Small Linewidth Research Articles

Laser diode-pumped compact hybrid lithium niobate microring laser.

We demonstrate a compact hybrid lithium niobate microring laser by butt coupling a commercial 980-nm pump laser diode chip with a high-quality Er3+-doped lithium niobate microring chip. Single-mode lasing emission at 1531-nm wavelength from the Er3+-doped lithium niobate microring can be observed with the integrated 980-nm laser pumping. The compact hybrid lithium niobate microring laser occupies the chip size of 3 mm × 4 mm × 0.5 mm. The threshold pumping laser power is 6 mW and the threshold current is 0.5 A (operating voltage 1.64 V) at atmospheric temperature. The spectrum featuring single-mode lasing with small linewidth of 0.05 nm is observed. This work explores a robust hybrid lithium niobate microring laser source which has potential applications in coherent optical communication and precision metrology.

Electro-optically tunable single-frequency lasing from neodymium-doped lithium niobate microresonators.

Tunable light sources are a key enabling technology for many applications such as ranging, spectroscopy, optical coherence tomography, digital imaging and interferometry. For miniaturized laser devices, whispering gallery resonator lasers are a well-suited platform, offering low thresholds and small linewidths, however, many realizations suffer from the lack of reliable tuning. Rare-earth ion-doped lithium niobate offers a way to solve this issue. Here we present a single-frequency laser based on a neodymium-doped lithium niobate whispering gallery mode resonator that is tuned via the linear electro-optic effect. Using a special geometry, we suppress higher-order transverse modes and hence ensure single-mode operation. With an applied voltage of just 68 V, we achieve a tuning range of 3.5 GHz. The lasing frequency can also be modulated with a triangular control signal. The freely running system provides a frequency and power stability of better than Δ ν=20MHz and 6 %, respectively, for a 30-minute period. This concept is suitable for full integration with existing photonic platforms based on lithium niobate.

Investigating the Influence of Material Extrusion Rates and Line Widths on FFF-Printed Graphene-Enhanced PLA

Fused filament fabrication (FFF) is a widely used additive manufacturing process that can produce parts from thermoplastics. Its ease of operation and wide variety of materials make it a popular choice for manufacturing. To leverage such benefits, the commonly used thermoplastics (e.g., PLA and ABS) are impregnated with nanoparticles, short or continuous fibers, and other additives. The addition of graphene nanoplatelets to PLA makes for a high-quality filament possessing enhanced mechanical, electrical, and thermal properties. Even with the advancement in materials, the optimisation of the process parameter remains the most complex aspect for FFF. Therefore, this study investigates the influence of two under-researched and overlooked processing parameters (material extrusion rates and line widths) on commercially available graphene-enhanced PLA (GPLA). Nine different material extrusion rates (70% to 150%) and five different line widths (0.2 mm to 1 mm) were used to manufacture GPLA specimens using a low-cost, desktop-based 3D printer, as per British and international standards. The study analyses the influence of these two processing parameters on mass, dimensional accuracy, surface texture, and mechanical properties of GPLA specimens. A non-destructive test has also been conducted and correlated with three-point flexural test to establish its applicability in evaluating flexural properties of GPLA. The results how that small line widths provide more accuracy with longer print times whereas large line widths offer more strength with shorter printing times. Increase in material extrusion rates adversely affect the surface finish and hardness but positively influence the flexural strength of GPLA specimens. The study shows that the manipulation of material extrusion rates and line widths can help designers in understanding the limitations of the default printing settings (100% material extrusion rate and 0.4 mm line width) on most desktop 3D printers and identifying the optimal combination to achieve desired properties using the FFF process.

