Underwater Laser Communication Research Articles

Echo scattering characteristics: Vortex beams reflected by rough surface in ocean turbulence

Optical field echo scattering characteristic is one of the essential technologies for the future integration of underwater laser communication and detection. Vortex beam (e.g. Laguerre-Gaussian (LG) beam) with a helical wavefront structure is better suited to reduce the effects of ocean turbulence. A double-path transmission model of LG beam scattering through rough surface in ocean turbulence is innovatively established. We developed a closed analytical expression for the echo speckle intensity of LG beams reflected by a rough surface with a Gaussian distribution in ocean turbulence based on the generalized Huygens-Fresnel principle. The autocorrelation function of intensity and complex degree of coherence (DOC) of echo speckle field were numerically simulated and analyzed. From the results we found a rule that when coherence length of rough surfaces (ρh) is greater than the coherence length (ρ0) of spherical waves propagating across ocean turbulence, the impact of a rough surface on DOC is much less than that of ocean turbulence. This conclusion provides the theoretical basis that the echo wave field cannot be obtained in target detection under ocean turbulence.

Error Characterization of Differential Detection and Non-Differential Detection for MIMO UWOC Systems in Seawater Turbulent Channels

Ocean turbulence is an important factor affecting the development of underwater wireless optical communication (UWOC). To improve the error characteristics of the underwater optical communication system, we propose a differential detection-based multi-receiver-multi-transmitter (MIMO) underwater laser communication transmission method. Additionally, we derive the expressions for calculating the average BER of the MIMO underwater wireless optical communication system with differential detection and non-differential detection in the case that the two transmitted beams are completely uncorrelated. The error characteristics of the MIMO system are simulated and analyzed from the perspective of ocean turbulence intensity and link distance. The simulation results show that the differential detection method has a lower average BER compared to the non-differential detection method in the case of moderate-to-strong ocean turbulence. In addition, the differential detection methods do not have the error floor effect, and non-differential detection methods have the error floor effect. The more the turbulence intensity affects the average BER of the MIMO UWOC system with the increase of the communication link distance, the more obvious is the effect of the turbulence intensity on the average BER of the MIMO UWOC system. Accordingly, the simulation analysis shows that the differential detection method is more suitable for the construction of communication links under long-distance and medium-strong turbulence.

Research on duplex underwater wireless laser communication system

To improve the bit error rate (BER) of underwater wireless laser communication and increase communication speed, a duplex underwater wireless laser communication system is designed. The system uses both blue and green lasers to transmit signals simultaneously, and adopts an improved modulation and demodulation algorithm to achieve separate modulation of the two sets of signals. A duplex underwater wireless laser communication system builds based on 440 and 550 nm lasers. On the basis of analyzing the absorption and scattering characteristics of seawater in laser communication, the influence of turbulence on communication efficiency is discussed. Improved the traditional duplex modulation algorithm and conducted communication testing at a depth of 2.0 m underwater. The experimental results show that when the communication distance increases from 5 to 50 m, the BER of the unoptimized duplex mode increases from 2.34 × 10−7 to 3.5 × 10−5. After adopting the optimization algorithm, the BER increases from 2.81 × 10−7 to 1.18 × 10−6, and the signal attenuation has been significantly suppressed. The duplex communication structure and algorithm can effectively reduce the impact of communication distance on bit BER.

Research on underwater wireless dynamic optical communication system based on PPM modulation

In order to achieve underwater wireless dynamic optical communication, a laser communication system is proposed based on Pulse Position Modulation (PPM). In order to achieve underwater laser communication accurately, the mathematical model of underwater laser communication was constructed with small angle analysis. The pulse position modulation demodulation algorithm is designed, and the workflow of modulation and demodulation is given in the transmit module and the receive module. In the experiment, Lumileds-470 nm light source was selected for data communication for testing at a communication rate of 15 Mbp/s. In the servo control process, the square wave signal used for stepping motor drive had a stable amplitude output and a stable time width. It can well simulate the testing process of underwater dynamic scanning. In the experiment, laser light spots were obtained under different attenuation states, and the characteristics of the light spot distribution were analyzed. The numerical reconstruction of the light spot energy was completed in MATLAB. Three types of light attenuators, 1.0%, 0.1%, and 0.01%, were used to simulate different light attenuations underwater. The test results show that the system error rate is better than 10−6 when attenuation chip is 1.0%. When attenuation chip is 0.1%, the error rate of the system is reduced to 10−4. When attenuation chip is 0.01%, a valid signal cannot be obtained by the system. The feasibility of the system is verified.

