Search

The Bering Land Bridge connecting North America and Eurasia was periodically exposed and inundated by oscillating sea levels during the Pleistocene glacial cycles. This land connection allowed the intermittent dispersal of animals, including humans, between Western Beringia (far north-east Asia) and Eastern Beringia (north-west North America), changing the faunal community composition of both continents. The Pleistocene glacial cycles also had profound impacts on temperature, precipitation, and vegetation, impacting faunal community structure and demography. While these palaeoenvironmental impacts have been studied in many large herbivores from Beringia (e.g., bison, mammoths, horses), the Pleistocene population dynamics of the diverse guild of carnivorans present in the region are less well understood, due to their lower abundances. In this study, we analyse mitochondrial genome data from ancient brown bears (Ursus arctos; n = 103) and lions (Panthera spp.; n = 39), two megafaunal carnivorans that dispersed into North America during the Pleistocene. Our results reveal striking synchronicity in the population dynamics of Beringian lions and brown bears, with multiple waves of dispersal across the Bering Land Bridge coinciding with glacial periods of low sea levels, as well as synchronous local extinctions in Eastern Beringia during Marine Isotope Stage 3. The evolutionary histories of these two taxa underscore the crucial biogeographic role of the Bering Land Bridge in the distribution, turnover, and maintenance of megafaunal populations in North America.

Radiation-induced attenuation (RIA) at a wavelength of $\lambda = 1.55\,\,\mu \text{m}$ as well as the RIA spectra were investigated in optical fibers during and after irradiation at the IVG.1M nuclear reactor (Republic of Kazakhstan). The fibers studied were single mode at $\lambda = 1.55\,\,\mu \text{m}$ , had an undoped silica core and an-F-doped silica cladding [pure-silica fibers (PSFs)], and were provided by different manufacturers. In addition to five PSFs, a single-mode revolver hollow-core fiber was also studied. The PSFs had different coating materials: polyimide, copper, or aluminum. All fibers were exposed to three runs of reactor irradiation at enhanced temperature (155–355 °C) so that by the end of the third run, the fast-neutron fluence amounted to $\Phi = 4.46\cdot 10^{17}$ n/cm2 and $\gamma $ -dose to $D = 2.91\cdot 10^{7}$ Gy. The total induced loss at $\lambda = 1.55\,\,\mu \text{m}$ was found to be composed of 1) RIA, 2) bending loss (BL), caused by thermal expansion of the copper cylinder the PSFs were wound on, and 3) thermo-induced microbending loss (ML) in Cu-coated PSFs. At the end of the irradiation campaign, the net RIA in PSFs proved to lie in the interval of 0.12–0.16 dB/m, which appears to be admissible to intrareactor transport fibers. It was confirmed that long-wavelength RIA tail is the only RIA origin at $\lambda = 1.55\,\,\mu \text{m}$ in PSFs in such strong radiation fields. The hollow-core fiber was found to feature zeroth RIA throughout the campaign.

This paper describes research in the field of fiber optic nondestructive testing and structural health monitoring (SHM) of carbon fiber reinforced polymers (CFRP) by the use of integrated optical fiber sensors (OFS) based on fiber Bragg gratings (FBG). Basic mathematical expressions that represent optical SHM of composites are presented. Some new relationships are derived by considering the non-linear character of the FBG-sensor response and the combined effect of temperature and deformation. Both linear and non-linear coefficients of the sensor elements, as well as combined strain–temperature coefficients before and after the sensors are embedded into the composite panels, have been obtained. Experimental results on the strain state of CFRP under static and dynamic loads demonstrate the effectiveness of the proposed non-linear model for evaluating deformation in composites. It is shown that integrated OFS’s allow SHM of composite parts when mechanically loaded to failure, and that they can provide the actual level of strain in the composite parts in real time. SHM improves the operational safety of highly loaded and/or critical aerospace structures by providing real-time stress data, which would permit data-based decisions on overload conditions or imminent failure. Additionally, actual stress data from CFRP samples, or from real parts in use, could show whether the design of the parts should be changed to improve safety margins or to reduce weight.

