Quasi-continuous Wave Research Articles

High-power mid-wave infrared LED using W-superlattices and textured surfaces

Efficient mid-infrared light output has been obtained by incorporating a W-superlattice into a cascaded mid-infrared LED structure and by thinning and roughening of the emission side of the structure. At cryogenic temperatures, a radiance of ∼13.4 W/cm2-sr is achieved. Compared to the best published InAs/GaSb mid-IR LED, the maximum radiance is improved by ∼2.0×, while the wallplug efficiency improvement at the maximum radiance is improved >10×. For room temperature measurements on an un-thinned 400 μm diameter diode, the radiance (light output power) for a quasi-continuous wave and 1% duty cycle were ∼ 0.48 W/cm2-sr (2.4 mW) and ∼1.35 W/cm2-sr (6.8 mW), respectively. When compared to previous room temperature 4.2 μm LEDs, at a 1% duty cycle, this LED has optical powers that are 3× brighter. When compared to thermal emitters used in gas sensors, in the quasi-continuous wave, this LED uses ∼100× less energy per measurement.

Effect of continuous wave, quasi-continuous wave and pulsed laser radiation on functional characteristics of fish spermatozoa

For the first time, using sturgeon sperm as a model system, sensitive to optical radiation, the comparative studies of biological effect of continuous wave, quasi-continuous wave, nano- and picosecond laser radiation under conditions with equal average irradiance (3 mW/cm2) and wavelength (532 nm) have been carried out. Analyzing the parameters of spermatozoa motion it has been shown that, depending on the energy dose and mode of laser operation, the radiation may have both stimulatory and inhibitory effect on the velocity of motion and spermatozoa motility duration as well as on sustaining of functional characteristics of cold-stored sperm. The possibility of increasing the fertilization rate due to use of the sperm preliminary treated with laser radiation is demonstrated. For the first time, the possibility of enhancement of biological effect going from continuous wave to quasi-continuous wave laser radiation at equal irradiance and wavelength has experimentally been proven. It is shown that the difference in biological effect of continuous wave, quasi-continuous wave, nano- and picosecond laser radiation is due to amplitude (peak) values of intensity. Using fluorescence analysis and luminol-dependent chemiluminescence assay, evidence for the participation of endogenous flavins and metal-free porphyrins in sensitized ROS formation (singlet oxygen, hydrogen peroxide, and hydroxyl radicals) in sturgeon sperm was obtained. Mechanisms of photochemical and photothermal reactions explaining the difference in efficacy of action of laser radiation in above modes are discussed.

Water-jet guided laser drilling of thermal barrier coated aerospace alloy

Drilling using quasi-continuous wave (QCW) millisecond pulse fibre laser is the state-of-the-art for producing film-cooling holes over uncoated aero-engine components. Laser drilling of coated components, including thermal barrier coated (TBC) components, is still a challenging task due to the risk of coating delamination. Water-jet guided (WJG) laser is nowadays increasingly used for machining of advanced materials such as ceramics and composites. This paper aims to study the basic fundamental characteristics of WJG lasers drilling of acute angular holes over TBC coated nickel superalloy and compare it with the state-of-the-art QCW millisecond fibre laser. Experiments were performed to investigate the characteristics of both QCW and WJG laser drilling in terms of TBC delamination, recast layer formation, hole surface topology and cycle time. Finite element analysis based numerical model was developed and used, to understand and confirm the mechanism of heat propagation and the subsequent material removal characteristics for both WJG and QCW laser. Experimental results show that the WJG laser drilling quality (in terms of thermal damages) is very similar to the process of cold-ablation (i.e. machining without inducing any thermal defects within the substrate material). The numerical model confirms the hypothesis that the high-temperature zone of WJG laser machining process is wholly confined within the small laser irradiation zone, which helps to avoid the TBC delamination typically observed with long pulse acute angle laser drilling process. The material removal mechanism of WJG laser machining seems to be the combination of melt ejection by vapour pressure, melt ejection by impulse shock pressure and by the action of water-jet pressure.

