Performance evaluation and comparative research of underwater wireless optical communication system by using different structured beams.

Structured beams have attracted increasing interest in free-space and fiber-based optical communications. Underwater wireless optical communication (UWOC) is becoming a prospective technique in marine exploration. We investigated UWOC performance using different representative structured beams. The transmission performances of the Gaussian, Bessel-Gaussian (BG), Ince-Gaussian (IG), and radially polarized Gaussian (RPG) beams were experimentally demonstrated and evaluated in underwater channels subjected to thermal gradient. The experimental results show that the BG, IG, and RPG perform better against the thermal gradient. Compared with the Gaussian beams, the beam wanders of BG, IG, and RPG beams under the thermal gradient have been reduced by 56.9%, 8.2%, and 59%, the scintillation indices have been decreased by 12.8%, 17.3%, and 28.9%, and the BER performance of the BG, IG, and RPG beams have been improved by ∼5.5, ∼3.7, and ∼5.2d B at the forward error correction threshold (FEC threshold). Based on the above results, the RPG beam is a more promising light source for UWOC. The experimental results provide a promising beam choice for UWOC.

Pulse Accumulation Approach Based on Signal Phase Estimation for Doppler Wind Lidar.

Coherent Doppler wind lidar (CDWL) uses transmitted laser pulses to measure wind velocity distribution. However, the echo signal of CDWL is easily affected by atmospheric turbulence, which can decrease the signal-to-noise ratio (SNR) of lidar. To improve the SNR, this paper proposes a pulse accumulation method based on the cross-correlation function to estimate the phase of the signal. Compared with incoherent pulse accumulation, the proposed method significantly enhances the correlation between signals from different periods to obtain high SNR gains that arise from pulse accumulation. Using simulation, the study evaluates the effectiveness of this phase estimation method and its robustness against noise in algorithms which analyze Doppler frequency shifts. Furthermore, a CDWL is developed for measuring the speed of an indoor motor turntable and the outdoor atmospheric wind field. The phase estimation method yielded SNR gains of 28.18 dB and 32.03 dB for accumulation numbers of 500 and 1500, respectively. The implementation of this method in motor turntable speed measurements demonstrated a significant reduction in speed error-averaging 9.18% lower than that of incoherent accumulation lidar systems. In experiments that measure atmospheric wind fields, the linear fit curve slope between the measured wind speed and the wind speed measured via a commercial wind-measuring lidar can be reduced from 1.146 to 1.093.

Optical adaptive power control based on atmospheric channel reciprocity for mitigating turbulence disturbances in free-space optics communication.

A continuous time-domain adaptive power model of transmitter optical and control algorithm based on atmospheric turbulence channel reciprocity are explored for mitigating the free-space optical communication (FSOC) receiver optical intensity scintillation and bit error rate (BER) deterioration. First, a transmitter optical adaptive power control (OAPC) system architecture using four wavelength optical signals based on atmospheric turbulence channel reciprocity is proposed, and electronically variable optical attenuator (EVOA) and erbium-doped fiber amplifier (EDFA) are employed as the main OAPC units for power adaptation. Moreover, a reciprocity evaluation model for gamma-gamma (G-G) continuous-time signals is generated using the autoregressive moving average (ARMA) stochastic process, which takes into account the delay time and system noise, and a reciprocity-based OPAC algorithm is proposed. Numerical simulations were also performed to analyze the signal reciprocity characteristics under different turbulence, noise, and sampling time mismatch at both ends, as well as the scintillation index (SI) performance under OAPC system operation. Simultaneously, the time-domain signals of continuous quadrature amplitude modulation -16 (QAM-16) and QAM-32 real states are fused with the gamma-gamma (G-G) reciprocal turbulence continuous signals to analyze the probability density function (PDF) and bit error ratio (BER) performance after OAPC correction. Finally, a 64 Gpbs QAM-16 OPAC communication experiment was successfully executed based on an atmospheric turbulence simulator. It is shown that the OAPC correction is carried out using reciprocity at millisecond sampling delay, the light intensity scintillation of the communication signal can be well suppressed, the signal-to-noise ratio (SNR) is greatly improved, the suppression is more obvious under strong turbulence, the overall BER reduction is greater than 2.8 orders of magnitude with the OAPC system, and this trend becomes more pronounced as the received power increases, even reach 6 orders of magnitude in some places. This work provides real time-domain continuous signal samples for real signal generation of communication signals in real turbulence environments, adaptive coding modulation using reciprocity, channel estimation, and optical wavefront adaptive suppression, which are the basis of advanced adaptive signal processing algorithms.

