Generation of ultrashort pulses with high average power and moderately high pulse energy generally requires a modelocked laser followed by several fiber amplifiers in a master-oscillator power-amplifier configuration. Recently, gainswitched diode lasers have emerged as a viable replacement to mode-locked oscillators as sources of sub-100 ps pulses in these systems, but the low output power available from the diodes necessitates the use of multiple costly amplifier stages. Here, we demonstrate the generation of 1.7 μJ pulses at 1030 nm, and 11.7 μJ pulses at 1064 nm from a gain-switched diode seeded compact MOPA with only two amplification stages. The final stage is a tapered fiber amplifier, whose geometry efficiently suppresses amplified spontaneous emission and allows reaching a gain of ~40 dB. This research work is still in progress, and further increase in pulse energy should be possible by optimizing the setup.
The monocrystalline Bi2Te3-xSex and Bi2-xSbxTe3-ySey films of various compositions and thickness were synthesized by MOCVD on sapphire substrates1 . The transmission of laser pulses (duration τ≈35 ps, wavelength λ=1064 nm) through the films was measured as a function of radiation intensity with the aim to optimize functionality of these films deposited on different substrates as saturable absorbers for the application in passively mode-locked 1-2 μm wavelength lasers. There was investigated nonlinear absorption of films of various compositions and thickness with measured saturation intensity ≈ 20 MW/cm2 for Bi2Te3-xSex and 45 MW/cm2 for Bi2-xSbxTe3-ySey. Data obtained can be explained by the classical nonlinearity – phase space filling in a quantum well2 . As it was established earlier3 , the semiconductor conductivity type of the film without light interaction can be changed to the metallic conductivity type in the case of light interaction with the film, that is strongly nonlinear depends on the light intensity. At the same time, with a metallic conductivity type, the film becomes vulnerable for high light intensity.
Distributed acoustic/vibration sensing schemes based on phase-OTDR are naturally sensitive to environmental perturbation. Nevertheless, further sensitivity enhancement is possible by using specialty fibers. In this paper, a nitrogen doped single-mode fiber with increased Rayleigh scattering properties is tested alongside a standard telecommunications single-mode fiber (SMF) for comparative phase-OTDR measurements. The high Rayleigh scattering fiber (HRF) does not only benefit from a higher numerical aperture, but also from a higher non-homogeneity of material density resulting in an enhanced scattering coefficient. For perturbations caused by shaker-induced vibration applied on a fiber section or by an acoustic signal emitted from a loudspeaker, the ability of localizing the perturbation and determining the frequency is studied simultaneously for the HRF and the SMF, using a direct detection phase-OTDR setup. Vibration frequencies in the range 100-7000 Hz with accelerations of up to 0.1g and acoustic signals in the frequency range 100-10000 Hz at sound pressure levels up to 115 dBC are tested. The signal-to-noise ratios (SNRs) for the differential phase-OTDR traces are calculated as the maximum difference signal level in the perturbation zone and a noise reference level outside the zone. Moving average methods are also employed for further enhancement. As expected, the HRF has superior performance for the localization and the frequency characterization, and it allows detection of signal levels that are undetectable with an SMF without using of denoising methods. On average, a 7 dB and a 3 dB improvement can be achieved for vibration detection and acoustic detection, respectively.
We demonstrates all-fiber master oscillator – power amplifier delivered 70W output power at 1033.33nm with 8 kHz FWHM linewidth without any problems with SBS. The anisotropic ytterbium doped tapered double clad amplifier with 50 μm MFD and polarization extinction ratio about 30 dB is developed as a burst stage. The output radiation demonstrated perfect beam quality (M2=1.03/1.08).
The powerful picosecond master oscillator – power amplifier (MOPA) with double clad ytterbium doped tapered fiber as a buster amplifier has been demonstrated in the presented paper. The developed MOPA has 60ps pulses with 0.3mJ pulse energy and 5MW peak power.
We report theoretical and experimental study of tapered double-clad fibers (T-DCF) and consider various amplifiers and lasers using this fiber as a gain medium.
The radiation resistant sensor system for measuring deformations of the fuel channels of the RBMK-1000 nuclear power reactor is by now developed and tested. Concept of the system is the application of metal-coated radiation resistant nitrogen-doped-silica-core fiber and Bragg gratings written in it. Performance of the fiber and sensor elements measured in conditions of the extreme radiation environment inside core of operating nuclear reactors is being reported.
We present a comparative theoretical and experimental study of different schemes of fiber lasers with tapered doubleclad
fiber (T-DCF) as an active medium. We have developed a theoretical model for obtaining the distribution of power
and power density inside the fiber for the considered laser schemes with different longitudinal shapes of T-DCFs. The
slope efficiencies, spectral characteristics, and beam quality deterioration associated with the mode conversion in the TDCF
were also studied experimentally for different laser schemes. In addition, a spontaneous transition to a self-pulsing
regime under certain conditions, and the associated problems have been investigated. Characteristics of T-DCF lasers
constructed in co- and counter-propagating schemes are discussed and compared to those of regular fiber lasers,
equivalent in active volume or in brightness.
