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We demonstrated a stable passively Q-switched fiber laser operation at 1531.57 nm using a double-transition metal MAX phase TiNbAlC saturable absorber. The prepared saturable absorber's modulation depth, saturation intensity, and non-saturation loss are 1.75%, 0.089 MW/cm 2 , and 32.81%, respectively. Using the TiNbAlC saturable absorber within an erbium-doped fiber laser ring cavity, stable Q-switched pulses with a central wavelength of 1531.57 nm, and a 3-dB bandwidth of 1.43 nm are acquired. The minimum pulse duration and the maximum single pulse energy are 3.18 μ s and 51.56 nJ, respectively. Furthermore, the maximum repetition rate is 37.9 kHz with a signal-to-noise ratio of 50 dB. Our results indicate that double-transition metal MAX phase TiNbAlC can be an excellent saturable absorber candidate for an ultrashort pulse fiber laser. [ABSTRACT FROM AUTHOR]

An external cavity wavelength-fiber ring laser (ECWTFL) based on a semiconductor optical amplifier and a combined wavelength scanning filter in the Littrow configuration is proposed and experimentally demonstrated. With the benefit of the combination of an external cavity wavelength filter and a Lyot filter, the laser achieves a single-mode narrow linewidth output with a linewidth of 1.75 kHz. The wavelength tuning range reaches 133 nm, covering the entire S + C band. The proposed ECWTFL is used for demodulation of a fiber EFPI sensor; the result shows that the proposed ECWTFL has the ability to demodulate the small cavity-length FPI sensor. [ABSTRACT FROM AUTHOR]

We investigate a new phenomenon, where a reciprocal fiber ring laser switches from bidirectional to unidirectional operation above a certain pump power threshold. Significant simplifications regarding earlier experiments are presented, which for the first time allow the identification of individual nonlinear effects. We highlight the unique role of stimulated Raman scattering in triggering unidirectional operation, and that additional conditions apply. The threshold is reduced from 30 to 3.8 W, which eases potential applications. [ABSTRACT FROM AUTHOR]

Quantum cascade lasers (QCLs) are a prominent semiconductor laser source operating in the mid-infrared and terahertz regimes. As is typical with semiconductor lasers, QCLs usually monolithically integrate the active gain material and the resonator. Hence, over nearly 30 years of QCL development, resonator geometries have developed alongside active region designs. Disk and ring geometries, in particular, have long been recognized for their unique attributes, which have, in turn, contributed to the demonstration of ultra-small cavities as well as surface emission from QCLs. In recent years, ring geometries have witnessed a resurgence as promising platforms for frequency comb and soliton generation as well as mid-infrared photonic integration. In this Perspective, we describe the attributes that make ring and disk QCLs unique by discussing key demonstrations. We present recent results, which indicate that these devices are poised to become building blocks of highly integrated, next-generation spectrometers operating in the mid-infrared. We discuss promising avenues for future research centered around monolithic ring and disk-type QCLs in applications ranging from gas sensing and spectroscopy to quantum optics and non-Hermitian photonics. [ABSTRACT FROM AUTHOR]

A new segmentation and topology denoising method is proposed wherein the laser scanning three-dimensional (3-D) point cloud data and datum points are extracted. The laser scanned 3-D point cloud model is cut in the horizontal plane. The minimum distance between the point cloud data on the horizontal cutting plane and the geodesic line of the datum point is the feature points. The skeleton of the 3-D point cloud model is constituted by the feature points and the point cloud data are segmented according to the skeleton feature points. In the 3-D space of laser scanning point cloud data the original point cloud is transformed in two-dimensional (2-D) space by the spatial topological mapping function. The distance weight is computed by segmented point cloud data. In the process of topological transformation, combining with the distance weight of the sampling point, the topology transformation function is built. The noise point and the non-noise point can be separated by the topology transformation. The large-scale noise points are removed by comparison with a set threshold. The fairing direction of small scale noise is calculated according to the curvature and point cloud density. The small scale noise point is moved along the fairing direction to achieve the effect of denoising of the point cloud data. The method is verified by the point cloud data of laser scanning ring forgings in this paper. [ABSTRACT FROM AUTHOR]

The temperature characteristics of ring lasers with a diameter of 480 μm of an original design with an active region based on 10 layers of InAs/InGaAs/GaAs quantum dots are studied. The lasers demonstrated a low threshold current density (200 A/cm2 at 20°C), the characteristic temperature of the threshold current in the range of 20–100°C was 68 K, the maximum lasing temperature was as high as 130°C. These values are only slightly inferior to the parameters of the edge-emitting lasers fabricated from the same epitaxial wafer. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a passively mode-locked all-fiber laser utilizing a multimode interference effect in graded-index multimode fiber (GIMF). The mode-locking is obtained due to the Kerr effect in GIMF, which induces the intensity modulation inside the ring cavity laser. We obtain stable soliton mode-locked pulses at a center wavelength of 1574.1 nm, with a fixed repetition rate of 22 MHz and a pulse duration of 650 fs, when the pump power is changed in the range from 66.2 to 140.0 mW. The maximum average output power and pulse energy are recorded at 9.82 mW and 448 pJ, respectively, at a pump power of 140 mW. This proves that a nonlinear optical response in GIMF can be used as a new modulation mechanism for obtaining ultra-short pulses based on all-fiber ring cavity. [ABSTRACT FROM AUTHOR]

The results of studies of 7.5–8.0 μm range surface-emitting ring quantum-cascade lasers are presented. A second- order diffraction grating with a calculated coupling coefficient of ~9 cm–1 is formed on the entire surface of the ring cavity by focused ion beam milling. Surface-emitting lasing at room temperature near 7.75 μm with a threshold current density of ~8 kA/cm2 and an outer radius of the ring cavity of 202 μm is demonstrated. The results of studying the intensity distribution in the far-field near the normal to the surface showed the presence of two maxima. It is shown that the implemented coupling coefficient is not sufficient to ensure single-mode lasing in the studied ring quantum-cascade lasers. [ABSTRACT FROM AUTHOR]

