Detection Bandwidth Research Articles

Broadband terahertz time-domain polarimetry based on air plasma filament emissions and spinning electro-optic sampling in GaP.

We report on a time-domain polarimetry (TDP) system for generating and detecting broadband terahertz (THz) waves of different polarization angles. We generate THz waves from two-color laser filaments and determine their polarization states with a detection bandwidth of up to 8 THz using a spinning gallium phosphide crystal. The polarization of THz emission can be controlled by adjusting the position and tilt angle of the β-barium borate crystal. We characterize the precision of this system for polarimetric measurements at fixed time delay to be and for complete time-domain waveforms. We also demonstrate the feasibility of our TDP system by measuring broadband optical properties of anisotropic samples in both transmission and reflection geometries. The THz-TDP technique can be easily integrated in conventional THz time-domain spectroscopy setups using nonlinear crystal detectors.

Resonance-Enhanced Quantum Well Micropillar Array with Ultra-Narrow Bandwidth and Ultra-High Peak Quantum Efficiency

Infrared cameras with narrow-band detection capability are widely used for SF6 gas detection, which is an essential part of power equipment inspection. Narrow-band detection is usually achieved by a combination of quantum well infrared photodetectors (QWIPs) and narrow-band filters. Improving the quantum efficiency of QWIPs and reducing the detection bandwidth are important ways to improve camera performance. In this study, a back-incident-type device of quantum well micropillar array targeting at a 10.5 μm central wavelength is designed and studied by three-dimensional simulation. The operating mechanism of the device was determined by investigating the effect of the device geometry on the quantum efficiency. The enhanced absorption capability of the device mainly comes from the Fabry–Pérot resonance and the antireflection effect. The final device exhibits a remarkable peak quantum efficiency of 83% at 10.5 μm and an ultra-narrow spectral bandwidth of 0.2 μm. These excellent properties are achieved without an antireflective film and narrow-band filter, which can significantly improve the narrow-band capability and integration of the system; the dark current reduces to be 0.2762 times due to the low-duty cycle. These properties indicate that the structure of the quantum well micropillar array is of great significance to the development of QWIPs used in gas detection.

Encapsulated Microbubble Fiber-Tip Fabry-Perot Cavity With Tunable Bandwidth for Sensitive Acoustic Detection

A photothermally generated microbubble on a single-mode fiber tip with tunable frequency bandwidth was demonstrated for sensitive detection of acoustic waves. The microbubble with spherical gas-water interface formed a flexible Fabry-Pérot cavity at the fiber tip and detected the acoustic waves by interferometric readout of the bubble deformation. To work in different liquids besides water, the sensor was encapsulated into a water-filled tube and the effects of the encapsulation process on the optical and acoustic characteristics of the sensor were investigated. A 40 μm-diameter microbubble based sensor exhibited a noise-equivalent pressure level of ∼1.0 mPa/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> at the frequency of ∼100 kHz. The bubble diameter was photothermally changed from 20 μm to 60 μm, which enabled online tuning of the sensor resonant frequency from 100 kHz to 300 kHz. This tunable feature is attractive to applications that requires the detection of acoustic waves covering different frequency bands. The encapsulated microbubble sensor with advantages of high sensitivity, tunable bandwidth and low-cost would be interesting for applications in biomedical and industrial areas involving various liquid environments.

Broadband Model‐Based Optoacoustic Mesoscopy Enables Deep‐Tissue Imaging beyond the Acoustic Diffraction Limit

AbstractOptoacoustic mesoscopy (OAM) retrieves anatomical and functional contrast in vivo at depths not resolvable with optical microscopy. Recent progress on reconstruction algorithms have further advanced its imaging performance to provide high lateral resolution ultimately limited by acoustic diffraction. In this work, a new broadband model‐based OAM (MB‐OAM) framework efficiently exploiting scanning symmetries for an enhanced performance is presented. By capitalizing on the large detection bandwidth of a spherical polyvinylidene difluoride film while accurately accounting for its spatial impulse response, the new approach significantly outperforms standard OAM implementations in terms of contrast and resolution, as validated by functional in vivo experiments in mice and human volunteers. Furthermore, L1‐norm regularization enables resolving structures separated by less than the theoretical diffraction‐limited resolution. This unique label‐free angiographic performance demonstrates the general applicability of MB‐OAM as a super‐resolution deep‐tissue imaging method capable of breaking through the limits imposed by acoustic diffraction.

