Efficient CW Research Articles

Continuous-Wave Self-Raman Vanadate Lasers Generating Versatile Visible Wavelengths

In this review, the developments of efficient high-power CW orange-lime-green lasers by using intracavity stimulated Raman Scattering (SRS) in Nd-doped vanadate lasers are systematically discussed. The overall properties of the spontaneous Raman spectra in Nd:YVO4 and Nd:GdVO4 crystals are overviewed. The critical phase matchings of using the lithium triborate (LBO) crystals for sum frequency generation (SFG) and second harmonic generation (SHG) are thoroughly reviewed. We make a detailed review for achieving the individual green-lime-orange emissions from the self-Raman Nd:YVO4 and Nd:GdVO4 lasers with LBO crystals. The following is to review the dual-wavelength operations of the lime-green and orange-green lasers. Finally, the procedure for generating the triple-wavelength operation of orange-lime-green simultaneous emissions is completely described. The present review is expected to be useful for developing compact, efficient, high-power CW visible lasers for applications including medical treatment, biology, spectroscopy, and remote sensing.

Study Filter Materials for Vertical Subsurface Flow Constructed Wetland to Treat Wastewater from the Da Mai Noodle Handicraft Village in Bac Giang Province

Vertical subsurface flow-constructed wetlands (VSF CW) are evaluated in this research as a potentially effective and cost-efficient solution for treating wastewater from Da Mai noodle handicraft village in Bac Giang province. Due to the high pollution load in this type of wastewater, there are concerns about clogging and the stability of the technology. In this study, we explored different sizes of limestone and gravel for the five medium composition formulas in the VSF CW system. The experiments were conducted over a period of three months. We used ANOVA, one-sample T-test, and Pearson correlation analysis methods to evaluate the experimental results, with statistical significance set at 0.05. The results showed a linear correlation between medium size, hydraulic conductivity, and treatment efficiency (p < 0.05). Smaller medium sizes had higher pollutant removal efficiency but lower porosity and hydraulic conductivity. For the same sizes, limestone demonstrated higher treatment efficiency compared to gravel (p < 0.05). The Cp5 formula (comprising sand, 1×2 cm limestone, and 3×5 cm gravel) was selected as the best filter medium for VSF CW, achieving COD, TSS, TN, and TP removal efficiencies of 86.02 ± 1.71%, 81.15 ± 2.24%, 64.46 ± 2.23%, and 69.76 ± 2.68%, respectively. After three months of operation, the treated wastewater consistently met QCVN 40:2011/BTNMT Column B standards, and there were no significant differences in hydraulic conductivity (p > 0.05), indicating stable operation of the VSF CW.

Hydrodynamic performance of a novel WEC-breakwater integrated system consisting of triple dual-freedom pontoons

A breakwater consisting of three pontoon-type wave energy converters equipped with Power Take-Off (PTO) systems to extract wave energy from pitch and heave motions is proposed. The proposed integrated system has the potential to reduce the cost of the wave energy conversion system by sharing the essential infrastructure with the floating breakwater. In the model, the eigenfunction expansion matching method and technique of variables separation are used. The effects of the geometrical parameters (including pontoon width, draft and spacing) on the hydrodynamic performance characterized by the wave energy conversion efficiency, transmission and reflection coefficients are investigated, respectively. It is found that the effective bandwidth (transmission coefficient KT < 0.5 and hydrodynamic efficiency Cw > 0.3) and the peak efficiency are enhanced when the pitch and heave motions are considered simultaneously, compared with the single DoF motion system. The effective bandwidth increases with the decrease of width and draft of the front pontoon. And the variation of pontoon spacing affects significantly the distribution of the efficiency due to the Bragg-type reflection. Additionally, it is found that the variation of the geometrical parameters of the front pontoon on wave transmission is limited by changing the geometrical parameters of the front pontoon only.

Development and validation of a steep incline and decline metabolic cost equation for steady-state walking.