Linewidths and energy shifts of electron-impurity resonant states in quantum wells with infinite barriers

The linewidths and the energy shifts of the resonant states of the impurity electron in GaAs-based quantum wells (QWs) with infinite barriers are calculated. The two-dimensional Schr\"{o}dinger equation for the charge impurity in the QW is solved by the developed finite-difference method combined with the complex-scaling technique. A dependence of the linewidths and energy shifts on the QW width for the impurity localized in the center of QW is studied. The calculated results extend and improve theoretical estimations of these quantities in the GaAs-based QW by Monozon and Schmelcher [Phys. Rev. B 71, 085302 (2005)]. In particular, in agreement with their studies we obtain that resonant states originating from the second quantum-confinement subband have negligibly small linewidths. In contrast to previous estimations, we show that for the QW widths of the order of the impurity's Bohr radius the linewidths of resonant states associated to the third quantum-confinement subband linearly depend on the thickness of the QW. We show how the previous theoretical predictions can be improved for such QW widths. We also calculate linewidths for the case when the electron impurity is localized away from the center of the QW.

A pulsed lock-in method for DC ensemble nitrogen-vacancy center magnetometry

This article proposes a scheme for DC nitrogen-vacancy (NV) center magnetometry that combines the advantages of the lock-in detection and pulse-type schemes. The proposed pulsed lock-in scheme does not lead to optical power broadening of optically-detected magnetic resonance, resulting in smaller linewidth, higher contrast, and better photon shot-noise-limited sensitivity compared with the conventional continuous-wave lock-in scheme. The optimal conditions, optimal sensitivity, and noise-suppression capability of the proposed method are compared with those of the conventional methods from both theoretical and experimental points of view. Through experimental measurements, an improvement by a factor of four in the sensitivity and an improvement by a factor of 60 in the minimum resolvable magnetic field (MRMF) compared with conventional method were obtained. Using a confocal experiment setup, the proposed scheme achieves an MRMF of 100 pT and a sensitivity of 3 nT·Hz-1/2 corresponding to a volume-normalized sensitivity of 2.5 pT/(Hz·mm−3)1/2, demonstrating the potential of this method in magnetic measurement applications with high sensitivity and high spatial resolution.

Effects of the linewidth enhancement factor on the microwave linewidth of the period-one oscillations of optically injected semiconductor lasers.

We investigate the microwave linewidth of the period-one (P1) nonlinear dynamics of an optically injected semiconductor laser as a function of the linewidth enhancement factor. Under proper conditions of optical injection, the destabilizing effect of a large value of the linewidth enhancement factor of the injected semiconductor laser is found to be unexpectedly suppressed in the P1 oscillation of the laser, thus resulting in a narrow P1 linewidth equal to that generated by a laser of a small linewidth enhancement factor. By properly choosing the operating point, P1 oscillations with microwave linewidths that are an order of magnitude narrower than the free-running optical linewidths of the master and slave lasers are observed.

Influence of process parameters on the characteristics of electrohydrodynamic-printed UV-curing conductive lines on the fabric

As a similar technology to the near-field static electrospinning, the emerging electrohydrodynamic (EHD) printing technology with digital printing process and compatibility of viscous particle-blended inks is one of the simplest methods of fabricating multifunctional electronic textiles. With increasing demands for textile-based conductive lines with controllable width and excellent electrical performance, it is of great importance to know the influence of key process parameters on the morphology and electrical properties of EHD-printed UV-curing conductive lines on the fabric. This work will systematically explore the effect of the EHD printing process parameters (i.e. applied voltage, direct-writing height, flow rate and moving velocity of the substrate) on the morphology and electrical performance of the EHD-printed textile-based conductive lines, especially focus on the diffusion and penetration of inks on the rough and porous fabric. The UV-curing nano-silver ink with low temperature and fast curing features was selected, and the line width and electrical resistance of printed lines under different process parameters were observed and measured. The results showed that, unlike previous results about EHD printing on smooth and impermeable substrates, the ink diffusion related to fabric textures had a greater effect on the fabric-based conductive line width than the applied voltage and direct-writing height in the case of a stable jet. Meanwhile, the relationship between the line width and the flow rate met the equation of , and the minimum volume on fabric per millimeter was 0.67 μl to form continuous line with low electrical resistance. Additionally, the higher substrate moving velocity resulted in a smaller line width, while it deteriorated the thickness uniformity and electrical property of printed lines. Generally, due to the effect of surface structure of the fabric on the spreading and penetrating behavior of inks, the flow rate and the substrate moving velocity are two significant parameters ensuring the electrical property of printed lines. It is believed that these findings will provide some guides for applying EHD printing technology into flexible electronics on the woven fabric.