Average capacity of an underwater wireless communication link with the quasi-Airy hypergeometric-Gaussian vortex beam based on a modified channel model.

Prompted by alleviating the random perturbation of underwater channel and enhancing the performance for the orbital angular momentum (OAM) -based underwater wireless optical communication (UWOC), the quasi-Airy Hypergeometric-Gaussian (QAHyGG) vortex beam is first proposed and demonstrated. Moreover, an underwater channel model is first modified for more accurate simulated results of the propagation property of various beams. Based on the modified model, the transmission and communication performance of three different OAM-carrying beams (the Gauss vortex (GV) beam, the Hypergeometric-Gaussian (HyGG) vortex beam, and the QAHyGG vortex beam) are comparatively studied. In addition, the parameters optimization of the QAHyGG vortex beam is made for further enhancing the average capacity. The results show that the QAHyGG vortex beam exhibits higher received power and lower crosstalk probability under different channel conditions. The average capacity of the QAHyGG vortex beam has enhanced by ∼8% and ∼27% compared with the HyGG vortex beam and the GV beam at 100m, respectively. The QAHyGG vortex beam is more suitable in an OAM-based UWOC system with a limited-size receiving aperture or lower transmit power. Besides, the average capacity will improve effectively at longer distances with the optimized beam parameters. These research results can provide advances in designing the practical OAM-based UWOC system.

Low-power consumption InP-based optical phase arrays with non-uniformly spaced output waveguides.

Optical Phase Arrays (OPAs) are expected to be an ideal solution to achieve beam shaping, laser radar (LIDAR), free-space optical communications, and spatially resolved optical sensors, etc. We demonstrated a low-power consumption 32-channel OPA with non-uniformly spaced waveguides based on InP substrate. The phase shifters are based on a p-i-n structure which are operated with reverse bias and have a low power consumption. Besides, in order to improve the performance especially to obtain larger steering angle and narrower beam divergence without increasing the number of channels, we have optimized the spacing between the output waveguides. The fabricated OPA achieved a steering angle of 35° with the side lobe suppression ratio more than 8.2 dB across the angle range from -20° to 20° in the far field, which is the largest phase tuning steering angle reported by InP-based OPAs as far as we know. The divergence angle is about 0.46° in the phase steering dimension and the power consumption of the OPA at each steering angle is lower than 7.5 mW.

Method for 10 Gbps near-ground quasi-static free-space laser transmission by nutation mutual coupling.

This study proposes a method to reduce link loss in near-ground space laser communication with fiber-nutation-based mutual coupling. We designed a transmit-receive coaxial laser terminal with a 50 mm aperture and implemented beam acquisition tracking and nutation coupling using a single-detector and single-actuator. Following an indoor experiment and a 1 km field test, the single-ended nutation coupling and the two-way nutation mutual coupling theories were compared and analyzed. In conclusion, the proposed method could significantly increase the reception efficiency by ∼8 dB. The bit error rate of 10 Gbps pseudo-random code transmission was 2.478E-9. We also demonstrated video transmission.

Generation of temporal fading envelope sequences for the FSOC channel based on atmospheric turbulence optical parameters.

