Detection Bandwidth Research Articles

Pulsed microwave photonic vector network analyzer based on direct sampling.

Pulsed VNAs enabling large dynamic measurement at a low pulse duty cycle are necessary for the characterization of active devices due to the overheating damage risk under continuous wave stimulation. In this paper, we proposed a pulsed microwave photonic vector network analyzer (p-MPVNA) based on direct sampling. With the broad system bandwidth of the p-MPVNA and undersampling technique, pulsed signals are received thru asynchronous wideband detection, and pulsed S-parameters are calculated thru vector superposition. In asynchronous wideband detection, the continuous spectrum of the pulsed signal is discretized into multiple frequency components. A low repetition rate optical pulse train undersamples the pulsed signal, and the discretized frequency components are aliased. The frequency components in the main lobe including most energy of the pulse signal are vector superimposed to calculate the pulsed S-parameter. The proposed p-MPVNA has a dynamic range decreases rate of 10log(duty cycle) when pulse duty cycle is below 10% , which is much slower than that of 20log(duty cycle) for classical narrowband detection. An experimental p-MPVNA is established for validation. A 6 to 18 GHz microwave amplifier is measured with continuous and pulsed power supply and the measured gain curves are consistent with the results from a commercial VNA. The system dynamic range decrease with pulse duty cycle is verified by the pulsed S-parameter measurement of a 10 GHz low pass filter under continuous and pulsed stimulation.

Detector bandwidth and polarization switching rates: spectrophotometric observations of the Sun by the Birmingham Solar Oscillations Network

Abstract The Birmingham Solar Oscillations Network (BiSON) observes acoustic oscillations of the Sun. The dominant noise source is caused by fluctuations of Earth’s atmosphere, and BiSON seeks to mitigate this effect by combining multiple rapid observations in alternating polarization states. Current instrumentation uses bespoke Pockels-effect cells to select the polarization state. Here, we investigate an alternative off-the-shelf solution, a liquid crystal (LC) retarder, and discuss the potential impact of differences in performance. We show through electrical simulation of the photodiode-based detectors, and assessment of both types of polarization device, that although the switching rate is slower the off-the-shelf LC retarder is a viable replacement for a bespoke Pockels-effect cell. The simplifications arising from the use of off-the-shelf components allow easier and quicker instrumentation deployment.

Frequency Stability Requirements in Quasi-Integer-Ratio Time-Expanded Phase-Sensitive OTDR

Time-expanded phase sensitive (TE-ϕ)OTDR is a recently reported technique for distributed fiber sensing that relies on the use of an electro-optic dual frequency comb (DFC) scheme. A distinctive feature of this approach is its ability to provide high spatial resolution (on the centimeter scale) with detection bandwidths orders of magnitude lower than those of conventional ϕOTDR systems. The stringent trade-off between resolution, range and sensing bandwidth that exists in TE-ϕOTDR has demonstrated to be substantially relaxed by implementing two frequency combs with quasi-integer-ratio repetition rates. However, employing very dissimilar line separations (with a ratio between them >100) is challenging due to the need of keeping the coherence over long sequences of interferograms, which eventually limits the attainable range. In this paper, we formulate the requirements for the frequency stability of the reference clock used in a quasi-integer-ratio DFC scheme. This analysis allows us to stablish the limits on the number of comb lines (i.e., on the number of available independent sensing points) for a particular reference clock. By using a rubidium atomic clock (with a relative frequency stability of ∼10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−13</sup> ), we demonstrate up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> sensing points along 2 km of fiber with tens of Hz sensing bandwidth.

Time-expanded φOTDR using low-frequency electronics.

