Nonlinear Detector Research Articles

A Two-Point Modulation Spread-Spectrum Clock Generator With FIR-Embedded Binary Phase Detection and 1-Bit High-Order ΔΣ Modulation

This paper describes a spread-spectrum clock generation method by utilizing a Δ∑ digital PLL (DPLL) which is solely based on binary phase detection and does not require a linear time-to-digital converter (TDC) or other linear digital-to-time converter (DTC) circuitry. A 1-bit high-order Δ∑ modulator and a hybrid finite-impulse response (FIR) filter are employed to mitigate the phase-folding problem caused by the nonlinearity of the bang-bang phase detector (BBPD). The Δ∑ DPLL employs a two-point modulation technique to further enhance linearity at the turning point of a triangular modulation profile. We also show that the two-point modulation is useful for the BBPLL to improve the spread-spectrum performance by suppressing the frequency deviation at the input of the BBPD, thus reducing the peak phase deviation. Based on the proposed architecture, a 3.2 ㎓ spread-spectrum clock generator (SSCG) is implemented in 65 ㎚ CMOS. Experimental results show that the proposed SSCG achieves peak power reductions of 18.5 ㏈ and 11 ㏈ with 10 ㎑ and 100 ㎑ resolution bandwidths respectively, consuming 6.34 ㎽ from a 1 V supply.

Warhead verification as inverse problem: Applications of neutron spectrum unfolding from organic-scintillator measurements

Verification of future warhead-dismantlement treaties will require detection of certain warhead attributes without the disclosure of sensitive design information, and this presents an unusual measurement challenge. Neutron spectroscopy—commonly eschewed as an ill-posed inverse problem—may hold special advantages for warhead verification by virtue of its insensitivity to certain neutron-source parameters like plutonium isotopics. In this article, we investigate the usefulness of unfolded neutron spectra obtained from organic-scintillator data for verifying a particular treaty-relevant warhead attribute: the presence of high-explosive and neutron-reflecting materials. Toward this end, several improvements on current unfolding capabilities are demonstrated: deuterated detectors are shown to have superior response-matrix condition to that of standard hydrogen-base scintintillators; a novel data-discretization scheme is proposed which removes important detector nonlinearities; and a technique is described for re-parameterizing the unfolding problem in order to constrain the parameter space of solutions sought, sidestepping the inverse problem altogether. These improvements are demonstrated with trial measurements and verified using accelerator-based time-of-flight calculation of reference spectra. Then, a demonstration is presented in which the elemental compositions of low-Z neutron-attenuating materials are estimated to within 10%. These techniques could have direct application in verifying the presence of high-explosive materials in a neutron-emitting test item, as well as other for treaty verification challenges.

III–V Semiconductor Nanoresonators—A New Strategy for Passive, Active, and Nonlinear All‐Dielectric Metamaterials

We demonstrate 2D and multilayer dielectric metamaterials made from III–V semiconductors using a monolithic fabrication process. The resulting structures could be used to recompress chirped femtosecond optical pulses and in a variety of other optical applications requiring low loss. Moreover, these III–V all-dielectric metamaterials could enable novel active applications such as efficient nonlinear frequency converters, light emitters, detectors, and modulators.

An automated gas chromatography time-of-flight mass spectrometry instrument for the quantitative analysis of halocarbons in air

Abstract. We present the characterization and application of a new gas chromatography time-of-flight mass spectrometry instrument (GC-TOFMS) for the quantitative analysis of halocarbons in air samples. The setup comprises three fundamental enhancements compared to our earlier work (Hoker et al., 2015): (1) full automation, (2) a mass resolving power R = m/Δm of the TOFMS (Tofwerk AG, Switzerland) increased up to 4000 and (3) a fully accessible data format of the mass spectrometric data. Automation in combination with the accessible data allowed an in-depth characterization of the instrument. Mass accuracy was found to be approximately 5 ppm in mean after automatic recalibration of the mass axis in each measurement. A TOFMS configuration giving R = 3500 was chosen to provide an R-to-sensitivity ratio suitable for our purpose. Calculated detection limits are as low as a few femtograms by means of the accurate mass information. The precision for substance quantification was 0.15 % at the best for an individual measurement and in general mainly determined by the signal-to-noise ratio of the chromatographic peak. Detector non-linearity was found to be insignificant up to a mixing ratio of roughly 150 ppt at 0.5 L sampled volume. At higher concentrations, non-linearities of a few percent were observed (precision level: 0.2 %) but could be attributed to a potential source within the detection system. A straightforward correction for those non-linearities was applied in data processing, again by exploiting the accurate mass information. Based on the overall characterization results, the GC-TOFMS instrument was found to be very well suited for the task of quantitative halocarbon trace gas observation and a big step forward compared to scanning, quadrupole MS with low mass resolving power and a TOFMS technique reported to be non-linear and restricted by a small dynamical range.

