Beam Wander Research Articles

High-resolution flying-PIV with optical fiber laser delivery

Implementation of non-intrusive optical measurement techniques, such as particle image velocimetry (PIV), in harsh environments requires specialized techniques for introducing controlled laser sheets to the region of interest. Large earthquake shake tables are a particularly challenging environment. Lasers must be mounted away from the table, and the laser sheet has to be delivered precisely and stably to the measurement station. Here, high-power multi-mode step-index fiber optics enable introduction of light from an Nd:YLF pulsed laser to a remote test section. Such lasers are suitable for coupling to optical fibers, which presents a portable, flexible, and safe manner to deliver a PIV light sheet. Best practices for their implementation are reviewed. Particular attention is focused on obtaining a collimated beam of acceptable quality at the output of the fiber. To achieve high spatial resolution, the PIV camera is directly mounted on the moving shake table with care to minimize its vibrations. A special arrangement of PIV planes is deployed for precise in-situ PIV alignment and to monitor and account for residual structure vibrations and beam wandering. The design of the instruments is detailed. Here, an experimental facility for the study of nuclear fuel bundle response to seismic forcing near prototypical conditions is instrumented. Only through integration of a high-resolution flying-PIV system can velocity fields be acquired. Data indicate that in the presence of a mean axial flow, a secondary oscillatory flow develops as the bundle oscillates. Instantaneous, phase-averaged, and fluctuating velocity fields illustrate this phenomenon.

Statistical Profile of Hermite–Gaussian Beam in the Presence of Residual Spatial Jitter in FSO Communications

In this letter, the statistics of the beam profile of a free-space optical (FSO) system is studied when the optical beam possesses a Hermite–Gaussian profile and the beam is subject to random pointing jitter. In particular, it is assumed that the received signal is impaired by the residual pointing error cause by the spatial tracking loop responsible for tracking the atmospheric beam wander and platform sway. The residual pointing jitter is assumed to have a Gaussian probability density function (pdf) with independent components in the two dimensions of the detector. The mean and variance of the beam profile along with the impact of spatial error on the signal-to-noise (SNR) loss and channel cross-talk are obtained assuming that the pointing jitter has a variance that is substantially smaller than the beam waist.

Chromatic effects in beam wander correction for free-space quantum communications

Beam wander caused by atmospheric turbulence can significantly degrade the performance of horizontal free-space quantum communication links. Classical beam wander correction techniques cannot be applied due to the stronger requirements of transmitting single photons. One strategy to overcome this limitation consists in using a separate wavelength from that of the quantum signal to analyze the beam wander and use this information for its correction. For this strategy to work adequately, both wavelengths should be affected equally by atmospheric turbulence, i.e. no chromatic effects should be originated from beam wander. In this letter, a series of experiments are performed to prove that this is the case for {\lambda} ~ 850 nm as the quantum signal and {\lambda} ~ 1550 nm as the tracking signal of a quantum communication system.

Beam wander of Gaussian-Schell model beams propagating through oceanic turbulence

For Gaussian-Schell model beams propagating in the isotropic turbulent ocean, theoretical expression of beam wander is derived based on the extended Huygens–Fresnel principle. The spatial coherence radius of spherical waves propagating in the paraxial channel of turbulent ocean including inner scale is also developed. Our results show that the beam wander decreases with the increasing rate of dissipation of kinetic energy per unit mass of fluid ϵ, but it increases as the increasing of the dissipation rate of temperature variance χt and the relative strength of temperature and salinity fluctuations ϖ. The salinity fluctuation has greater influence on the beam wander than that of temperature fluctuations. The model can be evaluated submarine-to-submarine/ship optical wireless communication performance.

Quality Metrics and Reliability Analysis of Laser Communication System

<p class="Abstract">Beam wandering is the main cause for major power loss in laser communication. To analyse this prerequisite at our environment, a 155 Mbps data transmission experimental setup is built with necessary optoelectronic components for the link range of 0.5 km at an altitude of 15.25 m. A neuro-controller is developed inside the FPGA and used to stabilise the received beam at the centre of detector plane. The Q-factor and bit error rate variation profiles are calculated using the signal statistics obtained from the eye-diagram. The performance improvements on the laser communication system due to the incorporation of beam wandering mitigation control are investigated and discussed in terms of various communication quality assessment key parameters.</p><p class="Abstract"> </p>

