Spectral Beam Research Articles

Kilowatt-level fiber amplifier with spectral-broadening-free property, seeded by a random fiber laser.

In this Letter, we demonstrate a kilowatt (kW) level high-power fiber laser amplifier with a clear sign of spectral-broadening-free property. The high-power fiber lasing is realized by employing a master oscillator power-amplifier (MOPA) configuration, seeded by a temporally stable random fiber laser (RFL) that utilizes Raman amplification and random distributed feedback from a long passive fiber. The output power reaches 1.03 kW with a 1070 nm wavelength and an optical-to-optical efficiency of 74.6%. Despite the typical nonlinear spectral broadening in most traditional MOPA systems, the output spectral linewidth is well maintained during the whole high-power amplification process. The suppressed linewidth broadening in the spectral domain during high-power amplification is significant for further power scaling, spectral beam combination, and other applications that require narrow-linewidth high-power lasing.

Fabrication of polarization independent gratings made on multilayer dielectric thin film substrates

We briefly present design parameters and fabrication procedures of polarization independent gratings made on multilayer dielectric thin film substrates for spectral beam combining applications. In the wavelength range of 1.044 m to 1.084 m the -1st Littrow order average diffraction efficiencies of TE polarization and TM polarization are 89.7% and 93.8% respectively.

Comparison of spectral beam combining based on an external cavity with and without microlens array

An experimental system of spectral beam combining based on external cavity with a microlens array has been designed and built. According to the theoretical models established, the influence of the transverse offsets on coupling efficiency and beam quality with and without a microlens array were compared. The simulation results show that the incorporation of microlens can significantly improve the coupling efficiency and beam quality. In the experiment of individual fiber laser, the fiber laser can be tunable in a range of 40.23nm with a microlens, wider than the one without the microlens (36.25nm). In the spectral beam combining experiment of two fiber lasers with a microlens array, the combined efficiency is 77% with the output power of 695mW and the beam quality factor (Mx2 ) of 1.27, and for the system without the microlens array, the combining efficiency is 75% with the output power of 660mW and the beam quality factor (Mx2) of 1.31. The experiment results show that the SBC system with a microlens array has slightly better performance than that without the microlens array, which is almost consistent with the theoretical analysis results for the fiber array width of 0–2.5mm.

Hybrid Beam Combination by Active Phasing and Bandwidth-Controlled Dichromatic Mirror

In this letter, we present what we believe to be a new design for simultaneously coherent and spectral beam combinations by active phasing and bandwidth-controlled dichromatic mirror. A principle of concept experiment is demonstrated by employing eight 20-W power-level polarization-maintained all fiber amplifiers, which generate a 142.1-W near diffraction limited ( $M^{2}_{x}\sim 1.1$ and $M^{2}_{y}\sim 1.1$ ) output power with the whole combining efficiency of $>90$ %. The new designed beam combining system is competent for both narrow-band and broadband gain elements with full linewidth of $ nm. Along with the near-diffraction-limited power scaling progress on monolithic polarization-maintained amplifier, we believe that the approach demonstrated has great potential for remarkable power boosting in both continuous- and pulsed-wave operations.

Analysis and simulation of silicon grating structure for spectral beam combining application

In this paper, the spectral beam combination using silicon grating structure is presented. Spectral beam combination is realised by the amalgamation of both reflected and transmitted beams. Here the beam of light having wavelength range, 1310–1550nm is taken owing to absorption loss is zero for silicon grating structure. This beams combination is analysed by simulation result, which is made by plane wave expansion method. Simulation result reveals that grating material, grating period, length of grating and thickness of odd and even layer play an important role to understand the beam combing application.

High-brightness spectral beam combining of diode laser array stack in an external cavity.

We demonstrate the spectral beam combining of a diode laser stack, which contains three 970nm Mini-Bars along the fast-axis direction, in an external cavity. At the pump current of 60 A, the output power of 127 W, the spectral bandwidth of 12 nm and the Electro-optical conversion efficiency of 48.35% are achieved. The measured beam qualities after the spectral beam combining are M(2) ≈10.2 along the slow axis and M(2) ≈11.5 along the fast axis. Under a maximum injection current of 75A, the laser output power of more than 159W is achieved. The beam quality deteriorated slightly with the rising of the current from 60A to 75A, but it is enough to be coupled into a 50µm core / 0.22NA fiber.

