Continuous Wave Research Articles

A microcomb-empowered Fourier domain mode-locked LIDAR.

Light detection and ranging (LIDAR) has emerged as an indispensable tool in autonomous technology. Among its various techniques, frequency-modulated continuous wave (FMCW) LIDAR stands out due to its capability to operate with ultralow return power, immunity to unwanted light, and simultaneous acquisition of distance and velocity. However, achieving a rapid update rate with submicrometer precision remains a challenge for FMCW LIDARs. Here, we present such a LIDAR with a sub-10-nanometer precision and a 24.6-kilohertz update rate by combining a broadband Fourier domain mode-locked (FDML) laser with a silicon nitride soliton microcomb. An ultrahigh-frequency chirp rate up to 320 petahertz per second is linearized by a 50-gigahertz microcomb to reach this performance. Our theoretical analysis also contributes to resolving the challenge of FMCW velocity measurements with nonlinear frequency sweeps and enables us to realize velocity measurement with an uncertainty below 0.4 millimeter per second. Our work shows how microcombs can unlock the potential of ultrafast frequency sweeping lasers.

Thermal diffusion in cement mortar exposed to continuous wave point laser

ABSTRACT .Laser Spot Thermography (LST) is a rapid, non-contact defect detection method, but its application to porous, heterogeneous materials like cement mortar is challenging due to complex heat diffusion. This study explores LST for cement mortar by capturing thermal images of both surfaces under continuous wave (CW) laser exposure. COMSOL simulations analyze three-dimensional heat diffusion, considering aggregate content, particle size, and laser power. A correction coefficient for composite thermal conductivity, derived using the Maxwell–Eucken model, is validated under varying conditions. Results show that increasing aggregate content reduces the front surface temperature difference (ΔT) by 6.5% at 20% and 11% at 30%, while ΔT at the back surface increases by 3.5% and 5.7%, respectively. The correction coefficient (μ) remains stable at 0.932 for aggregate fractions of 0.1–0.2 but decreases linearly beyond this range. These findings provide insights into optimizing LST for structural health monitoring of heterogeneous materials.

Multi-watt output power in-band-pumped Ho:KLu(WO4)2 laser

Multi-watt continuous-wave operation is reported for what is believed to be the first time for an in-band-pumped Ho:KLu(WO4)2 laser emitting at 2078 nm in N m polarization. The slope efficiency reached 33.5% with respect to the incident unpolarized pump radiation near 1959 nm, without pronounced roll-off effects. The total tuning range with a 1.5% output coupler covered 117 nm.

Confronting the dark matter capture rate with a continuous gravitational wave probe of local neutron stars

Continuous gravitational waves (CGWs) from various astrophysical sources are one of the many future probes of upcoming gravitational wave (GW) search missions. Neutron stars (NSs) with deformity are one of the leading sources of CGW emissions. In this work, for the first time, a novel attempt to estimate the dark matter (DM) capture rate is performed using CGW as the probe to the local NS population. Competitive bounds on DM capture from the local NS population are reported when compared with DM direct search experiments and other astrophysical observations. Published by the American Physical Society 2025

Design and Analysis of Dual-Band Metasurface Filter for Pulse Waves Based on Capacitive Nonlinear Circuits