Effect of oceanic turbulence on the spectral changes of diffracted chirped Gaussian pulsed beam

The analytical expression for the spectral intensity of chirped Gaussian pulsed beam diffracted by a circular aperture propagating through the oceanic turbulence is derived based on the extended Huygens-Fresnel principle, and the spectral behaviors are analyzed in detail for both cases with aperture and without one. The results obtained show that there is a great difference between the spectra with aperture and without one, spectral switch occurs according to the size of aperture or pulse duration, which can be applied for information encoding. It is also found that the aperture radius and the pulse duration are important controllable parameters which can be used in the practice for spectral diversity technique and information encoding, and the corresponding spectral changes occur according to the propagation distance of pulsed beam as a result of competition between source and turbulence correlation. The results of this work can have potential applications in underwater laser communication and imaging, in conditions where spectral diversity technique and information encoding are used.

Adaptive stochastic resonance detection of weak light signal in underwater laser communication

Adaptive stochastic resonance detection of weak light signal in underwater laser communication

Investigation of 3 dB Optical Intensity Spot Radius of Laser Beam under Scattering Underwater Channel.

An important step in the design of receiver aperture and optimal spacing of the diversity scheme for an underwater laser communication system is to accurately characterize the two-dimensional (2D) spatial distribution of laser beam intensity. In this paper, the 2D optical intensity distribution and 3 dB optical intensity spot radius (OISR) are investigated due to the dominating optical intensity of laser beam being within the 3 dB OISR. By utilizing the Henyey–Greenstein function to compute the scattering angles of photons, the effects of the scattering underwater optical channel and optical system parameters on 3 dB OISR are examined based on the Monte Carlo simulation method. We have shown for the first time that in the channel with a high density of scattering particles, the divergence angle of the laser source plays a negligible role in 3 dB OISR. This is an interesting phenomenon and important for optical communication as this clearly shows that the geometric loss is no longer important for the design of receiver aperture and optimal spacing of the diversity scheme for the underwater laser communication system in the highly scattering channel.

Underwater laser communication with sloped pulse modulation in turbid water

In this article, we propose the demonstration of an underwater wireless laser communication link operating at data rates of up to 100 Mbps over a band-limited turbid water channel using a modified pulse position modulation scheme with a sloped pulse. The communication link uses an avalanche photodiode module as the receiver. The bit error rate of a 5.4 × 10−6 by 450-nm blue laser diode in turbid water with 1.4 Nephelometric Turbidity Unit was below the forward error correction limit. Our demonstrated underwater wireless laser communication system is simple to use and maintains a high data transmission rate.

수중 레이저 통신에서 LDPC 부호화된 M-레벨 펄스 위치 변조 신호의 연판정 복호 기법

본 논문에서는 수중 레이저 통신에서 적용되는 M진 PPM 변조 방식의 복조 과정에서 연판정 기법을 제안하며, LDPC 부호화 방식과 M진 PPM 변조 방식을 적용하여 연판정 기법과 기존의 경판정 기법의 성능을 비교 분석하는 시뮬레이션을 수행하였다. 시뮬레이션 결과 본 논문에서 제안한 연판정 기법이 기존의 경판정 기법보다 0.2dB ~ 2.2dB의 성능이 향상됨을 확인하였다.

