High-power Laser Light Research Articles

Beam shaping of highly multimode sources with cascaded diffractive optical elements

Multimode laser sources, like multimode fiber lasers and many laser diodes, play a central role in material processing requiring high power and continuous laser light delivery. Their native beam profiles, however, may not be ideal for certain laser applications, therefore creating a demand for beam shaping technologies. For laser applications where beam shaping provides a technical advantage, it is essential that the beam transformation is performed with as little energy loss as possible. The current contribution presents an inverse design algorithm for the creation of beam transformers that meets these expectations by computing optical devices consisting of pure and smooth phase modulations. These devices allow arbitrary intensity shaping and are suitable for free-space or integrated optical realization.

High thermal stability AlN-YAG: Ce composite phosphor ceramics for high-power laser-driven lighting

In this paper, AlN-YAG: Ce composite phosphor ceramics with high thermal conductivity was prepared by accompanying YAG: Ce phosphor with an AlN matrix. The phosphor ceramics was densified at 1700 °C for 3 h, and YAG: Ce phosphor particles were uniformly dispersed in the AlN matrix without any chemical reaction and impure phases around the contact interfaces. The thermal conductivity of the sample with 10 wt. % YAG: Ce phosphor was as high as 68.8 W m−1 K−1. As a result, the saturation power density is up to 39.6 W mm−2 with a luminous emittance of 1516 lm mm−2, showing excellent resistance to high flux density photon irradiation. This work presents a method to enhance the thermal conductivity of phosphor ceramics for high power laser lighting.

AlN–Ce-doped yttrium aluminum garnet composite ceramic phosphor for high-power laser lighting

Ce-doped yttrium aluminum garnet (YAG:Ce) has been used as a phosphor emitting white light using a blue-light-emitting diode as a light source. Recently, studies on laser lighting as next-generation lighting have been carried out. However, because a high-power blue laser diode is used as the light source, the decrease in phosphor emission due to temperature increase (thermal quenching) has become a serious issue. Therefore, it is desirable to develop a method to suppress the temperature increase of the YAG:Ce phosphor. To increase the thermal conductivity of the YAG:Ce phosphor and suppress the temperature increase, we fabricated a composite ceramic of AlN (nonoxide), which has a high thermal conductivity, and YAG:Ce (oxide) using a modified vapor transport equilibration technique. The oxynitride was not completely removed, but we succeeded in adding up to 80 wt% AlN to YAG:Ce. The ceramic with 50% added AlN had a thermal conductivity up to 19.2 W m−1K−1 and emitted almost the same intensity as that of the nondoped YAG:Ce. This thermal conductivity exceeds the reported values of 12–13 W m−1K−1 for Al2O3–YAG:Ce composite ceramics. Moreover, the luminous efficacy exhibited by the ceramic with a 20 wt% AlN addition was 266.0 lmW−1, which is larger than the reported values obtained from composite ceramics.

Optical fibers for endoscopic high-power Er:YAG laserosteotomy

.Significance: The highest absorption peaks of the main components of bone are in the mid-infrared region, making Er:YAG and lasers the most efficient lasers for cutting bone. Yet, studies of deep bone ablation in minimally invasive settings are very limited, as finding suitable materials for coupling high-power laser light with low attenuation beyond is not trivial.Aim: The first aim of this study was to compare the performance of different optical fibers in terms of transmitting Er:YAG laser light with a wavelength at high pulse energy close to 1 J. The second aim was to achieve deep bone ablation using the best-performing fiber, as determined by our experiments.Approach: In our study, various optical fibers with low attenuation () were used to couple the Er:YAG laser. The fibers were made of germanium oxide, sapphire, zirconium fluoride, and hollow-core silica, respectively. We compared the fibers in terms of transmission efficiency, resistance to high Er:YAG laser energy, and bending flexibility. The best-performing fiber was used to achieve deep bone ablation in a minimally invasive setting. To do this, we adapted the optimal settings for free-space deep bone ablation with an Er:YAG laser found in a previous study.Results: Three of the fibers endured energy per pulse as high as 820 mJ at a repetition rate of 10 Hz. The best-performing fiber, made of germanium oxide, provided higher transmission efficiency and greater bending flexibility than the other fibers. With an output energy of 370 mJ per pulse at 10 Hz repetition rate, we reached a cutting depth of in sheep bone. Histology image analysis was performed on the bone tissue adjacent to the laser ablation crater; the images did not show any structural damage.Conclusions: The findings suggest that our prototype could be used in future generations of endoscopic devices for minimally invasive laserosteotomy.

