Continuous-wave Laser Research Articles

High-performance 320-nm continuous-wave solid-state laser

For the first time, to our knowledge, we realized a high-performance 320-nm continuous-wave (CW) laser with a fiber-coupled blue laser diode as the pump source. A V-shaped folded cavity is constituted by three mirrors, with a Pr3+:LiYF4 (Pr:YLF) as the laser crystal and a LiB3O5 (LBO) as the frequency doubling crystal, respectively. Under an absorbed pump power of 19.5 W, the maximum 320-nm output power reaches 4.26 W, corresponding to an optical conversion efficiency of 21.8% and a root-mean-square (RMS) power fluctuation of 0.61%, which give the best results of a 320-nm CW laser up to now. The excellent performance will make this ultraviolet laser source very suitable for practical applications, including semiconductor detection, spectral analysis, microscopic imaging, and biotechnology.

Laser-Induced Decomposition and Mechanical Degradation of Carbon Fiber-Reinforced Polymer Subjected to a High-Energy Laser with Continuous Wave Power up to 120 kW

Carbon fiber-reinforced polymer (CFRP), noted for its outstanding properties including high specific strength and superior fatigue resistance, is increasingly employed in aerospace and other demanding applications. This study investigates the interactions between CFRP composites and high-energy lasers (HEL), with continuous wave laser powers reaching up to 120 kW. A novel automated sample exchange system, operated by a robotic arm, minimizes human exposure while enabling a sequence of targeted laser tests. High-speed imaging captures the rapid expansion of a plume consisting of hot gases and dust particles during the experiment. The research significantly advances empirical models by systematically examining the relationship between laser power, perforation times, and ablation rates. It demonstrates scalable predictions for the effects of high-energy laser radiation. A detailed examination of the damaged samples, both visually and via micro-focused computed X-ray tomography, offers insights into heat distribution and ablation dynamics, highlighting the anisotropic thermal properties of CFRP. Compression after impact (CAI) tests further assess the residual strength of the irradiated samples, enhancing the understanding of CFRP’s structural integrity post-irradiation. Collectively, these tests improve the knowledge of the thermal and mechanical behavior of CFRP under extreme irradiation conditions. The findings not only contribute to predictive modeling of CFRP’s response to laser irradiation but enhance the scalability of these models to higher laser powers, providing robust tools for predicting material behavior in high-performance settings.

Multi-octave two-color soliton frequency comb in integrated chalcogenide microresonators.

Mid-infrared (MIR) Kerr microcombs are of significant interest for portable dual-comb spectroscopy and precision molecular sensing due to strong molecular vibrational absorption in the MIR band. However, achieving a compact, octave-spanning MIR Kerr microcomb remains a challenge due to the lack of suitable MIR photonic materials for the core and cladding of integrated devices and appropriate MIR continuous-wave (CW) pump lasers. Here, we propose a novel slot concentric dual-ring (SCDR) microresonator based on an integrated chalcogenide glass chip, which offers excellent transmission performance and flexible dispersion engineering in the MIR band. This device achieves both phase-matching and group velocity matching in two separated anomalous dispersion regions, enabling phase-locked, two-color solitons in the MIR region with a commercial 2-μm CW laser as the pump source. Moreover, the spectral locking of the two-color soliton enhances pump wavelength selectivity, providing precise control over soliton dynamics. By leveraging the dispersion characteristics of the SCDR microresonator, we have demonstrated a multi-octave-spanning, two-color soliton microcomb, covering a spectral range from 1156.07 to 5054.95nm (200 THz) at a -40dB level, highlighting the versatility and broad applicability of our approach. And the proposed multi-octave MIR frequency comb is relevant for applications such as dual-comb spectroscopy and trace-gas sensing.

Efficient Thermo-Optic Switching and Modulating in Nonlocal Metasurfaces.

