Detection Bandwidth Research Articles

Monitoring beam charge during FLASH irradiations

In recent years, FLASH irradiation has attracted significant interest in radiation research. Studies have shown that irradiation at ultra-high dose rates (FLASH) reduces the severity of toxicities in normal tissues compared to irradiation at conventional dose rates (CONV), as currently used in clinical practice. Most pre-clinical work is currently carried out using charged particle beams and the beam charge monitor described here is relevant to such beams. Any biological effect comparisons between FLASH and CONV irradiations rely on measurement of tissue dose. While well-established approaches can be used to monitor, in real time, the dose delivered during CONV irradiations, monitoring FLASH doses is not so straightforward. Recently the use of non-intercepting beam current transformers (BCTs) has been proposed for FLASH work. Such BCTs have been used for decades in numerous accelerator installations to monitor temporal and intensity beam profiles. In order to serve as monitoring dosimeters, the BCT output current must be integrated, using electronic circuitry or using software integration following signal digitisation. While sensitive enough for FLASH irradiation, where few intense pulses deliver the requisite dose, the inherent insensitivity of BCTs and the need for a wide detection bandwidth makes them less suitable for use during CONV “reference” irradiations. The purpose of this article is to remind the FLASH community of a different mode of BCT operation: direct monitoring of charge, rather than current, achieved by loading the BCT capacitively rather than resistively. The resulting resonant operation achieves very high sensitivities, enabling straightforward monitoring of output during both CONV and FLASH regimes. Historically, such inductive charge monitors have been used for single pulse work; however, a straightforward circuit modification allows selective resonance damping when repetitive pulsing is used, as during FLASH and CONV irradiations. Practical means of achieving this are presented, as are construction and signal processing details. Finally, results are presented showing the beneficial behaviour of the BCT versus an (Advanced Markus) ionisation chamber for measurements over a dose rate range, from <0.1 Gys−1 to >3 kGys−1.

Design of Light Emitting Diode-Based Fiber Optic Current Sensor and Its Application in Pollution Flash Prevention in Digitalized Substations

Prevention of pollution flash in electrical equipment in digitalized substations and transmission lines has always been a crucial part of power operation. In order to prevent pollution flash, it is essential to detect the contamination of electrical equipment. The measurement of leakage current is a fundamental aspect of preventive pollution flash work in electrical equipment. Applying fiber optic current sensors to online monitoring systems in the power system offers advantages such as resistance to harsh environments, immunity to electromagnetic interference, and resistance to chemical corrosion. This study designs a unidirectional half-wave fiber optic current sensor based on Light Emitting Diodes (LED) for selectively observing the positive and negative half-cycles of leakage current. Due to the light emission and unidirectional conduction characteristics of LED, with a well-designed circuit, LED performs photoelectric conversion on the current, allowing the sensor to selectively measure the positive and negative half-cycles of leakage current. Subsequently, a combination circuit of silicon detectors and amplifiers is used to complete the photoelectric conversion, facilitating voltage signal measurement. In experiments, the response of the unidirectional half-wave LED fiber optic current sensor reached 37.8 mV/mA, with a detection bandwidth range between [1.2 kHz, 9.8 kHz]. In leakage current tests, using the leakage current of insulators as a reference, experiments were conducted using the voltage boosting method. As the applied voltage increased, the leakage current displayed on the sampling resistor increased, and the measurement waveform indicated that the sensor could correctly select the unidirectional half-wave. In leakage current and salt density tests, the designed fiber optic sensor accurately reflected the exponential relationship between insulator leakage current and contamination, demonstrating the practical value of the designed fiber optic current sensor.

