Dual Frequency Modulation Research Articles

Design and Application of Laser Polarization Underwater Detection Equipment

With the increasing demand for precise identification of underwater targets, the development of advanced underwater detection technologies has become a pivotal area of research. This study presents the design and implementation of a laser-based underwater detection system that leverages polarization characteristics to significantly enhance detection accuracy and target identification capabilities in complex aquatic environments. A key innovation of this research lies in the application of a dual-frequency modulation technology using a 532 nm pulsed laser. By modulating the high-frequency characteristics of the laser, this technique effectively suppresses backscattering interference within the water medium, improves the efficiency of target signal extraction, and exhibits exceptional performance, particularly in long-range detection and highly turbid water conditions. This paper elucidates the principles of underwater laser detection and polarization measurement, outlines the design of an integrated optical and mechanical system for laser transmission and reception, and introduces an optimized signal processing methodology. Experimental results demonstrate that the proposed system can reliably distinguish targets composed of different materials while maintaining high detection accuracy across a range of challenging environmental conditions. A comparative analysis further highlights the system’s significant advantages over traditional technologies, including enhanced noise suppression and greater detection depth. These findings establish a solid foundation for advancing underwater detection technologies and broadening their practical applications.

Dual microwave frequency modulation of the VCSEL injection current for a CPT-based atomic clock.

Previously, we have proposed a method to control the emission spectrum of the vertical-cavity surface-emitting laser (VCSEL) with the synchronized modulation of the injection current at single and doubled frequencies. In this work, the above method is used to improve the metrological characteristics of the coherent population trapping (CPT) resonance in 87Rb. The dual-frequency (DF) modulation reduces the carrier power and suppresses the light shift of the resonance frequency, if it is unattainable with the single-frequency modulation. In addition, a higher resonance contrast is achieved by equalizing the powers of the resonant components of the spectrum.

Dual-frequency modulation based mutual inductance and load identification method for inductively coupled power transfer systems

Dual-frequency modulation based mutual inductance and load identification method for inductively coupled power transfer systems

High-precision characterization of quantum-cascade laser frequency response using wavelength modulation spectroscopy.

This paper investigates the impact of the quantum cascade laser's frequency modulation response on its tuning rate and tunability. We show a significant disparity in laser tuning rates and tunability between single and dual-frequency modulation schemes frequently used in typical direct absorption and wavelength modulation spectroscopy (WMS) techniques. We show that the DC-characterized tuning rate of a laser can be reduced significantly under a specific set of modulation frequencies of the laser injection current. We characterize these effects by simultaneous measurements of higher harmonic WMS of methane and nitrous oxide in the 7.8 µm spectral regions. We further show that WMS signal modulation broadening mechanisms and spectral structure, i.e., its zero-crossings and turning points, can be used to quantify such laser-modulation effects and validate laser frequency response under dual modulation schemes.

Enhancing weak-magnetic-field sensing of a cavity-magnon system with dual frequency modulation

Enhancing weak-magnetic-field sensing of a cavity-magnon system with dual frequency modulation

Simultaneous dual-harmonic detection for LITES with a multi-pass configuration based on combined flexural modes of a quartz tuning fork

The simultaneous dual-harmonic detection for light-induced thermoelastic spectroscopy (LITES) with a multi-pass configuration based on combined flexural modes of a quartz tuning fork (QTF) is demonstrated. To enhance the photothermal energy absorbed by a custom QTF, the laser beam is designed to pass through the QTF multiple times. With dual-frequency modulation methodology, the second harmonic (2f_fun) signal and the first harmonic (1f_ove) signal are simultaneously inspired at resonant frequencies of fundamental and overtone flexural modes of the QTF, respectively. Thus, precise light intensity and gas concentration can be retrieved concurrently with corresponding 1f and 1f-normalized 2f signals using frequency division multiplexing technology. In comparison to traditional LITES under the same condition sensor, our triple-pass configuration performs 1.84 times better in signal-to-noise ratio (SNR). According to Allan variance analysis, 1f-normalized 2f signal enables a longer integration time of 1100 s and a lower detection limit of 0.07 ppm. The corresponding normalized noise equivalent absorption coefficient (NNEA) is 0.8 × 10−9 cm−1W/Hz1/2.

