Quasi-continuous Wave Research Articles

Burst-Mode 355 nm UV Laser Based on a QCW LD-Side-Pumped Electro-Optical Q-Switched Nd: YAG Laser

In this paper, a high-repetition-rate, high-peak-power burst-mode nanosecond 355 nm UV laser was demonstrated. A quasi-continuous wave (QCW) laser diode (LD) side-pumped electro-optical (EO) Q-switched burst-mode Nd: YAG laser was performed as the fundamental laser source. Under the pumping duration of 250 μs and a burst repetition rate of 100 Hz, the pulse energy of 20 kHz burst-mode UV laser reached 5.3 mJ with a single pulse energy of 1.325 mJ, pulse width of 68 ns, resulting in a peak power of 19.49 kW. The as-generated millijoule burst-mode UV laser has great potential for high-end processing of laser lift-off, annealing and slicing in display semiconductor fields.

High power continuous-wave, Q-switched, and quasi-continuous-wave operation of a diode-pumped Tm,Ho:YAG laser oscillator at 2.09 μm

A high-power diode-pumped 2.09 μm Tm,Ho:YAG laser oscillator with multi-regime operation is demonstrated. In the continuous-wave (CW) regime, it delivers 44.68 W output power with M2 = 3.47, yielding a record-high brightness of 87.57 MW/cm2·sr. In the Q-switched mode, stable laser performance across pulse repetition frequencies (PRFs) from 1 kHz to 4 kHz is achieved. At a PRF of 1 kHz, single pulse energy reaches 22.85 mJ with a pulse width of 443.4 ns and M2 = 3.50. In the quasi-continuous-wave (QCW) operation, a pulse energy of 31.85 mJ is obtained at 200 Hz with a 370 μs pulse width and M2 = 4.49. These pulse energies are the highest reported to date for Tm-Ho co-doped laser oscillators at high PRF. This also marks the first demonstration of a versatile 2 μm laser that can operate at room temperature while delivering high-energy, high-repetition-rate pulses in both nanosecond and microsecond durations within a single device, making it highly appealing for scientific and industrial applications.

Novel numerical simulation method for quasi-continuous wave laser processing of carbon-carbon composites and its application in parameter inversion

Novel numerical simulation method for quasi-continuous wave laser processing of carbon-carbon composites and its application in parameter inversion

Stimulated Emission and Lasing from Bulk CdSe Nanocrystals.

Nanocrystals with a size in the regime of vanishing quantum confinement, or bulk nanocrystals (BNCs), have emerged recently as viable solution processable optical gain materials in the green part of the spectrum. Here, we show that these properties can be extended to the crucial red region using CdSe BNCs. Through quantitative time-resolved spectroscopy, we can model these nanocrystals as bulk semiconductors, thereby revealing that the gain originates from an unbound electron-hole plasma state. The gain is broadband in nature and is not capped by Auger processes, but by a slower second-order recombination resulting in nanosecond gain lifetimes. Finally, optically pumped lasers under femtosecond pulsed and quasi-continuous wave operation are demonstrated using a photonic crystal surface emitting laser cavity, thereby stretching from 635 to 720 nm. Our results indicate that compositional variation can indeed provide spectral versatility to the BNC concept, while preserving the excellent gain metrics associated with it.

Simultaneously achieving high repetition rate and high peak power in Er,Yb:YAl3(BO3)4 microchip laser

This study presents a dual-ended pumped 1.5 µm Er,Yb:YAl3(BO3)4 (Er,Yb:YAB) passively Q-switched microchip laser, designed to address the challenge of achieving high peak power and high repetition rates simultaneously. The dual-ended pumping method significantly reduces the thermal gradient within the crystal, allowing for higher pump incident power compared to single-ended pumping. Under quasi-continuous-wave (QCW) pumping conditions, the laser achieved a pulse energy of 14.33 µJ, a repetition rate of 470.08 kHz, and a peak power of 2.53 kW. Additionally, we conducted a detailed investigation of the pulse bifurcation phenomenon observed during the experiments, uncovering the underlying mechanisms and their potential impact on laser performance. The results not only demonstrate the effectiveness of dual-ended pumping in enhancing laser performance but also provide valuable insights into thermal management in Er,Yb:YAB crystals. These findings could serve as a foundation for developing more efficient and powerful Er,Yb:YAB lasers, which hold promise for applications in lidar, remote sensing, and range finding.