A Wide Frequency Response Fabry–Pérot Acoustic Sensor Based on the Self-Stabilization System

A self-stabilization system based on the operating point is built for measuring the membrane-free optical acoustic sensor constructed of a rigid Fabry-Pérot (F-P) cavity. The system can directly lock and track the quadrature point of the sensor’s spectrum to realize broadband acoustic detection with high sensitivity. Compared with the demodulation system based on phase modulation spectrum technology, the system is simple in construction and has the advantage of fast real-time demodulation rate and wide detection bandwidth. The experimental results show the acoustic sensor achieves a wide frequency response of 700 Hz-411 kHz with a flatness of ±1.78 dB, and a sensitivity of 31.83 mV/Pa, while the maximum detectable sound pressure reaches 136.9 dB. The sensor performs well in broadening the frequency response range, which includes a medium and high frequency range from 700 Hz to 20 kHz for noise monitoring, as well as an ultrasound range from 20 to 411 kHz for aeroacoustics and photoacoustic imaging. The improvement of the frequency band characteristics of the sensor in this research will further contribute to the development of application fields.

Bi-Functional High-Speed and Ultrabroad Bandwidth Detector

Bi-Functional High-Speed and Ultrabroad Bandwidth Detector

Parallel CV-QRNG with Strict Entropy Evaluation

Continuous-variable quantum random number generators (CV-QRNGs) have promising application prospects thanks to their advantages such as high detection bandwidth, robustness of system, and integratability. In major CV-QRNGs, the generation of random numbers is based on homodyne detection and discretization of the quadrature fluctuations of the EM fields. Any defectiveness in physical realization may leak information correlated with the generated numbers and the maximal amount of randomness that can be extracted in presence of such side-information is evaluated by the so-called quantum conditional min-entropy. The parallel CV-QRNG overcomes the rate bottleneck of the previous serial type scheme. As a type of device-trusted QRNG, its security needs to be better guaranteed based on self-testing or monitoring that can be rigorously enforced. In this work, four sideband modes of vacuum state within 1.6 GHz detection bandwidth were extracted parallelly as the entropy source, and 16-bit analog-to-digital conversion in each channel was realized. Without making any ideal assumptions, the transfer function of the homodyne and quantization system was measured based on beat method to calibrate the evaluation of the min-entropy. Based on the rigorous entropy evaluation with a hash security parameter of εhash = 2−110, a real-time generation rate of 7.25 Gbps was finally achieved.

Nonlinear confocal positioner for micron-scale target alignment.

This paper presents a novel target positioner system that exhibits high sensitivity and accuracy. Specifically, the system is capable of precisely locating rough target surfaces within a micron-scale in the focal plane. The high sensitivity comes from the nonlinear detection scheme which uses the two-photon-absorption process in a Si-photodiode and a CMOS sensor at 1550 [nm]. The setup employs a confocal configuration that is easy to align and does not require a conjugated focal plane selective aperture (pinhole), thus demonstrating its feasibility and tilt tolerance of the target. Moreover, the system offers high accuracy up to 5 [μm], which corresponds to the step size of the focus scanning. The presented positioner system has potential applications in microfabrication with lasers and laser-driven plasma accelerators even at high repetition rates, limited by the detection bandwidth of the photodiode. Additionally, the principle can be extended to cameras if spatial information is needed and the system design can be extended to other spectral ranges with minimal changes.

Demonstration of Turbulence-Resilient Self-Homodyne 12-Gbit/s 16-QAM Free-Space Optical Communications Using a Transmitted Pilot Tone