Generation of particles and fragments by quasicontinuous wave fiber laser irradiation of stainless steel, alumina, and concrete materials

An experimental characterization is presented on fine particles, droplets, and fragments produced at the interaction region between a 2.7 kW quasicontinuous wave (repetitive pulsed operation with a 10 ms pulse duration) fiber laser and stainless steel, alumina, and heavy concrete samples. In the samples, the recoil pressure induced by vaporization pushes particles and fragments into the ambient atmosphere. In order to preserve a safe working environment, in particular for nuclear decommissioning, special care should be taken to confine and retrieve such particles during laser processing. In the experiments, particle production from the vapor and the molten phase layer in the targeted material was observed with a high-speed camera, with fine particles collected and analyzed using an electron microscope. The observed results were qualitatively interpreted with the help of a simplified one-dimensional hydrodynamic code coupled with a stress computation code. Characterization and classification of the results are expected to provide a useful database that will contribute to the decommissioning of nuclear facilities and other industrial applications.

Fabrication, microstructures, and optical properties of Yb:Lu2O3 laser ceramics from co-precipitated nano-powders

The Yb:Lu2O3 precursor made up of spherical particles was synthesized through the co-precipitation method in the water/ethanol solvent. The 5 at% Yb:Lu2O3 powder is in the cubic phase after calcination at 1100 °C for 4 h. The powder also consists of spherical nanoparticles with the average particle and grain sizes of 96 and 49 nm, respectively. The average grain size of the pre-sintered ceramic sample is 526 nm and that of the sample by hot isostatic pressing grows to 612 nm. The 1.0 mm-thick sample has an in-line transmittance of 81.6% (theoretical value of 82.2%) at 1100 nm. The largest absorption cross-section at 976 nm is 0.96×10−20 cm2 with the emission cross-section at 1033 nm of 0.92×10−20 cm2 and the gain cross sections are calculated with the smallest population inversion parameter β of 0.059. The highest slope efficiency of 68.7% with the optical efficiency of 65.1% is obtained at 1033.3 nm in quasi-continuous wave (QCW) pumping. In the case of continuous wave (CW) pumping, the highest slope efficiency is 61.0% with the optical efficiency of 54.1%. The obtained laser performance indicates that Yb:Lu2O3 ceramics have excellent resistance to thermal load stresses, which shows great potential in high-power solid-state laser applications.

Measuring and modeling quasicontinuous wave operation of quantum cascade lasers for enhanced average brightness

Quasicontinuous wave operation of midinfrared quantum cascade lasers are shown to have increased average output power with good beam quality. The ability to enhance average power by a significant fraction of CW power motivates the development of a model to estimate and project performance at varying duty cycles based on a previously developed continuous wave power projection model. The model takes into account pulse to pulse changes in temperature profile to project a transient steady-state temperature distribution. This temperature distribution is used to project both peak and average power in agreement with measurements. Preliminary model projections suggest that high average brightness may be achieved using a reduced number of stages and a greater scaling of core width than would be permissible for CW lasing.

Hot isostatic pressing of transparent Yb3+-doped Lu2O3 ceramics for laser applications

Yb3+-doped Lu2O3 nanoparticles produced by laser ablation were used to fabricate transparent ceramics by a combination of pressureless sintering in vacuum (PS) followed by a hot isostatic pressing (HIPing). The samples were subjected to various PS and HIPing conditions and the microstructure evolution and its correlation with the transmittance were investigated. Relative densities of over 97% were achieved after PS at the temperatures of 1250–1700 °C. Rapid grain growth occurred within PS and HIPing temperatures above 1500 °C leading to formation of intragranular porosity which is deleterious for optical quality. Higher transmittance (81.7% at λ = 1080 nm) and ultrafine microstructure with an average grain size of 0.35 μm were obtained by PS at 1250 °C followed by HIPing at 1400 °C for 5 h under 207 MPa. Output power of 2.02 W with a slope efficiency of 46.5% was obtained under a quasi-continuous wave end pumping at 929.4 nm.

