5 kHz, 4.2mJ, 900 ps end-pumped Nd:YVO4 MOPA laser system.

A 5 kHz sub-nanosecond master oscillator power amplifier (MOPA) laser system was reported in this paper. The master oscillator was an electro-optically Q-switched Nd:YVO4 laser directly pumped at 879 nm, yielding a pulse energy of 520 µJ and a pulse width of 900 ps at 5 kHz. With two Nd:YVO4 amplifiers directly pumped at 914 nm, the pulse energy was further scaled up. Under the absorbed pump energy of 11.0 mJ, the pulse energy was amplified to 4.2 mJ, corresponding to a peak power of 4.7 MW. The optical-to-optical efficiency of the amplifiers reached 33.5%.

Building roof extraction from ASTIL echo images applying OSA-YOLOv5s.

Light detection and ranging (LiDAR) is a type of essential tool for urban planning and geoinformation extraction. Airborne streak tube imaging LiDAR (ASTIL) is a new system with great advantages in the rapid collection of remote sensing data. To the best of our knowledge, a new method to extract a building roof from the echo images of ASTIL is proposed. We improve YOLOv5s with a one-shot aggregation (OSA) module to improve efficiency. The experimental results show that the mean average precision of the OSA-YOLOv5s algorithm can reach 95.2%, and the frames per second can reach 11.74 using a CPU and 39.39 using a GPU. The method proposed can extract building objects efficiently from the echo images of ASTIL and acquire the building roof point cloud.

Generation and amplification of a multi-beam sub-nanosecond laser.

In this paper, a methodology to produce a multi-beam sub-nanosecond laser is proposed. Laser pulses with a pulse energy of 0.14 mJ and a pulse width of 490 ps are generated in a YAG/Nd:YAG/Cr4+:YAG microchip laser at a repetition rate of 200 Hz. After amplification with a laser diode (LD) side-pumped Nd:YAG module, four laser beams are generated because of the thermally induced birefringence. With a double-pass LD side-pumped amplifier, the single pulse energy of the four laser beams is amplified to 5.23 mJ with a peak power of ∼10.67 MW, and air breakdown with four points is achieved with a 2 × 2 lens array.

High-pulse-energy passively Q-switched sub-nanosecond MOPA laser system operating at kHz level.

A passively Q-switched sub-nanosecond master oscillator power amplifier (MOPA) laser system at 1064 nm has been reported in this paper. The master oscillator was a passively Q-switched YAG/Nd:YAG/Cr4+:YAG microchip laser, yielding a pulse energy of 0.14 mJ and a pulse width of ∼490 ps at repetition rates of 500 Hz and 1 kHz. After passing a double-pass side-pumped Nd:YAG amplification system, the pulse energy reached 7.6 mJ and 1.7 mJ at 500 Hz and 1 kHz, respectively. The spatial beam deformation caused by the thermally induced birefringence was investigated numerically and experimentally.

Voxel-based spatial elongation filtering method for airborne single-photon LiDAR data.

A novel voxel-based spatial elongation filtering method is proposed, to reduce noise in airborne single-photon lidar (SPL) data. In this method, six additional points are generated adjacent to each point of interest in the SPL data. Then, we count the number of points within each voxel and discriminate signals from noise via a predefined threshold. A filter performance evaluation index (taking into account the false alarm and signal loss rates, and the average distance between the residual noise points and their nearest signal points) is introduced. We compare the proposed and voxel-based spatial filtering method. The average false alarm rate found with our method (3.5%) is 18.6% smaller than that of the voxel-based spatial filtering method (4.3%).

Temporal Multilevel Luminescence Anticounterfeiting through Scattering Media.

