Unified performance analysis of interference-limited and interference-free dual-hop mixed RF/FSO systems with partial relay selection under pointing errors.

This paper investigates the performance of interference-limited and interference-free dual-hop mixed radio frequency (RF)/free space optical (FSO) systems with partial relay selection (PRS) for the variable-gain (VG) amplify-and-forward (AF) relaying scenario. We concentrate on the generalized channel model that not only describes different application scenarios but also allows a more accurate description of the channel characteristics. Specifically, the PRS-aided RF link is modeled by the κ-µ shadowed distribution, and the FSO link is expressed in terms of Fox's H-function, which unifies Fisher-Snedecor F, Gamma-Gamma (GG), and Malága (M) distributions for atmospheric turbulence along with pointing errors and detection modes. The interference signals at the selected relay are modeled by independent identically κ-µ shadowed distributions. Using our analytical framework, new unified closed-form expressions for the cumulative distribution function (CDF), the average bit error rate (BER), and the ergodic capacity are derived. Additionally, we provide asymptotic expressions of the average BER at high SNR. The analysis quantifies the impact of co-channel interference, pointing errors, number of relays, and rank of the selected relay on the considered system's performance. Finally, numerical results and Monte Carlo (MC) simulations are presented to confirm the effectiveness of the derived expressions. Note that our results provide a generalized framework for comprehensive studies of this kind of systems.

Secrecy performance analysis in the FSO communication system considering different eavesdropping scenarios.

In this paper, we numerically study the secrecy performance of the free-space optical (FSO) system by considering different eavesdropping scenarios. More precisely, we considered three possible eavesdropping scenarios for Eve: 1) Eve is between Alice and Bob; 2) Eve and Bob are in the same receiving plane; 3) Eve is behind Bob. We adopt the Málaga (M)-distribution channel to model atmospheric turbulence due to the presence of link blockage while considering the non-zero boresight pointing error and path loss. To do so, we obtain a novel probability density function (PDF) and cumulative distribution function (CDF) under different eavesdropping scenarios, based on which we derived the secrecy outage probability (SOP) analytical expressions as well as their asymptotic expressions at a high SNR regime. We verified the results using Monte Carlo simulations, which showed that the parameters related to atmospheric turbulence and pointing errors, as well as the location of the eavesdropper, have different effects on different eavesdropping scenarios.

Effect of Doppler shift on preamplifier DPSK receivers using balanced detection for optical satellite networks.

Optical satellite networks using laser intersatellite links are recognized as being capable of satisfying the increasing broadband communication demands. However, the Doppler shift due to relative movement between satellites must be taken into consideration. A convenient method that does not require complicated mathematical calculation is adopted to derive the relationship between the residual Doppler shift and the bit error rate (BER) for the balanced differential phase-shift keying (DPSK) detection system. It is shown that the DPSK receiver is sensitive to the residual Doppler shift; when the residual Doppler shift Δfres/Rb is 0.05, 0.10, and 0.15, the BER is reduced from a level of 10-17 to a level of 10-14, 10-11, and 10-6, respectively. The BER of a certain node pair under different link assignment schemes is calculated in a typical multi-hop LEO constellation. The results show that the communication BER of the same node pair is not only related to the step change of the Doppler but also related to the accumulation of the Doppler shift in the optical path. The results obtained here will be helpful in routing selection, link assignment, topology design, and system compensation.

Performance analysis of optical spatial modulation over a correlated gamma-gamma turbulence channel.

In this paper, theoretical closed expressions for the symbol error rate (SER) of two optical spatial modulation (OSM) schemes in a free-space optical (FSO) communication system-optical spatial pulse amplitude modulation (OSPAM) and optical spatial pulse position modulation (OSPPM)-over correlated gamma-gamma fading channels are derived. To study practical optical channels affected by atmospheric eddies, the effect of arbitrary atmospheric channel correlation is considered particularly, and it is shown that the correlation can degrade system performance. The parameters of atmospheric turbulence and the number of optical antennas are also under consideration in performance analysis of the FSO multiple input, multiple output system. All analytical results are validated with Monte Carlo simulations, which clearly illustrate the effect of spatial constellation and signal constellation on the average SER. In addition, it is shown by comparison that turbulence fluctuation and channel correlation have less influence on the OSPPM system than the OSPAM system. Overall, the framework proposed in this paper is proven to be effective and conducive to algorithm research on reducing the effect of correlated channels on OSM FSO systems.

