Shortest Pulse Width Research Articles

High peak power passively Q‐switched 2 μm solid‐state laser based on a MoS2 saturated absorber

AbstractA 2 μm passively Q‐switched Tm:YAP solid‐state laser with MoS2 as saturated absorber was demonstrated in this work. MoS2 two‐dimensional materials were prepared by using the method of chemical vapor deposition. The laser achieved high peak power of 21.2 W and high single pulse energy of 19.5 μJ, while maintaining a short pulse width of 916 ns. This is the first time that the peak power of a Tm:TAP solid‐state laser based on a MoS2 saturated absorber has reached 21.2 W. The beam quality factor was M2x = 1.08 and M2y = 1.10.

Advances in All-Solid-State Passively Q-Switched Lasers Based on Cr4+:YAG Saturable Absorber

All-solid-state passively Q-switched lasers have advantages that include simple structure, high peak power, and short sub-nanosecond pulse width. Potentially, these lasers can be applied in multiple settings, such as in miniature light sources, laser medical treatment, remote sensing, and precision processing. Cr4+:YAG crystal is an ideal Q-switch material for all-solid-state passively Q-switched lasers owing to its high thermal conductivity, low saturation light intensity, and high damage threshold. This study summarizes the research progress on all-solid-state passively Q-switched lasers that use Cr4+:YAG crystal as a saturable absorber and discusses further prospects for the development and application of such lasers.

Photonic-crystal lasers with two-dimensionally arranged gain and loss sections for high-peak-power short-pulse operation

Realizing high-peak-power (tens to hundreds of watts or higher) short-pulse (tens of picoseconds or less) operation in semiconductor lasers is crucial for state-of-the-art applications including eye-safe high-resolution remote sensing and non-thermal ultrafine material processing. However, it has been challenging to introduce mechanisms that enable stable high-peak-power short-pulse operation in conventional semiconductor lasers. Here, we propose photonic crystal lasers that have two-dimensionally arranged gain and loss sections to enable high-peak-power short-pulse operation in the fundamental mode while suppressing lasing in higher-order modes to avoid laser instability. On the basis of this concept, we experimentally realize a high peak power of ~20 W and a short pulse width of ~35 ps with an injection current of only 3-4 A using a 400-μm-diameter device and theoretically predict that even higher peak power (>300 W) can be achieved in a 1-mm-diameter device. Our results will contribute to the realization of next-generation laser sources for the aforementioned applications. By using engineered gain and loss sections in a photonic crystal laser, pulses with a peak power of ~20 W and pulse width of ~35 ps have been experimentally demonstrated and even higher peak power operation (>300 W) has been theoretically predicted.

Effects of NMES pulse width and intensity on muscle mechanical output and oxygen extraction in able-bodied and paraplegic individuals.

Neuromuscular Electrical Stimulation (NMES) is commonly used in neuromuscular rehabilitation protocols, and its parameters selection substantially affects the characteristics of muscle activation. Here, we investigated the effects of short pulse width (200µs) and higher intensity (short-high) NMES or long pulse width (1000µs) and lower intensity (long-low) NMES on muscle mechanical output and fractional oxygen extraction. Muscle contractions were elicited with 100Hz stimulation frequency, and the initial torque output was matched by adjusting stimulation intensity. Fourteen able-bodied and six spinal cord-injured (SCI) individuals participated in the study. The NMES protocol (75 isometric contractions, 1-s on-3-s off) targeting the knee extensors was performed with long-low or short-high NMES applied over the midline between anterior superior iliac spine and patella protrusion in two different days. Muscle work was estimated by torque-time integral, contractile properties by rate of torque development and half-relaxation time, and vastus lateralis fractional oxygen extraction was assessed by Near-Infrared Spectroscopy (NIRS). Torque-time integral elicited by the two NMES paradigms was similar throughout the stimulation protocol, with differences ranging between 1.4% (p = 0.877; able-bodied, mid-part of the protocol) and 9.9% (p = 0.147; SCI, mid-part of the protocol). Contractile properties were also comparable in the two NMES paradigms. However, long-low NMES resulted in higher fractional oxygen extraction in able-bodied (+ 36%; p = 0.006). Long-low and short-high NMES recruited quadriceps femoris motor units that demonstrated similar contractile and fatigability properties. However, long-low NMES conceivably resulted in the preferential recruitment of vastus lateralis muscle fibers as detected by NIRS.

Nonlinear Optical Response of Gold Nanobipyramids for a Doubly Q-Switched Ho-Doped Laser at a Wavelength of 2.1 µm.

