Shortest Pulse Width Research Articles

GHz-repetition-rate fundamentally mode-locked, isolator-free ring cavity Yb-doped fiber lasers with SESAM mode-locking.

A novel fundamentally mode-locked, GHz-repetition-rate ring cavity Yb-doped femtosecond fiber laser is demonstrated, which utilizes polarization-maintaining gain fiber and is enable by SESAM mode-locking. Thanks to the isolator-free structure, the ring cavity laser is operated bidirectionally and the two polarization-multiplexed output pulse trains are demonstrated synchronous. As a result, tunable waveforms one of which is with reduced pedestal and shorter pulse width in comparison with each individual, are generated by combination of the two orthogonal-polarized output pulses. Furthermore, a similar ring cavity structure that generates GHz picosecond pulses is demonstrated. We believe such high-repetition-rate polarization-multiplexed mode-locked fiber lasers could find further uses in various applications in need of gigahertz repetition rate and tunable waveforms.

Picosecond synchronously pumped diamond Raman laser

With diamond crystals as Raman media, picosecond synchronously pumped solid-state Raman laser is theoretically studied in detail for the first time. High efficient working point and effective pulse compression working point are investigated. For both 532 nm and 1064 nm pumping, high Raman conversion efficiency can be achieved for negative cavity length detuning (∆x) and diamond crystal length of 5 mm. The higher efficiency can be obtained with longer Raman crystal, longer pumping pulse width and higher pumping power. For 532 nm pumping, effective pulse width compression can be realized for ∆x = 0 nearby and diamond crystal length of 10 mm. Shorter pulse width and higher peak power of 1st Stokes laser can be achieved with longer Raman crystal, shorter pumping pulse width and higher pumping power. The findings can contribute to the design and optimization of picosecond synchronously pumped diamond Raman lasers.

Numerical modeling for the characteristics study of a focusing ultrashort spatiotemporal optical vortex.

Spatiotemporal (ST) wave packet carrying pure transverse orbital angular moment (OAM) with subwavelength spatial size has attracted increasing attentions in recent years, which can be obtained by tightly focusing a linear superposition of ST vortices with different topological charges. In this work, numerical models are proposed to explore the impact of the pulse width of the ST vortex on the characteristics of its focal field. We demonstrate that the rigorous model for calculating the focused ST wave packet is essential for ultrashort optical pulse, while the simplified model has the advantage of high efficiency but can only provide credible results when the pulse width of the illumination is long enough. Specifically, when the pulse width decreases from 100 fs to 5 fs, the accuracy of the simplified model would decrease significantly from 99% to 65.5%. In addition, it is found that the pulse duration would still lead to the collapse of transverse OAM structure near the focus of a high numerical aperture lens, even though the ST astigmatism has already been corrected. To analyze the physical mechanism behind this distortion, Levenberg-Marquardt algorithm is adopted to retrieve the OAM distribution of the focal field. It is shown that the contributions from undesired OAM modes would become nontrivial for short pulse width, leading to the formation of the focal field with hybrid OAM structures. These findings provide insight for the focusing and propagation studies of ultrashort ST wave packets, which could have wide potential applications in microscopy, optical trapping, laser machining, nonlinear light-matter interactions, etc.

Modulating Light Emission Performance of PCSEL via GaN HEMT Driving Circuit

In this study, a paradigm for modulating the light emission performance of photonic-crystal surface-emitting laser (PCSEL) via GaN high electron mobility transistor (HEMT) driving circuit is proposed for the first time. For light detection and ranging (LiDAR) system, a faster pulse repetition frequency with shorter pulse width can provide not only high resolution but also sufficiently precise range resolution. Hereupon, comprehensive analyses for such an integrated system are conducted with not only electro-optical responses but also the corresponding optical behaviors. The relevant electrical characteristics of the employed GaN HEMT are examined at first. Next, the integrated system on a matrix board with its corresponding circuit topology is discussed, illustrating the relevant operating principles. Thereby, sufficient systematical scrutinization for relevant light emissions is performed for both photodiode responses and the optical behaviors under different conditions, paving a holistic panorama for the LiDAR system. Thus, prospects for the next generation LiDAR system in high-power and high-speed operation can be expected.

Optical single-sideband modulation for a coherent Doppler lidar.