Time expanded phase-sensitive optical time-domain reflectometry (TE-φOTDR) is a recently reported technique for distributed optical fiber sensing based on the interference of two mutually coherent optical frequency combs. This approach enables distributed acoustic sensing with centimeter resolution while keeping the detection bandwidth in the megahertz range. In this paper, we demonstrate that TE-φOTDR can be realized with low-frequency electronics for both signal generation and detection. This achievement is possible thanks to the use of a couple of electro-optic comb generators driven by commercially available step recovery diodes. These components are fed by radio frequencies that are orders of magnitude lower than those involved in the signals so far originated by ultrafast waveform generation. The result is a simple, compact, low-cost and potentially field-deployable sensor that works without resorting to any decoding algorithm. Besides, high-resolution distributed sensing is carried out with no need of coding strategies or enhanced backscatter fibers. To check the capabilities of our system, we perform distributed strain sensing over a range of 20 m. The spatial resolution is 3 cm and the acoustic sampling rate can be increased up to 200 Hz. This performance reveals the prospective of the proposed approach for field applications, including structural health monitoring.

Phase-sensitive distributed Rayleigh fiber sensing enabling the real-time monitoring of the refractive index with a sub-cm resolution by all-optical coherent pulse compression.

We have developed a novel architecture enabling distributed acoustic sensing in a commercial single-mode fiber with a sub-cm spatial resolution and an interrogation rate of 20 kHz. More precisely, we report the capability of real-time and space-resolved monitoring of the distributed phase and of the refractive index variations along the sensing fiber. The system reported here is optimal in many aspects. While the use of broadband light waveforms enables a sub-cm spatial resolution, the waveforms are quasi CW, delaying the occurrence of non-linear effects. Coherent detection ensures direct access to the distributed phase and to the local variations of the refractive index. Moreover, an all-optical pulse compression feature enables to lower the detection bandwidth down to 10 MSa/s. Based on a bi-directional frequency shifting loop, the architecture makes use of a single CW laser, commercial telecom components, and low frequency electronics. It is expected to open new avenues in distributed acoustic sensing applications, where high spatial resolution and high interrogation rates are required.

Study on influence of ring hole array metamaterial on performance of pyroelectric terahertz detectors

In order to improve the detection performance of 0.1–1THz terahertz wave, a new lithium tantalate pyroelectric terahertz detector based on ring hole array metamaterial is proposed. The quantitative influences of the characteristic parameters such as inner diameter, outer diameter, period and thickness on the transmission bandwidth and transmittance of ring hole array metamaterials are analyzed by simulation. The mechanisms of the influences of different combinations of ring hole array metamaterials and pyroelectric detectors on the detection bandwidth and detection rate of terahertz waves are clarified. The lithium tantalate pyroelectric terahertz detectors of the ring hole array metamaterial ware are fabricated by the MEMS technology. The transmission of the ring hole array metamaterial and the noise equivalent power of the metamaterial detector at different frequencies are tested. The results show that the transmittance of the fabricated ring hole array metamaterial is greater than 40% at 0.25–0.65 THz, and bandpass filtering is realized. When the ring hole array metamaterial and the pyroelectric detector maintain a sufficient distance, the noise equivalent power of the detector at 0.315 THz is 11.29 μW/Hz&lt;sup&gt;0.5&lt;/sup&gt;, which is 6.3% of the 0.1 THz noise equivalent power (outside the bandpass band), so the bandpass detection is achieved. When the ring hole array metamaterial is attached to the detector, the noise equivalent power of the metamaterial detector at 0.315 THz is 4.64 μW/Hz&lt;sup&gt;0.5&lt;/sup&gt;, which is 29.4% that of the detector without the ring hole array metamaterial, so the narrowband detection is achieved. The above conclusions show that the pyroelectric terahertz detector based on the ring hole array metamaterial can realize the bandpass and narrowband detection of specific frequency band in applications such as biological imaging and macromolecular detection.

Two dimensional NbSe2/Nb2O5 metal-semiconductor heterostructure-based photoelectrochemical photodetector with fast response and high flexibility.

