Random Reflectors Research Articles

Wavelength-switchable L-band fiber laser assisted by random reflectors

A wavelength-switchable L-band erbium-doped fiber laser (EDFL) assisted by an artificially controlled backscattering (ACB) fiber reflector is here presented. This random reflector was inscribed by femtosecond (fs) laser direct writing on the axial axis of a multimode fiber with 50 μm core and 125 μm cladding with a length of 17 mm. This microstructure was placed inside a surgical syringe to be positioned in the center of a high-precision rotation mount to accurately control its angle of rotation. Only by rotating this mount, three different output spectra were obtained: a single wavelength lasing centered at 1574.75 nm, a dual wavelength lasing centered at 1574.75 nm and 1575.75 nm, and a single wavelength lasing centered at 1575.5 nm. All of them showed an optical signal-to-noise ratio (OSNR) of around 60 dB when pumped at 300 mW.

A Dual-Wavelength Fiber Laser Sensor with Temperature and Strain Discrimination.

This work presents a dual-wavelength C-band erbium-doped fiber laser assisted by an artificial backscatter reflector. This fiber-based reflector, inscribed by femtosecond laser direct writing, was fabricated into a single mode fiber with a length of 32 mm. The dual-wavelength laser obtained, centered at 1527.7 nm and 1530.81 nm, showed an optical signal-to-noise ratio over 46 dB when pumped at 150 mW. Another feature of this laser was that the power difference between the two channels was just 0.02 dB, regardless of the pump power, resulting in a dual emission laser with high equalization. On the other hand, an output power level and a central wavelength instability as low as 0.3 dB and 0.01 nm were measured, in this order for both channels. Moreover, the threshold pump power was 40 mW. Finally, the performance of this dual-wavelength fiber laser enhanced with a random reflector for sensing applications was studied, achieving the simultaneous measurement of strain and temperature with sensitivities around 1 pm/με and 9.29 pm/°C, respectively.

Wavelength-switchable L-band fiber laser assisted by ultrafast laser fabricated random reflectors

A wavelength-switchable L-band erbium-doped fiber laser (EDFL) assisted by an artificially controlled backscattering (ACB) fiber reflector is here presented. This random reflector was inscribed by femtosecond (fs) laser direct writing on the axial axis of a multimode fiber with 50 μm core and 125 μm cladding with a length of 17 mm. This microstructure was placed inside a surgical syringe to be positioned in the center of a high-precision rotation mount to accurately control its angle of rotation. Only by rotating this mount, three different output spectra were obtained: a single wavelength lasing centered at 1574.75 nm, a dual wavelength lasing centered at 1574.75 nm and 1575.75 nm, and a single wavelength lasing centered at 1575.5 nm. All of them showed an optical signal-to-noise ratio (OSNR) of around 60 dB when pumped at 300 mW.

Cauchy–Rician Model for Backscattering in Urban SAR Images

This letter presents a new statistical model for urban scene synthetic aperture radar (SAR) images by combining the Cauchy distribution, which is heavy tailed, with the Rician backscattering. The literature spans various well-known models most of which are derived under the assumption that the scene consists of multitudes of random reflectors. This idea specifically fails for urban scenes since they accommodate a heterogeneous collection of strong scatterers such as buildings, cars, and wall corners. Moreover, when it comes to analyzing their statistical behavior, due to these strong reflectors, urban scenes include a high number of high amplitude samples, which implies that urban scenes are mostly heavy-tailed. The proposed Cauchy–Rician model contributes to the literature by leveraging nonzero location (Rician) heavy-tailed (Cauchy) signal components. In the experimental analysis, the Cauchy–Rician model is investigated in comparison to state-of-the-art statistical models that include <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {G}_{0}$ </tex-math></inline-formula> , generalized gamma, and the lognormal distribution. The numerical analysis demonstrates the superior performance and flexibility of the proposed distribution for modeling urban scenes.

Enhanced Ultrasonic Imaging and Non-Destructive Evaluation Using Angle-of-Arrival Estimation in Dimension-Reduced Beam Space

Abstract Robust defect detection in the presence of grain noise originating from material microstructures is a challenging yet essential problem in ultrasonic non-destructive evaluation (NDE). In this paper, a novel method is proposed to suppress the gain noise and enhance the defect detection and imaging. The defect echo and grain noise are distinguished through analyzing the spatial location where the echo is originating from. This is achieved by estimation of the angle of arrival (AOA) of the returned echo and evaluation of the likelihood that the echo is reflected from the point where the array is focused or otherwise from the random reflectors like the grain boundaries. The method explicitly addresses the statistical models of the defect echoes and the spatial noise across the array aperture, as well as the correlation between the flaw signal and the interfering echoes; estimates the AOA and the likelihood in a dimension-reduced beam space via a linear transformation; and determines a weighting factor based on the mean likelihood. The factors are then normalized and utilized to correct and weigh the NDE images. Experiments on industrial samples of austenitic stainless steel and INCONEL Alloy 617 are conducted with a 5 MHz transducer array, and the results demonstrate that the grain noise is reduced by about 20 dB while the defect reflection is well retained, thus the great benefits of the method on enhanced defect detection and imaging in ultrasonic NDE are validated.