Measurement of Conducted Supraharmonic Emissions: Quasi-Peak Detection and Filter Bandwidth

In modern power systems, the integration of renewable energy sources relies on dedicated inverters whose power electronic circuitry switches at high frequencies and causes conducted emissions in the supraharmonic range, i.e., from 9 to 150 kHz. In this regard, the normative framework is still lacking a reference measurement method as well as a set of emission limits and performance requirements. From a metrological point of view, it is important to evaluate whether some of the power quality indices adopted for radiated emissions could be transposed also in this context. In particular, the paper considers a recent algorithm for the identification of supraharmonic components and discusses how its estimates affect the estimation of quasi-peak values. To this end, the paper describes the implementation of a fully digital approach and validates the results by means of an experimental comparison against a traditional quasi-peak detector. The proposed analysis confirms the potential of the considered approach and provides some interesting insights about the reliability of quasi-peak estimation in supraharmonic range.

Optical Arbitrary Waveform Measurement (OAWM) Using Silicon Photonic Slicing Filters

We demonstrate an optical arbitrary waveform measurement (OAWM) system that exploits a bank of silicon photonic (SiP) frequency-tunable coupled-resonator optical waveguide (CROW) filters for gapless spectral slicing of broadband optical signals. The spectral slices are coherently detected using a frequency comb as a multi-wavelength local oscillator (LO) and stitched together by digital signal processing (DSP). For high-quality signal reconstruction, we have implemented a maximum-ratio combining (MRC) technique based on precise calibration of the complex-valued opto-electronic transfer functions of all detection paths. In a proof-of-concept experiment, we demonstrate the viability of the scheme by implementing a four-channel system that offers an overall detection bandwidth of 140 GHz. Exploiting a femtosecond laser with precisely known pulse shape for calibration along with dynamic amplitude and phase estimation, we reconstruct 100 GBd QPSK, 16QAM and 64QAM optical data signals. The reconstructed signals show improved quality compared to that obtained with a single high-speed intradyne receiver, while the electronic bandwidth requirements of the individual coherent receivers are greatly reduced.

Water-Based Coherent Detection of Broadband Terahertz Pulses.

Both solids and gases have been demonstrated as the materials for terahertz (THz) coherent detection. The gas-based coherent detection methods require a high-energy probe laser beam and the detection bandwidth is limited in the solid-based methods. Whether liquids can be used for THz detection and relax these problems has not yet been reported, which becomes a timely and interesting topic due to the recent observation of efficient THz wave generation in liquids. Here, we propose a THz coherent detection scheme based on liquid water. When a THz pulse and a fundamental laser beam are mixed on a free-flowing water film, a second harmonic (SH) beam is generated as the plasma is formed. Combining this THz-induced SH beam with a control SH beam, we successfully achieve the time-resolved waveform of the THz field with the frequency range of 0.1-18THz. The required probe laser energy is as low as a few microjoules. The sensitivity of our scheme is 1 order of magnitude higher than that of the air-based method under comparable detection conditions. The scheme is sensitive to the THz polarization and the phase difference between the fundamental and control SH beams, which brings direct routes for optimization and polarization sensitive detection. Energy scaling and polarization properties of the THz-induced beam indicate that its generation can be attributed to a four-wave mixing process. This generation mechanism makes simple relationships among the probe laser, THz-induced SH, and THz field, favorable for robustness and flexibility of the detection device.

Thin film lithium niobate electric field sensors.

We present our results for using thin film lithium niobate devices for electric field sensing applications. Micro-ring modulator and Mach-Zehnder modulator-based electric field sensors are demonstrated. Micro-ring resonator sensors can be used for low frequency (up to several GHz) electric field sensing applications and achieve a high sensitivity of 80 mV/(m Hz1/2) with a very compact size of 300μm, as limited by the intensity and phase noise of the used distributed feedback laser. A measurement bandwidth of 2.5GHz is measured for these sensors and is limited by the detector bandwidth. Alternatively, Mach-Zehnder modulators allow for perfect phase matching between the radio frequency signals and optical signals, and they can be used for electric field sensing up to several THz. A sensitivity of 2.2 V/(m Hz1/2) was obtained using our Mach-Zehnder electric field sensor with an interaction length of 600μm. The Mach-Zehnder sensor can sense electric fields with frequencies reaching 0.6THz based on the calculated results.

