Modeling of phase volume diffractive gratings, part 1: transmitting sinusoidal uniform gratings

A detailed model of the diffraction of plane and Gaussian beams on plane uniform phase Bragg gratings based on Kogelnik\u27s theory of coupled waves is presented. The model describes transmitting gratings with arbitrary orientation in a plane-parallel plate taking into account spectral width and angular divergence of laser beams along with material dispersion of a photosensitive medium. The model results are compared with experimental data for high- efficiency Bragg gratings in a photothermorefractive (PTR) glass

Results of multidisciplinary survey in the Laptev Sea in August-September, 2015

Data on oceanographic conditions and species composition of plankton, benthic and demersal fish and invertebrates are presented, obtained in the complex survey over the external shelf of the Laptev Sea in August-September 2015. The zooplankton abundance was low, with only local increases up to 400 mg/m3. Species diversity of fish and nekton invertebrates in the bottom trawl catches was low, too: 26 fish species and 2 species of cephalopods. Mean biomass of fish was estimated as 4.3 t/km2 (in total 132. 103 t within the surveyed area of 30,500 km2). All fish species were distributed sparse. Arctic cod was the most abundant and occurred over the whole surveyed area, with large-sized fish dominating at the bottom and medium-sized (9-15 cm) fish - in the pelagic layer, other commercial species were greenland halibut and deepwater redfish caught on the continental slope. Bottom invertebrates in trawl catches were presented by 6 species of shrimp and 12 taxonomic groups of different rank, with predominance of starfish, brittle stars and sponges; gastropods were represented by 11 species, with Neptunea heros dominating by mass (42 %). Macrobenthos in samples of the bottom sampler was presented by 20 taxonomic groups, with predominance of polychaetes, bivalves and sipunculoids

Current state of stocks for demersal fish on the continental shelf in the exclusive economic zone of Russia in the northwestern Bering Sea

Stocks of demersal fishes on the depths of 20-200 m in the northwestern Bering Sea are considered, excluding walleye pollock Theragra chalcogramma , on the data of 10 bottom trawl surveys conducted in the summer-fall of 1985-2015. The mean total biomass of demersal fish in two parts of this area (Western Bering Sea zone and Chukchi zone) in this period was 747.78. 103 and 119.83. 103 t, with almost equal density distribution: 5757 and 5709 kg/km2, respectively. The biomass had some redistributions in recent times: the assessments were 832.97. 103 and 69.88. 103 t in 2012, and 946.31. 103 and 141.40. 103 t in 2015, respectively. Pacific cod Gadus macrocephalus is usually the dominant species in the Western Bering Sea zone (up to 57 % of the total biomass in 2015) followed by subdominant species as alaska plaice Pleuronectes quadrituberculatus , flathead sole Hippoglossoides spp., alaska skate Bathyraja parmifera , and great sculpin Myoxocephalus polyacanthocephalus with portions of each species between 5-13 %. Generally, the ichthyocenosis of both zones was monodominant; the polydominant structure was observed in the Western Bering Sea zone only in 2001 and 2005, in conditions of abnormally low cod abundance. The total stock of demersal fish is determined mostly by state of the cod stock. It was very high in the middle 1980s, then had a significant decline in the late 1990s - early 2000s, but rises again since the middle 2000s. A considerable part of the mass demersal species migrate in summer from the Western Bering Sea to the Chukchi zone for feeding that is the background for significant fluctuations of their biomass assessments, which depend on the environments variability. The densest aggregations of the mass demersal fish form mainly in the area between Cape Olyutorsky and Cape Navarin and in the Anadyr Bay in the waters of the Navarin Current

Modeling Of Gaussian Beam Diffraction On Volume Bragg Gratings In Ptr Glass

A detailed model of diffraction of Gaussian beams on plane uniform volume Bragg gratings based on a Kogelnik\u27s theory of coupled waves is presented. The model describes transmitting and reflecting gratings and takes into account spectral width and angular divergence of diffracted beams. Exact formulas for angular and spectral selectivity are derived. Conditions for Bragg diffraction based on comparison between beam quality (divergence and spectral width) and volume grating parameters (angular and spectral selectivity) are formulated. The model results are compared with experimental data for high-efficient Bragg gratings in photo-thermo-refractive (PTR) glass

Spectral Combining Of High-Power Fiber Laser Beams Using Bragg Grating In Ptr Glass

High-efficient volume Bragg gratings (VBG) in inorganic photo-thermo-refractive (PTR) glass were recently reported for the use in high-power laser systems. Both transmission and reflection gratings have shown diffraction efficiency greater than 95% from visible to near IR spectra in a wide range of spatial frequencies. Those gratings have exhibited perfect thermal, optical and mechanical stability. Spectral beam combining (SBC) using PTR Bragg grating with efficiency more than 92% for two 100 W Yb-fiber-laser beams with the 11 nm wavelength separation between them is reported. The paper presents results of modeling and experimental study of a beam combiner for high-power lasers with the only passive PTR grating component in it. Two laser beams illuminate a thick Bragg grating which has only two symmetric resonant angles providing total diffraction of a beam with a certain wavelength. Incidence angle for all transmitting beams should correspond to the Bragg angle for the diffracted beam. Transmitting beams are not diffracted by grating if spectral shift corresponds to zeros in a spectral selectivity curve, and propagate in the same direction as a diffracted beam. It is shown the efficient trade-off between grating period and refractive index modulation allows modeling of high-efficient combining setup for each of arbitrary chosen grating thickness. Comparison between calculation results and experimental data is given

Characterization Of Photo-Thermo-Refractive Bragg Gratings In High-Power Ir Laser Beams

Bragg gratings recorded in the volume of photo-thermo-refractive glass are stable in high-power laser beams while degradation of laser parameters occurs. These glasses are promising candidates for use in high-power laser systems. ©2002 Optical Society of America

Incoherent combining of 100-W Yb-fiber laser beams by PTR Bragg grating

Volume diffractive gratings (Bragg gratings) in photo-thermo-refractive (PTR) inorganic glass are proposed for incoherent laser beam combining because they have narrow spectral selectivity and diffraction efficiency greater than 95 % from visible to near IR regions. They showed no laser-induced damage, no thermal lens, and no Bragg angle shift under CW Yb-fiber laser (1096 nm) irradiation at 100 kW/cm2. It opens the way to rugged, low-cost, efficient optics for high-power laser systems. Based on theoretical modeling of PTR Bragg gratings, we have designed a high-efficient technology for incoherent combining of two or several laser beams with certain wavelength shift. Two 100 W beams of Yb-fiber lasers in the range of 1080-1100 nm with the wavelength separation of 11 nm were combined with efficiency exceeding 75 % while material losses did not exceed 2-4%. No fading or parameter change of PTR Bragg grating working in two 100 W beams were found. It was found that the process limiting efficiency of incoherent beam combining is the spectral widening of radiation of Yb-doped fiber lasers. At high power, their spectral width exceeds spectral selectivity of Bragg grating and causes a decrease of diffraction efficiency

Modeling of phase volume diffractive gratings, part 2: reflecting sinusoidal uniform gratings, Bragg mirrors

A detailed model of diffraction of plane and Gaussian beams on plane uniform phase Bragg gratings based on a Kogelnik\u27s theory of coupled waves is presented. The model describes reflecting gratings (Bragg mirrors) with arbitrary orientation in a plane-parallel plate having no material losses. It takes into account spectral width and angular divergence of laser beams. The results of modeling are compared with experimental data for Bragg mirrors in a photo-thermo-refractive glass