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• 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]

• 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]

• 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]