Combined system of synchrotron radiation and laser for solid-state research.

Studies have been taken into the breakdown voltage jitter of corona-stabilized switch at high repetition rate up to several kHz. It is thought that the gas recovery affect the jitter of breakdown voltage directly at high repetition rate, because the gas switch will breakdown at a low voltage if the gas cannot fully recover. The breakdown voltage jitter is called voltage jitter in the following paper. However, based on the experimental results, the voltage jitter still exists at low repetition rate, even at single-shot mode. This is not explained by the previous studies. In this paper, voltage jitter below 100 Hz of a corona-stabilized switch is investigated experimentally. A corona-stabilized switch is introduced, using a cylindrical electrode as the cathode and a plane electrode as the anode. The gas used between the electrodes is SF6. A negative pulse with a maximum width of $30~\mu \text{s}$ is used to charge this switch. The tested factors include gas pressure, gap space, repetition rate, and switch geometry. Experimental results show that voltage jitter is mainly affected by the gas pressure and switch geometry. It is seen that voltage jitter decreases as gas pressure increases and a suitable cathode geometry presents a low voltage jitter. Critical volume is a region close to the highly stressed electrode, where the presence of an electron will lead to an electron avalanche which attains a critical size and hence leads to a stabilizing corona. Critical volume is introduced to explain the experimental results. It is found that less voltage jitter can be achieved with a smaller and more concentrated critical volume, as well as efficient corona stabilization. The results indicate that voltage jitter may be reduced by optimize the critical volume at low repetition rate.

This work aims to enhance the precision and safety of refractive surgery by developing a compact, cost-effective femtosecond laser system with a high repetition rate. The motivation behind this research stems from the need to improve surgical accuracy, which is hypothesized to be achievable through higher laser repetition rates, a feature characteristic of compact resonators. In line with this, a low-energy Kerr-lens mode-locked Ti:Sapphire laser oscillator was developed, operating at a 1 GHz repetition rate. This laser, utilizing a bow-tie cavity with dispersioncompensating mirrors and a Peltier temperature regulation system, achieved an average output power of 0.94 W at 7.4 W of pump power, with a broadband emission centered at 756 nm, generating low-energy pulses of 1 nJ with a temporal width of 61 fs and a peak power of 15 kW. To evaluate the application of the laser in refractive surgery, an in-depth analysis of laser-PMMA interactions was conducted using Polymethyl methacrylate (PMMA) as a corneal surrogate. This analysis aimed to elucidate the laser-induced ablation essential parameters for surgical applications. The characterization of the surgical process was performed using commercially available laser for refractive surgery to better understand the processing method. The study involved micromachining techniques characterized by surface and volume analysis, conducted with two different femtosecond lasers, examining varying combinations of pulse overlap and energy levels to determine the threshold pulse energy required for damage. The results revealed that higher repetition rates with low-energy pulses produced more precise damage compared to high-energy, low repetition rate pulses. These findings indicate that compact, high repetition rate femtosecond lasers with low-energy pulses offer substantial potential for improving the precision of refractive surgeries, while also facilitating equipment miniaturization and cost reduction.

Changes in surface morphology and ablation rate induced on sapphire were investigated after interaction with femtosecond laser pulses in air at variable fluence (2 to 77 J cm−2) and repetition rate (10 to 1000 Hz). Multiple laser pulses at a wavelength of 785 nm and pulse width of 130 fs were fired at the surface of sapphire to produce craters whose depth, size and morphology were evaluated using optical and scanning electron microscopy. Ablation rate was found to depend on laser fluence, number of laser pulses and repetition rate. A rapid increase in ablation rate with fluence was observed for fluences lower than 5.9 J cm−2, followed by a slow increase up to fluence of 40.7 J cm−2. A drop in ablation rate occurred at fluence greater than 40.7 J cm−2. Craters produced at high repetition rate (1000 Hz) at fluence of 11.8 J cm−2 were deeper than those produced at low repetition rate (10 Hz) during the first 40 to 50 pulses. The situation was reversed for craters produced by greater than 50 laser pulses. The drop in ablation rate observed at high fluence and repetition rate can be attributed to attenuation of the laser energy due to plasma and particle shielding that result from interactions with the laser-generated particles that cannot be completely removed from the ablated crater. Defocusing effects associated with the non-equilibrium ionization of air which causes a divergence to the laser beam and consequently a reduction in the laser intensity at the sample surface can be another reason for the observed drop in the ablation rate at high fluence.

