Average Power Scaling Research Articles

Scaling of Average Power in All-Fiber Side-Pumped Sub-MW Peak Power ps-Pulses Yb-Doped Tapered Amplifier

Scaling of Average Power in All-Fiber Side-Pumped Sub-MW Peak Power ps-Pulses Yb-Doped Tapered Amplifier

Intelligent Laser Emitting and Mode Locking of Solid‐State Lasers Using Human‐Like Algorithms

AbstractSolid‐state lasers based on mode‐locked technology are widely studied for their strong ability of average or peak power scaling and wide wavelength coverage. However, it usually takes a long time and a lot of effort to manually align an ultrafast solid‐state laser to achieve laser emission and stable mode‐locking. Here, an approach based on intelligent human‐like algorithms is proposed for aligning a solid‐state ultrafast laser system. The intelligent system is based on multi‐algorithm fusion and can fully simulate the process of observation, analysis, decision, and action of an experienced experimenter in the adjustment of the laser alignment and mode‐locking. The intelligent adjustment starts from the state of no laser emission, adopts the neural network, the modified augmented random search (ARS) algorithm, random search, and sliding window strategy, and takes the fluorescence and speckle patterns as indications to realize the laser emission and stable mode‐locking automatically. Several validation experiments are conducted using this intelligent system, and the stable mode‐locked pulses can be achieved within 40 s. This technology provides an efficient solution to the ultrafast solid‐state laser that requires full automatic laser emitting and stable mode‐locking for the first time.

Laser Method for Studying Temperature Distribution within Yb:YAG Active Elements

Currently, laser systems based on active elements doped with Yb3+ with simultaneously high pulse repetition rates and high peak power are in demand for many applications. High thermal load of active elements is the primary limiting factor for average power scaling. Experimental investigation of temperature distribution in active elements is of particular importance for estimation of cooling efficiency and for thermal processes’ monitoring. In the present work, the method of dynamic laser thermometry is proposed for temperature distribution investigation within cryogenically cooled Yb3+-doped active elements. The method is based on the dependence of the Yb3+ ion absorption cross-section on temperature at a wavelength of 1030 nm. The method was tested to study the 2D temperature map of the Yb:YAG active element of the high-power, diode-pumped, cryogenically cooled laser amplifier. The best measurement accuracy ±3 K is achieved at the maximal temperature 176 K. The results of numerical simulation are in good agreement with the experimental data. On the basis of the investigation, the quality of the cooling system is evaluated. The advantages and other possible applications of the method are discussed.

8.7-W average power, in-band pumped femtosecond Ho:CALGO laser at 2.1 µm.

We report on an in-band pumped SESAM mode-locked Ho:CALGO bulk laser with a record-high average power of 8.7 W and an optical-to-optical efficiency of 38.2% at a central wavelength of 2.1 µm. At this power level, the bulk laser generates pulses with a duration of 369 fs at an 84.4-MHz repetition rate, corresponding to a pulse energy of 103 nJ and a peak power of 246 kW. To the best of our knowledge, this is the highest average power and pulse energy directly generated from a mode-locked bulk laser in the 2-3 µm wavelength region. Our current results indicate that Ho:CALGO is a competitive candidate for average power scaling of 2-µm femtosecond lasers.

Influence of Gain Saturation Effect on Transverse Mode Instability Considering Four-Wave Mixing

Transverse mode instability (TMI) has been recognized as onse of the primary limiting factors for the average power scaling of high-brightness fiber lasers. In this work, a static model of the TMI effect based on stimulated thermal Rayleigh scattering (STRS) is established while considering the four-wave mixing (FWM) effect. The focus of the model is to theoretically investigate the TMI phenomenon and threshold power dominated by FWM. The gain saturation effect and fiber laser system parameters, such as seed power, pumping direction, and core numerical aperture, which have not been considered in the previous perturbation theory model, are also investigated. This work will enrich the perturbation theory model and extend its application scope in TMI mitigation strategies, providing guidance for understanding and suppressing TMI.