Generation of a multi-wavelength Brillouin erbium fiber laser with low threshold in multiple frequency spacing configurations

Brillouin fiber lasers are a promising optical source technology for industrial and academic purposes that have experienced continuous progress in recent years. Currently, part of this development is focused on proposing laser solutions with low resonator loss, relatively low pump threshold, high efficiency, small linewidth, and multi-wavelength architecture. In this context, we experimentally demonstrate the generation of a cascaded multi-wavelength Brillouin erbium fiber laser source with low threshold at multiple configurations. By employing a laser source of narrow linewidth and a fiber with high nonlinearity, compact all-fiber setups with low loss, single, double, and triple frequency spacing configurations are obtained using a Brillouin pump (BP) power as low as 1 mW. In these proposed configurations, we obtain up to 15, 15, and 11 Stokes channels within a 30 dB range and with frequency spacing between two adjacent lasing lines of 0.08 nm, 0.16 nm, and 0.32 nm, respectively. All laser source configurations investigated exhibit stable temporal behavior without prominent signs of wavelength shifts or power fluctuations. In this way, these multi-wavelength fiber laser sources demonstrated in this paper could be considered experimental and potential source to support dense wavelength division multiplexing systems.

Radiation source based on an optical parametric oscillator with MgO : PPLN crystal and volume Bragg grating, tunable in ranges of 2050 – 2117 and 2140 – 2208 nm

A tunable radiation source based on an optical parametric oscillator (OPO) with a periodically poled lithium niobate (PPLN), generating light with a wavelength of ∼2.1 μm in a near-degenerate regime, is developed. The use of a volume Bragg grating (VBG) as a selective element makes it possible to reduce significantly (by a factor of more than 180) the radiation linewidth. Wavelength tuning from 2050 to 2117 nm for the signal wave and from 2140 to 2208 nm for the idler wave is demonstrated. The maximum average OPO output power is found to be 617 mW (123 μJ), which corresponds to a conversion efficiency of 10 %. The use of a VBG provides a fundamental possibility of designing a pump source with a small linewidth (of no more than 1 nm) and smooth wavelength tuning in a desired range for frequency converters based on non-oxide nonlinear crystals generating mid-IR light (5 – 15 μm).

A low-g omnidirectional MEMS inertial switch with load direction identification

In order to solve the problem that the low-g omnidirectional MEMS inertial switches cannot recognize the load direction, a nickel omnidirectional MEMS inertial switch with identifiable load direction and uniform in-plane threshold distribution was designed. The switch is mainly composed of a spring–mass system, four independent flexible radial electrodes and an axial electrode. The function of identifying the direction of acceleration is realized by detecting the closed states of different electrodes. According to the design results of MEMS inertial switch, the dynamic performance of the switch was analyzed by ANSYS software. The results show that the designed switch meets the functional requirements of load direction identification. Based on UV-LIGA multilayer photolithography, the nickel MEMS inertial switch was fabricated on a steel substrate. In the process of fabrication, the line width compensation method was used to reduce the dimensional error caused by swelling phenomenon and removal process of the photoresist. What's more, the thickness compensation method was used to reduce the process error caused by flattening treatment. The overall dimension of the switch is 5300 × 5300 × 270 μm and the smallest line-width is 20 μm. Ultimately, the fabricated switch was tested by the centrifuge device and dropping hammer in XOY plane and Z-axis direction. The dynamic threshold of the switch is between 7.9 and 11.3 g, and the response time in each direction is less than 2 ms. The switch can recognize the direction of acceleration in XOY plane and Z-axis. The switch has an excellent anti-overload performance.