The temporal characteristics of the free space optical communication (FSOC) turbulence fading channel are essential for analyzing the bit error rate (BER) performances and compiling the rationale of adaptive signal processing algorithms. However, the investigation is still limited since the majority of temporal sequence generation fails to combine the autocorrelation function (ACF) of the FSOC system parameters, and using the simplified formula results in the loss of detailed information for turbulence disturbances. In this paper, considering the ACF of engineering measurable atmospheric parameters, we propose a continuous-time FSOC channel fading sequence generation model that obeys the Gamma-Gamma (G-G) probability density function (PDF). First, under the influence of parameters such as transmission distance, optical wavelength, scintillation index, and atmospheric structural constant, the normalized channel fading models of ACF and PSD are established, and the numerical solution of the time-domain Gaussian correlation sequence is derived. Moreover, the light intensity generation model obeying the time-domain correlation with statistical distribution information is derived after employing the rank mapping, taking into account the association between the G-G PDF parameters and the large and small scales turbulence fading channels. Finally, the Monte Carlo numerical method is used to analyze the performances of the ACF, PDF, and PSD parameters, as well as the temporal characteristics of the generated sequence, and the matching relationships between these parameters and theory, under various turbulence intensities, propagation distances, and transverse wind speeds. Numerical results show that the proposed temporal sequence generation model highly restores the disturbance information in different frequency bands for the turbulence fading channels, and the agreement with the theoretical solution is 0.999. This study presents essential numerical simulation methods for analyzing and evaluating the temporal properties of modulated signals. When sophisticated algorithms are used to handle FSOC signals, our proposed temporal sequence model can provide communication signal experimental sample data generating techniques under various FSOC parameters, which is a crucial theoretical basis for evaluating algorithm performances.

1.7 µm gain-switched and mode-locked hybrid Tm-Ho codoped fiber laser signal generation and optimization.

We propose and experimentally demonstrate 1.7 µm gain-switched and mode-locked hybrid laser signal generation using a modulated pump and the nonlinear polarization rotation (NPR) effect. In the laser scheme, a 1.55 µm amplified modulated optical signal was used as a homemade pump. A bidirectional pumping configuration was adopted by splitting the homemade pump. A 1 m long thulium-holmium (Tm-Ho) codoped fiber was used as the gain medium. A fiber Bragg grating was employed as a spectral filter. The mode-locked laser pulse was obtained with a central wavelength of 1724 nm. The repetition rate was 11.81 MHz and the pulse width was 65.27 ps. Additionally, the gain-switched pulse sequences with a repetition rate from 50 kHz to 200 kHz were obtained by the modulated pump. Moreover, the mode-locked pulse train was filtered and modulated by the shape of the gain-switched pulse, and the hybrid pulse train was then obtained. Furthermore, the hybrid laser signals were analyzed and optimized by applying different waveforms of the modulated pump. The experimental results showed that the generated laser pulse driven by the sinusoidal signal has a better SNR (49.39 dB).

2.5 Gbps free-space optical transmission between two 5G airship floating base stations at a distance of 12 km.

We experimentally demonstrated a long-range, large-capacity-featured, airship-based, free-space optical transmission system by using key technologies, such as GPS/INS real-time precision laser pointing, coarse and fine compound high-bandwidth laser tracking, avalanche photodiode detector adaptive control turbulence channel compensation, and the aurora laser fifth-generation (5G) interface protocol, to solve the problems of laser tracking and targeting based on an airship motion platform, high-speed signal transmission under atmospheric channel perturbation, and interface protocol between a wireless laser link and 5G base station signal, respectively. To the best of our knowledge, the first FSO transmission of 5G base station signals between airship platforms with a rate of 2.5 Gbps and a distance of 12 km was realized, and channel turbulence jitter was also recorded for laser links of different altitudes, from 200 to 1000 m, which provides technical and data support for the application of wireless laser links in 5G floating base stations.

Analysis of scintillation effects along a 7 km urban space laser communication path.

In this study, the scintillation effects along a 7 km space laser communication path in an urban area were examined, including the relationship between the scintillation and bit error rate and the variation of the scintillation index with changes in the transmitting and receiving apertures. It was concluded that multi-aperture transmitting technology can effectively reduce the scintillation caused by atmospheric turbulence. For the investigated urban link, the scintillation index could meet the communication requirements for adjusting the receiving aperture. These results will facilitate space laser communication improvement and turbulence suppression along horizontal urban paths.

Single-/dual-pulse repetition rate variable supercontinuum light source with peak wavelength around 1.7 µm using a modulated pump.

