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We demonstrate selective excitation and coherent amplification in the multicore fiber of the out-of-phase supermode that has equal intensities but interleaved 0/π phases in the neighboring cores. The out-of-phase supermode is beneficial due to its stability at arbitrary high powers, whereas the fundamental supermode becomes unstable at relatively high powers. We fabricated a double-clad all solid silica fiber with six highly Yb-doped coupled cores arranged in a ring structure. We obtained amplification of 50 picosecond chirped pulses up to ∼1 μJ energy with measured content of the out-of-phase mode of >80%. The experimental results were supported by 3D numerical simulation demonstrating the possibility of the out-of-phase supermode amplification in the multicore fibers with real parameters without degradation of supermode composition for powers up to damage threshold in silica glass.

We report a new method for manufacturing a Fabry – Perot interferometer in the form of a microcavity in the optical fibre core. The method is based on the use of the process of electric discharge fusion splicing of a conventional optical fibre and a special composite fibre having a core with a high content of phosphorus oxide. The possibility of using this interferometer as a sensitive element of a strain sensor with high temperature stability is shown.

Results of studying photoinduced changes of a refractive index in the core of a composite erbium-doped phosphorosilicate optical fibre under pulsed UV irradiation at a wavelength of 193 nm and the annealing of these changes are presented and analysed. It is shown that the preliminary saturation of a composite optical fibre by molecular hydrogen substantially increases the photosensitivity. Analysis of thermal anneal kinetics revealed fundamental differences in the photosensitivity mechanisms between the optical fibres saturated with hydrogen and not subjected to hydrogen treatment. Initial and photoinduced absorption spectra of the phosphorosilicate glass are investigated. It is found that in the UV range, the induced absorption is mainly determined by a contribution from the intense absorption band of photoinduced phosphorus electron centres PO2 with a maximum at 4.7 eV (λ = 260 nm).

This paper reports results of a study aimed at producing a Raman fibre laser based on a multicore fibre with high-reflectivity fibre Bragg gratings inscribed in its peripheral cores using femtosecond laser pulses. We present spectral and power characteristics of a Raman laser whose output power at a wavelength of 1090 nm is 2.5 W. The laser emission linewidth has been shown to decrease as a result of the reduction in nonlinear effects owing to the larger effective mode area in the multicore fibre in comparison with a standard single-mode fibre Raman laser.

Population of the chemically active singlet 1 Δ g ( 0 ) state of molecular oxygen occurring due to direct laser excitation of the 1 Δ g ( 1 ) ← 3 Σ g − ( 0 ) transition has been observed for the first time, to the best of our knowledge, in oxygen molecules dissolved in organic solvents saturated with air under natural conditions (room temperature and normal atmospheric pressure). The data were obtained in 1 cm spectrophotometric cells due to the application of a set of high-power IR fiber and diode lasers. The rate of laser generation of the singlet ( 1 Δ g ( 0 ) ) states in oxygen molecules was monitored by a chemical trapping method. It was found that the action spectra of singlet oxygen generation have one distinct band with a maximum at 1070 nm and half-width of ∼ 10 n m . The absorption coefficients at 1070 nm were shown to be 100–110-fold lower than those at the main oxygen absorption peak (1273 nm) corresponding to the 1 Δ g ( 0 ) ← 3 Σ g − ( 0 ) transition. Under excitation at 810–1061 nm, very low trapping rates were observed, which did not depend on excitation wavelengths being probably caused by thermal effects. There was no reliable increase in the trapping rate under irradiation at 810 and 920 nm corresponding to the 1 Δ g ( 2 , 3 ) ← 3 Σ g − ( 0 ) transitions. This fact suggests that absorbance corresponding to these transitions is much lower than that at 1070 nm. The obtained results are important for both spectroscopy of oxygen and mechanistic studies of biological and therapeutic action of laser radiation.

Purpose: The article is devoted to the study of the peculiarities of the functioning of Indian toponyms in the text of the work of art. The analysis and interpretation of the current meaning of the toponym is the fundamental point in the development of linguistic and regional knowledge, without which it is impossible to successfully master the language.

We demonstrate selective excitation and coherent amplification in multicore fiber of the out-of-phase supermode that has equal intensities but interleaved 0/π phases in the neighboring cores. The out-of-phase supermode is beneficial due to its stability at arbitrary high powers, contrary to the fundamental supermode. We fabricated a double-clad all solid silica fiber with six highly Yb-doped coupled cores arranged in a ring structure. We obtained amplification of 50 picosecond chirped pulses up to ~1 µJ energy with measured content of the out-of-phase mode of >80%. We demonstrated in 3D numerical simulation the possibility of the out-of-phase supermode amplification for powers up to damage threshold in silica glass.