All-inorganic high efficiency LuAG:Ce3+ converter based on phosphor-in-glass for laser diode lighting

High-power, high-brightness laser lighting technology put forward new requirements for the service stability of color conversion materials. Phosphor-in-glass materials (PiGs) have emerged with their unique advantages of withstanding high power excitation density. High luminous efficiency (LE) and high internal quantum efficiency (IQE) LuAG:Ce PiG is still an urgent demand for high power laser lighting conversion materials. In this paper, LuAG:Ce PiGs was successfully prepared by careful design of glass matrix composition. The maximum internal quantum efficiency of LuAG:Ce PiG is 93.8%. The conversion efficiency (CE) of LuAG:Ce PiG from blue to green light is increased to 56.3%. The luminous efficiency (LE) even reaches 225 lm W−1, which is the best performance of LuAG:Ce in LD lighting so far. These results show that this work has taken a big step in improving the performance of LuAG:Ce Green converter and will greatly promote the development of LD lighting.

808nm Near-Infrared Light-Triggered Payload Release from Green Light-Responsive Donor-Acceptor Stenhouse Adducts Polymer-Coated Upconversion Nanoparticles.

Owing to deep activation in biotissues and enhanced targeting efficiency, developing photoresponsive polymer-upconversion nanoparticles (PP-UCNPs) nanovectors has witnessed rapid growth in the past decade. However, up to date, all developed nanovectors require high-order photon processes to initiate the release of cargos. The photodamage caused by high-power near-infrared laser light may be a critical obstacle to their clinical application. Here, for the first time, by leveraging absorption-emission spectral matching between donor-acceptor Stenhouse adducts (DASA) PP and UCNPs (λex , 808nm) in the green region (≈530nm), the designed nanovector is capable of releasing cargos at a low-power 808nm excitation (0.2 W). Considering the high molar absorptivity, biobenign, and synthetic tunability of DASA, DASA PP can be utilized as an up-and-coming candidate to design and synthesize the next generation of upconversion nanovectors without photodamage.

Watt-level blue light for precision spectroscopy, laser cooling and trapping of strontium and cadmium atoms.

High-power and narrow-linewidth laser light is a vital tool for atomic physics, being used for example in laser cooling and trapping and precision spectroscopy. Here we produce Watt-level laser radiation at 457.75 nm and 460.86 nm of respective relevance for the cooling transitions of cadmium and strontium atoms. This is achieved via the frequency doubling of a kHz-linewidth vertical-external-cavity surface-emitting laser (VECSEL), which is based on a novel gain chip design enabling lasing at > 2 W in the 915-928 nm region. Following an additional doubling stage, spectroscopy of the 1S0 → 1P1 cadmium transition at 228.87 nm is performed on an atomic beam, with all the transitions from all eight natural isotopes observed in a single continuous sweep of more than 4 GHz in the deep ultraviolet. The absolute value of the transition frequency of 114Cd and the isotope shifts relative to this transition are determined, with values for some of these shifts provided for the first time.

Laser safety assessments supported by analyses of reflections from metallic targets irradiated by high-power laser light

When using kilowatt-class lasers in outdoor environments, ensuring laser safety turns out to be a complex issue due to the large safety areas that must be respected. For the special cases of collimated or focused laser radiation reflected from ideally flat but naturally rough metallic surfaces, the classical laser hazard analysis is deemed insufficient. In order to investigate the corresponding hazard areas for the aforementioned cases, we performed experiments on laser-matter interactions. Using high-power laser radiation, we studied the spatial and temporal reflection characteristics from four different metallic samples. For the evaluation of total reflection characteristics, we performed curve-fitting methods comprising Gaussian-like specular components, diffuse scattering components according to the ABg-scatter model and Lambertian components. For the investigation of occurring caustics, we developed a dedicated model in order to assess the divergence of the contained structures as a function of distance. Our evaluations have shown that the majority of the reflected power is scattered and based on these findings, that resulting nominal optical hazard distance values, even under worst-case assumptions, are significantly smaller than those of the non-reflected laser beam.