A single-/dual-pulse repetition rate variable supercontinuum (SC) light source (SLS) with a peak wavelength of around 1.7 µm (SLS around 1.7 µm) is proposed and experimentally demonstrated. In our scheme, a 1.5 µm modulated pump source included a laser and an intensity modulator (IM). The pump source can generate pulse trains with different repetitions and pulse durations. A 1 km high nonlinear fiber (HNLF) was used as the nonlinear gain medium. A picosecond-pulsed SC signal was obtained by pumping the HNLF, and a wavelength division multiplexer was used for filtering residual pump. Additionally, a Sagnac loop was applied to create a multiwavelength pulse SC light source. The generated SC source covered from 1.59 to 1.96 µm, and its peak wavelength was around 1.7 µm. The single-/dual-pulse train can be produced and switched by adjusting the direct current bias and radio frequency driving voltages of the input signal to the IM. When the repetition rate of the generated pulse train was between 170 MHz and 2 GHz, the pulse duration of the dual-pulse train was between 60 ps and 180 ps. Additionally, the duty cycle of the dual-pulse operation was 40%. The single pulse SLS, around 1.7 µm, can be a choice to improve optical coherence tomography (OCT) performance, and the dual-pulse source will be a reference for laser drilling applications.

Statistical Scene-Based Non-Uniformity Correction Method with Interframe Registration.

The non-uniform response in infrared focal plane array (IRFPA) detectors inevitably produces corrupted images with a fixed-pattern noise. In this paper, we present a novel and adaptive scene-based non-uniformity correction (NUC) method called Correction method with Statistical scene-based and Interframe Registration (CSIR) , which realizes low delay calculation of correction coefficient for infrared image. This method combines the statistical method and registration method to achieve a better NUC performance. Specifically, CSIR estimates the gain coefficient with statistical method to give registration method an appropriate initial value. This combination method not only reduces the need of interactive pictures, which means lower time delay, but also achieves better performance compared to the statistical method and other single registration methods. To verify this, real non-uniformity infrared image sequences collected by ourselves were used, and the advantage of CSIR was compared thoroughly on frame number (corresponding to delay time) and accuracy. The results show that the proposed method could achieve a significantly fast and reliable fixed-pattern noise reduction with the effective gain and offset.

Analysis and evaluation of the performance between reciprocity and time delay in the atmospheric turbulence channel.

In order to better evaluate the relationship between reciprocity and time delay of the fiber receiving system in the atmospheric turbulence channel, a time-domain signal generation mathematical model is proposed for the first time. A numerical solution of Johnson SB probability density distribution (PDF) in time-domain is creatively given for evaluating the reciprocity of both communication ends, which relates to the normalized fluctuation variance of the light intensity and the Greenwood frequency. An experiment is then carried out for verifying the time-domain signal generation model and measuring reciprocity. It shows that the excellent fitting accuracy of Johnson SB PDF signal generation model is first experimentally verified. It also indicates that the system reciprocity is improved by 10% after eliminating the system time delay. Meanwhile, the relationship between time delay and reciprocity under different atmospheric environments are analyzed and the relationship between time delay and system reciprocity at different Greenwood frequencies are discussed. This work provides a time parameter reference for the design of adaptive system and free-space optical (FSO) communication system.

Secondary-Transferring Graphene Electrode for Stable FOLED.

In this work, sharp wrinkles on graphene films, caused by graphene duplicating the grain boundary cracks of copper foil during the preparation process, were carefully explored. A secondary-transferring graphene film process was proposed to re-transform the "Peak" morphology of graphene surface into "Valley" form. The process we have developed is highly effective and almost nondestructive to the graphene through testing the surface morphology and photo-electric properties before and after the secondary-transferring process. Flexible organic light-emitting device (FOLED) with PEDOT:PSS/SLG/NOA63 framework as a targeted application was fabricated to illustrate the value of our proposed method in fabricating stable devices, the maximum luminance can reach about 35000 cd/m2, and the maximum current efficiency was 16.19 cd/A. This method can also be applied to the roll-to-roll preparation of large area high-quality graphene.

Study on extended depth of field for a planar flow cytometric microimaging system.