Dual-frequency modulated heterodyne quartz-enhanced photoacoustic spectroscopy.

A novel dual-frequency modulated heterodyne quartz-enhanced photoacoustic spectroscopy (DFH-QEPAS) was demonstrated for what we believe to be the first time in this study. In traditional H-QEPAS, the frequency of modulated sinusoidal wave has a frequency difference (Δf) with the resonance frequency (f0) of a quartz tuning fork (QTF). Owing to the resonance characteristic of QTF, it cannot excite QTF to the strongest response. To achieve a stronger response, a sinusoidal wave with a frequency of f0 was added to the modulation wave to compose a dual-frequency modulation. Acetylene (C2H2) was chosen as the target gas to verify the sensor performance. The proposed DFH-QEPAS improved 4.05 times of signal-to-noise ratio (SNR) compared with the traditional H-QEPAS in the same environmental conditions.

Dual-Frequency Modulation to Achieve Power Independent Regulation for Dual-Load Underwater Wireless Power Connector

The power supply of underwater devices can be realized by the nonelectrical contact wireless power transfer (WPT) technology. The diversity and flexibility of devices carried by the remotely operated vehicle (ROV) can be increased by the WPT which can achieve convenient and safe device switching. Power independent regulation is a key for the underwater wireless power connector (UWPC) with multiload, and a superimposed dual-frequency modulation (SDFM) method is proposed in this article to achieve the load power independent regulation. The equal-area rule is first applied to SDFM, which can use one inverter to generate SDFM current at the transmitter. The theory of decoupled power transfer is analyzed in detail for the SDFM UWPC with dual-load (DL). All the above analyses are verified in a DL UWPC system prototype. The experimental results show that the SDFM current is generated using one inverter, and the load power can be independently regulated by adjusting the output voltage of the inverter. Furthermore, the load power is still independent and controllable under the conditions of load switching and mutual inductances changing. The DL UWPC system power can reach 50 W and dc–dc efficiency is about 68.73%.

Ground-Vibration Suppression by a Matched Center of Mass for Microthrust Testing in Spaceborne Gravitational-Wave Detection

High-precision and wide-band microthrust testing and evaluation on the ground is key to achieving precision, drag-free control, which is extremely demanding for spaceborne gravitational-wave detection missions. However, unlike a stable space environment, complex ground vibrations inevitably affect the evaluation of microthrust. To reduce the interference of ground vibrations, this paper proposes a suppression solution based on a hanging pendulum with an adjustable mass center. The theoretical analysis shows that the response of the pendulum to ground vibration can be effectively reduced with a matched mass center. The dual-frequency modulation experiment verified the linear relationship between vibration noise and mass-center-to-swing-point distance. By setting the mass-center distance from the swing point to 0.2 mm, the noise of the sub-micro-Newton balance is better than $0.1\phantom{\rule{0.1em}{0ex}}\text{\ensuremath{\mu}}\mathrm{N}\phantom{\rule{0.2em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}$ from 6 mHz to 20 Hz. This method effectively suppresses the interference of ground vibration, which helps us to obtain an accurate microthrust model for space gravitational-wave missions, while providing a solution for weak-force, ground testing over wide-band frequencies.

Control of the VCSEL spectrum by dual microwave frequency modulation.

We propose and investigate a method for controlling the spectrum of the vertical-cavity surface-emitting laser by simultaneous modulation of the injection current at single and doubled frequencies. We experimentally demonstrate the ability to control the power asymmetry of the first-order sidebands and to suppress the carrier by the proposed method. These possibilities are beneficial to improve frequency stability of atomic clocks based on the effect of coherent population trapping.