Particle dynamics in the rising plume at Piccard Hydrothermal Field, Mid‐Cayman Rise

AbstractProcesses active in rising hydrothermal plumes, such as precipitation, particle aggregation, and biological growth, affect particle size distributions and can exert important influences on the biogeochemical impact of submarine venting of iron to the oceans and their sediments. However, observations to date of particle size distribution within these systems are both limited and conflicting. In a novel buoyant hydrothermal plume study at the recently discovered high‐temperature (398°C) Piccard Hydrothermal Field, Mid‐Cayman Rise, we report optical measurements of particle size distributions (PSDs). We describe the plume PSD in terms of a simple, power‐law model commonly used in studies of upper and coastal ocean particle dynamics. Observed PSD slopes, derived from spectral beam attenuation and laser diffraction measurements, are among the highest found to date anywhere in the ocean and ranged from 2.9 to 8.5. Beam attenuation at 650 nm ranged from near zero to a rarely observed maximum of 192 m−1 at 3.5 m above the vent. We did not find large (>100 μm) particles that would settle rapidly to the sediments. Instead, beam attenuation was well‐correlated to total iron, suggesting the first‐order importance of particle dilution, rather than precipitation or dissolution, in the rising plume at Piccard. Our observations at Piccard caution against the assumption of rapid deposition of hydrothermal, particulate metal fluxes, and illustrate the need for more particle size and composition measurements across a broader range of sites, globally.

25-W Monolithic Spectrally Stabilized 975-nm Minibars for Dense Spectral Beam Combining

Low-fill-factor laser bars composed of five narrow-stripe broad-area lasers (30 $\mu \text{m} \,\, \times \,\, 6000~\mu \text{m}$ ) with monolithic spectral stabilization are presented. Each laser on the bar emits at a unique wavelength from 970 to 980 nm with a channel spacing of 2.5 nm. Narrow spectral line width $\kappa \text{L}~\sim $ 0.2, which is shown to be sufficient to suppress lasing in Fabry-Perot modes for narrow line operation across all wavelength channels. However, additional optical scattering losses of the gratings are found to limit output power and efficiency. The final realized minibars deliver 25-W continuous wave output power with a conversion efficiency >40% and operate with a beam parameter product of $\sim 1.6$ mm mrad per single emitter, twofold improved in comparison with typical 90- $\mu \text{m}$ wide broad area laser. Such laser minibars are an attractive source for brilliant dense spectral beam combining.

Channel density and efficiency optimization of spectral beam combining systems based on volume Bragg gratings in sequential and multiplexed arrangements.

Spectral-beam-combining (SBC) systems utilizing multiple volume Bragg gratings must be carefully analyzed to maximize channel density and efficiency, and thus output radiance. This analysis grows increasingly difficult as the number of channels in the system increases, and heuristic optimization techniques are useful tools for exploring the limits of these systems. We explore three classes of multigrating SBC systems: cascaded, where each grating adds a new channel to the system in sequence; sandwiched, where several individual gratings are placed together and all channels enter the system at the same facet; and multiplexed, where all of the gratings occupy the same holographic optical element (HOE). Loss mechanisms differ among these three basic classes, and our optimization algorithm shows that the highest channel density for a given minimum efficiency and fixed operating bandwidth is achieved for a cascaded grating system. The multiplexed grating system exhibits the lowest channel density under the same constraints but has the distinct advantage of being realized by a single HOE. For a particular application, one must weigh channel density and efficiency versus system complexity when choosing among these basic classes of SBC systems. Additionally, one may need to consider the effects of finite-width input beams. As input beam radius is reduced, angular clipping effects begin to dominate over spectral interference and crosstalk effects, limiting all three classes of SBC systems in a similar manner.

Power scaling of narrowband high-power all-fiber superfluorescent fiber source to 1.87 kW.

We demonstrate a high-power narrowband all-fiber superfluorescent fiber source employing three-stages master oscillator power-amplifier chain. Narrowband seed light is selected from a broadband-amplified spontaneous-emission source by a spectrum filter combing with fiber circulator and fiber Bragg grating. In the main amplifier, the maximal output power is 1.87 kW with an optical-to-optical conversion efficiency of 77.4% and a full width at half-maximum (FWHM) linewidth of 1.7 nm. The corresponding power fluctuation is ±1.4% in 100 s operation, and no parasitic oscillation or self pulsation is observed. A beam quality of M(2)=1.71 is measured at 1.4 kW output power. This manuscript investigates the spectral evolution in high-power amplification. The central wavelength shifted from 1079.5 nm to 1080.7 nm, and the FWHM linewidth narrowed from 2 nm to 1.7 nm at full power, respectively. These effects could be inferred as hybrid effects of many factors, such as wavelength shifting of pump LD, nonoptimized length of gain fiber, and enhancement of temperature level of gain fiber. The narrowband-maintained characteristic in spectral domain under high-power amplification is significant for applications such as spectral beam combination, and further power scaling is available as the output power is only limited by the pump power.