Highly thermal conductive red-emitting AlN-CaAlSiN3:Eu2+ composite phosphor ceramics for high-power laser-driven lighting

Thermally robust red-emitting phosphor ceramics are urgently required for laser lighting and displays with high luminance and better color saturation. The most promising CaAlSiN3:Eu ceramics have a low thermal conductivity of 4.2 W m−1 K−1 and a small luminance saturation of 0.5 W, making it hard to be used under high power laser irradiation. In this work, we incorporated AlN into the CaAlSiN3:Eu ceramic to produce red-emitting AlN-CaAlSiN3:Eu composite phosphor ceramics by spark plasma sintering. The fully densified phosphor ceramics have the highest thermal conductivity reported so far (53.5 W m−1 K−1), which is about 13 times higher than the reported one. The luminance saturation of the composite ceramics occurs at a high threshold of 4.2 W under blue laser excitation, enabling them to be used for high power laser lighting. This work provides an idea of tackling the microstructure of nitride phosphor ceramics and of preparing thermally robust red-emitting color converters.

High color rendering index composite phosphor-in-glass for high-power white laser lighting

The aim of this study is to investigate a thermally robust white color converter for high-power solid-state lighting, especially laser lighting. Lu3Al5O12:Ce3+/CaAlSiN3:Eu2+ phosphor-in-glass samples (LuAG&CASN-PiGs) were synthesized via a low temperature co-sintering technique. The prepared LuAG&CASN-PiGs exhibited remarkably high internal quantum efficiency of 87 %. Tunable warm white light-emitting diodes (wLED) were acquired by tailoring the sample thicknesses and phosphor contents. The optimized sample showed a high luminous flux of 183.68 lm under a blue laser diodes (LDs). In addition, the chromaticity of white LDs based on the LuAG&CASN-PiGs shifted from cool to warm white by changing the sample thicknesses. High quality white light in wLDs was achieved (Ra＝95). More importantly, the constructed LuAG&CASN-PiG converted LDs with a heat sink exhibited the luminous efficiency of 216.79 lm W−1. The results revealed that the prepared LuAG&CASN-PiG had great potential for application in solid-state laser light sources.

Characteristics of double-cascade photodiodes based on p-i-n AlGaAs/GaAs diodes connected by n++-GaAs/i-GaAs/i-AlGaAs/p++-AlGaAs tunnel diodes

Characteristics of double-cascade nonmonolithic p-i-n AlGaAs/GaAs photovoltaic converters of high-power laser radiation, electrically combined with n++-GaAs/i-GaAs/i-AlGaAs/p++-AlGaAs tunnel diodes, have been studied. The AlGaAs/GaAs p-i-n structures of the photovoltaic converters and n++-GaAs/i-GaAs/i-AlGaAs/p++-AlGaAs tunnel diodes were grown by molecular-beam epitaxy. The photoelectric characteristics of the double-cascade AlGaAs/GaAs photovoltaic converters were examined in the temperature range from 103 to 298 K under excitation with a laser light wavelength of 809 nm and a power density ⩽92 W/cm2. The double-cascade p-i-n AlGaAs/GaAs nonmonolithic photovoltaic converters of planar design had a fill factor of 0.88 and efficiency of 58.2% at a temperature of 103 K. It was shown experimentally that the current-voltage characteristics of the connecting n++-GaAs/i-GaAs/i-AlGaAs/p++-AlGaAs tunnel diodes affect the photoelectric parameters of the double-cascade photovoltaic converters of high-power laser light.

CaAlSiN3:Eu/glass composite film in reflective configuration: A thermally robust and efficient red-emitting color converter with high saturation threshold for high-power high color rendering laser lighting

To achieve high color rendering and proper color temperature, a red color converter is essential for phosphor-converted white lighting devices. CaAlSiN3:Eu2+ (CASN) is a highly suitable red phosphor for white light-emitting diodes. However, it can be hardly used in high-power laser lighting due to poor thermal/chemical performance of the phosphor/silicone resin mixture. A series of all-inorganic CASN-based phosphors (e.g., composite ceramic and phosphor-in-glass) were developed to avoid the use of resin. However, new challenges emerged: none of them showed sufficient luminous efficacy (i.e., >50 lm/W) and adequate saturation-threshold (i.e., >30 W or 10 W/mm2). Here, we report a facile fabrication of CASN/glass composite films using a simple and efficient blade-coating method. Upon 450 nm excitation, the resultant composite film presents a high internal quantum efficiency of ~83%, comparable to that of pristine CASN powder (~90%). When irradiated with a blue laser, the composite film shows a record high luminous efficacy of 82 lm/W. Furthermore, its saturation threshold was investigated in high power and high power density mode, respectively. When measured in high power mode, it shows a high saturation threshold over 29.7 W (1.75 W/mm2), thus achieving a high luminous flux of 1576 lm; when measured in high power density mode, it shows a saturation threshold of ~10.2 W/mm2 (1.13 W). With abovementioned excellent properties, the CASN/glass composite film has great potential for use in high-power and high color rendering laser lighting.