Enhancement of magnetic and dielectric properties of low temperature sintered NiCuZn ferrite by Bi2O3–CuO additives

With a series of 1.0 wt%Bi2O3–x wt% CuO (x = 0.0, 0.2, 0.4, 0.6, and 0.8) serving as sintering additives, Ni0.23Cu0.32Zn0.45Fe2O4 ferrites are successfully synthesized at a low temperature (900 °C) by using the solid state reaction method. The effects of the additives on the phase formation, magnetic and dielectric properties as well as the structural and gyromagnetic properties are investigated. The x-ray diffraction (XRD) results indicate that the added Bi2O3–CuO can lower the synthesis temperature significantly without the appearing of the second phase. The scanning electron microscope (SEM) images confirm that Bi2O3 is an important factor that determines the sintering behaviors, while CuO affects the grain size and densification. With CuO content x = 0.4 or 0.6, the sample shows high saturation magnetization, low coercivity, high real part of magnetic permeability, dielectric permittivity, and small ferromagnetic resonance linewidth (ΔH). The NiCuZn ferrites are a promising new generation of high-performance microwave devices, such as phase shifters and isolators.

Non-Abelian statistics in light-scattering processes across interacting Haldane chains

The $S=1$ Haldane state is constructed from a product of local singlet dimers in the bulk and topological states at the edges of a chain. It is a fundamental representative of topological quantum matter. Its well-known representative, the quasi-one-dimensional SrNi$_2$V$_2$O$_8$ shows both conventional as well as unconventional magnetic Raman scattering. The former is observed as one- and two-triplet excitations with small linewidths and energies corresponding to the Haldane gap $\Delta_H$ and the exchange coupling $J_c$ along the chain, respectively. Well-defined magnetic quasiparticles are assumed to be stabilized by interchain interactions and uniaxial single-ion anisotropy. Unconventional scattering exists as broad continua of scattering with an intensity $I(T)$ that shows a mixed bosonic / fermionic statistic. Such a mixed statistic has also been observed in Kitaev spin liquids and could point to a non-Abelian symmetry. As the ground state in the bulk of SrNi$_2$V$_2$O$_8$ is topologically trivial, we suggest its fractionalization to be due to light-induced interchain exchange processes. These processes are supposed to be enhanced due to a proximity to an Ising ordered state with a quantum critical point. A comparison with SrCo$_2$V$_2$O$_8$, the $S=1/2$ analogue to our title compound, supports these statements.

One-Pot Access to Ethylene-Bridged BODIPY Dimers and Trimers through Single-Electron Transfer Chemistry.

A Cu(I)-promoted oxidative dimerization of BODIPY dyes was developed to give a series of α,α- ethylene-bridged BODIPY dimers and trimers for the first time. This methodology does not need harsh conditions but relies on the singlet-electron-transfer process between alkylated BODIPYs and Cu(I) salt to generate BODIPY-based radical species, which undergo a selective radical homocoupling reaction. Moreover, these resultant dimers and trimers showed high attenuation coefficients, small line widths of the absorption and emission, and intense fluorescence.

The MACH Hi Absorption Survey. I. Physical Conditions of Cold Atomic Gas outside of the Galactic Plane

Abstract Tracing the transition between the diffuse atomic interstellar medium and cold, dense gas is crucial for deciphering the star formation cycle in galaxies. Here we present Measuring Absorption by Cold Hydrogen (MACH), a new survey of cold neutral hydrogen (HI) absorption at 21 cm by the Karl G. Jansky Very Large Array. We target 42 bright background sources with 60 < l < 110°, 30 < b < 62°, significantly expanding the sample of publicly available, sensitive 21 cm absorption outside the Galactic plane. With matching 21 cm emission data from the Effelsberg-Bonn Hi (EBHiS) Survey, we measure the total column density and cold Hi fraction, and quantify the properties of individual Hi structures along each sightline via autonomous Gaussian decomposition. Combining the MACH sample with results from recent Hi absorption surveys, we produce a robust characterization of the cool atomic medium at high and intermediate Galactic latitudes. We find that MACH Hi has significantly smaller column density relative to samples at similar latitudes, and the detected cold Hi structures have smaller line widths, temperatures, and turbulent Mach numbers, suggesting that MACH probes a particularly quiescent region. Using all available observations, we compute the cumulative covering fraction (c) of cold Hi at local velocities outside the disk: structures with τ > 0.001 are ubiquitous (c ∼ 100%), whereas high optical depths (τ > 1) are extremely rare (c ∼ 0%).