Two dimensional (2D) metal-semiconductor heterostructures are promising for high-performance optoelectronic devices due to fast carrier separation and transportation. Considering the superior metallic characteristics accompanied by high electrical conductivity in NbSe2, surface oxidation provides a facile way to form NbSe2/Nb2O5 metal-semiconductor heterostructures. Herein, size-dependent NbSe2/Nb2O5 nanosheets were achieved by a liquid phase exfoliation method and a gradient centrifugation strategy. These NbSe2/Nb2O5 heterostructure-based photodetectors show high responsivity with 23.21 μA W-1, fast response time of millisecond magnitude, and wide band detection ability in the UV-Vis region. It is noticeable that the photocurrent density is sensitive to the surface oxygen layer due to the oxygen-sensitized photoconduction mechanism. The flexible testing of the NbSe2/Nb2O5 heterostructure-based PEC-type photodetectors exhibits high photodetection performance even after bending and twisting. Beyond that, the solid-state PEC-type NbSe2/Nb2O5 photodetector also achieves relatively stable photodetection and high stability. This work promotes the application of 2D NbSe2/Nb2O5 metal-semiconductor heterostructures in flexible optoelectronic devices.

Fault-Tolerant Cooperative Signal Detection for Petahertz Short-Range Communication With Continuous Waveform Wideband Detectors

Motivated by unique scattering properties at petahertz frequencies, we present a novel statistical model of fault-tolerant cooperative signal detection for short-range optical petahertz wireless communications, where a single-scattering assumption holds valid. We characterize the received non-line-of-sight (NLOS) signal by a fault-tolerant continuous waveform wideband detector with a non-zero failure probability. To better reflect the physical properties of the petahertz communication, both signal-independent and signal-dependent noise sources are considered in characterizing the received signal. The distribution of the received signal is quantified with each scattered path following the Málaga distribution. We leverage the location flexibility of randomly distributed collaborative users, considering each user experiences an independent channel condition. Utilizing the Neyman-Pearson criterion, we develop the binary hypothesis testing problem and subsequently derive the likelihood ratio for the test statistics. Moreover, to quantify the performance, a framework is developed for the average area under the receiver operating characteristic (ROC) curve for both single user and cooperative scenarios. An optimal decision fusion with a majority rule for fault-tolerant signal detection is applied to exploit the maximum spectrum opportunity. It is found that with the optimal voting rule and for a given target error rate, the network requires fewer collaborative secondary users than the total number of users available in an optical network. However, in the limiting case, it is shown that, as the cost function approaches its minimum or maximum value within its allowable range, the optimal number of collaborative users becomes independent of the failure probabilities. With the realistic assumption of fault-tolerant users, it is found that the false alarm probability increases faster than the detection probability.

Analysis and Compensation of Vibration Induced Beat Frequency Shift in OFDR Based Fiber-Optic Distributed Acoustic Sensor

Optical frequency domain reflectometry (OFDR) is an attractive locating technology for distributed acoustic sensor (DAS) due to its high spatial resolution, but the detectable bandwidth of vibration is very limited up to date, because high frequency vibration will induce time-variant beat frequency shift within the probe, which can deteriorate or even invalidate the demodulation algorithm. In this work, the influence of vibration induced beat frequency shift in OFDR is analyzed and assessed by the crosstalk power spectral density (PSD). Then an adaptive two-dimensional cross-correlation algorithm is proposed for beat frequency shift compensation, in which the instantaneous beat frequency shift within the probe is estimated and compensated for precise strain demodulation. In the experiment, an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$18 \text{-}kHz$</tex-math></inline-formula> vibration signal is successfully detected with spatial resolution of 12 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\rm cm$</tex-math></inline-formula> and strain resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$594\;\rm p\varepsilon/\sqrt{Hz}$</tex-math></inline-formula> , whose frequency is an order of magnitude higher than previous works. The proposed beat frequency shift compensation algorithm is evaluated in different conditions, and a suppression of crosstalk PSD up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$40.5\rm \ dB$</tex-math></inline-formula> is proved.