Algorithms for Identification of Nearly-Coincident Events in Calorimetric Sensors

For experiments with high arrival rates, reliable identification of nearly-coincident events can be crucial. For calorimetric measurements to directly measure the neutrino mass such as HOLMES, unidentified pulse pile-ups are expected to be a leading source of experimental error. Although Wiener filtering can be used to recognize pile-up, it suffers errors due to pulse-shape variation from detector nonlinearity, readout dependence on sub-sample arrival times, and stability issues from the ill-posed deconvolution problem of recovering Dirac delta-functions from smooth data. Due to these factors, we have developed a processing method that exploits singular value decomposition to (1) separate single-pulse records from piled-up records in training data and (2) construct a model of single-pulse records that accounts for varying pulse shape with amplitude, arrival time, and baseline level, suitable for detecting nearly-coincident events. We show that the resulting processing advances can reduce the required performance specifications of the detectors and readout system or, equivalently, enable larger sensor arrays and better constraints on the neutrino mass.

A review: detection techniques for LTE system

Long term evolution (LTE) has become the fastest developing mobile system technology and is considered as the fourth generation of wireless communication systems. Multiple-input multiple-output spatial multiplexing is a key technique employed in LTE system to boost the system capacity. With this gain comes the challenge of designing efficient detectors at the receiver side to decouple the transmitted data streams. The exhaustive search detector is optimum in terms of performance but it has a high computational load in terms of time and power. Owing to its high computational load, many detection techniques have been proposed in order to achieve an efficient detection performance with low complexity in terms of computational power. This paper reviews the detection techniques used in LTE including the maximum likelihood (ML) detection, zero forcing (ZF) detection, minimum mean square error (MMSE) detection, successive interference cancellation (SIC) detection, sphere decoding detection, lattice reduction and list decoding. It is shown that Maximum Likelihood detection has the best performance, but the complexity is too high. Other detectors like ZF or MMSE and nonlinear detectors employing SIC have much lower complexity, however, they do not provide detection performance close to that of the ML detection. The sphere detection algorithm has been shown to have low average computational complexity and achieves a quasi-ML performance which gives it an advantage over to other types of detection techniques.

Impact of Improved Calibration of a NEO HySpex VNIR-1600 Sensor on Remote Sensing of Water Depth

This paper investigates at the example of bathymetry how much an application can profit from comprehensive characterizations required for an improved calibration of data from a state-of-the-art commercial hyperspectral sensor. A NEO HySpex VNIR-1600 sensor is used for this paper, and the improvements are based on measurements of sensor properties not covered by the manufacturer, in particular, detector nonlinearity and stray light. This additional knowledge about the instrument is used to implement corrections for nonlinearity, stray light, spectral smile distortion and nonuniform spectral bandwidth and to base the radiometric calibration on a SI-traceable radiance standard. Bathymetry is retrieved from a data take from the lake Starnberg using WASI-2D. The results using the original and improved calibration procedures are compared with ground reference data, with an emphasis on the effect of stray-light correction. For our instrument, stray-light biases the detector response from 416–500 nm up to 8% and from 700–760 nm up to 5%. Stray-light-induced errors affect bathymetry mainly in water deeper than Secchi depth, whereas in shallower water, the dominant error source is the calibration accuracy of the light source used for radiometric calibration. Stray-light correction reduced the systematic error of water depth by 19% from Secchi depth to three times Secchi depth, whereas the relative standard deviation remained stable at 5%.

Mammalian cochlea as a physics guided evolution-optimized hearing sensor.