An interharmonic dual switching frequency modulation strategy for impedance network inverters

SummaryRecently, a novel dual switching frequency modulation (DFM) has been proposed for impedance network inverters (INIs). The proposed approach allows the INIs to be designed optimally in size and efficiency, for motor drive applications. Moreover, DFM provides the possibility of operation with unity modulation index. However, DFM requires a high‐frequency ratio between pulse‐width modulation (PWM) and sinusoidal PWM (SPWM) switching frequencies. This paper proposes an improved modulation strategy to overcome this issue. The modulation called interharmonic dual frequency modulation (IDFM) utilizes the interharmonic areas of a conventional sinusoidal PWM to implement DFM. The proposed approach gives a low total harmonic distortion (THD) output voltage without increasing the frequency ratio of PWM and SPWM. IDFM provides the possibility of selecting a wide range of frequencies for both switching modulations. The approach is explained theoretically, compared with other switching strategies, and also the simulation and experimental results are used to validate its performance.

Machine Learning Assisted Multi-Functional Graphene-Based Harmonic Sensors

Real-time monitoring of multiple (bio)chemical agents, molecules and gases is in a high demand, particularly for the future internet-of-things (IoTs) and point-of-care tests (POCT), that are connected via the 5G ecosystem. Here, we propose a lightweight, multi-agent (bio)-chemical wireless sensor based on graphene field-effect transistor (GFET) circuits, taking advantage of GFET’s dual functionalities, i.e., frequency modulation and (bio-)chemical sensing. The GFET-based radio-frequency (RF) modulators circuits can convert the continuous wave (CW) monotonic signal to multiple harmonics, with conversion efficiencies sensitively depending on densities of (bio-)chemical agents. Specifically, we exploit a machine learning (ML)-based readout method to extract the concentration levels of the (bio-)chemical dopants from the harmonic spectrum. Further, we show that by increasing the order of GFET circuits and thus the number of detectable harmonics, the neural network performance and the overall readout accuracy can be enhanced. The proposed GFET-based wireless sensor could be ultracompact, ultralow-profile, portable and flexible, thus potentially benefiting a wide range of applications in IoTs, POCTs, and Industry 4.0.

Comprehensive Ranging Disambiguation for Amplitude-Modulated Continuous-Wave Laser Scanner With Focusing Optics

Amplitude-modulated continuous-wave (AMCW) laser scanner with focusing optics is expected to realize high-precision 3-D measurement, which requires accuracy of mm or less. Since such a modulation scheme employs periodical intensity modulation, the longitudinal resolution and the unambiguous range are in a tradeoff. Another problematic situation is the case that the target object is large so that the scanning range exceeds the unambiguous range. The acquired 3-D point clouds contain phase jumps at the boundary of the unambiguous range imposed by the modulation frequency. In this article, we propose multiple solutions to cope with these problems. First of all, our system utilizes dual-frequency modulation to overcome the tradeoff between the resolution and the unambiguous range. With appropriate electronics for demodulation, the hardware imposed a longitudinal resolution of 19.2 ± 38 $\mu \text{m}$ and the unambiguous range of 48 ± 614.3 $\mu \text{m}$ were realized. The unambiguous range can be shifted within 5-m range using a mechanical focusing optics. However, such an attractive laser scanner still suffers from ranging ambiguity. The noise on the laser light modulated by the lower modulation frequency results in phase deviation, which can incur ranging errors at the integer times of the half cycle of the higher modulation frequency. We have coped with such ranging ambiguity by synthesizing ranging errors with the data in the correct range. Especially, defocused 3-D point clouds contaminated by severe ranging errors were analyzed and restored. The measurement range can thus be elongated by >20 times of the depth-of-focus using such data processing without manipulating the mechanical focusing optics. Next, we developed an algorithm as a remedy to prevent phase jumps in the unambiguous range. With exploitation of the relationship between the intensity and spatial information, the phase unwrapping was performed to recover the spatial continuity. Thereafter, the unambiguous range can be elongated to be >48 cm. With all the abovementioned configuration and data processing, we have overcome the ranging ambiguity inherent in the AMCW laser scanner.