An End-to-End Deep Learning Framework for Wideband Signal Recognition

Successful management of the radio spectrum requires, as a first step, detailed information about spectrum occupancy. In this work, we present an end-to-end deep learning (DL) based framework to obtain information from wide spectrum bands through signal detection, localization, and modulation classification. By visually representing the radio signals in spectrograms, we formulate the wideband detection problem as an object detection task from the computer vision field. To this end, the proposed framework consists of two cascaded modules: an object detection network repurposed to detect and classify distinctive signals in wideband spectrograms, and a convolutional neural network (CNN) designed to extend the classification capabilities to support a wide range of analog and digital modulation schemes. To evaluate our framework, we use a public wideband recognition dataset, which we carefully analyze and curate through a series of preprocessing techniques. To tackle the challenges of insufficient training data and class imbalance observed in the dataset, we suggest a training strategy that includes data mixing and transfer learning. Our experimental results on a general test set demonstrate that the proposed approach can detect and classify a variety of narrowband signals with simultaneously high precision (77.1%), recall (81.8%), and localization accuracy, as indicated by an average Intersection over Union (IoU) of 86%.

Ultra-broadband all-optical sampling of optical waveforms.

Optical-field sampling techniques provide direct access to the electric field of visible and near-infrared light. The existing methods achieve the necessary bandwidth using highly nonlinear light-matter interaction that involves ionization of atoms or generation of charge carriers in solids. We demonstrate an alternative, all-optical approach for measuring electric fields of broadband laser pulses, which offers an advantage in terms of sensitivity and signal-to-noise ratio and extends the detection bandwidth of optical methods to the petahertzdomain.

Passive mode-locked cesium diode-pumped alkali laser.

A first, to the best of our knowledge, demonstration of passive mode-locking in diode-pumped alkali laser (DPAL) is reported in this paper. An intracavity cesium vapor cell, buffered by atmospheric pressure methane, is used to passively mode lock a continuously pumped cesium DPAL with a static gain medium. A train of short pulses with duration less than 460 ps was observed using a 2.0-GHz bandwidth detector that limited the real time duration measurements. The calculated minimum duration for these pulses is 57 ps.

Vivaldi antenna for early cancer detection based on THz spectroscopy: Comparison between response of breast and skin cancer

this paper suggests a Vivaldi antenna for the THz frequency range with a wide bandwidth (more than 120% bandwidth) for early cancer detection with dimensions of 1λ× 0.5λ× 0.034λ for 0.5 THz with special novel arrangement of photonic crystal and taper grating reflector. The photonic crystal walls are implemented on both sides of the antenna to make an electric shield to concentrate energy for increasing the gain and directivity of the antenna and reducing the side lobe. Front to back (F/B) ratio is another problem for this THz antenna where the grating parasitic elements are used as the reflector to reduce the back lobe of the basic antenna. This antenna covers 0.5–2 THz with a maximum gain of 8.8 dB in the broadside. The proposed antenna is used for detecting breast and skin cancers which are modeled by Debye’s second-order model. The reflection and transmission, pulse response, and phase variation for normal and tumor tissue are considered for the reorganization of cancers. The results show that reflection and transmission, pulse responses are not effective factors for breast cancer detection in comparison to skin cancer while the phase response is the best solution for this type of cancer.