High-Q optical resonances in nonlocal metasurfaces, benefiting from significantly enhanced light/matter interactions, feature strong responses even under a weak external stimulus. In this work, we leverage the high-Q resonances of quasi-guided modes (QGMs) supported by a photonic crystal slab (PCS) structure to achieve efficient optical switching/modulation. The QGMs with an experimentally measured Q-factor of ∼2200 are realized by shifting every second column of air holes in a rectangular lattice within a silicon slab. At a weak illumination intensity of less than 4.0 W/cm2 from a 532 nm continuous-wave pump laser, the QGM resonance around 1550 nm experiences a pronounced spectral shift, with modulation depth exceeding 55%. This is attributed to the thermo-optic response caused by photothermal heating of the metasurface triggered by the absorption of the pump laser in silicon, which is further verified by the electrical heating approach. Our reported results showcase a simple yet effective way of tailoring light propagation in nonlocal metasurfaces.

Continous-wave and passively Q-switched laser at 1.36 µm of a Nd3+-doped phosphate fiber with a Yb3+-doped inner cladding.

All-fiber continuous-wave (CW) and passively Q-switched lasers at 1.36 µm (4F3/2 → 4I13/2) by a Nd3+-doped double cladding phosphate fiber are demonstrated for the first time, to the best of our knowledge. To suppress the competitive 0.9 and 1.05 µm emission, the Yb3+ ions are intentionally introduced into the inner cladding of this Nd3+-doped fiber. CW laser at 1.36 µm with a signal-to-noise ratio over 50 dB is realized by using two fiber-type dielectric films as cavity mirrors. A 4.0% CW laser efficiency is obtained when the Nd3+-doped fiber length is 33 mm. The emission at 0.9 and 1.05 µm is well-suppressed, and no parasitic laser is observed. Taking a commercial semiconductor saturable absorber (SA) mirror as the SA, the compact 1.36-µm Q-switched laser is demonstrated, and the repetition rate of output pulses can be tuned from 230 to 522 kHz with a narrowest pulse duration of 152 ns. Our results may provide a promising way to realize 1.3 µm laser oscillation in Nd3+-doped fibers.

Highly Productive Laser Annealing Manufacturing Method Using Continuous Blue WBC (Wavelength Beam Combining) Technique

Blue laser annealing can be used to obtain a high-mobility thin-film transistor (TFT) through a laser annealing (i.e., LTPS: low-temperature Poly-Si) process. However, the laser annealing process’s low productivity (as well as high cost) is an issue because the high output power of blue lasers still needs to be addressed. Therefore, productivity can be improved if blue laser energy is efficiently supplied during the laser annealing process using a continuous wave laser instead of a conventional pulsed excimer laser. We developed a blue laser light source (440 ± 10 nm) using the wavelength beam combining (WBC) method, which can achieve a laser power density of 73.7 kW/cm2. In this semiconductor laser, when the power was increased s by 2.9 times, the laser scanning speed was increased by 5.0 times, achieving twice the productivity of conventional lasers. After laser annealing, the size of the crystal grains varied between 2 and 15 μm, resulting in a crystallization rate of 100% by Raman scattering rsult and low resistivity of 0.04 Ωcm. This increase in production capacity is not an arithmetic increase with increased power but a geometric production progression.

Broadband generation and tomography of non-Gaussian states for ultra-fast optical quantum processors

Quantum information processors benefit from high clock frequencies to fully harness quantum advantages before they are lost to decoherence. All-optical systems offer unique benefits due to their inherent 100-THz carrier frequency, enabling the development of THz-clock frequency processors. However, the bandwidth of quantum light sources and measurement devices has been limited to the MHz range, with nonclassical state generation rates in the kHz range. In this study, we demonstrated broadband generation and quantum tomography of non-Gaussian states using an optical parametric amplifier (OPA) as a squeezed light source and an optical phase-sensitive amplifier (PSA). Our system includes a 6-THz squeezed-light source, a 6-THz PSA, and a 66-GHz homodyne detector. We successfully generated non-Gaussian states at a 0.9 MHz rate with sub-nanosecond wave packets using a continuous-wave laser. The performance is currently limited by the jitter of superconducting detectors, restricting the usable bandwidth to 1 GHz. Our technique extends the bandwidth to GHz, potentially increasing non-Gaussian state generation rates for practical optical quantum processors using OPAs.

