Laser Bandwidth Research Articles

Theoretical investigations of population trapping phenomena in atomic four-color, three-step photoionization scheme

The four-color three-step selective photoionization process of atom is very important in laser isotope separation technology. The population trapping phenomena and their influences are studied theoretically in monochromatic and non-monochromatic laser fields based on the density matrix theory in this work. Time evolutions of the photoionization properties of the four-color, three-step process are given. The population trapping effects occur intensely in monochromatic excitation, while it gradually turns weak as the laser bandwidth increases. The effects of bandwidth, Rabi frequency, time delay, and frequency detuning on the population trapping effect are investigated in monochromatic and non-monochromatic laser fields. The effects of laser process parameters and atomic parameters on the effective selective photoionization are also discussed. The ionization probability and selectivity factors, as evaluation indexes, are difficult to improve synchronously by adjusting systematic parameters. Besides, the existence of metastable state may play a negative role when its population is low enough.

Modeling of melt pool and thermal field simulation for wide-band laser cladding

Modeling of melt pool and thermal field simulation for wide-band laser cladding

Phase-shifted fiber Bragg grating filters based on counter-propagating cladding modes coupling.

In this paper, we comprehensively analyze counter-propagating cladding mode assisted phase-shifted fiber Bragg gratings (FBGs) and propose an ultra-narrow bandwidth laser line filter based on such gratings. Full vector modal analysis has been used to obtain the mode guiding and its coupling characteristics. We show that the transmission spectrum of the counter-propagating cladding mode assisted FBGs can be tailored by incorporating single or multiple phase shifts along the grating length. Phase shifts open up narrowband transmission windows inside the stop band of the Bragg grating, and the transmitted wavelength can be altered by controlling the amount of phase shift. Unlike conventional FBGs, the grating discussed here has access to the evanescent field of the excited counter-propagating cladding mode, which opens up the possibility of refractive index based tuning of resonance wavelength. Further, the bandwidth of the proposed grating is two orders of magnitude smaller than that of the conventional LPGs. As an alternative application, we also show that with the right length ratio, the multiple phase shifts can also be used to create a very flattened single transmission peak inside the stop band. The effects of grating regulating factors such as the grating length, grating strength, location of the phase shift, and index apodization on the linewidth of the narrow central peak of π PS-FBG are also investigated. We present a clear physical explanation of various factors involved in the counter-propagating cladding mode coupling in such phase-shifted Bragg grating. Due to its extremely narrow transmission window, our study can find application in developing sensors for measuring parameters with greater accuracy. Such phase-shifted Bragg gratings can also be used to create an all-fiber demultiplexer for multichannel systems and fiber optic sensors as a particular application.

Modulation bandwidth enhancement in monolithic integrated two-section DFB lasers based on the detuned loading effect

Modulation bandwidth enhancement in a directly modulated two-section distributed feedback (TS-DFB) laser based on a detuned loading effect is investigated and experimentally demonstrated. The results show that the 3-dB bandwidth of the TS-DFB laser is increased to 17.6 GHz and that chirp parameter can be reduced to 2.24. Compared to the absence of a detuned loading effect, there is a 4.6 GHz increase and a 2.45 reduction, respectively. After transmitting a 10 Gb/s non-return-to-zero (NRZ) signal through a 5-km fiber, the modulation eye diagram still achieves a large opening. Eight-channel laser arrays with precise wavelength spacing are fabricated. Each TS-DFB laser in the array has side mode suppression ratios (SMSR) > 49.093 dB and the maximum wavelength residual < 0.316 nm.

Effects of frequency-modulated pump on stimulated Brillouin scattering in inhomogeneous plasmas

The effects of a frequency-modulated pump on stimulated Brillouin scattering (SBS) in a flowing plasma are investigated by theoretical analysis, three-wave simulations, and kinetic simulations. The resonance point of SBS oscillates in a certain spatial region with time when frequency modulations are applied. There exists a certain frequency modulation that causes the velocity of resonant points to be similar to the group velocity of the seed laser, which increases the SBS reflectivity. The SBS can also be suppressed by frequency modulation with larger bandwidth. In the kinetic simulations, the effects of the frequency-modulated pump on the reflectivity agree with our theoretical predictions. Multi-location autoresonance is also observed in the narrow-bandwidth frequency modulation case, which can also increase the SBS reflectivity. Our work provides a method for selecting the laser bandwidth to inhibit SBS in inhomogeneous plasmas.

