Free-space Communication Systems Research Articles

Robustness of partially coherent vortex beams to the impact of dynamic Kolmogorov kind of turbulence

The wave propagation characteristics of Gaussian-Schell model vortex beams passing through a dynamic Kolmogorov type of turbulence are analyzed at the laboratory level. The effect of a rotating pseudo-random phase plate, which simulates Kolmogorov-type atmospheric turbulence, on the Gaussian-Schell model beams carrying twist phase is characterized by calculating the scintillation index and intensity line profiles. Our analysis proves the resilience of Gaussian-Schell model vortex beams to the impact of dynamic turbulence. Simulation studies are further used to validate the experimental results. Because of the resemblance between our investigation conditions and real-world atmospheric turbulence, these findings have potential applications in free-space communication systems.

Design of optical phased array with low-sidelobe beam steering in thin film lithium niobate

Optical beam-steering devices are crucial components of complex optical control systems, which are widely used in diverse information-processing applications. Optical phased arrays (OPAs) as the promising beam-steering devices, have proven to alleviate current performance limitations in beam steering devices for ultra-small solid-state light detection and rangings (LiDARs) and free-space communication systems. The high electro-optic (EO) coefficient (γ33 = 30.9 pm/V) of lithium niobate (LiNbO3, LN) can be a useful platform for phase tuning of the OPAs, which is conducive to enable phase shifting for non-mechanical beam steering with high-speed and low power consumption. The design, configurations and LN-OPA corresponding performances are discussed in this paper. The OPA consisting of different waveguides array is optimized with a side lobe suppression lower than - 10 dB at the wavelength of 1.55 μm. Meanwhile, the OPA with grating emitters or adding silica cavity further improves the deflection quality of the beam. Then optical beam steering with an angle of 50° in one dimension controlled by the LN-OPA has been demonstrated. The proposed OPA on the LN platform in this work might hold significances for advanced LiDAR schemes based on EO tuning.

Simultaneously enhancing capacity and security in free-space optical chaotic communication utilizing orbital angular momentum

Optical chaotic signals emitted from an external-cavity feedback or injected laser diode enable small-signal information concealment in a noise-like carrier for secure optical communications. Due to the chaotic bandwidth limitation resulting from intrinsic relaxation oscillation frequency of lasers, multiplexing of optical chaotic signal, such as wavelength division multiplexing in fiber, is a typical candidate for high-capacity secure applications. However, to our best knowledge, the utilization of the spatial dimension of optical chaos for free-space secure communication has not yet been reported. Here, we experimentally demonstrate a free-space all-optical chaotic communication system that simultaneously enhances transmission capacity and security by orbital angular momentum (OAM) multiplexing. Optical chaotic signals with two different OAM modes totally carrying 20 Gbps on–off keying signals are secretly transmitted over a 2 m free-space link, where the channel crosstalk of OAM modes is less than −20 dB, with the mode spacing no less than 3. The receiver can extract valid information only when capturing approximately 92.5% of the OAM beam and correctly demodulating the corresponding mode. Bit error rate below the 7% hard-decision forward error correction threshold of 3.8×10−3 can be achieved for the intended recipient. Moreover, a simulated weak turbulence is introduced to comprehensively analyze the influence on the system performance, including channel crosstalk, chaotic synchronization, and transmission performance. Our work may inspire structured light application in optical chaos and pave a new way for developing future high-capacity free-space chaotic secure communication systems.

A novel 2D-PhC based ring resonator design with flexible structural defects for CWDM applications