The purpose of this study was to develop and validate a data-supported prediction equation (Lankford equation) for walking metabolic cost ([Formula: see text]), and to compare this equation to the ACSM, Pandolf, Minetti, and LCDA equations. The current study also investigated how kinematics of incline walking relates to mechanical efficiency and metabolic cost. Subjects consisted of 145 recreationally fit individuals. Walking speeds were between 1 AND 3 mph with grades ranging from - 18 to 40%. The Lankford equation was then compared to four other reference equations using adjusted R-squared (R2) and Root Mean Square Error (RMSE) as primary metrics to determine correlation with measured CW. Kinematics data collected from reflective markers placed on bony landmarks were compared to CW, incline, and metabolic efficiency to determine the interrelationship between these variables. The Lankford equation for estimating [Formula: see text] was validated with an adjusted R2 = 0.89 and a RMSE of 5.92Kjmin-1, shown to have the highest accuracy among all equations tested. A 0.21 efficiency plateau was observed above 15% incline, and hip, knee, HAT, thigh, and shank angles at foot touch down were found to be highly correlated with [Formula: see text] (r > 0.980). The Lankford equation is a validated and highly accurate prediction equation for steady-state walking across a wide range of inclines and speeds and is applicable to the general public. Altered leg swing observed above 15% incline was found to account for the mechanical efficiency plateau and the rectilinear increase in [Formula: see text] with increasing incline.

Alexandrite: an attractive thin-disk laser material alternative to Yb:YAG?

Yb:YAG thin-disk (TD) technology has enabled construction of laser/amplifier systems with unprecedented average/peak power levels, and has become the workhorse of many scientific investigations. On the other hand, for some applications, the narrow emission bandwidth of Yb:YAG limits its potential, and the search for alternative broadband TD gain media with suitable thermo-optomechanical parameters is ongoing. The alexandrite gain medium has a broad emission spectrum centered around 750 nm, possesses thermomechanical strength that even outperforms Yb:YAG, and has unique spectroscopic properties enabling efficient laser operation even at elevated temperatures. In this work, we have numerically investigated the power scaling potential of continuous-wave (cw) alexandrite lasers in TD geometry for the first time. Using a detailed laser model, we have compared the potential cw laser performance of Yb:YAG, Ti:Sapphire, Cr:LiSAF, Cr:LiCAF, and alexandrite thin-disk lasers under similar conditions and show that among the investigated transition metal-doped gain media, alexandrite is the best alternative to Yb:YAG in power scaling studies at room temperature. Our analysis further demonstrates that potentially Ti:Sapphire is also a good alternative TD material, but only at cryogenic temperatures. However, in comparison with Yb:YAG, the achievable laser gain is relatively low for both alexandrite and Ti:Sapphire, which then requires usage of low-loss cavities with small output coupling for efficient cw operation.

Electrically pumped efficient broadband CW frequency conversion in diode lasers using bulk χ2

Optical frequency conversion in chip-scale devices enables crucial applications in myriad fields, such as optical communication, spectroscopy, and sensing. With large nonlinearities and the possibility of monolithic pump laser integration coexisting within the same platform, compound semiconductor is a choice of material superior to many others. In this work, we demonstrate an intracavity difference frequency generation, electrically pumped diode lasers with external input signal beam. Thanks to exact phase matching and large bulk nonlinearity in the laser structures reported, there is a normalized internal conversion efficiency of 169% W−1 cm−2. We also demonstrate a broad tuning range between 1486 nm and 1686 nm, or 24 THz, which spans the S-C-L-U telecommunications bands.

The codopant assisted tunable photoluminescence and highly efficient CW lasers in Nd3+:SrF2 crystal

Generally, the well shielded f orbitals made the f-f transition of lanthanides be non-tunable and sharp lines. In the work, the spectral property of Nd3+:SrF2 crystal codoped with R3+ (R3+ = Y3+, La3+, Gd3+) were investigated. The results show that with no loss of crystal structure integrity, the photoluminescence were tuned by the codopants in large scale, and altered significantly with the codopant species and concentrations, which strongly indicates that the f-f transitions could be modulated and tuned rather than the general considerations, and the codopants prefer to work as local lattice structure regulators. At the viewpoint, the designing of new lanthanide-doped laser crystals through the local structure modulation was suggested. With help of the codopant, a quasi-single-center system of Nd3+,Y3+:SrF2 and Nd3+,Gd3+:SrF2 were obtained, and the peak emission cross section, lifetime, linewidth and quantum yields were manipulated to be about 5 × 10−20 cm2, 350 μs, 20 nm and 90%, respectively. The highly improved CW lasing efficiency and output power agreed well with the spectral properties. The modified photoluminescence parameters show that Nd3+,R3+:SrF2 crystal with high thermal conductivity were very promising candidate for generation of high repetition rate ultrashort 1 μm lasers.