16.3 W Peak-Power Pulsed All-Diode Laser Based Multi-Wavelength Master-Oscillator Power-Amplifier System at 964 nm

An all-diode laser-based master oscillator power amplifier (MOPA) configuration for the generation of ns-pulses with high peak power, stable wavelength and small spectral line width is presented. The MOPA emits alternating at two wavelengths in the spectral range between 964 nm and 968 nm, suitable for the detection of water vapor by absorption spectroscopy. The monolithic master oscillator (MO) consists of two slightly detuned distributed feedback laser branches, whose emission is combined in a Y-coupler. The two emission wavelengths can be adjusted by varying the current or temperature to an absorption line and to a non-absorbing region. The power amplifier (PA) consists of a ridge-waveguide (RW) section and a tapered section, monolithically integrated within one chip. The RW section of the PA acts as an optical gate and converts the continuous wave input beam emitted by the MO into a sequence of short optical pulses, which are subsequently amplified by the tapered section to boost the output power. For a pulse width of 8 ns, a peak power of 16.3 W and a side mode suppression ratio of more than 37 dB are achieved at a repetition rate of 25 kHz. The measured spectral width of 10 pm, i.e., 0.1 cm−1, is limited by the resolution of the optical spectrum analyzer. The generated pulses emitting alternating at two wavelengths can be utilized in a differential absorption light detection and ranging system.

Hybrid external-cavity lasers (ECL) using photonic wire bonds as coupling elements

Combining semiconductor optical amplifiers (SOA) on direct-bandgap III–V substrates with low-loss silicon or silicon-nitride photonic integrated circuits (PIC) has been key to chip-scale external-cavity lasers (ECL) that offer wideband tunability along with small optical linewidths. However, fabrication of such devices still relies on technologically demanding monolithic integration of heterogeneous material systems or requires costly high-precision package-level assembly, often based on active alignment, to achieve low-loss coupling between the SOA and the external feedback circuits. In this paper, we demonstrate a novel class of hybrid ECL that overcome these limitations by exploiting 3D-printed photonic wire bonds as intra-cavity coupling elements. Photonic wire bonds can be written in-situ in a fully automated process with shapes adapted to the mode-field sizes and the positions of the chips at both ends, thereby providing low-loss coupling even in presence of limited placement accuracy. In a proof-of-concept experiment, we use an InP-based reflective SOA (RSOA) along with a silicon photonic external feedback circuit and demonstrate a single-mode tuning range from 1515 to 1565 nm along with side mode suppression ratios above 40 dB and intrinsic linewidths down to 105 kHz. Our approach combines the scalability advantages of monolithic integration with the performance and flexibility of hybrid multi-chip assemblies and may thus open a path towards integrated ECL on a wide variety of integration platforms.

Importance of Long‐Range Channel Sr Displacements for the Narrow Emission in Sr[Li2Al2O2N2]:Eu2+Phosphor

AbstractThe recently discovered Sr[Li2Al2O2N2]:Eu2+red phosphor, candidate for the next generation of eco‐efficient white light‐emitting diodes, exhibits excellent emission spectral position and exceptionally small linewidth. It belongs to the UCr4C4‐structure family of phosphors containing many potential candidates for commercial phosphors, whose small linewidth, tentatively ascribed to the high‐symmetry cuboid environment of the doping site, has drawn the attention of researchers in the last five years. Density functional theory, ΔSCF method, and configuration coordinate models (CCM) are used to provide a complete characterization of this material. Using a multi‐dimensional CCM, an accurate description of the coupling of the vibronic structure with the electronic 5d→4f transition is obtained, including the partial Huang–Rhys factors and frequency of the dominant modes. It is shown that, in addition to the first‐coordination shell cuboid deformation mode, low‐frequency phonon modes involving chains of strontium atoms along the tetragonal axis shape the emission linewidth in Sr[Li2Al2O2N2]:Eu2+. This finding sheds new light on the emission properties of UCr4C4‐structure phosphors, possessing similar Ca/Sr/Ba channel. The approach provides a robust theoretical framework to systematically study the emission spectra of such Eu‐doped phosphors, and predict candidates with expected similar or even sharper linewidth.