In this paper, a novel dual-band metasurface filter (MSF) designed for accurately differentiating pulse waves (PWs) and continuous waves (CWs) is proposed, which is based on a complementary cross resonator (CSR) structure adhered on a dielectric substrate integrated with a capacitive nonlinear circuit. The unit cell of the designed dual-band MSF comprises two identical CSR structures: one of the capacitive nonlinear circuits is configured in parallel with a capacitor (C1) within one CSR structure. These structures loaded with nonlinear circuits are fabricated on a dielectric substrate. The simulation outcomes reveal that, for normally incident CWs with an input power of 10 dBm, the transmittance of the designed dual-band MSF reaches as high as 97.1% at 2.0 GHz and 93.9% at 3.45 GHz. In contrast, when it comes to 50 ns short PWs, the transmittance remains consistently below 6% throughout the entire frequency range from 1 GHz to 5 GHz. In addition, the transmittance of the dual-band MSF for normally incident PWs increases significantly as the pulse width widens at the aforementioned two discrete frequencies. The ensuing simulation data corroborates that within the input power range of −15 to 15 dBm, the transmittance difference between CWs and PWs of the dual-band MSF first rises and then falls as the input power increases. Specifically, when the input power is specified as 10 dBm and the angle of oblique incidence ranges from 0° to 60°, in the context of TE and TM modes, the transmittance of CWs exceeds 80% around both 2.0 GHz and 3.45 GHz, while that of PWs remains below 15%. Finally, the effects of resistance and capacitance on the transmittance of the dual-band MSF for the incident PWs and CWs are also studied. The dual-band MSF proposed herein showcases its potential applications in wireless communication as well as in the realm of anti-electromagnetic interference. The electromagnetic (EM) waveform modulation in the frequency band of 1–5 GHz has great development prospects in low-frequency working fields such as radar antennas and EM protection.

High-speed pulsing circuit for continuous wave laser beams

Abstract Tunable continuous wave (CW) lasers produce the highest optical resolution for pumping and probing atomic and molecular transitions. These lasers adopt sophisticated electronic control of optical components within the cavity to set the wavelength and ensure a narrow bandwidth. The low-cost circuit described here gates a CW laser beam on and off for high resolution timing experiments, using an electro-optic modulator that acts as a switchable waveplate between linear polarizers. The switch can operate at rates of up to 100 kHz with a minimum pulse width of 650 ns, or it can be configured to deliver laser pulses with widths ⩾ 35 ns at lower rates. Multiple laser pulses of equal or non-equal widths can also be delivered within each repetition cycle.

Long-term Evolution of Sco X-1: Implications for the Current Spin Frequency and Ellipticity of the Neutron Star

Abstract Sco X-1 is the brightest observed extrasolar X-ray source, which is a neutron star (NS) low-mass X-ray binary (LMXB) and is thought to have a strong potential for continuous gravitational waves (CW) detection due to its high accretion rate and relative proximity. Here, we compute the long-term evolution of its parameters, particularly the NS spin frequency (ν) and the surface magnetic field (B), to probe its nature and its potential for CW detection. We find that Sco X-1 is an unusually young (∼7 × 106 yr) LMXB and constrain the current NS mass to ∼1.4–1.6 M ⊙. Our computations reveal a rapid B decay, with the maximum current value of ∼1.8 × 108 G, which can be useful to constrain the decay models. Note that the maximum current ν value is ∼550 Hz, implying that, unlike what is generally believed, a CW emission is not required to explain the current source properties. However, ν will exceed an observed cutoff frequency of ∼730 Hz, and perhaps even the NS breakup frequency, in the future without a CW emission. The minimum NS mass quadrupole moment (Q) to avoid this is ∼(2–3) × 1037 g cm2, corresponding to a CW strain of ∼10−26. Our estimation of current ν values can improve the CW search sensitivity.

Fast passage effect in cw-ODMR of an ensemble of NV- centers.

A rapid scan technique of the magnetic field has been developed to improve excitation efficiency in continuous wave (cw)-electron spin resonance. The resulting passage effects induce significant responses of magnetizations, most notably an inversion of longitudinal magnetization. The change in longitudinal magnetization can be observed in optically detected magnetic resonance (ODMR), but the passage effect has not been investigated in cw-ODMR spectroscopy. In this study, we combine the ODMR technique with the rapid scan of the magnetic field to demonstrate the impact of fast passage effects on the field-swept cw-ODMR. An ensemble of negatively charged nitrogen-vacancy (NV-) centers in a diamond crystal is used as a model system. By selecting the fast passage adiabatic region among six distinct passage regions, efficient ODMR detection is achieved through the macroscopic control of longitudinal magnetization. Experimental results are explained by the numerical analyses of classical time-dependent Bloch equations and related rate equations under an optical cycle. This knowledge paves the way for the facilitation of versatile applications of ODMR for quantum sensors.