Optical anticounterfeiting, typically using luminescent materials to encode and unveil hidden patterns, plays an essential role in countering fraud in trademark, document security, food industry, and public safety. However, this technique is often realized through color-encoded fashion and in the visible range, preventing high-order encryption as well as visualization through scattering layers. Here, we describe a set of shortwave infrared (SWIR)-emitting lanthanide-doped nanoparticles with precisely controlled luminescence lifetime, which can be utilized as temporary codes for multilevel anticounterfeiting through opaque layers. To achieve this, we devise a core/shell/shell/shell structure of NaYF4:Yb3+/Er3+ @ NaYbF4 @ NaYF4 @ NaYF4:Nd3+, in which the inert NaYF4 shell acts as an energy-retarding layer to regulate energy flow from the outmost light-harvesting layer to the inner core domain to produce long-lived SWIR luminescence at 1532 nm. A precise control of the NaYF4 layer thickness enables yielding a precisely defined lifetime tunable between ∼3 and 10 ms, yet without compromising luminescence intensities. Importantly, optical patterns of these lifetime-encoded core/multishell nanoparticles are able to dynamically show a multitude of secured images in the time domain at defined time points through opaque plastic and biomimetic intralipid layers (about half a centimeter thick). Our temporal optical multiplexing results, demonstrated here in multilevel anticounterfeiting, have implications for optical data storage, biosensing, diagnostics, and nanomedicine.

Deblurring streak image of streak tube imaging lidar using Wiener deconvolution filter.

We use a Wiener deconvolution filter to deblur the streak image of streak tube imaging lidar to improve the spatial resolution of the system and reduce the edge blurring effect in point clouds. Experiments were performed to investigate the performance of the deconvolution method. Results show that the spatial resolution improved from 9 to 4.5 mm, and the root-mean-square errors of the edge regions are effectively reduced. Additionally, the transition section decreases from 14 to 5.6 mm when the target is 5 m away from the receiver.

Rare-earth-doped fluoride nanoparticles with engineered long luminescence lifetime for time-gated in vivo optical imaging in the second biological window.

Biomedicine is continuously demanding new luminescent materials to be used as optical probes for the acquisition of high resolution, high contrast and high penetration in vivo images. These materials, in combination with advanced techniques, could constitute the first step towards new diagnosis and therapy tools. In this work, we report on the synthesis of long lifetime rare-earth-doped fluoride nanoparticles by adopting different strategies: core/shell and dopant engineering. The here developed nanoparticles show intense infrared emission in the second biological window with a long luminescence lifetime close to 1 millisecond. These two properties make the here presented nanoparticles excellent candidates for time-gated infrared optical bioimaging. Indeed, their potential application as optical imaging contrast agents for autofluorescence-free in vivo small animal imaging has been demonstrated, allowing high contrast real-time tracking of gastrointestinal absorption of nanoparticles and transcranial imaging of intracerebrally injected nanoparticles in the murine brain.

14 GHz broadband and continuously frequency-tuned Nd:YVO4 laser with an RTP etalon.

A laser-diode-pumped broadband and continuously frequency-tuned all-solid-state Nd:YVO4 laser at 1064 nm with an output power of 200 mW is demonstrated. A RbTiOPO4 (RTP) etalon and a piezoelectric-transducer (PZT) are utilized for coarse and fine frequency tuning, respectively. Dependence of the frequency excursion on the applied voltage to the RTP etalon and the displacement of the PZT is theoretically and experimentally investigated. A continuous frequency tuning of 14 GHz is conducted by synchronous adjustment of the RTP etalon and the PZT. The tuning covers more than 6 times the longitudinal mode spacing of a laser resonator without any mode hops.

Impact of input field characteristics on vibrational femtosecond coherent anti-Stokes Raman scattering thermometry.

In this paper, three ultrashort-pulse coherent anti-Stokes Raman scattering (CARS) thermometry approaches are summarized with a theoretical time-domain model. The difference between the approaches can be attributed to variations in the input field characteristics of the time-domain model. That is, all three approaches of ultrashort-pulse (CARS) thermometry can be simulated with the unified model by only changing the input fields features. As a specific example, the hybrid femtosecond/picosecond CARS is assessed for its use in combustion flow diagnostics; thus, the examination of the input field has an impact on thermometry focuses on vibrational hybrid femtosecond/picosecond CARS. Beginning with the general model of ultrashort-pulse CARS, the spectra with different input field parameters are simulated. To analyze the temperature measurement error brought by the input field impacts, the spectra are fitted and compared to fits, with the model neglecting the influence introduced by the input fields. The results demonstrate that, however the input pulses are depicted, temperature errors still would be introduced during an experiment. With proper field characterization, however, the significance of the error can be reduced.

Linearly frequency-tuned LD-pumped Nd:YVO4 laser with an 18-GHz broadband tuning range.