Free-space optical (FSO) communication links can benefit from recovering both amplitude- and phase-encoded data (e.g., quadrature-amplitude-modulation, QAM) by mixing a local oscillator (LO) with a received data beam in a coherent detector. However, atmospheric turbulence can cause power coupling from a fundamental Gaussian mode of the data beam to higher-order modes, resulting in a significantly degraded mixing efficiency between the data and a fundamental Gaussian LO. Previously, a turbulence-resilient pilot-assisted self-coherent FSO QAM link has been demonstrated by transmitting a frequency-offset Gaussian pilot beam along with the Gaussian data beam. The pilot and data beams experienced similar modal coupling and thus can efficiently mix in a self-heterodyne detector. However, a frequency guard band was used to avoid signal-signal beating interference, thereby reducing the utilization efficiency of the detector's bandwidth. To increase the detector's bandwidth utilization, we experimentally demonstrate in this paper a turbulence-resilient 12-Gbit/s 16-QAM FSO link using pilot-assisted self-homodyne detection. The pilot beam is located at the same center optical frequency as the data channel but on an orthogonal polarization. At the receiver, we utilize self-homodyne detection to achieve the efficient mixing of the data and pilot. Compared to self-heterodyne, our approach has ∼2X the utilization efficiency of the detector's bandwidth. Compared to conventional LO-based coherent (homodyne or intradyne) detection, the pilot-assisted self-homodyne link shows up to ∼20-dB improvement of optical-to-electrical mixing efficiency under a turbulence strength of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2{{\bm{w}}}_0/{\bm{\ }}{{\bm{r}}}_0 = \ \sim 7.5$</tex-math></inline-formula> . The turbulence resilience of the link is demonstrated with a bit-error rate below the 7% forward error correction limit for 400 different turbulence realizations.

Time Expansion in Distributed Optical Fiber Sensing

Distributed optical fiber sensing (DOFS) technology has recently experienced an impressive growth in various fields including security, structural monitoring and seismology, among others. This expansion has been accompanied by a speedy development of the technology in the last couple of decades, reaching remarkable performance in terms of sensitivity, range, number of independent sensing points and affordable cost per monitored point as compared with competing technologies such as electrical or point optical sensors. Phase-sensitive Optical Time-Domain Reflectometry (ϕOTDR) is a particularly interesting DOFS technique, since it enables real-time monitoring of dynamic variations of physical parameters over a large number of sensing points. Compared to their frequency-domain counterparts (OFDR), ϕOTDR sensors typically provide higher dynamics and longer ranges but significantly worse spatial resolutions. Very recently, a novel ϕOTDR approach has been introduced, which covers an existing gap between the long range and fast response of ϕOTDR and the high spatial resolution of OFDR. This technique, termed time-expanded (TE) ϕOTDR, exploits an interferometric scheme that employs two mutually coherent optical frequency combs. In TE-ϕOTDR, a probe comb is launched into the fiber under test. The beating of the backscattered light and a suitable LO comb produces a multi-heterodyne detection process that compresses the spectrum of the probe comb, in turn expanding the detected optical traces in the time-domain. This approach has allowed sensing using ϕOTDR technology with very high resolution (in the cm scale), while requiring outstandingly low detection and acquisition bandwidths (sub-MHz). In this work, we review the fundamentals of TE-ϕOTDR technology and describe the recent developments, focusing on the attainable sensing performance, the existing trade-offs and open working lines of this novel sensing approach.

Artificial Bee Colony Based Iterative Heuristic Signal Detection Technique for Optical Wireless Communications

In this paper, we consider a problem of optical signal detection and present a novel artificial bee colony (ABC) based heuristic approach for optical spectrum sensing in a free-space optical wireless network that overcomes channel impairment due to noise uncertainty. In particular, we propose a generic ABC-based optimization framework in conjunction with eigenvalue-based detection (EVD). The uniqueness of the proposed algorithm is that it is executed iteratively by comparing various local solutions until an optimum solution is obtained. The channel is characterized as correlated multivariate fading and the received signal is quantified as a continuous waveform wideband detector. We derive the joint statistic of the correlated multivariate channel fading in the multiple-input multiple-output (MIMO) optical network. Subsequently, the marginal probability density function of the channel coefficients is obtained. For comparison purposes with existing optical spectrum sensing techniques, noise uncertainty is considered. It is demonstrated from simulations that the proposed ABC-EVD spectrum sensing yields performance improvement and the analytical results derived are verified. From the comparative study, it is also found that the proposed scheme outperforms existing optical spectrum sensing techniques, particularly in a regime of low signal-to-noise ratio. When compared to EVD, it is shown that, at a received signal-to-noise ratio of -15 dB, the performance gain achieved in terms of the probability of detection is 15% higher over the entire range of target probability of false alarm. When compared with the ED technique, the proposed technique outperforms it with approximately 50% higher probability of detection at the noise uncertainty of 1 dB and received SNR of -10 dB. The proposed algorithm offers fast convergence, strong robustness, and high flexibility.

