More complicated DSPs bring us extra costs; thus, the DSP-free coherent receivers are also investigated because of the high-power consumption of conventional DSP-based receivers, but the transceivers upgrading also results in extra costs. In this invited paper we implement a 4-dimentional modulation format based on Slepian sequences. We applied LDPC coding and experimentally investigated the BER performance in a two-dimensional (2D) 40 km fiber link transmission and demonstrate that being error free is possible without employing the complicated DSP. We compared our proposed modulation scheme with regular 16QAM and found it outperforms 16QAM with DSP over back-to-back transmission by 3.8 dB improvement in OSNR when BER = 10−5, while over 40 km metro network communication link our proposed 4D modulation signals are still successfully transmitted, and the LDPC-coding still works properly with such a new transmission strategy. On the other hand, DSP-free transmission of LDPC-coded 16-QAM exhibits an early error floor phenomenon. 
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