Dynamic properties of three-dimensional piezoelectric Kagome grids

Piezoelectric Kagome grids can be considered as a kind of functional material because they have vibration isolation performance and can transform mechanical energy to electric energy. In this study, the dynamic properties of three-dimensional (3D) piezoelectric Kagome grids without and with material defects are studied based on the frequency-domain responses. The spectral element method (SEM) is adopted to solve a 3D piezoelectric beam which contains bending components in two planes, tensional components, and torsional components. The dynamic stiffness matrix of a spectral piezoelectric beam is derived. Highly accurate solutions in the frequency-domain are obtained by solving the equation of motion of the whole structure. Compared with the results from the FEM and those in the existing literature, it can be seen that the SEM can be effectively used to study the 3D piezoelectric Kagome grids. The band-gap properties of Kagome grid and defect state properties of Kagome grid with material defects are analyzed. The effect of the piezoelectric parameter on the band-gap property is investigated further.

Design of broadband multilayer dichroic coating for a high-efficiency solar energy harvesting system.

We report on the design and performance of a broadband dichroic coating for a solar energy conversion system. As a spectral beam splitter, the coating facilitates a hybrid system that combines a photovoltaic cell with a thermal collector. When positioned at a 45° angle with respect to incident light, the coating provides high reflectance in the 40-1100nm and high transmission in the 1200-2000nm ranges for a photovoltaic cell and a thermal collector, respectively. Numerical simulations show that our design leads to a sharp transition between the reflection and transmission bands, low ripples in both bands, and slight polarization dependence.

Multi-octave spectral beam combiner on ultra-broadband photonic integrated circuit platform.

We present the design of a novel platform that is able to combine optical frequency bands spanning 4.2 octaves from ultraviolet to mid-wave infrared into a single, low M2 output waveguide. We present the design and realization of a key component in this platform that combines the wavelength bands of 350 nm - 1500 nm and 1500 nm - 6500 nm with demonstrated efficiency greater than 90% in near-infrared and mid-wave infrared. The multi-octave spectral beam combiner concept is realized using an integrated platform with silicon nitride waveguides and silicon waveguides. Simulated bandwidth is shown to be over four octaves, and measured bandwidth is shown over two octaves, limited by the availability of sources.

Common aperture spectral beam combination of fiber lasers with 5 kW power high-efficiency and high-quality output

Common Aperture Spectral Beam Combination (SBC) of multiple high power fiber lasers output into a composite beam via a multi-layer dielectric (MLD) gratingis a promising approach for the fiber laser system to scale the output power further while maintaining high efficiency and excellent beam quality simultaneously. A common aperture SBC system with dual-grating dispersion compensation configuration based on five kW-level narrow-linewidth fiber amplifier chains in all-fiber format is set up. A common aperture combing beam is achieved with high efficiency and excellent beam quality in the system by using China-made MLD gratings. The maximum average output power of 5.07 kW of the combined beam and the combination efficiency of 91.2% are obtained. The beam quality factor M2 at a power level of 5 kW is less than 3. The preliminary experimental results have shown that MLD grating, which can maintain high diffraction efficiency in a wide spectrum range under high power level operation, is a perfect element for SBC of high power fiber laser. The results also demonstrate that the beam quality and the linewidth of the individual fiber laser are the key factors affecting the beam quality of the spectral combining beam. Presently, the output power is limited by the amount of the narrow-linewidth fiber amplifier chains and the output power of the individual narrow-linewidth fiber amplifier. So it can be expected that the common aperture SBC system output power can be further scaled.

Spectral beam combining for a diode laser array by using a grating pair

A grating and a collimating lens are used to replace the transformer lens in the traditional spectral beam combining structure. As a result, the degradation of the beam quality and the cross-talk are prevented. A spectral beam combining with narrow line-width is achieved. A standard semiconductor laser array is employed in this experiment. An output power of 44.8 W and an electro-optic conversion efficiency of 38.9% are achieved. The spectral line-width is 4.1 nm. A beam quality factor of 1.3711.7 in fast and slow axis directions are obtained. The beam quality of the output is close to that of a single emitter of the array in both directions.

