Laser beam propagation underwater is becoming an important research topic because of high demand for its potential applications. Namely, ability to image underwater at long distances is highly desired for scientific and military purposes, including submarine awareness, diver visibility, and mine detection. Optical communication in the ocean can provide covert data transmission with much higher rates than that available with acoustic techniques, and it is now desired for certain military and scientific applications that involve sending large quantities of data. Unfortunately underwater environment presents serious challenges for propagation of laser beams. Even in clean ocean water, the extinction due to absorption and scattering theoretically limit the useful range to few attenuation lengths. However, extending the laser light propagation range to the theoretical limit leads to significant beam distortions due to optical underwater turbulence. Experiments show that the magnitude of the distortions that are caused by water temperature and salinity fluctuations can significantly exceed the magnitude of the beam distortions due to atmospheric turbulence even for relatively short propagation distances. We are presenting direct measurements of optical underwater turbulence in controlled conditions of laboratory water tank using two separate techniques involving wavefront sensor and LED array. These independent approaches will enable development of underwater turbulence power spectrum model based directly on the spatial domain measurements and will lead to accurate predictions of underwater beam propagation.
We have performed a series of experiments in order to simultaneously validate several devices and methods for measurement of the path-averaged refractive index structure constant ( 𝐶𝑛 2). The experiments were carried out along a horizontal urban path near the ground. Measuring turbulence in this layer is particularly important because of the prospect of using adaptive optics for free-space optical communications in an urban environment. On one hand, several commercial sensors were used: SLS20, a laser scintillometer from Scintec AG, BLS900, a largeaperture scintillometer, also from Scintec, and a 3D sonic anemometer from Thies GmbH. On the other hand, we measured turbulence strength with new approaches and devices developed in-house. Firstly, an LED array combined with a high-speed camera allowed for measurement of 𝐶𝑛 2 from raw- and differential image motion, and secondly a two-part system comprising a laser source, a Shack-Hartmann sensor and a PSF camera recoded turbulent modulation transfer functions, Zernike variances and angle-of-arrival structure functions, yielding three independent estimates of 𝐶𝑛 2. We compare the measured values yielded simultaneously by commercial and in-house developed devices and show very good agreement between 𝐶𝑛 2 values for all the methods. Limitations of each experimental method are also discussed.
A simple grid of 10×10 white-light LEDs allows for simultaneous measurement of several characteristics of atmospheric turbulence. With this device, an imaging sensor and the model of tilt anisoplanatism one can determine turbulence strength, anisotropy, outer scale and spectral slope of turbulence. We describe the theory and present preliminary results obtained over a 270-m path.
The efficiency of laser communications systems is significantly limited by atmospheric effects. Notably challenging scenarios like a long horizontal path or strong scintillation lead to high failure rates of the electro-optical systems. Adaptive optics (AO) methods and components developed for astronomical applications cannot fulfill these higher requirements. The so-called Holographic Wavefront Sensor (HWFS) is a promising alternative for measuring the wavefront deformation of a laser beam. The basic elements of the HWFS are a diffraction pattern and a fast photodetector like a photodiode array. With these components the aberrations present in the beam are measured directly. In this paper we present an optical setup to verify two important features of the sensor type experimentally: high measurement speed and the sensor’s response on present scintillation. Measurements with the HWFS were performed at a bandwidth of 11 kHz for a single aberration and of 2.5 kHz for 6 aberrations simultaneously. Furthermore we operated the HWFS in a closed-loop adaptive optics system with a bandwidth of 600 Hz. We show that the wavefront correction based on the HWFS measurements is insensitive to dynamic partial intensity fluctuations of the laser beam.
It is well known that a laser beam propagating through optical atmosphere is affected by atmospheric turbulence. In this paper, we describe an experimental double-passage system for laser beam propagation along a horizontal urban path that can be useful for applications such as free-space laser communications. The setup includes a telescope to focus a laser beam on a retro-reflector, which is located 410 meters away, and the optical-test bench with which we measure intensity and phase fluctuations of the reflected beam. In our measurements scintillation is decreasing with distance from the center of the pupil. This shows the need for further theoretical modelling of double-passage systems.
The ELI-beamlines project will use high power ultrafast lasers with high average powers up to 1kW and peak powers up
to 10PW. The project presents a major challenge in terms of damage threshold of ultrafast mirror coatings and gratings.
In order to assess the LIDT of ultrafast coatings in the expected environments a test station was constructed in the PALS
facility. The setup can use beams from a 25TW Ti:Sapphire laser system with a 10Hz repetition rate. Testing is
performed mainly with a secondary 1 kHz 40fs while contamination levels are investigated.
We present an overview of the projected and/or implemented laser systems for ELI-Beamlines. The ELI-Beamlines
facility will be a high-energy, high repetition-rate laser pillar of the ELI (Extreme Light Infrastructure) project. The
facility will make available high-brightness multi-TW ultrashort laser pulses at kHz repetition rate, PW 10 Hz repetition
rate laser pulses, and kilojoule nanosecond laser pulses that will be used for generation of 10 PW, and potentially higher,
peak power. These systems will allow meeting user requirements for cutting-edge laser resources for programmatic
research in generation and applications of high-intensity X-ray sources, in electron and proton/ion acceleration, and in
dense plasma and high-field frontier physics.
