Coherent Detection Research Articles

Demonstration of an 8-Gbit/s quadrature-phase-shift-keying coherent underwater wireless optical communication link using coherent heterodyne detection under scattering conditions.

In this paper, we experimentally demonstrate an 8-Gbit/s quadrature-phase-shift-keying (QPSK) coherent underwater wireless optical communication (UWOC) link under scattering conditions at 532 nm. At the transmitter, we generate the 532-nm QPSK signal using second-harmonic generation (SHG), where the 1064-nm signal modulated with four phase levels of an 8-phase-shift-keying (8-PSK) format is phase doubled to produce the 532-nm QPSK signal. To enhance the receiver sensitivity, we utilize a local oscillator (LO) at the receiver from an independent laser source. The received QPSK data beam is mixed with the independent LO for coherent heterodyne detection. Results show that the bit error rates (BERs) of the received QPSK signal can reach below the 7% forward error correction (FEC) limit under turbid water with attenuation lengths (γL) up to 7.4 and 6.1 for 2- and 8-Gbit/s QPSK, respectively. The corresponding receiver sensitivities are -34.0 and -28.4 dBm for 2- and 8-Gbit/s QPSK, respectively.

STRUCTURE FEATURES OF MINIMUM FREQUENCY SHIFT KEYING SIGNAL MODEMS

The article discusses specific features of constructing modems for minimum frequency shift keying signals using various schemes with high noise immunity levels. An analysis of the formation of these signals and frequency modulation signals, which are orthogonal if enhanced, is provided, including quantitative indicators and comparative characteristics. Two schemes of minimum frequency shift keying signal demodulators (coherent and autocorrelation) are examined, including their comparative features. Operation principles of these demodulator circuits with the use of timing diagrams are described, requirements for the reference oscillator generators included in coherent demodulators are formulated, as well as the requirements for minimum frequency shift keying signals, which ensure the greatest noise immunity in autocorrelation demodulators of minimum frequency shift keying signals with binary signals quantization.

Output Characteristics of External-Cavity Mode-Hop-Free Tunable Laser Source in C+L Band

Tunable laser sources with a wide wavelength tuning range, mode-hop-free (MHF) operation, and high spectral purity are essential for applications such as high-resolution spectroscopy, coherent detection, and intelligent fiber sensing. In this paper, we present a wide-range tunable laser source that operates without mode hopping, based on external cavity feedback using a semiconductor gain chip as the laser gain medium. The wavelength, power, and spectral characteristics of the laser are experimentally measured. A wide MHF continuous wavelength tuning range from 1480 nm to 1620 nm with a side-mode suppression ratio of more than 61.65 dB is achieved. An output optical power of more than 11.14 dBm with good power stability can also be realized in the full C+L band. This proposed external-cavity tunable laser source features a narrow intrinsic linewidth and MHF tunable radiation with a maximum sweep speed of 200 nm/s, enabling practical applications such as high-resolution vector spectrum analysis.

Security of discrete-modulated continuous-variable quantum key distribution

Continuous variable quantum key distribution with discrete modulation has the potential to provide information-theoretic security using widely available optical elements and existing telecom infrastructure. While their implementation is significantly simpler than that for protocols based on Gaussian modulation, proving their finite-size security against coherent attacks poses a challenge. In this work we prove finite-size security against coherent attacks for a discrete-modulated quantum key distribution protocol involving four coherent states and heterodyne detection. To do so, and contrary to most of the existing schemes, we first discretize all the continuous variables generated during the protocol. This allows us to use the entropy accumulation theorem, a tool that has previously been used in the setting of discrete variables, to construct the finite-size security proof. We then compute the corresponding finite-key rates through semi-definite programming and under a photon-number cutoff. Our analysis provides asymptotic rates in the range of 0.1−10−4 bits per round for distances up to hundred kilometres, while in the finite case and for realistic parameters, we get of the order of 10 Gbits of secret key after n∼1011 rounds and distances of few tens of kilometres.