High-power (500 mW) narrow-linewidth (21 kHz) low-RIN (−168 dB/Hz) distributed feedback laser

A high-power, narrow-linewidth, low-relative-intensity-noise (RIN) distributed feedback (DFB) laser is demonstrated. The laser employs four strained AlGaInAs quantum wells and three diluted waveguides to reduce internal loss, and enhance heat dissipation performance to reach a high power and low noise operation. At a test temperature of 15 °C, the DFB laser with a cavity length of 1.5 mm exhibited single mode output power of 517 mW in pulsed mode and 424 mW in continuous-wave (CW) mode. with far-field full width at half maximum (FWHM) divergence angles of 10°×20°. In addition, the DFB laser achieved an intrinsic linewidth of 21 kHz, and an ultra-low RIN of less than −168 dB/Hz.

Localized Heating Assisted Laser Shock Peening without Coating Enhances Mechanical Properties of Ti6Al4V Alloys

Room-temperature laser shock peening without coating (RT-LSPwoC) is the mainstream method for surface strengthening, yet it induces tensile residual stress on the surface during rapid heating and cooling. Warm LSPwoC, combining the benefits of dynamic strain aging and LSPwoC, exhibits higher performance than RT-LSPwoC. However, overall high-temperature is time- and resource-consuming, and it is prone to causing thermal stress relaxation during processing, making it unsuitable for large-scale applications. In this paper, a novel localized heating assisted LSPwoC (LHA-LSPwoC) method, utilizing a continuous wave laser for local heating, is proposed. Additionally, the deep physics-informed neural network model, embedded with physical formulas, is designed for process optimization. It enables rapid and accurate responses for an extensive number of cases (40 cases with an average deviation below 1%), overcoming the drawbacks of traditional numerical simulations that are time-consuming and difficult to efficiently handle numerous cases. Compared to RT-LSPwoC, LHA-LSPwoC samples have higher dislocation density and higher grain refinement, demonstrating higher hardness and residual stress, particularly superior fatigue performance. The mechanism of LHA-LSPwoC is discussed, and thermally coupled finite element simulations and molecular dynamics calculations are conducted to provide theoretical support. The proposed method is cost-effective and flexible, showing outstanding potential for industrial applications.

Effect of Electrical Heat Carrier Temperature on Bacterial Leakage of Endodontically Treated Teeth Using a Bioceramic Sealer.

The aim of this study is to investigate the effect of electrical heat carrier temperature on bacterial leakage of root canals obturated with the continuous wave of condensation (CWC) technique and a bioceramic sealer. This ex vivo experimental study was conducted on 92 extracted single-rooted teeth. The teeth were subjected to endodontic treatment and obturated with the Endoseal MTA bioceramic sealer by the CWC technique using two different temperature settings of the electrical heat carrier: 120°C (group G120, n = 41) and 200°C (group G200, n = 41). A positive and a negative control group were also considered (n = 5 each). Bacterial leakage was assessed over a 40-day period using a bacterial leakage model. The incidence of bacterial leakage was compared between the experimental groups using the log-rank test. The significance level was set at 0.05. The median survival rate was 39.0 (25.0) days in the G120 group and 34.0 (25.0) days in the G200 group. Despite a slightly higher survival rate in the G120 group, the difference between the two experimental groups was not statistically significant (p = 0.612). The tested temperatures of the electrical heat carrier (120°C and 200°C) did not have a significant effect on bacterial leakage of root canals obturated by the CWC technique and the Endoseal MTA bioceramic sealer.