Single-mode optical fibres (SMFs) are required for ITER in-vessel applications as transport fibres to deliver the signal at wavelength λ = 1.55 µm from/to optical fibre sensors. The paper describes the first comparison study of radiation resistance of six pure-silica-core SMFs of different manufacturers performed in the process of fission-reactor irradiation in the conditions corresponding to the whole ITER lifetime (fast-neutron fluence, flux, γ-dose and dose rate of up to 1.8·1020n/cm2, 1.08·1014n/(cm2·s), 2.32 GGy and 1.39 kGy/s, respectively, temperature of 170–190 °C and vacuum pumping). Polyimide- and acrylate-coated SMFs failed mechanically during the irradiation; therefore, only metal-coated fibres can be considered for the in-vessel applications. Induced optical loss in all three metal-coated SMFs (copper- and aluminum-coated ones) at the fast-neutron fluence of 1·1020n/cm2 was found to lie in the range ~1.5–4 dB/m, the lower value of this range allowing the employment of at least 10-m-long transport fibre lengths in the in-vessel applications, assuming the dynamic range of 30 dB. The temperature-dependent microbending optical loss in metal-coated SMFs is discussed, of which the contribution to the total induced loss may be comparable in value to the radiation-induced absorption of light. Neither radiation-induced luminescence, nor Cerenkov emission was detected in the SMFs in the near-infrared range.

Native cattle breeds represent an important cultural heritage. They are a reservoir of genetic variation useful for properly responding to agriculture needs in the light of ongoing climate changes. Evolutionary processes that occur in response to extreme environmental conditions could also be better understood using adapted local populations. Herein, different evolutionary histories of the world northernmost native cattle breeds from Russia were investigated. They highlighted Kholmogory as a typical taurine cattle, whereas Yakut cattle separated from European taurines approximately 5,000 years ago and contain numerous ancestral and some novel genetic variants allowing their adaptation to harsh conditions of living above the Polar Circle. Scans for selection signatures pointed to several common gene pathways related to adaptation to harsh climates in both breeds. But genes affected by selection from these pathways were mostly different. A Yakut cattle breed-specific missense mutation in a highly conserved NRAP gene represents a unique example of a young amino acid residue convergent change shared with at least 16 species of hibernating/cold-adapted mammals from six distinct phylogenetic orders. This suggests a convergent evolution event along the mammalian phylogenetic tree and fast fixation in a single isolated cattle population exposed to a harsh climate.

Structured light, in particular light possessing orbital angular momentum (OAM), has been actively studied in recent decades. Helical Bragg grating (HBG) is a reflecting optical element, which predicted to be able to convert the OAM of light in waveguides and fibers. However, the HBG has not been demonstrated experimentally yet. Here we report the first experimental manifestation of HBG created in the form of a waveguide with depressed-index cladding. Few-mode channel helical-depressed-cladding waveguides (HDCWs) have been written in a YAG crystal by a femtosecond laser beam. The HDCW pitches were comparable to near-IR wavelengths. The spectral and polarization characteristics of the transmitted and reflected light have been studied. It was shown that HDCWs behave like HBGs providing narrowband resonance coupling of the counter propagating modes, the OAM of the modes differing by the Bragg order or, what is the same, the topological charge of the helical cladding structure. Adjusting the HDCW parameters allows one to excite a reflected wave at the predetermined wavelength and with the predetermined OAM change with respect to the light coupled to the waveguide. Thus, for the first time, the HBGs suitable for the generation and filtration of the vortex light have been experimentally demonstrated. The experimental results obtained agree well with the coupled mode theory calculations.

Velimir Khlebnikov (1885-1922) aimed at understanding and integration of world culture riches to find new synthesis that involves solving key issues of mankind history, culture and its future. In his works of fiction, articles on various subjects, mathematical calculations, he sought to combine in a new mythology and a single "star language" theory pantheons, symbols, creativity of peoples living on different continents, representing different civilizations, in general – all humanity. This was especially evident in a number of his works and articles of the late 1910s and early 1920s: “Tuda, tuda…”, “Azy iz uzy. Edinaya kniga”, “Zangezi”, “Nasha osnova”, etc. On the other hand, "worldliness", increased in the last period of his work, focus on dialogue of cultures and their new synthesis were accompanied by other processes in the writer’s creativity and outlook. The picture of the united humanity conceivable by him was actually based on the discoveries of Russian culture, on the dominant cultural style of Russian Silver Age, and the world language was based on the Russian language and literature, including his own creative work. The worldliness, or "tendency to world-wide responsiveness" (F. Dostoevsky) of Khlebnikov was accompanied by strengthening of his creative thinking national orientation, which is found not in the sloganistic nationalist-chauvinist way, as in some of his early articles, but in sacrifice and a deep involvement in the fate of the Fatherland.