Enhancement of the laser emission efficiency of Yb:Y2O3 ceramics via multi-step sintering method fabrication

Highly transparent 5.0 at.% Yb:Y2O3 ceramics were successfully fabricated by using an optimal content of La2O3 and ZrO2 sintering additives and multi-step sintering method. The sintering process of the ceramics was based on a combination of two-step sintering in air followed by vacuum sintering. In-line optical transmission reached 78.5% at 1100 nm and 80% at 2000 nm. The absorption cross-section at 978 nm and the stimulated emission cross-section at 1032 nm were determined to be 2.23 × 10−20 cm2 and 1.42 × 10−20 cm2, respectively. Laser emission at 1.03 μm with a high slope efficiency of 33% was obtained from a 1.5 mm thick Yb:Y2O3 uncoated ceramic medium under quasi-continuous wave pumping at 971 nm with a fiber-coupled diode laser.

Study of long-pulse quasicontinuous wave INNOSLAB amplifier at 1319 nm

We report on a long-pulse, quasicontinuous wave partially end-pumped slab (INNOSLAB) amplifier at 1319 nm. The low-pulse-energy seed laser was amplified to 8.24 mJ at repetition of 500 Hz by five-pass amplification. The beam quality factors are 1.70 and 1.28 in the horizontal and vertical directions, respectively. The optical–optical efficiency was 1.16% under the pump power of 187.5 W. The experimental results match well with the numerical simulation. The optical–optical efficiency can be improved to about 10% with higher absorption efficiency of the pump source and higher overlapping efficiency between the pump light and 1319-nm seed laser. And further improvements can be realized by optimizing the input pulse energy density and the pass of amplification.

High-power repetition rate- and pulse width-tunable 589 nm versatile laser for adaptive optical systems.

Compact high-power yellow laser is a critical part for sodium beacon adaptive optical systems. A narrow-linewidth quasi-continuous-wave (QCW) solid-state 589 nm laser with high-power and high beam quality simultaneously is investigated here, operating in hundreds-microsecond pulse duration with a tunable repetition rate of 400 to 1 kHz, which is flexible to allow the telescope to move in observing direction. The laser source is based on employing sum-frequency generation between 1319 and 1064 nm QCW Nd:YAG amplifiers. For a 100 µs pulse duration and 400 Hz repetition rate, the yellow laser provides a highest output power of 86.1 W with beam quality M2 = 1.37. The central wavelength can be precisely tuned to sodium-D2a line at 589.159 nm with a ∼440 MHz linewidth. This is the maximum power-reported for all-solid-state sodium guide star laser demonstrated to date. The result represents a key step toward solving the requirement of multi-conjugate adaptive optics for large adaptive optical telescopes.

Adjustable slab-aberration compensator for high power slab laser.

An adjustable slab-aberration compensator (ASAC) with the ability to compensate the large magnitude inherent wavefront aberrations in the slab width direction is proposed and experimentally demonstrated. The ASAC has a size of 130mm×45mm (effective aperture of 75mm×28mm) and 11 actuators along the length with a contact spacing of 8 mm. The design is optimized by simulations in terms of the mirror's coupling coefficient with the contact areas, mechanical properties of the driving units, and the mirror thickness. The initial surface figure of the ASAC has PV and RMS values of 55 nm and 10 nm, and the dynamic range is 30 µm. In our experiments, a 20 kW Nd: YAG quasi-continuous wave (QCW) slab laser is further compensated by the ASAC system. The beam quality increases from 15× to 3.5× diffraction limit at 20 kW output after correction. Besides, the proposed ASAC can maintain the surface shape after power shutdown and have good thermal stability. The temperature rise of the ASAC is less than 7 °C in the 20 kW laser correction experiment.

Controlling spatiotemporal nonlinearities in multimode fibers with deep neural networks

Spatiotemporal nonlinear interactions in multimode fibers are of interest for beam shaping and frequency conversion by exploiting the nonlinear interaction of different pump modes from quasi-continuous wave to ultrashort pulses centered around visible to infrared pump wavelengths. The nonlinear effects in multi-mode fibers depend strongly on the excitation condition; however, relatively little work has been reported on this subject. Here, we present a machine learning approach to learn and control nonlinear frequency conversion inside multimode fibers. We experimentally show that the spectrum of the light at the output of the fiber can be tailored by a trained deep neural network. The network was trained with experimental data to learn the relation between the input spatial beam profile of the pump pulse and the spectrum of the light at the output of the multimode fiber. For a user-defined target spectrum, the network computes the spatial beam profile to be applied at the input of the fiber. The physical processes involved in the creation of new optical frequencies are cascaded stimulated Raman scattering as well as supercontinuum generation. We show experimentally that these processes are very sensitive to the spatial shape of the excitation and that a deep neural network is able to learn the relation between the spatial excitation at the input and the spectrum at its output. The method is limited to spectral shapes within the achievable nonlinear effects supported by the test setup, but the demonstrated method can be implemented to learn and control other spatiotemporal nonlinear effects.