Gold nanobipyramids (Au-NBPs) were successfully fabricated using the seed-mediated growth method. The saturable absorption performance of the Au-NBPs at a 2-μm band wavelength was characterized. Using excellent-quality, mature Ho:YLF crystals, a doubly Q-switched (DQS) laser joining an acousto-optic modulator (AOM) with an Au-NBP saturable absorber (SA) was achieved. When the modulation rate of the AOM was 1 kHz, the shortest pulse width (54 ns) was attained, corresponding to the highest peak power (3.87 kW). This was compared with a singly Q-switched laser joining an AOM with an Au-NBP SA, whereby the maximum pulse width compression ratio was 15.2 and the highest peak power enhancement factor was 541.3. Our study has shown that Au-NBPs are a potential saturable absorption nanomaterial, and the DQS laser has the benefit of compressing the pulse width and increasing the peak power at a wavelength of 2.1 μm.

FDTD Simulations of LEMP Propagation in the Earth-Ionosphere Waveguide Using Different Lightning Models

In this article, vertical electric fields (including skywaves) produced by lightning return strokes at distances ranging from 100 to 958 km have been computed using the finite-difference time-domain (FDTD) method in the 2-D spherical coordinate system. The return stroke is represented by the original transmission-line (TL) model (with no current decay with height and abrupt current termination at the channel top), the modified TL model with linear current decay with height (MTLL model), the modified TL model with exponential current decay with height (MTLE model), or the Hertzian dipole (HD) model. All the TL-type models include propagation delay, while in the HD model the current changes with time, but does so simultaneously in all channel sections; that is, without any propagation delay. The HD model predicts considerably higher fields than the TL-type models, if the same channel length is employed. The HD model with an unrealistically small channel length of 0.7 km yields electric field peaks similar to those produced by TL-type models with a 7-km channel. Ground waves and skywaves computed using the HD model have faster rise and decay times (shorter pulsewidths) than those computed using a TL-type model. Electric fields produced by a longer risetime current are less attenuated due to propagation over lossy ground when a TL-type model is used, while they are more attenuated if the HD model is employed. The opposite trends are related to the fact that the radiation field peak is proportional to the product of the current and propagation speed for a TL-type model, while for the HD model, it is proportional to the product of the time derivative of current and channel length. In the TL model, abrupt termination of significant current at the channel top results in the so-called mirror image (abrupt radiation field polarity change). This artifact does not occur in the framework of the MTLL model and is negligible in the MTLE model with λ = 2 km and H = 7 km.

Molecular Response of Skin to Micromachining by Femtosecond Laser

Pulsed lasers at the near infrared (NIR) range have been widely used in dermatology. Ultrashort pulsed picosecond lasers are found with the specific ability of very effective activation of skin repair and remodeling along with significant photodamage. Femtosecond lasers, with a shorter pulse width, may be a promising alternative to current NIR lasers in clinic. In this study, we performed optical micromachining by a femtosecond laser at 1,030 nm to skin of live mice in two modes of scanning of focused laser and direct irradiation by unfocused laser. The acute and one-day delayed immune molecular responses of the skin to the micromachining are studied by immunofluorescence microscopy of the skin sections. Our data shows the focused laser can activate remodeling of skin without any significant immune responses. In contrast, the direct irradiation by the unfocused laser activate significant immune responses in the deep dermis with high regulation of interleukin. Those results suggest focused femtosecond laser is of good promising potential in activation of skin remodeling and repairing with little immune or physical damage.

Physical mechanism of pulsed laser interaction with fused silica optics during CO2 laser mitigation process

The intrinsic physical mechanism of CO2 laser rapid ablation mitigation micro-defects on fused silica optics surface are studied through experiments and simulations. Based on the multi-physics coupling mathematical model involving the stages of phase transition, melting flow and evaporative ablation, the calculated depth of Gaussian ablation crater and protrusion height at the edge formed by single laser pulse are 5.05 μm and 25 nm respectively, which agrees well with the experiment. Then, the temperature distribution, heat affected zone, solid-liquid transition process and surface morphology evolution under the action of moving pulsed laser are analyzed. Through studies of the relationship between laser parameters and processed surface quality, it can be concluded that the periodic structural features at the bottom of surface morphology are formed by the superposition of residual height of linear laser processing. Laser with short pulse width is more likely to cause material redeposition on the surface of fused silica optics.