Dispersion response broadband tunable underwater FMCW blue chirped laser source

Frequency-modulated continuous-wave (FMCW) narrow linewidth lasers have served as the cornerstone behind applications such as autonomous driving, wearable technology, virtual reality, and remote sensing mapping. Strongly coherent lasers are typically used for these studies, with a clear demand for linear fast response and wide frequency tuning range. In this paper, profiting from the ultrahigh-quality factor of the crystalline whispering-gallery-mode resonator, by using a self-injection locking mechanism to suppress spontaneous emission noise and improve coherence, sub-kHz linewidth at 450 nm is obtained. Furthermore, based on the dispersive response principle, fast electrical tuning is realized by using the strain-influenced resonator, and the experimental test result reaches 81 pm/V. More importantly, we demonstrate the comprehensive performance of this type of FMCW laser in underwater detection, with a sensitivity of 319 MHz/m at a chirp frequency of 1 kHz.

A low quantum defect and power scalable Yb-doped glass waveguide laser

We present a continuous wave laser action in ytterbium (Yb)-doped phosphate glass buried channel waveguides. The 25 mm long waveguides were fabricated by the standard ion exchange method. When pumped with a 976 nm laser diode, the waveguide laser emits at 982.4 nm, yielding an ultralow quantum defect of 0.66 %. The impact of coupling and pump power variation on the laser emission wavelengths is demonstrated. Furthermore, the results of pulsing using a monolayer graphene as a saturable absorber (SA) are presented and a pulse train with a repetition rate of 1 MHz and a pulse width of 700 ns was obtained.

Silica nanofiber membrane with high reflectivity and low absorptivity for high-energy laser protection (29.0 kW/cm2)

To safeguard against high-energy continuous wave laser, the chosen strategy is a robust reflection to restrict heat generation instead of thermal protection. Based on this, the SiO2 nanofiber membranes (0.12 g/cm3) with nearly 100% reflectivity and an extremely low absorption coefficient, were fabricated by electrospinning under the optimal parameters and calcinating at 600-1000 °C. The membranes, with a thickness of 4 mm, exhibit the best laser protection property reported to date. Under laser irradiation of 29.0 kW/cm2, they remain intact and achieve a steady state after 10 s with a rear temperature of only 700 °C because the necessary conditions for excellent laser protection property—the absence of highly absorbing groups, moderate fiber stacking density, presence of fiber stacking structure, chemical inertness, and specific thickness—are all met. In addition, excellent mechanical properties, including a tensile strength of 16.6 MPa (normalized value through equivalent solid materials for comparison purposes), a compressive strength of 0.07 MPa, and good bending performance, provide guarantees for their practical applications.

Cavity-enhanced Faraday rotation spectroscopy for interference-free measurement of OH radical at 2.8 μm

An instrument based on cavity-enhanced Faraday rotation spectroscopy (CE-FRS) operating at 2.8 μm has been developed for interference-free measurement of OH radicals in the laboratory. By off-axis coupling of a continuous-wave laser into a high finesse optical cavity, FRS signal is obtained from balanced detection of time-integrated light intensity leaking out of the cavity in the presence of magnetic field. Radio-frequency white noise (5–520 MHz) was injected into laser current which reduced intensity fluctuations in cavity transmission, thus improved the signal-to-noise ratio of the spectroscopic signal by a factor of 2. The setup provides a simple and robust spectroscopic instrument for in-situ and highly-selective detection of paramagnetic species. We demonstrated the instrument’s capabilities using OH radical with concentration in the range of 1012 molecule.cm−3, generated by microwave discharge of water vapor at low pressure. The CE-FRS instrument exhibited a limit of detection of ∼ 1010 molecule.cm−3 in an integration time of 20 s, which is enhanced by a factor of 2.5 compared to cavity-enhanced wavelength modulation spectroscopy involving an off-axis integrated cavity output spectroscopy approach. A time-resolved FRS signal was recorded in a pulsed microwave discharge regime, giving a millisecond time resolution for the measurement of OH concentration profile. The developed instrument provides a potential analytical tool for the measurement of OH concentration for chemical kinetic study in reactor cells.