The first, to the best of our knowledge, implementation of optical single-sideband modulation (OSSBM) in coherent Doppler lidar (CDL) is demonstrated for use with short pulse widths. It is shown that by providing more bandwidth through a higher intermediate frequency (IF), the OSSBM CDL addresses cross talk between the IF and baseband spectra, reducing spectral distortion. Ultimately, a 4-ns pulse width with a 1-GHz IF is achieved and the effectiveness of the OSSBM CDL is confirmed through velocimetry experiments. This represents a several fold improvement over current fiber-based CDL implementations.

Ultrafast Tm-doped fiber laser based on hybrid fiber coupler

Tm-doped ultrafast fiber lasers have been widely used in scientific research and industrial applications. Here, with the usage of an novel integrated fiber coupler, we describe a reduced laser configuration to achieve compact and high-performance all-fiber Tm-doped mode-locked fiber laser. This novel fiber component can be used as wavelength division multiplexer (WDM), output coupler (OC) and optical modulator at the same time, fabricated by magnetron-sputtering vanadium dioxide (VO2) saturable absorber (SA) on a optical coupler. The bare coupler was laboratory-tapered performing WDM and OC simultaneously. With above-mentioned device, a mode-locked Tm-doped fiber laser was constructed. Fundamental solitons were formed and generated with a short pulse width of 478 fs and a high repetition rate of 160 MHz. In addition, Short-term and long-term laser stability was confirmed by the noise analysis and 4-h running test. 336 fs RMS time jitter ([10 kHz - 10 MHz]) and 0.036% RMS relative intensity noise ([1 Hz - 10 MHz]) was acquired. This new integrated design at 2 μm region simplifies implementation of the ultrafast Tm-doped fiber laser and paves the way for developing compact and high-performance all-fiber ring laser.

Gaussian-wave-packet model for single-photon generation based on cavity quantum electrodynamics under adiabatic and nonadiabatic conditions

For single-photon generation based on cavity quantum electrodynamics, we investigate a practical model assuming a Gaussian wavepacket. This model makes it possible to comprehensively analyze the temporal dynamics of an atom-cavity system with both adiabatic and nonadiabatic conditions using analytical expressions. These results enable us to clarify the relationship between pulse width and maximum success probability, over the full range of coupling regimes, for arbitrary detunings. We demonstrate how to achieve a high success probability while keeping a short pulse width by optimizing the cavity transmittance parameter. Our formulations provide a practical tool for efficient single-photon generation in a wide variety of experimental platforms.

Influence of unipolar pulse time offset between Ti and Zr dual cathodes in closed magnetic field high power impulse magnetron sputtering

Titanium/zirconium dual cathodes with a closed magnetic field configuration were used in unipolar high power impulse magnetron sputtering (HiPIMS). Different pulse width (pulse on time) and pulse time offset between the negative voltage pulses of two targets were applied. The target discharge current waveforms and consequent substrate current waveforms were recorded and compared. The results show that both ion fluxes emitted from the Ti and Zr cathodes increase with the argon pressure. However, the ion flux emitted from the Zr cathode increases more substantially at high argon pressure. The closed magnetic field raises the target discharge current and ion flux to the substrate when the pulses of the two targets are simultaneous synchronized, or followed closely. The TiZr film composition is influenced by the HiPIMS discharge behavior of each target material more than by the average power ratio of each target. For low working pressure and short pulse width, simultaneous synchronization of the pulses on the two targets might effectively use the electrons drifting between the dual targets to enhance the generation of ions from both targets. For high working pressure and long target pulse width, setting pulses on two targets following closely is recommended to additionally utilize the runaway ions as pre-ionized media for the follow-up pulse on the adjacent target.

Diode-pumped CaGdAlO4 crystal-assisted Kerr-lens mode-locked all-solid-state Pr:LiYF4 laser at 639 nm

At present, direct generation of an ultrafast laser with a pulse width shorter than 10 ps in the visible spectral region has been very rarely demonstrated. In this work, we focus on this issue by operating an InGaN blue-diode-pumped Kerr-lens mode-locked Pr:LiYF4 laser. Using an undoped CaGdAlO4 crystal as an additional Kerr medium in the resonator for providing enhanced third-order nonlinearity, a stable mode-locked Pr:LiYF4 laser with red emission at 639 nm is achieved with the shortest pulse duration of 2.6 ps at a repetition rate of 49.7 MHz. The maximum average output power, single pulse energy, and peak power reach 68.4 mW, 1.38 nJ, and 530 W, respectively. To the best of our knowledge, this is the shortest pulse width that has ever been achieved in this specific emission line of Pr3+ ions.