The ELI-beamlines project is expected to reach state of the art parameters in its laser systems. The Laser Induced Damage Threshold of the corresponding optical systems will have to sustain the expected fluences and repetition rates. The LIDT requirements for the ultrafast pulse compressors, vacuum transport mirrors and high average power optics are presented together with the current and planned capabilities for LIDT testing with a 25TW laser system at 800 and 1060nm.
We have examined the trial-to-trial variability of the reflectance spectra of surface coatings containing effect pigments. Principal component analysis of reflectances was done at each detection angle separately. A method for classification of principal components is applied based on the eigenvalue spectra. It was found that the eigenvalue spectra follow characteristic power laws and depend on the detection angle. Three different subsets of principal components were examined to separate the relevant spectral features related to the pigments from other noise sources. Reconstruction of the reflectance spectra by taking only the first subset indicated that reflectance variability was higher at near-specular reflection, suggesting a correlation with the trial-to-trial deposition of effect pigments. Reconstruction by using the second subset indicates that variability was higher at short wavelengths. Finally, reconstruction by using only the third subset indicates that reflectance variability was not totally random as a function of the wavelength. The methods employed can be useful in the evaluation of color variability in industrial paint application processes.
This paper presents the integration and first results for the CAMCAO NIR camera. The camera was built
for the ESO Multi-conjugate Adaptive optics Demonstrator, where it is presently operating, to evaluate the
feasibility of this Adaptive Optics technique. On a second phase it will work directly at the Nasmyth focus of the
VLT. CAMCAO is a high resolution, wide field of view NIR camera, that is using the 2k×2k HgCdTe HAWAII-
2 infrared detector from Rockwell Scientific, controlled by the ESO IRACE system. The camera operates in
the near infrared region between 1.0 μm and 2.5 μm wavelength using an eight position filter wheel with J, H,
K', K-continuum and Brγ filters. Both the integration experience and the results obtained in the mechanical,
vacuum, cryogenics and optical tests are presented, including all relevant parameters in the ESO specifications.
The requirement of mechanical stiffness together with light weight was achieved yielding a total weight of less
than 90 Kg. The camera fulfills both cryogenic and vacuum stability requirements. The temperature within
the detector is maintained at 80K by an accurate control loop, ensuring mK stability, after cooling down the
detector at a rate kept below 0.5 K/min. The optical performance tests were made using a Fizeau interferometer
both for the individual optical components and complete setup. The infrared optical validation measurements
were performed by re-imaging a point source in the camera focal plane and measuring the PSF with the detector.
The computed Strehl ratio reached 95% in the central region of the FoV, with values larger than 90% in a area
covering 88% of the focal plane.
The CAMCAO instrument is a high resolution near infrared (NIR) camera conceived to operate together with the new ESO Multi-conjugate Adaptive optics Demonstrator (MAD) with the goal of evaluating the feasibility of Multi-Conjugate Adaptive Optics techniques (MCAO) on the sky. It is a high-resolution wide field of view (FoV) camera that is optimized to use the extended correction of the atmospheric turbulence provided by MCAO. While the first purpose of this camera is the sky observation, in the MAD setup, to validate the MCAO technology, in a second phase, the CAMCAO camera is planned to attach directly to the VLT for scientific astrophysical studies. The camera is based on the 2kx2k HAWAII2 infrared detector controlled by an ESO external IRACE system and includes standard IR band filters mounted on a positional filter wheel. The CAMCAO design requires that the optical components and the IR detector should be kept at low temperatures in order to avoid emitting radiation and lower detector noise in the region analysis. The cryogenic system inclues a LN2 tank and a sptially developed pulse tube cryocooler. Field and pupil cold stops are implemented to reduce the infrared background and the stray-light. The CAMCAO optics provide diffraction limited performance down to J Band, but the detector sampling fulfills the Nyquist criterion for the K band (2.2mm).
Contact lens' fitting evaluation is of critical importance in the contact lens' prescription process. For the correction of eye's refraction problems the use of contact lens' is very appealing to the user. However its prescription is far more demanding than the one of eye glasses. The fitting of a contact lens to a particular cornea must be carefully assessed in order to reduce possible user's physical miscomfort or even medical situations.The traditional way of easily checking the fitting of a contact lens is to perform a fluorescein test. The simple visual evaluation of the 'smoothness' of the color/brightness distribution of the fluorescence at the contact lens' location gives the optometrist an idea of the fitting's quality. We suggested the automation of the process simply by the substitution of the optometrist's eye by a CCD camera, and the use of appropriated simple image processing techniques. The setup and the digitalization and processing routines will be described in this communication. The processed images may then be directly analyzed by the optometrist in a faster, easier and more efficient way. However, it is also possible to perform an automated fitting evaluation by working out the information given by the image's intensity histograms for the green and blue RGB' channels.