Pulsed Light Source Research Articles

Wavelet optical flow velocimetry of a scramjet combustor using high-speed frame-straddling focusing schlieren images

Scramjet combustors feature ultra-high-speed turbulent reacting flows with high temperatures and intense luminescence. Measurement of velocity fields under such extreme conditions presents great challenges. The present work demonstrated a seedless velocimetry approach using focusing schlieren images (FSIs) of high spatiotemporal resolution in a scramjet engine. Two fuel mass flow rates (Case1 and Case2) were investigated with corresponding global equivalence ratios of 0.27 and 0.13, respectively. The FSIs enabled by the employment of a high-speed pulsed LED light source are characterized by an effective exposure of 100 ns, and a 500-ns frame-straddling time interval with full resolution of 1280 × 800 pixels recorded at 76 kHz. The 100-ns exposure allows for capturing of transient high-speed flow motion without blurring, and the 500-ns time interval ensures an appropriate spatiotemporal correlation between subsequent schlieren images for high-speed reacting flows. A wavelet-based optical flow velocimetry (wOFV) algorithm was developed and applied to the FSIs. In contrast to the correlation-based algorithms widely employed in PIV for distinct particles, the wOFV algorithm suits better FSIs with continuous variation in brightness. The maximal velocities in the main duct of the scramjet combustor were measured to be approximately 550 m s-1 and 1100 m s-1 for Case 1 and Case2, suggestively corresponding to subsonic and supersonic combustion modes, respectively. The measured velocity inside the cavity is generally below 200 m s-1 for both cases. Recirculation regions and their dynamic motions inside the cavity were well resolved. In summary, the development of the novel velocimetry approach holds great potential for applications in extreme flow conditions. Novelty and Significance Statement: Present work demonstrates a seedless velocimetry approach based on high-speed frame-straddling focusing schlieren imaging coupled with the novel wavelet optical flow velocimetry (wOFV) algorithm. Velocity field measurement realized in a scramjet combustor with high spatiotemporal resolution (1280 × 800 pixels at 38 kHz) for the first time, showing great abilities to accommodate wide velocity range and to resolve dynamic flow characteristics with potentials for broader future applications.

Highly circularly polarized monochromatic high harmonic pulses via a reflective phase retarder at 27.9eV.

A circularly polarized (CP) single-order high harmonic pulse light source, operating at a photon energy of 27.9 eV, is developed. The production of CP harmonic pulses with a degree of polarization exceeding 99% is achieved by utilizing a high-throughput phase retarder composed of SiC mirrors. Notably, our phase retarder exhibits minimal deviation from quarter-wave phase retardation, with a precision of less than ±λ/50 across a 3.1 eV bandwidth. This wide bandwidth enables effective polarization conversion of attosecond pulses preserved. By implementing the phase retarder, we converted the linear polarization to circular with an ellipticity of 0.93.

Analysis of the extreme ultraviolet broadband spectral characteristics and spatiotemporal evolution of laser-produced germanium plasmas

This paper presents comprehensive spectral measurements and a theoretical analysis of laser-produced germanium (Ge) plasma within the 7.5–17 nm wavelength range, with the measurements being performed using spatiotemporally-resolved emission spectroscopy. We obtained and analyzed the extreme ultraviolet (EUV) spectra and their variations meticulously at various positions adjacent to the target surface. The 3p and 3d excitations of Ge ions spanning from Ge4+ to Ge13+ were calculated using the Cowan configuration interaction codes, with particular emphasis being placed on analysis of the spectral line distributions in the 3d-np, 3d-nf, and 3p-3d, 4s, 4d transition arrays. This analysis indicated the contributions made by these transition arrays to the broadband spectral features that were observed experimentally. By using a steady-state collisional radiation model along with a normalized Boltzmann distribution assumption, we clarified the predominant contributions of the 3d-4p and 3p-3d transitions from the Ge7+ to Ge9+ ions in close proximity to the target surface, along with those from the Ge5+ and Ge6+ ions at greater distances. Furthermore, by combining the results above with a plasma dynamics evolution analysis, we gained insights into how the plasma's transient and nonuniform properties influence the analysis of complex EUV band spectral profiles and examine their implications for spectral diagnostics. These results will be useful for plasma spectral diagnosis and for application to development of pulsed short-wavelength light sources.

Photoluminescence measurement of ruby excited by duty ratio-controlled pulse light-emitting diodes (LED) light source.