Growth, spectroscopy and high power laser operation of Yb:YAl3(BO3)4 crystal: Continuous-wave, mode-locking and chirped pulse regenerative amplification

We report on growth, spectroscopy and continuous-wave, mode-locking lasing and chirped pulse regenerative amplification of Yb:YAl3(BO3)4 (Yb:YAB) crystal. Efficient CW laser operation with 9.2 W output power, 46% optical-to-optical and 74% slope efficiencies was demonstrated. Efficient SESAM mode-locked Yb:YAB bulk laser was demonstrated for σ-polarization with 4 W average output power at 70 MHz pulse repetition frequency (PRF) and 195-fs pulse duration resulting in the pulse energy of 57 nJ and pulse peak power of 0.3 MW. Optical-to-optical efficiency of 27% in mode-locked regime is demonstrated. Chirped pulse regenerative amplifier based on Yb:YAB crystal demonstrated with maximum output power of 4.6 W (6 W of chirped pulses and optical-to-optical efficiency of 25%) and 695 fs pulse duration at 100 kHz PRF for the first time to the best of our knowledge.

Highly efficient CW operation of a diode pumped Nd:Y2O3 ceramic laser

High efficiency continuous-wave laser operation of an in-house made Nd:Y2O3 ceramic was demonstrated. The Nd:Y2O3 laser ceramic was fabricated by vacuum sintering plus the hot isostatic pressing process, whose in-line transmittance reaches 81.6% at the wavelength of 1000 nm. The 1.08 and 1.36 µm CW laser oscillation of an uncoated ceramic sample was experimentally investigated. Both achieved the highest slope efficiency and output power so far reported for the Nd:Y2O3 ceramic lasers. For the operation of 4F3/2-4I11/2 transitions, simultaneous oscillations at 1074.6 nm and 1078.8 nm were observed. At an absorbed pump power of 7.0 W, a maximum output power of 3.01 W was obtained with a slope efficiency of 49.6%. For the laser operation of 4F3/2-4I13/2 transitions, a 1.68 W single wavelength at 1357.7 nm with the slope efficiency of 29.3% was achieved at the absorbed pump power of 6.7 W.

A compact high efficient Tm:YLF laser dual-end-pumped by an equidirectional-polarizing fiber coupled laser diode at room temperature

We report a new compact high efficient CW Tm:YLF laser dual-end-pumped by an equidirectional-polarizing fiber coupled laser diode at room temperature. The maximum output power was 15.2 W at 1907.6 nm with the incident pump power of 47.4 W, corresponding to the optical-to-optical efficiency of 32.1% and the slope efficiency of 37.2%, the beam quality factors were M2 x = 3.19 and M2 y = 3.29, respectively.

Crystal growth, spectroscopic characterization, and sub-100 femtosecond mode-locked operation of a Yb:LiLuF_4 laser

We present a comprehensive growth, spectroscopic, and laser performance investigation of a Yb:LiLuF4 crystal, including CW laser experiments, and, for the first time to the best of our knowledge, mode-locked laser operation. Pumping with two 400 mW, polarization-combined, single-mode fiber-coupled laser diodes, we demonstrated efficient CW emission with 140 mW maximum output power and up to 59% slope efficiency. Using a semiconductor saturable absorber mirror for inducing the mode-locked regime and a pair of fused silica prisms for intracavity group delay dispersion compensation, we obtained soliton mode-locking pulses with 90 fs duration for the emission with polarization parallel to the crystal c-axis and 113 fs for the orthogonal polarization, with average power exceeding 30 mW in both cases at about a 90 MHz repetition rate.