Beam wander of laser beam propagating through oceanic turbulence

The analytical expressions for beam wander of collimated and focused beam in oceanic turbulence are derived. Compared with the previously integrating ones, it can be seen that the two results are in agreement with each other exactly for the collimated and focused beam, respectively. Further, the influences of three main oceanic parameters (i.e., the rate of dissipation of mean-squared temperature χT, the rate of dissipation of kinetic energy per unit mass of seawater ɛ and the ratio of temperature to salinity contribution to the refractive index spectrum w) and the beam radius W0 on beam wander are investigated in the collimated and focused beam cases. The results indicate that the beam wander increases as ɛ decreases, χT increases, salinity-induced predominates and W0 decreases in mentioned above cases. In addition, based on the dimensionless quantity BW, the relation between beam wander and the long-term spot size or turbulence-induced beam spot size is investigated. In particular, to distinguish beam wander among different beam types, the relative beam wander is defined. Based on this definition, the increment of beam wander between focused and collimated beam is larger than that of arbitrary beam type (i.e., 0<Θ0<1). It is beneficial to select the predominant beam type in laser propagation applications

Ultraviolet Stand-off Raman Measurements Using a Gated Spatial Heterodyne Raman Spectrometer

A spatial heterodyne Raman spectrometer (SHRS) is evaluated for stand-off Raman measurements in ambient light conditions using both ultraviolet (UV) and visible pulsed lasers with a gated ICCD detector. The wide acceptance angle of the SHRS simplifies optical coupling of the spectrometer to the telescope and does not require precise laser focusing or positioning of the laser on the sample. If the laser beam wanders or loses focus on the sample, as long as it is in the field of view of the SHRS, the Raman signal will still be collected. The SHRS is not overly susceptible to vibrations, and a vibration isolated optical table was not necessary for these measurements. The system performance was assessed by measuring stand-off UV and visible Raman spectra of a wide variety of materials at distances up to 18 m, using 266 nm and 532 nm pulsed lasers, with 12.4 in. and 3.8 in. aperture telescopes, respectively.

Statistical analysis of accurate prediction of local atmospheric optical attenuation with a new model according to weather together with beam wandering compensation system: a season-wise experimental investigation

Atmospheric parameters strongly affect the performance of Free Space Optical Communication (FSOC) system when the optical wave is propagating through the inhomogeneous turbulent medium. Developing a model to get an accurate prediction of optical attenuation according to meteorological parameters becomes significant to understand the behaviour of FSOC channel during different seasons. A dedicated free space optical link experimental set-up is developed for the range of 0.5 km at an altitude of 15.25 m. The diurnal profile of received power and corresponding meteorological parameters are continuously measured using the developed optoelectronic assembly and weather station, respectively, and stored in a data logging computer. Measured meteorological parameters (as input factors) and optical attenuation (as response factor) of size [177147 × 4] are used for linear regression analysis and to design the mathematical model that is more suitable to predict the atmospheric optical attenuation at our test field. A model that exhibits the R2 value of 98.76% and average percentage deviation of 1.59% is considered for practical implementation. The prediction accuracy of the proposed model is investigated along with the comparative results obtained from some of the existing models in terms of Root Mean Square Error (RMSE) during different local seasons in one-year period. The average RMSE value of 0.043-dB/km is obtained in the longer range dynamic of meteorological parameters variations.

Research progress of Airy beam and feasibility analysis for its application in FSO system

As one member of non-diffraction light family, Airy beam has attracted a great deal of interests due to its unique non-diffraction, transverse-accelerating and self-healing properties. And it has potential applications in many fields such as optical micro-particles manipulation, generation of optical bullets, creating curved electron beams, formation of Airy beam lasers and so on. This paper briefly reviews the recent development of Airy beams from four aspects: generating method, controlling of accelerating trajectory, auto-focusing of Airy beams arrays and propagating properties in atmospheric channel. The main methods of Airy beam generation are summarized and a deep analysis for each method is made from the cost of the system, how complex to come true and potential of applications, and give some suggestions for improving and perfecting. It shows that the methods with low cost of the system, easy to come true and the broad application prospect are hoped. This paper summarizes the main methods of Airy beam’s propagation trajectory control. And make a analysis for each method controlling principle of propagation trajectory in acceleration direction, self- bending degree, peak intensity position and so on. This paper makes a conclusion of advantages and disadvantages for each method. The research show that the Airy beam’s propagation trajectory control method by changing system parameters, that is simple and easily success, but cannot reverse acceleration direction and controlling ranges is limited. The control methods based on refractive-index gradient and the special medium, nonlinear photonic crystals, can reverse acceleration direction but difficultly come true. The acceleration direction and intensity distribution of Airy beam’s propagation trajectory can be controlled by the special transform and that easily come true. The research methods of Airy beam and Airy array beams’ propagation properties at the atmosphere turbulence are based on the Huygens-Fresnel principle, the multiple phase screens and Perturbation approximation methods and so on. The research show that the propagation properties of Airy beam at atmosphere, which include beams expand, beam drifts and Self focusing ability, are better than Gaussian beams. Furthermore, the properties of Gaussian beam and Airy beam were compared from the view of communication. The feasibility to use Airy beam as information carrier of FSO system was analyzed. As a conclusion, it was pointed out that auto-focusing Airy beams that combined by radial symmetrically distributing arrays cannot only enhance the light intensity at receive plane of FSO system, but also suppress effectively atmospheric effects such as beam spreading, scintillation, beam wander, etc. It would be a promising approach to use auto focusing Airy beams as light source to develop long rang, high bit rate FSO system.

