Search Results (153)

Dissipative sensing in a whispering-gallery-mode (WGM) microresonator entails monitoring changes in WGM throughput dip depth or linewidth due to analyte absorption. In our earlier work, we showed that dip depth sensitivity can be two orders of magnitude greater than linewidth sensitivity for sensing the broadband absorption of a dye in methanol. Here we experimentally demonstrate enhancement of absorption sensing of methane. Its narrowband absorption lines (a few GHz linewidth) necessitate strain tuning of the WGM of our hollow bottle resonator (HBR) to bring the WGM into resonance with the absorption line. Three asymmetric tapered fibers with different nonadiabaticities were designed to excite multiple fiber modes that couple into the WGM to interact with methane inside the HBR via the internal evanescent field. Measurements were carried out for both pure and trace (in 1 atm of air) methane at 1654 and 1651 nm. Enhancement factors as large as 141 were found; the experimental results agree with theoretical calculations and with the predictions of a limiting-case model. Effective absorption path lengths as large as 273 cm, more than ten thousand times the HBR diameter, were achieved for trace methane sensing, with detection limits estimated to be in the hundreds of ppm. Full article

Narrow-linewidth semiconductor lasers have become indispensable devices in high-precision measurement and detection. Among various available technologies, self-injection locking plays a crucial role due to its significant ability to reduce linewidth and enhance coherence. Here, we demonstrate a hybrid feedback narrow-linewidth laser based on fixed external cavity feedback combined with self-injection locking feedback. The laser consists of a semiconductor gain chip, fiber Bragg grating, and micro-ring resonator, achieving laser mode selection and linewidth compression. Ultimately, a single longitudinal mode narrow-linewidth laser output with a Lorentzian linewidth of 149 Hz and a side-mode suppression ratio of 65 dB was obtained. The demonstrated laser can be applied in applications such as coherent optical communication and high-precision coherent detection. Full article

As a key parameter that defines the spectral characteristics of lasers, the precise measurement of laser linewidth is crucial for a wide range of advanced applications. This review systematically summarizes recent advances in laser linewidth measurement techniques, covering methods applicable from GHz-level broad linewidths to sub-Hz ultranarrow regimes. We begin by presenting representative applications of lasers with varying linewidth requirements, followed by the physical definition of linewidth and a discussion of the fundamental principles underlying its measurement. For broader linewidth regimes, we review two established techniques: direct spectral measurement using high-resolution spectrometers and Fabry–Pérot interferometer-based analysis. In the context of narrow-linewidth lasers, particular emphasis is placed on the optical beating method. A detailed comparison is provided between two dominant approaches: power spectral density (PSD) analysis of the beat signal and phase-noise-based linewidth evaluation. For each technique, we discuss the working principles, experimental configurations, achievable resolution, and limitations, along with comparative assessments of their advantages and drawbacks. Additionally, we critically examine recent innovations in ultra-high-precision linewidth metrology. This review aims to serve as a comprehensive technical reference for the development, characterization, and application of lasers across diverse spectral regimes. Full article

The external cavity frequency doubling technique serves as a potent method for generating short-wavelength lasers, yet achieving high-power outputs remains challenging due to the thermal lens effect. This study systematically investigates the generation mechanism of the thermal lens effect and its impact on laser performance. By optimizing the bow-tie cavity design and leveraging a large beam waist of 106 µm to suppress thermal-induced distortions, we demonstrate a tunable 420 nm laser with up to 800 mW of output power and a peak conversion efficiency of 77%. The fundamental light source, a Ti:Sa laser locked to an ultra-stable cavity, ensures a narrow linewidth, flexible tunability, and long-term frequency stability. This high-performance blue laser enables the efficient Rydberg excitation of rubidium atoms, presenting critical applications in quantum computing, quantum simulation, and quantum precision measurement. Full article