The results of studying the effect of filamentary striae with birefringence 3 to 15 nm/cm in sitall [glass-ceramic] SO-115M on the quality of manufacturing of optical components of ring lasers and the accuracy characteristics of laser gyroscopes based on them are presented. It is shown that the presence of streaks with stresses not exceeding 15 nm/cm in sitall SO-115M does not affect the manufacturing accuracy of the resonator optical circuit and its assembly quality. The streaks in the output mirror of a ring laser serve as a source of additional backscattering and can increase the lock-in zone width. Proper adjustment of the output mirror makes it possible to reduce the amount of scattering and minimize the growth of the lock-in zone in a laser gyroscope. It is shown that the accuracy of a laser gyroscope is determined by the quality of the mirror substrates, the body geometry, and the level of backscattering inside the resonator. [ABSTRACT FROM AUTHOR]

The results of studies of ring quantum-cascade lasers with surface emission due to a second-order grating formed in the top cladding layers are presented. Surface emission near 7.85 μm with a low threshold current density (3.8 kA/cm2), in comparison with ridge quantum-cascade lasers of the same cavity length is demonstrated. The results of measurements of the intensity distribution of the near and far fields at different pumping levels are presented. The estimated value of the angle of beam extraction relative to the surface normal is in the range 5.7°–6.7°. [ABSTRACT FROM AUTHOR]

Rotational seismology opens a new avenue to study the deep interior of the Earth. Using data from the Wettzell Observatorium, Germany, where a ring laser gyroscope and a 3-component translational broadband seismometer are co-located, we report the presence of clear S, ScS and SdS signals on both rotational and translational seismograms. Using S wave arrivals, we propose a new methodology to extract information on velocity changes in the Earth mantle and we show that, by combining both translational and rotational data, we are able to solve the well known velocity-depth ambiguity inherent to classical inverse problems. The methodology is validated using ray theory and 2.5-D finite-difference synthetics. We provide a proof-of-concept showing that future studies of the Earth's deep interior can be improved by combining translational and rotational records. [ABSTRACT FROM AUTHOR]

Many real-world systems are connected together, in natural and man-made networks. A complex-valued laser network can simulate the working mechanism of human brain. However, amplitude control of a complex-valued laser network is seldom studied. In this paper, a ring network of complex-valued Lorenz laser systems is investigated. The ring network exhibits complex dynamics including hyper-chaos, quasi-periodic orbits, and coexisting hyper-chaos. Three kinds of single-parameter oriented amplitude controls are realized with varying or unvarying Lyapunov exponents in the ring network. Meanwhile, a simple knob can realize the amplitude rescaling of hyper-chaotic signals, which reduces the cost of circuit implementation. Moreover, a criterion of chaotic complete synchronization among all the nodes is established for a network with strong coupling. For relatively weak coupling, quasi-periodic complete synchronization is found, and the performance of chaotic synchronization is studied in terms of the cross-correlation coefficient. It is moreover revealed that the improvement and trend of synchronization performance are robust to the parity of the number of nodes for the same-scale laser networks. [ABSTRACT FROM AUTHOR]

Picosecond optical pulses generation in a ring fiber laser with a semiconductor optical amplifier was investigated. Optical spectrum width is minimal at a fundamental resonant repetition frequency of 45 MHz, corresponding to a fiber resonator length of 4.5 m. As the frequency increases, the spectral width increases by an order of magnitude while the pulse duration decreases to 18 ps, and the power increases to 0.5 W. The power of picosecond optical pulses at the output of the semiconductor amplifier is 7 times higher than in the CW mode due to the carrier accumulation effect that occurs with short pump pulses. [ABSTRACT FROM AUTHOR]

The Vernier effect created using an incorporated Lyot–Sagnac loop is used to create an ultra-high sensitivity temperature sensor based on a ring laser cavity. Unlike standard double Sagnac loop systems, the proposed sensor is fused into a single Sagnac loop by adjusting the welding angle between two polarization-maintaining fibers (PMFs) to achieve effective temperature sensitivity amplification. The PMFs are separated into two arms of 0.8 m and 1 m in length, with a 45° angle difference between the fast axes. The sensor's performance is examined both theoretically and experimentally. The experimental results reveal that the Vernier amplification effect can be achieved via PMF rotating shaft welding. The temperature sensitivity in the laser cavity can reach 2.391 nm/°C, which is increased by a factor of more than eight times compared with a single Sagnac loop structure (0.298 nm/°C) with a length of 0.8 m without the Vernier effect at temperatures ranging from 20 °C to 30 °C. Furthermore, unlike traditional optical fiber sensing that uses a broadband light source (BBS) for detection, which causes issues such as low signal-to-noise ratio and broad bandwidth, the Sagnac loop can be employed as a filter by inserting itself into the fiber ring laser (FRL) cavity. When the external parameters change, the laser is offset by the interference general modulation, allowing the external temperature to be monitored. The superior performance of signal-to-noise ratios of up to 50 dB and bandwidths of less than 0.2 nm is achieved. The proposed sensor has a simple structure and high sensitivity and is expected to play a role in biological cell activity monitoring. [ABSTRACT FROM AUTHOR]

The article discusses the generation of terahertz optical solitons using dispersion-compensated antenna-coupled planarized ring Quantum Cascade Lasers (QCL). The researchers address the challenges of achieving solitons in terahertz ring QCLs, including dispersion compensation and power extraction. They propose a double waveguide structure and a planarization technique to overcome these challenges.

