Balanced Detection Research Articles

Photonic generation of dual-band complementary linearly chirped microwave waveforms based on phase modulator

A simple scheme to generate a dual-band complementary linearly chirped microwave waveform using a Mach–Zehnder modulator (MZM) paralleled with a phase modulator (PM) is proposed. In the scheme, a five-line optical frequency comb, generated by MZM, is combined with a phase modulated optical chirp waveform by a baseband parabolic signal or a single-chirp signal as two beams with inverse relative phases via a 2 × 2 optical coupler. After balanced detection with heterodyne beating, the dual-band complementary linearly chirped microwave waveform is generated for the two driving cases, and they demonstrate different enhanced performances. The dual-band complementary linearly chirped microwave waveforms with center frequency of 20 and 40 GHz are generated simultaneously for both cases by simulation. The generated two complementary linearly chirped waveforms with a baseband parabolic driving signal have a larger bandwidth of 7.6 GHz, corresponding to larger pulse compression ratios (PCRs) of 1462.86 and 1365.33, which could effectively improve the range Doppler resolution in modern radar systems, although their peak-to-sidelobe ratios (PSRs) are only 12.98 and 12.97 dB. Although for the case with a single-chirp driving signal, the PSRs of the two waveforms with a bandwidth of 4 GHz are higher, 13.66 and 13.67 dB, respectively, but with the PCRs of only 787.69 and 758.52, which is of practical significance for radar detection in weak targets and multitargets.

Squeezed States of Light for Future Gravitational Wave Detectors at a Wavelength of 1550nm.

The generation of strongly squeezed vacuum states of light is a key technology for future ground-based gravitational wave detectors (GWDs) to reach sensitivities beyond their quantum noise limit. For some proposed observatory designs, an operating laser wavelength of 1550nm or around 2 μm is required to enable the use of cryogenically cooled silicon test masses for thermal noise reduction. Here, we present for the first time the direct measurement of up to 11.5dB squeezing at 1550nm over the complete detection bandwidth of future ground-based GWDs ranging from 10kHz down to below 1Hz. Furthermore, we directly observe a quantum shot-noise reduction of up to (13.5±0.1) dB at megahertz frequencies. This allows us to derive a precise constraint on the absolute quantum efficiency of the photodiode used for balanced homodyne detection. These results hold important insight regarding the quantum noise reduction efficiency in future GWDs, as well as for quantum information and cryptography, where low decoherence of nonclassical states of light is also of high relevance.

Sequential Attention-Based Distinct Part Modeling for Balanced Pedestrian Detection

Despite pedestrian detectors having made significant progress by introducing convolutional neural networks, their performance still suffers degradation, especially in occlusion scenes with more false positives (FPs) and false negatives (FNs). To alleviate the problem, we propose a novel Sequential Attention-based Distinct Part Modeling (SA-DPM) for balanced pedestrian detection. It takes one step further in constructing more robust representation that supports detection with fewer FNs and FPs. Specifically, the Sequential Attention serves as one internal perception process that captures several distinct part areas step by step from each pedestrian proposal (full-body). Different from the previous either-or feature selection, the following Joint Learning attempts to seek a reasonable trade-off between part and full-body features, and combines both features for more accurate classification and regression. Evaluation on the widely used pedestrian datasets including Caltech and Citypersons shows that the proposed SA-DPM achieves promising performance for both non-occluded and occluded pedestrian detection tasks, especially on Caltech Heavy Occlusion set, which yields a new state-of-the-art miss rate by 30.18% and outperforms the second best detector by 6.32%.

Electrolyte-Layer-Tunable ATR-SEIRAS for Simultaneous Detection of Adsorbed and Dissolved Species in Electrochemistry.

A balanced detection of both adsorbates and dissolved species is very important for the clarification of the electrochemical reaction mechanism yet remains a major challenge for different modes of electrochemical infrared (IR) spectroscopy. Among others, conventional attenuated total reflection-surface-enhanced IR absorption spectroscopy (ATR-SEIRAS) is far less sensitive to low-concentration solution species than to surface species. We report herein an electrochemical wide-frequency ATR-SEIRAS with a novel thin-layer flow cell design, fulfilling the simultaneous detection of the variations of surface and solution species. This setup consists of a silicon wafer (with one side micromachined and the other side metallized), a thin-layer electrolyte structure with tunable thickness and flow rate, and a tilt-correction system based on laser collimation, enabling a well-controlled mass transport within the electrolyte layer and the spectral differentiation of solution species from adsorbates. Using acidic methanol oxidation on a Pt film electrode as a model system, besides SEIRA bands for adsorbed CO and formate intermediates, IR spectral signals for dissolved products CO2, formic acid, and methyl formate can be readily identified for a quiescent electrolyte layer of ∼20 μm, which are otherwise undetected with conventional ATR-SEIRAS, as indicated by the trend of spectral features with increasing thickness or flow rate.