Specially designed composite heavily Er3+-doped fiber in combination with unique point-by-point inscription technology by femtosecond pulses at 1,026 nm enables formation of distributed-feedback (DFB) laser with ultra-short cavity length of 5.3 mm whose parameters are comparable and even better than those for conventional Er3+-doped fiber DFB lasers having much longer cavity. The composite fiber was fabricated by melting rare-earth doped phosphate glass in silica tube. The ultra-short DFB laser generates single-polarization single-frequency radiation at 1,550 nm with narrow linewidth (3.5 kHz) and 0.5 mW output power at 600 mW 980-nm pumping. The same fiber with conventional CW UV (244 nm) inscription technology using phase mask enables fabrication of 40-mm long DFB laser with > 18 mW output power at 3.3% pump conversion, which is a record efficiency for Er3+-doped fiber DFB lasers. The developed technologies form an advanced platform for Er3+-doped fiber DFB lasers operating around 1.55 µm with excellent output characteristics and unique practical features, in particular, the ultra-short DFB lasers are attractive for sensing applications.

We experimentally demonstrate spatial beam self-cleaning in an Yb-doped graded-index multimode fiber taper, both in passive and active configurations. The input laser beam at 1064 nm was injected for propagation from the small to the large core side of the taper, with laser diode pumping in a counterdirectional configuration. The Kerr effect permits to obtain high-beam quality amplification with no accompanying frequency conversions. As a result, our nonlinear taper amplifier may provide an important building block for multimode fiber lasers and amplifiers.

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.

We demonstrate Kerr self-cleaning of beams in an Ytterbium doped multimode fiber taper with exponentially decreasing nonlinearity, with no accompanying frequency conversion or spectral broadening.

Femtosecond point-by-point inscription of pi-shifted fiber Bragg grating in Er3+-heavily-doped phosphosilicate fiber with ultrashort length of 5.3 mm enables stable linearly-polarized single-frequency generation of 0.5 mW power and 3.5-kHz linewidth.

We report a study of composite fibres as gain media of all-fibre amplifiers. A distinctive feature of the fibres is an erbium – ytterbium doped phosphate core in a double silica cladding. The high Yb3+ concentration (above 1021 cm−3) ensures effective pump absorption from the cladding at fibre lengths under 40 cm and effective erbium ion sensitisation. The silica cladding facilitates fusion splicing to standard fibres. The composite fibres offer a large gain coefficient: about 0.4 – 0.5 dB cm−1 at a signal wavelength of 1550 nm. The maximum gain at a fibre length of just 36 cm and an input signal power of 0.37 mW at λ = 1550 nm is about 13.5 dB. Under such conditions, the amplified spontaneous emission level is lower than the input signal level by 10 dB.

Coherent propagation of supermodes in a multicore fiber is promising for power scaling of fiber laser systems, eliminating the need for the active feedback system to maintain the phases between the channels. We studied the propagation of broadband pulsed radiation at a central wavelength of 1030 nm in a multicore fiber with coupled cores arranged in a square array. We designed and fabricated a silica multicore fiber with a 5 × 5 array of cores. For controllable excitation of a desired supermode, we developed a beam-forming system based on a spatial light modulator. We experimentally measured intensity and phase distributions of the supermodes, in particular, the in-phase and out-of-phase supermodes, which matched well the numerically calculated profiles. We obtained selective excitation and coherent propagation of broadband radiation with the content of the out-of-phase supermode of up to 90% maintained without active feedback. Using three-dimensional numerical modeling with allowance for a refractive index profile similar to those of the developed fiber, we demonstrated stable propagation of the out-of-phase supermode and collapse of the in-phase supermode at a high signal power.

Analytical expressions for estimating the minimum acceptable bending diameter are derived as a function of the expected service life, probability of premature fracture, and optical fiber diameter, which can be used to estimate the multicore optical fiber functionality in the particular applications.

A fiber-optic delay line based on the successive transmission of a modulated optical signal through all seven cores of an optical fiber has been fabricated. The total loss of the signal after the transmission through all cores is less than 12 dB. The group delay in the range of modulation frequencies of 6 to 12GHz is found to be 5.62 μs for a fiber as short as 160 m.

We report a study of silica-clad composite optical fibres having a phosphate glass core doped with active rare-earth elements. The phosphate glass core allows a high concentration of active rare-earth ions to be obtained, and the silica cladding ensures high mechanical strength and facilitates fusion splicing of such fibres to silica fibres. Owing to the high concentration of active rare-earth ions, this type of fibre is potentially attractive for applications where a small cavity length and high lasing efficiency are needed.