Prediction of the bead profile for wide-band laser cladding via hybrid model of mathematics and thermal simulation

Wide-band laser cladding has high deposition rate and low dilution due to the large laser spot and rectangular powder nozzle. A mathematical model was proposed to predict the track cross-section profile for wide-band laser cladding. The melt pool morphology and powder spatial concentration were taken into consideration to improve the accuracy of the model. Thermal field simulation was carried out to study the melt pool evaluation with process parameters, while infrared images were used to verify the simulation fidelity. Powder stream photographs were taken via a CCD (charge coupled device) camera to obtain the powder spot size. Single and multi-track cladding experiments were conducted, and calculated contours of the model coincide well with the experimental results. Simulated and measured melt pool width and length agree with in 2.16% and 2.36% of error, respectively. The error of predicted cross section area is less than 9.35%. The model also has good ability to predict the powder efficiency and morphology of the cladding layer.

Parametric instabilities and hot electron generation in the interactions of broadband lasers with inhomogeneous plasmas

The development of parametric instabilities in the interaction of a large-scale inhomogeneous plasma with either a monochromatic or a broadband laser pulse is investigated theoretically and numerically. For a monochromatic laser at an intensity of W cm−2, the development of Stimulated Brillouin Scattering (SBS) in the relatively low density region will obviously dissipate pump laser and hence inhibit the development of Two-Plasmon Decay (TPD) and absolute Stimulated Raman Scattering (SRS) near the quarter-critical density. By using a laser with a moderate fractional bandwidth (∼1.0%) at the same averged intensity, it is found that the laser reflectivity will be greatly reduced since the SBS can be suppressed effectively due to its low linear growth rate. On the contrary, the TPD and absolute SRS are obviously enhanced since the in situ laser intensity near the quarter-critical density becomes stronger in this case. As a result, the hot electron generation due to the TPD and absolute SRS is dramatically enhanced as well. This indicates that the competition between various parametric instabilities in a large-scale inhomogeneous plasma makes it more challenging to simultaneously suppress all kinds of parametric instabilities by using broadband lasers. Particular attention should be paid to the TPD, which not only has a relatively large linear growth rate but also is efficient in generating harmful hot electrons. Increasing the laser bandwidth further, the hot electron generation will be finally reduced as long as the TPD and SRS are also suppressed with a sufficient bandwidth (3%) at the intensity of W cm−2 that may be encountered in some novel ignition schemes such as shock ignition. However, it is worth noting that the laser bandwidth required to mitigate parametric instabilities and hot electron production strongly depends on laser intensity. A moderate laser bandwidth (∼1%) may be sufficient to mitigate both the laser reflectivity and hot electron production for typical ICF target designs operated with laser intensities lower than 1015 W cm−2.

Transition from backward to sideward stimulated Raman scattering with broadband lasers in plasmas

Broadband lasers have been proposed as future drivers of inertial confined fusion (ICF) to enhance the laser–target coupling efficiency via the mitigation of various parametric instabilities. The physical mechanisms involved have been explored recently, but are not yet fully understood. Here, stimulated Raman scattering (SRS) as one of the key parametric instabilities is investigated theoretically and numerically for a broadband laser propagating in homogeneous plasma in multidimensional geometry. The linear SRS growth rate is derived as a function of scattering angles for two monochromatic laser beams with a fixed frequency difference δω. If δω/ω0 ∼ 1%, with ω0 the laser frequency, these two laser beams may be decoupled in stimulating backward SRS while remaining coupled for sideward SRS at the laser intensities typical for ICF. Consequently, side-scattering may dominate over backward SRS for two-color laser light. This finding of SRS transition from backward to sideward SRS is then generalized for a broadband laser with a few-percent bandwidth. Particle-in-cell simulations demonstrate that with increasing laser bandwidth, the sideward SRS gradually becomes dominant over the backward SRS. Since sideward SRS is very efficient in producing harmful hot electrons, attention needs to be paid on this effect if ultra-broadband lasers are considered as next-generation ICF drivers.