A 30-GHz pulse-train is generated using the rational harmonic mode-locking technique, experimentally, using a Mach–Zehnder Lithium Niobate modulator. The width of the pulses is then reduced from 5.8-ps to 1.9-ps by incorporating nonlinear polarization rotation. This phenomenon arises due to the very high nonlinear behavior of the photonic crystal fiber (PCF) added to the ring laser cavity. Numerically solving the Generalized Nonlinear Schrödinger Equation provided insights into the pulse evolution behavior. The relative polarization angle and length of the PCF were varied to study their effects on the pulse-width. [ABSTRACT FROM AUTHOR]

Optical pulses at high repetition rates are generated using rational harmonic mode locking and saturable absorber made of graphene nanoparticles in a fiber laser. The pulse generation from the fiber laser is modeled by solving the Generalized Nonlinear Schrodinger Equation. The computation involved varying the various saturable absorption parameters, such as linear and nonlinear absorption coefficients. Experimentally stable pulse trains at 20 GHz and 50 GHz are generated with a pulse width of ∼ 2.7 ps. This result agrees with the simulation. [ABSTRACT FROM AUTHOR]

The experimental results obtained for the sextuple Brillouin Stokes line (0.48 nm) based on stimulated Brillouin scattering phenomenon are presented in this paper. Three ring laser cavities with unidirectional propagation are used. In each ring cavity, a dispersion compensation fibre (DCF) spool is used as a Brillouin gain medium. Two erbium amplifiers are used to amplify the generated Stokes signal and to overcome the path loss for the propagation Stokes lines. A maximum pump power of 500 mW at a central wavelength of 1480 nm is used for these two amplifiers. In our new structure, up to three sextuple Stokes lines with a high signal power of 10 dBm and a large optical signal-to-noise ratio of more than 55 dB are achieved. The generated Stokes lines can be tuned over a wavelength range of 15 nm from 1560 to 1575 nm. [ABSTRACT FROM AUTHOR]

Stable isotope ratio analysis of tree rings has been widely and successfully applied in recent decades for climatic and environmental reconstructions. These studies were mostly conducted at an annual resolution, considering one measurement per tree ring, often focusing on latewood. However, much more information could be retrieved with high-resolution intra-annual isotope studies, based on the fact that the wood cells and the corresponding organic matter are continuously laid down during the growing season. Such studies are still relatively rare, but have a unique potential for reconstructing seasonal climate variations or short-term changes in physiological plant properties, like water-use efficiency. The reason for this research gap is mostly technical, as on the one hand sub-annual, manual splitting of rings is very tedious, while on the other hand automated laser ablation for high-resolution analyses is not yet well established and available. Here, we give an update on the current status of laser ablation research for analysis of the carbon isotope ratio (δ13C) of wood, describe an easy-to-use laser ablation system, its operation and discuss practical issues related to tree core preparation, including cellulose extraction. The results show that routine analysis with up to 100 laser shot-derived δ13C-values daily and good precision and accuracy (ca. 0.1‰) comparable to conventional combustion in an elemental analyzer are possible. Measurements on resin-extracted wood is recommended as most efficient, but laser ablation is also possible on cellulose extracted wood pieces. Considering the straightforward sample preparation, the technique is therefore ripe for wide-spread application. With this work, we hope to stimulate future progress in the promising field of high-resolution environmental reconstruction using laser ablation. [ABSTRACT FROM AUTHOR]

In this paper, we investigate multiple soliton interactions and other solitary wave solutions (SWS) for a perturbed nonlinear Schrödinger equation (NLSE) with negative index material having quadratic-cubic nonlinearity (NLSE-QCN). Due to its high order dispersion term, this model yields sub-picosecond impulses useful in mode-locked ring lasers. Hirota bilinear method (HBM) will be used to study soliton interaction. By controlling the parameters, we will obtain S , V , parabolic and anti-parabolic, butterfly, bright and dark shaped solitons. On the other hand, we will obtain some other solitary wave solutions with the help of Sine-Gordon expansion (SGE) scheme. [ABSTRACT FROM AUTHOR]

Optical vortices, which have been extensively studied over the last decades, offer an additional degree of freedom useful in many applications, such as optical tweezers and quantum control. Stimulated Brillouin scattering (SBS), providing a narrow linewidth and a strong nonlinear response, has been used to realize quasi-continuous wave lasers. Here, stable oscillation of optical vortices and acoustic modes in a Brillouin laser based on chiral photonic crystal fiber (PCF) is reported, which robustly supports helical Bloch modes (HBMs) that carry circularly polarized optical vortex and display circular birefringence. A narrow-linewidth Brillouin fiber laser that stably emits 1stand 2nd-order vortex-carrying HBMs is implemented. Angular momentum conservation selection rules dictate that pump and backward Brillouin signals have opposite topological charge and spin. Additionally, it is shown that when the chiral PCF is placed within a laser ring cavity, the linewidth-narrowing associated with lasing permits the peak of the Brillouin gain that corresponds to acoustic mode to be measured with resolution of 10 kHz and accuracy of 520 kHz. The results pave the way to a new generation of vortex-carrying SBS systems with applications in optical tweezers, quantum information processing, and vortex-carrying nonreciprocal systems. [ABSTRACT FROM AUTHOR]

Ultra-smooth surface with sub-nanometer roughness is of great significance in various fields, including Ring Laser Gyro (RLG), high-power laser apparatus, ultraviolet optical systems, and the semiconductor industry. However, the mechanism of the ultra-smooth polishing process remains to be studied, which is of great value for understanding and optimizing optical-fabrication methods. This paper establishes a relationship between surface densification characteristics and surface roughness in conventional polishing. Moreover, we conclude that the densification process created by the longitudinal pressure, which avoids non-uniformity, is beneficial to forming super-smooth surfaces in the conventional polishing methods. Under this guidance, we can stably fabricate ultra-smooth surfaces on fused silica with 0.4 Å roughness. This result overturns the mainstream view that the longitudinal pressure in polishing should be minimized. This conclusion is meaningful for the understanding of the ultra-smooth surface formation not only in conventional polishing but also in other optical-fabrication technologies. [ABSTRACT FROM AUTHOR]