1 kHz, 10 mJ, sub-200 fs regenerative amplifier utilizing a dual-crystal configuration of Yb:CaGdAlO4 featuring exceptional beam quality

In order to boost the energy of the femtosecond regenerative amplifier (RA), we adopt the chirped pulse amplification technique to stretch the seed pulses through the Martinez stretcher and inject them into our specially designed dual-crystal regenerative cavity based on the Yb:CaGdAlO4 (Yb:CGA) crystals. To avoid damaging the component coating, we meticulously regulate the size of the cavity mode on the surface of each component within the cavity to ensure that the energy density remains below the damage threshold. The final output of 11.3 mJ pulse energy was obtained at a 1 kHz repetition rate with a power stability of 0.35% over 1 hour, which is the highest energy we know of for the regenerative output of Yb:CGA crystals. Additionally, leveraging the wide gain spectrum of the Yb:CGA crystal, we achieve a spectrum full width at half maximum (FWHM) of 9 nm along with a compressed pulse duration of 198 fs. The combination of the dual-crystal setup, superior thermal properties of the Yb:CGA crystals, quasi-continuous wave pumping approach, and the thermal-insensitive design of the regenerative cavity effectively minimize the thermal impact on the crystals. The output beam exhibits near-diffraction-limited performance, with an M2 value of only 1.05 × 1.06.

Recent Advancements and Challenges in High‐Power Thulium‐Doped Laser

AbstractHigh‐power all‐fiber thulium lasers have gained considerable interest in recent times due to their distinct characteristics and versatile applications in the medical and industrial sectors. This review article presents a comprehensive examination of the advancements and challenges in this field. It begins with an overview of thulium‐doped silica fiber, which is a critical component for high‐power lasers operating at the 2 µm (micrometer) wavelength band. The research progress of essential high‐power thulium laser sources, including continuous‐wave (CW), quasi‐continuous wave (QCW), and pulsed lasers, is then thoroughly analyzed, highlighting their respective strengths and limitations. Additionally, the diverse applications of high‐power thulium fiber lasers in medical and industrial domains are summarized. Furthermore, the article emphasizes the current challenges in the advancement of high‐power thulium‐doped fiber lasers (TDFLs) and outlines potential avenues for future development. Despite TDFLs being the predominant laser source in lithotripsy and material processing applications, optimizing their performance and expediting further progress in thulium laser technology remain crucial objectives. This review article aims to provide valuable insights for researchers, engineers, and professionals working in the field of high‐power fiber lasers operating at 2 µm.

Realizing enhanced lithotripsy efficiency using 700 W peak power thulium-doped fiber laser

Despite its widespread use in laser lithotripsy, the Ho:YAG solid-state laser suffers from limited parameter combinations, poor beam quality, and low photoelectric conversion efficiency. The development of 2.0 μm fiber laser technology has positioned the thulium-doped fiber laser (TFL) as a superior alternative to the Ho:YAG laser in lithotripsy. The TFL excels because of its high-water absorption coefficient, all-fiber structure and pump modulation flexibility. However, the peak power reported for TFL in lithotripsy remains at most 500 W. This study presented a quasi-continuous wave (QCW) TFL featuring a wide tunable range of pulse width and pulse repetition rates, and a peak power of 700 W to realize enhanced ablation efficiency. A high-speed camera was used to acquire the laser-induced cavitation bubble images for the analysis of the interaction mechanism of a high peak power QCW TFL pulse with water and stones. While the synergistic action of photothermal and photomechanical effects constituted the mechanism of TFL lithotripsy, the laser-induced bubble channel was a prerequisite for TFL noncontact effective lithotripsy, with bubble length and width markedly influencing stone ablation rates. Ablation experiments on uric acid and calcium oxalate monohydrate stones conducted with various parameter combinations at a peak power of 700 W yielded rates of 8.9 mg/s for uric acid stones and 7.5 mg/s for calcium oxalate monohydrate stones. A comparison with prior findings confirmed that a high peak power TFL could notably improve stone ablation rates.