Compact Solid-State Marx Modulator With Fast Switching for Nanosecond Pulse

This article describes the compact solid-state Marx modulator (SSMM) with a fast rising time and a short pulsewidth based on the modular structure. By stacking of the TO-263-7 packaged silicon carbide <small>mosfet</small>s having high voltage and fast switching capabilities, the designed circuit consists of three components, including the Marx capacitor, an <small>on</small> switch for applying pulse to the load, and an <small>off</small> switch to pull down the pulse. The <small>off</small> switch connected in parallel with load is chosen as a <small>mosfet</small> instead of a diode that is used in the conventional Marx generator. Because the <small>off</small> switch provides a discharging path for the stored energy on the parasitic capacitance, it is highly related to the fast falling time. In order to provide complementary driving signal and power for <small>on&#x002F;off</small> switches, the simple control algorithm with a minimum component count and a reliable drive circuit against the noise are proposed. In addition, the synchronization of all the gate signals and a compact configuration for minimizing the stray inductance are essential to shorten the rising, falling, and pulsewidth. Based on the proposed circuit, the following specifications of the developed SSMM are achieved: the output voltage of 10 kV, the pulsewidth of less than 50 ns, and the rising and falling times of less than 15 ns.

Passively high-power Q-switching in Er- and Er/Yb doped fiber with CdTe

In this work, Cadmium Telluride (CdTe) thin-film was demonstrated as a saturable absorber (SA) to induce Q-switching in the erbium-doped fiber laser (EDFL) and erbium-ytterbium co-doped fiber laser (EYDFL) cavities. CdTe thin film was prepared by the liquid-phase exfoliation method and sandwiched between two fiber ferrules to complete the fiberized SA assembly. Firstly, the CdTe-SA was inserted into the EDFL cavity and stable Q-switched pulses were observed at a threshold pump power of 88mW. The shortest pulse width and the highest repetition rate measured for the EDFL cavity were 6.10 ± 0.02 μs and 42.3 ± 1.3 kHz respectively, at a maximum pump power of 295 mW. At a maximum pump power of 295 mW, the highest average output power produced by the Q-switched pulses in the EDFL cavity was 0.15 mW. Meanwhile, Q-switched pulses in the EYDFL cavity self-start at the threshold pump power of 1.4 W. The maximum repetition rate obtained was 99.5 ± 1.5 kHz and a pulse width of 2 μs at a maximum pump power of 3 W. At this pump power, the highest average output power produced by the Q-switched pulses in the EYDFL cavity was 70.0 ± 0.2 mW. This is the first demonstration of a high average power passively Q-switched fiber laser operating in the C-band employing EDF and EYDF as the gain medium and CdTe thin-film as SA that we are aware of.

Diode-pumped passively Q-switched Yb[formula omitted]:LuYLaVO4 laser with GaAs saturable absorber

A passively Q-switched (PQS) Yb3+:LuYLaVO4 laser based on a GaAs saturable absorber is reported. The nonlinear saturable absorption property of GaAs has been studied, the saturation fluence, modulation depth and non-saturation loss were characterized to be 3.3 MW/cm2, 4.5% and 54.7%, respectively. In the PQS laser operation, the highest pulse repetition rate, shortest pulse width, largest single pulse energy and highest pulse peak power were measured to be 40 kHz, 4 ns, 16.7 μJ and 4.2 kW. The peak power achieved nearly three orders of magnitude improvement compared with the former Yb3+:LuYLaVO4 PQS lasers.

A Randomized, Double-Blinded Crossover Trial of Short Versus Conventional Pulse Width Subthalamic Deep Brain Stimulation in Parkinson's Disease.

Background:Subthalamic nucleus deep brain stimulation (STN-DBS) is a well-established treatment for patients with Parkinson’s disease. Previous acute challenge studies suggested that short pulse widths might increase the therapeutic window while maintaining motor symptom control with a decrease in energy consumption. However, only little is known about the effect of short pulse width stimulation beyond the setting of an acute challenge.Objective:To compare 4 weeks of STN-DBS with conventional pulse width stimulation (60 μs) to 4 weeks of STN-DBS with short pulse width stimulation (30 μs) regarding motor symptom control.Methods:This study was a monocentric, double-blinded, randomized crossover non-inferiority trial investigating whether short pulse width stimulation with 30 μs maintains equal motor control as conventional 60 μs stimulation over a period of 4 weeks (German Clinical Trials Register No. DRKS00017528). Primary outcome was the difference in motor symptom control as assessed by a motor diary. Secondary outcomes included energy consumption measures, non-motor effects, side-effects, and quality of life.Results:Due to a high dropout rate, the calculated sample size of 27 patients was not met and 24 patients with Parkinson’s disease and STN-DBS were included in the final analysis. However, there were no differences in any investigated outcome parameter between the two treatment conditions.Conclusion:This study demonstrates that short pulse width settings (30 μs) provide non-inferior motor symptom control as conventional (60 μs) stimulation without significant differences in energy consumption. Future studies are warranted to evaluate a potential benefit of short pulse width settings in patients with pronounced dyskinesia.

