Single Mode Continuous Wave Research Articles

Study of Continuous-Wave Single-Mode Lasing Efficiency in a Longitudinally Pumped YLF:Nd3+ Laser

Study of Continuous-Wave Single-Mode Lasing Efficiency in a Longitudinally Pumped YLF:Nd3+ Laser

Continuous-wave pumped dispersion-compensated ultrashort-pulse doubly resonant oscillator studies

We have reported theoretical analysis of broadband phase-coherent spectral generation with stable short pulse output based on optical parametric oscillator (OPOs) pumped by low-cost single-mode continuous-wave (cw) laser considering the incorporation of mode-locking element in the cw dispersion-compensated OPO in doubly-resonant oscillator (DRO) configuration. Exploiting the control of dispersion in the cw mode-locked cavity, the intracavity round-trip phase shift, phase coherence among resonating waves, carrier envelope offset phase of mode-locked output and enhancement of efficiency of system using active mode-locking element driven at high modulation frequency has been studied. The study paves the way of realization of compact, low-cost, efficient ultrashort pulse OPO under cw pumping.

Constructing process maps for pulsed wave laser additive manufacturing with interpretable machine learning

Oxide dispersion strengthened (ODS) steels have found increasing applications in advanced fission and fusion reactors due to their excellent strength and resistance to radiation damage. The traditional powder metallurgy (PM) methods for fabricating ODS steels are costly and time-consuming. To address these limitations, attempts have been made to fabricate ODS steels with emerging additive manufacturing (AM) technology, such as laser powder bed fusion (L-PBF). However, up to now, the AM-fabricated ODS steels have not been able to match the quality of those from the conventional PM processes. In this work, we explore pulsed wave (PW) laser AM, as opposed to the conventional single mode continuous wave (CW) laser AM, for ODS steel fabrication. In particular, based on available experimental data of PW laser fabricated ODS steel parts, we construct a two-dimensional (2D) process map using two PW features (engineered using operating parameters) to define and visualize the parameter space that is directly linked to the quality assessment of the parts. Additional process maps are constructed to quantitatively model the density and nano-hardness of the parts. These process maps are constructed through rational feature engineering, supervised feature selection, and interpretable machine learning techniques. The constructed process maps for quality classification, density and nano-hardness regression analyses are validated by experiments. The process maps help us further understand the physical phenomena that produce good or bad quality parts.

Ultra-low threshold continuous-wave quantum dot mini-BIC lasers

Highly compact lasers with ultra-low threshold and single-mode continuous wave (CW) operation have been a long sought-after component for photonic integrated circuits (PICs). Photonic bound states in the continuum (BICs), due to their excellent ability of trapping light and enhancing light-matter interaction, have been investigated in lasing configurations combining various BIC cavities and optical gain materials. However, the realization of BIC laser with a highly compact size and an ultra-low CW threshold has remained elusive. We demonstrate room temperature CW BIC lasers in the 1310 nm O-band wavelength range, by fabricating a miniaturized BIC cavity in an InAs/GaAs epitaxial quantum dot (QD) gain membrane. By enabling effective trapping of both light and carriers in all three dimensions, ultra-low threshold of 12 μW (0.052 kW cm−2) is achieved at room temperature. Single-mode lasing is also realized in cavities as small as only 5 × 5 unit cells (~2.5 × 2.5 μm2 cavity size) with a mode volume of 1.16(λ/n)3. The maximum operation temperature reaches 70 °C with a characteristic temperature of T0 ~93.9 K. With its advantages in terms of a small footprint, ultra-low power consumption, and adaptability for integration, the mini-BIC lasers offer a perspective light source for future PICs aimed at high-capacity optical communications, sensing and quantum information.

