Search Results (14)

Buried depressed-cladding waveguides were fabricated in 0.7-at.% Nd:Ca₃Li_0.275Nb_1.775Ga_2.95O₁₂ (Nd:CLNGG) and 7.28-at.% Yb:CLNGG disordered laser crystals grown by Czochralski method. Circular waveguides with 100 μm diameters were inscribed in both crystals with picosecond (ps) laser pulses at 532 nm of 0.15 μJ energy at 500 kHz repetition rate. A line-by-line writing technique at 1 mm/s scanning speed was used. Laser emission at 1.06 μm (with 0.35 mJ pulse energy) and at 1.03 μm (with 0.16 mJ pulse energy) was obtained from the waveguide inscribed in Nd:CLNGG and Yb:CLNGG, respectively, employing quasi-continuous wave pumping with fiber-coupled diode lasers. The waveguide realized in RE³⁺-doped CLNGG crystals using ps-laser pulses at high repetition rates could provide Q-switched or mode-locked miniaturized lasers for a large number of photonic applications. Full article

A continuous-wave (CW), high-power, quasi-three-level Nd:YAG laser operating at 946 nm is reported. The beam quality of the laser is greatly improved. The laser consists of a composite Nd:YAG rod end pumped by a fiber-coupled diode laser inside a simple concave-plane cavity. At an incident pump power of 31 W, a maximum CW output of 9.98 W was obtained at 946 nm, with a beam quality factor of M²~5. The corresponding optical-to-optical efficiency was 32.2% with respect to the incident pump power. To the best of our knowledge, this is the highest output power at 946 nm with such a beam quality ever generated by diode-pumped Nd:YAG laser with bulk crystals. Full article

Traditional ytterbium-doped high-power fiber lasers generally use a unidirectional output structure. To reduce the cost and improve the efficiency of the fiber laser, we propose a bidirectional output fiber laser (BOFL). The BOFL has many advantages over that of the traditional unidirectional output fiber laser (UOFL) and has a wide application in the industrial field. In theory, the model of the BOFL is established, and a comparison of the nonlinear effect in the traditional UOFL and the BOFL is studied. Experimentally, high-power continuous wave (CW) and quasi-continuous wave (QCW) BOFLs are demonstrated. In the continuous laser, we first combine the BOFL with the oscillating amplifying integrated structure, and a near-single-mode bidirectional 2 × 4 kW output with a total power of above 8 kW is demonstrated. Then, with the simple BOFL, a CW bidirectional 2 × 5 kW output with a total power of above 10 kW is demonstrated. By means of pump source modulation, a QCW BOFL is developed, and the output of a near-single mode QCW laser with a peak output of 2 × 4.5 kW with a total peak power of more than 9 kW is realized. Both CW and QCW output BOFL are the highest powers reported at present. Full article

Up to now, transverse mode instability (TMI) and stimulated Raman scattering (SRS) have become the main factors limiting the power scaling of conventional ytterbium-doped fiber laser. Many technologies are proposed to suppress the SRS or TMI individually, but most of them are contradictions in practical application. In this article, we focus on the technologies that can balance the suppression of both SRS and TMI, including fiber coiling optimization, pump wavelength optimization, pump configuration optimization, and novel vary core diameter active fiber. Firstly, we validate the effectiveness of these technologies in both theoretical and relatively low-power experiments, and introduce the abnormal TMI threshold increasing in a few-mode fiber amplifier with fiber coiling. Then, we scale up the power through various types of fiber lasers, including wide linewidth and narrow linewidth fiber lasers, as well as quasi-continuous wave (QCW) fiber lasers. As a result, we achieve 5~8 kW fiber laser oscillators, 10~20 kW wide linewidth fiber laser amplifiers, 4 kW narrow linewidth fiber amplifiers, and 10 kW peak power QCW fiber oscillators. The demonstration of these new technical schemes is of great significance for the development of high-power fiber lasers. Full article

Thanks to the advantage of balancing nonlinear effects and transverse mode instability, vary core diameter active fiber (VCAF) has been widely used in high power ytterbium-doped fiber lasers in recent years. Up to now, VCAF has developed from the basic form of the original tapered fiber to the spindle-shaped and saddle-shaped fiber with different characteristics and has been applied in conventional fiber lasers, oscillating–amplifying integrated fiber lasers, and quasi-continuous wave fiber lasers and successfully improved the performance of these lasers. In the present study, a 6110 W fiber laser amplifier is realized based on a tapered fiber. The maximum output power of a fiber laser amplifier based on spindle-shaped fibers is 6020 W with a beam quality of M²~1.86. In this paper, we first introduce the basic concept of VCAF and summarize its main fabrication methods and advantages in high-power fiber laser applications. Then, we will present the recent research results of high-power fiber laser employing VCAF in our group and clarify the outstanding advantages of VCAF compared with the constant core diameter active fiber (CCAF). Full article

