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Highlights • High-power passively Q-switched Tm:CYA lasers at short wavelengths have been operated. • The operating wavelength was tuned from 1848 to 1876 nm, for the first time. • The tunable Tm:CYA laser is very potential in neural stimulation applications. Abstract Laser emission between about 1850 and 1880 nm is of great interest for neural stimulation applications. We report here on the diode-pumped, continuous-wave and passively Q-switched laser operation of Tm:CaYAlO 4 in this specific wavelength domain, for the first time to our knowledge. In the continuous-wave regime, the maximum output power reaches 5.35 W at 1864 nm with a laser slope efficiency of about 40.5%. Using a Cr:ZnSe saturable absorber, passive Q-switching is achieved with a maximum average output power of 2.37 W at 1857 nm. The narrowest laser pulse width is found to be about 59 ns for a repetition rate of 35.3 kHz and the corresponding laser pulse peak power and pulse energy are about 1.145 kW and 67.5 μJ, respectively. By introducing an undoped YAG crystal as intracavity etalon, tunable and passively Q-switched laser operation is obtained between 1848 and 1876 nm while keeping high pulse energies and peak powers. [ABSTRACT FROM AUTHOR]

• Spectral properties and high-efficiency broadband laser operation of Tm:CYGA crystal at 2 μm were demonstrated. • The maximum continuous-wave output power of 1.43 W was obtained. • A 284.7 nm tuning coverage was realized with a BF. We report on spectral properties and a high-efficiency broadband laser operation of Tm:CaY 0.9 Gd 0.1 AlO 4 (Tm:CYGA) crystal. The polarized absorption and emission spectra, gain cross sections and fluorescence lifetime are systematically studied, presenting a broad absorption and emission spectra and thus a smooth gain band. Furthermore, under an absorbed pump power of 4.35 W, a maximum continuous-wave (CW) laser output power of 1.43 W at 1967.8 nm is obtained, corresponding to a slope efficiency of 42.2 %. The beam quality factors (M2) in X-axis and Y-axis are measured to be 1.011 and 1.027, respectively. A tunable Tm:CYGA laser with a tuning range of 284.7 nm from 1837.5 to 2122.2 nm is achieved employing a birefringent filter (BF). The results indicate that the Tm:CYGA crystal is an excellent laser medium for generating high-efficiency broadband CW laser, indicating its potential for ultrashort pulse generation in the 2-μm spectral range. [ABSTRACT FROM AUTHOR]

We investigated the 1443.9 nm laser characteristics of Nd:GdLuAG crystal. Diode-end-pumping configuration was employed under both continuous-wave (CW) and passively Q- switched operations. For CW operation, the maximum average output power was 1.36 W with a slope efficiency of 15%. By using a V 3+ :YAG crystal as the saturable absorber, we obtained the maximum average output power of 164 mW under Q -switched operation. The corresponding pulse energy was 29.3 μJ and pulse duration was 59 ns. [ABSTRACT FROM AUTHOR]

We report on diode-pumped Nd:LuAG crystal lasers on 4 F 3/2 → 4 I 11/2 transition around 1.1 μm in continuous-wave and passive Q-switching regimes. For continuous-wave laser operation, a dual-wavelength laser at 1117 and 1122 nm is achieved with maximum output power of 5.82 W and slope efficiency of about 42.8%, which is believed to be the highest output power of Nd:LuAG lasers at 1.1 μm. Using an etalon to tune the lasing wavelength, a single-wavelength laser at 1114 nm is also realized, for the first time to our knowledge, with maximum output power of 4.11 W. Further wavelength tuning from 1113.9 to 1122.6 nm is also obtainable. Passively Q-switched laser operation at 1122 nm is finally performed with a maximum average output power of 0.73 W, the narrowest pulse width of 21.2 ns and a maximum pulse repetition rate of 14.12 kHz, which leads to a maximum pulse peak power of 2.43 kW and a maximum pulse energy of about 51.7 μJ. [ABSTRACT FROM AUTHOR]