Extraction of Bias-dependent Source and Drain Resistances in AlGaN/GaN MIS-HEMTs Using Pulsed Measurement Method

A new method for extraction of bias-dependent source and drain resistances in AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) is reported. The total source/drain access resistances are extracted by measuring the pulsed transfer characteristics of test devices with various quiescent biases. With short pulse width, the charge trapping and thermal effects will not be affected by the magnitude of pulse voltages, and thus the extracted resistances correspond to the quiescent bias condition. As a result, the dependences of total access resistances on the gate and drain voltages are obtained. While employing the dual-sweep combinational transconductance technique, the source and drain resistances are separated further. To investigate the variation of access resistances with applied voltages, AlGaN/GaN HEMTs are also illuminated with a 365-nm ultraviolet (UV) source during pulsed measurements. Under UV illumination, the resistance will be less dependent on the drain voltage, indicating the charge trapping in GaN buffer layer has a large impact on the access resistance.

Generation of High Peak Power Pulses With Controllable Repetition Rate in Doubly Q-Switched Laser With AOM/SnSe2

By simultaneously employing self-made SnSe2 saturable absorber (SA) and an acous-to-optic modulator (AOM), a dual-loss-modulated Q-switched Nd:YVO4 laser with short pulse width, high peak power and adjustable pulse repetition rate is presented. The maximum pulse peak power of 37.57 KW with the minimum pulse duration of 5.52 ns were obtained under the pump power of 8 W and the pulse repetition rate of 5 kHz. The experimental results demonstrate that the dual-loss modulation technology with AOM and SnSe2 SA is a simple and efficient method to generate short pulses with high peak power and adjustable low repetition rates.

High Repetition Rate, TEM00 Mode, Compact Sub-Nanosecond 532 nm Laser

As a critical transmitter, compact 532 nm lasers operating on high repetition and short pulse widths have been used widely for airborne or space-borne laser active remote sensing. We developed a free space pumped TEM00 mode sub-nanosecond 532 nm laser that occupied a volume of less than 125 mm × 50 mm × 40 mm (0.25 L). The fundamental 1064 nm laser consists of a passively Q-switched composite crystal microchip laser and an off-axis, two-pass power amplifier. The pump sources were two single-emitter semiconductor laser diodes (LD) with 808 nm wavelengths and a maximum continuous wave (CW) power of 10 W each. The average power of the fundamental 1064 nm laser was 1.26 W, with the laser operating at 16 kHz repetition rates and 857 ps pulse widths. Since the beam distortion would be severe in microchip lasers due to the increase in heat load, in order to obtain a high beam quality of 532 nm, the beam distortion in our experiment amplifying the fundamental laser was compensated by adjusting the distribution of the pumping beam. Furthermore, in the critical phase matching (CPM) regime for the second harmonic generation (SHG), a Type I LiB3O5 (LBO) crystal obtained 770 ps, a beam quality of M2 &lt; 1.2, and a 16 kHz pulse output at 532 nm, which was better than 0.6 W average power.

Long distance high resolution FMCW laser ranging with phase noise compensation and 2D signal processing.

A long distance high resolution frequency-modulated continuous wave (FMCW) laser rangefinder with phase noise compensation and two-dimensional (2D) data processing skills is developed. Range-finding ladar consists of a continuously chirped laser source, an auxiliary reference interferometer, and a monostatic optical transceiver for target illumination and return photon collection. To extend the range unambiguity and lower the electronic processing bandwidth, a two-step laser frequency chirping scheme is adopted, where a long pulse width, small frequency bandwidth laser chirping signal are used in step 1 for coarse distance estimation, and a short pulse width and large frequency bandwidth laser chirping signal are applied afterwards for step 2 high resolution distance realization. An auxiliary reference interferometer is to record the phase noise originated from the laser source to compensate for phase errors induced in the target return photons. The 2D data processing skill helps to coherently sum up all the phase noise removed echo photons to achieve high resolution range peak extraction with high detection sensitivity. Experimental demonstration shows that the proposed FMCW ladar at 1550 nm wavelength with a laser chirping bandwidth of 10 GHz and electronic processing bandwidth of 200 MHz can measure a corner cube test target in an outdoor atmospheric environment, and the measurement results are 12013.905 m with a 2.4 cm range resolution under strong return photon levels and 12013.920 m with a 2.5 cm range resolution under weak return photon levels.