Growth, spectroscopy properties, and laser operation of single crystal fiber: Nd3+-doped CNGG

Nd3+-doped calcium niobium gallium garnet single crystal fibers (Nd:CNGG SCFs) with varying Nd3+ concentrations were successfully grown using the laser-heated pedestal growth (LHPG) method. The study thoroughly examines the fundamental physical structure, optical properties, and laser characteristics of the as-grown Nd:CNGG SCFs. Results indicate that the Nd:CNGG SCF exhibits broader absorption and emission full width at half maximum (FWHM) compared to bulk single crystals. Moreover, pumped by a commercial 808 nm laser diode, continuous-wave laser operation at 1061 nm with a half-height width of 1.86 nm was achieved. The laser output power reached 1.04 W at an absorbed power of 7.46 W, with a slope efficiency of 25.4% relative to the absorbed pump power. These findings suggest that Nd:CNGG SCF holds promise as a practical and potent candidate for laser gain medium, offering significant research potential in the field of lasers.

Research on Phase Stabilization Algorithm of Femtosecond Timing System

This paper presents the design, implementation, and validation of a femtosecond timing system aimed at achieving precise time control and phase synchronization for large particle accelerators. A prototype system utilizing a continuous wave laser was developed, focusing on minimizing timing jitter and long-term phase drift. Key components include an optical delay line for coarse adjustments and a fiber stretcher for fine-tuning, achieving an adjustment precision of 1 femtosecond. The system incorporates a phase detection module with a non-In-phase/Quadrature downconversion approach, enabling high-accuracy phase measurements. A collaborative algorithm was designed to optimize the interplay between the optical delay line and the fiber stretcher, utilizing a proportional-integral-derivative (PID) control algorithm to enhance adjustment precision. A Field Programmable Gate Array (FPGA) served as the core interface converter, facilitating data communication and real-time phase information acquisition. Experimental results demonstrated significant improvements in phase stability, with average phase deviation reduced from 1374.104 fs to 15.782 fs, showcasing the effectiveness of the proposed system in achieving high precision and stability in phase control. This research provides a solid foundation for future advancements in timing systems for high-frequency reference signals.

Integrated Control of Thermal Residual Stress and Mechanical Properties by Adjusting Pulse-Wave Direct Energy Deposition.

Directed energy deposition with laser beam (DED-LB) components experience significant residual stress due to rapid heating and cooling cycles. Excessive residual tensile stress can lead to cracking in the deposited sample, resulting in service failure. This study utilized digital image correlation (DIC) and thermal imaging to observe the in situ temporal evolution of strain and temperature gradients across all layers of a deposited 316 L stainless steel thin wall during DED-LB. Both continuous-wave (CW) and pulsed-wave (PW) laser modes were employed. Additionally, the characteristics of thermal cracks and geometric dislocation density were examined. The results reveal that PW mode generates a lower temperature gradient, which in turn reduces thermal strain. In CW mode, the temperature-stress relationship curve of the additive manufacturing sample enters the "brittleness temperature zone", leading to the formation of numerous hot cracks. In contrast, PW mode samples are almost free of cracks, as the metal avoids crack-sensitive regions during solidification, thereby minimizing hot crack formation. Overall, these factors collectively contribute to reduced residual stress and improved mechanical properties through the adjustment of pulsed-wave laser deposition.