Unlocking the Power of Nanopores: Recent Advances in Biosensing Applications and Analog Front-End.

The biomedical field has always fostered innovation and the development of various new technologies. Beginning in the last century, demand for picoampere-level current detection in biomedicine has increased, leading to continuous breakthroughs in biosensor technology. Among emerging biomedical sensing technologies, nanopore sensing has shown great potential. This paper reviews nanopore sensing applications, such as chiral molecules, DNA sequencing, and protein sequencing. However, the ionic current for different molecules differs significantly, and the detection bandwidths vary as well. Therefore, this article focuses on current sensing circuits, and introduces the latest design schemes and circuit structures of different feedback components of transimpedance amplifiers mainly used in nanopore DNA sequencing.

Exploring nonlinearity to enhance the sensitivity and bandwidth performance of a novel tapered beam micro-gyroscope

In this paper, nonlinearity is used to enhance the sensitivity and bandwidth performance of tapered beam micro-gyroscopes. Considering the electrostatic force nonlinearity and fringing field effects, the governing equations of system are derived by Euler angular transformation and Hamilton principle. The influences of parameters such as the shape factors and DC voltage on static deflection, pull-in voltage and natural frequency of tapered beam micro-gyroscope are analyzed by the Adomian decomposition method (ADM). Galerkin discretization is performed on the governing equations of the tapered beam micro-gyroscope. Frequency-response equation of the system is deduced and solved by applying the method of multi-scale (MMS). The convergence analysis of the frequency response curve is performed, and then verified by the Runge-Kutta method. Finally, the performance of the tapered beam micro-gyroscope is analyzed in terms of bandwidth, sensitivity, and calibration curve nonlinearity under the different shape factors. The maximum amplitude of response in the driving and sensing directions increases significantly with the change of shape factors of the tapered beam in the dynamics analysis. Compared with the straight beam micro-gyroscope, the tapered beam micro-gyroscope has the higher amplitude and pronounced nonlinear characteristics. Due to the electrostatic force nonlinearity, the amplitude of motion in the driving direction has a high response over a wide frequency band, while also effectively increasing the detection bandwidth. In addition, the nonlinear micro-gyroscope has less loss of sensitivity performance during damping fluctuations. Therefore, in view of the contradiction between bandwidth and sensitivity of the traditional micro-gyroscope, this paper proposes a solution from the perspective of structural design and nonlinear characteristics of the tapered beam micro-gyroscope.

Photonic Signal Processing in Phase-Coded Lidar System

The next generation of lidar systems needs to adapt to variable environments with broadened bandwidth for increased resolution. Due to their digital components, conventional lidar systems, especially imaging lidar systems, suffer from limited detector bandwidth and sampling frequency. However, photonics devices can provide a reliable technical solution with high precision and ultra-broad bandwidth. This paper presents a photonic signal processing structure for a phase-coded lidar system. Two acousto-optic modulators (AOMs) are adopted in the proposed architecture. One is used for phase-coded laser signal modulation, and the other is used for demodulation. The echo laser signal is directed to the AOM performing demodulation before the sampling of the detector, accomplishing the multiplication of the echo laser signal and the electric reference signal. The detector is controlled to accumulate the demodulated laser signal. The AOM and detector transfer the correlation calculation from electrical signals processing to photonic signals processing. This photonics-based structure greatly decreases the sampling frequency of the detector without extending the width of the laser pulses, which achieves high resolution with low sampling speed. Photonic signal processing has the promising potential of simultaneously processing signals of multiple pixels. It is going to be an effective solution for imaging lidar systems to increase resolution with available low-cost devices.