Quantum computer-enabled receivers for optical communication

Optical communication is the standard for high-bandwidth information transfer in today’s digital age. The increasing demand for bandwidth has led to the maturation of coherent transceivers that use phase- and amplitude-modulated optical signals to encode more bits of information per transmitted pulse. Such encoding schemes achieve higher information density, but also require more complicated receivers to discriminate the signaling states. In fact, achieving the ultimate limit of optical communication capacity, especially in the low light regime, requires coherent joint detection of multiple pulses. Despite their superiority, such joint detection receivers are not in widespread use because of the difficulty of constructing them in the optical domain. In this work we describe how optomechanical transduction of phase information from coherent optical pulses to superconducting qubit states followed by the execution of trained short-depth variational quantum circuits can perform joint detection of communication codewords with error probabilities that surpass all classical, individual pulse detection receivers. Importantly, we utilize a model of optomechanical transduction that captures non-idealities such as thermal noise and loss in order to understand the transduction performance necessary to achieve a quantum advantage with such a scheme. We also execute the trained variational circuits on an IBM-Q device with the modeled transduced states as input to demonstrate that a quantum advantage is possible even with current levels of quantum computing hardware noise.

Spectral analysis based blind frequency offset estimation for digital subcarrier multiplexing systems with adjustable roll-off factors and various modulation formats

We proposed a blind frequency offset estimation (FOE) method for digital subcarrier multiplexing (DSM) signals. By utilising spectral information from the DSM signal and analysing the correlation between the frequency offset (FO) and the summed power of the signal spectrum, the proposed FOE method can accurately and effectively handle various modulation schemes assigned to each subcarrier. The proposed FOE method exhibits a flexibility to the roll-off factor of the root raised cosine (RRC) spectral shaping and can achieve a high level of accuracy in FOE under different roll-off factors and subcarrier numbers, with an FOE error of less than 30 MHz under a fast Fourier transform (FFT) size of 2048. Additionally, the proposed FOE method has advantages in terms of the computational complexity compared to the existing method. The performance of the proposed FOE method is experimentally verified for 48Gbaud 16QAM DSM signals in a coherent detection system.

Measurement of the Dispersion of χ(3)$\chi ^{(3)}$ of SiO2${\rm SiO}_2$ and SiN Across the THz and Far‐Infrared Frequency Bands

AbstractTerahertz (THz) radiation sources based on two‐color femtosecond plasmas in air are becoming a mature technology for coherent spectroscopy and strong‐field physics across the extended THz range to several tens of THz. The field‐resolved detection of such THz transients relies on the third‐order nonlinearity of the detection medium. Here, a comparative measurement is demonstrated with air‐biased coherent detection (ABCD) and solid‐state biased detection (SSBCD) as a novel method to measure the dispersion of the magnitude and phase of the relevant third‐order nonlinearity for fused silica () and silicon nitride (SiN). Based on the development of the ultrabroadband SSBCD device with a detection bandwidth exceeding 30 THz, measurements are obtained across the 1–28 THz frequency range, hence covering the THz and far‐infrared. It is shown that the vibrational modes in and SiN in the THz range lead to strong resonant enhancement and dispersion of the nonlinearity. The SSBCD devices operate down to nanojoule (nJ) probe energy, and their is demonstrated by measuring the dielectric function of the Lorentzian line profile of transverse‐optical (TO) phonon mode at 9 THz in single‐crystal gallium arsenide (GaAs) and observing the weak phonon combination bands near the TO phonon.