Average power scaling of THz spintronic emitters efficiently cooled in reflection geometry.

Metallic spintronic terahertz (THz) emitters have become well-established for offering ultra-broadband, gapless THz emission in a variety of excitation regimes, in combination with reliable fabrication and excellent scalability. However, so far, their potential for high-average-power excitation to reach strong THz fields at high repetition rates has not been thoroughly investigated. In this article, we explore the power scaling behavior of tri-layer spintronic emitters using an Yb-fiber excitation source, delivering an average power of 18.5 W (7 W incident on the emitter after chopping) at 400 kHz repetition rate, temporally compressed to a pulse duration of 27fs. We confirm that a reflection geometry with back-side cooling is ideally suited for these emitters in the high-average-power excitation regime. In order to understand limiting mechanisms, we disentangle the effects on THz power generation by average power and pulse energy by varying the repetition rate of the laser. Our results show that the conversion efficiency is predominantly determined by the incident fluence in this high-average-power, high-repetition-rate excitation regime if the emitters are efficiently cooled. Using these findings, we optimize the conversion efficiency and reach highest excitation powers in the back-cooled reflection geometry. Our findings provide guidelines for scaling the power of THz radiation emitted by spintronic emitters to the milliwatt-level by using state-of-the-art femtosecond sources with multi-hundred-Watt average power to reach ultra-broadband, strong-field THz sources with high repetition rate.

Milliwatt average power, MHz-repetition rate, broadband THz generation in organic crystal BNA with diamond substrate.

We demonstrate a 13.3 MHz repetition rate, broadband THz source with milliwatt- average power, obtained by collinear optical rectification of a high-power Yb-doped thin-disk laser in the organic crystal BNA (N-benzyl-2-methyl-4-nitroaniline). Our source reaches a maximum THz average power of 0.95 mW with an optical-to-THz efficiency of 4×10-4 and a spectral bandwidth spanning up to 6 THz at -50 dB, driven by 2.4 W average power (after an optical chopper with duty cycle of 10%), 85 fs-pulses. This high average power excitation was possible without damaging the crystal by using a diamond-heatsinked crystal with significantly improved thermal properties. To the best of our knowledge, this result represents the highest THz average power reported so far using the commercially available organic crystal BNA, showing the potential of these crystals for high average power, high repetition rate femtosecond excitation. The combination of high power, high dynamic range, high repetition rate and broadband spectrum makes the demonstrated THz source highly attractive to improve various time-domain spectroscopy applications. Furthermore, we present a first exploration of the thermal behavior of BNA in this excitation regime, showing that thermal effects are the main limitation in average power scaling in these crystals.

Single and combined effect of kinesio tape and warm-up on sprint cycling performance

BackgroundThe fact that kinesio tape may be capable to enhance muscle power would qualify it as practical tool to be considered during passive warm-up (WU) or coupled with active WU processes prior to power-based performance. Therefore, the aim of this study was to investigate the single and combined effect of kinesio tape (KT) and WU on sprint cycling performance.MethodsIn a repeated measure design, fifteen participants underwent six sessions to assess sprint cycling performance involving a combination of three taping conditions (without KT: NoKT; with KT positioned vertically over the thigh muscles KT; with KT positioned horizontally over the thigh muscles: Sham) with two pre-exercise routines (with WU: WU; without WU: NoWU) in a randomized order. Allometric scaling of peak power (PP) and average power (AP) values were considered for each sprint.ResultsKT-WU demonstrated the highest PP and AP with respect to the other conditions (p < 0.05), except for AP that was similar to Sham-WU (p > 0.05). Moreover, NoKT-NoWU showed the lowest PP and AP with respect to the other conditions (p < 0.05).ConclusionsOverall, our findings suggest that kinesio tape might be a possible tool to be combined with an active WU routine, inducing benefit on sprint performance. Moreover, KT may be considered a potential strategy to include within a passive WU, perhaps where an active WU is not feasible. However, as the influence of KT on muscle function is still unclear, our results should not be overinterpreted.