Impact of Laser Intensity Noise on Dual-Comb Absolute Ranging Precision.

Noise in mode-locked lasers has been a central issue for dual-comb metrological applications. In this work, we investigate the laser intensity noise on dual-comb absolute ranging precision. Two different dual-comb schemes based on linear optical sampling (LOS) and nonlinear asynchronous optical sampling (ASOPS) have been constructed. In the LOS scheme, the ranging precision deteriorates with the increase in laser relative intensity noise (RIN). This effect can be corrected by implementing a balanced photo-detection (BPD). In the ASOPS scheme, the experiment shows that the conversion from laser RIN to dual-comb ranging precision is negligible, making a balanced detection unnecessary for ranging precision improvement. The different manners of RIN’s impact on absolute ranging precision are attributed to the distinct cross-correlation signal patterns and the underlying time-of-flight (TOF) extraction algorithms.

Evaluation of Signal Degradation Due to Birefringence in a Multiple Reference Optical Coherence Tomography System With Polarization-Based Balanced Detection

Although time-domain optical coherence tomography (TD-OCT) systems are straightforward to realize, the imaging speed, sensitivity, and imaging depth limit their range of applications. Multiple reference optical coherence tomography (MR-OCT) based on TD-OCT increases imaging range by about tenfold while providing sensitivity to image highly scattering biological samples. The multiple path-delays and free-space construction make MR-OCT also interesting for hybrid and compact systems, filling the gap between fibre-based and wafer-level integrated optical systems. We describe an optical configuration using a balanced detection scheme and the resulting signal properties due to the required use of polarizing optical components. We numerically simulate the signal properties using Jones calculus and compare the results with measurements. We discuss the origin of signal degradation due to birefringence of the sample in OCT and show that the quarter-wave plate in the sample arm of the Michelson interferometer can be adjusted to optimize the signal returning from a birefringent sample thereby improving the visibility of structures of interest. The theory discussed will be useful to understand and minimize signal degradation due to birefringence in Time-Domain and Fourier-Domain OCT systems.

Detecting pine wilt disease at the pixel level from high spatial and spectral resolution UAV-borne imagery in complex forest landscapes using deep one-class classification

Pine wilt disease (PWD) poses a serious threat to the worldwide pine forest resources. Unmanned aerial vehicle (UAV) remote sensing has been widely used for PWD control, due to its flexibility and efficiency. Although pixel-level detection can obtain fine detection boundaries, there have been few related works in complex scenes because of the difficulty of setting a preset category system and the poor generalization. A preset category system establishes which categories are to be labeled, and is necessary for traditional pixel-level detection. However, the poor generalization leads to an obvious accuracy drop when detecting PWD in new scenes. In the proposed approach, to address the first issue, one-class classification (OCC) is introduced to detect diseased pixels, focusing only on the category of diseased pine trees. However, the numerous objects but low PWD pixel proportion makes the model optimization unbalanced, for which balanced unbiased detection risk estimation is proposed. To address the second issue, a novel model consisting of three-dimensional (3D) convolutional layers and transformer blocks is proposed to extract more robust features. A novel PWD detection framework based on deep OCC is finally proposed to achieve fine pixel-level PWD detection results. PWD detection experiments were conducted on eight UAV H2 (high spatial and spectral resolution) image strips. In total, 300 PWD samples from strip 1 (accounting for roughly 0.009 % of the total pixels) and 400 unlabeled pixels formed the training set. The test experiments were conducted in the remaining seven strips to validate the model generalization. Satisfactory quantitative results (F1-score greater than 0.9) were obtained for all the test strips. The results indicate that the proposed method has a powerful ability to detect PWD in pine trees, even when the PWD proportion is low, and shows better model generalization than the traditional pixel-level detection methods.

Impact of homodyne receiver bandwidth and signal modulation patterns on the continuous-variable quantum key distribution.