Design and Implementation of a Capacitive Energy Storage Pulse Drive Source

According to the requirement of driving power supply for pulsed semiconductor laser, a method of constant current output is proposed by combining large energy storage capacitance with MOS transistor working in linear region to solve the problem of generating high amplitude, short pulse width and high stability current pulse on low impedance load. The front stage uses the buck circuit to charge the energy storage capacitor, and through the hysteresis control of the buck circuit, the voltage of the energy storage capacitor is controlled. In the latter stage, the MOS transistor working in the linear region is used to realize the pulse output, and the PI module is used to adjust the output current to ensure the constant output current. Through PSPICE simulation, the key factors affecting the output pulse amplitude, pulse width and rise time are analyzed and verified by experiments. Using 155V DC power supply, the experimental results show that the capacitor energy storage pulse driver circuit can achieve a pulse constant current output with amplitude of 30A, pulse width of 300μs and rise time of less than 10μs on a 5 Ω low impedance load.

Effects of 445-nm Laser on Vessels of Chick Chorioallantoic Membrane with Implications to Microlaryngeal Laser Surgery.

ObjectivePrevious research has shown that effective application of angiolytic lasers in microlaryngeal surgery is determined by wavelength, pulse width (PW), and fluence. Recently, a 445‐nm (blue) laser (BL) has been developed with a potentially greater hemoglobin absorption than previous lasers. The chick chorioallantoic membrane (CAM) represents a suitable model for testing various settings to find out the most optimal settings of this laser. This study used the CAM model to examine whether successful photoangiolytic effects could be obtained using BL.MethodsSeven hundred and ninety three third‐order vascular segments of viable CAM were irradiated using BL via 400‐μm diameter fiber, 1 pulse/second, with PW and power varied systematically at standardized fiber‐to‐vessel distances of 1 and 3 mm. Outcome measures including vessel ablation rate (AR), rupture rate (RR), and visible tissue effects were analyzed using Chi‐square test.ResultsEnergy levels of 400, 540, and 600 mJ (per pulse) were most effective for vessel ablation. A working distance of 3 mm resulted in higher ablation and less vessel rupture compared with 1 mm at these optimal energy levels. At 3 mm, a longer PW resulted in higher AR. At 1 mm, AR increased with shorter PW and higher power. The 1‐mm working distance resulted in lower tissue effects than 3 mm.ConclusionFindings in this study showed that BL was effective in vessel ablation using relevant combination of working distance, PW, and energy levels. To obtain high AR, longer working distance plus longer PW was required and if working distance was reduced, shorter PW should be set.Level of EvidenceNA Laryngoscope, 131:E1950–E1956, 2021

Simulation and Analysis of Pulse Driving Circuit for Semiconductor Laser

Aiming at the problem of how to provide high amplitude, short pulse width and high stability driving power supply for semiconductor lasers, this paper proposes a method to realize constant current pulse output by combining buck circuit, energy storage network and linear regulation module. The hysteresis control of buck circuit is used to control the voltage of energy storage capacitor, and then the PI module is used to adjust the MOSFET working in the linear region to ensure the constant current output on the load. PSPICE software is used to simulate and analyze the designed pulse driving circuit. Through simulation, we can clearly and intuitively understand the influence of various parameters in the circuit on the circuit, so as to optimize the circuit design. The simulation results show that the circuit can achieve high-power pulse constant current output with amplitude of 30A and rise time of 3us on the semiconductor laser load, which provides a reference for the subsequent actual circuit test.

Short Pulse Width and High Peak Power 457 nm Deep Blue Laser with V-Type Cavity

An intracavity frequency doubling acousto-optically Q-switched Neodymium-doped Yttrium Orthvanadate (Nd:YVO4) 457 nm blue laser by employing a three-mirror folded cavity was demonstrated. With the incident pump power of 40.4 W, the maximum average output power of 439 mW 457 nm laser, and the minimum pulse duration of 86.14 ns and the maximum peak power of 510 W were achieved at 10 kHz. The M2 factors are 1.23 and 1.61 in X and Y directions, respectively. The power stability in two hours is better than 2%.

Design and Implementation of High Power Pulse Constant Current Source

According to the requirement of the semiconductor laser for driving power supply, aiming at the problem of how to produce the current pulse with high amplitude, short pulse width and high stability on the low impedance load, a method of realizing the constant current output of the pulse by using the combination of the buck transform circuit and the linear regulation module is proposed. The Buck circuit is hysteresis controlled to control the storage capacitance voltage, and then the MOSFET working in the linear region is adjusted by the PI module to ensure constant current output on the load. The key factors affecting the output pulse amplitude, pulse width, front and back edge parameters are also analyzed and verified by experiments. The results of 170V DC power supply show that the circuit designed in this paper can output 30A, 250us pulse width and rise time less than 10us pulse constant current output on 5Ω low impedance load.