Photoluminescence (PL) spectra from ruby were obtained using a highly stable LED light source, employing pulse width modulation technique for excitation. The temporal variation in PL intensity caused by the increasing temperature of the LED used for excitation can be mitigated by adjusting the duty ratio (%) of the pulsed LED light to below 10% for cooling the LED. Stable PL spectra measurements were achieved with a duty ratio of less than 10% using a duty ratio-controlled pulsed LED light source, as temperature fluctuations in LED light intensity are minimized at duty ratios less than 10%. Furthermore, fluctuations in the measured PL intensity were diminished by setting the frequency of the pulsed LED light source to greater than 1kHz. This method enables more reliable, cost-effective, and stable PL measurements for material characterization in semiconductors, photonics, and nanotechnology.

High-average-power single-frequency pulse optical parametric oscillator based on pulse-integrated seed-injection automatic locking.

Near-infrared nanosecond (ns) single-longitudinal-mode (SLM) pulse light generated from an optical parametric oscillator (OPO) is an important source in nonlinear optics and high-precision spectral analysis. In this Letter, a stable SLM near-infrared ns pulse light source generated from the OPO is presented, which is achieved by developing a seed-injection automatic locking technique based on a pulse-integrated photodetector (PIPD). Depending on the PIPD, the peak power of the pulse light detected by the photodiode is converted to the average power by integrating several pulses. As a result, the detector saturation is thoroughly eliminated, and the interference signal including the resonance point between seed and pulse lights can easily be attained by scanning the resonator length. On this basis, a microcontroller unit (MCU) is employed to realize automatic locking by looking for the minimum value of the interference signal. Finally, a SLM 824 nm pulse light source with an output power of 20.5 W and a linewidth of 51.42 MHz is obtained. The presented method can pave the way to implement a low-cost and compact high-average-power SLM pulse OPO.

Analysis of extreme ultraviolet radiation and hydrodynamics simulation at the core of laser-produced nickel plasmas

The non-uniformity and transient nature of laser-produced plasma are critical factors that affect the analysis of the extreme ultraviolet spectra of highly charged ions and the diagnosis of plasma states. This paper systematically investigates the characteristics of extreme ultraviolet radiation and the hydrodynamic evolution of laser-produced nickel plasmas from two perspectives: high-spatio-temporal-resolution extreme-ultraviolet spectroscopic measurement and radiation hydrodynamics simulation. The consistency between the four-band experimental spectra and their theoretically simulated spectra confirms the accuracy of the atomic structure parameters and plasma state parameters. We also analyze the significant contribution of the 3d-4f double-excited state radiation to the spectral profile and discuss the influence of the self-absorption caused by plasma opacity on the characteristics of extreme ultraviolet radiation. The findings are crucial for accurately understanding the characteristics of extreme ultraviolet radiation, the hydrodynamic evolution, and the application of medium- and high-Z laser-produced plasma as a pulsed short-wavelength light source.

Photodynamic therapy with redaporfin activated by pulsed laser light

SignificanceRedaporfin is a photosensitizer under clinical trials for head and neck cancer. In previous work, a continuous light source was used in several tumor models, either in vitro or in vivo. However, the use of pulsed light (PL) sources can bring advantages to photodynamic therapy, such as higher penetration depths. ApproachTo study the influence of the light source in the efficacy of redaporfin-PDT, balb/c mice were inoculated with subcutaneous CT26 tumors. Redaporfin was activated using a PL source at 750 nm, with a repetition rate of 20 Hz and an energy per pulse of 24 mJ/cm2. Illumination time ranged from 5 to 15 min. ResultsThe results show an acute inflammatory response followed by a fast shrinkage and necrosis of the tumors and no significant side effects. ConclusionsThe activation of redaporfin with PL leads to good response of the tumors and further studies are being held to optimize the treatment conditions.

Photoacoustic microscopy for real-time monitoring of near-infrared optical absorbers inside biological tissue.