Highly responsive SnSe/GaN heterostructure-based UVC-SWIR broadband photodetector

Broadband photodetection spans the ultraviolet-C (250 nm) to shortwave-infrared (1250 nm) range is essential and desirable for various applications such as optical communication, spectral switching, and memory storage. We have utilized ultraviolet-responsive gallium nitride (GaN) and Tin selenide (SnSe) as the infrared-activated material to fabricate a broadband heterojunction-based optical detector. Through SnSe/GaN heterostructure, the fabricated detector combines GaN's ultraviolet to visible, with SnSe's visible to shortwave-infrared detectability. The device also possesses great responsiveness and a low dark current due to edge contact geometry, which directly accesses the heterostructure interface for both materials. The fabricated device functions well from the ultraviolet-C to the shortwave-infrared region, exhibiting the highest responsivity of 128 AW-1 and the lowest noise equivalent power of 5.2 × 10−14 WHz−1/2, the quantum efficiency ∼5 × 104% with a response time ∼800 μs for ultraviolet illumination while the highest responsivity of 6.06 AW-1 and noise equivalent power of 1 × 10−12 WHz−1 for the device under infrared illumination. The research will open up new possibilities for optoelectronic applications utilizing heterostructure-based wideband detection.

Wideband Brillouin light scattering analysis of spin waves excited by a white-noise RF generator

Spin waves are studied intensively for their intriguing properties and potential use in future technology platforms for the transfer and processing of information and microwave signals. The characterization of devices and materials for magnonic systems is time-consuming, and thus, the development of instruments that can speed up the collection and analysis of spin-wave data is crucial. In this Letter, we report a straightforward approach to enhance the measurement throughput by fully exploiting the wideband detection nature of the Brillouin light scattering technique with a white-noise RF generator.

Design of Low Profile ATFSS and Antenna With In-Band and Out-of-Band RCS Reduction

Stealth is a very important property for antennas, not only in-band stealth, but out-of-band stealth will also greatly affect the overall stealth of antennas. Thus, a novel low-profile absorbing/transmitting integrated frequency selective surface (ATFSS) is proposed in this article, and then an antenna with in-band and out-of-band radar cross section (RCS) reduction is designed by combining it with an array antenna with integrated radiation and scattering. The profile height of the novel ATFSS is only 3.6 mm, and the gain of the in-band and out-of-band RCS reduced antenna is greater than 17.5 dBi and its insert loss is less than 1 dB in 8.5–10 GHz. Meanwhile, the antenna can achieve significant RCS reduction in the out-of-band of 4.8–8.5, 11.2–15.1 GHz, and in the in-band of 8.5–10.5 GHz. Excellent agreement is obtained between simulation and measurement for the in-band and out-of-band RCS reduced antenna. Such a method not only can significantly reduce the profile height of the traditional ATFSS, broaden the low detectable bandwidth and enhance the invisibility of the antenna, but also maintain the high gain radiation characteristics of the antenna, which has important practical application value.

Beyond fundamental resonance mode: high-order multi-band ALN PMUT for in vivo photoacoustic imaging

This paper reports on an aluminum nitride (AlN) piezoelectric micromachined ultrasound transducer (PMUT) array for photoacoustic (PA) imaging, where the high-order resonance modes of the PMUT are utilized to improve imaging resolution. A flexural vibration mode (FVM) PMUT is fabricated and applied in a photoacoustic imaging (PAI) system. Specifically, the microelectromechanical system (MEMS)-based PMUT is suitable for PA endoscopic imaging of blood vessels and bronchi due to its miniature size and high sensitivity. More importantly, AlN is a nontoxic material, which makes it harmless for biomedical applications. In the PAI system, the AlN PMUT array is used to detect PA signals, and the acousto–mechanical response is designed and optimized at the PMUT’s fundamental resonance. In this work, we focus on the high-order resonance performance of the PMUT PAI beyond the fundamental resonance. The acoustic and electrical responses of the PMUT’s high-order resonance modes are characterized and analyzed. The fundamental and three high-order resonance bandwidths are 2.2, 8.8, 18.5, and 48.2 kHz. Compared with the resolution at the fundamental resonance mode, the resolutions at third- and fourth-order resonance modes increase by 38.7% and 76.9% in a phantom experiment. The high-order resonance modes of the AlN PMUT sensor array provide higher central frequency and wider bandwidth for PA signal detection, which increase the resolution of PAI compared to the PMUT working at the fundamental resonance mode.

