Optimizing LERP systems: opto-thermal steady-state simulation analysis and experimental validation.

Laser-excited remote phosphor (LERP) systems are the next step in solid-state lighting technology. However, the thermal stability of phosphors has long been a major concern in the reliable operation of these systems. As a result, a simulation strategy is presented here that couples the optical and thermal effects, while the phosphor properties are modeled to temperature. A simulation framework is developed in which the optical and thermal models are defined in Python using appropriate interfaces to commercial software: the ray tracing software Zemax OpticStudio for the optical analysis and the finite element method (FEM) software ANSYS Mechanical for the thermal analysis. Specifically, the steady-state opto-thermal analysis model is introduced and experimentally validated in this study based on Ce:YAG single-crystals with polished and ground surfaces. The reported experimental and simulated peak temperatures are in good agreement for both the polished/ground phosphors in the transmissive and reflective setups. A simulation study is included to demonstrate the simulation's capabilities for optimizing LERP systems.

Midterm clinical outcome of uncemented short-stem reversed shoulder arthroplasty.

INTRODUCTION: While the incidence of reverse total shoulder arthroplasty (rTSA) is increasing constantly, newer implants with designs other than the classic Grammont geometry are gaining importance. More anatomic inclination angles and lateralization are supposed to have a positive impact on clinical results and complication rates. Presentation of midterm results therefore is important to support these assumptions. The aim of this study was to report the midterm clinical outcome of primary rTSA with an uncemented humeral short-stem prosthesis (USSP) with a humeral inclination angle of 145° and the analysis of different variables on the outcome. METHODS: This is a retrospective study of all patients with primary rTSA using an USSP and a combined humeral inclination angle of 145° (Ascend™ flex, Stryker) with a minimum clinical follow-up of 2 years. The implant combines a 132.5° inclination for the humeral stem with an additional 12.5° for the polyethylene inlay. Primary outcomes were patient-reported outcome measures: ASES score, simple shoulder test (SST) and subjective shoulder value (SSV). Secondary outcomes were complication and revision rates. We analyzed different variables: preoperatively gender, age, indication for surgery and status of rotator cuff. Also, the glenoid morphology was classified according to Walch and a proximal humerus cortical bone thickness measurement (CBT avg) of 6 mm was used as a threshold for osteoporosis. Postoperatively, we analyzed different radiologic parameters: filling ratio, distalization and lateralization angles according to Boutsiadis. RESULTS: A total of 84 out of 99 (85%) patients with a mean FU of 46.7 months (range 24-80 months) could be included: 62 women and 22 men with a mean age of 74.7 years. Mean ASES score significantly increased from 47 preoperatively to 85.8 at the last follow-up (p = 0.001). The postoperative SST reached an average of 65.3 and the mean SSV was 83%. None of the variable parameters analyzed could be identified as a risk factor for a lower outcome defined as a SSV < 70. Three patients (3.6%) had a complication: one incomplete lower plexus lesion, one dislocation and one major hematoma. Surgical revision was needed in two cases (2.4%). CONCLUSION: The midterm clinical outcome of primary reverse total shoulder arthroplasty (rTSA) with an uncemented humeral short stem and a humeral inclination angle of 145° showed good-to-excellent results with a low complication and revision rate independent from a wide range of pre- and postoperative variables. PROMs are comparable to those reported for anatomic TSA with a low complication rate, different to historical studies especially with the Grammont design. LEVEL OF EVIDENCE: Treatment study, Level IV.

Passively Q-switched microchip laser based picosecond light source in the visible-red to near-infrared band for semiconductor excitation.

We developed a visible-red to near-infrared wavelength tunable all-solid-state laser system utilizing an optical parametric generation process in a MgO doped PPLN crystal pumped at 532 nm by an amplified and frequency doubled picosecond passively Q-switched Nd:YVO4 microchip laser. A broad bandwidth, tuneable over 300 nm between 710 nm to 1015 nm, is accessible. Depending on the green pump light pulse energy, pulses with durations down to 69 ps as well as pulses with energies above 2 µJ were achieved with kHz repetition rates.

CO2-laser-ablation-assisted fabrication of signal-pump combiners with chirally coupled core fibers for co- and counter-pumped all-fiber amplifiers.