24.6 kW near diffraction limit quasi-continuous-wave Nd:YAG slab laser based on a stable-unstable hybrid cavity.

A 24.6 kW quasi-continuous-wave (QCW) Nd-doped yttrium aluminum garnet (Nd:YAG) slab laser is proposed in this Letter. The laser is based on a stable-unstable hybrid cavity. A stable and an unstable resonator were constructed along orthogonal directions in the aperture of the slab. Due to features of the hybrid cavity, the slab laser achieves both high efficiency of power extraction with excellent beam quality and compactness with simplicity. Average output power of 24.6 kW with 47% optical-to-optical efficiency is achieved in the experiment. The beam quality of the output beam is 1.5 times diffraction limits after correction of adaptive optics. The repetition frequency and pulse width of the laser are 400 Hz and 200 μs.

Localized infrared radiation-induced hyperthermia sensitized by laser-ablated silicon nanoparticles for phototherapy applications

Silicon (Si) nanoparticles (NPs) synthesized by methods of laser ablation in water are explored as sensitizers of photothermal therapy under a laser excitation in the window of relative tissue transparency. Based on theoretical calculations and experimental data, it is shown that the NPs can be heated up to temperatures above 42–50 °C by laser diode irradiation at 808 nm in continuous wave (CW) and quasi-continuous wave (QCW) regimes. Profiting from the laser-induced heating, a high efficiency Si-NPs as sensitizers of the hyperthermia of cells in Paramecium Caudatum model is demonstrated. The QCW regime is found to be more efficient, leading to complete cell destruction even under relatively mild laser irradiation conditions. The obtained data evidence a great potential in using laser-ablated Si-NPs as sensitizers of photohyperthermia in antibacterial or cancer therapy applications.

Thermoelectric properties of SiGe thin films prepared by laser sintering of nanograin powders

Silicon-germanium is an important high-temperature thermoelectric material for thermal to electric energy conversion. This paper focuses on the study of nanograin thermoelectric Si80Ge20 thin films. The nanograin Si80Ge20 thin films were prepared by a novel method of ball milling to generate nanoparticles and sintered by a quasi-continuous wave, near-infrared laser of wavelength 1070 nm. The electrical and thermal properties variations with different laser sintering power were measured, and the sintering mechanism is discussed. The Si/Ge wetting to substrate for a percolation morphology and the control of carrier concentration are found to be the key factors for good electrical conductivity. The stoichiometric ratio of Si and Ge was found to be unchanged in the ball milling and laser sintering process. We demonstrate that the laser sintering was capable of creating well alloyed thermoelectric materials while minimizing grain growth. The thermoelectric parameters like electrical conductivity, thermal conductivity, and figure of merit were measured, and the results are compared with other reported bulk and thin-film studies, and the advantages of our method are discussed. We demonstrate that low-cost nanograin SiGe thin films prepared by ball milling and laser sintering process have a comparable thermoelectric figure of merit to other SiGe thermoelectric materials and which can be further improved.

6.2 kW quasi-continuous-wave diode-pumped Yb:YAG ceramic slab laser

We demonstrate a high power quasi-continuous-wave (QCW) diode-pumped Yb:YAG ceramic slab laser. The wavefront distortion of the assembled large-sized Yb:YAG ceramic slab module is evaluated firstly. Through a short cavity setup, the average output power of a single ceramic slab laser oscillator is up to 6.2 kW. The corresponding optical-to-optical efficiency with respect to the absorbed pump power is 64.8% and a slope efficiency of 72.1%, operating at a pulse duration of 560 μs and a repetition rate of 160 Hz. To the best of our knowledge, this represents a highest output power for a QCW Yb:YAG ceramic single slab laser oscillator ever reported to date. This result proves the way to the further development of high power solid-state Yb:YAG ceramic slab laser.