Self-seeded quasi-continuous-wave Er:ZBLAN fiber laser

A self-seeding phenomenon is forecasted in a pulse-pumped Er:ZBLAN fiber laser. A non-zero lasing power, serving as a seeding power for subsequent output pulse, between two adjacent output pulses, will appear in a pulse-pumped Er:ZBLAN fiber laser when the average pump power is increased to a certain value. The seeding power plays an essential role for the transformation of gain-switching operation to quasi-continuous-wave (QCW) operation in a pulse-pumped Er:ZBLAN fiber laser, and is responsible for the suppression of chaotic spikes in the output QCW pulse as well. Under certain conditions, moreover, stable QCW output pulses are expected to be generated in pulse-pumped the Er:ZBLAN fiber laser with the assistance of the self-seeding power. The pulse-pumped Er:ZBLAN fiber laser, referred to as a self-seeded Er:ZBLAN fiber laser, has great potential for stable QCW pulse generating with nearly the same duration and temporal profile as those of pump pulse.

High energy and high repetition rate QCW-LD end-pumped electro-optical Q-switched Yb:YAG laser

The quasi-continuous wave laser diode (QCW-LD) pumping is considered as the most efficient way for generating high energy nanosecond pulsed laser with repetition rate ranging from 50 to 1000 Hz. Here, a high efficiency, high repetition rate and high energy QCW-LD end-pumped electro-optical (EO) Q-switched Yb:YAG laser was demonstrated. The shortest pulse duration of 17.4 ns with a maximum single pulse energy of 5.1 mJ was obtained at the repetition rate of 100 Hz. As the repetition rate increases up to 500 Hz, the maximum output single pulse energy was obtained to be 5.38 mJ with the pulse width of 18.7 ns, corresponding to the slope efficiency of 21.6% and beam quality factor of M x 2=1.04, and M y 2=1.01. In addition, a theoretical study was carried by numerical simulation of the rate equations, and the results were agreed well with the experimental ones. The results show that QCW-LD pumping with Yb-doped crystals is a promising way for highly efficient, high-energy and high repetition rate nanosecond pulsed laser generation.

Buried Depressed-Cladding Waveguides Inscribed in Nd3+ and Yb3+ Doped CLNGG Laser Crystals by Picosecond-Laser Beam Writing.

Buried depressed-cladding waveguides were fabricated in 0.7-at.% Nd:Ca3Li0.275Nb1.775Ga2.95O12 (Nd:CLNGG) and 7.28-at.% Yb:CLNGG disordered laser crystals grown by Czochralski method. Circular waveguides with 100 μm diameters were inscribed in both crystals with picosecond (ps) laser pulses at 532 nm of 0.15 μJ energy at 500 kHz repetition rate. A line-by-line writing technique at 1 mm/s scanning speed was used. Laser emission at 1.06 μm (with 0.35 mJ pulse energy) and at 1.03 μm (with 0.16 mJ pulse energy) was obtained from the waveguide inscribed in Nd:CLNGG and Yb:CLNGG, respectively, employing quasi-continuous wave pumping with fiber-coupled diode lasers. The waveguide realized in RE3+-doped CLNGG crystals using ps-laser pulses at high repetition rates could provide Q-switched or mode-locked miniaturized lasers for a large number of photonic applications.

THz-wave multilevel frequency modulation by a single wavelength-tunable laser

Photomixing is one of the promising THz-wave generation methods. A conventional photomixing system with two lasers results in increasing the volume and power consumption of the system. For downsizing the system and reducing its power consumption, we have previously proposed a method to generate a THz-wave by a single wavelength-tunable laser. This technique can generate a quasi-continuous THz wave as well as a pulsed THz wave, based on which we are studying toward pulse frequency-amplitude modulation as a multi-level modulation method. In this paper, we have successfully generated a THz wave with frequency modulation between 286 GHz and 322 GHz by a single wavelength-tunable laser.

Quasi-Continuous Wave Pumped Slab Laser Based on Nd:CTGS Crystal

Quasi-Continuous Wave Pumped Slab Laser Based on Nd:CTGS Crystal

A Load-Adaptive Driving Method for a Quasi-Continuous-Wave Laser Diode.