Gbps key rate passive-state-preparation continuous-variable quantum key distribution within an access-network area

The conventional Gaussian-modulated coherent-state quantum key distribution (QKD) protocol requires the sender to perform active modulations based on a true random number generator. Compared with it, the passive-state-preparation (PSP) continuous-variable quantum key distribution (CVQKD) equivalently performs modulations passively by exploring the intrinsic field fluctuations of a thermal source, which offers the prospect of chip integration QKD with low cost. In this paper, we propose and experimentally demonstrate a high-rate PSP-CVQKD scheme within an access-network area using high-bandwidth detectors in a continuous wave encoding and decoding way. By proposing effective methods for suppressing the noises during the PSP process and polarization multiplexing to decrease the photon leakage noises, we realize the high-intensity local oscillator transmission, thereby achieving coherent detection with high efficiency, low noise, and high bandwidth. The secure key rates over transmission distance of 5.005 km with and without consideration of the finite-size effect are 273.25 Mbps and 1.09 Gbps. The use of the PSP method boosts the asymptotic secret key rate of CVQKD to Gbps level for the first time, to our knowledge, within the range of the access network, which provides an effective and secure key distribution strategy for high-speed quantum cryptography access communication.

Coherent homodyne detection for amplified cross-beam electric-field induced second harmonic

The electric-field induced second harmonic (E-FISH) signal sensing is commonly used as a nonlinear optical technique to probe electric fields inside a plasma region. Cross-beam E-FISH is being investigated to improve spatial resolution by defining the interaction volume via a controlled geometry of two overlapping noncolinear optical beams. This drastic reduction in interaction length of the electric field and laser region results in a significant signal reduction. To overcome this signal reduction, we introduce coherent amplification of the cross-beam E-FISH signal by mixing the low E-FISH signal with a phase-locked bright local oscillator. We demonstrate enhancement of the signal. By introducing a local oscillator we can now derive the polarity of the measured electric field through the phase of the homodyne signal. To illustrate the technique, we, for the first time, to the best of our knowledge, measure the magnitude and the direction of the electric field in a cold atmospheric pressure plasma jet, which dynamically follows the profile of the applied bias current.

Semantic versus perceptual priming: dissecting their impact on intuitive judgments of semantic coherence.

This research explores the mechanisms underlying the intuitive processing of semantic coherence, focusing on the effects of semantic and perceptual priming on semantic coherence detection. Two studies examined how these priming types influence individuals' abilities to discern semantic incoherence. In Study 1, we used solutions to semantically coherent triads as primes, finding that such priming significantly improves participants' accuracy and confidence in identifying incoherent elements within word tetrads. These results corroborate the hypothesis that intuitive judgments in linguistic tasks are closely tied to the processing fluency elicited by semantic connections. In Study 2, we show that perceptual priming does not significantly enhance accuracy, albeit it does increase the confidence with which individuals make their judgments. Distinct effects of semantic and perceptual priming on intuitive judgments highlight the complex interplay between processing fluency and affect in shaping intuitive judgments of semantic coherence. We discuss the nuanced roles of semantic and perceptual factors in influencing the accuracy and confidence of intuitive decisions.

Linewidth Measurement of a Narrow-Linewidth Laser: Principles, Methods, and Systems.

Narrow-linewidth lasers mainly depend on the development of advanced laser linewidth measurement methods for related technological progress as key devices in satellite laser communications, precision measurements, ultra-high-speed optical communications, and other fields. This manuscript provides a theoretical analysis of linewidth characterization methods based on the beat frequency power spectrum and laser phase noise calculations, and elaborates on existing research of measurement technologies. In addition, to address the technical challenges of complex measurement systems that commonly rely on long optical fibers and significant phase noise jitter in the existing research, a short-delay self-heterodyne method based on coherent envelope spectrum demodulation was discussed in depth to reduce the phase jitter caused by 1/f noise. We assessed the performance parameters and testing conditions of different lasers, as well as the corresponding linewidth characterization methods, and analyzed the measurement accuracy and error sources of various methods.

Empowering the Blind: Contactless Activity Recognition with Commodity Software-Defined Radio and Ultra-High-Frequency Radio Frequency Identification.