Effects of Laser Bandwidth in Direct-Drive High-Performance DT-Layered Implosions on the OMEGA Laser.

In direct-drive inertial confinement fusion, the laser bandwidth reduces the laser imprinting seed of hydrodynamic instabilities. The impact of varying bandwidth on the performance of direct-drive DT-layered implosions was studied in targets with different hydrodynamic stability properties. The stability was controlled by changing the shell adiabat from (α_{F}≃5) (more stable) to (α_{F}≃3.5) (less stable). These experiments show that the performance of lower adiabat implosions improves considerably as the bandwidth is raised indicating that further bandwidth increases, beyond the current capabilities of OMEGA, would be greatly beneficial. These results suggest that the future generation of ultra-broadband lasers could enable achieving high convergence and possibly high gains in direct drive ICF.

Effect of Rayleigh backscattering on the multiwavelength random fiber laser bandwidth

We investigated the effect of Rayleigh backscattering on the bandwidth of a multiwavelength random fiber laser (MWRFL). This MWRFL is based on an erbium-doped fiber amplifier (EDFA) as the gain medium and a Lyot filter as the wavelength-selective device. A high number and stable lasing line is generated with the assistance of a nonlinear polarization rotation effect induced by highly nonlinear fiber (HNLF) and optimized by a polarization controller. Longer single-mode fiber (SMF) produces wider multiwavelength bandwidth as compared to shorter SMF lengths, as 10 km, 3 km, and 0.1 km obtained 6.24 nm, 5.76 nm, and 2.75 nm wavelength bandwidth, respectively. When the SMF is removed from the setup, no Rayleigh backscattering is provided in the cavity, which led to a narrower multiwavelength bandwidth of 2.69 nm. Subsequently, the alteration of EDFA power, half-wave plate (HWP) angle, and quarter-wave plate (QWP) angles affected the degradation of multiwavelength performance. The best output spectrum is achieved at an EDFA power of 18 dBm, a HWP angle of 60°, and a QWP angle of 105°. The laser stability over a period of 120 min was excellent with a peak power deviation of 1.33 dB.

Exploration of cross-beam energy transfer mitigation constraints for designing an ignition-scale direct-drive inertial confinement fusion driver

The compression of direct-drive inertial confinement fusion (ICF) targets is strongly impacted by cross-beam energy transfer (CBET), a laser-plasma instability that limits ablation pressure by redirecting laser energy outward and that is projected to be mitigated by laser bandwidth. Here, we explore various CBET mitigation constraints to guide the design of future ICF facilities. First, we find that the flat, Gaussian, and Lorentzian spectral shapes have similar CBET mitigation properties, and a flat shape with nine spectral lines is a good surrogate for what can be obtained with other spectral shapes. Then, we conduct a comprehensive study across energy scales and ignition designs. 3D hydrodynamic simulations are used to derive an analytical model for the expected CBET mitigation as a function of laser and plasma parameters. From this model, we study the bandwidth requirements of conventional and shock ignition designs across four different energy scales and find that they require between 0.5 and 3±0.2% relative bandwidth. Best mitigation is achieved when the beam radius over critical radius Rb/Rc is kept low during the drive while the plasma temperature is kept high. In a steady state, we find that the bandwidth required to mitigate 85% of CBET scales as (Rb/Rc)2.15Ln−0.58I0.7, where Ln is the density scale length, and I the laser intensity. Finally, we find that the chamber beam port layout does not influence CBET mitigation. In the case of a driver using many monochromatic beamlets, we find that ∼10 beamlets per port is required, with diminishing returns above ∼20.

Numerical simulation study on the formation mechanism of hydroxyapatite-silver gradient bioactive ceramic coatings under wide-band laser cladding

Numerical simulation study on the formation mechanism of hydroxyapatite-silver gradient bioactive ceramic coatings under wide-band laser cladding

High-accuracy wide-range refractive index demodulation based on under-sampled fiber F-P cavity length spectrum.