Experimental Study on the In-Band Amplified Spontaneous Emission in the Single-Mode Continuous-Wave Yb-Doped Fiber Amplifier Operating near 980 nm

In this paper, the in-band amplified spontaneous emission (ASE) in the Yb-doped continuous-wave (CW) fiber amplifier operating near 980 nm is experimentally studied for the first time, to the best of our knowledge. A core-pumped single-mode Yb-doped fiber amplifier is fabricated and the effects of pump power, seed power, and active fiber length on the in-band ASE are investigated in the experiment. It is found that a strong in-band ASE around 980 nm can be observed even with no obvious ASE around 1030 nm present. It is also found that with the increment of pump power, the in-band ASE can be enhanced faster than the signal light. By studying the effects of seed power and active fiber length, it is found that, although increasing the seed power and shortening the active fiber can both suppress the in-band ASE, the latter method is less effective than the former one. The theoretical study is also carried out in order to understand the difference between the in-band ASE and 1030-nm ASE. The experimental observations are also discussed qualitatively with the theoretical results. We believe that the pertinent results and discussions can provide significant guidance for understanding the in-band ASE in the Yb-doped fiber amplifier operating near 980 nm.

Surface removal of stainless steel using a single-mode continuous wave fiber laser to decontaminate primary circuits

Removing radioactive contaminated metal materials is a vital task during the decommissioning of nuclear power plants to reduce the cost of the post-dismantling process. The laser decontamination technique has been recognized as a key tool for a successful dismantling process as it enables a remote operation in radioactive facilities. It also minimizes exposure of workers to hazardous materials and reduces secondary waste, increasing the environmental friendless of the post-dismantling processing. In this work, we present a thorough and efficient laser decontamination approach using a single-mode continuous-wave (CW) laser. We subjected stainless steels to a surface-removal process that repetitively exposes the laser to a confined region of ∼75 μm at a high scanning rate of 10 m/s. We evaluate the decontamination performance by measuring the removal depth with a 3D scanning microscope and further investigate optimal removal conditions given practical parameters such as the laser power and scan properties. We successfully removed the metal surface to a depth of more than 40 μm with laser power of 300 W and ten scans, showing the potential to achieve an extremely high DF more than 1000 by simply increasing the number of scans and the laser power for the decontamination of primary circuits.

Continuous wave vertical emission from terahertz microcavity lasers with a dual injection scheme.

Quantum cascade lasers (QCLs) represent a most promising compact source at terahertz (THz) frequencies, but efficiency of their continuous wave (CW) operation still needs to be improved to achieve large-scale exploitation. Here, we demonstrate highly efficient operation of a subwavelength microcavity laser consisting of two evanescently coupled whispering gallery microdisk resonators. Exploiting a dual injection scheme for the laser cavity, single mode CW vertical emission at 3.3 THz is obtained at 10 K with 6.4 mA threshold current and 145 mW/A slope efficiency up to 320 μW emitted power measured in quasi-CW mode. The tuning of the laser emission directionality is also obtained by independently varying the pumping strength between the microdisks. By connecting the resonators through a suspended gold bridge, the laser out-coupling efficiency in the vertical direction is strongly enhanced. Owing to the high brightness, low-power consumption and CW operation, the proposed microcavity laser design could allow the realization of high-performance CW THz QCLs ready for massive parallelization.

Comparison of QCW pulsed laser and single-mode CW laser on the welding of power cell lugs

Aluminum alloy is the main material of pole ear plate and bus bar of power cells for new energy electric vehicles. In this paper, quasi-continuous wave (QCW) pulsed laser and single-mode continuous wave (CW) laser were used to conduct laser lap welding experiments on a 0.5 mm thick 1060 aluminum alloy and 2 mm thick 6061 aluminum alloy. The effects of these two laser sources on weld zone morphology and tensile force were compared. The experimental results show that the QCW pulsed laser welding is more likely to have porosity cracks and other defects, while the overall weld performance of the single-mode CW laser welding is obviously better than the former. In addition, the maximum tensile strength of the weld joint in QCW welding is about 861.02 N, while it can reach 1521.36 N in CW welding. The tensile force with CW laser welding is much higher than that required by the power battery industry standard, i.e., the tensile force of 800 N.