A growing number of researchers studying horse domestication come to a conclusion that this process happened in multiple locations and involved multiple wild maternal lines. The most promising approach to address this problem involves mitochondrial haplotype comparison of wild and domestic horses from various locations coupled with studies of possible migration routes of the ancient shepherds. Here, we sequenced complete mitochondrial genomes of six horses from burials of the Ukok plateau (Russia, Altai Mountains) dated from 2.7 to 1.4 thousand years before present and a single late Pleistocene wild horse from the neighboring region (Denisova cave). Sequencing data indicates that the wild horse belongs to an extinct pre-domestication lineage. Integration of the domestic horse data with known Eurasian haplotypes of a similar age revealed two distinct groups: the first one widely distributed in Europe and presumably imported to Altai, and the second one specific for Altai Mountains and surrounding area.

The Bering Land Bridge connecting North America and Eurasia was periodically exposed and inundated by oscillating sea levels during the Pleistocene glacial cycles. This land connection allowed the intermittent dispersal of animals, including humans, between Western Beringia (far north-east Asia) and Eastern Beringia (north-west North America), changing the faunal community composition of both continents. The Pleistocene glacial cycles also had profound impacts on temperature, precipitation, and vegetation, impacting faunal community structure and demography. While these paleoenvironmental impacts have been studied in many large herbivores from Beringia (e.g., bison, mammoths, horses), the Pleistocene population dynamics of the diverse guild of carnivorans present in the region are less well understood, due to their lower abundances. In this study, we analyze mitochondrial genome data from ancient brown bears (Ursus arctos; n = 103) and lions (Panthera spp.; n = 39), two megafaunal carnivorans that dispersed into North America during the Pleistocene. Our results reveal striking synchronicity in the population dynamics of Beringian lions and brown bears, with multiple waves of dispersal across the Bering Land Bridge coinciding with glacial periods of low sea levels, as well as synchronous local extinctions in Eastern Beringia during Marine Isotope Stage 3. The evolutionary histories of these two taxa underscore the crucial biogeographic role of the Bering Land Bridge in the distribution, turnover, and maintenance of megafaunal populations in North America.

In the paper we investigate the characteristics of a short-cavity fiber laser based on a new silica fiber with a composite erbium-doped phosphosilicate core without additional co-doping with ytterbium. For the first time, the operation of such laser in the continuous-wave lasing mode with 1490-nm pump is shown. The laser operation in the self-Q-switching mode, changing to the continuous-wave mode with increasing the pumping power, is demonstrated.

Several new finds of the Late Pleistocene porcupine (Hystrix brachyura vinogradovi) in the Altai Mountains of southern Siberia and the Urals occur far north of previously assigned range for porcupine. These finds have necessitated a renewed study of this species's chronology and spatial distribution. We conclude that the oldest records of this porcupine in the Ural Mountains date to MIS 5e, and its geographic range possibly included also the Altai at that time. Directly radiocarbon-dated porcupine bones in the Altai fall in MIS 3 (ca. 32,000–41,000 cal BP). It is the northernmost record of this species and the youngest find outside its current geographic range.

The Altai region of Siberia was inhabited for parts of the Pleistocene by at least two groups of archaic hominins—Denisovans and Neanderthals. Denisova Cave, uniquely, contains stratified deposits that preserve skeletal and genetic evidence of both hominins, artefacts made from stone and other materials, and a range of animal and plant remains. The previous site chronology is based largely on radiocarbon ages for fragments of bone and charcoal that are up to 50,000 years old; older ages of equivocal reliability have been estimated from thermoluminescence and palaeomagnetic analyses of sediments, and genetic analyses of hominin DNA. Here we describe the stratigraphic sequences in Denisova Cave, establish a chronology for the Pleistocene deposits and associated remains from optical dating of the cave sediments, and reconstruct the environmental context of hominin occupation of the site from around 300,000 to 20,000 years ago. Optical dating of sediments from Denisova Cave establishes a chronology for its Pleistocene deposits and the associated artefacts, hominin remains and environmental records, which date to between about 300,000 and 20,000 years ago.