The realization of on‐chip microlasers with out‐of‐plane, controllable emission direction has been one of the core research themes in modern photonics. Traditional approaches for directional light scattering, including plasmonic nanoantennas and passive dielectric metasurfaces, are not suitable for on‐chip integration due to potential metallic contamination and reliance on external light sources. Meanwhile, achieving directional emission in active III–V microcavity lasers remains challenging due to geometrical symmetry and large free‐space coupling losses. Here, a novel approach is presented to realize directional lasing in an all‐dielectric, bottom‐up grown material system by coupling the laser emission from an InP microring cavity into a vertical nanowire at the ring center, which function as the photon source and directional antenna in the system, respectively. Efficient optical coupling is facilitated by enhanced light scattering at specific sidewall facets of the microring cavity, which can be engineered deterministically during epitaxial growth. Through Fourier imaging, out‐of‐plane laser emission with antenna‐like far‐field directivity in the coupled system is demonstrated. Furthermore, the emission directivity and side mode suppression in the coupled system can be improved significantly by tuning the geometric parameters of the system. The way for low power consumption, on‐chip microlasers with tunable emission directionality is paved here. [ABSTRACT FROM AUTHOR]

Intracavity phase interferometry is a powerful phase sensing technique using two correlated, counter-propagating frequency combs (pulse trains) in mode-locked lasers. Generating dual frequency combs of the same repetition rate in fiber lasers is a new field with hitherto unanticipated challenges. The large intensity in the fiber core, coupled with the nonlinear index of glass, result in a cumulative nonlinear index on axis that dwarfs the signal to be measured. The large saturable gain changes in an unpredictable way the repetition rate of the laser impeding the creation of frequency combs with identical repetition rate. The huge amount of phase coupling between pulses crossing at the saturable absorber eliminates the small signal response (deadband). Although there have been prior observation of gyroscopic response in mode-locked ring lasers, to our knowledge this is the first time that orthogonally polarized pulses were used to successfully eliminate the deadband and obtain a beat note. [ABSTRACT FROM AUTHOR]

Laser light scattering systems with volume Bragg grating (VBG) filters, which act as spectral/angular filters, have often been used as a point measurement technique, with spatial resolution as low as a few hundred μm, defined by the beam waist. In this work, we demonstrate how VBG filters can be leveraged for spatially resolved measurements with several μm resolution over a few millimeters along the beam propagation axis. The rejection ring, as determined by the angular acceptance criteria of the filter, is derived analytically, and the use of the ring for 1D laser line rejection is explained. For the example cases presented,i.e., for a focused probe beam waist with a diameter of ∼ 150 μm, the rejection ring can provide resolution up to several millimeter length along the beam propagation axis for a 1D measurement, which is also tunable. Additionally, methods to further extend the measurable region are proposed and demonstrated, using a collimation lens with a different focal length or using multiple VBG filters. The latter case can minimize the scattering signal loss, without the tradeoff of the solid angle. Such use of multiple VBGs is to extend the measurable region along the beam axis, which differs from the commonly known application of multiple filters, to improve the suppression of elastic interferences. 1D rotational Raman and Thomson scattering measurements are carried out on pulsed and DC discharges to verify this method. The system features compactness, simple implementation, high throughput, and flexibility, to accommodate various experimental conditions. [ABSTRACT FROM AUTHOR]

Frequency lock-in-induced deadband phenomena are major problems of ring laser gyroscopes (RLGs), which deteriorate linear responses to changes in the applied rotation rate. In this work, the frequency lock-in phenomenon occurring in the RLG was successfully investigated by compensating for the Sagnac effect through frequency analysis using a newly defined error function. Integrative and generalized viewpoints from the analyzed results provide new possibilities for relevant performance improvements of optical gyroscopes, as well as a deeper understanding of locked states in principle aspects. [ABSTRACT FROM AUTHOR]

Bound solitons have become a hot topic in the field of nonlinear optics due to their potential applications in optical communication, information processing and radar systems. However, the trapping of the cascaded bound soliton is still a major challenge up to now. Here, we propose and experimentally demonstrate a multi-pulse bound soliton fiber laser based on MoTe2 saturable absorber. In the experiment, MoTe2 nanosheets were synthesized by chemical vapor deposition and transferred to the fiber taper by optical deposition. Then, by inserting the MoTe2 saturable absorber into a ring cavity laser, the two-pulse, three-pulse and four-pulse bound solitons can be stably generated by properly adjusting the pump strength and polarization state. These cascaded bound solitons are expected to be applied to all-optical communication and bring new ideas to the study of soliton lasers. [ABSTRACT FROM AUTHOR]

A high-energy, alignment-insensitive, injection-seeded Q-switched Ho:yttrium aluminum garnet (YAG) single-frequency laser is developed. Both the slave Q-switched laser and the seed laser are Ho:YAG ring lasers based on a pair of corner cubic reflectors. The seed laser has an available power of 830 mW at 2096.667 nm. At 100 Hz, the Q-switched Ho:YAG laser provides a single-frequency pulsed output using injection-seeded technology. The 7.3 mJ single-frequency pulse energy from the slave laser has a pulse width of 161.2 ns and is scaled to 33.3 mJ after passing through the Ho:YAG single-pass amplifier. According to the measurement results of the heterodyne beating technique, the single-frequency pulse has a half-width of 4.12 MHz. [ABSTRACT FROM AUTHOR]