Design of Two Layer Depth-encoding Detector Module with SiPM for PET

A depth-encoding detector module with silicon photomultipliers(SiPMs) using two layers of scintillation crystal array was designed, and the position measurement capability was verified using DETECT2000. The depth of interaction of the crystal pixels with the gamma rays was tracked through the image acquired with the combination of surface treatment of the crystal pixels and reflectors. The bottom layer was treated as a reflector except for the optically coupled surfaces, and the crystals of top layer were optically coupled each other except for the outer surfaces so that the light sharing was made easier than the bottom layer. Flood images were obtained through the combination of specular reflectors and random reflectors, grounded and polished surfaces of crystal pixels, and the positions at which layer images were generated were measured and analyzed. The images were reconstructed using the Anger algorithm, whose the SiPM signals were reduced as the 16-channels to 4-channels. In the combination of the grounded surface and all reflectors, the depth positions were discriminated into two layers, whereas it was impossible to separate the two layers in the all polished surface combinations. Therefore, using the combination of grounded surface crystal pixels and reflectors could improve the spatial resolution at the outside of the field of view by measuring the depth position in preclinical positron emission tomography.

Linearly Polarized Multi-Wavelength Fiber Laser Comb via Brillouin Random Lasing Oscillation

A linearly polarized multi-wavelength Brillouin laser comb in the 1.5- $\mu \text{m}$ telecom spectral window is established by cascading multiple Brillouin random lasing oscillations in a semi-open polarization maintaining fiber (PMF)-based composite cavity. Distributed Rayleigh scattering offers a broadband random reflector to build up efficient Brillouin lasing resonance along kilometer-long PMFs, yielding four orders of Stokes radiation with high degree of polarization (>99.67%) and polarization extinction ratio of >20 dB. Each Stokes component with kilohertz linewidth as well as over 40-dB optical signal-to-noise ratio has been validated. Compared with single mode fiber-based laser design, the proposed PMF-based laser comb exhibits highly suppressed relative intensity noise, thanks to the immunity to external perturbation and random modes density reduction within polarization-matched Brillouin gain.

Lithographically fabricable, optimized three-dimensional solar cell random structure

The optimized random reflector is highly preferred for solar cells due to its superiority over an un-optimized totally random surface and its potential to exceed the Lambertian limit. There are some obstacles to overcome for realizing optimized random reflectors, including the feasibility from the process viewpoint and the intensive computational demand for large-scale random reflector design. Here a binary random grating is proposed which can be easily fabricated using common lithographic techniques. By using a global optimization algorithm and three-dimensional (3D) EMW simulation, the solar cell structure with 4 × 4 quasi-random binary grating can provide 23% higher integrated absorbance than its periodic grating counterpart and 103.5% higher integrated absorbance than a planar cell, approaching the Lambertian limit. Broad-band transmission improvement at short wavelength and a broad-band waveguiding effect at long wavelength is observed for the optimized 3D geometry. Additionally, the optimized random grating surpasses the periodic grating at all incident angles. The absorbance of the large-scale, fully optimized binary pattern can potentially exceed the Lambertian limit while its computational demand is shown to be manageable.

Deterministic approximations of random reflectors

Within classical optics, one may add microscopic to a macroscopically flat mirror so that parallel rays of a given angle are reflected at different outgoing angles. Taking the limit (as the roughness becomes increasingly microscopic) one obtains a flat surface that reflects randomly, i.e., the transition from incoming to outgoing ray is described by a probability kernel (whose form depends on the nature of the microscopic roughness). We consider two-dimensional optics (a.k.a. billiards) and show that every random reflector on a line that satisfies a necessary measure-preservation condition (well established in the theory of billiards) can be approximated by deterministic reflectors in this way.

Superresolution imaging in ultrasound B-scan imaging

A number of imaging systems exhibit speckle, which is caused by the interaction of a coherent pulse reflecting off of random reflectors. The limitations of these systems are quite serious since the speckle phenomenon creates a pattern of nulls and peaks from subresolvable scatterers or targets that are difficult to interpret. Another limitation of these pulse-echo imaging systems is that their resolution is dependent on the full spatial extent of the propagating pulse, usually several wavelengths in the axial or propagating dimension and typically longer in the transverse direction. This limits the spatial resolution to many multiples of the wavelength. This paper focuses on the particular case of ultrasound B-scan imaging and develop an inverse filter solution that eliminates both the speckle phenomenon, and the poor resolution dependency on the pulse length and width, to produce SURUS (super-resolution ultrasound) images. The key to the inverse filter is the creation of pulse shapes that have stable inverses. This is derived by use of the standard Z-transform and related properties. Although the focus of this paper is on examples from ultrasound imaging systems, the results are applicable to other pulse-echo imaging systems that also can exhibit speckle statistics.