Recent developments in high-throughput sequencing (HTS) technologies have enabled the reconstruction of complete genome sequence and nucleosome and methylation maps from ancient individuals. A diversity of degradation reactions affect DNA molecules after an organism dies, introducing specific nucleotide misincorporation patterns in HTS data that are typically exploited to distinguish modern contaminants and authentic DNA templates. In this study, we used two different DNA polymerases to amplify Illumina TruSeq DNA libraries built on DNA extracts from ancient equines. The AmpliTaq Gold, widely used in ancient DNA studies, generated amplified libraries showing typical misincorporation patterns. Such patterns were partially lost following AccuPrime Pfx amplification, for which a two-fold reduction of endogenous content was also observed. This is explained by the inability of the AccuPrime Pfx to bypass uracils, which represent the most common post-mortem base modification and derive from the post-mortem deamination of cytosines. Our study, therefore, reveals that amplification of TruSeq DNA libraries with AccuPrime Pfx increases the cost of whole-genome sequencing for samples showing substantial levels of DNA degradation and creates atypical nucleotide misincorporation patterns for data authentication. The method can, however, be exploited to identify ancient methylation marks, and potentially, nucleosome occupancy maps. Statement of significance After an organism dies, DNA molecules are subjected to degradation resulting in fragmentation and bases modifications. After high- throughput sequencing (HTS), it is possible to detect the molecular signature of these damages, demonstrating that the data generated is authentic. DNA extracts from fossil remains indeed contain, besides endogenous ancient molecules, a variety of modern contaminants, but only the former generally exhibits molecular signatures typical of post-mortem damage. Our study reveals that these signatures depend on the protocol used for amplifying DNA samples. It thus helps defining better guidelines for authenticating ancient HTS data. Our results also reveal one molecular approach that limits our ability to exploit the whole complexity of ancient DNA molecules preserved in fossil specimens but can help track ancient methylation marks. Data availability The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are contained within the paper.

The cleavage of dsDNA fragments in aqueous solution after irradiation with UV laser pulses at 193 nm has been studied. Samples were investigated using polyacrylamide gel electrophoresis. The intensity of damage of particular phosphodiester bond after hot alkali treatment was shown to depend on the base pair sequence. It was established that the probability of cleavage is twice higher for sites of DNA containing two or more successively running guanine residues. A possible mechanism of damage to the DNA molecule connected with the migration of holes along the helix is discussed.

IR absorption and spontaneous Raman scattering spectra of germanosilicate optical fibers loaded with molecular hydrogen at pressures of 150–170 MPa, as well as the change of these spectra during hydrogen out-diffusion from the fibers are investigated. Purely rotational transitions of molecular hydrogen in the Raman spectra of optical fibers are observed for the first time. Changes in spectral characteristics of the fiber Bragg gratings subjected to hydrogen processing are analyzed. It is found that after loading at so high pressures a decrease in hydrogen concentration in the fiber core exhibits two different stages, the initial stage being faster as compared with that for the fibers loaded at 10–15 MPa. To explain this phenomenon, the influence of gradient of internal stress caused by hydrogen dissolved in the glass network as well as the ortho–para hydrogen conversion in germanosilicate glass are considered. An increase in solubility of molecular hydrogen in the glass network subjected to UV irradiation is revealed. � 2005 Elsevier B.V. All rights reserved.

The aerospace education is the basic task of the Program (PSEMS’ 2002–2007) of the Scientific-Educational Micro-Satellite (SEMS) pursues solely humane objectives associated with directional evolution of interests of the students and extension of knowledge in a selected area through a wide use of practically received space information, the use of space and computer technologies. The main objective of the PSEMS is to introduce a new, highly efficient method of education for schoolchildren and students based on the development, launch of satellites and their use through school centre of reception of the telemetery information (SCRI), data receiving, processing and physical interpretation. Cosmonautics as a field of science and technology is a unique area of research and educational activity where interests of all branches of scientific knowledge cross. The PSEMS solves the tasks in three directions—educational, scientific, technical—and is based on sequential evolution of tasks—from a simple to a more complex one. The PSEMS is not commercial: it does not pursue deriving a profit. Money received from the PSEMS implementation will be invested to projects of new satellites, new research programs and development of logistics based on organizations involved in the activities.