Investigation on the influence of intensity scintillation and beam wander in space optical uplink DWDM communication system

Dense wavelength division multiplexing (DWDM) has been widely applied in ground optical communication. However, the technology of DWDM is still not mature enough in the space optical communication system. In order to further advance the use of DWDM into space optical communication, the probability density function (PDF) and the bit-error rate (BER) performance of DWDM is investigated in uplink communications under the influence of atmospheric turbulence, consisting of intensity scintillation and beam wander caused by atmospheric turbulence. Numerical results show that the atmospheric turbulence has a great impact on BER and PDF, and wavelength, divergence angel, and other relevant parameters should be carefully considered in this DWDM system. This work can be conducive for improving DWDM design of space optical uplink communication systems.

Thin laser beam wandering and intensity fluctuations method for evapotranspiration measurement

We compare in this study two simple optical setups to measure the atmospheric turbulence characterized by the refractive index structure parameter Cn2. The corresponding heat flux values sensed by the laser beam propagation are calculated leading to the plant evapotranspiration. The results are discussed and compared to measurements obtained with a well-known and calibrated eddy-covariant instrument. A fine analysis gives a good insight of the accuracy of the optical devices proposed here to measure the crop evapotranspiration. Additional evapotranspiration values calculated with meteorological sensor data and the use of different models are also compared in parallel.

External error modelling with combined model in terrestrial laser scanning

Terrestrial laser scanning (TLS) is a promising tool in geodetic applications. Data quality of outdoor TLS measurements is considerably limited by external errors caused by atmospheric variations and object related factors. In this study, external models related to atmospheric refraction, beam wander and incidence angle were introduced and integrated into a seven-parameter similarity transformation (7PST) model to compose a combined model. The combined model was rigorously adjusted by an iterative Gauss–Helmert (GH) model primarily to estimate external calibration parameters and transformation parameters simultaneously. In the application on a dam surface, a t-test showed that a significant external calibration parameter was the range scale, which mainly corresponded with the incidence angle model. The precision of estimated representative points was improved by approximately 50% with the proposed method. The presented models show a great potential for detecting external errors and registering multiple scans in outdoor applications.

不同下垫面下光束漂移和光斑扩展的实验研究

不同下垫面下光束漂移和光斑扩展的实验研究

海上大气湍流中光束漂移模型分析

海上大气湍流中光束漂移模型分析

Effects of aperture averaging and beam width on a partially coherent Gaussian beam over free-space optical links with turbulence and pointing errors.

Joint effects of aperture averaging and beam width on the performance of free-space optical communication links, under the impairments of atmospheric loss, turbulence, and pointing errors (PEs), are investigated from an information theory perspective. The propagation of a spatially partially coherent Gaussian-beam wave through a random turbulent medium is characterized, taking into account the diverging and focusing properties of the optical beam as well as the scintillation and beam wander effects. Results show that a noticeable improvement in the average channel capacity can be achieved with an enlarged receiver aperture in the moderate-to-strong turbulence regime, even without knowledge of the channel state information. In particular, it is observed that the optimum beam width can be reduced to improve the channel capacity, albeit the presence of scintillation and PEs, given that either one or both of these adverse effects are least dominant. We show that, under strong turbulence conditions, the beam width increases linearly with the Rytov variance for a relatively smaller PE loss but changes exponentially with steeper increments for higher PE losses. Our findings conclude that the optimal beam width is dependent on the combined effects of turbulence and PEs, and this parameter should be adjusted according to the varying atmospheric channel conditions. Therefore, we demonstrate that the maximum channel capacity is best achieved through the introduction of a larger receiver aperture and a beam-width optimization technique.