This study presents a novel computational radio frequency identification (RFID) system designed specifically for assisting blind individuals, utilising software-defined radio (SDR) with coherent detection. The system employs battery-less ultra-high-frequency (UHF) tag arrays in Gen2 RFID systems, enhancing the transmission of sensed information beyond standard identification bits. Our method uses an SDR reader to efficiently manage multiple tags with Gen2 preambles implemented on a single transceiver card. The results highlight the system's real-time capability to detect movements and direction of walking within a four-meter range, indicating significant advances in contactless activity monitoring. This system not only handles the complexities of multiple tag scenarios but also delineates the influence of system parameters on RFID operational efficiency. This study contributes to assistive technology, provides a platform for future advancements aimed at addressing contemporary limitations in pseudo-localisation, and offers a practical, affordable assistance system for blind individuals.

Google Earth Engine and Machine Learning for Flash Flood Exposure Mapping—Case Study: Tetouan, Morocco

Recently, the earth’s climate has changed considerably, leading to several hazards, including flash floods (FFs). This study aims to introduce an innovative approach to mapping and identifying FF exposure in the city of Tetouan, Morocco. To address this problem, the study uses different machine learning methods applied to remote sensing imagery within the Google Earth Engine (GEE) platform. To achieve this, the first phase of this study was to map land use and land cover (LULC) using Random Forest (RF), a Support Vector Machine (SVM), and Classification and Regression Trees (CART). By comparing the results of five composite methods (mode, maximum, minimum, mean, and median) based on Sentinel images, LULC was generated for each method. In the second phase, the precise LULC was used as a related factor to others (Stream Power Index (SPI), Topographic Position Index (TPI), Slope, Profile Curvature, Plan Curvature, Aspect, Elevation, and Topographic Wetness Index (TWI)). In addition to 2024 non-flood and flood points to predict and detect FF susceptibility, 70% of the dataset was used to train the model by comparing different algorithms (RF, SVM, Logistic Regression (LR), Multilayer Perceptron (MLP), and Naive Bayes (NB)); the rest of the dataset (30%) was used for evaluation. Model performance was evaluated by five-fold cross-validation to assess the model’s ability on new data using metrics such as precision, score, kappa index, recall, and the receiver operating characteristic (ROC) curve. In the third phase, the high FF susceptibility areas were analyzed for two-way validation with inundated areas generated from Sentinel-1 SAR imagery with coherent change detection (CDD). Finally, the validated inundation map was intersected with the LULC areas and population density for FF exposure and assessment. The initial results of this study in terms of LULC mapping showed that the most appropriate method in this research region is the use of an SVM trained on a mean composite. Similarly, the results of the FF susceptibility assessment showed that the RF algorithm performed best with an accuracy of 96%. In the final analysis, the FF exposure map showed that 2465 hectares were affected and 198,913 inhabitants were at risk. In conclusion, the proposed approach not only allows us to assess the impact of FF in this study area but also provides a versatile approach that can be applied in different regions around the world and can help decision-makers plan FF mitigation strategies.

Investigation on the performance of photonics-aided W-band millimeter-wave wireless transmission

W-band (75–110 GHz) is a potential radio frequency band to provide long-distance wireless links for mobile data transmission. This paper proposes and experimentally demonstrates high-speed wireless transmission at W-band using photonics-aided method, including optical heterodyne, photonics-aided down-conversion without RF oscillator and coherent detection. A comparison between the photonics-aided method and the conventional electronic method employing solid-state electronic devices is conducted for the first time. The photonics-aided method is shown to offer advantages such as lower harmonic components, spur, reduced nonlinearity, and no local oscillator leakage, results in a 2.5 dB better performance of the photonic-aided W-band mm-wave transmitter compared to the electronic one. In the terms of receiver, the photonics-aided method can surpass the electronic method, with the help of larger electro-optical modulator bandwidth and lower drive voltage in the photonic down-conversion stage. Ultimately, using the photonics-aided method, a recorded equivalent transmission distance of 29 km@84 GHz and 45km@75.6GHz is achieved respectively for 1Gbaud QPSK signal.