Spectral filtering effect on multi-pulsing induced by chirped fiber Bragg grating in dispersion-managed mode-locked Yb-doped fiber lasers

Abstract We numerically and experimentally investigate the multi-pulsing mechanism in a dispersion-managed mode-locked Yb-doped fiber laser. Multi-pulsing occurs primarily owing to the inherent filtering effect of the chirped fiber Bragg grating. The spectral filtering effect restricts the spectral broadening induced by self-phase modulation and causes extra loss, leading to a decreased pump power threshold for the multi-pulsing state. Numerical simulations show that multi-pulsing emerges at a lower pump power when the spectral filter bandwidth becomes narrower. In the experiment, the spectral width increases as the net cavity dispersion approaches zero. Pulses with wider spectral widths experience more loss from the spectral filtering effect, leading to a decreased pump power threshold for multi-pulsing. Therefore, the net cavity dispersion also has an impact on the multi-pulsing threshold. Based on this conclusion, we devise a strategy to obtain single-pulsing operation with the shortest pulse width and the highest pulse energy.

Research progress of high-order harmonics and attosecond radiation driven by interaction between intense lasers and plasma

The realizing of the detection and control of ultrafast process conduces to understanding and remoulding the physical world at a microcosm level. The attosecond light source with attosecond temporal resolution and nanometer spatial resolution can realize real-time detection and manipulation of the atomic-scale electronic dynamics and relevant effects of the substances. Therefore, attosecond science is considered as one of the most important milestones in the history of laser science. and has been listed as an important scientific and technological development direction in the coming 10 years. High-order harmonic generation (HHG) from intense laser-matter interaction is one of the most important routes to breaking through the femtosecond limit and achieving brilliant attosecond pulse radiations, and thus having aroused great interest in recent years. After more than 20-year development, the research about attosecond pulse generation by laser-gas interaction has reached a mature stage. This method produces the shortest isolated pulse in the world to date, with a pulse width being only 43 as. However, this method based on ionization-acceleration-combination encounters inevitable difficulties in pursuing the relativistically intense attosecond pulses and the highest possible photon energy. Quite a lot of studies have proved that the HHG efficiency from laser-plasma interaction can be a few orders of magnitude higher than that in gaseous media, which makes it possible to produce pulses with shorter pulse width and higher photon energy. In this article, we introduce the main generation mechanisms, research progress and frontier applications of HHG through the laser-plasma interaction process. In Section 2, we introduce the HHG generation mechanisms, including coherent wake emission, which is used to describe the HHG process driven by a nonrelativistic laser; relativistic oscillating mirror, which can well explain most of HHG processes generated from plasma-vacuum interface in relativistic regime; coherent synchrotron emission, which is suited to explain the HHG synchronously emitted from isolated electron sheets. The research progress is summarized in Section 3 from the aspects of radiation efficiency, polarization characteristics, phase characteristics, generation and diagnosis of isolated attosecond pulses, etc. Frontier applications of these ultra-broadband intense attosecond pulses are presented in the last section, such as the study of electronic dynamics, process, coherent diffraction imaging, diagnosis of extreme states of matter, the generation of extremely intense fields, etc. Finally, an outlook on the future development trends and innovation breakthroughs is also presented.

7 kHz sub-nanosecond microchip laser amplified by a grazing incidence double pass slab amplifier

To obtain short pulse width and high peak power laser, a 7 kHz sub-nanosecond microchip laser amplified by a grazing incidence double pass slab amplifier is experimentally demonstrated in this Letter. We use a compact side-pumped Nd:YVO4 bounce amplifier with grazing incidence beam for achieving high gains and power extraction. Laser output power of 7.37 W at 7 kHz, 1.2 MW pulse peak power with 877 ps duration and 1.05 mJ energy, 25 pm spectral width, and near diffraction limited mode beam quality are achieved, and the optical-to-optical efficiency is 18%. The laser is packaged in a volume of 356 mm × 226 mm × 84 mm and may be used for applications such as laser altimeters and ladar systems.