An injection-locking diode laser at 671 nm with a wide tuning range up to 6 GHz

We present a compact injection-locking diode laser module to generate 671 nm laser light with a high output power up to 150 mW. The module adopts a master–slave injection-locking scheme, and the injection-locking state is monitored using the transmission spectrum from a Fabry–Pérot interferometer. Beat frequency spectrum measurement shows that the injection-locked slave laser has no other frequency components within the 150-MHz detection bandwidth. It is found that without additional electronic feedback, the slave laser can follow the master laser over a wide range of 6 GHz. All the elements of the module are commercially available, which favors fast construction of a complete 671-nm laser system for the preparation of cold 6Li atoms with only one research-grade diode laser as the seeding source.

Microscopic single-pixel polarimetry for biological tissue

Single-pixel imaging (SPI) is a computational imaging modality that reconstructs images from information collected with a single-pixel detector. Due to the relatively large bandwidth and cheap price of single-pixel detectors over pixel-array detectors, SPI has been demonstrated with great success in many imaging scenarios. In this work, we further extended SPI to microscopic Mueller-matrix polarimetry, which is capable of providing polarization-sensitive properties of biological tissue with a millimeter-size field of view and micrometer-size resolution. As a proof of concept, we applied polarimetry on normal and cancerous esophagus tissues from patients and reconstructed their Mueller matrices. Effects of depolarization, retardation, and diattenuation were also examined by decomposing Mueller matrices. We envision that the developed computational technique can bring new possibilities for both SPI and Mueller-matrix polarimetry.

Fisheye piezo polymer detector for scanning optoacoustic angiography of experimental neoplasms

A number of optoacoustic (or photoacoustic) microscopy and mesoscopy techniques have successfully been employed for non-invasive tumor angiography. However, accurate rendering of tortuous and multidirectional neoplastic vessels is commonly hindered by the limited aperture size, narrow bandwidth and insufficient angular coverage of commercially available ultrasound transducers. We exploited the excellent flexibility and elasticity of a piezo polymer (PVDF) material to devise a fisheye-shape ultrasound detector with a high numerical aperture of 0.9, wide 1–30 MHz detection bandwidth and 27 mm diameter aperture suitable for imaging tumors of various size. We show theoretically and experimentally that the wide detector’s view-angle and bandwidth are paramount for achieving a detailed visualization of the intricate arbitrarily-oriented neovasculature in experimental tumors. The developed approach is shown to be well adapted to the tasks of experimental oncology thus allows to better exploit the angiographic potential of optoacoustics.

Improving the ranging performance of chaos LiDAR.

Chaos lidar has gained significant attention due to its high spatial resolution, natural anti-interference capability, and confidentiality. However, constrained by the power of the chaos laser, the sensitivity of the linear detector, and the hardware bandwidth, chaos lidar is greatly restricted in the application of long-distance target detection and imaging. To overcome these constraints, we propose a novel, to the best of our knowledge, chaos lidar based on Geiger mode avalanched photodetectors (GM-APDs) in a previous study called chaos single-photon (CSP) lidar. In this paper, we compare the CSP lidar with the linear mode chaos lidars by combining with lidar equation. Regarding the ranging principle, the CSP lidar is fully digital and breaks through the constraints of a detector's bandwidth and ADC's sampling rate. The simulation results indicate that the detection range of the CSP lidar is approximately 35 times and 8 times greater than that of a continuous-wave chaos lidar and pulsed chaos lidar, respectively. Although the detection accuracy of the CSP lidar is only at the centimeter level and is lower than the linear mode chaos lidars, its consumption of storage resources and power is greatly reduced due to 1-bit quantization in the GM-APD. Additionally, we investigate the impact of GM-APD parameters on the signal-to-noise ratio (SNR) of the CSP lidar system and demonstrate that the dead time difference between GM-APDs has a negligible effect. In conclusion, we present and demonstrate a new chaos lidar system with a large detection range, high SNR, low storage resources and power consumption, and on-chip capability.