Nonlinear physics plays an essential role in hearing. We demonstrate on a mesoscopic description level that during the evolutionary perfection of the hearing sensor, nonlinear physics led to the unique design of the cochlea observed in mammals, and that this design requests as a consequence the perception of pitch. Our insight challenges the view that mostly genetics is responsible for the uniformity of the construction of the mammalian hearing sensor. Our analysis also suggests that scaleable and non-scaleable arrangements of nonlinear sound detectors may be at the origin of the differences between hearing sensors in amniotic lineages.

Development, validation and comparison of two stability-indicating RP-LC methods using charged aerosol and UV detectors for analysis of lisdexamfetamine dimesylate in capsules

Two new stability-indicating liquid chromatographic methods using two detectors, an ultraviolet (UV) and a charged aerosol detector (CAD) simultaneously connected in series were validated for the assessment of lisdexamfetamine dimesylate (LDX) in capsule. The method was optimized and the influence of individual parameters on UV and CAD response and sensitivity was studied. Chromatography was performed on a Zorbax CN column (250mm×4.6mm, 5μm) in an isocratic elution mode, using acetonitrile and 20mM ammonium formate at pH 4.0 (50:50, v/v) as mobile phase and UV detection at 207nm. The developed method was validated according to ICH guidelines and the parameters’ specificity, limit of detection, limit of quantitation, linearity, accuracy, precision and robustness were evaluated. CAD is designated to be a non-linear detector in a wide dynamic range, however, the method was linear over the concentration range of 70–130μgmL−1 in both detectors. The method was precise and accurate. Robustness study was performed by a Plackett–Burman design, delivering results within the acceptable range. Neither the excipients nor the degradation products showed interference in the method after studies of specificity as well as under stress conditions. The results of the LC-UV and LC-CAD methods were statistically compared through ANOVA and showed no significant difference (p>0.05). Both proposed methods could be considered interchangeable and stability-indicating, and can be applied as an appropriate quality control tool for routine analysis of LDX in capsule.

Performance evaluation of the time delay digital tanlock loop architectures

This article presents the architectures, theoretical analyses and testing results of modified time delay digital tanlock loop (TDTLs) system. The modifications to the original TDTL architecture were introduced to overcome some of the limitations of the original TDTL and to enhance the overall performance of the particular systems. The limitations addressed in this article include the non-linearity of the phase detector, the restricted width of the locking range and the overall system acquisition speed. Each of the modified architectures was tested by subjecting the system to sudden positive and negative frequency steps and comparing its response with that of the original TDTL. In addition, the performance of all the architectures was evaluated under noise-free as well as noisy environments. The extensive simulation results using MATLAB/SIMULINK demonstrate that the new architectures overcome the limitations they addressed and the overall results confirmed significant improvements in performance compared to the conventional TDTL system.

Independent Laboratory Characterization of NEO HySpex Imaging Spectrometers VNIR-1600 and SWIR-320m-e

The Remote Sensing Technology Institute (Institut fur Methodik der Fernerkundung) of the German Aerospace Agency (DLR) operates two sensors for airborne hyperspectral imaging, i.e., a Norsk Elektro Optikk A/S (NEO) HySpex VNIR-1600 and a NEO HySpex SWIR-320m-e. Since these sensors are used for the development of physically based inversion algorithms, atmospheric correction algorithms and for calibration/ validation activities, their properties need to be characterized in detail, and an accurate calibration is mandatory. The characterization is performed at the calibration laboratory of DLR for imaging spectrometers in Oberpfaffenhofen. Key results of the characterization are assessments of the radiometric, spectral, and geometric performances, including the typical optical distortions prevalent in pushbroom imaging spectrometers, keystone and smile, and the associated measurement uncertainties. Potential sources of systematic error, the detector nonlinearity and the polarization sensitivity are discussed. The radiometric calibration is traceably performed to the German national metrology institute Physikalisch-Technische Bundesanstalt, whereas the spectral measurements can be traced back to the spectral properties of atomic line lamps. The implemented level 0 to level 1 calibration procedure is presented as well.