Scaling of average power in sub-MW peak power Yb-doped tapered fiber picosecond pulse amplifiers.

Prospects for average power scaling of sub-MW output peak power picosecond fiber lasers by utilization of a Yb-doped tapered fiber at the final amplification stage were studied. In this paper, it was shown experimentally that a tapered fiber allows the achievement of an average power level of 150 W (limited by the available pump power) with a peak power of 0.74 MW for 22 ps pulses with no signs of transverse mode instability. Measurements of the mode content using the S2 technique showed a negligible level of high order modes (less than 0.3%) in the output radiation even for the maximum output power level. Our reliability tests predict no thermal issues during long-term operation (105 hours) of the developed tapered fiber laser up to kilowatt output average power levels.

Intensity noise as a driver for transverse mode instability in fiber amplifiers

The effect of transverse mode instability (TMI) is currently the main limitation for the further average-power scaling of fiber laser systems with diffraction-limited beam quality. In this work a main driving force for TMI in fiber amplifiers is identified. Our experiments and simulations illustrate that the performance of fiber laser systems in terms of their diffraction-limited output power can be significantly reduced when the pump or seed radiation exhibit intensity noise. This finding emphasizes the fact that the TMI threshold is not only determined by the active fiber but, rather, by the whole system. In the experiment an artificially applied pump intensity-noise of 2.9% led to a reduction of the TMI threshold of 63%, whereas a similar seed intensity-noise decreased it by just 13%. Thus, even though both noise sources have an impact on the TMI threshold, the pump intensity-noise can be considered as the main driver for TMI in saturated fiber amplifiers. Additionally, the work unveils that the physical origin of this behavior is linked to the noise transfer function in saturated fiber amplifiers. With the gained knowledge and the experimental and theoretical results, it can be concluded that a suppression of pump-noise frequencies below 20 kHz could strongly increase the TMI threshold in high-power fiber laser systems.

High gain in a dual-pass rod-type fiber amplifier

Single-pass amplification using rod-type fibers has become a common route to pulsed laser sources around 1030 nm with high average and peak power. Average-power scaling is currently limited by the dynamic thermo-optic phenomenon of “transverse mode instability.” In comparison, double-pass amplifier configurations have not been extensively studied. Recent theoretical and experimental work has shown both static and dynamic mode degradation phenomena, including an unexpected nonlinear polarization rotation effect. Here we present new results obtained with a modified setup using polarization filtering between the first and the second pass. We obtain up to 113 W output power, i.e., more than 40 dB of amplification from a single amplifier module seeded by 10 mW of 20 ps/20 MHz/1030 nm pulses. We observe excellent beam quality and polarization extinction ratio. Finally, we investigate a wide range of seed powers and report a strong increase in the static mode deformation threshold with decreasing seed power. The experimental results are corroborated by numerical simulations.

Security-aware dual-hop communication for amplify-and-forward relay networks

The authentication scheme for vehicular ad hoc networks aims to improve the security and integrity of message delivery. The base station manages a large number of vehicular nodes, so the security communications are non-trivial. In this article, we propose an amplify-and-forward strategy for a dual-hop cooperative network in order to improve secure communications for vehicular ad hoc networks. We assume that each vehicular node equipped with a single antenna and derive closed-form expressions for the secure communication rate calculation. Moreover, we propose a cooperative strategy by jointly considering average power scaling and instantaneous power scaling, which are proved to be able to achieve information security. The simulation result shows that the proposed scheme can achieve better performance in scenarios with different signal-to-noise ratio.

