Search Results (15)

We present an investigation of the thermal damage threshold of passivated Bi₂Se₃ films upon laser illumination, with a focus on their employment in terahertz (THz) spectroscopic applications. Passivation was achieved by depositing a thin 3 nm Al capping layer which, exposed to the ambient, forms a natural oxide. In THz transient emission experiments, the samples were exposed to a train of 100 fs wide laser pulses with 800 nm wavelength at 78 MHz repetition rate and peak power density up to 295 mW/µm². For the sake of comparison, the films were also exposed to continuous wave laser light with a wavelength of 532 nm in the average optical power density range from 5 × 10⁻² mW/µm² to 50 mW/µm². In both cases, changes in film appearance, detected by optical microscopy, or even film removal in a small area close to the center of the illuminated spot could be induced. Raman spectroscopy provided evidence that the crystalline phase of Bi₂Se₃ films is present in areas that have been exposed but not damaged. Conversely, in the film region illuminated with the highest peak power density no Raman signal was detected in the range under investigation which we ascribe to material removal. At the perimeter of this ablated area, we observed a dominant Raman mode at approximately 255 cm⁻¹ that we can attribute to selenium and indicates partial Bi₂Se₃ decomposition. In contrast, we observed Raman spectra corresponding to as-deposited Bi₂Se₃ only a few micrometers away from the laser-damaged area. Hence, the observed THz radiation originates from this illuminated but undamaged region. This detailed knowledge is expected to serve as a guide for designing the emitter’s thermal management and choosing laser parameters for optimal operation. Full article

Ytterbium-doped femtosecond fiber lasers are widely used in scientific research, industrial processing, and other fields due to their high quantum efficiency, wide gain bandwidth, and compact structure. This article addresses the problems of low processing efficiency and difficulty in increasing the average power of femtosecond lasers. A high repetition rate fiber chirped pulse amplification system is built, which uses a high repetition rate Figure-9 fiber laser as the seed source and an acousto-optic modulator (AOM) to shape the dense pulse train in the time domain. The main amplification stage uses a large mode field ytterbium-doped fiber to achieve full fiberization of the amplification system, and a volume grating (VBG) is selected as the pulse compressor to make the laser system highly integrated. When the repetition rate is 67.5 MHz, the compressed output laser has an average power of 20.5 W, a pulse width of 447 fs, a pulse train energy of 750 μJ, a spot ellipticity of 0.96, and a beam quality M² better than 1.4 ($M_x^2=1.33$, $M_y^2=1.16$). Full article

We report the simultaneous generation of conventional solitons (CSs) and dissipative solitons (DSs) in an erbium-doped mode-locked fiber laser with a dual-branch cavity configuration based on the nonlinear polarization rotation (NPR) technique. By incorporating fibers with different dispersion properties in two propagation branches, the laser can establish simultaneous operation in the normal and anomalous dispersion regimes within the respective loops, enabling the generation of two distinct soliton types. The CSs exhibit a 3 dB spectral bandwidth of 9.7750 nm and a pulse duration of 273 fs, while the DSs have a quasi-rectangular spectrum spanning 18.7074 nm and a pulse duration of 2.2 ps, which can be externally compressed to 384 fs. The fundamental repetition rate is approximately 21 MHz, with a repetition rate difference of 216 Hz for the two pulse trains. Stable second-order, third-order, and fourth-order harmonic mode-locking (HML) can be achieved through optimization of pump power and intracavity polarization states. The laser we build in this work has significant potential for applications in high-precision spectroscopy and asynchronous optical sampling. Full article