A planar flow cytometric microimaging system is mainly used for cell recognition and classification in urinary sediment and gynecological secretion analysis. The depth of field (DOF) of the microscope seriously restricts its imaging range in the direction of the optical axis, rendering it incapable of imaging all the cells in the whole laminar thickness of the planar flow cytometric microimaging system. In this paper, the DOF is extended by using dual sensors with a common light path, and imaging of high-speed moving cells at a large DOF is realized, thus solving the difficulty that the multifocus super-depth technique can only be used in a static observation sample. A fusion algorithm based on saliency detection and multiscale image decomposition is developed to fuse the dual-depth-of-field images. The multiscale image decomposition uses L0 smoothing for multiscale image decomposition. L0 smoothing is particularly effective in sharpening major edges by increasing the steepness of transition, while eliminating a manageable degree of low-amplitude structures. It can globally control the number of non-zero gradients that result in an approximately prominent structure in a sparsity-control approach; this does not depend on the local features, instead it locates important edges globally. Experimental results show that our approach can enlarge the DOF 1.89 times, and the dual-DOF fusion algorithm can fuse two images with different DOFs into one image with clear multiple targets.

Performance of M-PAM FSO communication systems in atmospheric turbulence based on APD detector.

We characterize the performance of the optical signal propagation model of multi-level pulse amplitude modulation (M-PAM) based on avalanche photodiode (APD) detector in the free-space link for the first time. When the number of photons absorbed by the active surface of the APD is large enough, the bit error rate (BER) performance relationship of the systems based on the signal intensity and the photon characteristics are depicted. We use the Gamma-Gamma (G-G) channel model to analysis the communication systems with joint parameter constraints, and demonstrate the atmospheric turbulence intensity, link lengths, optical wavelength, symbol transmission rate, temperature of APD and pointing errors (PEs) impact on the system average bit error rate (ABER) performance. Moreover, the relationship between signal-to-noise-ratio (SNR) and ABER rate is given. The numerical results show that the 4-PAM free-space optical (FSO) communication is suitable for medium-to-weak turbulence, and the high gain of APD can mitigate the influence of ABER. The best detection condition of the 4-PAM optical signal is at least 20 dB SNR, when the ABER is under the 7% forward error correction (FEC) limit of 3.8 × 10-3. This work provides a reference for parameter designing and evaluating in M-PAM FSO communication systems.

Label-Free Sensitive Detection of Microcystin-LR via Aptamer-Conjugated Gold Nanoparticles Based on Solid-State Nanopores.

A versatile and highly sensitive strategy for nanopore detection of microcystin-LR (MC-LR) is proposed herein based on the aptamer and host-guest interactions by employing a gold nanoparticle (AuNP) probe. The aptamer of MC-LR and its complementary DNA (cDNA) are respectively immobilized on AuNPs with distinct sizes (5 nm AuNPs for the aptamer and 20 nm for the cDNA), and the constructed polymeric AuNP network via the hybridization of the aptamer and cDNA was disintegrated upon the addition of MC-LR. The specific interactions between the aptamer and MC-LR disrupt and release the cDNA-AuNPs that were then removed by centrifugation, leaving the MC-LR-aptamer-AuNP species in the supernatant for subsequent nanopore determination. By monitoring the current blockade of released MC-LR-aptamer-AuNPs using a specific tailored nanopore (10 and 20 nm in diameter, generated by current dielectric breakdown), we could deduce the presence of MC-LR, as the bulky NP network could not pass through a nanopore with a relatively smaller size. We realized the detection of MC-LR with a concentration as low as 0.1 nM; additionally, we have proved the specificity of the interaction between the aptamer and MC-LR by replacing MC-LR with other congener toxins (MC-RR and MC-YR), chlorophyll (a component abundantly coexists in water), and the mixture of the four.

Design of a delayed XOR phase detector for an optical phase-locked loop toward high-speed coherent laser communication.