A method of width gauging of surface slot using laser-generated Rayleigh waves in time domain is presented. A two-step detection is employed in this method, Rayleigh waves are first generated on one side of the surface slot and then on the other side. Incident and reflected Rayleigh waves from surface slot are detected respectively on both sides of the slot in the two detections. Width of surface slot is calculated based on the arrival time of incident and reflected Rayleigh waves. Experiment results agree well with the measured results by digital microscope and validate the feasibility of the proposed method. The approach will open the way for simultaneous measurement of the depth and width of surface slot and provide a potential application for characterization of surface slot in extreme environment and width gauging of subsurface structure. [ABSTRACT FROM AUTHOR]

• A transient thermal grating (TTG) method is induced by pulsed laser. • The time–frequency scalograms obtained by wavelet transform are used to train a DCNN model. • The proposed method can characterize the penetration depth of cracks reliably and flexibly in automatic detection. An effective nondestructive testing technique that enables the detection and quantification of surface and subsurface defects is highly demanded for assuring the safety and reliability of materials. In this paper, we classified and regressed defects with different depths. A transient thermal grating (TTG) method induced by a pulsed laser is used to characterize the properties of surface defects. Based on thermo-elasticity theory, a theoretical model with different types of surface defects is established with the finite element method (FEM). Surface acoustic waves (SAWs) excited by the TTG method propagate at the surface of a sample with defects at different penetration depths and are measured by a vibrometer. By combining the simulation and experimental results, the time–frequency scalograms of 8200 images were obtained by wavelet transform analysis. 70 % of the images are randomly selected to train the residual network (Resnet) model, and the remaining data (2460 images), as well as 10 scalograms in an independent experiment, are used as previously unknown experimental signals to test the robustness of the Resnet model. The results indicate that the Resnet with scalograms via wavelet transform achieves a 96.21 % classification accuracy on the validation set, with a prediction accuracy of 90.00 % for the 10 scalograms. Moreover, compared with three widely used machine learning algorithms, VGG16, back propagation (BP), and support vector machine (SVM), this method achieves the highest classification accuracy. Finally, 500 data selected randomly from the previously trained experimental database and 10 data in an independent experiment are used for regression prediction. The result shows that the data are well located along or close to a straight line with a slope of 1. The statistical results suggest that 80.6 % (66.0 %) of the residuals of the experimental data fell within the size range ±10 μm (±5 μm). This work proves the feasibility and reliability of the combination of the Resnet model with wavelet transform and numerical simulation to identify the depth of surface defects using laser ultrasonics. [ABSTRACT FROM AUTHOR]

We report diode-pumped continuous-wave (CW) and Q-switched Nd:GYSO lasers using a compact two-mirror linear laser cavity. Single-wavelength laser emissions at 1074.11 nm with 4.1-W power and at 1058.27 nm with 1.47-W power have been obtained in CW mode. The slope efficiencies with respect to the absorbed pump powers are 48.5% and 22.9%, respectively. Wavelength tunability is also demonstrated with range of about 8 nm. Using a MoS 2 saturable absorber, maximum average output power up to 410 mW at 1074 nm can be yielded with absorbed pump power 6.41 W and the maximum pulse energy reaches 1.20 μJ with pulse repetition rate of 342.5 kHz and shortest pulse width of 810 ns. The CW laser results represent the best laser performance and the Q-switching also present the highest output power for Q-switched Nd 3+ lasers with MoS 2 as saturable absorber. [ABSTRACT FROM AUTHOR]

The generation of 72 fs hyperbolic secant pulses centered at 1050 nm with 17.8 nm bandwidth from a diode pumped Kerr-lens mode-locked Yb: GSO laser is demonstrated. With the help of a semiconductor saturable absorber mirror, stable mode-locking with an average output power of 85 mW at a repetition rate of 113 MHz is realized. To the best of our knowledge, this is the first demonstration of Kerr-lens mode-locking in Yb: GSO laser. [ABSTRACT FROM AUTHOR]

Diode-pumped CW and passively Q -switched lasers of Nd:GdLuAG mixed garnet at 1123 nm were demonstrated. The maximum average output power of CW operation was 4.13 W. For Q -switched operation, the average output power was 800 mW, the corresponding single pulse energy was 133.8 µJ. The Nd:GdLuAG laser emitting at 1123 nm was obtained for the first time to the best of our knowledge, which proves that the Nd:GdLuAG mixed garnet has a better ability of energy storage than Nd:YAG in 1123 nm oscillation. [ABSTRACT FROM AUTHOR]