Time-dependent measurement of high-power laser light reflection by low-Z foam plasma

Abstract Porous materials have many applications for laser–matter interaction experiments related to inertial confinement fusion. Obtaining new knowledge about the properties of the laser-produced plasma of porous media is a challenging task. In this work, we report, for the first time to the best of our knowledge, the time-dependent measurement of the reflected light of a terawatt laser pulse from the laser-produced plasma of low-Z foam material of overcritical density. The experiments have been performed with the ABC laser, with targets constituted by foam of overcritical density and by solid media of the same chemical composition. We implemented in the MULTI-FM code a model for the light reflection to reproduce and interpret the experimental results. Using the simulations together with the experimental results, we indicate a criterion for estimating the homogenization time of the laser-produced plasma, whose measurement is challenging with direct diagnostic techniques and still not achieved.

Femtosecond laser machining of hollow-core negative curvature fibres.

Hollow core negative curvature fibres (NCFs) are a relatively new class of microstructured optical fibre with potential applications in areas such as the delivery of high power laser light and gas sensing. For sensing, it is necessary for the measurand to interact with the guided mode. To facilitate this, a novel femtosecond laser-based machining protocol has been developed that allows the precision sculpting of access slots into the NCF core along the length of the fibre. The process is a direct-write process using a digitally defined scanning strategy with no need for physical masks or additional processing such as wet etchants and/or focussed ion beam machining. Due to the inherent flexibility of the machining strategy and the high level of control over the depth of material removal, it is likely that this new technique will be transferable to a wide range of microstructured fibres.

Ternary solid solution phosphors Ca1-x-yLixAl1-x-ySi1+x+yN3-yOy:Ce3+ with enhanced thermal stability for high-power laser lighting

The discovery of thermally robust phosphors is essential to achieve efficient and stable luminescence in solid-state lighting devices, especial in high-power devices. Here, a novel ternary solid solution phosphor enabling to address this challenge is designed by introducing isostructural LiSi2N3 and Si2N2O into CaAlSiN3 lattice. The compound, Ca1-x-yLixAl1-x-ySi1+x+yN3-yOy:Ce3+ (CALSON:Ce), exhibits an excellent thermal stability with an integrated emission loss of only 9% at 200 °C, far smaller than that of the parent CaAlSiN3:Ce3+ (25%). The enhanced thermal stability is assigned to the broadened band gap (5.21 → 5.50 eV) upon the formation of solid solutions. Moreover, the new phosphor presents a broadband (full width at half maximum of 135 nm) yellow-orange emission peaking at 580 nm and shows a high external quantum efficiency of 41.5%. The excellent photoluminescence properties and thermal stability of CALSON:Ce thus enable it to be used under high-power laser excitation, and to produce high-brightness white light having a luminous flux of 392 lm (luminous emittance of 781 lm mm−2), a low correlated color temperature of 4349 K, and a high color rendering index of 70. This work suggests that creating multi-component solid solutions would be a general strategy for designing novel phosphors.

YAGG:Ce Phosphor-in-YAG Ceramic: An Efficient Green Color Converter Suitable for High-Power Blue Laser Lighting

Thermally robust phosphor ceramics are an ideal color converter for high-power laser lighting applications. However, high-quality white light is different to achieve due to the absence of green and...