In continuous-variable quantum key distribution (CV-QKD), the key information are encoded on quadratures of the optical field, which are measured via balanced homodyne detector (BHD). The bandwidth of the BHD is one of key parameters for precise characterization of quantum states. We establish a theoretical model to analyze the impact of the BHD bandwidth and signal modulation patterns on the channel parameters estimation of CV-QKD systems. Based on the proposed model, the secure key rate of a practical CV-QKD system under different BHD bandwidths and signal modulation patterns are investigated. Our results show that insufficient BHD bandwidth will result in wrong estimate of the transmission loss and excess noise, which significantly affects the performance of CV-QKD systems. Given the BHD bandwidth, there exists an optimal signal repetition rate that maximizes the secure key rate. The BHD bandwidth requirement of the QKD system increases with the transmission distance for large duty cycle pulse. Furthermore, the root raised-cosine pulse signal modulation performs better than the square pulse signal modulation in general.

Quantum Cascade Laser-Based Vibrational Circular Dichroism Augmented by a Balanced Detection Scheme.

Vibrational circular dichroism (VCD) constitutes a powerful technique, enabling the determination of the absolute configuration of molecules without the need for specialized reagents. While delivering critical information, VCD signals commonly are several orders of magnitude weaker than classical absorbance signals, which so far necessitated long measurement times to achieve acceptable signal-to-noise ratios (SNRs) in VCD experiments. We present here an improved setup for the measurement of VCD in the range between 5.6 and 6.5 μm. Employing an external cavity quantum cascade laser (EC-QCL) as a high-power light source, we collected spectra with competitive noise levels in less than 5 min. The basis for this improvement was a balanced detection module combined with an optical path catered to VCD measurements. With the stabilization provided by the two-detector setup, noise originating from the laser source could be suppressed effectively. Noise level improvement up to a factor of 4 compared to the classical single detector EC-QCL-VCD could be reported. Compared to commercial Fourier transform infrared (FT-IR) instruments, the presented setup offers measurement time reductions of a factor of at least 6, with comparable noise levels. The applicability of the setup for qualitative and quantitative VCDs was proven. With the comparatively high temporal resolution provided, the monitoring of optically active processes will be possible in future applications.

10dB Quantum-Enhanced Michelson Interferometer with Balanced Homodyne Detection.

Future generations of gravitational-wave detectors (GWD) are targeting an effective quantum noise reduction of 10dB via the application of squeezed states of light. In the last joint observation run O3, the advanced large-scale GWDs LIGO and Virgo already used the squeezing technology, albeit with a moderate efficiency. Here, we report on the first successful 10dB sensitivity enhancement of a shot-noise limited tabletop Michelson interferometer via squeezed light in the fundamental Gaussian laser mode, where we also implement the balanced homodyne detection scheme that is planned for the third GWD generation. In addition, we achieved a similarly strong quantum noise reduction when the interferometer was operated in higher-order Hermite-Gaussian modes, which are discussed for the GWD thermal noise mitigation. Our results are an important step toward the targeted quantum noise level in future GWDs and, moreover, represent significant progress in the application of nonclassical states in higher-order modes for interferometry, increased spatial resolution, and multichannel sensing.

High-speed two-dimensional terahertz spectroscopy with echelon-based shot-to-shot balanced detection.

By using a reflective-echelon-based electro-optic sampling technique and a fast detector, we develop a two-dimensional terahertz (THz) spectrometer capable of shot-to-shot balanced readout of THz waveforms at a full 1-kHz repetition rate. To demonstrate the capabilities of this new detection scheme for high-throughput applications, we use gas-phase acetonitrile as a model system to acquire two-dimensional THz rotational spectra. The results show a two-order-of-magnitude speedup in the acquisition of multidimensional THz spectra when compared to conventional delay-scan methods while maintaining accurate retrieval of the nonlinear THz signal. Our report presents a feasible solution for bringing the technique of multidimensional THz spectroscopy into widespread practice.

Cross-polarized common-path temporal interferometry for high-sensitivity strong-field ionization measurements.

Absolute density measurements of low-ionization-degree or low-density plasmas ionized by lasers are very important for understanding strong-field physics, atmospheric propagation of intense laser pulses, Lidar etc. A cross-polarized common-path temporal interferometer using balanced detection was developed for measuring plasma density with a sensitivity of ∼0.6 mrad, equivalent to a plasma density-length product of ∼2.6 × 1013 cm-2 if using an 800 nm probe laser. By using this interferometer, we have investigated strong-field ionization yield versus intensity for various noble gases (Ar, Kr, and Xe) using 800 nm, 55 fs laser pulses with both linear (LP) and circular (CP) polarization. The experimental results were compared to the theoretical models of Ammosov-Delone-Krainov (ADK) and Perelomov-Popov-Terent'ev (PPT). We find that the measured phase change induced by plasma formation can be explained by the ADK theory in the adiabatic tunneling ionization regime, while PPT model can be applied to all different regimes. We have also measured the photoionization and fractional photodissociation of molecular (MO) hydrogen. By comparing our experimental results with PPT and MO-PPT models, we have determined the likely ionization pathways when using three different pump laser wavelengths of 800 nm, 400 nm, and 267 nm.