A tunable high-power 60 MHz ultraviolet pulse laser directly produced by the extra-cavity fourth-order harmonic generation of a self-similar amplification infrared pulse laser is reported in this study. Utilizing the self-similar pulse evolution and the self-phase modulation in a self-similar amplifier, the system generates a 58.9 W pulse train with a spectral half-width of 85.4 nm, corresponding to a pulse duration of 36 fs. To obtain the ultraviolet pulses from the infrared pulses, a single-pass frequency quadrupling system comprising two cascaded β-BBOs was used. The ultraviolet spectra can be tuned within a spectral range of 253.6 to 275 nm owing to the broadband infrared seed spectra. The maximum ultraviolet average power of 1.44 W was achieved at 275 nm with spectral half-width and an infrared-to-ultraviolet efficiency of 1.1 nm and 2.44%, respectively. To the best of our knowledge, this is the first demonstration of tunable high-power ultraviolet pulse generation from a self-similar amplification Yb-fiber laser. Full article

Ytterbium laser sources are state-of-the-art systems that are increasingly replacing Ti:Sapphire lasers in most applications requiring high repetition rate pulse trains. However, extending these laser sources to THz Time-Domain Spectroscopy (THz-TDS) poses several challenges not encountered in conventional, lower-power systems. These challenges include pump rejection, thermal lensing in nonlinear media, and pulse durations exceeding 100 fs, which consequently limit the detection bandwidth in TDS applications. In this article, we describe our design of a THz-TDS beamline that seeks to address these issues. We report on the effectiveness of temperature controlling the Gallium Phosphide (GaP) used to generate the THz radiation and its impact on increasing the generation efficiency and aiding pump rejection while avoiding thermal distortions of the residual pump laser beam. We detail our approach to pump rejection, which can be implemented with off-the-shelf products and minimal customization. Finally, we describe our solution based on a commercial optical parametric amplifier to obtain a temporally compressed probe pulse of 55 fs duration. Our study will prove useful to the increasing number of laboratories seeking to move from the high-energy, low-power THz time-domain spectroscopy systems based on Ti:Sapphire lasers, to medium-energy, high-power systems driven by Yb-doped lasers. Full article

Traditional Chinese medicine (TCM) has relied on pulse diagnosis as a cornerstone of healthcare assessment for thousands of years. Despite its long history and widespread use, TCM pulse diagnosis has faced challenges in terms of diagnostic accuracy and consistency due to its dependence on subjective interpretation and theoretical analysis. This study introduces an approach to enhance the accuracy of TCM pulse diagnosis for diabetes by leveraging the power of deep learning algorithms, specifically LeNet and ResNet models, for pulse waveform analysis. LeNet and ResNet models were applied to analyze TCM pulse waveforms using a diverse dataset comprising both healthy individuals and patients with diabetes. The integration of these advanced algorithms with modern TCM pulse measurement instruments shows great promise in reducing practitioner-dependent variability and improving the reliability of diagnoses. This research bridges the gap between ancient wisdom and cutting-edge technology in healthcare. LeNet-F, incorporating special feature extraction of a pulse based on TMC, showed improved training and test accuracies (73% and 67%, respectively, compared with LeNet’s 70% and 65%). Moreover, ResNet models consistently outperformed LeNet, with ResNet18-F achieving the highest accuracy (82%) in training and 74% in testing. The advanced preprocessing techniques and additional features contribute significantly to ResNet18-F’s superior performance, indicating the importance of feature engineering strategies. Furthermore, the study identifies potential avenues for future research, including optimizing preprocessing techniques to handle pulse waveform variations and noise levels, integrating additional time–frequency domain features, developing domain-specific feature selection algorithms, and expanding the scope to other diseases. These advancements aim to refine traditional Chinese medicine pulse diagnosis, enhancing its accuracy and reliability while integrating it into modern technology for more effective healthcare approaches. Full article

In ultrashort-pulsed laser processing, surface modification is subject to complex laser and scanning parameter studies. In addition, quality assurance systems for monitoring surface modification are still lacking. Automated laser processing routines featuring machine learning (ML) can help overcome these limitations, but they are largely absent in the literature and still lack practical applications. This paper presents a new methodology for machine learning classification of self-organized surface structures based on light microscopic images. For this purpose, three application-relevant types of self-organized surface structures are fabricated using a 300 fs laser system on hot working tool steel and stainless-steel substrates. Optical images of the hot working tool steel substrates were used to learn a classification algorithm based on the open-source tool Teachable Machine from Google. The trained classification algorithm achieved very high accuracy in distinguishing the surface types for the hot working steel substrate learned on, as well as for surface structures on the stainless-steel substrate. In addition, the algorithm also achieved very high accuracy in classifying the images of a specific structure class captured at different optical magnifications. Thus, the methodology proposed represents a simple and robust automated classification of surface structures that can be used as a basis for further development of quality assurance systems, automated process parameter recommendation, and inline laser parameter control. Full article