Room-temperature continuous-wave vertical-cavity surface-emitting lasers based on 2D layered organic–inorganic hybrid perovskites

Two-dimensional (2D) layered lead halide perovskites with large exciton binding energies, efficient radiative recombination, and outstanding environmental stability are regarded as supreme candidates for realizing highly compact and ultralow threshold lasers. However, continuous-wave (CW) pumped lasing of 2D lead halide perovskites, as the precondition for the electrically pumped lasing, is still challenging. Here, we tackled this challenge by demonstrating lasing emission in phenylethylammonium lead iodide [(PEA)2PbI4] embedded in a vertical microcavity under continuous pumping at room temperature. The millimeter-sized (PEA)2PbI4 single crystal was obtained from a two-step seed-growth method, showing high crystallization, excellent thermal stability, and outstanding optical properties. We used the exfoliated (PEA)2PbI4 thin flake as the gain medium to construct a vertical-cavity surface-emitting laser (VCSEL), showing robust single-mode CW lasing operation with an ultra-low threshold of 5.7 W cm−2 at room temperature, attributed to strong optical confinement in the high-Q cavity. Our findings provide a strategy to design and fabricate solution-based 2D perovskite VCSELs and mark a significant step toward the next-generation of coherent light sources.

High‐resolution continuous‐wave coherent anti‐Stokes Raman spectroscopy in a CO2‐filled hollow‐core photonic crystal fiber

AbstractWe demonstrated high‐resolution continuous‐wave (CW) coherent anti‐Stokes Raman spectroscopy of gaseous samples in a single ring hollow‐core photonic crystal fiber. Pump beam and Stokes beam were provided by CW single longitudinal mode lasers. These narrow linewidth lasers brought the benefit of a spectral resolution of 99 MHz. The high resolution allowed a measurement of a linewidth 0.3 GHz of the ν1 transition of CO2 in a gas mixture with partial pressure of 0.15 MPa and total pressure of 0.9 MPa. Pump beam from a tunable dye laser and Stokes beam from an external cavity diode laser were coupled into the fiber containing gaseous CO2. Fundamental mode phase matching was achieved by tuning gas pressure. The long interaction length provided by the hollow fiber yielded a 60‐dB increase in the anti‐Stoke signal. The attained signal to noise ratio of the spectrum of ν1 component of 0.6‐MPa CO2 was ~25,000 with 152‐mW pump power and 1.4‐mW Stokes power.

Continous-wave lasing operation of 1.3-μm wavelength InP-based photonic crystal surface-emitting lasers using MOVPE regrowth.

We report on electrically driven InP-based photonic-crystal surface-emitting lasers (PCSELs), which possess a deep-air-hole photonic crystal (PC) structure underneath an active region formed by metal-organic vapor-phase-epitaxial (MOVPE) regrowth. Single-mode continuous-wave (CW) lasing operation in 1.3-μm wavelength is successfully achieved at a temperature of 15°C. It is shown that the enhancement of lateral growth during the MOVPE regrowth process of air holes enables the formation of deep air holes with an atomically flat and thin overlayer, whose thickness is less than 100 nm. A threshold current of 120 mA (threshold current density = 0.68 kA/cm2) is obtained in a device with a diameter of 150 μm. A doughnut-like far-field pattern with the narrow beam divergence of less than 1° is observed. Strong optical confinement in the PC structure is revealed from measurements of the photonic band structure, and this strong optical confinement leads to the single-mode CW lasing operation with a low threshold current density.

Single-longitudinal-mode pumped pulsed-dye amplifier for high-resolution laser spectroscopy.

The In-Gas-jet Laser Ionization and Spectroscopy (IGLIS) technique relies on narrow-bandwidth, high-peak-power, short-pulse-length (≈10 ns), and high-repetition-rate laser pulses to probe, precisely and efficiently, the hyperfine structure of medium-heavy and heavy isotopes, embedded in a supersonic jet. The power and repetition rate requirements of the laser system are met by combining ≈100 W, 8 ns pulse width, 10 kHz commercial Nd:YAG pump lasers with a single-mode continuous wave seeded Pulsed Dye Amplifier (PDA). The common multi-longitudinal-mode operation of these Nd:YAG pump lasers causes, however, undesirable frequency sidebands in the output spectrum of the PDA system, hindering the attainable spectral resolution, a correct interpretation, and an accurate analysis of the hyperfine spectra. In this article, a new prototype Nd:YAG laser is presented, which combined with the PDA system is capable of providing quasi-transform-limited laser pulses at 10 kHz, with only limited losses in laser power. This system reduces any spectral sideband amplitude below a proven upper limit of 0.2% with one order of magnitude extra reduction expected based on simulations. A full characterization of both the Nd:YAG and PDA laser systems is done by studying the temporal and frequency behavior in detail. This study is finalized by a performance benchmark of this combined laser system in the hyperfine spectroscopy of copper isotopes, showcasing its applicability for future IGLIS studies.