Cytokinesis requires the constriction of an actomyosin-based contractile ring and involves multiple F-actin crosslinkers. We show that partial depletion of the C. elegans cytokinetic formin generates contractile rings with low F-actin levels that constrict but are structurally fragile, and we use this background to investigate the roles of the crosslinkers plastin/PLST-1 and β-heavy-spectrin/SMA-1 during ring constriction. We show that the removal of PLST-1 or SMA-1 has opposite effects on the structural integrity of fragile rings. PLST-1 loss reduces cortical tension that resists ring constriction and makes fragile rings less prone to ruptures and regressions, whereas SMA-1 loss exacerbates structural defects, leading to frequent ruptures and cytokinesis failure. Fragile rings without SMA-1 or containing a shorter SMA-1, repeatedly rupture at the same site, and SMA-1::GFP accumulates at repair sites in fragile rings and in rings cut by laser microsurgery. These results establish that β-heavy-spectrin stabilizes the constricting ring and reveals the importance of β-heavy-spectrin size for network connectivity at low F-actin density. [ABSTRACT FROM AUTHOR]

In underwater turbulent medium, field correlations are found when the incidence is a laser beam array. Variations of the field correlations against the variations in the ring radius of laser array beam, number of beamlets composing the laser array, source size, underwater turbulence parameters, i.e. the ratio of temperature to salinity contributions to the refractive index spectrum, rate of dissipation of mean-squared temperature and rate of dissipation of kinetic energy per unit mass of fluid, are investigated. Field correlations of laser arrays are found to be larger than the field correlations of the single beams. The effect of underwater turbulence is to reduce the field correlation of laser arrays. [ABSTRACT FROM AUTHOR]

In this study, an erbium fiber ring laser is demonstrated by using multiple-fiber-ring (MFR) scheme to reach single-longitudinal-mode (SLM) operation. The proposed MFR architecture can not only mitigate the multi-longitudinal-mode (MLM) induced by the longer fiber length and homogeneous gain broadening of erbium, but also can elongate the tuning scope from C- to part of L-bands. Furthermore, the obtained optical signal to noise ratio (OSNR), output power, laser linewidth and stability of the presented fiber laser are also performed experimentally. [ABSTRACT FROM AUTHOR]

We report an ytterbium (Yb) doped fiber Mach Zehnder interferometer (MZI) based on the up-taper fiber structure in a fiber ring laser (FRL) cavity. Different from the traditional FRL sensing system, in which additional filters are required, the designed structure simultaneously acts as a filter, sensor and gain medium. Furthermore, thanks to the high thermal–optical coefficient of Yb doped fiber, the temperature sensitivity of 0.261 nm/°C can be achieved in the range of 10–50 °C. In addition, benefiting from the unique characteristics of the laser system itself, the designed structure has a narrower linewidth (−0.2 nm) and a higher signal-to-noise ratio (SNR) (−40 dB) than the sensor system based on a broadband light source (BBS). Meanwhile, the refractive index (RI) response and stability of the system are measured. The RI sensitivity is up to 151 nm/RIU, and the wavelength fluctuation range within two hours is less than 0.2 nm. Therefore, the designed structure is expected to play a significant role in human life safety monitoring, aircraft engine temperature monitoring, etc. [ABSTRACT FROM AUTHOR]

In sensors, the highest precision in measurements is given by vacuum fluctuations of quantum mechanics, resulting in a shot noise limit. In a Mach–Zenhder interferometer (MZI), the intensity measurement is correlated with the phase, and thus, the precision measurement ( Δ n ) is coupled with the phase resolution ( Δ φ ) by the Heisenberg uncertainty principle. Quantum metrology offers a different solution to this precision measurement using nonclassical light such as squeezed light or higher-order entangled-photon pairs, resulting in a smaller Δ φ and sub-shot noise limit. Here, we propose another method for the high precision measurement overcoming the diffraction limit in classical physics, where the smaller Δ φ is achieved by phase quantization in a coupled interferometric system of coherence de Broglie waves. For a potential application of the proposed method, a quantum ring laser gyroscope is presented as a quantum version of the conventional ring laser gyroscope used for inertial navigation and geodesy. [ABSTRACT FROM AUTHOR]

We have studied the ultrafast saturation behavior of a high-density well-aligned single-walled carbon nanotubes saturable absorber (HDWA-SWCNT SA), obtained by a high-pressure and high-temperature treatment of commercially available single-wall carbon nanotubes (SWCNTs) and related it to femtosecond erbium-doped fiber laser performance. We have observed the polarization dependence of a nonlinear optical saturation, along with a low saturation energy level of <1 fJ, limited to the detector threshold used, and the ultrafast response time of <250 fs, while the modulation depth was approximately 12%. We have obtained the generation of ultrashort stretched pulses with a low mode-locking launching threshold of ~100 mW and an average output power of 12.5 mW in an erbium-doped ring laser with the hybrid mode-locking of a VDVA-SWNT SA in combination with the effects of nonlinear polarization evolution. Dechirped pulses with a duration of 180 fs were generated, with a repetition rate of about 42.22 MHz. The average output power standard deviation was about 0.06% RMS during 3 h of measurement. [ABSTRACT FROM AUTHOR]