In this article, a chain-shaped photonic crystal-based ring resonator (PhCRR), which can function as a channel drop filter (CDF), is designed using two-dimensional photonic crystals. Silicon rods with a refractive index of 3.44 are chosen, and they are perforated in the air with a refractive index of 1 for the PhCRR layout design. Silicon material is selected for realizing the PhCRR-based channel drop filter because it exhibits nearly zero absorption in the C-band (1530–1565 nm) spectral region. The PhCRR structure is established within two-dimensional hexagonal lattices. Transmission efficiency, quality factor, and sensitivity are critical parameters in the design of optical components. Our proposed ring resonator achieves a transmission efficiency of 99.7% with a quality factor of 4550 at a wavelength of 1551 nm. To analyze the drop filter’s functionality, we calculate the electric field distribution of two-dimensional photonic crystals at 1551 nm and 1553 nm. We employ the FDTD numerical analyser to extract simulation results, and the plane wave expansion solver method is used to estimate the photonic band gap of the designed resonator. The chain-shaped photonic crystal-based ring resonator is designed to operate in the third optical window wavelength, which offers very low loss (less than 0.2 dB). The proposed PhCRR is designed to operate within the conventional band range of 1530 to 1565 nm and can be used for various applications such as tunable add/drop multiplexing, optical filters, signal routing and switching in CWDM applications, free space communication systems, and photonic integrated circuits.

Bi-Directional Free-Space Visible Light Communication Supporting Multiple Moveable Clients Using Light Diffusing Optical Fiber.

In this work, we put forward and demonstrate a bi-direction free-space visible light communication (VLC) system supporting multiple moveable receivers (Rxs) using a light-diffusing optical fiber (LDOF). The downlink (DL) signal is launched from a head-end or central office (CO) far away to the LDOF at the client side via a free-space transmission. When the DL signal is launched to the LDOF, which acts as an optical antenna to re-transmit the DL signal to different moveable Rxs. The uplink (UL) signal is sent via the LDOF towards the CO. In a proof-of-concept demonstration, the LDOF is 100 cm long, and the free space VLC transmission between the CO and the LDOF is 100 cm. 210 Mbit/s DL and 850 Mbit/s UL transmissions meet the pre-forward-error-correction bit error rate (pre-FEC BER = 3.8 × 10-3) threshold.

Anisotropic plasma sheath turbulence effects on the mode states of Whittaker–Gaussian beams

In this paper detection probability and crosstalk probability variance of the orbital angular momentum states of the diffraction-free Whittaker–Gaussian (WG) beams passing through an anisotropic plasma sheath turbulence (PST) are theoretically formulated. In this regard, using modified von Karman spectrum in the frame of Rytov theory the spiral spectrum of the WG beams is derived, and the role of the anisotropy parameters, refractive index fluctuation variance, initial topological charge, receiving aperture radius, the outer scale of turbulence, beam wavelength and waist in the mitigation of the turbulence-induced disturbance is numerically explored. The remarkable findings indicate that WG beams in anisotropic PST are less influenced than isotropic one, and increasing the anisotropy feature of plasma medium leads to alleviate modulated distortion. Furthermore, it is found out that the propagation performance of WG vortex beams with high-order initial topological charge is significantly better than those with lower order. We expect that our results will be helpful for designing optimal free-space communication systems.

Analysis of LDPC Code in Hybrid Communication Systems

Free-space optical (FSO) communication is of supreme importance for designing next-generation networks. Over the past decades, the radio frequency (RF) spectrum has been the main topic of interest for wireless technology. The RF spectrum is becoming denser and more employed, making its availability tough for additional channels. Optical communication, exploited for messages or indications in historical times, is now becoming famous and useful in combination with error-correcting codes (ECC) to mitigate the effects of fading caused by atmospheric turbulence. A free-space communication system (FSCS) in which the hybrid technology is based on FSO and RF. FSCS is a capable solution to overcome the downsides of current schemes and enhance the overall link reliability and availability. The proposed FSCS with regular low-density parity-check (LDPC) for coding techniques is deliberated and evaluated in terms of signal-to-noise ratio (SNR) in this paper. The extrinsic information transfer (EXIT) methodology is an incredible technique employed to investigate the sum-product decoding algorithm of LDPC codes and optimize the EXIT chart by applying curve fitting. In this research work, we also analyze the behavior of the EXIT chart of regular/irregular LDPC for the FSCS. We also investigate the error performance of LDPC code for the proposed FSCS.