3D coherent single shot lidar imaging beyond coherence length

Advancements in remote sensing and autonomous vehicle technologies made lidars equally important for unmanned objects alongside cameras. Therefore, precise 3D lidar imaging and point cloud generation have become important subjects. Although existing coherent lidar technologies provide precise imaging results, the spectral linewidth of the laser sources becomes a key limitation over long distances as it defines the maximum detection range. Here, we present long-distance 3D lidar imaging which removes the coherence length limitations and therefore the necessity of high-coherence laser sources. Mainly, we generate optical sidebands, by modulating a continuous wave (CW) laser source with multiple radio-frequency (RF) tones. Then, using our own post-processing and triangulation methods, we use the relative phase changes between the sidebands which are free from laser phase noise to determine the target distance. We prove that the multi-tone coherent Lidar technique can perform precise 3D imaging and point cloud generation of various targets at sub-10pW optical power reception and distances up to ∼12× beyond the coherence length of the CW laser employed in the lidar architecture. Overall, it is demonstrated that coherence length restriction is removed by the suggested method, which makes precise long-distance 3D lidar imaging possible, particularly for applications such as spacecraft and aerial coherent lidars.

Trapping light, revealing properties: Laser trapping as a powerful tool for photoluminescence spectroscopy

Laser trapping, a non-contact technique for manipulating microscopic objects, has gained prominence in scientific research. When coupled with fluorescence spectroscopy, it offers a powerful tool for exploring material properties. This study demonstrates the application of laser trapping in the crystallization of MAPbBr3 perovskites and its simultaneous use for photoluminescence imaging. MAPbBr3 perovskites are a class of materials with exceptional optical properties, making them attractive for various optoelectronic applications. However, conventional excitation methods using UV light can lead to phase segregation and mechanical distortion of these materials. Two-photon excitation, on the other hand, offers advantages such as deeper penetration and reduced scattering interference. In this research, we utilize a 1064 nm continuous wave laser for both trapping and excitation purposes. The MAPbBr3 perovskite, with its absorption band ranging from 400 to 540 nm, exhibits two-photon excitation at 532 nm. By focusing the laser beam at the air-solution interface, we successfully crystallize MAPbBr3 perovskites from an unsaturated precursor solution. Simultaneously, the same laser source is used for photoluminescence imaging, allowing for real-time analysis of the crystal's emission properties. This approach eliminates the need for additional excitation sources and simplifies the experimental setup. The combination of laser trapping and two-photon excitation opens up new possibilities for studying perovskite materials. It provides a gentle and non-invasive method for manipulating and characterizing hybrid perovskites materials, paving the way for advancements in various fields such as optoelectronics and energy harvesting.

Plasmon-enhanced two photon excited emission from edges of one-dimensional plasmonic hotspots with continuous-wave laser excitation.

One-dimensional junctions between parallelly and closely arranged multiple silver nanowires (NWs) exhibit a large electromagnetic (EM) enhancement factor (FR) owing to both localized and surface plasmon resonances. Such junctions are referred to as one-dimensional (1D) hotspots (HSs). This study found that two-photon excited emissions, such as hyper-Rayleigh, hyper-Raman, and two-photon fluorescence of dye molecules, are generated at the edge of 1D HSs of NW dimers with continuous-wave near-infrared (NIR) laser excitation and propagated through 1D HSs; however, they were not generated from the centers of 1D HSs. Numerical EM calculations showed that FR of the NIR region for the edges of 1D HSs was larger than that for the centers by ∼102 times, resulting in the observation of two-photon excited emissions only from the edge of 1D HSs. The analysis of the NW dimer gap distance dependence of FR revealed that the lowest surface plasmon (SP) mode, compressed and localized at the edges of 1D HSs, was the origin of the large FR in the NIR region. The propagation of two-photon-excited emissions was supported by the higher-order coupled SP mode.

Study of Solvent Dependent on Third-Order Nonlinear Optical and Thermo-Optic Coefficient of Malachite Green Dye.