We demonstrate a single-frequency distributed Bragg reflector (DBR) fiber laser based on the novel erbium-doped composite fiber fabricated by melting phosphate glass in a silica tube. The fabricated composite fiber was single-mode at the wavelength of 1.55 μm; the measured cutoff wavelength was 1.4 μm. The composite fiber was photosensitive to irradiation at the wavelength of 193 nm. Using the phase mask method, the DBR fiber laser cavity with the total length of 21 mm was inscribed directly into the erbium-doped composite fiber. A stable single-frequency regime of the fabricated DBR laser at the wavelength of 1565 nm is demonstrated.

We have studied the influence of cross-sectional structure and bends on optical cross-talk in a multicore fibre. A reduced refractive index layer produced between the cores of such fibre with a small centre-to-centre spacing between neighbouring cores (27 μm) reduces optical cross-talk by 20 dB. The cross-talk level achieved, 30 dB per kilometre of the length of the multicore fibre, is acceptable for a number of applications where relatively small lengths of fibre are needed. Moreover, a significant decrease in optical cross-talk has been ensured by reducing the winding diameter of multicore fibres with identical cores.

A fiber-optic delay line based on a multicore optical fiber is fabricated for the first time. Due to the optical pulse sequential passage over all cores, the time delay of the optical signal of 45.0 μs is obtained at the optical fiber length of only 1300 m. The use of the seven-core optical fiber allows a sevenfold reduction in the used fiber length in comparison with single-core fibers, which is promising for developing lines with long signal delays.

A way of measuring the difference in group delays between the cores of multi-core optical fiber is proposed. The method is based on a low-coherence Mach–Zehnder interferometer and a broadband radiation source. The corresponding experimental setup is constructed and a seven-core optical fiber with identical cores manufactured at the Fiber Optics Research Center, Russian Academy of Sciences, is studied.

Nonlinear spatiotemporal dynamics of femtosecond pulses in a hexagonal 7-core silica fiber pumped by a sub-uJ 370 fs Er:fiber CPA system was studied. Two orders of magnitude contrast enhancement of the pulses trapped in the central core of a multicore fiber was obtained. Further compression of pulses spectrally broadened in the multicore fiber down to 53 fs was demonstrated.

Characteristics of the continuous wave fiber laser based on a high-concentration Er-doped phosphate core silica cladding fiber were investigated. The laser demonstrated a maximum output power about 105 mW at 1535 nm with a fiber length of only 15 cm. The slope efficiency was about 25.5 %.

The results on core-selective inscription of fiber Bragg gratings in different multicore fibers by femtosecond laser pulses are presented. Applications, including multiple parameters sensors and fiber lasers, are discussed.

The first continuously accessible long-term fiber optic radio-frequency memory based on distributed structure of low-power emitters with wavelength division multiplexing, multicore optical fiber as retarding medium, optoelectronic repeater unit, and optical recirculation time-delay circuit are developed and investigated.

Nonlinear spatiotemporal dynamics of femtosecond pulses in a hexagonal seven-core silica fiber pumped by a sub-μJ 370 fs Er:fiber chirped pulse amplification (CPA) system were studied. Nonlinear pulse shaping and compression in the central core and pedestal coupling to the outer cores were directly measured by frequency-resolved optical gating. Further compression of the pulses spectrally broadened in the multicore fiber down to 53 fs was demonstrated. Two orders of magnitude contrast enhancement of the pulses after compression was observed.

Using sintering of powders in combination with vibration mixing of the melt, glasses for cores of preforms of optical fibers of the composition SiO2-Al2O3-P2O5-Yb2O3 with a rather high (up to 0.5 at. %) ytterbium content were fabricated. Lasing on ytterbium ions was obtained in light guides with a core of these glasses. To increase the lasing efficiency, further optimization of the process parameters is required to reduce optical losses. It is shown that one of the causes of optical losses is the inhomogeneity of the glass microstructure at the nanoscale level and the formation of nanocrystalline inclusions, which are SiO2, YbP3O9, and YbPO4.

We describe composite optical fibers with rare-earth co-doped phosphate-glass core in a silica-glass cladding. High RE-ion concentrations in the phosphate core of the composite fiber allow fiber length reduction in comparison with silica fibers. The silica cladding provides high mechanical strength and protects the phosphate core from air moisture while making it easier to splice with silica fibers.