This study developed a novel frequency-doubled Fabry–Perot cavity method based on a femtosecond laser of 1030 nm, 190 fs, 1 mJ, and 1 kHz. The time interval (60–1000 ps) and attenuation ratio (0.5–0.9) between adjacent sub-pulses of the 515 nm pulse train were able to be easily adjusted, while the efficiency was up to 50% and remained unchanged. Extremely high-quality low-spatial-frequency LIPSS (LSFL) was efficiently fabricated on an indium tin oxide (ITO) film using a pulse train with a time interval of 150 ps and attenuation ratio of 0.9 focused with a cylindrical lens. Compared with the LSFL induced by the primary Gaussian pulse, the uniformity of the LSFL period was enhanced from 481 ± 41 nm to 435 ± 8 nm, the divergence of structural orientation angle was reduced from 15.6° to 3.7°, and the depth was enhanced from 74.21 ± 14.35 nm to 150.6 ± 8.63 nm. The average line edge roughness and line height roughness were only 7.34 nm and 2.06 nm, respectively. The depths and roughness values were close to or exceeded those of resist lines made by the interference lithography. Compared with the common Fabry–Perot cavity, the laser energy efficiency of the pulse trains and manufacturing efficiency were enhanced by factors of 19 and 25. A very colorful “lotus” pattern with a size of 30×28 mm² was demonstrated, which was covered with high-quality LSFLs fabricated by a pulse train with optimized laser parameters. Pulse trains can efficiently enhance and prolong the excitation of surface plasmon polaritons, inhibit deposition particles, depress ablation residual heat and thermal shock waves, and eliminate high-spatial-frequency LIPSS formed on LSFL, therefore, producing extremely high-quality LSFL on ITO films. Full article

We present a new scheme of picosecond optical parametric chirped pulse amplification (OPCPA) in which a Fourier-transform-limit 5.0 ps pulse is optically sheared from a single-longitudinal-mode 1064 nm CW laser. The pulse is amplified and frequency-doubled as the pump in order to maintain the pump narrow bandwidth and picosecond duration simultaneously, which is very important to ensure the high temporal contrast for an OPCPA amplifier. Combined with the cross-polarized wave generation (XPW), a compound frontend for the high-power femtosecond laser system that delivers a 1 Hz chirped pulse train is established. The experiments provide an output pulse energy of 17.1 mJ, a spectrum bandwidth 71 nm (FWHM), and a pulse duration 16.4 fs. The pulse contrast reaches 1:10⁻¹² several picoseconds before the peak of the main pulse, which is the best value of the available measuring instruments. Full article

In this paper, we design a silica-cladded Germania-doped ring-core fiber (RCF) that supports orbital angular momentum (OAM) modes. By optimizing the fiber structure parameters, the RCF possesses a near-zero flat dispersion with a total variation of <±30 ps/nm/km over 1770 nm bandwidth from 1040 to 2810 nm for the OAM_1,1 mode. A beyond-two-octave supercontinuum spectrum of the OAM_1,1 mode is generated numerically by launching a 40 fs 120 kW pulse train centered at 1400 nm into a 12 cm long designed 50 mol% Ge-doped fiber, which covers 2130 nm bandwidth from 630 nm to 2760 nm at −40 dB of power level. This design can serve as an efficient way to extend the spectral coverage of beams carrying OAM modes for various applications. Full article