Dental lasers represent a promising therapeutic tool in the treatment of periodontal and peri-implant diseases. However, their clinical application remains still limited. Here, we investigated the potential biostimulatory effect of low pulse energy neodymium:yttrium-aluminum-garnet (Nd:YAG) laser irradiation on different cells representative of the oral microenvironment and elucidated the underlying molecular mechanisms. Saos-2 osteoblasts, H-end endothelial cells, and NIH/3T3 fibroblasts pre-treated or not with photosensitizing dye methylene blue (MB), were irradiated with low pulse energy (20 mJ) and high repetition rate (50-70 Hz) Nd:YAG laser, and evaluated for cell viability and proliferation as well as for the expression of specific differentiation markers by confocal immunofluorescence and real-time RT-PCR. Changes in intracellular Ca(2+) levels after laser exposure were also evaluated in living osteoblasts. Nd:YAG laser irradiation did not affect cell viability in all the tested cell types, even when combined with pre-treatment with MB, and efficiently stimulated cell growth in the non-sensitized osteoblasts. Moreover, a significant induction in the expression of osteopontin, ALP, and Runx2 in osteoblasts, type I collagen in fibroblasts, and vinculin in endothelial cells could be observed in the irradiated cells. Pre-treatment with MB negatively affected cell differentiation in the unstimulated and laser-stimulated cells. Notably, laser irradiation also caused an increase in the intracellular Ca(2+) in osteoblasts through the activation of TRPC1 ion channels. Moreover, the pharmacologic or genetic inhibition of these channels strongly attenuated laser-induced osteopontin expression, suggesting a role for the laser-mediated Ca(2+) influx in regulating osteoblast differentiation. Low pulse energy and high repetition rate Nd:YAG laser irradiation may exert a biostimulative effect on different cells representative of the oral microenvironment, particularly osteoblasts. Pre-treatment with MB prior to irradiation hampers this effect and limits the potential clinical application of photosensitizing dyes in dental practice.

Cell oxygenation and nutrition are crucial for the viability of tissue-engineered constructs, and different alternatives are currently being developed to achieve an adequate vascularisation of the engineered tissue. One of the alternatives is the generation of channel-like patterns in a bioconstruct. Here, the formation of full-formed channels inside hydrogels by laser-induced cavitation was investigated. A near-infrared, femtosecond laser beam focused with a high numerical aperture was employed to obtain intra-volume modifications of a block of gelatine hydrogel. Characterisation of the laser-processed gelatine was carried out by optical microscopy and epifluorescence microscopy right after and 24h after the laser process. Rheology analyses on the unprocessed gelatine blocks were conducted to better understand the cavitation mechanism taking place during the intense laser interaction. Different cavitation patterns were observed at varying dose values by changing the repetition rate and the overlap between successive pulses while keeping the laser fluence and the number of passes fixed. This way, cavitation bubble features and behaviour can be controlled to optimise the formation of intra-volume channels in the gelatine volume. Results showed that the generation of fully formed channels was linked to the formation of large non-spherical cavitation bubbles during the laser interaction at high dose and low repetition rates. In conclusion, the formation of fully formed channels was made possible with a near-infrared, femtosecond laser beam strongly focused inside gelatine hydrogel blocks through laser-induced cavitation at high dose and low repetition rates.

Reduction of laser-induced pathologic tissue injury using pulsed energy delivery

High harmonic generation (HHG) in gas occurs when atoms are excited by an intense laser field. This technique enables to generate short coherent and directional extreme UV (XUV) radiation. Due to these unique properties, HHG presents a promising source for many applications such as time-dependent XUV spectroscopy. So far, mainly high energy systems such as Ti-sapphire laser amplifier have been used but are associated with a low repetition rate (a demonstration has been still reported at 100 kHz [1]). However, some applications require detecting in practice one event per 10 shots or less [2]. With low repetition rate systems, acquisition times may become unacceptable for recording exploitable statistics. Fiber laser-based HHG technology offers now the possibility to drive such experiments at high repetition rate. Indeed, it has been demonstrated that Yb-doped fiber chirped-pulse amplification (FCPA) systems can deliver both high average power and high energy subpicosecond laser pulses at higher repetition rate (100 kHz – 1 MHz) with good enough characterization for HHG [3,4]. Very high harmonics were recently observed despite moderate conversion efficiency [5].

Thermal optics distortion of face pump slab lasers with two pump arrangements

In this paper, a novel all-solid-state Marx generator (SSMG) is proposed that offers fast recharge times and thus high repetition rates with little impact on the output rise time. The topology consists of an inverted buck converter combined with a Marx generator. The proposed topology is mostly suitable for the generation of pulses with low pulse width and high repetition rates since the inductor of the buck charger needs to block the full output pulse voltage. It has been shown that for air purification applications, pulses with shorter rise times result in higher yields. Therefore, the proposed generator is optimized for short output rise time through accurate stage timing and utilization of an impedance-matched layout. Finally, a prototype capable of generating <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5 \text{kV}, 100 ~\mathrm{A}$</tex> pulses has been realized to validate the proposed topology. Results show the generator can achieve 10 kHz repetition rate when generating 100 ns full load output pulses with 8.57 ns output voltage rise time.