This study reveals the solvent dependent third-order nonlinear optical (NLO) features of malachite green (MG) dye using Z-scan technique. MG dye is dissolved in polar solvents, including DMF, DMSO, ethanol, acetone, methanol, and 1-propanol. The study employs the Z-scan instrument, using a continuous wave (CW) laser as the excitation source, operating at a wavelength of 650nm. The closed aperture (CA) and open aperture (OA) Z-scan techniques are used to assess the nonlinear indices of refraction (n2) and nonlinear coefficient of absorption (β) of MG dye. MG dye demonstrates both saturable absorption (SA) and reverse saturable absorption (RSA) in the OA Z-scan results, due to negative and positive NLO absorption coefficients. MG dye proves a self-defocusing NLO refractive index while operating under low-power conditions. The third-order NLO susceptibility (χ3) of MG dye in various liquids are calculated to be the order of 10-6 esu. Multilinear regression fit is applied here to investigate the various intermolecular interactions between the solute and solvent. MG dye exhibited large thermo-optic coefficient of the order of 10- 3 K- 1 is observed. The results give important new information about the characteristics of MG dye and indicate potential sources of material for NLO applications in future.

Concentration-Dependent Photoluminescence of Carbon Quantum Dots Useable in LED.

We synthesized carbon quantum dots (CQDs) using a solvothermal method with o-phenylenediamine as the carbon and nitrogen source. The sample was characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. When we continued the optical characterization of the CQDs, we were surprised to discover that the colors of the synthesized CQDs changed with the dilution of the original solution. In addition, the photoluminescence (PL) of CQDs under 405 nm continuous wave laser excitation was also investigated. It was found that CQDs with different concentrations exhibited different PL spectra. In order to explain the mechanism of different PL spectra, chemical characterization of the CQDs at different concentrations was performed again, revealing that the color change is independent of particle size and surface functional groups. Systematic optical characterization and theoretical analysis indicate that this color change results from the interparticle distance. Furthermore, we investigated the PL lifetimes of CQDs using time-resolved PL measurements and found that the PL lifetime values change with the concentration of CQDs, which is attributed to nonradiative transitions. Finally, we fabricated warm white-light-emitting diodes with CQDs that are proportionally adjusted in concentrations. The investigation developed a simple and effective method to tune the color of CQDs by adjusting the concentration through dilution of the original solution, which provides a new approach for the preparation and regulation of multicolor CQDs.

Continuous wave laser ablation for tailored titanium nanoparticle synthesis: temperature and liquid medium effects

We reported for the first time the generation of titanium (Ti) nanoparticles (NPs) in different liquids (deionized (DI) water and sodium dodecyl sulfate (SDS) solution) and at a range of temperatures (5 °C, 28 °C, 60 °C, and 80 °C) using continuous-wave high-power laser ablation in liquid (CWLAL). The CWLAL technique is a convenient and cost-effective way to generate NPs. The key outcomes of our investigation are the effects of temperature and the liquid mediums on the average size, generation rate, shape, surface charges, and crystallographic structure of the NPs. Generated NPs show consistent spherical shape regardless of liquid medium changes and temperature variation. SDS solution notably impacts NP size and generation rate with higher surface charges than DI water. For instance, at temperatures of 28 °C and 80 °C, the generation rates in SDS solution are 316 mg/hr and 309 mg/hr, with average NP sizes of 33 nm and 34 nm, respectively. In contrast, the generation rates in DI water are 96 mg/hr and 302 mg/hr, with NP sizes of 13 nm and 20 nm, respectively. The weaker crystallographic structure observed in NPs generated in SDS solution, compared to the more robust crystallographic structure of NPs synthesized in DI water. Liquid temperature plays a significant role in determining surface charges, average particle size, and molecular structure of NPs. The choice of the liquid medium and temperature can be critical for tailoring NP characteristics to specific applications. Ongoing work is being conducted to explore the possibilities of further progress in this area to generate efficient and customized NPs.