Taking advantage of the short pulse duration and the high repetition rate of the fs pulse train, amplification without inversion (AWI) can be achieved in two- or three-level atomic systems. A modulation method using a static magnetic field to control such AWI gain in an atomic system is proposed. The schematic model of the $5^2{S}_{1/2}-5^2{P}_{1/2}$ transition of ⁸⁷Rb is adopted, and the results show that the system gain can be significantly enhanced and modulated by the magnetic field, while the population inversion can be almost completely suppressed. Moreover, the pulse number, pulse repetition period, and pulse area can also be utilized to modulate the AWI gain. Full article

Gallium sulfide (GaS), with a hexagonal structure, has received extensive attention due to its graphene-like structure and derived optical properties. Here, high-quality GaS was obtained via chemical vapor synthesis and then prepared as a saturable absorber by the stamp-assisted localization-transfer technique onto fiber end face. The stability of the material and the laser damage threshold are maintained due to the optimized thickness and the cavity integration form. The potential of the GaS for nonlinear optics is explored by constructing a GaS-based Erbium-doped mode-locked fiber laser. Stable femtosecond (~448 fs) mode-locking operation of the single pulse train is achieved, and the robust mode-locked operation (>30 days) was recorded. Experimental results show the potential of GaS for multi-functional ultrafast high-power lasers and promote continuous research on graphene-like materials in nonlinear optics and photonics. Full article

We investigated the possibility of making ever-shorter optical pulses by using the nonlinearity of ambient air. We produced a broad spectrum consisting of mutually coherent optical sidebands via collinear Raman generation driven by two picosecond laser pulses that are Raman-resonant with molecular vibrations of nitrogen. We demonstrated the ability to adjust the sideband phases via dispersion control which we accomplished by changing the optical path length of the generated multi-color beam through a pair of tilted glass plates. The resultant measured phases suggest the generation of a 3-fs optical pulse train. Full article

We report on laser performances obtained in Q-switch mode operation from buried depressed-cladding waveguides of circular shape (100 μm diameter) that were inscribed in Nd:YAG and Nd:YVO₄ media by direct writing with a femtosecond laser beam. The Q-switch operation was realized with a Cr⁴⁺:YAG saturable absorber, aiming to obtain laser pulses of moderate (few μJ) energy at high (tens to hundreds kHz) repetition rate. An average power of 0.52 W at 1.06 μm consisting of a train of pulses of 7.79 μJ energy at 67 kHz repetition rate, was obtained from a waveguide realized in a 4.8 mm long, 1.1-at % Nd:YAG ceramics; the pulse peak power reached 1.95 kW. A similar waveguide that was inscribed in a 3.4 mm long, 1.0-at % Nd:YVO₄ crystal yielded laser pulses with 9.4 μJ energy at 83 kHz repetition rate (at 0.77 W average power) and 1.36 kW peak power. The laser performances obtained in continuous-wave operation were discussed for each waveguide used in the experiments. Thus, a continuous-wave output power of 1.45 W was obtained from the circular buried depressed-cladding waveguide inscribed in the 1.1-at %, 4.8 mm long Nd:YAG; the overall optical-to-optical efficiency, with respect to the absorbed pump power, was 0.21. The waveguide inscribed in the 1.0-at %, 3.4 mm long Nd:YVO₄ crystal yielded 1.85 W power at 0.26 overall optical efficiency. This work shows the possibility to build compact laser systems with average-to-high peak power pulses based on waveguides realized by a femtosecond (fs) laser beam direct writing technique and that are pumped by a fiber-coupled diode laser. Full article

A two-color pump beam (800 and 1200 nm) was introduced into hydrogen for molecular phase modulation, and a probe beam (267 nm) to generate Raman sidebands, by coherent frequency modulation based on four-wave Raman mixing. The phase and temporal profile were evaluated by means of a self-diffraction frequency resolved optical gating (SD FROG) system. The relative phases among the Raman sidebands were controlled by changing the angle of a thin CaF₂-plate inserted into the 267-nm beam path, suggesting that a train of 2.6-fs pulses was generated in the deep-ultraviolet region. Full article