Superresolution imaging of scatterers in ultrasound B-scan imaging

A number of imaging systems exhibit speckle, which is caused by the interaction of a coherent pulse reflecting off of random reflectors. The limitations of these systems are quite serious because the speckle phenomenon creates a pattern of nulls and peaks from subresolvable scatterers or targets that are difficult to interpret. Another limitation of these pulse-echo imaging systems is that their resolution is dependent on the full spatial extent of the propagating pulse, usually several wavelengths in the axial or propagating dimension and typically longer in the transverse direction. This limits the spatial resolution to many multiples of the wavelength. This paper focuses on the particular case of ultrasound B-scan imaging and develops an inverse filter solution that eliminates both the speckle phenomenon and the poor resolution dependency on the pulse length and width to produce super-resolution ultrasound (SURUS) images. The key to the inverse filter is the creation of pulse shapes that have stable inverses. This is derived by use of the standard Z-transform and related properties. Although the focus of this paper is on examples from ultrasound imaging systems, the results are applicable to other pulse-echo imaging systems that also can exhibit speckle statistics.

Ultrasound Nondestructive Evaluation (NDE) Imaging with Transducer Arrays and Adaptive Processing

This paper addresses the challenging problem of ultrasonic non-destructive evaluation (NDE) imaging with adaptive transducer arrays. In NDE applications, most materials like concrete, stainless steel and carbon-reinforced composites used extensively in industries and civil engineering exhibit heterogeneous internal structure. When inspected using ultrasound, the signals from defects are significantly corrupted by the echoes form randomly distributed scatterers, even defects that are much larger than these random reflectors are difficult to detect with the conventional delay-and-sum operation. We propose to apply adaptive beamforming to the received data samples to reduce the interference and clutter noise. Beamforming is to manipulate the array beam pattern by appropriately weighting the per-element delayed data samples prior to summing them. The adaptive weights are computed from the statistical analysis of the data samples. This delay-weight-and-sum process can be explained as applying a lateral spatial filter to the signals across the probe aperture. Simulations show that the clutter noise is reduced by more than 30 dB and the lateral resolution is enhanced simultaneously when adaptive beamforming is applied. In experiments inspecting a steel block with side-drilled holes, good quantitative agreement with simulation results is demonstrated.

15-μm 128×128 GaAs/Al/sub x/Ga/sub 1-x/As quantum well infrared photodetector focal plane array camera

In this paper, we discuss the development of very sensitive, very long wavelength infrared GaAs/Al/sub x/Ga/sub 1-x/As quantum well infrared photodetectors (QWIPs) based on bound-to-quasi-bound intersubband transition, fabrication of random reflectors for efficient light coupling, and the demonstration of a 15-/spl mu/m cutoff 128/spl times/128 focal plane array imaging camera. Excellent imagery, with a noise equivalent differential temperature (NE/spl Delta/T) of 30 mK has been achieved.

Diffusion with random traps: Transient one-dimensional kinetics in a linear potential

The problem of one-dimensional diffusion with random traps is solved without and with a constant field of force. Using an eigenvalue expansion for long times and the method of images for short times we give an exact, straightforward solution for the time dependence of the mean survival probability and the mean probability density for returning to the origin. Using the backward equation approach, we determine the mean survival time and the mean residence time density at the origin. We comment on the relation between these solutions and those for one-dimensional diffusion with random reflectors.

One-dimensional normal-incidence inversion: A solution procedure for band-limited and noisy data

In this paper we present a one-dimensional normal-incidence inversion procedure for reflection seismic data. A lossless layered system is considered which is characterized by reflection coefficients and traveltimes. A priori knowledge for the unknown parameters, in the form of statistics, is incorporated into a nonuniform layered system, and a maximum a posteriori estimation procedure is used for the estimation of the system's unknown parameters (i.e., we assume a random reflector model) from noisy and band-limited data. Our solution to the inverse problem includes a downward continuation procedure for estimation of the states of the system. The state sequences are composed of overlapping wavelets. We show that estimation of the unknown parameters of a layer is equivalent to estimation of the amplitude and detection of the time delay of the first wavelet in the upgoing state sequence of the layer. A suboptimal maximum-likelihood deconvolution procedure is employed to perform estimation and detection. The most desirable features of the proposed algorithm are its layer-recursive structure and its ability to process noisy and band-limited data.

