A multilayer antireflection coating based on titanium oxide and silicon oxide has been developed. The dispersions of the refractive index and absorption coefficient of these materials are obtained in a wide spectral region of 500-8000 nm. Taking into account these dispersions, a calculation was made of a four-layer interference coating with ZGP substrate antireflection in the range of 2097 nm and 3500-5000 nm, with residual reflection R≤0.2% and R≤2.1%, respectively. The modes of coating deposition by the method of ion-beam sputtering on the substrate are selected.
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.
The paper shows an increase in the threshold energy density of laser radiation at a wavelength of 2091 nm as the temperature decreases from 0º С to -60º С until the optical breakdown of a ZnGeP2 (ZGP) single crystal sample is reached. With a decrease in temperature from 0º to -60º C, a sharp increase in the threshold energy density is observed from 1.6 to 2.6 J/cm2, with a laser beam diameter of 270 μm, and from 3.2 to 10.2 J/cm2 with a diameter of 100 μm (by 1.5 and 3 times, respectively).
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.
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.
Volumetric defects in ZnGeP2 single crystal have been visualized by using the digital holography camera. Type, geometrical sizes and positioning of founded defects were determined. Based on this information researching zones were chosen on the test ZnGeP2 plate. Measurements of absorption coefficient and refractive index in THz range were carried out. It is shown that in the frequency range 0,3-1 THz the difference of absorption coefficients reaches the value Δα = 0.2 cm-1 between “clear” zones and zones with volumetric defects. In same time, maximum difference by refractive index is about Δnmax = 0,0015.
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.
In paper the device – set of hardware and software for non-contact investigation of marine particles is presented. The device is designed on digital holography principles. Hardware features of submersible holocamera (or DHC sensor) are considered. Methods are described for the processing of holograms and retrieve information. Results of approbation of the device are presented.
Optical scheme of three-stage converter of middle IR range laser radiation to the terahertz (THz) radiation of range is proposed. In the first cascade the 2-micron emission in Ho:YAG laser is generated. In the second cascade it is supposed using frequency conversion of radiation of the Ho:YAG laser in ZnGeP2 single crystal to radiation with a wavelength of ~ 4-4.5 μm, and in the second cascade it is expecting of using parametric frequency conversion of the two parametric sources ~ 4-4.5 μm range to the THz range by generating radiation of difference frequency in the nonlinear optical crystal ZnGeP2. Estimation of the characteristics of the output terahertz radiation is given in this paper.
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