A scheme of fiber Fabry-Perot (F-P) cavity refractive index (RI) demodulation named under-sampled length spectrum retrieval (ULSR) is proposed. Unlike the wavelength spectrum method, ULSR can be used for physical quantity detection with just a monochromatic laser and photodetectors, avoiding the need for wideband lasers or expensive infrared spectrometers. Eight F-P cavities of different lengths were fabricated to sample the cavity length spectrum, and then the obtained under-sampled length spectrum was used to demodulate the RI of F-P cavity fillings. It was demonstrated that the ULSR system can achieve an index measurement accuracy of 1 × 10-4 in the glucose solution index range of 1.3294-1.3746 at wavelength λ = 1.55 µm. An index demodulation with higher accuracy and wider range is expected when more than 8 F-P cavities are used. The proposed scheme, with advantages of low system complexity, low cost, high reliability, high detecting accuracy, and wide detecting range, holds great promise for facilitating the wide application of F-P cavity sensors. Additionally, ULSR liberates wavelength freedom, making it a strong candidate for multiplexed sensing based on wavelength division multiplexing.

Study on Mechanical Properties of AlFeCrMoNi1.8Nb1.5 Eutectic High-Entropy Alloy Coating Prepared by Wide-Band Laser Cladding

In this study, AlFeCrMoNi1.8Nb1.5 (at.%) eutectic high-entropy alloy (EHEA) coating was successfully prepared on the surface of M2 high-speed steel (HSS) by wide-laser cladding. The effects of laser defocusing amount, laser power, scanning speed, and preset powder thickness on the formation quality of the EHEA coating were studied by the orthogonal experimental design, then the mechanical properties of the coating prepared by water-cooled solidification under optimal process parameters were studied. The experimental results showed that the optimal laser cladding process parameters are defocusing with an amount of −30 mm, laser power of 4 kW, scanning speed of 3 mm/s, and preset powder thickness of 1.5 mm. The substrate exhibited a favorable metallurgical bond with the coating, characterized by a stable interface devoid of any holes or cracks. Furthermore, the coating, which was prepared using water cooling, displayed a finer lamellar eutectic structure comprising FCC and Laves phases. The microhardness of the coating (544 HV0.2) was significantly higher than that of the substrate M2 HSS (~220 HV0.2), accompanied by good wear resistance.

Widely Tunable Femtosecond Sources with Continuously Tailorable Bandwidth Enabled by Self‐Phase Modulation

AbstractGenerating ultrafast pulses with better spectrotemporal control is crucial for optimizing and characterizing nonlinear light–matter responses, yet it is limited by the gain bandwidth of laser media or the phase‐matching geometry of nonlinear processes. This work proposes a simple approach to independently manage a femtosecond source's spectral location and bandwidth. Self‐phase‐modulation‐enabled spectral broadening is first analyzed, which is potentially energy‐scalable using hollow‐core capillaries or multipass cells. It is demonstrated that the outmost lobes in the broadened spectrum show different dependencies on the initial pulse energy and duration. A simple yet effective toy model is introduced that successfully predicts broadband spectral tuning, and the impact of other nonlinear effects, dispersion, and input pulse asymmetry on the experimental scenario is also discussed. Thus a fiber‐based versatile source is demonstrated, which is compressible down to its transform‐limit duration, as short as 12.2 fs centered at 920 nm. In addition, bandwidth‐dependent third‐harmonic generation spectroscopy is performed from a dielectric metasurface with an optimized nonlinear response, and the dependency of laser bandwidth and pulse duration is investigated on the signal‐to‐background ratio of two‐photon images. It is believed that this demonstration will advance the investigation of bandwidth‐dependent nonlinear spectroscopy and microscopy.

Dual-HR-FBG based single-frequency fiber laser at 1030 nm

We demonstrate a single-frequency fiber laser operating at 1030 nm with a distributed Bragg reflector cavity consisting of two high-reflectivity fiber Bragg gratings (HR-FBGs). By proper thermal adjustment, the gain bandwidth of the fiber laser is significantly compressed through the spectral overlap near the edges of the reflection band of two HR-FBGs, and the single-longitudinal-mode lasing is achieved. The effective cavity length of the fiber laser with the proposed cavity configuration is calculated, and its variation with the temperature difference between two HR-FBGs is also verified through the observation of the relaxation oscillation frequency shift. The measured laser linewidth is about 4.45 kHz at a temperature difference of 48.2 °C. The single-frequency output power of 14.4 mW is achieved at a pump power of 540 mW.