Performance of dual-hop FSO/RF systems with fixed-gain relaying over Fisher-Snedecor F and κ-µ shadowed fading channels.

We investigate the performance of a dual-hop mixed free space optical (FSO) /radio frequency (RF) system with fixed-gain relaying under direct detection and heterodyne detection techniques. The FSO link is modeled by the Fisher-Snedecor F distribution, which matches well with the experimental data under weak-to-strong turbulence regimes. The RF link experiences κ-µ shadowed fading, which unifies popular RF fading models. The κ-µ shadowed distribution is approximated by an α-µ distribution. Capitalizing on this approximation, closed-form approximate expressions for the cumulative distribution function, the average bit error rate of different modulation schemes, and the ergodic capacity are derived in terms of the bivariate Fox's H function. Moreover, asymptotic analysis is carried out at a high signal-to-noise ratio to further illustrate the obtained diversity order and the influence of system and channel parameters. Numerical results and Monte Carlo simulations are presented to validate the derived approximate expressions.

Biphoton engineering using modal spatial overlap on-chip.

Photon pairs generated by spontaneous parametric downconversion are essential for optical quantum information processing, in which the quality of biphoton states is crucial for the performance. To engineer the biphoton wave function (BWF) on-chip, the pump envelope function and the phase matching function are commonly adjusted, while the modal field overlap has been considered as a constant in the frequency range of interest. In this work, by using modal coupling in a system of coupled waveguides, we explore the modal field overlap as a new degree of freedom for biphoton engineering. We provide design examples for on-chip generations of polarization entangled photons and heralded single photons. This strategy can be applied to waveguides of different materials and structures, offering new possibilities for photonic quantum state engineering.

Spectrally pure photon pair generation in asymmetric heterogeneously coupled waveguides.

In this work, we develop a design methodology to generate spectrally pure photon pairs in asymmetric heterogeneously coupled waveguides by spontaneous parametric down conversion. Mode coupling in a system of waveguides is used to directly tailor the group velocity of a supermode to achieve group velocity matching that is otherwise not allowed by material dispersion. Design examples based on thin film lithium niobate waveguides are provided, demonstrating high spectral purity and temperature tunability. This approach is a versatile strategy applicable to waveguides of different materials and structures, allowing more versatility in single-photon source designs.

Multifield acquisition technology for a narrow beacon laser on the non-orbit determination platform.

The application of free space optical communications technology between unmanned airborne vehicles (UAVs) can significantly improve its communications rate and capacity. However, it will produce strong disturbance due to the change of external conditions when the UAV is flying, which will affect the acquisition probability of a light beacon. The wide beacon can generally increase the acquisition range and acquisition probability, but there are disadvantages of increased weight and energy consumption. We report on an advanced method to improve the performance of narrow beacon spatial acquisition under the condition of UAV drift. The acquisition probability model has been established to study narrow beacon spatial acquisition considering UAV drift. A multiscanning method is proposed to improve the acquisition probability instead of simply increasing the scanning area to overcome severe UAV drift, which is also proven by experimental validation. Moreover, through the control method of active disturbance rejection control (ADRC), the pointing accuracy of multifield fast scanning is improved under the condition of external interference. This technique is extremely useful to develop a smaller, lighter terminal for UAV optical communications in the future.

High-dynamic wavelength tracking and millimeter-level ranging inter-satellite laser communication link with feedback-homodyne detection.

The Integrated Laser Communication/Ranging System, which uses a coded signal as the ranging information carrier, is of great importance to the next large-capacity inter-satellite information network. In this paper, a system design with a high-sensitivity feedback-homodyne detection scheme and an asynchronous ranging algorithm is demonstrated with real-time field-programmable gate array-implementation (FPGA). The parallel fast Fourier transformation (FFT) estimation is applied to improve the speed and the range of the wavelength drift tracking, which can handle a dynamic wavelength drift up to 2.4 pm/s (300 MHz/s). Meanwhile, for clock sources with subtle dynamic frequency offset and sufficient stability, the proposed fractional symbol ranging method is proven to achieve millimeter-level measurement accuracy. The designed system is shown to perform well in terms of both laser linewidth tolerance and noise resistance.