Design, development and testing of IF section for FMCW reflectometry diagnostics for tokamaks

Reflectometry diagnostics is essential for determining plasma position, density, and density fluctuations to characterize tokamak plasmas. An Ultra-fast Frequency Modulated Continuous Wave reflectometry system for measuring time evolution of density profiles is operational on ADITYA-U. This paper discusses the development of an in-house IF section, signal acquisition and processing has already been reported separately. Initially, a modular design of the IF section was implemented. However, due to the higher cost and bulkiness of the modular design, it was replaced by IF section card developed in-house to achieve a compact and cost-effective solution. This paper details the circuit simulations, printed circuit board layout, design, and testing of the developed IF electronics. Both modular and card based IF sections were used and were found to be in excellent agreement with expected values of beat frequency. Specifically, modular IF design achieves a 2.5% error, and the card-based IF design measures with a 2% error. In addition to having a lower error compared to the modular IF design, the card-based design also offers a lower cost in a more robust and compact form factor.

Continuous-Wave Room-Temperature External Cavity Quantum Cascade Lasers Operating at λ~8.5 μm

External cavity quantum cascade lasers (EC-QCLs) utilizing the Littrow configuration and operating at an approximate wavelength of 8.5 μm have been successfully demonstrated in continuous wave operations at room temperature. Our work provides ideas and experimental support for the optimization of the EC-QCL which indicate optimal EC-QCL performance with an external cavity length of 25 cm and investigates the impact of various parameters, including injection current and temperature on the performance of the EC-QCL. In the absence of anti-reflection (AR) coating, the tuning range at 25 °C extends up to 103.3 cm−1, while the maximum side mode suppression ratio (SMSR) reaches 30.8 dB, accompanied by a full width half maximum linewidth (FWHM) of 0.76 nm.

Evaluating waist-to-hip ratio in youth using frequency-modulated continuous wave radar and machine learning

Waist-to-hip ratio (WHR) is an essential predictor of cardiometabolic diseases, but traditional tape-based WHR measurements in children and adolescents can cause discomfort due to direct contact and are prone to measurer variation. This study aimed to develop a non-invasive, precise, and convenient alternative for WHR measurement and central obesity assessment using frequency modulated continuous wave (FMCW) radar, and to evaluate its accuracy by comparing it with traditional measurement methods. We included 100 participants aged 7–18 and radar data were analyzed using point cloud generation processed through convolutional neural networks for estimating WHR. The radar-based WHR measurements were compared to conventional clinician measurements. Participants were classified into low (WHR < 0.86), moderate (≥ 0.86, < 0.91) and high WHR (≥ 0.91) groups, and the classifications were compared. Strong agreement was observed between the two methods, with an intraclass correlation coefficient of 0.83 (p = 0.023995). The radar system achieved 82% accuracy in classifying participants into the correct abdominal obesity risk groups. Our findings demonstrate that FMCW radar can be a reliable tool for routine monitoring of central obesity. This technology addresses concerns about privacy and discomfort, making it suitable for widespread application in both clinical and non-clinical settings.

Electron paramagnetic resonance spectroscopy: toward the path of dihydrogen isotopologue detection in porous materials.

Electron paramagnetic resonance (EPR) spectroscopy is a powerful method to characterize the local framework structure of nanoporous materials during the dihydrogen isotopologue adsorption process. It also allows for exploring the adsorption sites of the dihydrogen isotopes and monitoring their desorption characteristics on the microscopic scale. The paramagnetic spin probes in the form of transition metal ions or organic radicals are required for EPR spectroscopy and are introduced either at the framework lattice position or in the pores of the metal-organic frameworks. This review highlights current advancements within the field of dihydrogen isotopologue detection as well as key findings related to the versatility of in situ continuous wave EPR and pulsed EPR experiments as toolkits for monitoring the adsorption-desorption process of dihydrogen isotopologues from the perspective of the framework as well as studying the host-guest interactions based on high-resolution advantages offered by using a pulsed EPR approach.

Movement Compensation in Dual Continuous Wave Radar Using Deep Learning

Movement Compensation in Dual Continuous Wave Radar Using Deep Learning

Pulsed red light photobiomodulation ameliorates oxytocin-induced primary dysmenorrhea in mice by inhibiting oxidative stress and lipid accumulation.