A high quantum efficiency CaAlSiN3:Eu2+ phosphor-in-glass with excellent optical performance for white light-emitting diodes and blue laser diodes

CaAlSiN3:0.2%Eu2+ (CASN:Eu2+), an excellent red phosphor, playsaleadingpart in wide color gamut and high color rendition white light-emitting diodes (wLEDs). However, this phosphor or silicone resin mixture possesses a fatal defect of an intrinsic low thermal performance, which makes it hardly useful in high-power laser lighting and displays. Here, to improve the thermal performance of the CASN:Eu2+ phosphor, a series of red-emitting phosphor-in-glass materials (R-PiGs) with different CASN:Eu2+ concentrations (5–50 wt%) and sintering temperatures (750–850 °C) were successfully synthesized by dispersing the CASN:Eu2+ phosphor into a B2O3-Na2O-SiO2-CaO glass matrix. Surprisingly, the prepared R-PiGs showed a high external quantum efficiency (EQE) of 53%, and still maintained 82% efficiency compared with the CASN:Eu2+ phosphor (65%). The R-PiGs are superior to red ceramics for low cost and rapid batch preparation. Considering the densification of the R-PiGs, a systematic study was conducted on 10 wt% R-PiG sample. A facile chromaticity tuning for the configured wLEDs was obtained within the color range change from cool white to warm white when using the R-PiGs with different CASN:Eu2+ concentrations. Besides, we have successfully assembled a warm wLED with high luminous efficacy (90.3 lm/W) and color rendering (Ra = 94.2 and CCT = 5053 K). When irradiated by a blue laser, an optimized R-PiG can show enhanced luminous flux (49.27 lm) at a high laser incident power of 1.90 W/mm2. Therefore, with their excellent performances, R-PiGs are highly promising for use as red color converters for potential applications in tunable wLEDs and high-power laser lighting.

Development of a wide-width raster scanning optical system for high-resolution ultraviolet laser direct micro-patterning

We developed F-theta raster scanning optics for high-resolution ultraviolet (UV) beam direct micro-patterning, capable of machining on a 300-mm wide image with a confined beam spot of less than 20 μm, a 13.5% spot size. To achieve the imaging performance with resolution, F-theta optics based on synthetic silica glass were designed using a high-power laser light source with a wavelength of 355 nm. In addition, to improve the difficulty of optical alignment in conventional systems and the scan linearity between the scan angle and exposing position, F-theta scanning optics were designed to contain two lenses with a rotationally asymmetric high-order polynomial surface. The designed optical system showed a performance of 20 ± 2 μm at the full width of a 1/e2 maximum intensity value through a 0.8 mm focal depth. The scanning linearity error and root-mean-square (RMS) wavefront error were designed to be less than 0.2% and 0.05 λ, respectively. The design performance was verified using optical characteristic measurements and actual patterning experiments with the optics composed of proto lenses

Design of a CaAlSiN3:Eu/glass composite film: Facile synthesis, high saturation-threshold and application in high-power laser lighting

Even though CaAlSiN3:Eu2+ (CASN) is, in many regards, a highly suitable red phosphor that can be used in white light-emitting diodes, it can hardly be used in high-power laser-lighting because of its low saturation-threshold. By using CASN-based composite ceramics, it is possible to increase the threshold but new difficulties appear. These include complex and expensive synthesis, while the saturation-threshold still has room for improvement. In this study, we prepare a CASN/glass composite film, using an industry-friendly blade-coating method. The film has a high internal quantum efficiency of 79%, which suggests low conversion loss. Under 1.17 W blue laser excitation, a high luminous efficacy of 21.0 lm/W can be obtained. More importantly, the composite film shows a record-high saturation-threshold of more than 12.7 W (∼320 W/cm2) blue laser excitation. With these outstanding properties, CASN/glass composite films may open doors towards commercially viable red color converters for high-power laser-lighting applications.

Comparative study of Al2O3-YAG:Ce composite ceramic and single crystal YAG:Ce phosphors for high-power laser lighting

Recent studies revealed that both the Al2O3–Y3Al5O12:Ce (YAG:Ce) composite ceramic (YAG-CC) and single crystalline YAG:Ce (YAG-SC) possess very high luminescence-saturation thresholds (LSTs), which makes them promising for laser lighting applications. The present study is aimed to conduct a comparative investigation between YAG-CC and YAG-SC on aspects of the laser-pumped luminescence and the associated thermal behavior. The colorimetric and photometric properties (e.g. luminous flux/efficacy, correlated color temperature, and color rendering index) were investigated systematically, by conducting the measurements using a custom-built sphere-spectroradiometer system in the transmission mode. It was found that YAG-SC has a superior LST and it could maintain a high luminous efficacy (106 lm/W) under the irradiation of a 15.33 W blue laser, while the YAG-CC saturated at around 7.76 W. Moreover, by combining the thermal imaging data with the spectral variation of the YAG-CC, novel experimental insights into the origin of luminescence saturation were revealed and proposed as the “thermally-induced absorption saturation”.