查看更多>>摘要：A standard single mode input fiber combined with a graded index multimode sensing fiber (GI-MMF) with large numerical aperture is used as a distributed sensing fiber to realize bend-tolerant and low-cost structural safety and health monitoring of large infrastructures. In the designed distributed sensing fiber, the single mode fiber at the near end of the GI-MMF is used to excite the fundamental mode as much as possible in the GI-MMF to achieve good bending resistance, and filter out the high-order modes in the backscattering signal that may degrade the bending resistance and deteriorate the sensing performance, so as to realize the bend-tolerant distributed sensing. The principle of bend loss is analyzed, and the bending resistance of different sensing structures are discussed on the basis of theoretical and experimental comparisons. By measuring the Brillouin frequency shift and scattering spectrum width of the sensors, a stable and reliable optical coupling method is determined for the single mode fiber and GI-MMF combination, and a frequency-shifted local heterodyne Brillouin optical time domain reflectometry system is constructed for temperature measurements to evaluate the bending resistance of the sensors. The experimental results show that the sensor with the single mode fiber alignment fusion to a 5 km long and 62.5 mu m diameter GI-MMF has the best bending resistance, and the achieved minimum bend radius, temperature coefficient and measurement accuracy are 1.75 mm, 0.933 MHz/C and 1.4 degrees C, respectively, which indicates that the proposed sensing structure is capable of realizing bend-tolerant sensing and has huge potential in the field of high-accuracy structural safety and health monitoring of large infrastructures.
查看更多>>摘要：In multispectral transmission images, the strong scattering characteristics of biological tissues make the acquired signal weak and the image blurred, which brings difficulties to the contour detection of heterogeneity. The precision of image data can be improved by the modulation-demodulation technique, but the improvement of data effective bits also brings data redundancy. This paper proposes the "terrace compression method"and applies it to the detection of heterogeneity contours in transmission images. Experiments are designed to demonstrate the effectiveness of the proposed method. Four types of LEDs with different center wavelengths loaded with sinusoidal signals are used as the illumination sources respectively to acquire image sequences, and four single-wavelength images are demodulated by fast Fourier transform. The grayscale values of the demodulated images are sorted from low to high, several grayscale hard thresholds are set based on the quantity threshold obtained from the minimum size of the predicted detection target, and the grayscale values are divided into different intervals based on the grayscale hard thresholds. Then, the grayscale values below the threshold in each interval are combined to the minimum value of that interval. Finally, the Sobel operator is used to extract the heterogeneity contours. This method can realize image filtering, reduce data redundancy and enhance image gradient, which provides a new idea for heterogeneity contour detection of multispectral transmission images.
Chi, MingjunMueller, AndreHansen, Anders K.Jensen, Ole B....
5页
查看更多>>摘要：Solar radiation is a challenge for laser-based daylight imaging since it decreases the signal-to-noise ratio (SNR) of the imaging. Here we demonstrate a micro-integrated external-cavity tapered diode laser system for daylight imaging. It emits light at a wavelength of 762 nm, chosen because of its overlap with an oxygen absorption band in the solar radiation spectrum. The integrated laser system consists of a tapered diode amplifier as gain medium and an external volume Bragg grating for spectrum stabilization and narrowing, thus a narrow bandpass optical filter can be used to improve the SNR further. The laser system can be operated in both continuous wave (CW) mode and pulsed mode by modulating the injected current to the amplifier. In CW mode operation, 1.3 W of output power is obtained with an emission spectral linewidth of 4 pm, and a beam propagation factor in the slow axis, M-2, of 1.7 (4 sigma). In pulsed mode operation with a trigger signal of a 50 mu s pulse width and a 10 kHz repetition rate, 2.0 W of peak output power is achieved with an emission spectral bandwidth of 0.2 nm, and an M2 in the slow axis of 1.9 (4 ). The modulation depth is almost 100%.
查看更多>>摘要：Here, we observed the local angular chiroptical effects with unidirectional emission based on exquisitely designed plasmonic nanopillar antennas. Firstly, the scattering spectra for the asymmetric nanoantennas show double resonant peaks when illuminated by circular polarized light. The calculated circular dichroism reaches the maximum value at one resonant peak while almost disappears at the other peak. Secondly, when we turn to the momentum space for the CD-disappeared peak, the asymmetric nanoantennas exhibit different angular emission patterns that are sensitive to the handedness of light, which indicate the existence of the local angular chiroptical effects. The emission patterns also show unidirectional emission lobes in momentum space, which is distinct to other chiral phenomenon. Multipolar expansion and Green's function method in stratified medium is used to demonstrate the inner mechanism of the observed local angular chiroptical effects, and we found the phase difference of the multipolar resonance is the origin of the distinct local angular chiroptical effects. Our work investigating the local angular chiroptical effects based on nanopillar will shed new light on optical chirality and will be widely applied to wavelength router, photon-detection, and other optoelectronic devices.