Estimating Reflected Ultrasound Spectra from Quantized Signals

Quantization errors are inevitably introduced whenever analog-to-digital conversion of data is employed. The effect of these errors on estimating power spectra from ultrasound signals reflected from within the body is examined. Reflected data were simulated by convolving a pulse sequence with a filter model of the random reflectors encountered within the liver. The data were then quantized with an N-bit quantizer model, with 1 N 8, multiplied by a Hamming data window and used to estimate the power spectrum. For typical diagnostic signals reflected from 1 cm of tissue, the results indicate that the quantization errors limit the frequency range over which unbiased estimates of the spectra are observed for N 8. For N 8, the sidelobes in the Hamming window spectrum limit this range. To illustrate the implication of these results, the problem of estimating the slope of the acoustic attenuation coefficient, denoted by s, for liver tissue from reflected ultrasound signals is examined. Three sestimators are considered: 1) a new correlation method, 2) a zero-crossing count analysis, and 3) the slope of the log-spectral difference. The simulated results indicate under which conditions a particular sestimator should be employed.

STRUCTURE INDEPENDENT VELOCITY ESTIMATION

Many commonly used velocity estimation procedures assume that the reflectors are horizontal. Because of this, their performance tends to degrade as the reflectors become curved or discontinuous. Much of this degradation can be traced to the fact that data recorded over nonhorizontal reflectors need not resemble in detail the subsurface in the area where they were recorded. Diffraction and scattering are the major complicating factors. Beginning with the scalar wave equation and using a small dip assumption, approximate wave equations which quite accurately model both near‐ and wide‐angle reflections generated by one or more sources can be found. Finite difference formulations of these equations can be used to demonstrate that surface recorded seismic reflections which have been downward continued to the depth of their source reflectors must resemble those reflectors in detail. This property of downward continuation can be exploited to improve velocity estimates by using downward continuation as a preprocessor for velocity estimation techniques. Both synthetic and field data examples show that estimates based on downward continued data do not exhibit diffraction effects and are not dependent upon reflector dip. Synthetic data examples also illustrate that the use of downward continuation allows accurate velocity estimates to be made from no record data recorded over an earth in which the reflectors are random functions of the horizontal and vertical coordinates. For reasonable data parameters, theoretical considerations indicate that the coherence of properly downward continued random reflector data measured along the true velocity hyperbolic should be greater than a similar measure on the corresponding surface data. This coherence increase should make velocity estimates based on downward continued random reflector data less susceptible to noise than estimates based on surface recorded data.

Imperfections in Active Transmission Lines

The effect of discrete imperfections on the behavior of active transmission lines (i.e., lines with distributed gain) is considered. Two cases are studied: 1. Lines with identical, equally spaced reflectors. The transmission and reflection gains versus frequency are studied as functions of the magnitude of the reflectors. Limits on the magnitude of the reflectors to guarantee stability are investigated. 2. Lines with random reflectors, having random position and/or magnitude. The statistics of the transmission are studied; in particular, the average value and the variance and covariance of the transmission are determined for small reflections. If the reflections become large enough, instability may occur, and these calculations may become invalid. Stability of active distributed systems is studied in a companion paper.1

Stability of Active Transmission Lines with Arbitrary Imperfections

Two sufficient conditions for the stability of one-dimensional active transmission lines with arbitrary imperfections (i.e., discrete or continuous reflections) are derived. The first stability condition guarantees stability for any arbitrary distribution of reflection. The second stability condition is restricted to a special case of interest that includes discrete reflectors with nominally equal magnitude and spacing; the stability condition for this restricted class is greatly improved over the general stability condition described above. These results, aside from their own interest, provide rigorous justification for previous calculations for the gain statistics of such a device with random discrete reflectors. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> They may also be used to find an upper bound on the probability of instability of such a device with random reflectors. Certain types of optical maser amplifiers and traveling-wave tubes provide examples of practical devices with distributed gain to which these results, or similar ones, might be applied.

A unified theory of interaction for two arbitrarily mixed multi-group arrays set in well-defined and random reflectors

An interaction theory is given for calculating space arrays of fissile units of arbitrarysize, shape and distribution which uses the same core and reflector constants as are employed in ordinary reactor calculations for single units. Mixed arrays of fast and thermal units are considered. The reflecting material between units may be well defined or random in distribution. A comparison with experiment suggests that the theory is reasonably accurate and errs on the side of safety. Unlike Monte Carlo methods the method presented requires trivial computation (10 seconds per case computing time on a medium-size computer). Unlike the interaction parameter method it does not require an experiment on a simple assembly of units before a prediction for a complicated array can be made.