We report on the development of a side-fused signal-pump combiner with an integrated feed-through 34/250-µm chirally coupled core fiber. The manufacturing process involves a novel rotationally symmetrical cladding restructuring using a CO2-laser beam. The signal-pump combiner exhibits the pump-to-signal fiber coupling efficiency of 90%, signal-to-pump isolation of 30 dB, and is high-power tested at a pump input power of >500 W. Additionally, a signal feed-through loss of 0.23 dB is measured and the S2-method is used to confirm non-degradation of the spatial modes. The side-fused combiner technique has the advantage of an uninterrupted signal core and can be used in co- and counter-pumped configurations.

Generation of 15 nJ pulse energy by a sub-150 fs thulium-doped fiber Mamyshev oscillator.

Mamyshev oscillators have pushed the frontiers in output parameters of ytterbium- and erbium-based ultrafast fiber oscillators in the spectral region around 1 µm and 1.5 µm within the last few years tremendously. In order to expand the superior performance toward the 2 µm spectral region, we present in this Letter an experimental investigation of the generation of high-energy pulses by a thulium-doped fiber Mamyshev oscillator. Generating highly energetic pulses is enabled by a tailored redshifted gain spectrum in a highly doped double-clad fiber. The oscillator emits pulses with an energy of up to 15 nJ, which can be compressed to 140 fs.

VOILA on the LUVMI-X Rover: Laser-Induced Breakdown Spectroscopy for the Detection of Volatiles at the Lunar South Pole.

The project Lunar Volatiles Mobile Instrumentation-Extended (LUVMI-X) developed an initial system design as well as payload and mobility breadboards for a small, lightweight rover dedicated for in situ exploration of the lunar south pole. One of the proposed payloads is the Volatiles Identification by Laser Analysis instrument (VOILA), which uses laser-induced breakdown spectroscopy (LIBS) to analyze the elemental composition of the lunar surface with an emphasis on sampling regolith and the detection of hydrogen for the inference of the presence of water. It is designed to analyze targets in front of the rover at variable focus between 300 mm and 500 mm. The spectrometer covers the wavelength range from 350 nm to 790 nm, which includes the hydrogen line at 656.3 nm as well as spectral lines of most major rock-forming elements. We report here the scientific input that fed into the concept and design of the VOILA instrument configuration for the LUVMI-X rover. Moreover, we present the measurements performed with the breadboard laboratory setup for VOILA at DLR Berlin that focused on verifying the performance of the designed LIBS instrument in particular for the detection and quantification of hydrogen and other major rock forming elements in the context of in situ lunar surface analysis.

Low noise 400 W coherently combined single frequency laser beam for next generation gravitational wave detectors.

Design studies for the next generation of interferometric gravitational wave detectors propose the use of low-noise single-frequency high power laser sources at 1064 nm. Fiber amplifiers are a promising design option because of their high output power and excellent optical beam properties. We performed filled-aperture coherent beam combining with independently amplified beams from two low-noise high-power single-frequency fiber amplifiers to further scale the available optical power. An optical power of approximately 400 W with a combining efficiency of more than 93% was achieved. The combined beam contained 370 W of linearly polarized TEM00-mode and was characterized with respect to the application requirements of low relative power noise, relative beam pointing noise, and frequency noise. The noise performance of the combined beam is comparable to the single amplifier noise. This represents, to our knowledge, the highest measured power in the TEM00-mode of single frequency signals that fulfills the low noise requirements of gravitational wave detectors.

Wedged Nd:YVO4 crystal for wavelength tuning of monolithic passively Q-switched picosecond microchip lasers.

We present a monolithic integrated passively Q-switched sub-150 ps microchip laser at 1064 nm with a wedged Nd:YVO4 crystal operating up to a repetition rate of 1 MHz. The wedge enables to change the cavity length by a small amount to fine tune the spectral cavity mode position over the full gain bandwidth of Nd:YVO4 and hence to optimize the output power. This additional degree of freedom may be a suitable approach to increase the wafer scale mass production yield or also to simplify frequency tuning of CW single-frequency microchip lasers.

Mode-locked pulses from a Thulium-doped fiber Mamyshev oscillator.