We developed a high-speed optical-resolution photoacoustic microscopy (OR-PAM) system using a high-repetition-rate supercontinuum (SC) light source and a two-axes Galvano scanner. The OR-PAM system enabled real-time imaging of optical absorbers inside biological tissues with excellent excitation wavelength tunability. In the near-infrared (NIR) wavelength range, high-speed OR-PAM faces limitations due to the lack of wavelength-tunable light sources. Our study aimed to enable high-speed OR-PAM imaging of various optical absorbers, including NIR contrast agents, and validate the performance of high-speed OR-PAM in the detection of circulating tumor cells (CTCs). A high-repetition nanosecond pulsed SC light source was used for OR-PAM. The excitation wavelength was adjusted by bandpass filtering of broadband light pulses produced by an SC light source. Phantom and in vivo experiments were performed to detect tumor cells stained with an NIR contrast agent within flowing blood samples. The newly developed high-speed OR-PAM successfully detected stained cells both in the phantom and in vivo. The phantom experiment confirmed the correlation between the tumor cell detection rate and tumor cell concentration in the blood sample. The high-speed OR-PAM effectively detected stained tumor cells. Combining high-speed OR-PAM with molecular probes that stain tumor cells in vivo enables in vivo CTC detection.

Time-Dependent Resonant Inelastic X-ray Scattering of Pyrazine at the Nitrogen K-Edge: A Quantum Dynamics Approach.

We calculate resonant inelastic X-ray scattering spectra of pyrazine at the nitrogen K-edge in the time domain including wavepacket dynamics in both the valence and core-excited state manifolds. Upon resonant excitation, we observe ultrafast non-adiabatic population transfer between core-excited states within the core-hole lifetime, leading to molecular symmetry distortions. Importantly, our time-domain approach inherently contains the ability to manipulate the dynamics of this process by detuning the excitation energy, which effectively shortens the scattering duration. We also explore the impact of pulsed incident X-ray radiation, which provides a foundation for state-of-the-art time-resolved experiments with coherent pulsed light sources.

CW or Pulsed laser – That is the Question: Comparative Steady‐state Photocrystallographic Analysis of Metal Nitrosyl Linkage Isomers

AbstractThe investigation of photo‐induced effects is profoundly influenced by the characteristics of photo‐excitation sources. In this study, we present a comprehensive analysis of the structures of two photoinduced linkage isomers (PLI) in a ruthenium nitrosyl complex, trans‐[Ru(py)4F(NO)](ClO4)2, following irradiation with both pulsed and continuous wave (CW) light sources under low temperature conditions. The X‐ray (photo)diffraction analysis shows that the resulting PLI generated from the two types of irradiation sources, an isonitrosyl configuration of the nitrosyl ligand in the so‐called metastable state MS1, and a side‐on configuration of the nitrosyl ligand in the metastable state MS2, are identical. In‐situ optical absorption spectroscopy was employed during CW and pulsed irradiation, enabling the monitoring of the population process of these PLI. The results obtained from the infrared spectroscopic analysis after pulsed irradiation give insight into the population mechanism illustrating that the generation of the isonitrosyl MS1 occurs through a two‐step process, via the second PLI, the side‐on configuration MS2.

Vacuum Packaging Sensor Based on Time-Resolved Phosphorescence Spectroscopy

Intelligent food packaging with the multisensory analysis is promising as the next generation technology of food packaging. The oxygen content in food packaging is one of the crucial parameters affecting the food quality and shelf life. Caviar is among the most nutritious and costly food sources. Here, a photonic oxygen-sensing system, based on the time-resolved phosphorescence spectroscopy of a platinum complex, is developed for non-contact, non-intrusive, and real-time vacuum packaging quality control, and implemented for caviar packaging. The sensor is embedded in protective polyethylene layers and excited with a short-pulsed light emitting diode (LED) source. Integration of a blue pulsed light source, a fast and amplified silicon photodiode controlled by the Spartan-6 field programmable gate array (FPGA), and a long lifetime platinum complex results in a photonics-based oxygen sensor with a fast response and high sensitivity to the vacuum packaging damage, which is suitable for caviar. It is revealed that applying the polyethylene layers protects the caviar from the platinum complex, leaching while not interfering with the sensor functionality. Characterizing the photonic system based on its sensitivity, repeatability, stability, and long-term operation demonstrates its capability for this application.

Integrating 2D Materials and Plasmonics on Lithium Niobate Platforms for Pulsed Laser Operation at the Nanoscale

AbstractThe current need for coherent light sources for integrated (nano)photonics motivates the search for novel laser designs emitting at technologically relevant wavelengths with high‐frequency stability and low power consumption. Here, a new monolithic architecture that integrates monolayer MoS2 and chains of silver nanoparticles on a rare‐earth (Nd3+) doped LiNbO3 platform is developed to demonstrate Q‐switched lasing operation at the nanoscale. The localized surface plasmons provided by the nanoparticle chains spatially confine the gain generated by Nd3+ ions at subwavelength scales, and large‐area monolayer MoS2 acts as saturable absorber. As a result, an ultra‐compact coherent pulsed light source delivering stable train pulses with repetition rates of hundreds of kHz and pulse duration of 1 µs is demonstrated without the need of any voltage‐driven optical modulation. Moreover, the monolithic integration of the different elements is achieved without sophisticated processing, and it is compatible with LiNbO3‐based photonics. The results highlight the robustness of the approach, which can be extended to other 2D materials and solid‐state gain media. Potential applications in communications, quantum computing, or ultra‐sensitive sensing can benefit from the synergy of the materials involved in this approach, which provides a wealth of opportunities for light control at reduced scales.