The Effect of Pulse Discharge Control on Ozone Production

In this study, the effect of waveform control of pulsed discharge on ozone production for water treatment under atmospheric pressure is investigated and optimized. Using solid-state linear transformer driver (LTD) which has flexible waveform control capability, the applied voltage pulse has been separated into two parts, and each can be independently controlled while keeping the total input energy constant. The experimental results have shown that if the two parts are consecutive to each other without time interval, the second part of the discharge is inefficient for producing ozone. However, by introducing an interval between two voltage parts, making it effectively a double-pulse discharge, an increase in the energy yield of ozone production is observed. In addition, the ozone producing energy yield increases when the pulse interval is lengthened, up to 10 ms. Therefore, in conclusion, it has been observed that shorter pulsewidth and longer pulse interval lead to more efficient ozone production by pulsed discharge under atmospheric pressure.

Bubble Dynamics during Laser Irradiated Thermo-Mechanical Response of Pigmented Skin Phantom

During the laser treatment of pigmented dermatosis such as Nevus of Ota, vapor bubbles will be generated by the laser with short pulse width and high energy density. Laser irradiation is efficacious for the clinical treatment of Ota’s Nevus caused by hyperplasia of melanosomes in dermis. Since the mechanism of the laser–melanosome interaction is not yet clear, the clearance rate is generally low and bleeding of irradiated skin frequently occurs. This work conducted a flow visualization experiment to investigate the laser–melanosome interaction mechanism by using high-speed imaging. Pigmented phantom was prepared to simulate the diseased dermis tissue, where agar acted as substrate and synthetic melanin particles was infused as hyperplastic melanosomes. Putting the phantom into water, its thermo-mechanical responses to single-pulse 1064-nm Nd:YAG laser irradiation with energy density of 4–7 J/cm2 and pulse duration of 6 ns were recorded. The results indicated that laser-induced bubble formation caused by the gasification of tissue moisture is the key mechanism of laser–melanosome interaction, and an optimal energy density of 6 J/cm2 is recommended.

Liquid-Phase Exfoliation of Ta2NiS5 and Its Application in Near-Infrared Mode-Locked Fiber Lasers with Evanescent Field Interactions and Passively Q-Switched Bulk Laser.

We report on the application of a 1 μm solid-state passively Q-switched (PQS) laser and 1, 1.5 μm mode-locked (ML) fiber lasers based on ternary chalcogenide Ta2NiS5 saturable absorber (SA), which were successfully fabricated by liquid-phase exfoliation method (LPE). The nonlinear absorption of the Ta2NiS5-SA was characterized by 0.32 GW/cm2 and 0.25 GW/cm2 saturation intensities with 7.3% and 5.1% modulations depths at 1 μm and 1.5 μm, respectively. A PQS solid-state laser operating at 1.0 μm has been realized with the Ta2NiS5-SA. The maximum average output power, shortest pulse width, pulse energy, and pulse peak power from the PQS laser are 0.257 W, 180 ns, 1.265 μJ, and 7 W. Moreover, highly stable femtosecond laser centered at 1.5 μm, and picosecond centered at 1 μm, ML fiber lasers were obtained using the Ta2NiS5-SA. A 70 dB signal-to-noise ML laser with a pulse duration of 781 fs was observed in the telecommunication window, which is better than the duration of the previously reported lasers based on Ta2NiS5. The corresponding maximum single pulse energy and peak power are 0.977 nJ and 1251 W, respectively. The Ta2NiS5-SA fabricated by the LPE method was applied in near-infrared (NIR) ML fiber lasers (evanescent field interactions) and PQS bulk lasers. The results indicate that Ta2NiS5-SA prepared by the LPE method can be applied in a 1 μm bulk PQS laser and improved by the new combination mode (evanescent field interactions) for better output performance of the fiber laser.