Photobiomodulation (PBM) has gained attention as a kind of anti-pain or anti-inflammation therapy, yet its efficacy in mitigating the symptoms and underlying metabolic disturbances of primary dysmenorrhea remains underexplored. Here, 630nm light reduced menstrual pain and prostaglandin F2a/prostaglandin E2 dysregulation, regulated oxidation and lipid peroxidation levels, and improved uterus damage in oxytocin-induced mice. Notably, pulsed wave (PW) treatment exhibited superior efficacy compared to continuous wave application. Hence, this research focused on the effects of 630nm PW on oxytocin-induced mice by examining changes in the uterine transcriptome and plasma metabolome. Results from integrated analyses revealed significant modifications primarily in antioxidant and lipid metabolism pathways, alongside shifts in biomarkers related to arachidonic acid metabolism. Quantitative real-time PCR confirmed the downregulation of critical genes associated with oxidative stress and inflammation, as well as the suppression of uterine smooth muscle contractions and lipid overaccumulation. These findings support the potential of 630nm PW PBM as a viable option for clinical interventions in dysmenorrhea management.

Analytic weak-signal approximation of the Bayes factor for continuous gravitational waves

Abstract We generalize the targeted B-statistic for continuous gravitational waves by modeling the h0 -prior as a half-Gaussian distribution with scale parameter H. This approach retains analytic tractability for two of the four amplitude marginalization integrals and recovers the standard B-statistic in the strong-signal limit (H → ∞). By Taylor-expanding the weak-signal regime (H → 0), the new prior enables fully analytic amplitude marginalization, resulting in a simple, explicit statistic that is as computationally efficient as the maximum-likelihood F-statistic, but significantly more robust. Numerical tests show that for day-long coherent searches, the weak-signal Bayes factor achieves sensitivities comparable to the F-statistic, though marginally lower than the standard B-statistic (and the Bero-Whelan approximation). In semi-coherent searches over short (compared to a day) segments, this approximation matches or outperforms the weighted dominant-response FABw-statistic and returns to the sensitivity of the (weighted) Fw-statistic for longer segments. Overall the new Bayes-factor approximation demonstrates state-of-the-art or improved sensitivity across a wide range of segment lengths we tested (from 900 s to 10 days).

A Compact Time Domain Diffuse Optical Tomography System for Cortical Neuroimaging

Abstract Recent years have witnessed a rise in research utilizing neuroimaging for precision neuromedicine, but clinical translation has been hindered by scalability and cost. Time Domain functional Near Infrared Spectroscopy (TD-fNIRS), the gold standard of optical neuroimaging techniques, offers a unique opportunity in this domain since it provides superior depth sensitivity and enables resolution of absolute properties unlike its continuous wave counterparts. However, current TD systems have limited commercial availability, slow sampling rates, and sparse head coverage. Our team has overcome the technical challenges involved in developing a whole-head time-domain diffuse optical tomography (TD-DOT) system. Here, we present the system characterization results using standardized protocols and compare them to the state-of-the-art. Furthermore, we showcase the system performance in retrieving cortical activation maps during standard hemodynamic, sensory, and motor tasks. A combination of the system performance, signal quality, and ease-of-use can enable future studies aimed at investigating TD-DOT clinical applications.

Online monitoring of a high-power CW laser optical mirror based on infrared and visible image features visualization

In high-power CW (continuous wave) laser systems, laser damage to optical mirrors often occurs, and it is urgent to develop timely and effective monitoring technologies to avoid laser damage or emergency stop at the early damage stage. In this paper, an online monitoring method based on the combination of infrared and visible images is proposed. The proposed method is based on the feature recognition of infrared and visible images. After setting an appropriate threshold, a decision model can be employed to enable real-time monitoring and health status evaluation of optical mirrors. Here, the thresholds for abnormal and dangerous temperatures were established at 100°C and 200°C, respectively. The results show that the temperature of the optical mirror would significantly increase after damage, with the surface temperature of the element reaching up to 250°C. At the same time, the damaged area could be captured by a visible camera and the changes in gray values were displayed in the visible image. Online monitoring of the health status of optical elements can be achieved by assessing changes in infrared image temperature, visible image spot position, and area. This monitoring method serves as an early warning method for potential optical elements damage.