Solution-processed antimonene integrated arc-shaped fiber for mode-locked pulse laser generation at 1.9 μm spectral region

A mono-element 2D material named antimonene has more favorable physical and optical characteristics than its bulk counterpart, making it an excellent saturable absorber (SA) material. Few layers of 2D antimonene solution were synthesized using the ultrasonic-assisted liquid-phase exfoliation (LPE) approach. The nonlinear absorption properties were measured using a balanced twin detection method, with saturable absorption of 9.6% and saturation intensity of 0.11 MW/cm2. The drop-casting technique incorporated a few drops of 2D antimonene solution onto the fabricated arc-shaped fiber. The laser performances were observed upon integrating the antimonene/arc-shaped fiber-based SA into the thulium/holmium-doped fiber (THDF) laser cavity. A soliton optical spectrum was produced at the pump power of 292.7 mW with a central wavelength of 1914.95 nm and 3-dB bandwidth of 2.41 nm. In addition, stable mode-locked pulses were produced with a frequency of 11.1 MHz and pulse width of 1.61 ps. The high stability of generated pulses was evidenced by the high signal-to-noise ratio of 62.7 dB and no significant fluctuation on central wavelength and 3 dB bandwidth throughout the 3 h of the observation period. These findings imply the potential of 2D antimonene in developing desired photonic applications, particularly in broadband ultrafast photonics.

Relationship between the sensitivity and beam splitting ratio, conversion efficiency, and local oscillator power in balanced detection.

Balanced detection, which is becoming increasingly essential for wireless communication and optical fiber communication, has been extensively studied in recent years. However, the relationships between the sensitivity and the other parameters have not yet been comprehensively ascertained. In this work, the relationship between the sensitivity and the local oscillator power Plo, consistency parameter Δα, and beam splitting ratio ε in balanced detection is explored through numerical and communication system simulations. If ε decreases, the sensitivity increases, and the corresponding Plo decreases. With the increase or decrease in Δα, ε corresponding to the minimum sensitivity shifts toward the right or left, respectively. This shift increases with the increase in the absolute value of Δα, and the minimum value of the sensitivity increases. When the absolute values of Δα are equal, their curves are almost symmetrical. As ε approaches 0.5, the tolerable maximum of Plo becomes higher. At any instant, the average value of the quantum efficiency of the two photodiodes is more critical than the maximum quantum efficiency and balance. This work facilitates thorough understanding of the sensitivity of balanced detection, which can be beneficial for future optical communication design.

Rigorous calibration of homodyne detection efficiency for continuous-variable quantum key distribution.

We propose a rigorous calibration method for homodyne detection efficiency, which combines all the factors that affect detection efficiency to calibrate together through the actual homodyne detection. With this method, the transmittance converted from electronic noise in the one-time calibration method of the shot noise can be attributed to the detection inefficiency. Thus, a trusted detection noise-free model for continuous-variable quantum key distribution (CV-QKD) can be established, which simplifies the calibration of shot noise while having the same performance as the trusted detection noise model. We demonstrate this calibration method with a balanced detector based on a transimpedance amplifier. Experimental results show that detection efficiency will be overestimated if the integration factor of the detector is overlooked. The overestimation of the detection efficiency leads to an underestimation of modulation variance and excess noise when the modulation variance is monitored by the balanced detector, which opens security loopholes. Our method may prove a necessary method in the calibration of detection efficiency for CV-QKD.

Neural Network-Powered Nonlinear Compensation Framework for High-Speed Continuous Variable Quantum Key Distribution

In this paper, we present a neural network-based framework to compensate for the nonlinear distortion of high-speed continuous variable quantum key distribution (CV-QKD) systems. In practice, the nonlinear impairment of imperfect devices could bring interference to the measured quadrature values and thus compromise the parameter estimation procedure and the secure key rate. Specifically, for a high-speed CV-QKD system, the balanced homodyne detector (BHD) and the following analog-to-digital converter are the primary sources of nonlinear impairment. The nonlinear distortion effect will be more pronounced as the pulse repetition rate increases. We propose an autoencoder-based network to learn the inner patterns of distorted data and then compensate the quadrature measurements in real-time at the receiver’s side. The numerical simulation demonstrates that the excess noise induced by nonlinear impairment is reduced to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-3}$</tex-math></inline-formula> level in the shot noise unit. Consequently, such a software approach has the ability to significantly boost the bandwidth efficiency of the BHD and the secure key bit rate.