Soft X-ray monochromators for synchrotron radiation sources have been continuously developed over the years, improving energy resolution and stability. Great effort has been made in improving the surface quality of the optics involved, reaching values permitting diffraction-limited images. Still, one problem has not been solved, nor fully understood, yet: groove placing errors. Nowadays, these are one of the major factors responsible for resolving the power reduction of diffraction-gratings-based X-ray monochromators. Despite decades of use of gratings, there is not yet a universally established method for predicting and simulating the effect of groove placing errors on monochromator performance. This is especially important in the new scenario of high-coherent X-ray sources, i.e. diffraction-limited storage rings and free-electron lasers. To address this problem, in this article an approach based on WISER (Wavefront propagatIon Simulation codE libRary) is presented. WISER is a physical optics simulation package, also available in the user-friendly Orange Synchrotron Radiation Suite - OASYS. Even though it was originally conceived to assess the focusing performance of X-ray mirrors in the presence of height defects, it perfectly simulates the performance of a periodic (or quasi-periodic) structure like a diffraction grating. In this article, the way to use WISER and its application to a specific case, e.g. the design of a monochromator for the upgrade of a beamline at the Advanced Light Source, are shown. A simple rule for estimating how well the grooves are placed on a grating, based on calculation of the Strehl ratio, is also presented. [ABSTRACT FROM AUTHOR]

We demonstrate anti-misalignment acousto-optically (AO) Q-switched Ho: YLF ring cavity laser with a single corner cube prism (CCP) as the cavity mirror and a diode-pumped 1940 nm solid state laser as the pump source. We study the polarization state of oscillating light of ring cavity with the CCP by simulation calculations, which matched with the experimental results. In the continuous wave (CW) operation, the Ho: YLF laser achieves a total bidirectional output power of 3.8 W at the wavelength of 2063.5 nm, corresponding to an optical-to-optical conversion efficiency of 15% and a slope efficiency of 50.7%. The maximum allowable deviations of the CCP are 0.57° (pitch angle) and 1.65° (horizontal angle), respectively. We obtain a single pulse energy of 17 mJ and a pulse width of 113 ns at a pulse repetition frequency (PRF) of 100 Hz. The beam quality factors in the horizontal and vertical directions are M x 2 = 1.01 and M y 2 = 1.06, respectively, which are approached to the diffraction limit. [ABSTRACT FROM AUTHOR]

This paper presented a new kind of salinity and temperature dual-parameter sensor based on a fiber ring laser (FRL) with tapered side-hole fiber (SHF) embedded in a Sagnac interferometer. The sensing structure is majorly composed of tapered SHF located in the middle of SHF inside the Sagnac interferometer loop structure. The influences of the SHF's diameters of different tapered in the Sagnac interferometer loop on the FRL sensing system are studied. The presence of air holes in the SHF makes the cladding mode easier to excite, and the interaction between the cladding mode with its surroundings is enhanced, thus having higher salinity sensitivity. Besides, the unique advantages of high resolution, narrower linewidth, and high signal-to-noise ratio (SNR) of fiber laser make the measurement results more accurate. In this experiment, the SHF with different taper diameters was made, and it was found that reducing the diameter of the taper waist diameter could further improve the salinity sensitivity. When the waist diameter was 9.70 μm, the maximum salinity sensitivity of 0.2867 nm/‰ was achieved. Temperature sensing experiments were also carried out. The maximum temperature sensitivity of the FRL sensing system was −0.3041 nm/°C at the temperature range from 20 to 30 °C. The sensor has the characteristics of easy manufacture, good selectivity, and high sensitivity, proving the feasibility of simultaneous measurement of seawater salinity and temperature. [ABSTRACT FROM AUTHOR]

For safe operations and regular maintenance of pipelines in industrial and domestic usages, it is extremely important to extensively monitor gigantic pipeline systems and accurately detect harmful defects. Non-destructive evaluation (NDE) methods for inline inspection of these modern, massive pipe systems generate huge amount of data and need successful incorporation of automated defect detection algorithm that can conduct large-scale data analysis and provide instantaneous diagnosis by locating and identifying defects of varying characteristics. Here, we develop a novel NDE methodology for inspecting plastic pipelines used in natural gas distribution. Our proposed method is based on optical imaging and uses laser profiling to collect inline inspection data. In our laser profiling images, deformities due to defects lead to local perturbations in the circular rings produced by the otherwise non-defective inner walls of cylindrical pipes. Using experimental data collected by the proposed methodology, we studied the performance of a gamut of defect classification algorithms including conventional machine learning classifiers as well as deep learning architectures. Special attention was paid to take into account both the spatial and temporal features in our laser-scan image, particularly exploiting the circular shape of non-deformed laser rings. This led to the development of a hierarchical bilinear pooling (HBP) based deep learning framework that can precisely identify local perturbations in our laser-scan data by assimilating information across multiple cross-layer features. Equipping our endoscopic laser profiling system with the HBP based defect classification algorithm, we develop an integrated diagnosis method for inline data collection and automated defect characterization. Based on experimental data collected under varying profiling conditions, we demonstrate superior performance of our proposed integrated method in damage localization and classification. [ABSTRACT FROM AUTHOR]

The absolute measurement of the Earth angular rotation rate with ground-based instruments becomes challenging if the 1 part in 10 9 of precision has to be obtained. This threshold is important for fundamental physics and for geodesy, to investigate effects of General Relativity and Lorentz violation in the gravity sector and to provide the fast variation of the Earth rotation rate. High sensitivity Ring Laser Gyroscopes (RLG) are currently the only promising technique to achieve this task in the near future, but their precision has been so far limited by systematics related to the laser operation. In this paper we analyze two different sets of observations, each of them three days long. They were obtained from the G ring laser at the Geodetic Observatory Wettzell. The applied method has been developed for the GINGERINO ring laser in order to identify and extract the laser systematics. For the available data sets the residuals show mostly white noise behavior and the Allan deviation drops below 1 part in 10 9 after an integration time of about 10 4 s. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a novel high-temperature-resistant vector accelerometer, consisting of a ring cavity laser and sensing probe (i.e., fiber Bragg gratings (FBGs)) inscribed in a seven-core fiber (SCF) by using the femtosecond laser direct writing technique. A ring cavity laser serves as a light source. Three FBGs in the outer cores of SCF, which are not aligned in a straight line, are employed to test the vibration. These three FBGs have 120° angular separation in the SCF, and hence, vibration orientation and acceleration can be measured simultaneously. Moreover, the FBG in the central core was used as a reflector in the ring cavity laser, benefiting to resist external interference factors, such as temperature and strain fluctuation. Such a proposed accelerometer exhibits a working frequency bandwidth ranging from 4 to 68 Hz, a maximum sensitivity of 54.2 mV/g, and the best azimuthal angle accuracy of 0.21° over a range of 0–360°. Furthermore, we investigated the effect of strain and temperature on the performance of this sensor. The signal-to-noise ratio (SNR) only exhibits a fluctuation of ~1 dB in the range (0, 2289 με) and (50 °C, 1050 °C). Hence, such a vector accelerometer can operate in harsh environments, such as in aerospace and a nuclear reactor. [ABSTRACT FROM AUTHOR]