This paper deals with the development of a novel single-frequency tunable diode laser with fiber-optic output for gas-analysis applications. The approach we propose is a convenient, simple and cheap solution for spectroscopy of single absorption lines of any gases having absorption bands in the optical fiber transparency window (0.7 μm/1.7 μm). The presence of fiber-optic output is an additional advantage for remote sensing applications. The laser operation is demonstrated as applied to R7 line of 2ν3 methane absorption band at λ=1.645 μm. The mode-hop-free tuning range of 35 GHz (1.2 cm−1) has been achieved.

The first radiocarbon dates from the unique early Scythian monument Arzhan-2, discovered in 2001, are presented. The monument contained a royal burial (grave nr 5). Unfortunately, precise dating is hampered by the Hallstatt plateau in the calibration curve. However, using both accelerator mass spectrometry measurements from buried materials and conventional dates for floating tree rings from the burial chamber, we were able to date the construction of the monument to the 7th century BC. This is consistent with archaeological expectations. Other graves located inside the barrow were also dated. Grave nr 11, located on the edge of the barrow, is younger, showing that the monument was a place of burial ritual for many years for this ancient population.

The design of a fiber-optic dosimeter, which determines the radiation dose from the difference of radiation-induced attenuation (RIA) Δα measured in a P-doped silica fiber at λ = 413 and 470 nm, is presented along with its first test results under gamma-radiation (dose rates 0.00064 and 0.0066 Gy/s, maximal dose ~2Gy). The dose-dependence of Δα as well as of RIA at individual wavelengths is found to be well described by a power law, the exponent lying in the range 0.90-0.94. In contrast to RIA at individual wavelengths, Δα is found not to depend on dose rate and to decay only slightly on termination of irradiation. Therefore, using Δα for dosimetry is argued to be more promising.

In-fiber Bragg and long-period gratings as well as Mach-Zehnder interferometers based on germanium- and nitrogen-doped silica fibers have been investigated under /spl gamma/-rays. The majority of the experimental results suggest that both types of gratings in both types of fibers are stable with respect to /spl gamma/-ray doses of up to 1.47 MGy.

A bismuth-doped-pure-silica holey fiber is fabricated using a fiber preform made by the furnace chemical vapor deposition method. The spectroscopic properties of the fiber are studied, and laser action at λ=1450 nm with an efficiency of 12% is demonstrated.

Fiber Bragg gratings written in polarization-maintaining fiber are proposed for wavelength selection in actively mode-locked Er-doped fiber lasers. Combined with single-polarization optical circulator, they form unidirectional transmission filters that can be incorporated in polarization-maintaining laser cavities. We have fabricated such a grating in hydrogen-loaded PANDA fiber and we have incorporated it in a polarization-maintaining actively mode-locked Er-doped fiber laser designed to generate soliton-like pulses. Dependencies of pulse duration and spectral width on average intracavity power were measured. The power range over which soliton-like pulses were generated without pedestals was found to be ultimately limited by the grating's bandwidth.