We present experimental results of the generation of ultrashort pulses in the 2 µm wavelength region by a fiber Mamyshev oscillator, along with the simulation of the pulse propagation in the cavity. The Mamyshev oscillator emitted pulses with energies of 3.55 nJ at a repetition rate of 15 MHz and optical spectra with bandwidths of 48 nm. The pulses propagated in anomalous dispersive Thulium-doped fiber sections with dispersion compensation sections of normal dispersive fibers.

High-repetition rate, mid-infrared, picosecond pulse generation with µJ-energies based on OPG/OPA schemes in 2-µm-pumped ZnGeP2.

We report on two different concepts to generate µJ-level mid-infrared laser pulses with sub-10 ps pulse duration via nonlinear parametric generation at high pulse repetition rates. Both schemes rely on the recent development of compact and efficient CPA-free, Ho:YLF-based 2-µm laser sources pumping the highly nonlinear crystal ZnGeP2 used for parametric amplification. The first concept comprises a simplified OPG/OPA scheme efficiently producing signal and idler radiation at fixed wavelengths of 3.0 and 6.5 µm, respectively. In the second concept, we demonstrate a wide spectral tunability of the picosecond, µJ pulses over the entire tuning range between 2.5 and 12 µm maintaining a constant bandwidth of sub-20 cm-1 by integrating a two-color pumping scheme and a spectral shaper into the setup. The presented compact and efficient mid-IR picosecond radiation sources offer a way towards low-cost mid-IR material processing and spectroscopic applications.

Single-frequency chirally coupled-core all-fiber amplifier with 100 W in a linearly polarized TEM00 mode.

Large mode area fibers have become indispensable in addressing the power requirements of laser sources in gravitational wave detectors. Besides high power capabilities, the system must provide an excellent beam quality and polarization. In this Letter, we present the characterization of a monolithic high-power fiber amplifier at 1064 nm, built using an ytterbium-doped chirally coupled-core fiber, which achieves an output power of 100 W in a linearly polarized $ {{\rm TEM}_{00}} $TEM00 mode in an all-fiber setup.

Performance study of a high-power single-frequency fiber amplifier architecture for gravitational wave detectors.

The next generation of interferometric gravitational wave detectors will use low-noise single-frequency laser sources at 1064 nm. Fiber amplifiers are a promising design option because of high efficiency, compact design, and superior optical beam properties compared to the current generation of laser sources for gravitational wave detectors. We developed a reliable 200 W single-frequency fiber amplifier architecture to meet the application requirements regarding relative power noise, relative pointing noise, frequency noise, linear polarization, and beam quality. We characterized several of these amplifiers and discuss performance variations resulting from manufacturing tolerances and variations in amplifier architecture. This study serves as a baseline for further power scaling via e.g., coherent beam combining experiments.

High power, single-frequency, monolithic fiber amplifier for the next generation of gravitational wave detectors.

Low noise, high power single-frequency lasers and amplifiers are key components of interferometric gravitational wave detectors. One way to increase the detector sensitivity is to increase the power injected into the interferometers. We developed a fiber amplifier engineering prototype with a pump power limited output power of 200 W at 1064 nm. No signs of stimulated Brillouin scattering are observed at 200 W. At the maximum output power the polarization extinction ratio is above 19 dB and the fractional power in the fundamental transverse mode (TEM 00) was measured to be 94.8 %. In addition, measurements of the frequency noise, relative power noise, and relative pointing noise were performed and demonstrate excellent low noise properties over the entire output power slope. In the context of single-frequency fiber amplifiers, the measured relative pointing noise below 100 Hz and the higher order mode content is, to the best of our knowledge, at 200 W the lowest ever measured. A long-term test of more than 695 h demonstrated stable operation without beam quality degradation. It is also the longest single-frequency fiber amplifier operation at 200 W ever reported.

Sub-50 fs, µJ-level pulses from a Mamyshev oscillator-amplifier system.

In this Letter, we present a Mamyshev oscillator with an output power of 332 mW and a pulse energy of 31 nJ, which is directly amplified in an additional fiber section. By balancing the gain-narrowing effect with self-phase modulation during the amplification, we achieved an average output power of 11 W, corresponding to a pulse energy of more than 1 µJ. The pulse duration can be compressed to sub-50 fs behind the amplification stage.