100 W high-repetition-rate near-infrared optical parametric chirped pulse amplifier.

New high-repetition-rate x-ray free electron lasers (XFELs) require for their operation highly reliable ultrafast laser systems with high pulse energy, high repetition rate, and high average power. In this Letter, we present high-average-power scaling of near-infrared optical parametric chirped pulse amplification (OPCPA) in potassium titanyl arsenate (KTA) with tunable center wavelengths from 1.5 to 2.0μm. Using a three-stage amplification scheme and a kW-level InnoSlab Yb:YAG pump amplifier for the final non-collinear KTA stage, we demonstrate an amplified output power of 106.2W at a center wavelength of 1.75μm and 100kHz. Idler absorption introduces a potential upper limit on the average power scaling of center wavelengths <1.70 μm. Future scaling of average power to hundreds of Watts is possible at center wavelengths ≥1.70 μm.

Short-Term Delayed Effects of Kinesio Taping on Sprint Cycling Performance.

Trecroci, A, Formenti, D, Rossi, A, Esposito, F, and Alberti, G. Short-term delayed effects of Kinesio taping on sprint cycling performance. J Strength Cond Res 33(5): 1232-1236, 2019-The aim of this study was to assess short-term delayed effects of Kinesio taping (KT) on maximal cycling performance. A design with repeated measures was used. Fifteen healthy trained subjects underwent 3 conditions: no KT (without tape), KT (Kinesio tape applied longitudinally on the thighs), and sham taping (Kinesio tape applied horizontally on the thighs). Each subject performed 2 sets of 3 × 6-second sprints (separated by 3 minutes) interspersed by 30 minutes of rest on a cycle ergometer. Allometric scaling of peak power (PP) and average power (AP) values were computed and analyzed for each sprint performance. Both PP and AP decreased significantly after 30 minutes (between set 1 and set 2) in all conditions, with a greater decrease in no KT condition compared with KT and sham (p < 0.01). No significant differences were observed between KT and sham conditions. The application of KT on thigh muscles attenuated the performance decrease that occurred after 30 minutes of rest between the 2 sets compared with the no KT condition. This finding suggests athletes may use KT to better manage their performance during delays in competition events.

Peak power & average power scaling via fourier domain OPA (FOPA)

FoTaking advantage of pulse shortening upon amplification, we demonstrate 2.5TW pulses (30mJ, 2 cycle, 1.8 μm) based on TiSa pumping, while for boosting average power we utilize a 500W Yb platform for pump and seed pulses.

High repetition rate nanosecond optical parametric oscillator pumped by the third harmonic of a DPSSL

A high repetition rate (5 kHz), nanosecond, type-I phase matched β-BaB2O4 (BBO) crystal-based optical parametric oscillator (OPO), pumped by the third harmonic (355 nm) of a diode-pumped solid state laser, is demonstrated for the first time. The high threshold pump pulse energy, which impedes the high repetition rate operation of OPO, is reduced to ≤ 1 mJ using cylindrical focusing of the pump beam. The performance of the OPO is compared for different gain lengths of the type-I BBO crystal and the optimum length of the crystal for efficient operation of the OPO is determined. We have obtained a stable output power of ~ 2.6 W at a signal wavelength of 492 nm, and at a conversion efficiency of 26%, when operated with pump laser at 5 kHz. Furthermore, the narrowband (~ 0.15 nm) operation of the OPO at a repetition rate of 2 kHz was achieved by employing type-II phase-matched BBO crystal in double pass pump beam configuration. Thermal loading in the BBO crystal arising due to high repetition rate operation was studied by measuring the temperature of the crystal for determining the average power scaling limit of visible nanosecond OPOs.