A simple, f lexible method of postfabrication positioning of resonance wavelengths of long-period cladding-modecoupled gratings is proposed. This method is based on changing the outer fiber diameter. Reducing the diameter by etching the fiber in HF acid shifts the loss peaks to higher wavelengths. A shift as large as 130 nm after 5 min of HF etching was observed for our strongest grating peak, corresponding to the cladding mode HE19. The experimental results are in excellent agreement with model calculations. © 1996 Optical Society of America Photoinduced long-period cladding-mode-coupled gratings 1 can successfully be used in optical f iber communications systems, for example, for gain spectrum f lattening of Er-doped f iber amplifiers 2 and for removing undesirable Stokes orders in cascaded Raman amplifiers. The grating resonance wavelengths, the intensities of resonance peaks, and the bandwidth can be chosen in accordance with the application requirements. These parameters are related to one another, and we define them simultaneously by selecting the following parameters: amplitude of the grating-induced refractive index, length, and period. In practice, the grating writing procedure is limited by the discrete set of available amplitude masks. Moreover, during the long-period grating fabrication there is a strong displacement of resonance peaks owing to the increase of the mean refractive index of the f iber core. 3 For standard writing techniques the accuracy in positioning the peak for a desired peak intensity is limited to a few nanometers. In this situation it is very important to have a procedure to decorrelate peak position and intensity after the grating preparation. In this Letter we present a simple technique to tune precisely and continuously the resonance wavelengths of long-period cladding-mode-coupled gratings without strongly affecting the coupling strength. The main principle of this method is based on the strong dependence of the propagation constants and hence the peak positions of the cladding modes on the cladding diameter. Therefore one can shift the grating resonance wavelength by changing the cladding dimension. We have achieved this by etching the f iber in a HF acid solution. Long-period gratings were written in a single-mode germanosilicate f iber with a cutoff wavelength of 0.92 mm. The f iber has a step-index profile with 12 mol. % GeO2 in the core and a cladding diameter of 125 mm. The fiber was irradiated by a KrF excimer laser sl › 248 nmd through an amplitude mask with a period of L › 200 mm. An energy density of 85 mJycm 2 ypulse, a pulse repetition rate of 50 Hz, and an irradiation time of 1 h were typically applied. A halogen lamp and a 1-m focal-length monochromator were used for characterizing the gratings. A typical resolution of our transmission measurements was 1 nm. We carried out computer modeling, taking into account real fiber parameters, assuming a step-index profile in the f iber core, and considering the rectangular distribution of the induced refractive-index change in the core along the f iber axis. Fundamental sbcored and cladding-mode sbcladd propagation constants were calculated as described in Ref. 4. The resonance wavelengths were obtained from the phase-matching conditions between the modes considered

An all-fiber electrically tunable loss filter that is based on photoinduced long-period grating coated by Ti-Pt metal coating was developed and investigated. Maximum wavelength tuning of 11 nm with an applied power of 0.67 W was achieved for the HE17 cladding mode resonance peak.

The group refractive-index difference of cladding modes excited by a long-period fiber grating is characterized by use of the technique of optical low-coherence reflectometry, with a precision of10(-4) . Very good agreement between theoretical and experimental results is demonstrated.

We report a Raman laser with an output power of 4.2 W at 2.1 μm based upon a 75 mol.% GeO2 doped fiber pumped with CW Thulium doped fiber laser. Potential of 2.3 μm operation is discussed.

Germania-glass-core silica-glass-cladding single-mode fibers (deltan as great as 0.143) with a minimum loss of 20 dB/km at 1.85 microm were fabricated by modified chemical-vapor deposition. The fibers exhibit strong photorefractivity, with type IIa index modulation of 2 x 10(-3). A Raman gain of 300 dB/(kmW) was determined at 1.12 microm. Only 3 m of such fibers is sufficient for constructing the 10-W Raman laser at 1.12 microm with a 13-W pump at 1.07 microm.

This paper deals with development of novel single-frequency tunable diode laser with fiber-optic output for gas-analysis applications. The approach we proposed is convenient, simple and cheap solution for spectroscopy of a single absorption lines of any gases having absorption bands in the optical fiber transparency window (0,7 ÷ 1,7 μm). The presence of fiber-optic output is an additional advantage for remote sensing applications. The laser operation was demonstrated in the case of the line R7 of methane absorption band 2v3 at wavelength λ=1,645 μm. The mode-hop-free tuning range of 35 GHz (1,2 sm-1) was achieved.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The group index of well-defined cladding modes has been characterized for the first time to our knowledge by optical low-coherence reflectometry (OLCR) with a precision

Effects of UV-dose and fiber strain on annealing properties of fiber gratings written in Ge-doped fibers have been experimentally investigated. A new band in the distribution of the activation energy of the grating thermal decay with a center at 2.9 eV has been observed.

A tunable loss filter based on a long-period fiber grating and a Ti-Pt coating is presented. Electrical wavelength tuning of 11 nm with an efficiency of 16.4 nm/W and modulation with 2 s rise time were achieved.

Summary form only given. Novel nitrogen-doped silica-core pure-silica-cladding fibers provide exceptional possibilities to achieve abrupt variations of the refractive-index profile by thermo-diffusion of nitrogen. A high diffusion coefficient of nitrogen in silica makes it possible to create deep changes in refractive index by means of short-time heat action on a fiber section with a length comparable to the fiber diameter. Thus, N-doped silica fibers are most appropriate for various fiber-optic devices produced by thermo-diffusion.