Effect of Voltage Pulse Width and Synchronized Substrate Bias in High-Power Impulse Magnetron Sputtering of Zirconium Films

The Zr film microstructure is highly influenced by the energy of the plasma species during the deposition process. The influences of the discharge pulse width, which is the key factor affecting ionization of sputtered species in the high-power impulse magnetron sputtering (HiPIMS) process, on the obtained microstructure of films is investigated in this research. The films deposited at different argon pressure and substrate biasing are compared. With keeping the same average HiPIMS power and duty cycle, the film growth rate of the Zr film decreases with increasing argon pressure and enhancing substrate biasing. In addition, the film growth rate decreases with the elongating HiPIMS pulse width. For the deposition at 1.2 Pa argon, extending the pulse width not only intensifies the ion flux toward the substrate but also increases the fraction of highly charged ions, which alter the microstructure of films from individual hexagonal prism columns into a tightly connected irregular column. Increasing film density leads to higher hardness. Sufficient synchronized negative substrate biasing and longer pulse width, which supports higher mobility of adatoms, causes the preferred orientation of hexagonal α-phase Zr films from (0 0 0 2) to (1 0 1¯ 1). Unlike the deposition at 1.2 Pa, highly charged ions are also found during the short HiPIMS pulse width at 0.8 Pa argon.

Principle and technology of attosecond pulse characterization

Attosecond pulses have potential applications in the fields of physics, chemistry, biology, and medicine owing to the extremely high time resolution brought by their extremely short pulse widths. The generation and measurement technology of attosecond pulses provides a powerful means for the study of electron dynamical process in atoms. Since the date of its birth, the time width of an attosecond pulse has become shorter and shorter, making its measurement a challenging task. Although attosecond pulses are more and more widely used in the study of ultrafast dynamics and more interest is attracted into the field of attosecond science, it is undeniable that there are still difficulties and challenges for the measurement of attosecond pulses. This review is divided into two parts. The first part presents the development of technics for characterizing attosecond pulses, and briefly describes several experimental principles, setups and results for acquiring time-domain information of attosecond pulses, including RABITT (reconstruction of attosecond beating by interference of two-photon transitions) and laser-assisted lateral X-ray photoionization for measuring attosecond pulse train, attosecond streak camera for measuring single attosecond pulses, TIPTOE (tunneling ionization with a perturbation for the time-domain observation of an electric field) for measuring time-domain information of periodic pulses, as well as theoretical proposals such as attosecond cross correlation, asymmetric photoionization method, and SPIDER (attosecond spectral phase interferometry for direct electric-field reconstruction). The second part introduces the principle and structure of inversion algorithms for extracting time-domain information of attosecond pulses from spectrograms obtained with an attosecond streak camera, including FROG-CRAB (frequency resolved optical gating for complete reconstruction of attosecond bursts), PROOF (phase retrieval by omega oscillation filtering), VTGPA (Volkov transform generalized projections algorithm), PROBP-AC (phase retrieval of broadband pulses with autocorrelation) and CVGN (conditional variational generative network). We also evaluate the calculation time and accuracy of the algorithms. Finally, we summarize the experimental and theoretical challenges of the characterization of attosecond pulses, and look forward to the future development of attosecond pulse measurement.

Passively Q-switched Nd:YVO4 laser with few-layered MoSe2 saturable absorber

We fabricated few-layered MoSe2 nanosheets via the simple ultrasonic-assisted liquid phase exfoliation (UALPE) method. Typical characterizations and testing were exploited to confirm the nonlinear absorption properties of MoSe2 nanoflakes. A passively Q-switched Nd:YVO4 laser emitting 1 [Formula: see text]m wavelength was demonstrated with the as-prepared MoSe2 saturable absorber (SA). The stable pulses were generated with a maximum output power of 215 mW and a shortest pulse width of 192 ns. The experimental data provided a solid base for the photonics applications of MoSe2 nanomaterials.

Passive mode-locking of p-doped quantum dot semiconductor lasers

Quantum dot based monolithic edge-emitting semiconductor lasers at 1.25 µm are ideal sources for the generation of broad optical frequency combs for optical communication applications. In this work, InAs/InGaAs quantum dot lasers with different total laser length to absorber length ratio and with different p-doping concentrations in the GaAs barrier sections are investigated experimentally in dependence on the gain injection current and absorber reverse bias voltage. A smaller mode-locking area is found for the p-doped device in dependence on the laser biasing conditions. For the undoped active region 1.3 ps short pulse widths at a pulse repetition rate of 20 GHz with a pulse-to-pulse timing jitter of 111 fs are reported for an absorber section length of 12% to the total cavity length. For an undoped and p-doped device short pulse emission between 2.5 ps and 5.5 ps is attained and a shorter absorber section length of 8% or 5%.