The paper defines the laser-induced damage threshold from the fluence and the peak power of GaSe and GaSe:In single crystals upon exposure to nanosecond radiation in the two-micron range and assesses the influence of test radiation energy parameters (pulse repetition rate, pulse duration) on the damage threshold. Laser-induced damage threshold was determined with the parameters of the incident radiation close to the pump radiation parameters of promising dual-wavelength optical parametric oscillators (effective pump sources for THz difference frequency oscillators): wavelength is ~ 2.1 μm, pulse repetition rate is 12 and 20 kHz, pulse duration is 18-22 ns.
The paper shows the effect of magneto-rheological (MPO) polishing of a ZnGeP2 on the surface roughness. The extreme level of surface roughness of the ZnGeP2 Ra=1.54Å was recorded. Analysis of the topography of the sample surface before and after magnetic polishing showed that after the classical polishing technology, the relief of the surfaces was formed under the influence of multidirectional movement of the working tool, there are single extended scratches up to 1.3 nm deep. The topography of the sample surface after MPO polishing does not contain these scratches and is represented by a less textured profile formed under the influence of MPO fluid.
The effect of the designed antireflection coatings based on Nb2O5/Al2O3 and Nb2O5/SiO2 pairs of materials on the laserinduced damage threshold (LIDT) of ZnGeP2 crystals at a wavelength of 2097 nm is shown. The coating was applied using the ion-beam sputtering (IBS) method. The LIDT of the sample coated with Nb2O5/SiO2 was W0d = 1.8 J/cm2. The LIDT of a sample coated with Nb2O5/Al2O3 is W0d = 2.35 J/cm2. The presence of silicon conglomerates in the antireflection coating leads to a decrease the LIDT of a nonlinear crystal due to local thermomechanical stresses and scattering of the incident laser radiation.
KEYWORDS: Holography, Digital holography, Cameras, Ecology, Organisms, Digital cameras, Particles, Image classification, Ecosystems, Temperature metrology
The establishment of stationary monitoring stations is a timely and topical issue. The coastal zone of Lake Baikal is particularly affected by global climate processes and human activities. During the observations carried out in August 2021 in the coastal zone of Lake Baikal, it was reliably established that the miniDHC (Digital Holographic Camera) can be used to monitor qualitative and quantitative changes in coastal plankton. Lake Baikal is a good test site for improving the technique of monitoring changes in the world ocean. Despite their diversity, freshwater plankton can be better identified than marine plankton. The miniDHC holographic camera, if used over a long period of time, will provide new data on the ecology of the coastal area. The combination of new and traditional methods in the study of the coastal zone of Lake Baikal is necessary and extremely effective.
KEYWORDS: Digital holography, Holograms, Digital recording, Pulsed laser operation, 3D image reconstruction, Particles, Image registration, Transmittance, Signal to noise ratio, Digital cameras
The quality of particle images reconstructed from digital holograms depends on the choice of parameters for digital holograms recording — the exposure time of a digital camera, the laser radiant power, and the laser pulse duration. The problem of automatically choosing the parameters for recording a digital hologram under conditions of a changing optical transmission of the aquatic medium while studying plankton in its habitat arises. Using the maximum of digital hologram contrast as a criterion for choosing the parameters for recording a digital hologram to obtain the best quality particles images reconstructed from a digital hologram is proposed.
The holographic system is usually used for imaging, so it can be attributed to imaging optical systems. This analogy allows using a well-developed computational optics technique to design and analyze real digital holographic systems, as well as to solve the measuring tasks of digital holography. The work presents a mathematical model that establishes a one-to-one correspondence between dimensional and spatial parameters of a digital holographic image and a holographing object for the given case of an in-line scheme. The values of the model constants used to determine the real size and the longitudinal coordinate of an object according to its holographic image are found through calibration. The described approach is used to calibrate and analyze the imaging properties of a submersible digital holographic camera designed to study plankton in its habitat. The paper also shows the results obtained in situ using the holographic sensor of plankton.