Характеристики обнаружения когерентной системы при рассогласовании квадратурных каналов

Introduction – The operation of an adaptive coherent detection system is considered. The structure of a processing device is presented, containing a series-connected shaper of quadrature signal components that convert high-frequency input radio signals to video frequency, an adaptive multi-channel coherent storage device with threshold devices. Goal – To study the influence of amplitude and phase mismatches of quadrature channel parameters, which are inevitable in practice, on the operation of an adaptive coherent detection system. Formulation of the problem – Determine the spectral-correlation characteristics of the processed signals when the parameters of the quadrature channels are mismatched. Assess the influence of amplitude and phase deviations of the parameters of quadrature channels on the characteristics of a coherent detection system. Method – The complex variable method is used, in which the input high-frequency signals are represented at the video frequency by a pair of complex conjugate real components, shifted in phase by ninety degrees. Results – A model of the processed signal is presented at the video frequency, taking into account possible mismatches in the parameters of quadrature channels in practice. Expressions are obtained and graphs of the spectral density of the complex signal are plotted separately for the amplitude and phase mismatch of the quadrature channels. It is shown that the mismatch of quadrature channels leads to the appearance in the spectrum of the output signal of phase detectors of a component at a frequency that is the mirror frequency of the signal, which leads to an increase in the probability of false detection. The results of calculations of the probability of false detection of a signal depending on the deviation of the transmission coefficients of the quadrature channels and the phase mismatch of the reference voltages of the quadrature channels are presented. Conclusion – The significant influence of amplitude and phase mismatches of the parameters of quadrature channels on the spectral-correlation characteristics of the complex signal is shown. The mismatch of the parameters of the quadrature channels leads to deterioration in the detection characteristics of the coherent processing system. The research methodology used allows us to develop requirements for the quality of tuning quadrature channels.

Thin-film lithium niobate-based electro-optic comb cloning for self-homodyne coherent communication.

As the optical communication industry advances, metropolitan area networks (MANs) and radio access networks (RANs) are extensively deployed on a large scale, demanding energy-efficient integrated light sources and simplified digital signal processing (DSP) technologies. The emergence of thin-film lithium niobate (TFLN) has given rise to high-performance, energy-efficient on-chip modulators, making on-chip optical frequency comb (OFC) more appealing. Owing to the phase uniformity and stability of this chip-scale device, it has been possible to eliminate the carrier frequency phase estimation (CPE) in DSP stacks using comb-clone-enabled self-homodyne detection. Here we report the first use, to our knowledge, of a TFLN on-chip electro-optic (EO) frequency comb to realize comb cloning and self-homodyne coherent detection. We transmit three optical pilot tones and eight data channels encoded with 20 Gbaud polarization-multiplexed 16-ary quadrature amplitude modulation (PM-16-QAM) over 10 km and 80 km standard single-mode fibers. The bit error ratios (BERs) of the eight channels reach below 10-3, a result made possible by our on-chip comb. The scalability and mass producibility of on-chip EO combs, combined with the simplified DSP, show potential in our proposed fifth-generation (5G) RAN and MAN transmission scheme.

Unraveling Electronic and Vibrational Coherences Following a Charge Transfer Process in a Photosystem II Reaction Center

A reaction center is a unique biological system that performs the initial charge separation within a Photosystem II (PSII) multiunit enzyme, which eventually drives the catalytic water-splitting in plants and algae. The possible role of quantum coherences coinciding with the energy and charge transfer processes in PSII reaction center is one of the active areas of research. Here, we study these quantum coherences by using a numerically exact method on an excitonic dimer model, including linear vibronic coupling and employing optimal parameters from experimental two-dimensional coherent spectroscopic measurements. This enables us to precisely capture the excitonic interaction between pigments and the dissipation of the energy from electronic and charge-transfer (CT) states to the protein environment. We employ the time nonlocal (TNL) quantum master equation to calculate the population dynamics, which yields numerically reliable results. The calculated results show that, due to the strong dissipation, the lifetime of electronic coherence is too short to have direct participation in the charge transfer processes. However, there are long-lived vibrational coherences present in the system at frequencies close to the excitionic energy gap. These are strongly coupled with the electronic coherences, which makes the detection of the electronic coherences with conventional techniques very challenging. Additionally, we unravel the strong excitonic interaction of radical pair (PD1 and PD2) in the reaction center, which results in a long-lived electronic coherence of >100 fs, even at room temperature. Our work provide important physical insight to the charge separation process in PSII reaction center, which may be helpful for better understanding of photophysical processes in other natural and artificial light-harvesting systems.