Fabrication, microstructure, and optical properties of Tm:Y3ScAl4O12 laser ceramics

AbstractTransparent 4 at.% Tm:Y3ScAl4O12 (Tm:YSAG) laser ceramics were fabricated by solid‐state reaction combined with vacuum sintering method. The 4 at.% Tm:YSAG ceramic sample sintered at 1800°C for 30 hours possesses homogenous microstructure and excellent optical properties, showing a transmittance of 79.3% at 2000 nm. The absorption and emission spectra of the Tm:YSAG ceramics are studied and compared with those of 4 at.% Tm:Y3Al5O12 ceramics. The introduction of Sc3+ greatly affects the energy levels of the Tm3+, causing the disappearance and degeneration of some absorption and emission peaks in the middle infrared region. The laser performance of the 4 at.% Tm:YSAG ceramics is also tested in the Quasi‐continuous‐wave (QCW) mode by pumping with a 790 nm laser diode (LD). A maximum laser output power of 0.54 W with a slope efficiency of 4.8% is achieved, which is the first laser output for Tm:YSAG ceramics.

Investigation of Chaotic and Spiking Dynamics in Mid-Infrared Quantum Cascade Lasers Operating Continuous-Waves and Under Current Modulation

This study investigates chaotic and spiking dynamics of mid-infrared quantum cascade lasers operating under external optical feedback and emitting at 5.5 $\mu {\text{m}}$ and 9 $\mu {\text{m}}$ . In order to deepen the understanding, the route to chaos is experimentally studied in the case of continuous-wave and current modulation operation. The non-linear dynamics are analyzed with bifurcation diagrams. While for quasi-continuous wave operation, chaos is found to be more complex, pure continuous wave pumping always leads to the generation of a regular spiking induced low-frequency fluctuations dynamics. In the latter, results show that by combining external optical feedback with periodic forcing and further induced current modulation allows a better control of the chaotic dropouts. This work provides a novel insight into the development of future secure free-space communications based on quantum cascade lasers or unpredictable optical countermeasure systems operating within the two transparency atmospheric windows hence between 3 $\mu {\text{m}}$ –5 $\mu {\text{m}}$ and 8.5 $\mu {\text{m}}$ –11 $\mu {\text{m}}$ .

Influences of the Sc3+ content on the microstructure and optical properties of 10 at.% Yb:Y3ScxAl5-xO12 laser ceramics

10 at.% Yb:Y3ScxAl5-xO12 (x = 0.25, 0.5, 1.0 and 1.5) transparent ceramics have been successfully fabricated by the solid state reaction method combined with vacuum sintering, whose transmittances reach 80% at 1100 nm with thickness of 3.0 mm. The full width at half maximum (FWHM) of the emission peaks located at about 1030 nm increases for increasing the Sc3+ content, and a maximum FWHM of 14.53 nm is obtained for 10 at.% Yb:Y3Sc1.5Al3.5O12 ceramics, which is 1.4 times larger than that of the 10 at.% Yb:Y3Al5O12 (Yb:YAG) ceramics. Finally, the laser performance of the 10 at.% Yb:Y3Sc1.5Al3.5O12 ceramics was tested and compared with that of a standard 10 at.% Yb:YAG sample. In quasi continuous wave (QCW) regime, a maximum laser output power of 10.8 W was obtained with the slope efficiency of 66.4%. The tuning range significantly broadens in the case of the Yb:YSAG sample, reaching a width of 86.2 nm.

Generation of ultrashort laser pulses through a resonant interaction of quasi-continuous wave packet with running refractive index wave

Modulation instability followed by generation of subpicosecond pulses could be obtained with quasi-continuous wave packets propagating in optical fibers with running refractive index wave. We report on comprehensive studies of this process demonstrating that the peak power of the pulses exceeds the power of the pumping radiation by orders of magnitude. Practically, the effect could be implemented through interaction of the surface optical wave with an acoustic wave in a 2 cm cylindrical waveguide in a robust all-fiber format.