Conditions for the formation of THz radiation in ZnGeP2 single crystals when generating a difference frequency are considered. It is shown that effective THz radiation requires two-frequency laser pumping sources with a generation pulse duration of < 1 ns. It is proposed to use IR radiation as such a source dual-wavelength optical parametric oscillator based on the KTP singlecrystal with pumping by Nd:YAG lasers with active Q-switching with simultaneous synchronization mode.
The paper presents the underwater digital holographic sensor that can solve the tasks of monitoring the ecosystem biodiversity and bioproductivity for fisheries. The sensor is adapted for use in the accompanying measurement mode. The results of in situ testing of the sensor are presented.
Quite extensive test sites are needed to provide remote sensing methods of the Earth with reliable information on the electrical characteristics of the surface layer and on their daily-seasonal changes. Observation is constantly made on them for the main indicators that determine the results of sensing. The paper discusses a set of tools using radiophysical and optical methods for long-term studies of the properties of the environment at the test station "Kaybasovo".
A submersible digital holographic camera for measuring plankton and other particles is described. The camera provides underwater recording of digital holograms of water volume containing plankton followed by automatic restoration of holographic images of plankton species, determination of their sizes, shapes, and concentrations, and their recognition and classification. Particles with sizes of 200 μm and larger are analyzed. The water volume registered per exposure is about 1 L. The special features of the software for automatic information retrieval from digital holograms are discussed. Examples of application of the camera as an integral part of the hardware-software complex for field measurements are given. Prospects for application of this complex for ecological monitoring are discussed. The recognition criterion of the digital holographic camera and the data volume and the averaging time required for obtaining statistically reliable data on plankton species are also given.
KEYWORDS: Digital holography, Particles, Holograms, 3D image reconstruction, Image segmentation, Holography, Digital imaging, Image processing, Digital recording, Data acquisition
A technique for data acquisition from digital holograms of particle ensembles, including preprocessing of the digital hologram, construction of a two-dimensional display of the holographic image of investigated volume, and segmentation and measurement of particle characteristics is considered. The proposed technique is realized in automatic regime and can work in real time. Results of the technique approbation using digital holograms of sand, plankton particles in water, and air bubbles in oil are presented.
Laboratory works on holograms recording, reconstruction and interpretation are useful for two reasons. Firstly, holography is widely used in science and engineering. Secondly, training labs in holography require complex applying of knowledge on interference, diffraction, coherency and other domains of optics. Educational kit and methodological instructions for optical experiments were presented in the previous paper 1. The desktop holographic camera described in this paper is one of the additional functional units of the kit. The desktop holographic camera does not require additional protection against vibrations even if the exposure time is several minutes. This is a compact holographic installation for recording Denisyuk holograms. Two experiments are described in the paper to illustrate the usefulness of holographic laboratory works. The first one is a recording and reconstruction of a Denisyuk hologram. The second one is a recording and interpretation of a double-exposure interferogram when the holoplate is sagged due to loading between exposures. Also included in the paper are holographic setup and laboratory works on digital holography. These experiments require, in addition, complex applying of knowledge on photo receivers, CCD and other domains of photonics.
KEYWORDS: Digital holography, Holograms, 3D image reconstruction, Holography, Video, Particles, Reconstruction algorithms, CCD cameras, Digital video recorders, Charge-coupled devices
The use of digital holography for plankton detection is discussed, namely - for evaluation of the plankton particles shapes, orientation, and 3-D location. Algorithms are considered for improve the quality of the reconstructed plankton holographic image. The algorithms are based on the digital hologram pre-processing before reconstruction.
Algorithm for recording and reconstruction of digital holographic video is described. The possibility is shown for reconstruction either fixed plane of volume contained plankton, or reconstruction the plane moving together with plankton species.
Experimental results as well as their discussion are presented.
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