• The stable CW ML pulsed with pulse duration as short as 1.97 ps was experimentally obtained. • We proved it experimentally that under the increasing saturation parameters , output pulses could be narrowed in the steady state cavity. • We used the generalized Ginzburg-Landau equation for the mode-locked pulse dynamic evolutions. • We showed that the pulse-narrowing mechanism in solid-state cavity at water absorption band. Ultrafast solid-state laser sources operating in the mid-infrared (mid-IR) region are of significant importance in a variety of applications. However, the realization of ultrafast lasers with narrow pulse width at wavelengths between 1.8 and 2 µm has remained a big challenge due to the strong water molecule absorption. Here, we report a stable mode-locking (ML) pulsed lasing at 1967 nm and narrow pulse duration of 1.97 ps in a diode-pumped Tm:CaYAlO 4 (Tm:CYA) crystal laser oscillator. By employing the SESAMs with different modulation depths as mode-lockers, we show that under the increasing saturation parameters, output pulses could be narrowed in the steady state cavity. Furtherly, the generalized Ginzburg-Landau equation is used for the mode-locked pulse dynamic evolutions. It shows that the pulse- regulation mechanism in solid-state cavity at water absorption band is governed by the balance of the laser gain, the modulation depth, and the saturation parameter. To our knowledge, this is the first demonstration of pulse regulation mechanism in the 2 μm water absorption band by a commercial laser diode in a Tm:CYA laser. Our results provide an effective method for pulse regulation of ultra-short mid-IR pulses and even few-cycle pulses with controllable optical spectra in the water absorption band. [ABSTRACT FROM AUTHOR]

• We report the highest output power (12.36 W) of diode-pumped Tm:CALYO laser at 1938 nm with a slope efficiency of 53.9%. • We report the broadest tunability (271 nm) for Tm:CALYO lasers. • Passively Q-switched operation was also achieved with a maximum average output power of 3.81 W and pulse width of 25.9 ns. We report on the recent progress on high-power and broadband wavelength tunability of diode-pumped Tm:CaYAlO 4 (Tm:CALYO) lasers. Using a double-end pumping geometry, we have achieved a maximum output power of 12.36 W at 1938 nm, which is believed to be the highest power that have ever achieved for Tm:CALYO lasers. Moreover, we have operated 1861 nm, 1982 nm and 2032 nm lasers with maximum output powers of 8.73 W, 5.36 W and 0.73 W, respectively, by using differently coated end-face cavity mirrors. Furthermore, using a MgF 2 birefringent filter, we have tuned the lasing wavelengths from 1807 nm to 2078 nm. The 271 nm wavelength range is also believed to be the broadest tunability for Tm:CALYO lasers. Passively Q-switched Tm:CALYO laser has also been demonstrated with a maximum average output power of 3.81 W. The shortest pulse width reached 25.9 ns at a repetition rate of 10.1 kHz, which led to a pulse energy of 377.22 μJ and a peak power of 14.56 kW. These results indicate clearly that the Tm:CALYO crystal is excellent as a high-power and broadband laser gain medium operating at the important 2 μm wavelength region. [ABSTRACT FROM AUTHOR]

• Defects in additive manufactured samples are characterized by laser ultrasound. • The detection accuracy can reach 300μm, confirmed by X-ray computed tomography. • Build theoretical model and simulation results are confirmed by the experimental. With the development of 3D printing technique, additive manufactured (AM) components are becoming more and more important in the fields of manufacturing industry. However, in the process of manufacturing and utilizing, it is necessary to inspect the appearance of internal defects to ensure safety and to reduce economic losses. In this paper, a pulsed laser is used to generate the ultrasonic waves inside Nickel (Ni) and Titanium (Ti) based alloy samples with internal defects of different diameters, and the propagating ultrasonic waves are detected by an optical interferometer. By analyzing the experimental results, defects with diameters greater than 300 μm in the Ni based alloy sample and that in the Ti based alloy sample can be detected, and this can be validated by X-ray computed tomography. In addition, the effects of ultrasonic waves propagating inside the samples and interacting with the defects are studied theoretically. The experimental results are consistent with the numerical simulation results. These results present that laser ultrasonic technology can evaluate the reliability of AM components, and has the potential to inspect AM components during their production as a non-contact method. [ABSTRACT FROM AUTHOR]