Free radicals in G-361 human melanoma malignum control cells and the cells cultured with simvastatin were examined by EPR spectroscopy. The proliferation of the cells was determined. The aim of this work was to examine the influence of simvastatin used at different concentrations in the G-361 cell culture on its free radicals. The concentrations of simvastatin—0.1 μM, 1 μM, 3 μM, and 5 μM—were tested. EPR spectra of free radicals were measured by an X-band (9.3 GHz) spectrometer. Amplitudes, integral intensities, linewidths, and g factors were determined. Melanin biopolymers are the main source of o-semiquinone free radicals in G-361 human melanoma malignum cells, for which the EPR lines show characteristic g values of 2.0046–2.0059, but also, free radicals occurring in other cellular structures may contribute to these signals. The amount of free radicals decreases after interactions of simvastatin with the G-361 cells, and this effect depends on the concentration of simvastatin. The highest amounts of free radicals exist in G-361 cells cultured with simvastatin at concentrations of 3 μM and 5 μM. The relatively lower amounts of free radicals occur in G-361 cells cultured with simvastatin at concentrations of 0.1 μM and 1 μM. The fast spin–lattice relaxation processes exist in the control G-361 cells and in the cells cultured with simvastatin, regardless of simvastatin concentration. Full article

In the measurement process of cold atomic gravimeters, Raman laser plays an important role both in the state preparation stage and in the atomic interference stage. This paper discusses Raman laser generation techniques. The optical phase-locked loop (OPLL) method and the electro-optical modulation (EOM) method are compared from a theoretical point of view. An OPLL system and an EOM system were constructed separately. The two schemes were tested in terms of linewidth, phase noise and long-term stability. The experimental results were analyzed and discussed. Based on the results, recommendations are given for the selection of Raman laser schemes under different scenarios. Full article

By measuring the transfer function of the single-mode multi-aperture vertical-cavity surface-emitting laser (SM MA VCSEL) transmitting over a long single-mode fiber at 850 nm, we confirm that the chirp of the SM MA VCSEL under study is dominated by transient chirp with an alpha value of −3.81 enabling a 19 GHz bandwidth over 10 km of single-mode fiber. The detailed measurement of the VCSEL with different bias currents also allows us to recover other key characteristics of the VCSEL, thereby enabling us to practically construct the optical eye diagrams that closely match the experimentally measured ones. The link-level transfer function can be obtained using an analytical equation including effects of modal dispersion and laser chirp–chromatic dispersion (CD) interaction for an MMF of a given length and bandwidth grade. The narrow linewidth and chirp characteristics of the SM MA VCSEL enable transmission performance that surpasses that of conventional MM VCSELs, achieving comparable transmission distances at moderate modal bandwidths for OM3 and OM4 fibers and significantly longer reaches when the modal bandwidth is higher. The transmission performance was also confirmed with the modeled eye diagrams using extracted VCSEL parameters. The chirp properties also provide sufficient bandwidth for SM MA VCSEL transmission over kilometer-scale lengths of single-mode fibers at a high data rate of 100G or above with sufficient optical power coupled into the fibers. Advanced transmission distances are possible over multimode and single-mode fibers versus chirp-free devices. Full article

We present high-angular resolution observations, conducted with the Atacama Large Millimeter/Submillimeter Array (ALMA) in Band 6, of high-excitation molecular lines of ${\mathrm{CH}}_3\mathrm{CN}$, CH₃OH, and the H29$\alpha$ radio recombination line, towards the G333.6-0.2 ultracompact (UC) H ii region. Our observations reveal three hot molecular cores: A, B, and C, where emission is detected in ten components of the $J=14\to 13$ rotational ladder of ${\mathrm{CH}}_3\mathrm{CN}$ and in the CH₃OH $J=5_{1,4}\to 4_{1,3}$ transition. Rotational diagram analysis of ${\mathrm{CH}}_3\mathrm{CN}$ reveals excitation temperatures ranging from 380 to 430 K. First-order moment maps of ${\mathrm{CH}}_3\mathrm{CN}$ and CH₃OH reveal distinct velocity gradients in all cores, suggesting rotating structures, with core A also showing evidence of expansion motions. The H29$\alpha$ recombination line shows a linewidth of $30.2\pm 0.12$ km s⁻¹, dominated by dynamical and thermal broadening, indicative of large-scale motions in ionized gas. Analysis of the ionized gas properties yields an electron density of $(4.8\pm 0.4)\times {10}^5$ cm⁻³, an emission measure of $(1.23\pm 0.06)\times {10}^9$ pc cm⁻⁶, and a Lyman continuum photon flux consistent with an O5–O6 V (Zero-Age Main Sequence; ZAMS) star. Our results suggest that G333.6-0.2 is in an intermediate evolutionary stage between hypercompact (HC) and ultracompact (UC) H ii regions, hosting active high-mass star formation with rotating hot cores and ionized gas dynamics. Full article