Semiconductor Laser Linewidth Theory Revisited

More and more applications require semiconductor lasers distinguished not only by large modulation bandwidths or high output powers, but also by small spectral linewidths. The theoretical understanding of the root causes limiting the linewidth is therefore of great practical relevance. In this paper, we derive a general expression for the calculation of the spectral linewidth step by step in a self-contained manner. We build on the linewidth theory developed in the 1980s and 1990s but look from a modern perspective, in the sense that we choose as our starting points the time-dependent coupled-wave equations for the forward and backward propagating fields and an expansion of the fields in terms of the stationary longitudinal modes of the open cavity. As a result, we obtain rather general expressions for the longitudinal excess factor of spontaneous emission (K-factor) and the effective α-factor including the effects of nonlinear gain (gain compression) and refractive index (Kerr effect), gain dispersion, and longitudinal spatial hole burning in multi-section cavity structures. The effect of linewidth narrowing due to feedback from an external cavity often described by the so-called chirp reduction factor is also automatically included. We propose a new analytical formula for the dependence of the spontaneous emission on the carrier density avoiding the use of the population inversion factor. The presented theoretical framework is applied to a numerical study of a two-section distributed Bragg reflector laser.

EPR study of paramagnetic centers in SiO2:C: Zn nanocomposites obtained by infiltration of fumed silica with luminescent Zn(acac)2 solution

Silica-carbon with zinc (SiO2:C:Zn) nanocomposites obtained via infiltration with aged luminescent zinc acetylacetonate (Zn(acac)2) ethanol solution of two concentrations (1 or 4%) into the fumed silica (SiO2) matrix have been studied using EPR within the temperature range 6…296 K before and after thermal annealing. The EPR spectrum of SiO2:C:Zn nanocomposites consists of three signals with the Lorentzian lineshape corresponding to paramagnetic centers with S = 1/2, which are related to carbon dangling bonds (CDB) (g = 2.0029(3)), silicon dangling bonds (g = 2.0062(3)) and oxygen-centered carbon-related radicals (CRR) (g = 2.0042(3)). A small EPR linewidth (<1 mT) observed for CDB and oxygen-centered CRR allows us to conclude that they are in the sp3-hybridized state. It was found that the temperature dependence of the EPR signal integrated intensity of the CDB and oxygen-centered CRR follows the Curie–Weiss law with a small positive value of the Curie–Weiss constant, which indicates that the weak ferromagnetic exchange interaction takes place in the spin system of CDB and oxygen-centered CRR. It was supposed that the carbon-related centers are clustered in SiO2:C:Zn nanocomposites. We assume that the oxygen-centered CRR in the sp3-hybridized state are associated with luminescent centers in previously reported aged Zn(acac)2/C2H5OH solution.

Numerical analysis of the pump’s spectral linewidth impact on single and multi-wavelength Brillouin fiber lasers

Brillouin fiber lasers (BFLs) are a promising optical source technology for industrial and academic purposes that have experienced continuous progress in recent years. Currently, part of this development is focused on delivering laser solutions with low resonator loss, relatively low pump threshold, high efficiency, very small linewidth, and multi-wavelength architecture. In this perspective, this study employs an accurate numerical model based on an analytical solution to investigate the effect of the pump’s spectral linewidth on BFLs working both in single and cascaded multi-wavelength configurations. Output powers, thresholds, and the Stokes lines number simulated are analyzed here using a standard single-mode optical fiber as the Brillouin gain medium. The numerical results obtained not only allow identifying the regions of the pump’s spectral linewidth in which there is greater, moderate, or even no suppression of the stimulated Brillouin scattering inside the laser cavity but also provides a deep understanding of the internal dynamics of the BFLs cavities.