Ultrafast impulsive Raman spectroscopy across the terahertz–fingerprint region

Broadband Raman spectroscopy (detection bandwidth >1000 cm − 1) is a valuable and widely used tool for understanding samples via label-free measurements of their molecular vibrations. Two important Raman spectral regions are the chemically specific “fingerprint” (200 to 1800 cm − 1) and “low-frequency” or “terahertz” (THz) (<200 cm − 1; <6 THz) regions, which mostly contain intramolecular and intermolecular vibrations, respectively. These two regions are highly complementary; broadband simultaneous measurement of both regions can provide a big picture comprising information about molecular structures and interactions. Although techniques for acquiring broadband Raman spectra covering both regions have been demonstrated, these methods tend to have spectral acquisition rates <10 spectra / s, prohibiting high-speed applications, such as Raman imaging or vibrational detection of transient phenomena. Here, we demonstrate a single-laser method for ultrafast (24,000 spectra / s) broadband Raman spectroscopy covering both THz and fingerprint regions. This is achieved by simultaneous detection of Sagnac-enhanced impulsive stimulated Raman scattering (SE-ISRS; THz-sensitive) and Fourier-transform coherent anti-Stokes Raman scattering (FT-CARS; fingerprint-sensitive). With dual-detection impulsive vibrational spectroscopy, the SE-ISRS signal shows a >500 × enhancement of <6.5 THz sensitivity compared with that of FT-CARS, and the FT-CARS signal shows a >10 × enhancement of fingerprint sensitivity above 1000 cm − 1 compared with that of SE-ISRS.

Study of Capacitive Coupling Sensor Fused With High Voltage XLPE Cable Joint

A capacitive coupling sensor for partial discharge detection with the fusion of high voltage XLPE cable joint is designed in this paper. The sensor is to address partial discharge signals leading to transmission attenuation and external interference causing poor field detection sensitivity. First, a coaxial waveguide transmission model was established of high-frequency electrical signals in the body and joint. The result showed that the signal transmission attenuation was minimized while the sensor electrodes were closely attached to the outer semi-conductive layer of the body. Second, the equivalent circuit model was constructed of the capacitive coupling sensor fused with the 110 kV straight passing joint. The specific installation location, main structure size, detection bandwidth, and sensitivity of the sensor in the joint area were determined, which was to maximize the coupling output signal amplitude and transfer function amplitude. Finally, a lightning surge voltage test was carried out with the integration of the fusion of the joint voltage thermal cycling. Simulation and measurement show the following: while the sensor is installed in the cable metal sleeve break and the electrodes are closed to the overall semi-conductive layer, there is excellent performance for partial discharge detection in the frequency range of 1–300 MHz, with a sensitivity of 5 pC.

Optical interface for a hybrid magnon–photon resonator

We study optical detection of magnetic resonance of a ferrimagnetic sphere resonator, which is strongly coupled to a microwave loop gap resonator. Optical fibers are employed for coupling the sphere resonator with light in the telecom band. We find that magnetic resonance can be optically detected near the region of anti-crossing between the loop gap and the ferrimagnetic resonances. The detection bandwidth is found to be limited by a ferrimagnetic damping rate.

Detection of weak optical absorption by optical-resolution photoacoustic microscopy

Optical-resolution photoacoustic microscopy (OR-PAM) is one of the major implementations of photoacoustic (PA) imaging. With tightly focused optical illumination and high-frequency ultrasound detection, OR-PAM provides micrometer-level resolutions as well as high sensitivity to optical absorption contrast. Traditionally, it is assumed that the detected PA signal in OR-PAM has a linear dependence on the target’s optical absorption coefficient, which is the basis for quantitative functional and molecular PA imaging. In this paper, we demonstrate that, due to the limited detection bandwidth and detection view, OR-PAM can have a strong nonlinear dependence on the optical absorption, especially for weak optical absorption (<10 cm−1). We have investigated the nonlinear dependence in OR-PAM using numerical simulations, analyzed the underlining mechanisms, proposed potential solutions, and experimentally confirmed the results on phantoms. This work may correct a traditional misunderstanding of the OR-PAM signals and improve quantitative accuracy for functional and molecular applications.

Gain-switched dual frequency comb at 2 µm.