Grating assisted glass fiber coupler for mode selective co-directional coupling.

To keep pace with the increasing demand of transmission capacity, space division multiplexing technologies are currently intensively investigated. In this context, mode selective glass fiber couplers are of great interest due to their compatibility with existing glass fiber networks. In this work, we present a novel type of mode selective glass fiber coupler for co-directional coupling based on fiber gratings and fused asymmetric fibers. The achieved mode selective coupling efficiency agrees well with numerical simulations performed for comparison. The benefits of the grating approach are a lower mode crosstalk and a simple adaption of the propagation constants through changing of the grating-period.

High repetition rate, µJ-level, CPA-free ultrashort pulse multipass amplifier based on Ho:YLF.

We report on CPA-free multipass amplification of ps-pulses in Holmium-doped yttrium lithium fluoride (Ho:YLF) crystals up to µJ pulse-energy-covering repetition rates from 10 kHz up to 500 kHz. The seed pulses at a wavelength of 2.05 µm are provided by a Ho-based all-fiber system consisting of a soliton oscillator and a subsequent pre-amplifier followed by a free-space AOM as pulse-picker. Considering the achieved pulse peak power at MW-level, this system is a powerful tool for efficient pumping of parametric conversion stages addressing the highly demanded mid-IR spectral region.

Pump wavelength dependence of ASE and SBS in single-frequency EYDFAs.

In this Letter, the pump wavelength dependence of the amplified spontaneous emission (ASE) and the threshold of stimulated Brillouin scattering (SBS) in a typical single-frequency continuous wave Er3+:Yb3+-codoped fiber amplifier is investigated numerically. The Er3+:Yb3+ system is modeled as coupled two- and three-level systems, linked by a Förster resonance energy transfer process and described by the corresponding rate equations. The evolution of the pump and signal power along the fiber is modeled by differential equations, taking into account the steady-state population densities. Since the absorption at 976 nm can exceed the Yb3+-to-Er3+ energy transfer rate in high-power operation, unsaturated gain at around 1.0 µm can generate excessive ASE. Off-peak pumping with commercially available pump diodes at 915 or 940 nm spatially distributes the energy over a longer distance. For the studied amplifier configuration, energy transfer bottlenecking is prevented without the onset of SBS.

All-fiber, single-frequency, and single-mode Er3+:Yb3+ fiber amplifier at 1556 nm core-pumped at 1018 nm.

Emerging applications, such as gravitational wave astronomy, demand single-frequency lasers with diffraction-limited emission at 1.5 µm. Fiber amplifiers have greatly evolved to fulfill these requirements. Hundreds of watts are feasible using large-mode-area and specialty fibers. However, their application in a few watts to tens of watts in monolithic systems is unnecessarily complex due to the poor commercial availability of fiber components and standard integration procedures. In this Letter we propose and experimentally demonstrate a novel and simple method to amplify single-frequency signals at 1.5 µm up to tens of watts by core-pumping single-mode Er3+:Yb3+ fiber amplifiers at 1018 nm. The proof-of-principle system is tested with different active fibers, lengths, and seed power levels. Over 11 W with an efficiency of more than 48% versus launched power is achieved. Additionally, performance degradation during operation was observed for which photodarkening due to P1 defects might be an explanation.

Millijoule-level, kilohertz-rate, CPA-free linear amplifier for 2 µm ultrashort laser pulses.

The generation of millijoule-level ultrashort laser pulses at a wavelength of 2.05 µm in a compact chirped pulse amplification-free linear amplifier based on Holmium-doped YLF gain medium is presented. More than 100 MW of pulse peak power has been achieved. We show the capabilities of this laser amplifier from a 1 kHz to 100 kHz repetition rate. A detailed numerical description supports the experimental work and verifies the achieved results.

Microstructured fiber cladding light stripper for kilowatt-class laser systems.

An all-glass microstructured high-power cladding mode stripper capable of handling cladding light of up to a power of approximately 350 W with stripping efficiencies >22 dB is presented. An optimized graded structure pattern increased the device's reliability and its power-handling capabilities. Subjected to a 500 h stress test, the device shows no degradation.