Secure hybrid fiber optic and free optical space communication systems

This work investigates the integration of Electrical to Optical (E/O) and Optical to Electrical (E/O) communication systems. Two ways of transition, namely, Hybrid Fiber Optic (FO) and Free Optical Space (FSO) transmission system have been proposed. This work aims to evaluate the performance of these two transition ways. FO has the optimum transmission due to its lower attenuation which can be reached to 350 km. However, for further assurance of reaching data to the receiver side a second way of transition has been investigated. FSO path has been employed under different weather conditions of clear, moderate dust, heavy dust and heavy fog. Matlab and Optisystem7 software’s have been used to simulate Optical Secure Data Transition (SDT). The results show that an optimum transmission could be obtained using FO with RGB image encryption where the Mean Square Error (MSE) reach to 9.88 ×10^7 in read channel and it can reach to 4.93 ×10^9 using Discrete Wavelet Transform (DWT).

High-sensitivity multi-channel DPSK system with real-time phase lock controller for free-space optical communication

To overcome the power jitters in satellite-to-ground communications caused by atmospheric turbulence, a type of DPSK free-space communication system, assisted by a self-designed real-time phase lock controller, has been established. The system can effectively compensate for power swings in communication links and hence achieve high sensitivity. The wavelength division multiplexing technique is applied to a four-channel DPSK system to provide greater link capacity. With the data rate of a single channel as 2.5 Gbps and unencoded BER as 1 × 10–3, reception sensitivity has been obtained at −53.58 dBm (13.69 photons/bit), −53.59 dBm (13.66 photons/bit), −53.61 dBm (13.59 photons/bit), and −53.63 dBm (13.53 photons/bit) for each independent channel, respectively. The gap between our sensitivity result and the theoretical limit has narrowed to about −3.5 dB. Simultaneously, the DPSK receiver, with our self-designed phase lock controller, has stabilized reception of optical power fluctuations that range from 0 to 40 dB. Additionally, the impact of a four-wave mixing effect on multi-channel system performance has been investigated in detail. Our experimental results present a novel solution for the superior performance of free-space communication links.

Quantum key Distribution with a Hand-Held Sender Unit

Quantum key distribution (QKD) is a crucial component for truly secure communication, which enables to analyze leakage of information due to eavesdropper attacks. While impressive progress was made in the field of long-distance implementations, user-oriented applications involving short-distance links have mostly remained overlooked. Recent technological advances in integrated photonics now enable developments towards QKD also for existing hand-held communication platforms. In this work we report on the design and evaluation of a hand-held free-space QKD system including a micro-optics based sender unit. This system implements the BB84-protocol employing polarization-encoded faint laser pulses at a rate of 100 MHz. Unidirectional beam tracking and live reference-frame alignment systems at the receiver side enable a stable operation over tens of seconds when aiming the portable transmitter to the receiver input by hand from a distance of about half a meter. The user-friendliness of our system was confirmed by successful key exchanges performed by different untrained users with an average link efficiency of about 20 % relative to the case of the transmitter being stationarily mounted and aligned. In these tests we achieve an average quantum bit error ratio (QBER) of 2.4 % and asymptotic secret key rates ranging from 4.0 kbps to 15.3 kbps. Given its compactness, the versatile sender optics is also well suited for integration into other free-space communication systems enabling QKD over any distance.

Exploring for New Insights in the Performance of a 3D Orbital Angular Momentum Mode-Sorter

Optical vortex beams are light beams that can carry orbital angular momentum (OAM). Hence, such beams may serve as potential candidates for carriers of information in optical communication and quantum optics applications. This is owing to their spatial orthogonality, as these beams can be combined (multiplex) or separated (demultiplexed). We recently demonstrated a new method to detect OAM states by using a 3D-direct laser printing fabrication process. Measuring the mode-sorter performance was challenging, mainly due to mechanical and optical sensitivities originated from misalignments. In this work, this sensitivity was thoroughly examined. Pure OAM states having lateral and angular misalignments relative to the mode-sorter were introduced, and cross-talk between resolved states was theoretically simulated. The system is relatively vulnerable to small misalignments, which challenge its implementations in free-space communication systems. However, this might be an advantage for counseled communication, in which eavesdropping becomes more challenging, due to the angle-dependent increased modal cross-talk.