Designing and optimizing highly efficient grating for high-brightness laser based on spectral beam combining

A highly efficient nano-periodical grating is theoretically investigated for spectral beam combining (SBC) and is experimentally implemented for attaining high-brightness laser from a diode laser array. The rigorous coupled-wave analysis with the S matrix method is employed to optimize the parameters of the grating. According the optimized parameters, the grating is fabricated and plays a key role in SBC cavity. The diffraction efficiency of this grating is optimized to 95% for the output laser which is emitted from the diode laser array. The beam parameter product of 3.8 mm mrad of the diode laser array after SBC is achieved at the output power of 46.3 W. The optical-to-optical efficiency of SBC cavity is measured to be 93.5% at the maximum operating current in the experiment.

Robust spectrometer-based methods for characterizing radiant exitance of dental LED light curing units

ObjectivesFirstly, to assess light output, from a representative range of dental light curing units (LCUs), using a new portable spectrometer based instrument (checkMARC™) compared with a “gold standard” method. Secondly, to assess possible inconsistency between light output measurements using three different laboratory-grade thermopile instruments. MethodsThe output of four blue-dental LCUs and four polywave blue-and-violet-LCUs was measured with two spectrometer-based systems: a portable spectrometer instrument and a benchtop Integrating Sphere fiber-coupled spectrometer system. Power output was also recorded with three thermopiles according to ISO 10650-2. Beam profile images were recorded of LCU output to assess for spatial and spectral beam uniformity. ResultsPower recorded with the portable spectrometer instrument closely matched the ‘gold standard’ Integrating Sphere apparatus calibrated according to International Standards. Radiant exitance for the eight LCUs differed significantly between the three thermopiles. Light source to thermopile sensor distance influenced recorded power significantly (p<0.05), indicating the severe limitations of thermopiles for absolute measurements. Polywave LCU beam profiles demonstrated output spectral heterogeneity. SignificanceSpectrometer-based methods are capable of overcoming the limitations inherent with thermopile-based measurement techniques. Spectrometer based measurements can fulfill the intention of ISO 10650.

Vacuum ultraviolet spectropolarimeter design for precise polarization measurements.

Precise polarization measurements in the vacuum ultraviolet (VUV) region provide a new means for inferring weak magnetic fields in the upper atmosphere of the Sun and stars. We propose a VUV spectropolarimeter design ideally suited for this purpose. This design is proposed and adopted for the NASA-JAXA chromospheric lyman-alpha spectropolarimeter (CLASP), which will record the linear polarization (Stokes Q and U) of the hydrogen Lyman-α line (121.567nm) profile. The expected degree of polarization is on the order of 0.1%. Our spectropolarimeter has two optically symmetric channels to simultaneously measure orthogonal linear polarization states with a single concave diffraction grating that serves both as the spectral dispersion element and beam splitter. This design has a minimal number of reflective components with a high VUV throughput. Consequently, these design features allow us to minimize the polarization errors caused by possible time variation of the VUV flux during the polarization modulation and by statistical photon noise.

Wavelength beam combining of VECSELs using multiplexed volume Bragg gratings in a compound cavity

A method of wavelength beam combining to enhance the total output brightness of vertical external cavity surface emitting lasers (VECSELs) using multiplexed volume Bragg gratings (MVBGs) is presented. The MVBGs are designed to introduce wavelength-specific feedback for the 1020 nm near-infrared wavelength VECSELs to achieve both spectral stabilisation and beam combining effects. In conjunction with the compound cavity arrangement, a combined output of >6 W is achieved with >98% combining efficiency using two VECSELs. The beam combining scheme produces near-diffraction limited output with enhanced spectral density and wavelength-stabilised characteristics.

High-power spectral beam combining of linearly polarized Tm:fiber lasers.

To date, high-power scaling of Tm:fiber lasers has been accomplished by maximizing the power from a single fiber aperture. In this work, we investigate power scaling by spectral beam combination of three linearly polarized Tm:fiber MOPA lasers using dielectric mirrors with a steep transition from highly reflective to highly transmissive that enable a minimum wavelength separation of 6 nm between individual laser channels within the wavelength range from 2030 to 2050 nm. Maximum output power is 253 W with M(2)<2, ultimately limited by thermal lensing in the beam combining elements.