Rapid photo-bleaching gamma irradiated Yb-doped optical fibers by high-energy ns pulsed laser

A rapid and efficient photo-bleaching process was demonstrated with a high-energy nanosecond pulse to recover existing and/or revealed color centers on 10 kGy Gamma-irradiated Yb-doped optical fiber. Multi-mJ pulsed laser based on an optical parametric amplifier system operating at wavelengths of 532 nm, 680 nm and 793 nm was used. The photo-bleaching performance is investigated as a function of the wavelength and energy of the pulsed light source. It was observed that the photo-bleaching level of the Yb-doped optical fiber increased when the exposure time of the pulsed laser light and the photon energy was increased. The maximum PB occurred in the pulsed laser of 532 nm wavelength in the optical fiber. Also, a drastically increase in the PB was observed due to the increasing laser energy at the wavelength of 532 nm and 680 nm pulsed laser. The results show that the recovery levels of color centers in the Yb-doped optical fibers could be reached up to 96 % in a shorter time (h) by using the pulsed laser compared to that of the studies using continuous laser.

A BaGa4Se7 crystal based pulsed mid-infrared light source with a narrow linewidth in 4-12 µm.

We present a BaGa4Se7 (BGSe) crystal based coherent pulsed light source for high resolution mid-infrared (MIR) spectroscopy in the 4-12 µm region. The all-solid-state system consists of an injection seeded optical parametric generator (OPG) and an optical parametric amplifier (OPA) using two KTiOPO4 crystals. The idler output of OPG-OPA and the fundamental output (1064nm) of a wavelength stabilized Nd:YAG laser are employed for difference frequency generation of MIR pulses in the BGSe crystal. Pulsed MIR radiation in the 4-12 µm range is obtained with typical pulse energies higher than 100 µJ and pulse durations of ∼5ns. By measuring H2O absorption lines in the 8 µm region with this MIR light source and a cavity ring-down spectrometer, the linewidth of the MIR source is inferred as 120 ± 10MHz, which is very close to the Fourier-transform limited linewidth of 5ns laser pulses.

SNR enhanced high-speed two-photon microscopy using a pulse picker and time gating detection

Two-photon microscopy (TPM) is an attractive biomedical imaging method due to its large penetration depth and optical sectioning capability. In particular, label-free autofluorescence imaging offers various advantages for imaging biological samples. However, relatively low intensity of autofluorescence leads to low signal-to-noise ratio (SNR), causing practical challenges for imaging biological samples. In this study, we present TPM using a pulse picker to utilize low pulse repetition rate of femtosecond pulsed laser to increase the pulse peak power of the excitation source leading to higher emission of two-photon fluorescence with the same average illumination power. Stronger autofluorescence emission allowed us to obtain higher SNR images of arterial and liver tissues. In addition, by applying the time gating detection method to the pulse signals obtained by TPM, we were able to significantly reduce the background noise of two-photon images. As a result, our TPM system using the pulsed light source with a 19 times lower repetition rate allowed us to obtain the same SNR image more than 19 times faster with the same average power. Although high pulse energy can increase the photobleaching, we also observed that high-speed imaging with low total illumination energy can mitigate the photobleaching effect to a level similar to that of conventional illumination with a high repetition rate. We anticipate that this simple approach will provide guidance for SNR enhancement with high-speed imaging in TPM as well as other nonlinear microscopy.

Time- and wavelength-division multiplexed interrogation of fiber Bragg gratings based on dual-wavelength differential detection

The dual-wavelength differential detection technique is used to interrogate fiber Bragg grating sensors. A directly modulated distributed-feedback laser array acts as a multi-wavelength, frequency-scanning pulsed light source. Pulse pairs with frequency drift and identical intensities are generated at three wavelengths. By interrogating 10 serially connected Bragg gratings at the three wavelengths using these pulse trains, we realize a time- and wavelength-division multiplexed temperature sensor system based on dual-wavelength differential detection using a simple setup.

