Current Issue : October - December Volume : 2017 Issue Number : 4 Articles : 5 Articles
A compact, highly sensitive optical fiber displacement and curvature radius sensor is\npresented. The device consists of an adiabatic bi-conical fused fiber taper spliced to a single-mode\nfiber (SMF) segment with a flat face end. The bi-conical taper structure acts as a modal coupling\ndevice between core and cladding modes for the SMF segment. When the bi-conical taper is bent by\nan axial displacement, the symmetrical bi-conical shape of the tapered structure is stressed, causing a\nchange in the refractive index profile which becomes asymmetric. As a result, the taper adiabaticity is\nlost, and interference between modes appears. As the bending increases, a small change in the fringe\nvisibility and a wavelength shift on the periodical reflection spectrum of the in-fiber interferometer is\nproduced. The displacement sensitivity and the spectral periodicity of the device can be adjusted by\nthe proper selection of the SMF length. Sensitivities from around 1.93 to 3.4 nm/mm were obtained\nfor SMF length between 7.5 and 12.5 cm. Both sensor interrogations, wavelength shift and visibility\ncontrast, can be used to measure displacement and curvature radius magnitudes....
The evolution of a fiber-fuse phenomenon in a single-mode optical fiber was studied theoretically. To clarify both the silica-glass\ndensification and cavity formation, which have been observed in fiber fuse propagation, we investigated a nonlinear oscillation\nmodel using the Van Der Pol equation. This model was able to phenomenologically explain both the densification of the core\nmaterial and the formation of periodic cavities in the core layer as a result of a relaxation oscillation....
Many of the next generation of adaptive optics systems on large and extremely large\ntelescopes require tomographic techniques in order to correct for atmospheric turbulence over\na large field of view. Multi-object adaptive optics is one such technique. In this paper, different\nimplementations of a tomographic reconstructor based on a machine learning architecture named\nââ?¬Å?CARMENââ?¬Â are presented. Basic concepts of adaptive optics are introduced first, with a short\nexplanation of three different control systems used on real telescopes and the sensors utilised.\nThe operation of the reconstructor, along with the three neural network frameworks used, and\nthe developed CUDA code are detailed. Changes to the size of the reconstructor influence the\ntraining and execution time of the neural network. The native CUDA code turns out to be the best\nchoice for all the systems, although some of the other frameworks offer good performance under\ncertain circumstances....
In this work, a beat-frequency encoded fiber laser hydrophone is developed for\nhigh-resolution acoustic detection by using an elastic corrugated diaphragm. The diaphragm\nis center-supported by the fiber. Incident acoustic waves deform the diaphragm and induce a\nconcentrated lateral load on the laser cavity. The acoustically induced perturbation changes local\noptical phases and frequency-modulates the radio-frequency beat signal between two orthogonal\nlasing modes of the cavity. Theoretical analysis reveals that a higher corrugation-depth/thickness\nratio or larger diaphragm area can provide higher transduction efficiency. The experimentally\nachieved average sensitivity in beat-frequency variation is 185.7 kHz/Pa over a bandwidth of\n1 kHz. The detection capability can be enhanced by shortening the cavity length to enhance the\nsignal-to-noise ratio. The minimum detectable acoustic pressure reaches 74 �¼Pa/Hz1/2 at 1 kHz,\nwhich is comparable to the zeroth order sea noise....
A theoretical analysis upon the four-dimensional (4D) spatio-temporal temperature\ndependent dynamics of 888 nm pumped Nd:YVO4 dual-rod laser is established, which is valid in\nboth continuous-wave (CW) and acousto-optic (AO) Q-switched pulse lasers conditions. Our model\ncan accurately solve the 4D thermal generation and temperature evolution not only in the steady\nQ-switched state, but also in the first few unstable giant or dwarf pulses region. Factors including\nground state depletion (GSD), energy transfer upconversion (ETU), fluorescence branching ratios,\ntemperature-dependent cross sections and nonradiative relaxations processes are comprehensively\nconsidered for precisely estimating thermal effects, valid in both the steady pulse region and the\nunstable region at the beginning. Moreover, temporal and spatial temperature profiles and their\ncoupling effect on output properties at different repetition-rates are discussed. Experiments of\nhigh-power high-repetition-rate 888 nm end-pumped Nd:YVO4 dual-rod CW and AO Q-switched\nlasers are also firstly presented and the experimental results enjoy good consistency with our theory....
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