Recent progress in Rydberg atoms has enabled a wide range of applications in quantum sensing and quantum computation. In these applications, ultra-stable optical reference systems are essential to meet the requirements of a narrow linewidth and low technical noise. However, most existing systems are confined to laboratory settings and are not suitable for measurement-site-independent applications, such as external electric field sensors. This paper presents a transportable ultra-stable optical reference system for Rydberg excitation. The system is based on a 10 cm long ultra-low-expansion glass optical cavity, and it achieves finesse values of 410 k (at 1018 nm) and 200 k (at 852 nm). After being assembled in the experimental environment, it can still operate with only minor adjustments even after being transported over 400 km. Thanks to its vibration-insensitive design, two-stage temperature control, and hybrid locking unit, the system can maintain its locking status for up to 2 h. Full article

Electron beam lithography (EBL) is a pivotal technology in the fabrication of nanoscale devices, renowned for its high precision and resolution capabilities. This paper explores the effect of EBL process parameters on various substrate materials, including silicon dioxide, silicon-on-insulator (SOI), and silicon nitride. We specifically investigate the impact of the charging effect and reveal the narrow exposure dose windows necessary to achieve optimal pattern fidelity. Based on the measurement results of linewidth, the relationship between exposure dose and the width of the structure pattern after development was analyzed. The optimum exposure dose window for each substrate is identified. Furthermore, through simulations of the charge effect, we demonstrate strategies for mitigating this effect on different substrates, even in complex structural configurations. Our findings contribute to enhancing the capabilities of EBL in semiconductor and insulator manufacturing and research. Full article

Narrow-linewidth lasers play a crucial role in nonlinear optics, atomic physics, optical metrology, and high-speed coherent optical communications. Precise linewidth measurement is essential for assessing laser noise characteristics; however, conventional methods are often bulky, costly, and unsuitable for integrated applications. This paper presents a compact and cost-effective delay self-homodyne system for laser linewidth measurement, leveraging a field-programmable gate array (FPGA)-based data acquisition circuit. By employing fast Fourier transform (FFT) analysis, the system achieves high-precision linewidth measurement in the kHz range. Additionally, by optimizing the fiber length, the system effectively suppresses low-frequency and 1/f noise, providing an integrated and efficient solution for advanced laser characterization with enhanced performance and reduced cost. Full article

The combination of unmanned aerial vehicles and atomic magnetometers can be used for detection applications such as mineral resource exploration, environmental protection, and earthquake monitoring, as well as the detection of sunken ships and unexploded ordnance. A dark-resonance atomic magnetometer offers the significant advantages of a fully optical probe and omnidirectional measurement with no dead zones, making it an ideal choice for airborne applications on unmanned aerial vehicles. Enhancing the sensitivity of such atomic magnetometers is an essential task. In this study, we sought to enhance the sensitivity of a dark-state resonance atomic magnetometer. Initially, through theoretical analysis, we compared the excitation effects of coherent population trapping (CPT) resonance on the D₁ and D₂ transitions of ¹³³Cs thermal vapor. The results indicate that excitation via the D₁ line yields an increase in resonance contrast and a reduction in linewidth when compared with excitation through the D₂ line, aligning with theoretical predictions. Subsequently, considering the impact of various quantum system parameters on sensitivity, as well as their interdependent characteristics, two experimental setups were developed for empirical investigation. One setup focused on parameter optimization experiments, where we compared the linewidth and contrast of CPT resonances excited by both D₁ and D₂ transitions; this led to an optimization of atomic cell size, buffer gas pressure, and operating temperature, resulting in an ideal parameter range. The second setup was employed to validate these optimized parameters using a coupled dark-state atom magnetometer experiment, achieving approximately a 10-fold improvement in sensitivity. Full article