A quasi-continuous-wave (QCW) laser diode (LD) driver is commonly used to drive diode bars and stacks designed specifically for QCW operations in solid-state lasers. Such drivers are optimized to deliver peak current and voltage pulses to LDs while maintaining low average power levels. As a result, they are widely used in laser processing devices and laser instruments. Traditional high-energy QCW LD drivers primarily use capacitors as energy storage components and pulsed constant-current sources with op-amps and power metal-oxide-semiconductor field-effect transistors (MOSFETs) as their core circuits for generating repeated constant-current pulses. The drawback of this type of driver is that the driver's output voltage needs to be manually adjusted according to the operating voltage of the load before use to maximize driver efficiency while providing a sufficient current. Another drawback is its inability to automatically adjust the output voltage to maintain high efficiency when the load changes during the driver operation. Drastic changes in the load can cause the driver to fail to function properly in extreme cases. Based on the above traditional circuit structure, this study designed a stability compensation circuit and realized a QCW LD driver for driving a GS20 diode stack with a maximum repetition rate of 100 Hz, a constant current of approximately 300 A, a load voltage of approximately 10 V, and a pulse width of approximately 300 μs. In particular, a high-efficiency, load-adaptive driving method was used with the MOSFETs in the critical saturation region (i.e., between the linear and saturated regions), controlling its power loss effectively while achieving maximum output current of the driver. The experiments demonstrated that the driver efficiency could be maintained at more than 80% when the load current varied from 50 to 300 A.

Research on gas pore formation and inhibition mechanism of high nitrogen steel during laser direct metal deposition

Nitrogen pore formation is an inherently tricky problem that limits the application of high nitrogen steel (HNS) in various fields. In the present study, a CA-LBM coupling model was proposed to understand the nitrogen pore formation mechanism during laser direct metal deposition (DMD) of HNS. Based on the proposed model and theoretical analysis, the effects of pulse frequency and shielding gas nitrogen content on the porosity were clarified systematically. Results showed that after the onset of solidification, the ferrite phase with extremely low nitrogen solubility precipitated first, inducing the continuous enrichment of nitrogen in the residual liquid phase, which in turn provided the prerequisite for nitrogen pore nucleation. The application of quasi-continuous-wave (QCW) laser mode accelerated the molten pool solidification, and shortened the nitrogen supersaturated region, allowing more nitrogen to be solid dissolved in the matrix. On this basis, the porosity was inhibited to a certain extent. Compared with the high-frequency pulse, the low-frequency pulse showed better porosity inhibition. In addition, following the classic nucleation theory, nitrogen segregation and nitrogen solubility synergistically dominated nitrogen pore formation. The nitrogen partial pressure, determined by the shielding gas nitrogen content, played a dual role in porosity defects. Elevated nitrogen partial pressure served to increase nitrogen solubility while promoting the nitrogen absorption process to intensify nitrogen segregation. However, due to the short duration of the molten pool, the nitrogen absorption process could not be fully processed. Nitrogen solubility enhancement was the key factor in inhibiting porosity during the DMD process.

An LED-Side-Pumped Intracavity Frequency-Doubled Nd,Ce:YAG Laser Producing a 2W Q-Switched Red Beam

We report a high-average-power acousto-optic (AO) Q-switched intracavity frequency-doubled red laser based on a high-efficiency light-emitting-diode (LED) pumped two-rod Nd,Ce:YAG laser module. Under quasi-continuous wave operation conditions, a maximum output power of 1319.08 nm wavelength was achieved at 11.26 W at a repetition rate of 100 Hz, corresponding to a maximum optical efficiency of 13.9% and a slope efficiency of 17.9%. In the active Q-switched regime, the pulse energy of the laser was as high as 800 μJ at a repetition rate of 10 kHz with a pulse width of 1.5 μs. Under non-critical phase-matched KTP crystal conditions, an average power of 2.03 W of 658.66 nm through intracavity frequency-doubling was obtained at a repetition frequency of 10 kHz with a duration of 1.3 μs, and the M 2 factor was measured to be about 5.8. To the best of our knowledge, this is the highest average power of an LED-pumped AO Q-switched 1319 nm laser and intracavity frequency-doubled red laser reported to date.