Analysis of earth‐to‐satellite free‐space optical link performance in the presence of turbulence, beam‐wander induced pointing error and weather conditions for different intensity modulation schemes

In this study, the authors evaluate the performance of an earth-to-satellite free-space optical link in terms of bit error rate (BER) for three intensity modulation (IM) schemes [on–off keying (OOK), M-ary pulse position modulation (M-PPM) and M-ary differential PPM (M-DPPM)] and direct detection receiver. The performance is analysed in the presence of atmospheric turbulence, beam-wander induced pointing error and weather conditions. Turbulence with beam wander effect is modelled using gamma-gamma distribution and the weather effects are incorporated using the Beer-Lambert law. They obtain closed form BER expressions for the above IM schemes using combined channel state probability density functions. They show that for the same average power, M-PPM offers the best performance, followed by M-DPPM and OOK schemes. Link performance degrades for all IM schemes with increase in the value of ground level refractive index structure parameter and bit rate. Presence of weather conditions like moderate, light and thin fog increase signal attenuation and this increase may be very high in case of thick and dense fog or clouds and can lead to link failure.

Optimal beam focusing through turbulence.

Beam spread and beam wandering are the most perceptible effects of atmospheric turbulence on propagating laser beams. The width of the mean irradiance profile is typically used to characterize the beam spread. This so-called long-term (LT) statistic allows for a relatively simple theoretical description. However, the LT beam size is not a very practical measure of the beam spread because its measurements are sensitive to the movements of the source and detector, and to the large-scale variations of the refractive index that are not associated with turbulence. The short-term (ST) beam spread is measured relative to the instantaneous position of the beam center and is free of these drawbacks, but has not been studied as thoroughly as the LT spread. We present a theoretical model for the ST beam irradiance that is based on the parabolic equation for the beam wave propagation in random media, and the Markov approximation for calculation of the statistics of the optical field, and discuss an approximation that allows introduction of the isoplanatic ST point spread function (PSF). Unlike the LT PSF, the ST PSF depends on the overall beam geometry. This allows optimization of the initial beam field in terms of minimizing the ST beam size at the observation plane. Calculations supporting this conjecture are presented for the simple case of the coherent Gaussian beam, and Kolmogorov turbulence.

Theoretical expressions of long term beam spread and beam wander for Gaussian wave propagating through generalized atmospheric turbulence

Effects of the generalized atmospheric turbulence on the beam spread and beam wander for Gaussian wave are studied in detail both analytically and numerically. New analytic expressions of long term beam spread and beam wander are derived. They cover finite turbulence inner and outer scales and have a general spectral power law value in the range 3–4 instead of standard power law value of 11/3 (for Kolmogorov turbulence), and reduce correctly to the previously published analytic expressions for the case of Gaussian beam propagation through weak conventional Kolmogorov turbulence. The final results indicate that, turbulence inner and outer scales, and the general spectral power law value have significant influences on the Gaussian beam's propagation.

Numerical laser beam propagation using large eddy simulation of a jet engine flow field

In certain directed infrared countermeasure (DIRCM) situations, the laser beam path may have to pass close to the engine exhaust plume of the aircraft and models of plume turbulence are needed for DIRCM performance simulations. The jet engine plume was modeled using large eddy simulation (LES), providing time resolved information about the large scale turbulent eddies. The refractive index data from the LES calculations were integrated along the propagation path to produce time resolved phase screens for optical beam propagation. The phase screens were used to calculate laser beam parameters including beam wander and power-in-bucket (PIB). Numerical beam propagation resulted in a root-mean-square beam wander of 200 μrad for the small turbojet engine studied. The PIB was calculated for beams with 80 μrad and 2 mrad divergence having equal beam diameter when passing through the plume. For the beam with low divergence, the average PIB was reduced from 0.23 to 0.040, while the beam with wider divergence showed no significant reduction. In both cases, the plume introduced significant temporal variation of the instantaneous PIB. The beam wander is not affected by the divergence, but only depends on beam size.

Turbulence-induced persistence in laser beam wandering.

We have experimentally confirmed the presence of long-memory correlations in the wandering of a thin Gaussian laser beam over a screen after propagating through a turbulent medium. A laboratory-controlled experiment was conducted in which coordinate fluctuations of the laser beam were recorded at a sufficiently high sampling rate for a wide range of turbulent conditions. Horizontal and vertical displacements of the laser beam centroid were subsequently analyzed by implementing detrended fluctuation analysis. This is a very well-known and widely used methodology to unveil memory effects from time series. Results obtained from this experimental analysis allow us to confirm that both coordinates behave as highly persistent signals for strong turbulent intensities. This finding is relevant for a better comprehension and modeling of the turbulence effects in free-space optical communication systems and other applications related to propagation of optical signals in the atmosphere.