查看更多>>摘要：Stronger light intensity fluctuations are pursued by related applications such as optical resolution, image enhancement, and beam positioning. In this paper, an Nth-order light intensity fluctuation amplifier is proposed, which was demonstrated by a four-wave mixing process with different statistical distribution coupling lights. Firstly, its amplification mechanism is revealed both theoretically and experimentally. The ratio R of statistical distributions and the degree of second-order coherence g((2))(0) of beams are used to characterize the affected modulations and the increased light intensity fluctuations through the four-wave mixing process. The results show that the amplification of light intensity fluctuations is caused by not only the fluctuating light fields of incident coupling beams, but also the fluctuating nonlinear coefficient of interaction. At last, we highlight the potentiality of applying such amplifier to other Nth-order nonlinear optical effects.
查看更多>>摘要：We propose a photonic approach for measuring the angle-of-arrival (AOA) with a large measurement range using a dual-drive Mach-Zehnder modulator (DDMZM). The echo signals received by two antennas are driven into the DDMZM. By measuring the power of output RF signal, the phase difference of two echo signals can be acquired. Moreover, the DC port of the DDMZM is fed by a two-level electrical signal to construct two mapping curves, which extends the AOA measurement range. The unambiguous AOA measurement with a range of -70.81 degrees to 70.81 degrees and errors of no more than +/- 2 degrees is implemented. The key novelty of our scheme is that two mapping curves are acquired by a two-level electrical signal and only one channel is established. Thus, the two channels previously established by optical multiplexing elements to generate two mapping curves are no longer needed.
查看更多>>摘要：One way to restore scattered optical imaging is retrieving its phase through the speckle autocorrelation. However, the commonly used methods have the disadvantages such as poor recovery quality, the need to select the initial value multiple times and insufficient noise immunity. A method named imaging through scattering layers by exploiting the sum of gradient amplitude guided phase retrieval is proposed in this paper. Sparsity enhancement operation is added to the solution obtained in each iteration and a value called the sum of gradient amplitudes is used to control the number of iterations. This method can effectively improve the quality of restored images. It can also effectively recover the object with any initial iteration value and have better noise resistance by a specific designed parameter in the iteration.
查看更多>>摘要：This study proposes a gradient descent method for enhancing the quality of reconstructed images encoded by double-phase holograms (DPHs). Holographic displays are often noisy because of optical hardware problems. However, owing to the conversion from complex-amplitude holograms to phase-only holograms, the reconstructed images still have noise and cannot be ignored. In this paper, the proposed method uses a spectrum loss function instead of image loss function in the gradient descent optimization process. The spectrum loss function ensures that DPHs can be optimized by gradient descent method, and this method requires only several iterations. The simulations and experimental results have validated the effectiveness of the proposed method.
查看更多>>摘要：The potential applications of complex optical fields for environmental detection have attracted a lot of attention. In this study, dusty medium is equivalent to a uniformly charged sphere and a non-uniform concentric layered sphere that satisfying the single scattering condition, respectively. The interaction between a high-order polarized Bessel Gaussian beam and a spherical particle is investigated using the generalized Lorenz- Mie theory and the angular spectrum decomposition method. The location of incident beam is considered for the first time. The extinction efficiency results of a wet charged particle are compared with those of a wet neutral particle. Simulations show that the difference in extinction between these two kinds of particles mentioned above is more evident in the smaller size parameter range, which is similar to the case of plane wave incidence. In addition, the influence of complex refractive index and size proportion of heterogeneous medium on the extinction effect is analyzed. The characteristic parameters of the incident beam are considered and find out that how they affect the differences in extinction is independent of the illuminated particle. This study lays the foundation for analyzing the transmission capacity of a Bessel Gaussian vortex beam with various parameters in a dust particle cluster.
查看更多>>摘要：The laser-driven white emission of graphene foam in a vacuum bulb was investigated. Thanks to high brightness of light emission attributed to multiphoton ionization, it was possible to construct a compact, spatially coherent, white light source. A new graphene foam bulb was designed to efficiently couple white light into a multimode fiber. The efficiency of coupling light to multimode fibers with varying core diameters was measured. It is concluded that the performance of the graphene foam bulb is close to that simulated for an ideal geometrical point source.
NSTL
Elsevier