A variety of optical components are fabricated by the so called “Heat and Pull” technique in which optical fibers are fused and tapered. A numerical model simulating the temporal and spatial material distribution in such components is presented and validated by comparison with experimental results.
Over the years, numerous models and tools have been developed to simulate the optical behavior of fused fiber-optic components. While these models are well established, their predictions depend on accurate knowledge of the component’s physical structure and its refractive index distribution. Unfortunately, no such generic simulation tools are readily available. The need of a high fidelity structural simulation tool for such components is further emphasized in complex systems, which are difficult to fabricate and are optically sensitive to small structural variation. In view of the above, we developed a novel numerical methodology based on Immersed Boundary (IB) Method specifically designed to simulate flows in the presence of complex geometries and moving boundaries. In the present formulation pressure and interface curvature are implicitly embedded into the system of incompressible Navier-Stokes equations as distributed Lagrange multipliers. The developed methodology is currently capable to simulate two phase flows in two dimensions and is also adapted to solve quasi-3D evolutions. For validation, the simulation output is compared to the cross-sectional material distribution of a real component fabricated at our lab. The developed model, as well as the experimental results and the comprehensive analysis predicting the structure of symmetric and non-symmetric optic fiber components are presented and discussed.
The German-Israeli intercomparison experiment on the investigation of vertical profiles of horizontal wind speed and optical turbulence in the lower atmospheric boundary layer from 4th to 7th May 2015 was characterized by frontal activity in the atmosphere. The newly developed remote LIDAR-device of the Soreq institute for the investigation of the vertical wind and turbulence field was compared to the routinely performed measurements at the VerTurM (Vertical Turbulence Measurements) field site in Meppen, Germany. The long-term experiment VerTurM is focused on measurements of the optical turbulence and comprises scintillometer measurements close to the ground (1.15 m height), sonic anemometer measurements on a tall tower at 4 m, 8 m, 32 m, and 64 m and a SODAR-RASS-system. The temporal development of the vertical profiles of horizontal wind speed and optical turbulence Cn 2 during the frontal passage is investigated. Additional radiosonde measurements were performed to characterize the boundary layer height during the day.
We have compared measured angle-of-arrival (AOA) fluctuations to the prediction of various models, for a laser beam propagating through a turbulent atmosphere at ground level. Three models have been investigated: a simple small perturbation model, a model which incorporates also inner and outer scale effects and a third model which takes into account the contribution of additional spatial scales and is able to predict a saturation regime. Data were collected in an approximately ten year time span. We have used near infra-red LIDAR systems to determine the AOA fluctuations by measuring the short term movement of a laser spot in the receiver plane, reflected from targets placed at various distances. In parallel, we have also measured the turbulence strength with a short-range scintilometer and recorded the average wind speed along the laser path. Our analysis indicates that the simple model predictions are quite good for weak turbulence and short distances, however on the majority of the scenarios the conditions (turbulence strength and distance) are such that the AOA fluctuations deviate from the simple model and even approach saturation. In these cases the fluctuations follows the general form of the third model. We also found some differences between the day and night behavior which wasn't considered by any of the models.
A series of experiments have been conducted to evaluate the accuracy of single station, optical systems for measurement
of average crosswind velocity. By analyzing the spatial-temporal cross-correlation function of signals caused by
turbulence-induced refractive index irregularities, the wind velocity perpendicular to the line of sight is determined. This
method allows also for direct measurement of the turbulence structure parameter Cn2 by the angle-of arrival technique.
These real-time turbulence values are incorporated into the wind vector estimation to achieve higher accuracy. The
evaluated systems include an active technique which measures the backscatter of the transmitted laser pulses and a
passive technique where the naturally illuminated scene serves as the light source. Both active and passive systems have
been compared to a series of ultrasonic anemometers located along the measurement path. The experiments were
performed along a uniform path in various locations. Very good fits (about 0.5 m/sec) have been obtained at all
turbulence conditions along a 1000m path.
This paper presents a new method for remote sensing of cross-wind by using a naturally illuminated scene as a light
source. The method is based on spatial and temporal correlations of the intensity fluctuations measured by a passive
imaging device such as a video camera. Adaptable spatial filtering, taking into account variations of the dominant scales
of the turbulence (due to changes in meteorological conditions or imaging device performance) is integrated into this
method. The major merits of the proposed technique lie in its simple implementation for a wide range of imaging
systems and ability to remotely sense the crosswind with naturally occurring targets. Experimental comparison with
independent wind measurement using anemometers shows good agreement.
We report on developing of a prototype LIDAR for remote measurements of cross-wind profile, using backscattering
from aerosol in a single-ended scheme. The system contains a pulsed Nd:YAG laser with 500 Hz repetition rate and ~20
nsec pulse width, as a transmitter, and a matrix of 7 detectors, placed at the focus of Cassegrain-type telescope, as a
receiver. To realize detection of signals with high-sampling resolution, a high-sampling-rate digitizer with 8
simultaneously sampled channels and 60 Msamples/sec-per-channel sampling rate was incorporated into the system. To
check the ability to detect a local cross-wind flow, we performed an experiment using the cooling tower of Soreq reactor
as a vertical wind simulator. We recorded the detector signals from aerosol scattering at different distances, and analyzed
the temporal-spatial cross-correlation function. The analysis of the asymmetry and shift of the cross-correlation function
shows good ability for qualitative mapping of local cross-wind. Next development steps will include improvement of the
electronic circuits, in order to increase the sampling resolution along the line of sight, performing of additional controlled
experiments and developing of wind-profile algorithm.
We report on remote measurements of cross-wind and atmospheric turbulence, using a one-station scheme. As most
remote wind-sensing methods, our method is based on observing the drift of the scintillation pattern across the line of
sight. The scintillations are caused by naturally-occurring turbulence-induced refractive index irregularities in the
atmosphere, which drift at wind speed. Analyzing spatial-temporal cross-correlation function of the signals of two
elements in the array, it is possible to obtain the cross-wind speed. We use the zero-crossings technique for measuring
the cross-wind value, while the cross-wind direction is determined by comparing areas from both sides of the peak of the
cross-correlation function. Here we present results obtained using these techniques in comparison to independent
measurements of the anemometers. The experiments were performed along a uniform path over a flat beach parallel to
the Mediterranean Sea shore. Four white-screen diffusive targets were placed at distances of 300, 600, 850 and 1200m.
Five anemometers were placed along the laser beam path, one near each target and at the measurement station. Each
target was illuminated with a beam from a glass fiber pulsed infrared laser with a repetition rate of several thousand Hz,
and a sub-microsecond pulse-length, and output beam divergence of ~300 μrad. The receiver has an entrance aperture of
80mm, and the incoming radiation is focused onto an array of four 50×250um InGaAs detectors by a lens with
f=500mm. The results show good agreement. From the fluctuations of the signal on the detector array, our system also
measures the turbulence structure parameter Cn
2, using the angle-of arrival technique. The obtained results show
reasonable agreement with independent scintillometer measurements of Cn2, performed with a CW He-Ne laser in a
two-station setup with a detector at a distance of 60m from the laser.