Performance analysis of turbo-coded MIMO–OFDM system for underwater communication

In this article, we investigate the performance of a coded multiple-input multiple-output (MIMO) multi-carrier (MC) system in underwater communication, where acoustic interference and ambient noise are the two major channel impairments. The channel model considered in this work is based on the shallow water model that has eleven paths. In our work, the acoustic signal is spread with the aid of a high rate spreading sequence to alleviate the effects of acoustic interference. At the receiver, non-linear detectors based on Zero-Forcing (ZF) and Minimum-Mean-Square-Error (MMSE) are employed for signal detection. The simulation results reveal that the system under consideration with the MMSE detector provides better performance in terms of the achievable bit error rate than the system with the ZF detector while achieving higher data rates.

Calibration of single-particle inductively coupled plasma-mass spectrometry (SP-ICP-MS)

Incomplete vaporization and non-linear detector response in pulse counting mode cause non-linearity in single-particle ICP-MS.

CHARACTERIZATION OF THE ATMOSPHERE OF THE HOT JUPITER HAT-P-32Ab AND THE M-DWARF COMPANION HAT-P-32B

We report secondary eclipse photometry of the hot Jupiter HAT-P-32Ab, taken with Hale/WIRC in H and Ks bands and with Spitzer/IRAC at 3.6 and 4.5 micron. We carried out adaptive optics imaging of the planet host star HAT-P-32A and its companion HAT-P-32B in the near-IR and the visible. We clearly resolve the two stars from each other and find a separation of 2.923" +/- 0. 004" and a position angle 110.64 deg +/- 0.12 deg. We measure the flux ratios of the binary in g' r' i' z' and H & Ks bands, and determine Teff = 3565 +/- 82 K for the companion star, corresponding to an M1.5 dwarf. We use PHOENIX stellar atmosphere models to correct the dilution of the secondary eclipse depths of the hot Jupiter due to the presence of the M1.5 companion. We also improve the secondary eclipse photometry by accounting for the non-classical, flux-dependent nonlinearity of the WIRC IR detector in the H band. We measure planet-to-star flux ratios of 0.090 +/- 0.033%, 0.178 +/- 0.057%, 0.364 +/- 0.016%, and 0.438 +/- 0.020% in the H, Ks, 3.6 and 4.5 micron bands, respectively. We compare these with planetary atmospheric models, and find they prefer an atmosphere with a temperature inversion and inefficient heat redistribution. However, we also find that the data are equally well-described by a blackbody model for the planet with Tp = 2042 +/- 50 K. Finally, we measure a secondary eclipse timing offset of 0.3 +/- 1.3 min from the predicted mid-eclipse time, which constrains e = 0.0072 +0.0700/-0.0064 when combined with RV data and is more consistent with a circular orbit.

Breast tissue decomposition with spectral distortion correction: a postmortem study.

To investigate the feasibility of an accurate measurement of water, lipid, and protein composition of breast tissue using a photon-counting spectral computed tomography (CT) with spectral distortion corrections. Thirty-eight postmortem breasts were imaged with a cadmium-zinc-telluride-based photon-counting spectral CT system at 100 kV. The energy-resolving capability of the photon-counting detector was used to separate photons into low and high energy bins with a splitting energy of 42 keV. The estimated mean glandular dose for each breast ranged from 1.8 to 2.2 mGy. Two spectral distortion correction techniques were implemented, respectively, on the raw images to correct the nonlinear detector response due to pulse pileup and charge-sharing artifacts. Dual energy decomposition was then used to characterize each breast in terms of water, lipid, and protein content. In the meantime, the breasts were chemically decomposed into their respective water, lipid, and protein components to provide a gold standard for comparison with dual energy decomposition results. The accuracy of the tissue compositional measurement with spectral CT was determined by comparing to the reference standard from chemical analysis. The averaged root-mean-square error in percentage composition was reduced from 15.5% to 2.8% after spectral distortion corrections. The results indicate that spectral CT can be used to quantify the water, lipid, and protein content in breast tissue. The accuracy of the compositional analysis depends on the applied spectral distortion correction technique.

