查看更多>>摘要：In this paper, we proposed a microwave photonic frequency down-conversion scheme with the functions of self interference cancellation (SIC) and signal-signal beat interference (SSBI) mitigation. The scheme is based on a dual-polarization quadrature phase-shift keying (DP-QPSK) modulator. One of the dual-parallel Mach-Zehnder modulator (DP-MZM) which is located in the upper part of the DP-QPSK modulator is used to achieve SIC by properly biasing the modulator. A local oscillator signal is loaded on the other DP-MZM which is located in the lower part of the DP-QPSK modulator. After photoelectric conversion of the combined optical signal by a photo-detector (PD), the signal of interest (SOI) is down-converted to an intermediate frequency (IF) signal. The SSBI is mitigated by Kramers-Kronig (KK) detection to achieve lower IF down-conversion. Experimental results show that the SIC for a single-tone interference signal is around 53 dB, and for an interference signal with a bandwidth of 100 MHz, the SIC achieved at least 29 dB. A simulation verifies the effect which the least mean square (LMS) algorithm mitigated on the influence of multipath After SIC, we used KK algorithm to reduce error vector magnitude (EVM) of the down-converted 16-QAM signal from 17.54% to 9.07%, which indicates a remarkable SSBI mitigation effect.
查看更多>>摘要：A novel method to realize single-transverse-mode emission by incorporating a built-in index guide is employed on vertical-cavity surface-emitting lasers (VCSELs) with a large oxide aperture. The etched index guide provides modal loss discrimination for mode selection and transverse optical field confinement for light propagating in the longitudinal direction. A single-transverse-mode VCSEL with side-mode suppression ratio >30 dB under the full range of bias currents is achieved. The thermal resistance of the device is lower than one half of the counterpart of the conventional GaAs-based single-mode VCSELs. Moreover, compared with an ordinary VCSEL with the same oxide aperture diameter, the beam quality of the device incorporated a built-in index guide has been significantly improved. The divergence angle of the output beam estimated from the full-width-half-maximum of the far-field pattern is as small as 9 degrees.
查看更多>>摘要：Laser speckle rheology is a powerful method to measure viscoelasticity of biological fluids. To date, the in vivo assessment of viscoelastic properties of biofluids has been limited by several reasons such as combined speckle fluctuation and insufficient detection depth. In order to avoid these shortcomings, in particular, the problems associated with detection depth during in vivo viscoelastic testing of biological fluids, the transmission method has been adapted in this work. According to the experimental results, the modified laser speckle rheology technique has enabled one to increase a speckle fluctuation range by nine times at keeping the same thickness of the biomimetic materials, which is of great significance for further in-vivo research.
Miguel Varga, J. J.Lasa-Alonso, JonMolezuelas-Ferreras, MartinTischler, Nora...
6页
查看更多>>摘要：In this work we study the spatio-temporal correlations of photons produced by spontaneous parametric down conversion. In particular, we study how the waists of the detection and pump beams impact on the spectral bandwidth of the photons. Our results indicate that this parameter is greatly affected by the spatial properties of the detection beam, while not as much by the pump beam. This allows for a simple experimental implementation to control the bandwidth of the biphoton spectra, which only entails modifying the optical configuration to collect the photons. Moreover, we have performed Hong-Ou-Mandel interferometry measurements that also provide the phase of the biphoton wavefunction, and thereby its temporal shape. We explain all these results with a toy model derived under certain approximations, which accurately recovers most of the interesting experimental details.
查看更多>>摘要：In this report, we demonstrate a novel method to achieve random lasing in the diffusion regime. One of the missing features of a conventional random laser is directionality. Here, a novel cylindrical microstructure cavity made of silica polymer combination has been used to overcome this barrier. The proposed structure makes use of a doped PMMA as a core and silica as a cladding layer. This waveguide structure can be easily used as an integrated micro random laser as the outside layer material is silica. A random laser mediated by metal nanowire is demonstrated in a cladded microstructure for the first time. We also observed the lowering of threshold and spectral shift of such lasers with the nanowire concentration. A directional laser emission with a narrow spectral line FWHM = 0.19 nm was achieved in our experiments. The lasing modes from the microstructure were characterized in terms of emission wavelength, lasing threshold as well as the temporal evolution of the modes.