In conclusion the results obtained have shown the possibility of fiber grating fabrication using more commonly used lasers operating in the near-UV region. Besides these results gain insight into the phenomenon of the photo-induced glass refractive-index change.

Summary form only. We pump the Ho/sup 3+/-doped fiber laser into the core using a high-power double-clad Yb/sup 3+/-doped fiber laser operating at 1.15 /spl mu/m. This wavelength corresponds to a sufficiently strong absorption band of Ho/sup 3+/ ions due to the laser transition. Also we use a Bragg grating with a resonance wavelength of 2 /spl mu/m as the input mirror of the laser.

Bragg gratings were written in H2-loaded polarization- maintaining fiber (PM-FBGs) and inserted in an actively mode-locked Er-doped fiber laser. The use of PM-FBG in sigma laser cavity allows to effectively build all polarization- maintaining fiber laser. Long term stabilization of the laser was ensured by a feedback loop that controlled the cavity length. Peak wavelengths, reflection bandwidths and reflectivity values of the grating were equal to 1545.5 nm and 1540.5 nm, 1.6-nm and 0.8-nm, of 99% and 90%, respectively. At a 3-GHz repetition rate, pulses of 9.4-ps and 27.9-ps duration were generated with first and second gratings, respectively. By inserting additional long piece of dispersion-shifted fiber in the cavity, nonlinear pulse shortening was observed due to formation of average soliton inside the cavity. In this case, pulses of 7.8-ps and 22.7- ps duration were generated with first and second gratings, respectively. With the first grating, generated pulses were close to transform limit for hyperbolic secant pulse shape. Longer pulse duration obtained with second grating is attributed to narrower bandwidth and residual chirp of the grating.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The fabrication of π-shifted long-period gratings is reported. Such gratings exhibit a pass band within their core-cladding mode resonance peaks. Their application as filters in actively mode-locked Erbium fiber lasers is presented for the first time.

OSDR techniques for spatial characterization of fiber Bragg gratings based on local phase induction by IR or UV radiation have been developed. Both radiation types provide longitudinally and cross-sectionally uniform fiber excitation, high spatial resolution and good sensitivity of measurements.

Optical fiber technology is considered now for communication and sensing applications in various radiation environments, like space and nuclear industry. We report on results from an on-going experimental program, which aims at using multiplexed Fiber Bragg gratings (FBGs), essential photonic components, for in-pile temperature monitoring in a nuclear reactor. To the best of our knowledge, it is the first time that multiplexed FBG-sensors are used in such conditions.

The article overviews the main properties, fabrication techniques and ares of application of long-period fiber gratings. The basic theoretical equations describing spectral properties of the gratings are given. Experimental investigation of the cladding modes excited by long-period gratings, as well as sensitivity of the long-period grating spectrum to external perturbations are discussed. The most common fabrication techniques of long-period gratings are examined with reference to their advantages and disadvantages. The most important applications of long- period gratings are discussed. Particular attention is paid to the recent results obtained with participation of the authors.

The results of experimental research and numerical modeling of the 1.3 micrometers Raman fiber amplifier based on the high Gao 2 doped fiber are presented. The Raman amplifier was pumped by the P 2 O 5 -doped fiber Raman laser. The measurements of gain and noise figure in broad range of experimental conditions are fulfilled. The amplifier gain coefficient was measured to be 42 dB/W.

Single-mode germanosilicate-core optical fibers were fabricated by the MCVD process modified by sintering of the deposited soot in a reduced atmosphere (helium- or nitrogen- containing one). The fibers showed a higher photoinduced refractive index change and a higher efficiency of (chi) (2) formation as compared to an ordinary germanosilicate fiber. The sintering process in nitrogen and helium atmospheres was shown to increase the concentration of germanium oxygen-deficient centers. Besides, nitrogen appears to enter into the glass network in a sufficient concentration to modify the glass network and to additionally increase photosensitivity of the fibers.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A 1.43 micrometers fiber laser with an output power of 1.4 W is reported. The laser is based on Raman conversion of the Yb- fiber laser emission in a phosphorous doped fiber. The light conversion is based on both phosphorous and silica Raman shifts in the same P-doped fiber. This device can find medical applications, since its emission wavelength coincides with one of the water absorption bands.