Laser isotope separation of 192Ir

The following two-step isotope selective photoionization process has been studied for the separation of 192Ir isotope from natural Iridium irradiated in a medium flux reactor for 28 days.5d76s2F9/240.0cm−1→247.5871nm5d76s6p40389.83cm−1J=9/2→307.9nmIr+Density matrix formalism has been used for the investigation of laser isotope separation process. Optimum system parameters such as peak power densities of the excitation and ionization lasers, bandwidth of the excitation laser, angular divergence limit to the atomic ensemble have been derived. Effects of number density and ion travel distance prior to collection on charge exchange probability have also been studied. It has been shown that it is possible produce ∼98% enriched 192Ir at rate of 82 μg/h (27.9 GBq/h or 0.76 Ci/h) which can provide activity of 334 TBq/g or 9027 Ci/g. The enriched 192Ir can be used in both high-dose brachytherapy and high speed radiography. This is the first ever study on the laser based enrichment of Iridium isotopes.

High-speed directly modulated distributed feedback laser based on detuned loading and photon–photon resonance effect

A monolithic integrated two-section distributed feedback (TS-DFB) semiconductor laser for high-speed direct modulation is proposed and analyzed theoretically. The grating structure of the TS-DFB laser is designed by the reconstruction-equivalent-chirp (REC) technique, which can reduce the manufacturing cost and difficulty, and achieve high wavelength controlling accuracy. The detuned loading effect and the photon–photon resonance (PPR) effect are utilized to enhance the modulation bandwidth of the TS-DFB laser, exceeding 37 GHz, while that of the conventional one-section DFB laser is only 16 GHz. When the bit rate of the non-return-to-zero (NRZ) signal reaches 55 Gb/s, a clear eye diagram with large opening can still be obtained. These results show that the proposed method can enhance the modulation bandwidth of DFB laser significantly.

Spectral shaping of an ultrafast ytterbium fiber laser via a passive intracavity optical filter: a simple and reliable route to sub-45 fs pulses.

We investigate the use of passive intracavity optical filters for controlling the laser output spectrum of a polarization-mode-locked, ultrafast ytterbium fiber laser. The overall lasing bandwidth is increased or extended by strategic choice of the filter cutoff frequency. Overall laser performance, including pulse compression and intensity noise, is investigated for both shortpass and longpass filters with a range of cutoff frequencies. The intracavity filter not only shapes the output spectra, but also provides a route for overall broader bandwidths and shorter pulses in ytterbium fiber lasers. These results demonstrate that spectral shaping with a passive filter is a useful tool to routinely achieve sub-45fs pulse durations in ytterbium fiber lasers.

Inserting self-assembled InAs quantum dots into quantum cascade lasers to achieve a broadband free-running frequency comb and effective radio-frequency injection.

We present what we belive to be a new band design in which self-assembled InAs quantum dots (QD) are embedded in InGaAs quantum wells (QW) to fabricate broadband single-core quantum dot cascade lasers (QDCLs) operating as frequency combs. The hybrid active region scheme was exploited to form upper hybrid QW/QD energy states and lower pure QD energy states, which expanded the total laser bandwidth by up to 55 cm-1 due to a broad gain medium provided by the inherent spectral inhomogeneity of self-assembled QDs. The continuous-wave (CW) output power of these devices was as high as 470 mW with optical spectra centered at ∼7 µm, which allowed CW operation at temperatures up to 45 °C . Remarkably, measurement of the intermode beatnote map revealed a clear frequency comb regime extending over a continuous 200 mA current range. Moreover, the modes were self-stabilized with intermode beatnote linewidths of approximately 1.6 kHz. Furthermore, what we believe to be a novel π-shaped electrode design and coplanar waveguide transition way were used for RF signal injection. We found that RF injection modified the laser spectral bandwidth by up to 62 cm-1. The developing characteristics indicate the potential for comb operation based on QDCLs as well as the realization of ultrafast mid-infrared pulse.