Outage Probability Analysis of Free Space Communication System Using Diversity Combining Techniques

Recently, free space optical (FSO) communication is gaining much attention towards the research community. The reason for this attention is the promises of high data-rate, license-free deployment, and non-interfering links. It can, however, give rise to major system difficulties concerning alignment and atmospheric turbulence. FSO is the degradation in the signal quality because of atmospheric channel impairments and conditions. The worst effect is due to fog particles. Though, Radio Frequency (RF) links are able to transmit the data in foggy conditions but not in rain. To overcome these issues related to both the FSO and RF links. A free space communication system (FSCS) is proposed, in which the hybrid technology is based on the individual FSO and RF channel. FSCS is a capable solution to overcome the difficulties of the existing systems (FSO and RF) as well as to enhance the overall link reliability and availability. In this paper, FSCS is investigated in terms of performance throughput (i.e., outage probability and bit-error-rate (BER)) by implementing the receive diversity combining techniques. An analysis of the outage probability of the proposed system along with the individual FSO and RF system is developed. Simulation results are presented to support the analysis. It is shown that the proposed system outperforms the individual FSO and RF system and gives a power gain of 3dB over a distinct number of receive antennas.

Utilizing accelerating plane-wave beams for bendable light communications

Self-accelerating optical beams have attracted much attention for potential applications in obstruction-evading bendable free-space communication systems. In this work, accelerating plane-wave beams are used to enable a spatially multiplexed multi-channel free-space optical communication scheme. A method for generating and sorting an accelerating plane-wave beam family is proposed and experimentally verified. Employing a family of Airy plane-wave beams in particular, a 7-channel optical communication system with 56-Gbit/s OOK signal per channel transmitted along a parabolic trajectory is demonstrated with an increase in the ability to evade obstructions of up to 0.6 beam waist over that of Gaussian beams.

Feasibility of continuous-variable quantum key distribution through fog.

Free-space quantum key distribution (QKD) can be very attractive due to the possibility of its flexible and rapid deployment. In spite of the advantages of free-space transmission, there is still a risk of encountering bad weather such as fog. Here we experimentally demonstrate free-space QKD using continuous variables under foggy conditions and estimate achievable transmission distances based on the experimental results. Pessimistically, a weather condition that has visibility of 1 km allows about 0.6-km transmission. An optimistic result, on the other hand, shows that a transmission distance of 1.8 km at visibility of 1.5 km can be achieved. The results suggest that free-space continuous-variable quantum communication systems are potentially applicable in the presence of fog.

On the Spectral Efficiency of Orbital Angular Momentum With Mode Offset

Thanks to its low transceiver complexity, mode division multiplexing (MDM) using orbital angular momentum (OAM) has been recently investigated as a new physical layer wireless transmission technique. This is due to the fact that different OAM modes are spatially orthogonal to each other, thus, perfectly suitable for spatial multiplexing and/or diversity. The orthogonality amongst different OAM modes is achieved in the absence of any mode offset between the transceivers phase-shifting feeding networks. However, the inherent presence of OAM mode offset destroys the orthogonality between different modes, which gives rise to inter-mode interference (IMI). Therefore, in this paper, in order to theoretically analyze the negative impact of OAM mode offset of MDM-OAM systems on the spectral efficiency of line-of-sight free-space wireless communication systems, an explicit signal-to-interference-plus-noise ratio expression for each OAM mode is derived, through which the spectral efficiency degradation is evaluated. Furthermore, in order to support our theoretical analysis, a 9 x 9 MDM-OAM practical communication experiment for different OAM mode settings is carried out at a microwave frequency of 5.8 GHz. In addition, by leveraging the fact that all circulant matrices always have the same eigendirections, regardless of the entries of the matrix, a low-complexity optimal OAM mode selection scheme is proposed which significantly improves the spectral efficiency of the system. Furthermore, in order to completely null-out the impact of OAM mode offset, a baseband zero-forcing stage is embedded at the receiver.

A High Speed Retro-Reflective Free Space Optics Links With UAV

In this work we report the design and implementation of a very high speed retro-reflective free space communication system between a ground station and a commercial unmanned aerial vehicle (UAV). The system uses a pixelated electro-absorption modulator (EAM) modulating retro-reflector (MRR) to establish a data link operating at 500 Mbps at a range of 560 m and a bit error rate (BER) of 7.6·10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> . The MRR provides an effective aperture of 11 mm and full field of view (FFOV) of 6.4°. To the best of our knowledge, this is the fastest demonstration of an outdoor link of this type. In this paper the design and implementation of the system is described, as well as results from experimental trials.