Monolithic semiconductor chips as a source for broadband wavelength-multiplexed polarization entangled photons.

Generating entangled photons from a monolithic chip is a major milestone towards real-life applications of optical quantum information processing including quantum key distribution and quantum computing. Ultrabroadband entangled photons are of particular interest to various applications such as quantum metrology and multi-party entanglement distribution. In this work, we demonstrate the direct generation of broadband wavelength-multiplexed polarization entangled photons from a semiconductor chip for the first time. Without the use of any off-chip compensation or interferometry, entangled photons with a signal-idler separation as large as 95 nm in the telecom band were observed. The highest concurrence of 0.98±0.01 achieved in this work is also the highest, to the best of our knowledge, comparing to all previously demonstrated semiconductor waveguide sources. This work paves the way for fully integrated, ultrabroadband sources of polarization entangled photons.

Widely tunable frequency conversion in monolithic semiconductor waveguides at 2.4 µm.

We report on the generation of continuous-wave widely tunable light between 2360 and 2530 nm using difference-frequency generation with a pump tuned between 938 and 952 nm and a signal tuned between 1490 and 1590 nm in a type-II phase-matched monolithic semiconductor waveguide. The device internal conversion efficiency is estimated to be 0.29% W(-1) cm(-2). This design which uses a single-sided Bragg reflection waveguide has the potential for on-chip spectroscopy, as well as environmental monitoring applications, where a tunable source of coherent radiation tuned between 2 and 3 µm wavelength is desired.

Inherent polarization entanglement generated from a monolithic semiconductor chip.

Creating miniature chip scale implementations of optical quantum information protocols is a dream for many in the quantum optics community. This is largely because of the promise of stability and scalability. Here we present a monolithically integratable chip architecture upon which is built a photonic device primitive called a Bragg reflection waveguide (BRW). Implemented in gallium arsenide, we show that, via the process of spontaneous parametric down conversion, the BRW is capable of directly producing polarization entangled photons without additional path difference compensation, spectral filtering or post-selection. After splitting the twin-photons immediately after they emerge from the chip, we perform a variety of correlation tests on the photon pairs and show non-classical behaviour in their polarization. Combined with the BRW's versatile architecture our results signify the BRW design as a serious contender on which to build large scale implementations of optical quantum processing devices.

Monolithic source of photon pairs.

The creation of monolithically integratable sources of single and entangled photons is a top research priority with formidable challenges: The production, manipulation, and measurement of the photons should all occur in the same material platform, thereby fostering stability and scalability. Here we demonstrate efficient photon pair production in a semiconductor platform, gallium arsenide. Our results show type-I spontaneous parametric down-conversion of laser light from a 2.2 mm long Bragg-reflection waveguide, and we estimate its internal pair production efficiency to be 2.0×10(-8) (pairs/pump photon). This is the first time that significant pair production has been demonstrated in a structure that can be electrically self-pumped and which can form the basis for passive optical circuitry, bringing us markedly closer to complete integration of quantum optical technologies.

Generation of polarization entangled photons using concurrent type-I and type-0 processes in AlGaAs ridge waveguides.

A technique to generate polarization entangled photons using concurrent type-I and type-0 second-order nonlinear processes in monolithic Bragg reflection waveguides is presented and analyzed. Concurrent phase matching is achieved by lithographic tuning of the waveguide ridge width. Nearly perfect entanglement is achievable on-chip through appropriate epistructure design without the need of spectral filtering and group velocity compensation. Theoretical calculations predict that a high quantum interference visibility could be experimentally achieved with the pair generation rate of each process being approximately 3.0×10(6) pairs/s/mW/GHz.

Difference-frequency generation in AlGaAs Bragg reflection waveguides.

We demonstrate Type II difference-frequency generation (DFG) around 1550 nm in AlGaAs Bragg reflection waveguides using a pump around 778 nm and a signal within the C-band range. Difference-frequency power of 0.95 nW was obtained using a pump power of 62.9 mW and a signal power of 2.9 mW. Nonlinear conversion efficiency was estimated to be 2.5 x 10(-2)%/W(-1) cm(-2) in a 1.5-mm-long sample. Using numerical simulations, the phase-matching bandwidth was predicted to be 100 nm, while the measured DFG showed no sign of bandwidth limitation across a wavelength span of 40 nm, which was limited by instrumentation.