This article shows a dual frequency comb in the 2 µm wavelength region using mutually injection locked gain-switched semiconductor lasers. Strained InGaAs multi-quantum-well discrete mode lasers and gain switching were used to generate two optical frequency combs with repetition rates of 2 GHz and 2.0001 GHz respectively, centred at 2.002 µm. Each optical comb spanned approximately 100 GHz. Through mutual injection locking to an edge comb line common in both combs, a phase locked dual frequency comb was demonstrated with 44 beating tones unique to single comb line pair interactions. This scheme allows for the comb information to be compressed into a 5 MHz detection bandwidth and captured with millisecond acquisition times, which could be of benefit to a number of sensing applications.

Bandwidth Expansion Through Large-Amplitude Modulation and Proportional Feedback for Single-Beam Atomic Magnetometers

Atomic magnetometers are widely acknowledged as one of the most sensitive devices for magnetic field measurement. Our study presents a novel method that enables bandwidth expansion of single-beam magnetometers in the spin-exchange relaxation-free regime. This method bases on increasing the modulation amplitude of magnetic field combining with negative feedback with proportional gain. Through our method, the magnetometer&#x2019;s total relaxation rate is moderately increased by elevating modulation amplitude and frequency, and correspondingly, the detection bandwidth experiences dramatic improvement from 230 Hz to 1380 Hz. The application of negative feedback with proportional gain further increases this bandwidth to higher than 4 kHz, which is over 17 times of its original value. The sensitivity of the magnetometer under this condition is measured as 25 fT/<inline-formula> <tex-math notation="LaTeX">$\text {Hz}^{\text {1/2}}$ </tex-math></inline-formula> for open-loop scheme and 35 fT/<inline-formula> <tex-math notation="LaTeX">$\text {Hz}^{\text {1/2}}$ </tex-math></inline-formula> for closed-loop scheme. Comparatively, the sensitivity measured on conventional conditions in our experiment is 12 fT/<inline-formula> <tex-math notation="LaTeX">$\text {Hz}^{\text {1/2}}$ </tex-math></inline-formula>. This approach offers a profitable trade-off that extraordinary expansion of detection bandwidth can be acquired with the price of a little sensitivity loss. Our method is advantageous for miniaturization of atomic magnetometers as no extra hardware is added, which is of special interest for applications including biomagnetic measurement where large detection bandwidth is sometimes in need.

Reaching the True Shot-Noise-Limited Phase Sensitivity in Self-Heterodyne Interferometry

The use of an acousto-optic modulator in an interferometer enables heterodyne detection and can alleviate technical issues such as laser intensity noise and photodetector flicker noise. However, it also introduces at least a 3 dB penalty to the shot-noise limited phase sensitivity when compared to that attainable through homodyne detection. In this paper, we show theoretically that this penalty can be lifted by implementing detection and demodulation schemes that exploit the cyclostationary nature of shot noise observed at the outputs of the self-heterodyne interferometer. We also discuss how expected departures from ideality, namely imperfect interferometric visibility, non-zero stationary noise, and finite detection bandwidth, affect the performance of these schemes and propose a simplified procedure relaxing the requirement for a rigorous instrument characterization. While two independent detectors are required to reach the true (classical-light) shot-noise limit for phase, single-detector instruments can also benefit from the introduced ideas to achieve up to 3 dB improvement in phase sensitivity.

Dual-plane coupled phase retrieval for non-prior holographic imaging

Accurate depiction of waves in temporal and spatial is essential to the investigation of interactions between physical objects and waves. Digital holography (DH) can perform quantitative analysis of wave–matter interactions. Full detector-bandwidth reconstruction can be realized based on in-line DH. But the overlapping of twin images strongly prevents quantitative analysis. For off-axis DH, the object wave and the detector bandwidth need to satisfy certain conditions to perform reconstruction accurately. Here, we present a reliable approach involving a coupled configuration for combining two in-line holograms and one off-axis hologram, using a rapidly converging iterative procedure based on two-plane coupled phase retrieval (TwPCPR) method. It realizes a fast-convergence holographic calculation method. High-resolution and full-field reconstruction by exploiting the full bandwidth are demonstrated for complex-amplitude reconstruction. Off-axis optimization phase provides an effective initial guess to avoid stagnation and minimize the required measurements of multi-plane phase retrieval. The proposed strategy works well for more extended samples without any prior assumptions of the objects including support, non-negative, sparse constraints, etc. It helps to enhance and empower applications in wavefront sensing, computational microscopy and biological tissue analysis.