In order to stabilize the self-biasing state of a laser gyroscope, a dual-longitudinal-mode asymmetric frequency stabilization technique was studied. The special frequency stabilization is based on the accurate control of the intensity tuning curve in the prism ring laser. In this study, the effects of the ratio of the Ne isotopes, the inflation pressure, and the frequencies coupling on the intensity tuning curve in a laser gyro were examined. The profiles of the intensity tuning curve were simulated under the mixing ratios of Ne20 and Ne22 of 1:1 and 7:3, and the inflation pressures were 350 Pa, 400 Pa, and 450 Pa. The mixing ratio of Ne20 and Ne27 was dealt with similarly. The method for precisely adjusting the profiles of the intensity tuning curve was analyzed. The profiles were verified by experiments under different isotope ratios and pressures. Finally, based on a prism ring laser with an optical length of 0.47 m, the proposed frequency stabilization method was preliminarily verified. [ABSTRACT FROM AUTHOR]

The composition, operating principle, and main metrological characteristics of GET 138-2021 State Primary Refractive Index Standard are presented. This standard provides uniform measurements of the spectral dependence and spatial distribution of the refractive index in solid and liquid substances within the extended wavelength range of 0.4–1.55 μm by means of using 12 replaceable light sources, a dynamic gonio-spectrometer equipped with a ring laser and an autocollimator-based null-indicator. Performance of highly accurate and reliable refractometric measurements constitutes an important scientific field that is crucial in improving the resolution of lenses and other optical instruments used in the optical industry, as well as in controlling quality and determining the composition of substances in the food, chemical, and pharmacological industries. [ABSTRACT FROM AUTHOR]

• The effect of different FRM laser powers on weld morphology has been investigated. • The difference in the suppression of welding spatter for different laser combination modes of FRM laser welding has been studied. • The relation between microstructure and mechanical properties of the FRM welded joints has been revealed. Flexible Ring Mode (FRM) laser welding is a relatively new composite laser welding technology. The present study focused on the FRM laser welded joints produced from 4.5 mm-thick 304 stainless steel. The effects of various laser modes on the macroscopic morphology, microstructure, microhardness, and tensile properties of the produced welded joints were systematically investigated. The results showed that, when the ring laser power was set at 1000 W and the center laser power at 3000 W, a maximum weld area of 6.23 mm2 was obtained. Employing a composite laser with a center laser power greater than the ring laser power resulted in the fewest number of welding spatter particles on both sides of the weld bead, indicating an effective inhibition of welding spatter by the ring laser. Conversely, using a composite laser with a center laser power equal to the ring laser power still led to the presence of small welding spatter particles, indicating differences in the laser combination modes in suppressing welding spatter for the welding of 304 stainless steel. Increased cooling and a large presence of small turbulence contributed to the grain refinement in the fusion zone of the welded joints produced using a composite laser with a central laser power greater than the ring laser power. Consequently, this welded joint exhibited the highest maximum tensile strength among all tensile test specimens reaching 95.2 % of the base metal's maximum tensile strength. However, the fractured surfaces of the tensile test specimens produced using a composite laser with a center laser power equal to the ring laser power exhibited the highest number density and largest size of oxide particles, resulting in a joint elongation of only 77.3 % of the base metal's elongation. This study emphasizes the potential application of this FRM laser and provides theoretical guidance for selecting the optimal laser combination mode for FRM laser welding of stainless steel. [ABSTRACT FROM AUTHOR]

We report a widely wavelength tunable mode locked noise-like pulse (NLP) source from an erbium doped fiber ring laser. By making use of the reflection of an asymmetric Fabry- Perot interferometer (FPI), continuous tunability of the central wavelength over the entire C-band (1530 nm to 1565 nm), with an independent control over bandwidth (3.5 nm to 17 nm) is demonstrated. Using a single control parameter, i.e. the length of the FPI, it is thus possible to select from a range of output spectral bandwidths and desired central wavelengths. Using the same control on the length of the FPI and intracavity polarization settings, generation of dual wavelength NLPs, temporally shifted noise-like pulse pairs, harmonic mode locked NLPs and loosely bound NLP doublets and NLP triplets are reported. Owing to the simplicity of the tuning mechanism and the adaptable output spectrum, we believe, such a laser will be useful for various applications like sensing, spectroscopy and imaging. The novel combination of a reflecting FPI-based filter, and the complex nonlinear polarization rotation (NPR) based mode locking mechanism, also offers the opportunity to further explore the rich physics of NLPs. [ABSTRACT FROM AUTHOR]