151W monolithic diffraction-limited Yb-doped photonic bandgap fiber laser at ~978nm.

We demonstrate that the strong 4-level Yb emission in a fiber laser can be almost completely suppressed in an Yb all-solid double-clad photonic bandgap fiber, resulting in highly efficient high-power monolithic Yb fiber lasers operating at the 3-level system. We have achieved single-mode continuous wave laser output power of ~151W at ~978nm with an efficiency of 63% with respect to the launched pump power in a practical monolithic fiber laser configuration for the first time. The demonstrated power in this work are setting new records for diffraction-limited double-clad fiber lasers operating at ~978nm.

Tunable single-mode cw energy-transfer dye laser directly optically pumped by a diode laser

Within this work a tunable single-mode continuous wave energy transfer dye laser operation in green-yellow spectral region was achieved under direct optical pumping by an economy-class diode laser at λ = 445 nm. The laser is operated on a binary dye mixture: Coumarin 498 (donor) and Pyrromethene 556 (acceptor); the latter exhibits only negligible absorption at the pump wavelength, and thus its lasing is almost exclusively due to the excitation energy transfer. This binary energy transfer dye mixture was studied for the first time. Basic lasing properties at various acceptor concentrations were examined; donor concentration was fixed at the value optimum for its lasing under the optical pumping conditions applied. For the final acceptor concentration the optimum output coupling was determined for selected wavelengths from the measurements of the output power in variable output coupling scheme. Single-mode operation sufficiently stable for high-resolution spectroscopic applications was also achieved and tested. Further investigations of the energy transfer mechanisms and efficiency in the binary dye mixture applied are in progress and will be reported separately.

Evaluation of Luminescence Properties of Single Hydrophilic Upconversion Nanoparticles by Optical Trapping

By using multifunctional optical tweezers (OT) equipped with a 980 nm continuous-wave single-mode diode laser and multiple detectors, we are able to trap single upconversion nanoparticles stably as...

Laser Welding of Thin Copper and Aluminum Sheets: Feasibility and Challenges in Continuous-Wave Welding of Dissimilar Metals

The present paper defines process windows for laser welding of thin copper and aluminum sheets in Al-Cu and Cu-Al lap-joint configurations, exploiting different process strategies and parameters. A single-mode continuous-wave (CW) laser source was exploited with both linear and wobbling strategies. In both cases the influence of several parameters, such as, laser power, spot dimensions and scanning speed on the resulting microstructure and joint strength was analyzed. The influence of one, two and three parallel beads on the maximum shear strength was also evaluated. The optimum process configurations were finally determined based on maximum joint strength in shear tests and the mixing of the two parent metals occurring after welding was determined by means of SEM-EDS analysis.

Generation and characteristics of shock optical pulses based on a fiber-loop time-lens system

<sec>The shock ignition scheme has the advantages of low ignition energy threshold, high gain, and good hydrodynamic stability, which has become one of the key schemes for the potentially successful ignition of inertial confinement fusion. The crucial element of shock ignition is how to achieve a highly efficient shock laser pulse. We propose a new scheme based on a time-lens system combining the fiber-loop phase modulation and the grating-pair compression to generate a highly controllable shock pulse. Based on the asymmetric phase modulation in time-domain followed by linear dispersion compensation in frequency domain, the shock pulse can be actively controlled with high precision in both pulse duration and pulse contrast (peak power ratio of the compression part to the shock part of the pulse). We construct a theoretical model based on the nonlinear Schrödinger equation to simulate the evolution of the spectrum and temporal shape of the shock laser pulse. The influences of various key parameters of the proposed system on the characteristics of the generated shock pulse are analyzed in depth. </sec><sec>The time lens system consists of three parts, i.e. the seed pulse carving part, the phase modulation loop, and the chirp-compensating grating pair. The operation principle of this system for generating shock pulse is as follows. First, a single-mode continuous wave 1053 nm distributed feedback seed laser is chopped into pulses with a Mach-Zehnder intensity modulator. Then the pulses enter into a fiber-loop for phase modulation. Owing to different modulation frequencies exerted on the left and right side of the pulse, the amount of spectral broadening of these two sides of the spectrum are also different after phase modulation. The spectrally broadened pulses are linearly chirped when the phase-modulation function has a parabolic shape. Finally, the pulse transits through a grating pair system for chirp compensating. Just like an anomalous dispersion delay line, the grating pair applies an anomalous group velocity dispersion to the passing optical pulse. When the chirp is compensated for appropriately, the pulse will be compressed. What the target pulse can be finally shaped into is dependent on the combined optimization of all the above processes.</sec><sec>The simulation results show that by systematically designing the parameters such as chopping function, phase modulation function, modulation depth, modulation frequency, and chirp compensating, the target shock pulse can be actively controlled with high-precision in the pulse width, pulse rising edge, and peak-power contrast. In addition, we can also tune only one parameter (such as the pulse width) of the pulse, with the other parameters kept unchanged. This new design idea and the proposed system can actively and independently adjust the two key parameters (the peak power contrast and the pulse width) of the generated shock pulse, which is not only helpful in deepening our understanding of the principle of laser-pulse shaping, but also significant for the subsequent practical implement of shock ignition of inertial confinement fusion.</sec>