All-synchronized picosecond pulses and time-gated detection improve the spatial resolution of two-photon STED microscopy in brain tissue imaging.

Super-resolution in two-photon excitation (2PE) microscopy offers new approaches for visualizing the deep inside the brain functions at the nanoscale. In this study, we developed a novel 2PE stimulated-emission-depletion (STED) microscope with all-synchronized picosecond pulse light sources and time-gated fluorescence detection, namely, all-pulsed 2PE-gSTED microscopy. The implementation of time-gating is critical to excluding undesirable signals derived from brain tissues. Even in a case using subnanosecond pulses for STED, the impact of time-gating was not negligible; the spatial resolution in the image of the brain tissue was improved by approximately 1.4 times compared with non time-gated image. This finding demonstrates that time-gating is more useful than previously thought for improving spatial resolution in brain tissue imaging. This microscopy will facilitate deeper super-resolution observation of the fine structure of neuronal dendritic spines and the intracellular dynamics in brain tissue.

Qualification of DIRICH readout chain

The DIRICH front-end board (FEB) is a 32+1 channel FPGA based TDC readout module for Multianode Photomultipliers (MAPMT), and also Multianode MCPs. Due to its low cost and excellent timing precision in single photon measurement applications it will be used in the RICH detector of the Compressed Baryonic Matter (CBM) experiment at the future FAIR facility at GSI Darmstadt, Germany.In the regions of highest photon density at the CBM RICH one expects an average hit rate of ≃300 kHz per MAPMT pixel (6 mm × 6 mm) and ∼15% occupancy for minimum bias Au+Au collisions at 35 A GeV at an interaction rate of 10 MHz. In order to validate the high-rate capability of the DIRICH readout, a dedicated lab setup simulating realistic detector signals by employing a pulsed picosecond laser light source in combination with a constant current driven LED was commissioned. In this paper the setup is introduced and first results are discussed. We show that individual readout channels can withstand photon rates up to 2.2 MHz per pixel, limited only by maximum data rate capability and buffer size on the front-end board. Using the same setup, also effects of high photon occupancy on the MAPMTs are investigated, which might cause additional signals due to capacitive cross-talk within the MAPMT or readout chain. Occupancies of up to 55% (simultaneous photon hits on more than half of the MAPMT pixels) are investigated, indicating that in the expected occupancy range of 10%–15% the readout works flawlessly with very low cross-talk.

Nonlinear Refraction, Excited State Absorption, and Multiphoton Absorption of 1,3-Thiazolium-5-thiolate Mesoionic Compound.

We synthesized the mesoionic compound 2-(4-chlorophenyl)-3-methyl-4-(4-methylphenyl)-1,3-thiazole-5-thiolate and measured its refractive and absorptive nonlinear optical response in different temporal and spectral regimes. The experiments were performed by using the Z-scan technique with two pulsed light sources: the second harmonic (at 532 nm) of a mode-locked and Q-switched Nd-YAG laser (100 ps, 10 Hz) and a Ti: Sapphire laser system (100 fs, 1 kHz) operating at 800 nm. The observation and characterization of nonlinear refraction, two- and three-photon absorption, and excited state absorption of the mesoionic compound dissolved in dimethyl sulfoxide, in different concentrations, are presented and discussed with basis on the population redistribution in a three-energy-level model that allows the determination of the parameters which characterize the nonlinear response.

Public exposure to artificial optical radiation in the aesthetics and the entertainment sector in Greece. Risk management actions.

Safety issues are raised from the use of low- and high-power optical radiation sources, both laser and non-laser, by non-experts for aesthetic and entertainment purposes. The Greek Atomic Energy Commission relied on the ISO 31000:2018 framework to manage the public exposure risk to such cases. Risk was evaluated as (1) intolerable for lasers and intense pulsed light sources at aesthetic procedures and in the case of laser pointers, (2) severe for lasers at laser shows and (3) moderate for light-emitting diodes (LEDs) at aesthetic procedures, home-use intense pulsed light sources/LEDs and laser/LED projectors. Operators' training, public awareness campaigns, intensive market surveillance actions and the enhancement of the regulatory framework have been proposed as risk treatment/control measures and have been prioritised in this order, according to their effectiveness in reducing the exposure risk and their urgency of implementation. The Greek Atomic Energy Commission developed public awareness campaigns regarding exposure safety to laser and non-laser light sources at aesthetic procedures and the use of laser pointers.