A continuously frequency-tuned laser diode end-pumped Nd:YVO4 laser at 1064 nm is demonstrated. A coated etalon and a piezoelectric-transducer (PZT) are utilized for coarse and fine frequency tuning, respectively. Broadband and linear frequency tuning without mode hops is conducted by the synchronous adjustment of the etalon and the PZT. Dependence of the frequency excursion on the displacement of the PZT and the tilting angle of the etalon are theoretically and experimentally investigated. A linear frequency tuning range up to 18 GHz without mode hops or frequency overlaps in a one-way non-stopped scanning is obtained. The maximum output power is 930 mW at 1064 nm, and the average frequency tuning speed is 1.24 GHz/s. Standard deviation of the frequency variation to a linear frequency tuning is estimated to be 186 MHz, indicating a high tuning linearity.

100 kHz, 3.1 ns, 1.89 J cavity-dumped burst-mode Nd:YAG MOPA laser.

We demonstrated a cavity-dumped burst-mode 1.06 µm side-pumped Nd:YAG laser and its dual-stage dual-pass amplified laser performance. The cavity dumping process has been theoretically studied and the output performance has been experimentally investigated. At the pumping duration of 2 ms and pumping frequency of 10 Hz, burst energy, peak power and pulse width of the amplified laser reached 1.89 J, 2.87 MW and 3.1 ± 0.3 ns, respectively, at the Q-switch repetition rate of 100 kHz. The maximum energy extraction efficiency reaches to 30%.

2.4 THz/s continuously linearly frequency-modulated Nd:YVO4 laser.

We demonstrate a linearly frequency-modulated laser from a laser diode (LD) pumped Nd:YVO4 laser. A fast frequency tuning of 2.40 THz/s with a tuning range of 6 GHz is achieved in LD pumped Nd:YVO4 1064 nm laser by using RbTiOPO4 (RTP) crystals as the frequency modulator. The continuous tuning range is more than 3 times the longitudinal mode spacing of the resonator. The maximum output power of frequency-modulated laser reaches 160 mW at 1064 nm. Linewidth of the single frequency laser is measured to be 190 kHz by a delay self-heterodyne interferometer. A deviation lower than 60 MHz is obtained during linear modulation.

Heterogeneous core/shell fluoride nanocrystals with enhanced upconversion photoluminescence for in vivo bioimaging.

We report on heterogeneous core/shell CaF2:Yb(3+)/Ho(3+)@NaGdF4 nanocrystals of 17 nm with efficient upconversion (UC) photoluminescence (PL) for in vivo bioimaging. Monodisperse core/shell nanostructures were synthesized using a seed-mediated growth process involving two quite different approaches of liquid-solid-solution and thermal decomposition. They exhibit green emission with a sharp band around 540 nm when excited at ∼980 nm, which is about 39 times brighter than the core CaF2:Yb(3+)/Ho(3+) nanoparticles. PL decays at 540 nm revealed that such an enhancement arises from efficient suppression of surface-related deactivation from the core nanocrystals. In vivo bioimaging employing water-dispersed core/shell nanoparticles displayed high contrast against the background.

A novel miniaturized passively Q-switched pulse-burst laser for engine ignition.

A novel miniaturized Cr4⁺:YAG passively Q-switched Nd:YAG pulse-burst laser under 808 nm diode-laser pulse-pumping was demonstrated for the purpose of laser-induced plasma ignition, in which pulse-burst mode can realize both high repetition rate and high pulse energy simultaneously in a short period. Side-pumping configuration and two different types of laser cavities were employed. The pumping pulse width was constant at 250 µs. For the plane-plane cavity, the output beam profile was flat-top Gaussian and the measured M² value was 4.1 at the maximum incident pump energy of 600 mJ. The pulse-burst laser contained a maximum of 8 pulses, 7 pulses and 6 pulses for pulse-burst repetition rate of 10 Hz, 50 Hz and 100 Hz, respectively. The energy obtained was 15.5 mJ, 14.9 mJ and 13.9 mJ per pulse for pulse-burst repetition rate of 10 Hz, 50 Hz and 100 Hz, respectively. The maximum repetition rate of laser pulses in pulse-burst was 34.6 kHz for 8 pulses at the incident pump energy of 600 mJ and the single pulse width was 13.3 ns. The thermal lensing effect of Nd:YAG rod was investigated, and an plane-convex cavity was adopted to compensate the thermal lensing effect of Nd:YAG rod and improve the mode matching. For the plane-convex cavity, the output beam profile was quasi-Gaussian and the measured M2 value was 2.2 at the incident pump energy of 600 mJ. The output energy was 10.6 mJ per pulse for pulse-burst repetition rate of 100 Hz. The maximum repetition rate of laser pulses in pulse-burst was 27.4 kHz for 6 pulses at the incident pump energy of 600 mJ and the single pulse width was 14.2 ns. The experimental results showed that this pulse-burst laser can produce high repetition rate (>20 kHz) and high pulse energy (>10 mJ) simultaneously in a short period for both two different cavities.