• A fiber laser system able to generate pulse bursts with an adjustable number of pulses, from 1 to 7, is presented. • Cascaded Raman scattering with generation of up to for Stokes components is demonstrated. • Experimental setup allows the generation of a broad and flat spectrum over more than 200 nm directly at the laser output. • The results obtained have potential applications in materials processing, bioinspired robotics, and biophotonics. In this work, we study the pulse burst regime of a fiber laser, which allows generating packets of multiple pulses whose number can be precisely controlled by modifying the polarization state by means of two retarder plates (QWR, HWR). The proposed experimental scheme is a highly energetic Yb-doped all-fiber ring pulsed laser operating in the 1064 nm wavelength region and using a nonlinear polarization rotation (NPR)-based passive mode locking technique. Raman scattering effects can be found by increasing pump power to achieve the formation of up to four Stokes, resulting in a spectral broadening over more than 200 nm. [ABSTRACT FROM AUTHOR]

• This paper proposes a wavelength tunable ring cavity erbium-doped fiber laser based on Fabry Perot filter fabricated by hydrofluoric acid corrosion for stretch sensing. • In the design of the laser, hydrofluoric acid is employed for chemical etching to prepare the F-P cavity, and tapering is applied to the F-P cavity to enhance sensitivity. In this study, the erbium-doped fiber laser with a ring cavity and a Fabry-Perot (F-P) filter was proposed and experimentally verified. In the design of the laser, hydrofluoric acid was employed for chemical etching to prepare the F-P cavity, and tapering was applied to the F-P cavity to enhance sensitivity. The experimentally prepared F-P filter was placed in the resonant cavity and served as a frequency-selective device for the fiber laser. At a laser threshold of 48 mW, we obtained a single-wavelength laser output at 1565.28 nm, with a 3 dB linewidth of 0.29 nm. To evaluate the stretching sensing performance of the EDFL system, we performed stretching of the F-P filter. The experimental results demonstrated that within a stretching length range of 0 to 3 mm, we could obtain a single-wavelength laser that could be tuned from 1559.57 to 1567.04 nm, corresponding to a tuning range of 7.47 nm. The laser showed a sensitivity of 2.73 nm/mm and a goodness of fit 0.999. Monitoring the stability of a single-wavelength laser within 30 min, the signal-to-noise ratio was greater than 27.212 dB, and the power fluctuation was less than 0.712 dB. Additionally, the polarization controller was adjusted to obtain two different dual-wavelength lasers with a power fluctuation of less than 0.71 dB. [ABSTRACT FROM AUTHOR]

• The localization of gray losses during chemical etching has been determined. • A comparison is made of the effect of the type of Q-switch mode. • SA was created based on Bi2Te3 nanopowder - polymer. • The pulse duration in this case was 5 ns with a repetition rate of 4 MHz. • Taper-based Q-switch fiber laser has a pulse energy of more than 1000 nJ. The presented work shows an influence of geometry passive Q-switch based on a nanopowder-polymer on the characteristics of an erbium ring fiber laser. The saturable absorber Bi 2 Te 3 was applied to two types of specially prepared samples of thinned sections of optical fiber, and the resulting structures were built into the ring circuit of an erbium fiber laser. In the first case, the surface of the light guide segment was thinned by chemical etching, and in the second, by mechanical grinding. For the first time, the time-domain optical reflectometry technique was used to control the quality of the resulting sample surface and the resulting losses. For a more complete comparison, a set of different optical splitters was used, providing a change in the feedback coefficient in the resonator from 5 to 95 %. A study in the mode of passive Q-switching using polarized light showed that for both types of modulators there are optimal ratios of the amount of energy output from the resonator to obtain the highest values of the average generation power. The d-shape fiber modulator allows mode-locking to be observed without any additional changes to the laser circuit. The pulse duration in this case was 5 ns with a repetition rate of 4 MHz. A Q-switch based on a chemically thinned fiber segment made it possible to obtain narrower pulses in Q-switching mode (up to 700 ns at 38 kHz), a stable peak position in the transmission spectrum, and an average power of 34 mW. [ABSTRACT FROM AUTHOR]

• A tunable narrow linewidth fiber ring laser based on parity-time (PT) symmetric double rings and variable optical attenuator (VOA) is demonstrated. • By tuning the VOA, the single-longitudinal-mode oscillation can be achieved when the PT symmetric is broken. • The tunable laser can be tuned from 1525 nm to 1563 nm, and the linewidth of the laser is at the sub-kHz level in the whole tuning process. A tunable narrow linewidth fiber ring laser based on parity-time (PT) symmetric double rings and variable optical attenuator (VOA) is demonstrated. The laser is composed of a fiber loop mirror (FLM), which creates two coupled rings: the reflective ring and the transmissive ring. The loss of the transmissive ring can be controlled by tuning the VOA, which balances with the gain of the reflective ring. When the PT symmetry is broken, a single-longitudinal-mode (SLM) laser is realized. The laser can be tuned by an optical tunable filter (OTF), and the tuning range is 38 nm, spanning from 1525 nm to 1563 nm. In the whole tuning process, the linewidth of the laser is at the sub-kHz level, and the optical signal to noise ratios (OSNR) is over 53 dB. The fluctuation of wavelength and power are less than 0.02 nm and 0.52 dB in an hour, respectively. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically studied the interband transitions in a three-dimensional structure of GaAs/Al 0.3 Ga 0.7 As triple concentric quantum rings approximating a reported experimental case. An intense non-resonant laser field and/or a static electric field were appropriately chosen to investigate the heavy-hole spectra and the interband transitions. For the combined actions of the two fields acting in the horizontal plane of the structure, we considered parallel and perpendicular directions. In each study, the effects on the heavy-hole energy spectra were discussed concerning their relationship with wave function localization. The radiative recombination energies, overlap integrals, and interband transition lifetime emphasize regions of interest for efficient control of the radiative decay, which is important for practical applications. • We study interband transition in triple quantum ring in laser and electric fields. • We performed 3D numerical computation for energy spectra and wave functions. • Recombination energy decreases in electric field and increases with laser field. • Transition lifetime can be efficiently tuned by fields values and orientation. [ABSTRACT FROM AUTHOR]