A bound state in the continuum (BIC) is a localized state in a continuous spectrum of radiating waves. In this study, the coupled-mode theory is introduced to more precisely interpret a symmetry-protected BIC and the quasi-BIC in a facile trimeric metasurface, compared with Fano formula, which is introduced to generate a high-quality factor resonance in a trimeric metallic slit metasurface. The multipole decomposition method and the near field distribution of the structure further illustrate that the underlying physics of the high-Q is mainly originated from the interference between the electric quadrupole mode and the magnetic toroidal mode. Physical mechanism shows that the resonance arises from the perturbation of symmetry-protected BICs. The result may play a role in the applications of lasers, optical sensors, and low-loss fibers. Full article

We report on a millijoule-level fiber–solid hybrid hundred-picosecond laser system with a stable performance and compact structure. The laser system is based on a master oscillator power amplifier structure containing an all-fiber master oscillator, a quasi-continuous-wave side-pumped Nd:YAG regenerative amplifier, and a double-pass amplifier. By using the filtering effect of fiber Bragg grating and the dispersion characteristics of single-mode fiber stretcher, the spectrum broadening caused by self-phase modulation effect is effectively suppressed. Thus, the gain linewidth of the Yb-doped fiber seed source and Nd:YAG laser amplifiers is accurately matched. The reason for thermally induced depolarization in the solid-state laser amplifier is theoretically analyzed, and a more flexible depolarization compensation structure is adopted in amplifier experiment. Furthermore, the pulse energy of 14.58 mJ and pulse width of 228 ps is achieved at 500 Hz repetition rate. The central wavelength is 1064.1 nm with a 3 dB bandwidth of 0.47 nm. The beam quality factors in the horizontal and vertical directions are 1.49 and 1.51, respectively. This laser system has a simple and compact structure and has a power stability of 1.9%. The high pulse energy and beam quality of this hundred-picosecond laser are confirmed by latter theoretical simulation of copper laser ablation. It is a very practical laser system for material processing and laser-induced damage. Full article

This paper presents a novel system for generating and receiving quasi-continuous (QC) TeraHertz (THz) waves. A system design and theoretical foundation for QC-THz signal generation are presented. The proposed QC-THz system consists of commercially available photo-conductive antennas used for transmission and reception of THz waves and a custom-designed QC optical signal generator, which is based on a fast optical frequency sweep of a single telecom distributed-feedback laser diode and unbalanced optical fiber Michelson interferometer used for a high-frequency modulation. The theoretical model for the proposed system is presented and experimentally evaluated. The experimental results were compared to the state-of-the-art continuous-wave THz system. The comparison between the continuous-wave THz system and the proposed QC-THz system showed the ability to transmit and receive QC-THz waves up to 300 GHz. The upper-frequency limit is bounded by the length of the used Michelson interferometer. The presented design of THz signal generation has a potential for industrial application because it is cost-efficient and can be built using commercially available components. Full article

Single-frequency lasers are essential for high-resolution spectroscopy and sensing applications as they combine high-frequency stability with low noise and high output power stability. For many of these applications, there is increasing interest in power-scaling single-frequency sources, both in the near-infrared and visible spectral range. We report the second-harmonic generation of 670 µJ at 532 nm of a single-frequency fiber amplifier signal operating in the quasi-continuous-wave mode in a 10-mm periodically poled Mg-doped lithium niobate (MgO:PPLN) crystal, while increasing compactness. To the best of our knowledge, this is the highest pulse energy generated in this crystal, which may find applications in the visible and UV such as remote Raman spectroscopy. Full article

Control of solidification structure and crystallographic texture during metal additive manufacturing is a challenging work which attracts the increasing interest of researchers. In the present work, two kinds of scanning strategies (i.e., single-directional scanning (SDS) and cross-directional scanning (CDS) were used to control the solidification structure and crystallographic texture during quasi-continuous-wave laser additive manufacturing (QCW-LAM) of Inconel 718. The results show that the solidification structure and texture are strongly dependent on scanning strategies. The SDS develops a typical fiber texture with unidirectional columnar grains, whereas the CDS develops a more random texture with a mixture of unidirectional and multidirectional grains. In addition, the SDS promotes the continuously epitaxial growth of columnar dendrites and results in the linearly distributed Laves phase particles, while the CDS leads to the alternately distributed Laves phase particles with chain-like morphology and discrete morphology. The changed stacking features of molten-pool boundary and the switched heat flow direction caused by different scanning strategies plays a crucial role on the epitaxial growth of dendrites and the final solidification structure of the fabricated parts. Full article