Terahertz light-emitting graphene-channel transistor toward single-mode lasing

AbstractA distributed feedback dual-gate graphene-channel field-effect transistor (DFB-DG-GFET) was fabricated as a current-injection terahertz (THz) light-emitting laser transistor. We observed a broadband emission in a 1–7.6-THz range with a maximum radiation power of ~10 μW as well as a single-mode emission at 5.2 THz with a radiation power of ~0.1 μW both at 100 K when the carrier injection stays between the lower cutoff and upper cutoff threshold levels. The device also exhibited peculiar nonlinear threshold-like behavior with respect to the current-injection level. The LED-like broadband emission is interpreted as an amplified spontaneous THz emission being transcended to a single-mode lasing. Design constraints on waveguide structures for better THz photon field confinement with higher gain overlapping as well as DFB cavity structures with higher Q factors are also addressed towards intense, single-mode continuous wave THz lasing at room temperature.

Photodarkening-Free Yb-Doped Saddle-Shaped Fiber for High Power Single-Mode 976-nm Laser

We propose a novel saddle-shaped Yb-doped fiber design for high power generation in the spectral range near 0.976 $\mu \text{m}$ . The fiber has a central single-mode part with a core diameter of approximately $12~\mu \text{m}$ and an ultra-thin silica clad of approximately $49~\mu \text{m}$ . At both ends of the fiber, the core/clad diameters were adiabatically increased up to 20/80 $\mu \text{m}$ to be compatible with ordinary passive fibers. A monolithic 976-nm single-mode continuous wave laser based on the proposed fiber was created. The laser’s power was a record high compared with all-fiber laser schemes, with an output power of 10.6 W. Utilization of a photodarkening-free core glass matrix has allowed us to demonstrate perfect long-term stability (without any power degradation) of the developed laser over 45 h.

6-mW Single-Mode High-Speed 1550-nm Wafer-Fused VCSELs for DWDM Application

This paper presents data on wafer-fused 1550-nm vertical-cavity surface-emitting lasers (VCSELs) based on the active region and distributed Bragg reflectors (DBRs) grown by molecular beam epitaxy. VCSELs with a tunnel junction aperture diameter of 8 μm show lasing at a threshold current density j th <; 3 kA/cm 2 , an output optical power of ~4 mW, and a -3 dB bandwidth of approximately 7 GHz at a 10-mA bias current. The devices demonstrate single-mode continuous wave operation with the transverse side-mode suppression ratio (SMSR) varying in the range of 40-45 dB up to roll-over currents. The increase in mirror losses due to the etching of the top DBR makes the output optical power increase to 6 mW and causes the wallplug efficiency value to reach 20%, but SMSR remains in the range of 40-45 dB. This also makes it possible to reduce both the photon lifetime and, as a result, the effect of damping and increase the modulation bandwidth to 9 GHz. The observed open and clear eye diagrams indicate that non-return-to-zero operation is possible at bit rates of up to 30 Gbps without equalization or forward error correction. The high-output optical power and modulation performance pave the way for the dense wavelength division multiplexing application of wafer-fused 1550-nm VCSELs.