Transmission characters of wide-spectrum OAM beam in tunable atmospheric turbulence

In this paper, a wide-spectrum orbital angular momentum (OAM) based free space communication system is experimentally demonstrated. The wide-spectrum laser can be generated using narrow-linewidth pulses to pump nonlinear fiber and the OAM beam can be generated with spatial light modulator (SLM). In the experiment, a controllable atmospheric turbulence simulation device was used to ensure a unified test environment. The results show that with the combination of wide-spectrum laser and OAM beam, the received sensitivity can achieve a 4.23 dB gain compared with narrow-linewidth Gaussian beam-based communication system in weak atmospheric turbulence environment.

Statistical model for the weak turbulence-induced attenuation and crosstalk in free space communication systems with orbital angular momentum

A novel statistical model connected with turbulence strength is proposed to describe the attenuation and crosstalk in a vortex-based multi-channel free space optical (FSO) communication system. In this model, self-channel fading and interference between different orbital angular momentum (OAM) modes are characterized by the mixture exponential-generalized-gamma (EGG) distribution, and the analytical relations between turbulence strength and the distribution function's parameters are expressed by piecewise functions. The problems of obtaining parameters of this model are converted into optimization problems, and the algorithms based on the trust trigon algorithm are proposed to achieve more optimized parameters. This model is confirmed to have a good fit with the emulated data of OAM attenuation and crosstalk calculated by the square of the scalar product between the fields of two OAM modes. Furthermore, the application of the statistical model to the OAM-multiplexing FSO system with quadrature-phase-shift-keying modulation is presented, in which the theoretical average bit-error rate results match well with Monte Carlo simulation. This model can be used for FSO system design with OAM for continuous weak turbulence condition.

Performance Optimization of Hybrid ROFSO Link Using Hybrid Optical Amplifiers (RAMAN\\EDFA) for Long-Haul Transmission with the C-band and L-band Comparison

In this paper, a hybrid system is proposed consisting of a radio-frequency (RF) over free space optic/fiber optic (RFOFSO/FO). A free-space communication system is an alternative to optical fibers if it fails in performance or if it is difficult to propagate. This hybrid system is an effective solution for developing new generation networks, by transmitting the optical signal over long distances within the C-band, and L-band. The proposed system sends the radio frequency signal through the optical free-space link then extends to 80 km of optical fiber, of which 40 km is amplified by the Raman/EDFA hybrid optical amplifier. This paper aims to test the performance of the free space optic link under different weather conditions. The atmospheric attenuation, which reduces visibility, was considered as the main challenge. Several copies were sent in different paths in FSO to meet this challenge. The submitted system was implemented in the case of transmission in a wavelength within the C-band and also in L-band. The results of the hybrid system were compared by measuring the bit error rate (BER) and Q-factor under dust and fog conditions relative to Kim’s standard model by using Optisystem. The results obtained showed an effective sensation at a long haul, inn addition to not using an additional light source when transmitting for such long distances.

Identification of independent modes in two inputs free space communications system

The study proves the concept of using a system with two inputs laser-beam, modulated independently by helical and conical phase distributions followed by their superposition in the same physical path for free space optical communications. Both phase distributions generate annular intensities with specific geometry according to the modulating parameters. The modes are multiplexed in an emission unit and propagate via laboratory free space as rings with optimized radii, without interfering, to the reading phase mask of the receiving unit. The reading mask acts as matching filter which embeds planar phase information such that to distribute the helical and conical components on mutual perpendicular directions. Identification occurs when the parameters of the reading mask match those of the input modes as predicted by a 2-dim inner product model developed here and confirmed experimentally by the occurrence of maximum intensities in the specific centers. Splitting the matrix-like diffracted beam in enabler and processing branches allows to double check and to identify the matching cases by the properties of the associated histogram without affecting subsequent signal retrieval. Crosstalk levels are examined.