Cavity enhanced parametric homodyne detection of a squeezed quantum comb.

A squeezed state with higher-order sidebands is a valuable quantum resource for channel multiplexing quantum communication. However, balanced homodyne detection used in nonclassical light detection has a trade-off performance between the detection bandwidth and clearance, in which the verification of a highly squeezing factor faces a challenge. Here, we construct two optical parametric amplifiers with cavity enhancement; one is for the generation of a -10.5 dB squeezed vacuum state, and the other is for all-optical phase-sensitive parametric homodyne detection. Finally, -6.5 dB squeezing at the carrier with 17 pairs of squeezing sidebands (bandwidth of 156 GHz) is directly and simultaneously observed. In particular, for the cavity-enhanced parametric oscillation and detection processes, we analyze the limiting factors of the detectable bandwidth and measurement deviation from the generated value, which indicates that the length difference and propagation loss between two optical parametric amplifiers should be as small as possible to improve the detection performance. The experimental results confirm our theoretical analysis.

Inter-spacecraft offset frequency setting strategy in the Taiji program.

For controlling the beat frequency of heterodyne interferometry so that the Taiji program can detect gravitational waves in space, an offset frequency setting strategy based on a linear programming algorithm is proposed. Considering factors such as Doppler frequency shift, phase-locking scheme, laser relative intensity noise, and phase detector bandwidth, inter-spacecraft offset frequency setting results suitable for the Taiji program are obtained. During the six years of running the detection process, the use of frequency bounds in the range of [5 MHz, 25 MHz] showed that offset frequencies will remain unchanged for a maximum of 1931 days. If the upper and lower bounds are adjusted, and the relative motion between spacecraft is further constrained, the offset frequencies do not need to change during the time of the mission. These results may provide insights into selecting the phase detector and designing operation parameters such as orbit and laser modulation frequency in the Taiji program.

High-Performance Broadband Phototransistor Based on TeOx/IGTO Heterojunctions.

Ultraviolet to infrared broadband spectral detection capability is a technological challenge for sensing materials being developed for high-performance photodetection. In this work, we stacked 9 nm-thick tellurium oxide (TeOx) and 8 nm-thick InGaSnO (IGTO) into a heterostructure at a low temperature of 150 °C. The superior photoelectric characteristics we achieved benefit from the intrinsic optical absorption range (300-1500 nm) of the hexagonal tellurium (Te) phase in the TeOx film, and photoinduced electrons are driven effectively by band alignment at the TeOx/IGTO interface under illumination. A photosensor based on our optimized heterostructure exhibited a remarkable detectivity of 1.6 × 1013 Jones, a responsivity of 84 A/W, and a photosensitivity of 1 × 105, along with an external quantum efficiency of 222% upon illumination by blue light (450 nm). Simultaneously, modest detection properties (responsivity: ∼31 A/W, detectivity: ∼6 × 1011 Jones) for infrared irradiation at 970 nm demonstrate that this heterostructure can be employed as a broadband phototransistor. Furthermore, its low-temperature processability suggests that our proposed concept might be used to design array optoelectronic devices for wide band detection with high sensitivity, flexibility, and stability.

Numerical Study of Statistical Properties for Semiconductor Laser Chaos by Exploring the Injection Parameter Space

The statistical properties of semiconductor laser chaos are of great importance for practical applications. By exploring the injection parameter space of an optically injected semiconductor laser, the associated probability density functions (PDFs) are numerically investigated for generation of chaotic waveforms with rogue waves (RWs) or Gaussian distributions. First, generation of RWs are demonstrated by considering chaotic waveforms with long-tailed PDFs. The dynamical behaviors for RW generations are systematically investigated by exploring the entire injection parameter space, where a pattern is identified for parameter regions of nonzero RWs. The occurrence of RWs is also investigated for different injection parameters by evaluating the effects of laser bias current, spontaneous emission noise, and detection bandwidth. Alternatively, Gaussian PDFs are generated by bandwidth-limited detection of chaotic waveforms. The bandwidth-dependent statistical properties are investigated through tuning the injection parameters for different detection bandwidths. Generation of Gaussian PDFs are demonstrated for a large region of injection parameters and for a detection bandwidth as comparable to the laser relaxation resonance frequency. This numerical study reveals the importance of injection parameters for tuning the statistics associated with chaotic lasers, and confirms the feasibility of generating RWs and Gaussian PDFs on demand for potential applications in various fields.