分集阵列式水下激光通信光学接收天线设计

分集阵列式水下激光通信光学接收天线设计

Viterbi equalization for long-distance, high-speed underwater laser communication

In long-distance, high-speed underwater laser communication, because of the strong absorption and scattering processes, the laser pulse is stretched with the increase in communication distance and the decrease in water clarity. The maximum communication bandwidth is limited by laser-pulse stretching. Improving the communication rate increases the intersymbol interference (ISI). To reduce the effect of ISI, the Viterbi equalization (VE) algorithm is used to estimate the maximum-likelihood receiving sequence. The Monte Carlo method is used to simulate the stretching of the received laser pulse and the maximum communication rate at a wavelength of 532 nm in Jerlov IB and Jerlov II water channels with communication distances of 80, 100, and 130 m, respectively. The high-data rate communication performance for the VE and hard-decision algorithms is compared. The simulation results show that the VE algorithm can be used to reduce the ISI by selecting the minimum error path. The trade-off between the high-data rate communication performance and minor bit-error rate performance loss makes VE a promising option for applications in long-distance, high-speed underwater laser communication systems.

Evolution properties of partially coherent four-petal Gaussian beams in oceanic turbulence

Based on the extended Huygens–Fresnel integral formula, the analytical expressions for partially coherent four-petal Gaussian beam propagating in oceanic turbulence are derived, and the influences of coherence length, beam order N and the parameters of oceanic turbulence (the rate of dissipation of turbulent kinetic energy per unit mass of fluid, the rate of dissipation of mean square temperature and the relative strength of temperature and salinity fluctuations) on average intensity properties are investigated using numerical examples in detail. The results show that the beam with the higher beam order N or coherence length will lose its initial four-petal profiles slower. It is also indicated that the beam will evolve into a Gauss-like beam more rapidly with increasing oceanic turbulence strength. The results have the potential application in underwater laser communication using a partially coherent four-petal Gaussian beam.

The influence of oceanic turbulence on the spectral properties of chirped Gaussian pulsed beam

Based on the extended Huygens-Fresnel principle, the spectral behaviors of a chirped Gaussian pulsed beam propagating in oceanic turbulence are illustrated. The influence of the parameters of oceanic turbulence (the rate of dissipation of turbulent kinetic energy per unit mass of fluid, rate of dissipation of mean-square temperature, relative strength of temperature and salinity fluctuations), relative position parameter and propagation distance on the spectra shift is analysed and given by numerical examples. The research results have the potential application in underwater wireless laser communication and remote sensing.

水下激光通信最大比合并分集接收性能分析及仿真

水下激光通信最大比合并分集接收性能分析及仿真

面向水下应用的改进型光子计数通信方法

Underwater laser communication is influenced by the absorption and scattering of water, which causes severe signal energy attenuation during propagation. Laser communication based on the photon counting is considered as an effective way to resist signal loss and increase communication distance because of its ultra-high detection sensitivity. However, since communication data recovery is usually realized by detecting electrical pulse at the output of single photon detector directly in traditional photon counting communication, the communication bit error rate would be easily influenced by background light noise. In this paper, an improved method is proposed and studied to solve this problem. In our approach, the photons arrived at the communication receiver are converted into electrical pulses by single photon detector first. Then, communication data is recovered through counting the electrical pulse number on unit time. The experimental result shows that detection sensitivity of 84.24 bit-1can be realized by the proposed method, when the communication wavelength is 532 nm, the communication rate is 50 kb/s, and the signal to noise ratio is 5.14.The novel approach proposed in this paper provides a new technical idea for high-sensitivity underwater laser communication. © 2016, Chinese Lasers Press. All right reserved.

Effect of scattering albedo on attenuation and polarization of light underwater

Recent work on underwater laser communication links uses polarization discrimination to improve system performance [Appl. Opt.48, 328 (2009)] [in Proceedings of IEEE Oceans 2009 (IEEE, 2009), pp. 1-4]. In the laboratory, Maalox antacid is commonly used as a scattering agent. While its scattering function closely mimics that of natural seawaters, its scattering albedo can be much higher, as Maalox particles tend to be less absorbing. We present a series of experiments where Nigrosin dye is added to Maalox in order to more accurately recreate real-world absorption and scattering properties. We consider the effect that scattering albedo has on received power and the degree of depolarization of forward-scattered light in the context of underwater laser communication links.