Optical phase-locked loops are an effective detection method in high-speed and long-distance laser communication. Although this method can detect weak signal light and maintain a small bit error rate, it is difficult to perform because identifying the phase difference between the signal light and the local oscillator accurately has always been a technical challenge. Thus, a series of studies is conducted to address this issue. First, a delayed exclusive or gate (XOR) phase detector with multi-level loop compound control is proposed. Then, a 50 ps delay line and relative signal-to-noise ratio control at 15 dB are produced through theoretical derivation and simulation. Thereafter, a phase discrimination module is designed on a 15 cm×5 cm printed circuit board board. Finally, the experiment platform is built for verification. Experimental results show that the phase discrimination range is -1.1 to 1.1 GHz, and the gain is 0.82 mV/MHz. Three times the standard deviation, that is, 0.064 V, is observed between the test and theoretical values. The accuracy of phase detection is better than 0.07 V, which meets the design standards. A coherent carrier recovery test system is established. The delayed XOR gate has good performance in this system. When the communication rate is 5 Gbps, the system realizes a bit error rate of 1.55×10-8 when the optical power of the signal is -40.4 dBm. When the communication rate is increased to 10 Gbps, the detection sensitivity drops to -39.5 dBm and still shows good performance in high-speed communications. This work provides a reference for future high-speed coherent homodyne detection in space. Ideas for the next phase of this study are presented at the end of this paper.

10.23 Mcps laser pseudo-code ranging system with 0.33 mm (1σ) pseudo-range measurement precision.

The inter-satellite laser link is the backbone of the next inter-satellite information network, and ranging and communication are the main functions of the inter-satellite laser link. This study focuses on the inter-satellite laser ranging based on the pseudo-code correlation technology. In this paper, several typical laser-ranging methods have been compared and we determined that the laser pseudo-code ranging architecture is more suitable for the inter-satellite laser communication link. The pseudo-code ranging system is easy to combine with a digital communication system, and we used it to calculate integer ambiguity by modulating the time information. The main challenge of the ranging system is range precision, which is the main focus of this paper. First, the framework of the pseudo-code ranging system is introduced; the ranging architecture of dual one-way ranging is used to eliminate the clock error between the two transceivers, and then the uncertainty of the phase detector is analyzed. In the analysis, the carrier to noise ratio and the ranging code rate are constrained by the laser communication link margin and the electronic hardware limitation. Therefore, the relationship between the sampling depth and the phase detector uncertainty is verified. A series of optical fiber channel laser pseudo-code ranging experiments demonstrated the effects of sampling depth on the ranging precision. By adjusting the depth of storage, such as the depth of 1.6 Mb, we obtained a pseudo-range measurement precision of 0.33 mm (1σ), which is equivalent to 0.0001 times code subdivision of 10.23 Mcps pseudo-code. This paper has achieved high precision in a pseudo-range measurements, which is the foundation of the inter-satellite laser link.

Changes in orbital-angular-momentum modes of a propagated vortex Gaussian beam through weak-to-strong atmospheric turbulence.

The radial average-power distribution and normalized average power of orbital-angular-momentum (OAM) modes in a vortex Gaussian beam after passing through weak-to-strong atmospheric turbulence are theoretically formulated. Based on numerical calculations, the role of the intrinsic mode index, initial beam radius and turbulence strength in OAM-mode variations of a propagated vortex Gaussian beam is explored, and the validity of the pure-phase-perturbation approximation employed in existing theoretical studies is examined. Comparison between turbulence-induced OAM-mode scrambling of vortex Gaussian beams and that of either Laguerre-Gaussian (LG) beams or pure vortex beams has been made. Analysis shows that the normalized average power of OAM modes changes with increasing receiver-aperture size until it approaches a nearly stable value. For a receiver-aperture size of practical interest, OAM-mode scrambling is severer with a larger mode index or smaller initial beam radius besides stronger turbulence. Under moderate-to-strong turbulence condition, for two symmetrically-neighboring extrinsic OAM modes, the normalized average power of the one with an index closer to zero may be greater than that of the other one. The validity of the pure-phase-perturbation approximation is determined by the intrinsic mode index, initial beam radius and turbulence strength. It makes sense to jointly control the amplitude and phase of a fundamental Gaussian beam for producing an OAM-carrying beam.