In this work, we have constructed an actively Q-switched T-DCF laser with an acousto-optic modulator (AOM).
Actively Q-switched laser with T-DCF generated stable emission at 1065 nm over a wide repetition rate range of 0 Hz -
150 kHz, with the highest measured pulse energy of 1.6mJ limited by stimulated Brillouin scattering (SBS). We attribute
the robust operation at low duty cycles to the intrinsically low amplified spontaneous emission (ASE), provided by the
tapered fiber shape.
In addition to the advantage of the large mode area at the output end of the fiber, the T-DCF provides several other
attractive features. First, the vignetting of co-propagating ASE results in ASE power loss in wide-to-narrow end
propagation. Second, the low-mode nature of counter-propagating spontaneous emission, arising from mode selection in
the narrow part of the fiber, leads to weaker amplification of the spontaneous emission. These two effects contribute to
the low ASE background. Finally, fiber diameter modulation is a known method for SBS suppression, and another
inherent property of the T-DCF. These characteristics allowed for generation of 1.6mJ, 64 ns pulses at very low duty
cycles, up to single shot operation, illustrating the potential of the T-DCF for high-energy pulse generation and
amplification.
We report the theoretical and experimental study of the properties of an active tapered double clad fiber (T-DCF).
Optimization of the most important parameters of T-DCF such as tapering ratio, longitudinal profile, core/clad ratio and
absorption have been considered. Using optimized T-DCF design we have demonstrated a high-power (750W) and
highly efficient (80%) ytterbium fiber laser.
PANDA-type optical fibre with nitrogen-doped silica as core and pure silica as cladding material is fabricated and tested.
The fibre demonstrates birefringence as high as 5·10-4, polarization holding h-parameter 2·10-5 m-1, loss 1.5 dB/km at a
wavelength of 1.56 microns and 250 dB/km at 0.4 microns. Gamma radiation induced loss is found not exceeding 5-10
dB/km at a dose of 2-10 kGy, which is by an order of magnitude lower as compared to similar fibre with Ge-doped silica
in the core.
In situ monitoring of gratings' characteristics during continuous gamma irradiation to a dose of 9 MGy revealed by an
order of magnitude weaker Bragg wavelengths drift as compared to gratings written in Ge-doped silica-core fibres.
Nitrogen-doped silica-core fibre is discussed in the context of a technological basis for the fabrication of Bragg gratings for sensors with enhanced temperature resistance. To estimate applicability of this fibre type for high-temperature Bragg grating sensors manufacturing, the following features are analyzed: fibre preforms fabrication technology, fibre characteristics, and particularities of Bragg gratings writing in it. Practical application of this sensor type in thermometry is given as an example.
The results of thermal tests of high-temperature Bragg gratings written in a nitrogen-doped-silica-core fiber are presented. The gratings were produced by the conventional technique using an ArF excimer laser and a phase mask at various exposure regimes. The possible physical mechanisms responsible for thermal decay of the gratings are analyzed. Based on the results obtained, high-temperature sensors of physical quantities have been developed. The sensors can withstand heating up to 900 °C.
The light emission intensity at the wavelength of 400-750 nm in the KU-1 silica core (OH content 1000 ppm) fiber waveguide under irradiation at BARS-6 pulsed fission reactor (pulse duration 80 microsecond(s) , dose per pulse <5.5x1012 n/cm2 (9 Gy), dose rate <7x1016 n/cm2s (1.1x105 Gy/s) have been measured. The intensity of radiation-induced light emission has been found to depend on intensity of probing light. Lower intensity of the light emission has been observed for higher intensity of probing light (lasers, wavelength 532 and 632 nm). The light emission quenching occurs at the wavelengths shorter and longer than the wavelength of the probing light, and also at the equal wavelengths.
Aluminum-coated optical fibers are loaded with molecular hydrogen by keeping the fibers in an H2 atmosphere at high pressure and temperature. A high H2 concentration in the fiber glass (up to 1 at. %) has been obtained by this technique. A drastic reduction of radiation-induced absorption in the visible spectral region in H2- containing Al-coated fibers is demonstrated. The formation mechanism is the H(I)-center is analyzed.
The potential for intrinsic dosimetry with three different fibre types through the gamma ray radiation induced attenuation is investigated with respect to annealing, temperature dependencies, photo-bleaching and, to a limited extent, dose-rate. The radiation sensitivity of these three fibres covers an effective dose range of 0.1 Gy to 100 kGy which makes them suitable for a broad range of nuclear applications. With detailed analysis in both the spectral and temporal domain, it is shown that the long-term annealing effects can be suppressed by exploiting the stability of certain colour centres in the fibre core and adequate interrogation of the induced attenuation at a discrete set of wavelengths. Photo-bleaching is observed by employing a low power white light source (a few microwatts in the fibre) with Erbium doped fibres and when Germanium is added as a co-dopant with Phosphorous fibres. Temperature dependencies during irradiation and annealing are addressed from room temperature to 80 ?C. The Phosphorous and Erbium doped samples exhibit a reverse behaviour: the optical absorption increases with temperature.