An all-optical single-longitudinal-mode (SLM) forward Brillouin microwave oscillator (FB-MO) with an unbalanced Fiber Mach–Zehnder interferometer (UF-MZI) for microwave photonics (MWP) generation is proposed and experimentally investigated. UF-MZI consists of an optical coupler (OC), a polarization controller (PC), and two asymmetric length arms with 5 km and 500 m single-mode fibers (SMFs), which implements two unbalanced length feedback rings that are connected to one another. One long-length ring with a forward Brillouin gain cooperates with the other short-length ring to maintain a spectral Vernier effect and improve the effective free spectral range (FSR). By contrast with traditional optoelectronic oscillators (OEOs), this design does not require any photoelectric conversion devices and additional modulation, avoids external electromagnetic interference, and side-mode suppression and linewidth are favorable. Experimental results reveal that the 3-dB linewidth of the all-optical SLM FB-MO with UF-MZI is about 140 Hz. The acoustic-mode and side-mode suppression ratios are 26 dB and 31 dB. Within 60 min of the stability experiment, the power and frequency stability fluctuation were ±1 dB and ±100 Hz. Thanks to its long main ring cavity length, our all-optical SLM FB-MO with UF-MZI maintains good phase-noise performance. The measurement shows that a phase noise as low as −120 dBc/Hz at an offset frequency of 100 kHz is achieved. This SLM MWP generation technology holds great potential for applications in radar monitoring and wireless communication systems. Full article

Single-mode (SM) vertical-cavity surface-emitting lasers (VCSELs) have often been demonstrated with an unusually long transmission reach at very high data rates while today’s multimode VCSEL transmission has been limited by the fiber modal bandwidth and bandwidth contributed by the VCSEL–chromatic dispersion interaction under typical encircled flux launch condition. By using the same launch condition for VCSEL and modal bandwidth measurements, we studied the link bandwidth capability of SM multi-aperture (MA) VCSEL transmission. Using a multimode fiber with modal bandwidth under actual launch conditions moderately lower than OM4 threshold, we observed that the link bandwidth, with contributions from both modal bandwidth and laser–chromatic dispersion interaction, is higher than the corresponding modal bandwidths, which is very counter-intuitive. A detailed analysis reveals that the enhanced link bandwidth is contributed by both narrow laser linewidth and favorable laser–chromatic dispersion interaction. Through the study, we demonstrate that OM4 can meet link bandwidth requirements for 200/100 G/lane transmission over 100/200 m using SM MA VCSELs. Full article

A narrow linewidth tunable laser source is a critical component in various fields, including laser radar, quantum information, coherent communication, and precise measurement. Tunable external cavity diode lasers (ECDLs) demonstrate excellent performance, such as narrow linewidth, wide tunable range, and low threshold current, making them increasingly versatile and widely applicable. This article provides an overview of the fundamental structures and recent advancements in external cavity semiconductor lasers. In particular, we discuss external cavity semiconductor lasers based on quantum well and quantum dot gain chips. The structure of the gain chip significantly influences laser’s performance. External cavity quantum well laser has a narrower linewidth, higher power, and better mode stability. Conversely, external cavity quantum dot laser provides a wider tunable range and a remarkably lower threshold current. Furthermore, dual-wavelength external cavity tunable diode lasers are gaining importance in applications such as optical switching and terahertz radiation generation. With the continuous optimization of chips and external cavity structures, external cavity diode lasers are increasingly recognized as promising light sources with narrow linewidth and wide tunability, opening up broader application prospects. Full article