A Compact Combinatorial Device for Measurement of Nonlinearity of Radiation Detectors

A new compact computer-controlled device using a combinatorial technique for measuring the nonlinearity of radiation detectors is described. The device consists of two sets of four beam-splitter cubes optically cemented together and arranged so that radiation from a single source is split into four separate paths, then recombined after passing through one of five neutral density filters placed in each path. This allows for the measurement of 625 approximately equi-spaced inter-related flux levels based on only 16 unknown transmittance values. These can be solved for by least-squares fitting, leaving 609 degrees of freedom remaining to determine the nonlinearity of the detector. A novel aspect of the design is the use of neutral density glass plates optically cemented along all the external faces of the beam-splitter cubes, which act as beam dumps for any reflected or scattered radiation. The cube faces in the desired beam paths have clear glass plates with an anti-reflection coating applied at the wavelength of interest optically cemented to them. Operation at other wavelengths is achieved by simply replacing these plates with plates coated for the new wavelength. The performance of the device has been tested using a silicon photodiode with a collimated 650 nm LED as the source. The results demonstrate that the device is able to measure linearity to better than 1 part in \(10^{4}\).

Low-noise mid-IR upconversion detector for improved IR-degenerate four-wave mixing gas sensing.

We compare a nonlinear upconversion detector with a conventional cryogenic InSb detector for the detection of coherent infrared light showing near-shot-noise-limited performance in the upconversion system. The InSb detector is limited by dark noise, which results in a 500 times lower signal-to-noise ratio. The two detectors are compared for the detection of a coherent degenerate four-wave mixing (DFWM) signal in the mid-infrared, and applied to measure trace-level acetylene in a gas flow at atmospheric pressure, probing its fundamental rovibrational transitions. In addition to lower noise, the upconversion system provides image information of the signal, thus adding new functionality compared to standard point detection methods. We further show that the upconversion detector system can be implemented as a simple replacement of the cryogenic detector.

Two-photon quantum well infrared photodetectors below 6 THz

Two-photon quantum well infrared photodetectors (QWIPs) are nonlinear detectors for the mid-infrared and terahertz regimes optimized for resonant two-photon absorption. Here we present first results on two-photon QWIP samples based on the GaAs/AlGaAs material system with intersubband energies between 25 and 12meV (6–3THz) confirmed by photocurrent spectra. The dark current showed large discontinuities, presumably caused by impact ionization. We performed interferometric autocorrelation experiments at the free-electron laser FELBE and observed nonlinear interferograms for all samples.

Fourier analysis of the imaging characteristics of a CMOS active pixel detector for mammography by using a linearization method

Active pixel design using the complementary metal-oxide-semiconductor (CMOS) process is a compelling solution for use in X-ray imaging detectors because of its excellent electronic noise characteristics. We have investigated the imaging performance of a CMOS active pixel photodiode array coupled to a granular phosphor through a fiber-optic faceplate for mammographic applications. The imaging performance included the modulation-transfer function (MTF), noise-power spectrum (NPS), and detective quantum efficiency (DQE). Because we observed a nonlinear detector response at low exposures, we used the linearization method for the analysis of the DQE. The linearization method uses the images obtained at detector input, which are converted from those obtained at detector output by using the inverse of the detector response. Compared to the conventional method, the linearization method provided almost the same MTF and a slightly lower normalized NPS. However, the difference between the DQE results obtained by using the two methods was significant. We claim that the conventional DQE analysis of a detector having a nonlinear response characteristic can yield wrong results. Under the standard mammographic imaging condition, we obtained a DQE performance that was competitive with the performances of conventional flat-panel mammography detectors. We believe that the CMOS detector investigated in this study can be successfully used for mammography.

Far-infrared spectroscopy of the troposphere: calibration with a cold background.

The far-infrared spectroscopy of the troposphere (FIRST) instrument is a Fourier-transform spectrometer developed to measure the Earth's thermal emission spectrum with a particular emphasis on the far-infrared. FIRST has observed the atmosphere from both the ground looking up and from a high-altitude balloon looking down. A recent absolute laboratory calibration of FIRST under ground-like operating conditions showed accuracy to better than 0.3K at near-ambient temperatures (270-325K) but reduced accuracy at lower temperatures. This paper presents calibration results for balloon-flight conditions using a cold blackbody to simulate the space view used for on-board calibration. An unusual detector nonlinearity was discovered and corrected, and stray light was measured and removed. Over most of the range of Earth scene temperatures (205-300K), the accuracy of FIRST is 0.35-0.15K (one sigma).