A high-efficiency acquisition method of LED-multispectral images based on frequency-division modulation and RGB camera

LED illumination based multispectral imaging (LEDMSI) is one of the promising techniques of fast and effective spectral image acquisition. RGB camera based LEDMSI can acquire 3-band images in a single exposure, which has practical, high-efficiency and low-cost advantages. Frequency-division modulation technology can achieve the simultaneous acquisition of multi-wavelength images. In this paper, a high-efficiency acquisition method of LED-multispectral images based on frequency-division modulation and RGB camera is proposed. And its effectiveness is verified by the experiment with dual-frequency modulation of six wavelengths (two multiplexed illuminations) as example. In the experiment, two multiplexed illuminations modulated by two carrier frequencies are used as the light source to acquire an image sequence, and grayscale images are obtained by “R/G/B” 3-channel separation. Further, the Fast Fourier Transform is used to demodulate the time series of corresponding pixels of each grayscale image, the images of each multiplexed illumination are obtained, respectively. In the result and discussion section, the multispectral images obtained by the method in this paper is compared with the multispectral images obtained without combining frequency-division modulation. Through the result, it can be concluded that the method of combining frequency-division modulation with RGB camera is a high-efficiency acquisition method of high-quality multispectral images, which provides a reference for the LED-multispectral imaging technology.

Pulsed triple frequency modulation for frequency stabilization and control of two lasers to an optical cavity

We present a method to stabilize two lasers to an optical cavity using pulsed triple frequency modulation. The setup allows simultaneous Pound-Drever-Hall stabilization, as well as independent frequency control, while removing interference terms that limit the frequency scan range and allowing for smaller modulation depths. A review of single, dual, and triple frequency modulation is also presented in addition to a discussion of how to effectively turn pulsed triple frequency modulation into independent dual frequency modulation for each laser. This method would increase the scan range to half the free spectral range.

Optimizing a dual-frequency and phase modulation method for SSVEP-based BCIs

Objective. The design of the stimulation paradigm plays an important role in steady-state visual evoked potential (SSVEP)-based brain-computer interface (BCI) studies. Among various stimulation designs, the dual-frequency paradigm in which two frequencies are used to encode one target is of importance and interest. However, because the number of possible frequency combinations is huge, the existing dual-frequency modulation paradigms failed to optimize the encoding towards the best combinations. Thus, this work aiming at designing a new dual-frequency and phase modulation paradigm with the best combinations stimuli. Approach. This study proposed a dual-frequency and phase modulation method, which can achieve a large number of targets by making different combinations of two frequencies and an initial phase. This study also designed a set of methods for quickly optimizing the stimulation codes for the dual-frequency and phase modulation method. Main results. An online 40-class BCI experiment with 12 subjects obtained an accuracy of 96.064.00% and an averaged information transfer rate (ITR) of 196.0915.25 bits min−1, which were much higher than the existing dual-frequency modulation paradigms. Moreover, an offline simulation with a public dataset showed that the optimization method was also effective for optimizing the single-frequency and phase modulation paradigm. Significance. These results demonstrate the high performance of the proposed dual-frequency and phase modulation method and the high efficiency of the optimization method for designing SSVEP stimulation paradigms. In addition, the coding efficiency of the optimized dual-frequency and phase modulation paradigm is higher than that of the single-frequency and phase modulation paradigm, and it is expected to further realize the BCI paradigm with a large amount of targets.

Constant-Frequency and Noncommunication-Based Inductive Power Transfer Converter for Battery Charging

Compared with conductive charging, wireless inductive-power-transfer (IPT) charging exhibits higher potential as it avoids physical contact and provides convenient user experience. Regrettably, it is challenging for IPT converters to comply with the constant current (CC) and constant voltage (CV) charging profiles, while optimizing power efficiency. To achieve such goals, the existing IPT converters can apply multistage converter, dual side, or variable frequency modulation with feedback wireless communication. However, applying multistage converter increases cost and loss, while the stability of the IPT converter with dual side or variable frequency modulation can be at risk if communication fails. This article proposes a single-stage IPT converter for battery charging. With a constant operating frequency and without feedback wireless communication, the receiver side directly regulates the output to comply with the CC/CV charging profile, while the transmitter side aids in the reduction of the modulated phase shift angle at the receiver side, thus improving the efficiency. No wireless communication between the transmitter and receiver sides benefits both the hardware cost and stability. Also, we implement implicitly an output voltage regulation, further avoiding the need of an extra dc–dc converter. We verify experimentally the proposed control method in a 1-KW charging platform with a measured peak efficiency up to 94.35%.