Performance Analysis of Hybrid FSO Systems Using FSO/RF-FSO Link Adaptation

Free-space optical (FSO) links are considered as cost-effective, noninvasive alternative to fiber optic cables for 5G cellular backhaul networking. For FSO-based backhaul networks, we propose an additional millimeter-wavelength (MMW) radio-frequency (RF)-FSO link, used as a backup. Uninterrupted and reliable network connection is possible by switching between primary FSO link and the secondary RF-FSO link; when the primary link is under atmospheric turbulence, the secondary link maintains connectivity as the MMW RF link exhibits complementary characteristics to atmospheric effects. In order to analytically assess the improvement, we also derive concise mathematical expressions for different performance metrics, such as outage probability, average bit error rate (BER), and capacity. Our results demonstrate that the FSO/RF-FSO topology performs better than a single FSO link in terms of outage probability and BER. The dual-hop mixed RF-FSO link is realized with an amplify and forward (AF) relay that adapts an average power scaling strategy. The irradiance fluctuations in the FSO links are modeled by gamma-gamma distribution, assuming strong atmospheric turbulence while it is assumed that the RF link experiences multipath Rayleigh fading. For switching between links, a single FSO threshold is considered first, followed by a dual FSO threshold to prevent unnecessary switching.

High average power coherent femtosecond pulse combining system based on an all fiber active control method

We experimentally demonstrate a high average power coherent polarization beam combining system of two-channel femtosecond fiber chirped-pulse amplifiers based on our all fiber active control method comprising an active phase locking loop and adaptive control loop of optical path drift for the first time. A coherent combining efficiency of 87% is achieved at the combined power of 4.3 W, which drops with the average power scaling. The combined average power finally reaches 313 W with a coherent combining efficiency of 79% and the corresponding compressed pulse duration is 827 fs. The reasons for the loss in combining efficiency are experimentally investigated. Further power scaling is restricted by the deterioration of the laser mode, but it is still possible if the performance of the single femtosecond fiber chirped-pulse amplifier is improved.

Design of High Average Power OPCPA Based on Simultaneous Temperature and Wavelength Insensitive Phase Matching

Optical parametric chirped-pulse amplification (OPCPA) provides an efficient route to push ultrafast pulses toward ultrahigh peak powers. However, in high average power regime, the unavoidable thermal effects in nonlinear crystals will intrinsically destroy the phase-matching (PM) condition, which fundamentally limit average power scaling. Here, we present a theoretical design of high average power OPCPA based on simultaneous temperature and wavelength insensitive PM. By regulating the temperature set for PM as well as the wavelength of interacting waves in lithium triborate crystal, the noncollinear angles for temperature insensitive PM and wavelength insensitive PM will coincide and thus ensures simultaneous temperature and wavelength insensitive PM condition. We investigate the performances of the proposed PM scheme in comparison with traditional wavelength insensitive noncollinear PM. Because of the larger temperature bandwidth, higher average power can be anticipated in OPCPA with the proposed PM scheme. Besides, the large spectral bandwidth also allows the amplification of sub-10 fs pulses in the visible spectral range. The proposed PM scheme will provide a promising approach in generating ultrashort pulses with high peak and average powers.

Thermal analysis of Yb-doped high-power fiber amplifiers with Al:P co-doped cores.

It has been recently shown that photodarkening can significantly reduce the mode instability threshold in high power Yb-doped fiber amplifiers, thus resulting in an even more severe limitation to the scaling of the output average power of these systems. Therefore, an efficient reduction of photodarkening in an Yb-doped active fiber will lead to very significant gains in the output average power delivered by such systems. In this context, it has been reported that photodarkening can be significantly mitigated when co-doping a fiber core with Al and P, which makes this approach potentially appealing to increase the TMI threshold. Unfortunately co-doping the fiber core with Al and P also alters the effective cross-sections of the fiber, which has repercussion in the amplification efficiency. Thus, a fiber with a higher P concentration will exhibit lower cross-sections, therefore requiring a higher Yb-ion concentration to reach a certain desired amplification efficiency. However, increasing the Yb-ion concentration leads to higher photodarkening losses, which might potentially counteract the benefits of using P co-doping. In this paper we present a comparative analysis of the expected performance of different fiber amplifiers for a given constant average heat-load and amplification efficiency as a function of the ratio of Al:P concentration in the fiber core. This study indicates which core compositions are more beneficial for increasing the mode instability threshold in Yb-doped high-power fiber amplifier systems.