High-repetition-rate and short-pulse-width electro-optical cavity-dumped YVO4/Nd:GdVO4 laser.

In this paper, an electro-optical cavity-dumped 1.06 µm laser using YVO4/Nd:GdVO4 composite crystal under 808 nm diode-laser pumping was reported. Theoretical calculations showed that the temperature distribution in YVO4/Nd:GdVO4 crystal was lower than that in GdVO4/Nd:GdVO4 and Nd:GdVO4 crystals under the same conditions. A constant 3.8±0.3 ns pulse width was obtained and the repetition rate could reach up to 50 kHz with a maximum average output power of 5.6 W and slope efficiency of 40.7%, corresponding to a peak power of 31.1 kW.

Laser induced spark ignition of coaxial methane/oxygen/nitrogen diffusion flames.

We report the laser induced spark ignition (LSI) of coaxial methane/oxygen/nitrogen diffusion flames using the 1064 nm output of a Q-switched Nd:YAG laser. The minimum ignition energy (MIE) and ignition time of the LSI has been determined by measuring the emission signals due to the ignited flames. The effects of the gas mixture properties, including the overall equivalence ratio (Ф), oxygen concentration and flow rate, and the ignition positions on the two parameters have been investigated systematically. The variation of the MIE and ignition time with the experimental conditions has been compared with the existing results and discussed with a special concentration on the effects of the local Ф.

Intense ultraviolet upconversion emission from water-dispersed colloidal YF3:Yb3+/Tm3+ rhombic nanodisks.

Intense ultraviolet upconversion emission has been observed in water-dispersed uniform rhombic nanodisks (side length of ~14 nm and thickness of ~2.5 nm) of YF3 co-doped with Yb(3+) sensitizer and Tm(3+) activator ions, when excited at ~980 nm.

Laser ablation ignition of premixed methane and oxygen-enriched air mixtures using a tantalum target.

We report the laser ablation ignition of premixed methane and oxygen-enriched air mixtures using a tantalum target. The minimum laser pulse energy (MPE) of the ablation ignition was obtained as 2-4 mJ, which was reduced by one order of magnitude compared with that of the direct laser-induced gas breakdown ignition. The ignition time of the ablation ignition was investigated for the first time, to our best knowledge, by measuring the emission signal profiles due to the successfully ignited flames, and an ignition time as short as ~50 µs was obtained. The reduction in MPE will promote the miniaturization and, thus, the practical applications of laser ignition systems.

Comparison on performance of passively Q-switched laser properties of continuous-grown composite GdVO4/Nd:GdVO4 and YVO4/Nd:YVO4 crystals under direct pumping.

The comparison on performance of passively Q-switched laser properties of continuous-grown composite GdVO(4)/Nd:GdVO(4) and YVO(4)/Nd:YVO(4) crystals under direct pumping to the emitting level was demonstrated. A Cr(4+):YAG crystal was used as saturable absorber. At an incident pump power of 10 W, the average output power, the pulse width, the repetition rate, the pulse energy, and the peak power for a GdVO(4)/Nd:GdVO(4) laser were 1.22 W, 48.1 ns, 121 kHz, 10.1 µJ, and 209.6 W, respectively. And for a YVO(4)/Nd:YVO(4) laser under the same conditions, these output characteristics were 1.26 W, 44.9 ns, 218 kHz, 5.8 µJ, and 128.7 W, respectively.