Non-linear Phased Array Imaging of Flaws Using a Dual and Tri Frequency Modulation Technique

Recently, there has been high interest in the capabilities of nonlinear ultrasound techniques for damage/defect detection as these techniques have been shown to be more sensitive than linear ultrasound techniques for certain types of damage. This paper presents a nonlinear ultrasound phased array modulation method based on the principles of frequency and amplitude modulation, for the detection and imaging of material defects/damage. The proposed method requires the use of standard ultrasound phased array systems, which use multiple transmitting and receiving elements. An adjusted dual frequency method and tri-frequency is employed, which focuses on the evaluation of harmonic sidebands. A pump signal at a frequency of f2 is used to initialise a ‘breathing/ringing’ crack scenario after which a second frequency at f1 is used to further excite the crack and improve the probability that harmonic sidebands are generated. The modulation method employs a subtraction method which is used to filter out the fundamental frequencies. The technique adds benefits in ensuring that equipment based nonlinearities produced by single frequency setups are eliminated, ensuring that only defect related nonlinearities are present. A closed fatigue crack was evaluated using multiple linear and nonlinear ultrasound phased array techniques with the suggested method showing clear benefits. Furthermore, it is shown that modulation techniques with more than two driving frequencies could further improve damage detection capabilities of phased array systems.

Dual-frequency modulation quartz crystal tuning fork-enhanced laser spectroscopy.

An innovative trace gas-sensing technique utilizing a single quartz crystal tuning fork (QCTF) based on a photoelectric detector and dual-frequency modulation technique was demonstrated for the first time for simultaneous multi-species detection. Instead of traditional semiconductor detectors and lock-in amplifier, we utilized the piezoelectric effect and resonant effect of the QCTF to measure the light intensity. A fast signal analysis method based on fast Fourier transform (FFT) algorithm is proposed for overlapping signal extraction. To explore the capabilities of this technique, a gas-sensing system based on two lasers having center emission wavelength of 1.653 µm (a DFB laser diode in the near-IR) and 7.66 µm (an EC QCL in the mid-IR) is successfully demonstrated for simultaneous CH4 spectroscopy measurements. The results indicate a normalized noise equivalent absorption (NNEA) coefficients of 1.33×10-9 cm-1W·Hz-1/2 at 1.653 µm and 2.20×10-10 cm-1W·Hz-1/2 at 7.66 µm, were achieved. This proposed sensor architecture has the advantages of easier optical alignment, lower cost, and a compactness compared to the design of a conventional TDLAS sensor using multiple semiconductor detectors for laser signal collection. The proposed technique can also be expanded to common QEPAS technique with multi-frequency modulation for multiple species detection simultaneously.

A Parallel-Phase Demodulation-Based Distance-Measurement Method Using Dual-Frequency Modulation

Time-of-flight (ToF) measurement technology based on the amplitude-modulated continuous-wave (AMCW) model has emerged as a state-of-the-art distance-measurement method for various engineering applications. However, many of the ToF cameras employing the AMCW process phase demodulation sequentially, which requires time latency for a single distance measurement. This can result in significant distance errors, especially in non-static environments (e.g., robots and vehicles) such as those containing objects moving relatively to the sensors. To reduce the measurement time required for a distance measurement, this paper proposes a novel, parallel-phase demodulation method. The proposed method processes phase demodulation of signal in parallel rather than sequentially. Based on the parallel phase demodulation, 2π ambiguity problem is also solved in this work by adopting dual frequency modulation to increase the maximum range while maintaining the accuracy. The performance of proposed method was verified through distance measurements under various conditions. The improved distance measurement accuracy was demonstrated throughout an extended measurement range (1–10 m).