Longitudinally diode-pumped planar waveguide YAG/Yb:LuAG/YAG ceramic laser at 10307 nm

Composite YAG/15 at. % Yb:LuAG/YAG transparent ceramic planar waveguide was fabricated by a tape casting method and vacuum sintering technology. Under a 970 nm diode laser pumping, the absorbed efficiency of 85.4% was achieved, and efficient CW laser operation at 1030.7 nm was accomplished with a good beam quality with Gaussian spatial profile. A maximum output power of 288 mW was obtained under a pump power of 4.69 W, corresponding to a slope efficiency of 9% and an O-O conversion efficiency of 5%.

Spectroscopic properties and high-power laser operation of Yb0.14:Y0.77Gd0.09Ca4O(BO3)3 mixed crystal

We report, for the first time to our knowledge, on the spectroscopic properties and continuous-wave laser performance of Yb0.14:Y0.77Gd0.09Ca4O(BO3)3, a mixed rare earth calcium oxyborate Yb-ion crystal. Under simple end-pumping conditions with a 976-nm diode, efficient CW laser operation was demonstrated at room temperature, producing an output power of 14.1W at 1084.4nm with an optical-to-optical efficiency of 48%; while operating around 1045nm, the laser could generate an output power as high as 23.0W, with optical-to-optical and slope efficiencies amounting, respectively, to 57% and 70% with respect to incident pump power. The polarized absorption and emission cross sections are also presented. The impressive results demonstrated reveal the great potential of these mixed oxyborates in developing new promising Yb-ion laser crystals.

High-power CW and passively Q-switched laser operation of Yb:GdCa4O(BO3)3 crystal

We demonstrate efficient high-power CW and passively Q-switched operations of Yb:GdCa4O(BO3)3 lasers. An output power of 18.2W is generated at 1031.5nm in CW mode, with optical-to-optical and slope efficiencies being respectively 55% and 70%, with respect to incident pump power. In passively Q-switched operation with a Cr4+:YAG crystal as saturable absorber, a maximum average output power of 15.6W is produced at a pulse repetition rate of 91kHz, with an optical-to-optical efficiency of 44%. Low-repetition-rate Q-switched action is also realized, generating an average output power of 4.2W at 5.7kHz, the resulting pulse energy and duration are 737μJ and 3.6ns, leading to a peak power amounting to 205kW.

Research on all-solid-state blue lasers

Blue lasers have many important applications such as high-density optical data storage, biological and medical diagnostics, color displays, and submarine laser imaging and communication. High efficiency, high brightness and compactness all-solid-state blue lasers have attracted much attention for their merits. In this paper, four main ways for achieving all-solid-state blue lasers are reviewed and compared in detail, such as the semiconductor laser, the frequency-doubling of the semiconductor laser, the up-conversion blue laser, the nonlinear optical frequency conversion of Nd3+-doped or optical parametric oscillating lasers and so on. Results show that the nonlinear optical frequency conversion of the Nd3+-doped lasers is a potential way to realize a high power, high efficiency cw or pulsed blue laser output.

Municipal Wastewater Treatment with Phragmites australis L. and Typha latifolia L. for Irrigation Reuse. Boron and Heavy Metals

In this work, we compared the performance of Phragmites australis and Typha latifolia for depurating primary-treated urban wastewater and evaluated their suitability for irrigation reuse. Macrophytes were planted in two pilot-scale constructed wetland systems (CWs) and monitored during a 2-year experiment (2002–2003). CW efficiency was evaluated in terms of both mass removal and water quality considering boron (B) and the following heavy metals: aluminium (Al), arsenic (As), beryllium (Be) manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), lead (Pb), selenium (Se) and vanadium (V). The accumulation of the elements, both in plant tissues and sandy substrate layer, and their offtake with the macrophyte harvest were also measured. In quantitative terms, the established CW systems showed high removal efficiency for Al (96 %), Cu (91 %), Pb (88 %) and Zn (85 %), while lower efficiencies were observed for Fe (44 %), Co (31 %) and B (40 %). The sediment played a strategic role in the adsorption and accumulation of wastewater pollutants, while plants acted as phytostabilizers since element root concentrations were generally from one to two orders of magnitude higher than those observed in the other parts. The results were less favourable in terms of water quality because the high evapotranspiration counteracted the depuration process by concentrating the elements in the outflow water. Outflow water contained more B (68 %), Mn (196 %) and, in the case of CW managed with Phragmites, also Fe (73 %) than inflow water, breaking the Italian guidelines for irrigation reuse. Integrating solutions to reduce the high evapotranspiration of CWs with more efficient pre-cleaning systems are necessary to obtain better removal efficiencies that reduce the effect of ET on water quality.