Relic gravitons and high-frequency detectors

Cosmic gravitons are expected in the MHz–GHz regions that are currently unreachable by the operating wide-band interferometers and where various classes of electromechanical detectors have been proposed through the years. The minimal chirp amplitude detectable by these instruments is often set on the basis of the sensitivities reachable by the detectors currently operating in the audio band. By combining the observations of the pulsar timing arrays, the limits from wide-band detectors and the other phenomenological bounds we show that this requirement is far too generous and even misleading since the actual detection of relic gravitons well above the kHz would demand chirp and spectral amplitudes that are ten or even fifteen orders of magnitude smaller than the ones currently achievable in the audio band, for the same classes of stochastic sources. We then examine more closely the potential high-frequency signals and show that the sensitivity in the chirp and spectral amplitudes must be even smaller than the ones suggested by the direct and indirect constraints on the cosmic gravitons. We finally analyze the high-frequency detectors in the framework of Hanbury-Brown Twiss interferometry and argue that they are actually more essential than the ones operating in the audio band (i.e. between few Hz and few kHz) if we want to investigate the quantumness of the relic gravitons and their associated second-order correlation effects. We suggest, in particular, how the statistical properties of thermal and non-thermal gravitons can be distinguished by studying the corresponding second-order interference effects.

Ultrabright polarization-entangled photon pair source for frequency-multiplexed quantum communication in free-space.

The distribution of entanglement via satellite links will drastically extend the reach of quantum networks. Highly efficient entangled photon sources are an essential requirement towards overcoming high channel loss and achieving practical transmission rates in long-distance satellite downlinks. Here we report on an ultrabright entangled photon source that is optimized for long-distance free-space transmission. It operates in a wavelength range that is efficiently detected with space-ready single photon avalanche diodes (Si-SPADs), and readily provides pair emission rates that exceed the detector bandwidth (i.e., the temporal resolution). To overcome this limitation, we demultiplex the photon flux into wavelength channels that can be handled by current single photon detector technology. This is achieved efficiently by using the spectral correlations due to hyper-entanglement in polarization and frequency as an auxiliary resource. Combined with recent demonstrations of space-proof source prototypes, these results pave the way to a broadband long-distance entanglement distribution network based on satellites.

THz optical beat-note detection with a fast superconducting hot electron bolometer operating up to 31 GHz.

We study the performance of a hot-electron bolometer (HEB) operating at THz frequencies based on superconducting niobium nitride films. We report on the voltage response of the detector over a large electrical detection bandwidth carried out with different THz sources. We show that the impulse response of the fully packaged HEB at 7.5 K has a 3 dB cutoff around 2 GHz. Remarkably, detection capability is still observed above 30 GHz in an heterodyne beating experiment using a THz quantum cascade laser frequency comb. Additionally, the HEB sensitivity has been evaluated and an optical noise equivalent power NEP of 0.8 pW/√H z has been measured at 1 MHz.

Photoacoustic Imaging Based on Broadened Bandwidth Aluminum Nitride Piezoelectric Micromachined Ultrasound Transducers

This letter reports broadened bandwidth aluminum nitride (AlN) piezoelectric micromachined ultrasound transducers (BB-PMUT) for photoacoustic imaging (PAI). PMUT based on microelectromechanical systems (MEMS) technology has gained significant attention for PA endoscopic imaging inside colorectal and even blood vessels due to its miniature size, high sensitivity, and versatility. Broadened bandwidth, resulting from variable membrane cavity size during the fabrication process, provides benefits to imaging spatial resolution improvement. Moreover, AlN thin film is a nontoxic material, which makes it harmless for biomedical applications with potential of complementary metal-oxide semiconductor (CMOS) MEMS integration. In this letter, a BB-PMUT is designed and fabricated. The vibration modes, impedance, and bandwidth are analyzed. The BB-PMUT provides a wider fractional bandwidth (65%) for signal detection, which significantly increases the resolution compared with traditional single frequency PMUT design. We have conducted experimental validation on an agar sample to present the sensor's performance in improving axial resolution.