Pressure-Tailored Self-Driven and Broadband Photoresponse in PbI2.

Photoelectric devices based on the photothermoelectric (PTE) effect show promising prospects for broadband detection without an external power supply. However, effective strategies are still required to regulate the conversion efficiency of light to heat and electricity. Herein, significantly enhanced photoresponse properties of PbI2 generated from a PTE mechanism via a high-pressure strategy are reported. PbI2 exhibits a stable, fast, self-driven, and broadband photoresponse at ≈980nm. Intriguingly, the synergy of the photoconductivity and PTE mechanism is conducive to enhancing the photoelectric properties, and extending the detection bandwidth to the optical communication waveband (1650nm) with an external bias. The dramatically enhanced photoresponse characteristics are attributed to narrowing of the band gap and a significantly decreased resistance, which originate from the enhancement of atomic orbital overlap owing to pressure-induced Pb-I bond contraction. These findings open up a new avenue toward designing self-driven and broadband photoelectric devices.

Contactless AC/DC Wide-Bandwidth Current Sensor Based on Composite Measurement Principle.

With the accelerated construction of the smart grid, new energy sources such as photovoltaic and wind power are connected to the grid. In addition to power frequency, the current signal of power grid also includes several DC signals, as well as medium-high and high-frequency transient signals. Traditional current sensors for power grids are bulky, have a narrow measurement range, and cannot measure both AC and DC at the same time. Therefore, this paper designs a non-intrusive, AC-DC wide-bandwidth current sensor based on the composite measurement principle. The proposed composite current detection scheme combines two different isolation detection technologies, namely tunneling reluctance and the Rogowski coil. These two current sensing techniques are complementary (tunneling magnetoresistive sensors have good low-frequency characteristics and Rogowski coils have good high-frequency characteristics, allowing for a wide detection bandwidth). Through theoretical and simulation analysis, the feasibility of the composite measurement scheme was verified. The prototype of composite current sensor was developed. The DC and AC transmission characteristics of the sensor prototype were measured, and the sensitivity and linearity were 11.96 mV/A, 1.14%, respectively. Finally, the sweep current method and pulse current method experiments prove that the designed composite current sensor can realize the current measurement from DC to 17 MHz.

Feasibility of photoacoustic-guided ultrasound treatment for port wine stains.

Port wine birthmark, also known as port wine stain (PWS) is a skin discoloration characterized by red/purple patches caused by vascular malformation. PWS is typically treated by using lasers to destroy abnormal blood vessels. The laser heating facilitates selective photothermolysis of the vessels and attenuates quickly in the tissue due to high optical scattering. Therefore, residual abnormal capillaries deep in the tissue survive and often lead to the resurgence of PWS. Ultrasound (US) has also been proposed to treat PWS, however, it is nonselective with respect to the vasculature but penetrates deeper into the tissue. We aim to study the feasibility of a hybrid PWS treatment modality combining the advantages of both modalities. In this manuscript, we propose a photoacoustic (PA) guided US focusing methodology for PWS treatment which combines the optical contrast-based selectivity with US penetration to focus the US energy onto the vasculature. The PA signals collected by the transducers, when time-reversed, amplified, and transmitted, converge onto the PWS, thus minimally affecting the neighboring tissue. We performed two- and three-dimensional simulations that mimic realistic transducers and medium properties in this proof of concept study. The time-reversed PA signals when transmitted from the transducers converged onto the vasculature, as expected, thus reducing the heating of the neighboring tissue. We observed that while the US focus is indeed affected due to experimental factors such as limited-view, large detector separation and finite detection bandwidth, and so forth, the US did focus completely or partially onto the vasculature demonstrating the feasibility of the proposed methodology. The results demonstrate the potential of the proposed methodology for PWS treatment. This treatment method can destroy the deeper capillaries while minimally heating the neighboring tissue, thus reducing the chances of the resurgence of PWS and as well as cosmetic scarring.