Effect of Doppler shift on preamplifier DPSK receivers using balanced detection for optical satellite networks.

Optical satellite networks using laser intersatellite links are recognized as being capable of satisfying the increasing broadband communication demands. However, the Doppler shift due to relative movement between satellites must be taken into consideration. A convenient method that does not require complicated mathematical calculation is adopted to derive the relationship between the residual Doppler shift and the bit error rate (BER) for the balanced differential phase-shift keying (DPSK) detection system. It is shown that the DPSK receiver is sensitive to the residual Doppler shift; when the residual Doppler shift Δfres/Rb is 0.05, 0.10, and 0.15, the BER is reduced from a level of 10-17 to a level of 10-14, 10-11, and 10-6, respectively. The BER of a certain node pair under different link assignment schemes is calculated in a typical multi-hop LEO constellation. The results show that the communication BER of the same node pair is not only related to the step change of the Doppler but also related to the accumulation of the Doppler shift in the optical path. The results obtained here will be helpful in routing selection, link assignment, topology design, and system compensation.

Preimplantation genetic testing for structural rearrangement based on low-coverage next-generation sequencing accurately discriminates between normal and carrier embryos for patients with translocations

Research questionCan preimplantation genetic testing for structural rearrangement (PGT-SR) based on low-coverage next-generation sequencing (NGS) accurately discriminate between normal and carrier embryos of reciprocal translocation (RecT) and Robertsonian translocation (RobT)? DesignA total of 109 couples with RecT or RobT were included in this study. The ages, bad obsteric histories (BOH), blood karyotype and IVF cycle information, including the number of cumulus–oocyte complexes, metaphase II oocytes, two pronuclei oocytes and blastocysts were recorded. 0.1 × whole genome sequencing (WGS) of embryos followed by copy number variation (identifying unbalanced/balanced) and 2 × WGS of parents and embryos followed by haplotype analysis (discriminating between normal and carrier) were carried out in PGT-SR cycles. The embryos without translocation were transferred and clinical outcomes evaluated. ResultsAmong all the couples in this study, 67 patients had RecT and 42 had RobT. After unbalanced and balanced detection, 103 balanced embryos underwent a further normal and carrier discrimination procedure, and 53 normal embryos were identified. Finally, 32 normal embryos were transferred, with an ongoing pregnancy rate of 46.88% (15/32). All ongoing pregnancies underwent amniocentesis, and the amninocentesis karyotyping results showed 100% concordance with PGT-SR diagnosis. ConclusionsOur low-coverage NGS-based PGT-SR method can accurately discriminate between normal and carrier status of balanced embryos. The method is cost-effective and has broad clinical applicability.

A dual-channel microwave photonic mixer with large dynamic range

A microwave photonic mixer has the advantages of large transmission bandwidth, good frequency tunability, high isolation, and anti-electromagnetic interference. For the practical application of the multi-input multi-output (MIMO) communication system, a dual-channel, large dynamic range microwave photonic mixer is proposed in this paper. By controlling the bias angle of the modulator and applying a dual-polarization coherent receiver, the quadrature modulation and demodulation of the dual-channel radio frequency (RF) signal is realized. By constructing a heterodyne polarization light source and adjusting the laser power difference, the third-order intermodulation distortion (IMD3) component in the dual-channel frequency conversion signal can be suppressed. Moreover, due to the balanced detection technology, the second-order spurious-free dynamic range (SFDR2) and the third-order spurious-free dynamic range (SFDR3) of the system are simultaneously improved. Simulation results show that this scheme can obtain local oscillator-radio frequency (LO-RF) isolation of more than 40 dB, SFDR2 of more than 98 dB·Hz1/2, and SFDR3 of more than 115 dB·Hz2/3. In the 16quadrature amplitude modulation (QAM) vector signal test with 450 MHz bandwidth, the adjacent channel power ratio (ACPR) of the system is up to 53 dB and the error vector amplitude (EVM) is as low as 0.3%. Finally, the error analysis of the system is also carried out.

A 600 MHz Balanced Homodyne Detector for Quantum Information Processing

Balanced homodyne detector (BHD) has been widely used in quantum information processing. In this paper, we design a wide bandwidth BHD and implement an optical layout to test the performance of our BHD. The test results show that the bandwidth is about 600 MHz, the quantum to classical noise ratio (QCNR) is 15.2 dB and the common mode rejection ratio (CMRR) is 41.16 dB. The performance of BHD shows that it can meet the requirements of high-speed applications in quantum information processing.