Welding of thin stainless-steel sheets using a QCW green laser source

Bipolar plates are structured thin metal sheets and are, next to the membrane electrode assembly (MEA), one of the main components of polymer electrolyte membrane fuel cells. One of the production steps of such bipolar plates is the joining process of its two halves. Laser welding is a suitable method for such an application since it is fast, non-contact, automatable, and scalable. Particularly important aspects of the weld seam are the weld seam width and depth. In this paper, welding of stainless-steel material analogous to materials used in bipolar plates is examined. For this purpose, a newly developed quasi continuous wave (QCW) green laser source with higher beam quality is employed to assess the effect of the wavelength and the spot diameter on the welding of stainless-steel material. By using various focusing lens, different sized beam diameters below 20 µm are achieved and their influence on the final welding result—specifically concerning the seam width—are analyzed. With welding speeds starting at 500 mm/s, reduced weld seam widths (≤ 100 µm) are realized, particularly with a focusing lens of 200 mm focal distance. The suitability of such a process for thin channels of under 75 µm width is examined.

Operation of 2 K cryogenic system with cold compressors for the SHINE test facility

The Shanghai high repetition rate X-ray free electron laser (XFEL) and extreme light facility (SHINE), a quasi-continuous wave hard XFEL facility, is currently under construction. In order to test all the superconducting components, assembled cryomodules and undulators, the SHINE test facility that consists of various test benches has been built. A superfluid cryoplant with 1 kW@2K was installed, commissioned, acceptance tested and turned into operation to support the cryogenic tests. The plant consists not only of a standard compressor system and cold box but also of a process vacuum system for 2 K operation. In the 2 K cryogenic system, three-stage cold compressors are firstly used to compress helium vapor at a mass flow of up to 50 g/s from 28 mbar to about 400 mbar. Afterwards, the sub-atmospheric helium is continually pressurize to the suction pressure of the warm compressors by five parallel process vacuum pumps. During the first eight months of 2 K operations, the tuning of the cold compressor regulation has been implemented to adapt the different static and dynamic heat loads. A helium pressure stability of superconducting cavities better than ±0.15 mbar at 31 mbar was achieved inside the vertical test bench with additional heating power of 160 W. In this paper, the pump down procedure and the operation results of cryogenic system at 2 K will be described.

Novel Bidirectional Output Ytterbium-Doped High Power Fiber Lasers: From Continuous to Quasi-Continuous.

Traditional ytterbium-doped high-power fiber lasers generally use a unidirectional output structure. To reduce the cost and improve the efficiency of the fiber laser, we propose a bidirectional output fiber laser (BOFL). The BOFL has many advantages over that of the traditional unidirectional output fiber laser (UOFL) and has a wide application in the industrial field. In theory, the model of the BOFL is established, and a comparison of the nonlinear effect in the traditional UOFL and the BOFL is studied. Experimentally, high-power continuous wave (CW) and quasi-continuous wave (QCW) BOFLs are demonstrated. In the continuous laser, we first combine the BOFL with the oscillating amplifying integrated structure, and a near-single-mode bidirectional 2 × 4 kW output with a total power of above 8 kW is demonstrated. Then, with the simple BOFL, a CW bidirectional 2 × 5 kW output with a total power of above 10 kW is demonstrated. By means of pump source modulation, a QCW BOFL is developed, and the output of a near-single mode QCW laser with a peak output of 2 × 4.5 kW with a total peak power of more than 9 kW is realized. Both CW and QCW output BOFL are the highest powers reported at present.

Application of Quasi-Continuous Waveform Coding in Spaceborne Synthetic Aperture Radar

Quasi-continuous wave radar is an attempt to give consideration to the performance of pulse and continuous wave radar signals. However, it also has the shortcomings of both. This paper aims to add a new quasi-continuous-wave coding method to the spaceborne synthetic aperture radar (SAR) system. The technology of improving spaceborne SAR imaging performance by coding quasi-continuous-wave pulses is studied, and some shortcomings of this algorithm are improved. Firstly, the application of quasi-continuous-wave radar in the SAR system is studied, and the coding and reconstruction scheme is provided so that this technology can be successfully applied in spaceborne SAR. Secondly, the effects of different quasi-continuous-wave coding methods on SAR imaging performance are evaluated, including signal-to-noise ratio, resolution, and integration time. Then, several coding schemes are given, and the characteristic changes of the signal after quasi-continuous-wave coding are analyzed. The transmit–receive conversion loss function and azimuth Doppler ambiguity function of the design scheme are analyzed, which proves the advantages of the scheme. Finally, we design the hardware implementation scheme and carry out the practical test.