Precise U[sbnd]Pb dating of grandite garnets by LA-ICP-MS: Assessing ablation behaviors under matrix-matched and non-matrix-matched conditions and applications to various skarn deposits

The thermal accumulation effect is an important phenomenon for ultrafast laser material processing and was believed to happen only at high repetition rates over MHz. This study discovers that thermal accumulation can occur at low repetition rates in the kHz range in certain materials. It is found the threshold repetition rates required to initiate thermal accumulation are intrinsically determined by the thermal properties of the material and insensitive to ambient conditions. A simple analytical method is developed to predict the thermal accumulation threshold repetition rate, and the prediction agrees well with the experimental results.

The copper halide laser (CHL) is an efficient gas discharge laser that emits short pulses at two discrete wavelengths in the yellow and green spectral region. The laser pulse results from transitions in excited copper atoms. The CHL has produced pulses of up to 10 mJ in a double-pulsed mode at low repetition rates and has been operated at very high repetition rates (8 to over 35 kHz) in a continuously pulsed mode with a maximum average-power of 15 watts. In this paper, the development of the copper halide laser is reviewed along with a brief description of the copper laser operating principles. In the final section, a number of applications of the CHL are identified.

Measurement of Internet Protocol Packet Delay Variation (IPDV) is a relevant issue in real-time video-streaming services. To assess the Quality of Service (QoS) to the end user, IPDV measures are commonly compared to conventional thresholds, whose values depend on the application requirements. However, exceeding these thresholds only for short time intervals and with relatively low repetition rates does not generally imply a significant decrease of the quality perceived by the users. Vice versa, IPDV values not exceeding the admitted threshold but very close to this value for long time intervals and characterized by relatively high repetition rates can significantly affect the quality perceived by the users. Consequently, studying the correlation between IPDV measures and some performance metrics directly related to the quality perceived by the end user is a very useful and challenging issue. Literature concerning with IPDV evaluation does not provide relevant information about the measurement procedure, such as measurement rate, way of collecting the obtained results in a single estimator, time interval during which the estimator is evaluated, and so on. The absence of these important aspects makes the comparison between IPDV measures and the aforementioned thresholds very critical, thus making even more ambiguous the evaluation of the QoS to the end user. In this framework, the aim of this paper is twofold. From one side, it aims at experimentally assessing and correlating the values that some performance metrics assume at different layers of a wide area network (WAN) supporting video-streaming applications for several values of IPDV; from the other side, it aims at deducing helpful and practical hints for designers and technicians, in order to efficiently assess and enhance the performance of a WAN supporting video streaming, in some suitable setup conditions affected by IPDV. The final goal of the research activity is to identify and fine tune an IPDV estimator able to reliably estimate and detect the objective service degradation from the user perspective. The obtained estimator should be sensitive to service degradation that disregards the user's expectations, and at the same time, it should be insensitive to disturbs that do not cause a perceptible service degradation.

Low repetition rate of a mode locked Nd:YAG laser using quadratic polarization switching

An acoustooptic loss modulator made by bonding an X-cut quartz transducer to a fused silica scattering medium offers a practical means for repetitively Q-switching continuously pumped Nd : YAlG lasers at repetition rates up to 50 kHz. The peak power output of a multitransverse mode laser is typically enhanced by a factor of about 500 relative to CW operation at low (≪5 kHz) repetition rates and by larger factors when the transverse mode structure is suitably restricted. At high repetition rates, the peak power becomes smaller and the average power output approaches the CW level. The theoretical Q-switching behavior derived from the rate equations governing an ideal four-level laser is presented in graphical form, and is found to describe the observed behavior well when the laser is restricted to oscillation in the TEM <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</inf> transverse mode. Similar calculations for Q-switched intracavity second harmonic generation indicate that the peak output power available at the second harmonic slightly exceeds that available at the fundamental, and that the harmonic coupling necessary to optimally couple the Q-switched laser is several orders of magnitude smaller than that required for CW intracavity conversion. Experiments using Ba <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> NaNb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</inf> For intracavity Q-switched harmonic generation roughly verified these predictions. A peak power output of 220 watts at 0.532 µ in the TEM <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</inf> mode was achieved. Surface pitting of the nonlinear crystal after a few hours of operation prevented a thorough comparison with the calculated results. Preliminary experiments using the doubled Q-switched Nd : YAlG laser and an ammonia dihydrogen phosphate crystal to generate ultraviolet (the 0.266-µ harmonic) yielded a peak output power of 40 watts.