We have studied the properties of silica-core photonic bandgap fibres with a small diameter to pitch ratio (no greater than 0.4) of high-index rods and small refractive-index difference (no greater than 0.03) in the cladding. Theoretical analysis and experimental data demonstrate that the number of core modes depends only on the number of pure-silica elements that form the core and is independent of cladding parameters. If there is one such element, only one dispersion curve of a core mode falls in the fundamental bandgap and the fibre is single-mode. The fibres having seven such elements are multimode.

A photonic bandgap fibre with an ytterbium-doped silica glass core is fabricated and investigated. The possibility of implementing single-mode operation of such fibres in a wide spectral range at a large (above 20 μm) mode field diameter makes them promising for fibre lasers and amplifiers. To ensure a high quality of the beam emerging from the fibre, particular attention is paid to increasing the optical homogeneity of the ytterbium-doped core glass.

Spectral broadening in the regime of normal dispersion in a large-mode-area all-silica photonic band-gap fiber (PBGF) is shown to enable compression of an optical parametric amplifier output with a peak power of 3 MW, a central wavelength of 1140 nm, and an initial pulse width of about 200 fs to a minimum pulse width of 20 fs, corresponding to a maximum peak power of 20 MW. The spectral bandwidth of the PBGF output generated in our experiments is sufficient to support compression to a two-cycle pulse width.

A two-stage Raman fibre converter (1.089/1.273/1.533 μm) based on a P2O5-doped silica fibre is fabricated and studied. The spectral broadening of the first Stokes component is investigated. The Raman converter is simulated numerically. By using the experimental data, the method of Raman converter simulation is improved by taking into account the additional power loss of the first Stokes component. The results of calculations by the improved method are in good agreement with the experiment. It is shown that the additional power loss of the first Stokes component results in a change in the region of the optimal resonator length from 300–600 m to 600–800 m.

A fibre Raman amplifier operating at a wavelength of 1.65-μm is fabricated. A two-stage phosphosilicate fibre Raman converter of 1.53-μm radiation for pumping the Raman amplifier is designed and tested. The amplifier pumped by a phosphosilicate fibre Raman converter can operate with a maximum of the gain band ranging from 1.6 to 1.7 μm. A fibre with a core of a high (25%) molecular content of GeO2 serves as an active medium for the Raman amplifier. The gain obtained for different powers of the input signal is 22 — 25 dB. Our calculations show that the use of a standard telecommunication fibre (with a lower content of GeO2) allows one to increase the gain in this scheme by reducing fibre splicing losses.

We present the method of fabrication of Al- and P-codoped silica optical fiber preforms by sintering SiO2 and AlPO4

An efficient fibre-based Raman laser was developed on the basis of a phosphosilicate optical fibre with low optical losses. Bragg fibre gratings, constituting the laser cavity, were formed directly in the active fibre. The parameters, mathematical modelling, and optimisation of the laser were investigated. As a result of the optimisation, the output power reached 2.4 W for pumping with radiation from a neodymium fibre-based laser with a power of 3.5 W, which corresponds to a 77% quantum efficiency of the Raman laser.

Fabrication and laser tests of a composite optical fiber with phosphate glass core in a silica cladding are described. The fiber was made by drawing a high-purity silica tube filled with Yb, Er co-doped phosphate glass. 1.54 μm laser action was demonstrated both with core and cladding pumping by ytterbium fiber laser and by laser diode respectively.

Using neighboring cores with different mode propagation constants (indexes) is a well-known way to reduce crosstalk in multicore fiber (MCF). However, in actual field-deployed fiber, random bends can cause a reduction in the difference between the mode indexes of neighboring cores, which consequently increases crosstalk. The level of crosstalk induced by bending in both rectangular cross-section and circular cross-section heterogeneous MCF with cores arranged in a line was investigated. The experimental results obtained indicate that in contrast to circular cross-section MCF, no bending-induced crosstalk occurs in rectangular cross-section MCF wound on the mandrel without special control of cross-section orientation. Thus, to eliminate undesirable bending-induced crosstalk in heterogeneous MCF a rectangular cross-section should be employed.

A procedure for the preparation of optically homogeneous glass for fiber preforms through sintering of coarse oxide particles and further processing of the resultant glass, including several drawing and stacking steps, is described. Reducing the pressure to 10 -2 Torr during sintering considerably reduced the amount of gas bubbles in Yb/Al-doped silica glass and decreased the background loss to 100 dB/km after the third drawing-stacking-consolidation cycle. For comparison, a fiber singly doped with alumina was fabricated by the same procedure as above. The level of wavelength- independent losses in that fiber was 65 dB/km.