A 5.0-at.% Nd-doped La_0.64Gd_0.41Sc_2.95(BO₃)₄ (Nd:LGSB) borate laser crystal was successfully grown by the Czochralski method, for the first time to our knowledge. The spectroscopic properties of the grown crystal are discussed and 1 µm laser emission, under end-pumping with a fiber-coupled diode laser at 807 nm, is reported. A c-cut Nd:LGSB medium yielded 1.35 W continuous-wave output power at 0.63 overall optical-to-optical efficiency, with respect to the absorbed pump power, together with the high 0.68 slope efficiency. With an a-cut Nd:LGSB sample, 0.81 W output power at 0.52 optical-to-optical efficiency was obtained. The laser emission performances under quasi-continuous wave pumping are presented as well, for both c-cut and a-cut crystals. Passive Q-switching was investigated with a semiconductor saturable absorber mirror (SESAM). Laser pulses with 2.2 µJ energy and 32.8 ns durations were recorded from a-cut Nd:LGSB. The average output power reached 0.36 W at 1.55 W absorbed pump power. Passive mode-locking with SESAM was achieved in a long Z-type resonator. Ultrashort pulses with 0.19 W average power, 1.63 nJ energy, and 1.43 ps pulse duration, at 118 MHz repetition rate, are demonstrated for the a-cut Nd:LGSB medium. Full article

The interest in random fiber lasers (RFLs), first demonstrated one decade ago, is still growing and their basic characteristics have been studied by several authors. RFLs are open systems that present instabilities in the intensity fluctuations due to the energy exchange among their non-orthogonal quasi-modes. In this work, we present a review of the recent investigations on the output characteristics of a continuous-wave erbium-doped RFL, with an emphasis on the statistical behavior of the emitted intensity fluctuations. A progression from the Gaussian to Lévy and back to the Gaussian statistical regime was observed by increasing the excitation laser power from below to above the RFL threshold. By analyzing the RFL output intensity fluctuations, the probability density function of emission intensities was determined, and its correspondence with the experimental results was identified, enabling a clear demonstration of the analogy between the RFL phenomenon and the spin-glass phase transition in disordered magnetic systems. A replica-symmetry-breaking phase above the RFL threshold was characterized and the glassy behavior of the emitted light was established. We also discuss perspectives for future investigations on RFL systems. Full article

The failure response of aluminum alloys (Al-6061 and Al-7075) under the condition of simultaneously pre-stressing and laser heating was investigated. Specimens were subjected to predetermined preloading states and then irradiated by continuous wave fiber (Yb) laser. For all specimens, it was found that the yield stress decreased with increasing laser power density. This implies that the load-bearing capacity of the specimens reduced under increased thermal or tensile loading. Consequently, the specimen’s failure time was shortened by increasing either laser power density or preloaded speed. For Al-6061, a remarkable reduction in failure time by the increase of laser power density is found. However, for Al-7075, under higher preloaded speeds, comparatively smaller impact of laser power density on the failure time is reported. Moreover, for Al-6061, relatively a more non-uniform variation in the average failure time with the increase of laser power density or preloaded speed is observed. The failure mode of Al-6061 turned from brittle to ductile at higher laser power densities; whereas for Al-7075, it changed from quasi-brittle to ductile. At higher preloaded speeds, a greater degree of melting and ablation phenomenon can be seen due to relatively higher temperatures and higher heating rates. Full article

Although photoluminescence from gallium-arsenide (GaAs) deep-centers was first observed in the 1960s, semiconductor lasers have always utilized conduction-to-valence-band transitions. Here we review recent materials studies leading to the first GaAs deep-center laser. First, we summarize well-known properties: nature of deep-center complexes, Franck-Condon effect, hotoluminescence. Second, we describe our recent work: insensitivity of photoluminescence with heating, striking differences between electroluminescence and photoluminescence, correlation between transitions to deep-states and absence of bandgap-emission. Room-temperature stimulated-emission from GaAs deep-centers was observed at low electrical injection, and could be tuned from the bandgap to half-the-bandgap (900–1,600 nm) by changing the electrical injection. The first GaAs deep-center laser was demonstrated with electrical injection, and exhibited a threshold of less than 27 mA/cm2 in continuous-wave mode at room temperature at the important 1.54 μm fiber-optic wavelength. This small injection for laser action was explained by fast depopulation of the lower state of the optical transition (fast capture of free holes onto deep-centers), which maintains the population inversion. The evidence for laser action included: superlinear L-I curve, quasi-Fermi level separations satisfying Bernard-Duraffourg’s criterion, optical gains larger than known significant losses, clamping of the optical-emission from lossy modes unable to reach laser action, pinning of the population distribution during laser action. Full article