查看更多>>摘要：The reflection and transmission of a wave falling onto a stratified medium is analyzed in terms of the RT-matrixes establishing a relation between the wave amplitudes in the asymptotic regions z -> +/-infinity. The RTmatrixes are considered as a generalization of the characteristic matrixes introduced by Abeles with elements expressed via the amplitude reflectance and transmittance of a wave falling onto the medium from the right and left sides. If the medium is divided into several parts, the product of their RT-matrixes is equal to the matrix of the whole medium, whose reflectance and transmittance are thus expressed via the reflectance and transmittance of its parts. The easy use of the RT-matrix in practical calculations was shown through analysis of the wave reflection from periodic and aperiodic multilayer structures with abrupt or smooth interfaces.
查看更多>>摘要：Operation states of multiple solitons (MS), noise-like pulses (NLP), and a special pattern of MS coexist with NLP have been observed in a passively mode-locked erbium-doped fiber laser. By increasing the pump power or adjusting the polarization state of the cavity, the operation state from MS to NLP will go through three states: pure MS, MS & NLP, and pure NLP. The experiment results show that the state transition from pure MS to MS & NLP is mutational, while the state transition from MS & NLP to pure NLP is gradual. To our knowledge, this is the first time detailed exploration of the conversion process from MS to NLP operation in mode-locked fiber laser.
查看更多>>摘要：Frequency modulated continuous wave (FMCW) lidar is a high-precision absolute distance measurement technology in which the resolution is proportionate to the frequency modulation (FM) bandwidth of the light source. The distributed feedback (DFB) array exhibits advantages that meet FMCW lidar, such as a wide FM range, narrow linewidth, high FM speed, and low cost. However, the existing range extraction method for DFB array-based FMCW lidar has problems such as signal splicing error, residual FM nonlinearity, high sample rate requirement, and dispersion mismatch. Here, we propose and demonstrate an FM nonlinear kernel function based range extraction method, which improves the measurement precision of DFB array-based FMCW lidar. The distance to a target located at 5 m is experimentally measured using this method, and a precision of 1.9 mu m is obtained, which is much better than the existing range extraction method under the same conditions.
查看更多>>摘要：Fourier ptychographic microscopy (FPM) is a recently developed computational imaging method for high resolution and wide field-of-view. In this paper, we propose a new FPM method based on an untrained neural network (FuNN), which integrates a physical reconstruction model for FPM into a convolutional neural network, to solve the FPM reconstruction problem. The weights and bias of FuNN are optimized based on the interaction of the network and the physical reconstruction model so that the requirement for thousands of training datasets is eliminated. To reduce the size of the network, a set of low-resolution images are preprocessed to synthesize a complex object field before fed as the input to FuNN. Pupil recovery is also included and the final input of FuNN is a 3D image tensor that consists of synthesized field and coherent transfer function. The output of FuNN is modeled as reconstructed high-resolution complex object field and pupil function. The function of the pupil recovery is achieved mainly by the physical reconstruction model, so that the main structure of the neural network does not need to be adjusted according to the pupil recovery process as other iterative-based FPM neural network does. The performance of FuNN is evaluated by both simulation and experiment. Compared with other iterative-based FPM neural network, which just utilize network to calculate the gradient, FuNN can improve the quality of reconstructed complex object field without a large number of training datasets.
查看更多>>摘要：We propose dual Butterfly-Gauss vortex beams with uniform polarization, which are composed of two finite energy Butterfly-Gauss beams with vortex cores. Based on the paraxial vector theory, the analytical propagation expressions of the proposed beams in uniaxial crystals orthogonal to the optical axis are derived and used to investigate the evolutions of autofocusing behavior, polarization states and Stokes vortices. The results show that the autofocusing behavior and its autofocusing length can be affected by topological charge, and the input uniform polarization is disordered into non-uniform polarization owing to anisotropic diffraction. The symmetry and rotation of polarization states are also found for the case of vortex. The Stokes parameters on the Poincare sphere can be manipulated by adjusting the refractive index ratio, azimuthal angle and phase retardation angle. Their Stokes vortices, e.g. S-12 vortices, present the behavior of merging, separation and reversal. The presented work may be useful for manipulating polarization dynamics of catastrophe optical field in uniaxial crystal and has potential values in polarization-sensitive detection.
NSTL
Elsevier