Efficient upconversion-pumped continuous wave Er3+:LiLuF4 lasers

We report on detailed spectroscopic investigations and efficient visible upconversion laser operation of Er3+:LiLuF4. This material allows for efficient resonant excited-state-absorption (ESA) pumping at 974nm. Under spectroscopic conditions without external feedback, ESA at the laser wavelength of 552nm prevails stimulated emission. Under lasing conditions in a resonant cavity, the high intracavity photon density bleaches the ESA at 552nm, allowing for efficient cw laser operation.We obtained the highest output power of any room-temperature crystalline upconversion laser. The laser achieves a cw output power of 774mW at a slope efficiency of 19% with respect to the incident pump power delivered by an optically-pumped semiconductor laser. The absorption efficiency of the pump radiation is estimated to be below 50%.To exploit the high confinement in waveguides for this laser, we employed femtosecond-laser pulses to inscribe a cladding of parallel tracks of modified material into Er3+:LiLuF4 crystals. The core material allows for low-loss waveguiding at pump and laser wavelengths. Under Ti:sapphire pumping at 974nm, the first crystalline upconversion waveguide laser has been realized. We obtained waveguide-laser operation with up to 10mW of output power at 553nm.

Highly efficient 10 cm long fiber laser.

We investigate an innovative multipass-pumping technique in a short photonic crystal fiber laser. The technique is based on simple optical elements and shows, in a power oscillator configuration, highly efficient CW and Q-switched operation of an extremely short fiber laser. We report significantly higher absorption of the pump light, compared to a single-pass configuration, which results in an impressive fourfold increase in slope efficiency, up to 60%. Moreover, we show pulse energy of 1.22mJ and pulse duration shorter than 12ns, corresponding to a peak power of more than 100kW.

Efficient CW Dual-Wavelength and Passively $Q$ -Switched Tm:LuAG Lasers

Efficient diode-pumped Tm:LuAG lasers operating in continuous wave, dual-wavelength,and passive $Q$ -switching regimes have been demonstrated. A maximum output power of 4.42 W with a beam quality of $\mathrm{M}^{\mathbf {2}}\le 1.45$ at 2021.2 nm was obtained, corresponding to a slope efficiency of 49.5%. Using a birefringent quartz plate, the Tm:LuAG laser emitted the highest output power of 2.3 W at dual-wavelength of 2022 and 2030 nm. With a SWCNT-SA employed as saturable absorber, passively $Q$ -switched Tm:LuAG laser was realized, which generated the shortest pulses of 405 ns at 2021.6 nm and a maximum single pulse energy of $40.6~\mu $ J at 2019.7 nm.

Efficient tunable multi-wavelength CW Nd:GGG laser at 1.06 and 1.4 μm regions

We demonstrat a diode-pumped, efficient tunable multi-wavelength CW Nd:GGG laser simultaneously oscillated at 1.06 and 1.4μm optical region for the first time. Besides an output power of 1.31W at 1.06μm, a maximum output power of 226mW at 1.4μm region is obtained under a diode pump power of 6.52W. The corresponding slope efficiency and diode-to-optical conversion efficiency are 4.73% and 3.47%, respectively. By using a quartz plate as frequency filter, the laser around 1.4μm can be tuned in the range from 1407.6 to 1436.8nm, indicating that Nd:GGG laser at 1.4μm region is a promising candidate for femtosecond pulse generation. The M2 factor of the Nd:GGG laser beam at 1.4μm is measured to be 1.21 and 1.23 in x- and y-direction, respectively. By adjusting the tilt angle, the stable multi-wavelength operation with comparable intensity can be obtained.