The technology, properties and applications of novel germanium-free nitrogen-doped silica fibers are reviewed. The key features of the plasmachemical synthesis of low- hydrogen N-doped silica fiber preforms are discussed. Optoelectronic components fabricated from N-doped silica fibers are considered: thermostable Bragg and long-period gratings, mode field converters, and filters to smooth the erbium amplifier gain spectrum. Other important properties of the new fibers, such as high resistance to ionizing radiation and efficient third harmonic generation, are also discussed.
The mechanisms of radiation-induced absorption in silica optical fibers in the visible spectral region and in the telecom spectral windows as well as the technological means to lower the induced absorption are analyzed. Hydrogen loading of large-core silica optical fibers is shown to drastically reduce the induced absorption at megagray doses. It is shown that low-dose transient absorption can degrade the performance of pure-silica-core fibers at (lambda) approximately 1.55 micrometers . N-doped silica fibers are argued to be the best candidates for low-dose applications (e.g. in space). At megagray doses, the long-wavelength induced absorption is found to be the main induced absorption mechanism. Its origin is not known with certainty, whereas its value may be different in pure-silica-core fibers obtained under different preform fabrication conditions. Different types of radiation-sensitive fibers are investigated with the aim to develop fiber-optic dosimeters. An optimum wavelength region for the operation of P-doped silica fiber dosimeters has been determined. Novel types of dosimeters and neutron detectors are proposed based on the effect of irreversible radiation-induced increase of OH- group absorption.
Nuclear industry may benefit from the implementation of Optical Fiber Sensor (OFS) technology. This is obvious if the OFS integrates features that meet specific nuclear sensing needs, such as distributed dose monitoring in underground radioactive waste disposals. The use of radiation sensitive fibers for dosimetry has already been proposed in literature. In this paper, we report on our gamma irradiation of such dedicated Ge-P co-doped and pure P-doped optical fiber, fabricated by the Fiber Optic Research Center in Moscow. We show that after a careful selection of the wavelength at which the radiation-induced attenuation is measured, it is possible to reconstruct the dose within 20% accuracy by means of a linear model or a second order polynomial, depending on the temperature.
Fluorine-doped silica active and passive optical fibers for different purposes have been fabricated by two novel modifications of the plasmachemical technology. These versions of the technology are shown to be convenient in laboratory conditions and very promising in developing various types of special fibers with non-standard parameters based on pure and fluorine-doped silica.
Spectra and kinetics of decay of (gamma) -irradiation induced loss in optical fibers with pure silica and Ge- and/or F-doped silica core and F-doped cladding made by MCVD and SPCVD methods were measured at room temperature. Induced loss in short-wavelength range is shown to decrease at Ge-content decrease and at F addition to the core glass. Minimum of induced loss is located in long-wavelength range, the best result (about 2 dB/km after the dose 10 kGy) being achieved on pure silica core MCVD fiber. Induced absorption band at 630 nm is observed in pure silica core fibers and it is suppressed by F-doping of the core glass.
Metal-coated (Al) tapered all-silica fibers with core diameter up to 1.2 mm, the input/output diameter ratio up to 3:1, the tapered length 1.5 - 2 m, and the core-clad refractive index difference up to 0.017 have been prepared for high energy laser pulses delivery. The increased input diameter allows us to launch higher energy into the fiber keeping the surface power density well below the threshold of radiation-induced surface damage. Using laser radiation with the beam divergence of 3 mrad, the transmission of the fibers has been found to be 85%. Both free running and Q-switched TEM00-mode regimes of Nd:YAG laser have been used to demonstrate stable operation of the fibers for the following set of input parameters: 40 mJ pulse energy, 25 ns pulse duration, and 15 Hz pulse repetition rate.
The newly developed optical fibers are described designed to transmit intense radiation in the visible and near IR spectral regions. The main characteristic feature of the fibers is a core made of unique purity silica with extremely low OH content not exceeding 20 ppb. As a result, the fiber spectral attenuation in the infrared is substantially reduced: the absorption spectrum is a smooth monotonously rising curve with typical loss levels of less than 0.2 dB/m at 2.1, 10 dB/m--at 2.7 and 27 dB/m--at 2.8 microns. Thin (approx. 10% of the fiber diameter) fluorinated silica cladding together with the high purity core ensure excellent optical damage resistivity of the fibers and greatly facilitate their coupling to high power lasers. The plasma outside deposition technology used for clad fabrication can provide fluorine doping levels sufficient to produce the fibers with numerical aperture NA up to 0.24. The fibers can have either hermetic metal (In, Sn, Pb, Zn, Al) or plastic protective coating. For the Al-coated fibers the mechanical properties and environmental durability are characterized by the following figures: tensile strength--more than 7 GPa, minimal bend radius--40 times the fiber radius (long term), operating environment--from--196 up to 400 degree(s)C, 100% humidity.
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