Random raman fiber laser based on a twin-core fiber with FBGs inscribed by femtosecond radiation

Narrowband Raman lasing in a polarization-maintaining two-core fiber (TCF) is demonstrated. Femtosecond point-by-point inscription of fiber Bragg gratings (FBGs) in individual cores produces a half-open cavity with random distributed feedback. The laser linewidth in the cavity with a single FBG inscribed in one core of the TCF reduced by ∼2 times with respect to the cavity with a fiber loop mirror. It is shown that the inscription of two FBGs in different cores leads to the formation of a Michelson-type interferometer, leading to the modulation of generation spectra near threshold. This technique offers new possibilities for spectral filtering or multi-wavelength generation

Experimental Method of Temperature and Strain Discrimination in Polymer Composite Material by Embedded Fiber-Optic Sensors Based on Femtosecond-Inscribed FBGs

Experimental method of temperature and strain discrimination with fiber Bragg gratings (FBGs) sensors embedded in carbon fiber-reinforced plastic is proposed. The method is based on two-fiber technique, when two FBGs inscribed in different fibers with different sensitivities to strain and/or temperature are placed close to each other and act as a single sensing element. The nonlinear polynomial approximation of Bragg wavelength shift as a function of temperature and strain is presented for this method. The FBGs were inscribed with femtosecond laser by point-by-point inscription technique through polymer cladding of the fiber. The comparison of linear and nonlinear approximation accuracies for array of embedded sensors is performed. It is shown that the use of nonlinear approximation gives 1.5–2 times better accuracy. The obtained accuracies of temperature and strain measurements are 2.6–3.8°C and 50–83 με in temperature and strain range of 30–120°C and 0–400 με, respectively

Personality profiles and the "russian soul": Literary and scholarly views evaluated

Many domestic and foreign observers have claimed that Russians have a unique constellation of personality traits that mirrors their distinctive historical and cultural experience. To examine the hypothesized uniqueness of Russian personality, members of the Russian Character and Personality Survey collected data from 39 samples in 33 administrative areas of the Russian Federation. Respondents (N = 7,065) identified an ethnically Russian adult or college-aged man or woman whom they knew well and rated the target using the Russian observer-rating version of the Revised NEO Personality Inventory. The mean personality profile of Russians was very similar to the international average based on 50 different countries, debunking the myth of a unique Russian soul.The small variations from world norms did not converge with depictions of Russian national character in fiction and the scholarly literature. New items intended to capture distinctive, emic aspects of Russian personality provided no new information beyond the familiar Big Five dimensions. Religion, ethnicity, and beliefs about the uniqueness of the Russian character and the malleability of personality traits had little effect on personality ratings. Perceptions of the Russian soul do not seem to be based on the personality traits of Russians

PID controlling approach based on FBG array measurements for laser ablation of pancreatic tissues

In this paper, we propose a temperature-based proportional–integral–derivative (PID) controlling algorithm using highly dense fiber Bragg grating (FBG) arrays for laser ablation (LA) of ex vivo pancreatic tissues. Custom-made highly dense FBG arrays with a spatial resolution of 1.2 mm were fabricated with the femtosecond point-by-point writing technology and optimized for LA applications. In order to obtain proper PID gain values, finite element method-based iterative simulation of different PID gains was performed. Then, the proposed algorithm, with numerically derived PID gains, was experimentally validated. In the experiments, the point temperature was controlled at different distances from the laser fiber tip (6.0 mm, 7.2 mm, 8.4 mm, and 10.8 mm). The obtained results report robust controlling and correlation between controlled distance and the resulting area of ablation. The results of the work encourage further investigation of FBG array application for LA control

Femtosecond writing of refractive index structures in multimode and multicore optical fibres

The possibility of writing fibre Bragg gratings (FBGs) in optical fibres with a complex spatial structure using femtosecond IR laser pulses is considered. In particular, selective writing of uniform FBGs in individual cores of a seven-core fibre and writing of serial and parallel FBG arrays in the core of a multimode fibre with a gradient refractive index profile are demonstrated

LIPSS on thin metallic films: New insights from multiplicity of laser-excited electromagnetic modes and efficiency of metal oxidation

Thin Cr films 28-nm thick deposited on glass substrates were processed by scanning low-intensity femtosecond laser pulses with energy well below single-pulse damage threshold. Two types of laser-induced periodic surface structures (LIPSS) were produced, depending on the scanning velocity, (1) parallel to laser light polarization with periodicity somewhat smaller than laser wavelength and (2) perpendicular to polarization with spatial period much smaller than wavelength. All structures are formed as protrusions above the initial film surface and exhibit a high degree of oxidation. To explain formation of the LIPSS and their conversion from one to another type, a rigorous numerical approach for modeling surface electromagnetic waves in thin-film geometry has been developed, which takes into account the change of optical properties of material due to laser-induced oxidation and porosity. The approach addresses the multiplicity of electromagnetic modes allowed for thin films. It has been found that the low spatial frequency LIPSS parallel to laser polarization, which are formed at low scanning velocities, are well described by the Sipe theory for surfaces of low roughness. The SEW mode responsible for high spatial frequency LIPSS formation at high scanning velocities has been identified. The mechanisms of optical feedback and transformation between types of LIPSS with scanning velocity have been proposed