• Diode-pumped CW Pr,Mg:SRA lasers at 643 nm, 724 nm and 622 nm are demonstrated. • The highest output power of the diode-pumped Pr,Mg:SRA laser is 675 mW. • Passively Q-switched Pr,Mg:SRA lasers are also demonstrated, for the first time. • The shortest pulse width is 105.6 ns for the 622 nm laser, while 211.7 ns for the 643 nm laser. We report on compact blue-diode-pumped Pr,Mg:SrAl 12 O 19 (SRA) lasers in continuous-wave and passively Q-switched regimes. For continuous-wave mode, three lasers have been achieved with maximum output powers of 675 mW, 566 mW and 312 mW at wavelengths of 643 nm, 724 nm and 622 nm, respectively. The maximum slope efficiencies with respect to absorbed power for the three lasers are 18.7%, 16.6% and 11.1%. Using a uniaxial Co2+-doped strontium, lanthanum and magnesium aluminate (Co:ASL) crystal as saturable absorber with remarkable absorptions at 643 nm and 622 nm, we have achieved passively Q-switched Pr,Mg:SRA lasers at the two emissions with maximum output powers of 175 mW and 118 mW, respectively, for the first time to our knowledge. The shortest pulse widths for the two lasers are 211.7 ns and 105.6 ns. Correspondingly, we estimated that pulse energies and peak power are 2.13 μJ and 10.1 W for the 643 nm laser as well as 0.98 μJ and 9.27 W for the 622 nm laser. This work reveals that Pr,Mg:SRA as one of the few reported Pr3+-doped oxides could be potential in generating compact and efficient lasers in visible spectral range. [ABSTRACT FROM AUTHOR]

• A Pr:YLF-based single-frequency orange laser at 607 nm has been first demonstrated. • A Pr:YLF-based single-frequency orange laser at 604 nm has been first demonstrated. • The single-frequency orange lasers have potential application in biomedicine. We report on blue diode-pumped single-frequency Pr:YLF lasers in orange spectral region, for the first time to the best of our knowledge. For a single-frequency σ-polarized 607 nm laser, a maximum output power up to 175 mW is achieved with a slope efficiency of about 16.6%. Linewidth of the 607 nm single-frequency laser is measured to be about 6 MHz. Single-frequency operation has also been obtained at π-polarized 604 nm laser with a maximum output power up to 91 mW and a slope efficiency of about 8.9%. The corresponding linewidth of the 604 nm laser is about 22 MHz. The two single-frequency orange lasers exhibit wavelength tuning ranges of about 0.15 nm and 0.35 nm, respectively. This work has proposed the simplest way for single-frequency laser generation in orange spectral region, which could be obtained by complicated and low-efficiency sum-frequency mixing of Nd3+ lasers. This work paves a practical way for various orange-related applications. [ABSTRACT FROM AUTHOR]

• The performance of diode-pumped (Nd3+-doped) Ca 0.7 La 0.3 Al 12 O 19 (Nd:CLA) lasers was studied for the first time. • By inserting a Cr4+:YAG absorber, we successfully achieved a passively Q-switched laser. • Comparation between a- and c-cut disordered Nd:CLA lasers were made. A new neodymium-doped (Nd3+-doped) Ca 0.7 La 0.3 Al 12 O 19 (Nd:CLA) disordered crystal was successfully grown by the Czochralski method. The performance of diode-pumped Nd:CLA (a-cut and c-cut) lasers was demonstrated for the first time. For the c-cut crystal continuous-wave (CW) operation, we obtained the highest output power of 2.08 W at the wavelength of 1053.2 nm, and the slope efficiency was 23% versus the incident pump power. By inserting a Cr4+:YAG absorber, we successfully achieved a passively Q-switched laser, and the shortest pulse width was 18.36 ns with a peak power of 364.6 W. Compared with a-cut crystal, c-cut crystal has superior laser characteristics. [ABSTRACT FROM AUTHOR]