Non-line-of-sight optical wireless communication system enabled by wavefront shaping for multi-user indoor access.

In this Letter, we experimentally investigate a non-line-of-sight (NLOS) optical wireless communication (OWC) system that utilizes wavefront shaping techniques to realize simultaneous data transmission for multiple users. Wavefront shaping techniques are employed to address the issue of low intensity of diffusely reflected light at the receiver in NLOS scenarios for indoor high-speed access. To achieve communication path planning and tracing for two different users in free-space optical communication, the pixels of the spatial light modulator (SLM) are divided into two halves to separately manipulate the wavefront of two independent data carriers centered at different wavelengths. The maximum received optical power can be effectively improved by more than 15 dB with the wavefront shaping technique. To avoid power enhancement of non-target wavelength, the wavelength difference of two different users is experimentally studied. The difference in power enhancement ratio (DPER) is increased with the wavelength difference, and 14.95 dB DPER is obtained with a 10 nm wavelength difference. Under the aforementioned wavelength planning strategy, successful transmission and reception of 2 × 160 Gbit/s 16-QAM signals for two users with coherent detection is achieved using wavelengths of 1550 and 1560 nm in an indoor access scenario.

A low-complexity ISAC method for LFM-PSK signals using de-chirping and ISAC phase unwrapping techniques

Integrated Sensing and Communication (ISAC) has the potential to revolutionize wireless technology in areas such as autonomous vehicles, the Internet of Things, and unmanned aerial vehicle navigation technology. Nonetheless, conventional processing methods for Linear Frequency Modulation-Phase Shift Keying (LFM-PSK) signals predominantly depend on matched filtering, leading to high sampling rates and computational complexity. In response to these challenges, our research proposes a method to process LFM-PSK signals, which enables the use of de-chirping techniques. Applying the proposed ISAC unwrapping technique makes it possible to selectively recover the phase of the de-chirped LFM-PSK signal. Subsequently, the distance can be calculated by analyzing the slope of the recovered phase. Moreover, by adjusting the threshold in the ISAC unwrapping technique, communication information can be extracted by detecting phase hops associated with PSK. This method offers an attractive alternative to traditional methods such as matched filtering and coherent demodulation techniques. We derive the Cramér-Rao lower bound (CRLB) for ranging. Theoretical analysis and simulation indicate that our method effectively reduces the required sampling rate and provides higher-ranging performance than the matched filtering technique in radar sensing. In high signal-to-noise ratio scenarios, our proposed method delivers a ranging performance close to that of CRLB. Regarding communication, our method has the advantage of lower synchronization requirements, making it more feasible for practical engineering applications than the conventional coherent demodulation method. Furthermore, it also necessitates lower computational resources compared to matched filtering techniques. Importantly, our method allows high device sharing, promoting cost-effective ISAC implementation.

Interference fading suppression with a multi-subcarrier pulse in a distributed acoustic sensor

A low-complexity multi-subcarrier pulse generation scheme is proposed to suppress the interference fading in a phase-sensitive optical time-domain reflectometer (Φ-OTDR) based distributed acoustic sensor (DAS) with heterodyne coherent detection. The multi-subcarrier pulse is generated in the digital domain based on the proper clipping operation of a sine signal. The localization and recovery of the disturbance signal are realized by the spectrum extraction and rotated vector sum (SERVS) method. The experimental results show that the occurrences of interference fading can be significantly reduced. The intensity fluctuation is reduced from ∼75 dB to ∼25 dB. Multiple disturbance signals are successfully demodulated to verify the effectiveness of the proposed method.