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High power cladding mode stripper device has been developed in robust configurations, which demonstrated high attenuation and low temperature rise, allowing to remove 2.16 kW of power with efficiency being 21.7 dB. The maximum temperature in packaging did not exceed 55 °C at a room temperature of 20 °C. The signal insertion loss was measured to be below 0.05 dB, indicating the ability to strip the cladding light efficiently but keep the core light almost intact. A 40-min stress test has been carried out, which confirmed the working stability and reliability of the proposed cladding mode stripper. Moreover, the device presented is in a compact and closed pack, more than 2 kW of power could be completely dissipated inside a housing with merely 200 mm of length, which could not only be benefit for suppressing nonlinear effects during power delivering but also be easily integrated into high power monolithic fiber laser systems.

CTFBGs with different center wavelengths are cascaded to broad the bandwidth to 20 nm (FWHM). The homemade duplexed CTFBG assists a 20.88 kW output of single fiber laser system with 18.6 dB of SRS suppression.

Based on generalized nonlinear Schrodinger and mode decomposition method, SRS-induced mode distortion in large-mode area passive fibers has been theoretically studied for the first time, which agree well with the experimental observations.

Five-hundred-watt cylindrical vector beams (CVBs) at 1030 nm with the 3 dB linewidth being less than 0.25 nm have been generated from a narrow linewidth all-fiber linearly polarized laser by metasurface extracavity conversion. At maximum output power, the transmission efficiency and polarization extinction ratio of radially polarized cylindrical vector beams (RP-CVBs) are beyond 98% and 95%, respectively. The average power is approximately an order higher than previously reported high-power narrow-linewidth CVBs generated from fiber lasers. The temperature rise of the metasurface is less than 10°C at 500 W output power, which means that the system can be further power-scaled in the near future. The high-power, high-purity, and high-efficiency RP-CVBs generated by the metasurface demonstrate potential application of a metasurface in high-power CVBs lasers.

In this paper, we present experimental studies on newly developed multiplexed chirped tilted fiber Bragg gratings (MCTFBGs) for stimulated Raman scattering (SRS) suppression for high-power fiber laser systems. The MCTFBG device is composed of five continuous segments of chirped tilted fiber Bragg gratings (CTFBGs), which are inscribed into the large-mode-area (LMA, 25/400μm) fibers. The SRS suppression capability of the MCTFBG device has been successfully demonstrated with a master oscillator power amplification (MOPA) fiber laser system at the output level of 3.4 KW. The experimental observation thus indicates the MCTFBG’s excellent SRS suppression capability at a very high power level (∼15 dB under >3 kW) and high thermal handling capacity (∼1.48℃/kw). Our work thus provides a key development of essential fiber grating components that can effectively suppress the SRS suppression at a very high power level.

In this work, quasi-static mode degradation in high power fiber amplifiers has been investigated experimentally. An increase of M2 from 1.3 to 2.6 with distortion of the beam profile is observed, which results in the signal spectra and backward light characterization departing from the traditional phenomena. The amplifier has been operated at the same input pump power of 705 W for nearly 2.2 hours to investigate the relationship between quasi-static mode degradation and photodarkening. The evolution of M2 factor/beam profile, mode correlation coefficient and output laser power at different working times indicate that the quasi-static mode degradation in the high power fiber amplifiers is dependent on photodarkening and evolves on the scale of tens of minutes. A visible green light has been injected to photobleach the gain fiber for 19 hours, which reveals that the quasi-static mode degradation has been suppressed simultaneously. To the best of our knowledge, this is the first detail report of photodarkening-induced quasi-static degradation in high power fiber amplifiers.

A 20μm-to-50μm monolithic MFA has been designed and fabricated by only one-step-tapering. With the MFA, 1kW laser beam with nearly diffraction-limited beam quality has been demonstrated after 15m propagation in highly multimode 50μm/0.11 NA fiber with different bend diameters.

We present a theoretical study of mode evolution in high-power distributed side-coupled cladding-pumped (DSCCP) fiber amplifiers. A semi-analytical model taking the side-pumping schemes, transverse mode competition, and stimulated thermal Rayleigh scattering into consideration has been built, which can model the static and dynamic mode evolution in high-power DSCCP fiber amplifiers. The mode evolution behavior has been investigated with variation of the fiber amplifier parameters, such as the pump power distribution, the length of the DSCCP fiber, the averaged coupling coefficient, the number of the pump cores and the arrangement of the pump cores. Interestingly, it revealed that static mode evolution induced by transverse mode competition is different from the dynamic evolution induced by stimulated thermal Rayleigh scattering. This shows that the high-order mode experiences a slightly higher gain in DSCCP fiber amplifiers, but the mode instability thresholds for DSCCP fiber amplifiers are higher than those for their end-coupled counterparts. By increasing the pump core number and reducing the averaged coupling coefficient, the mode instability threshold can be increased, which indicates that DSCCP fibers can provide additional mitigation strategies of dynamic mode instability.

Models of thermally-induced mode instability, taking the backward reflection into consideration, has been derived, and influence of backward reflection on thermally-induced mode instability has been studied numerically for the first time. Numerical results reveal that one percent of backward reflection can result in 33% reduction of mode instability threshold.

A backward (6+1)×1 pump and signal combiner has been presented, which can combine >5kW pump power in brightness-loss operation mode. By optimizing the fabrication technology, the beam quality (M2) degradation has been controlled to be less than 0.1.

We report a lossless all-fiber 7x1 signal combiner, which can be used to combine more than 10 kW laser power. The measured power transmission efficiency is larger than 98.1% and power handle capability is more than 2 kilowatt (kW) for each port. When the combiner is put on a 20°C water cool plate, the average temperature rise is less than 3°C/kW. Due to the nearly lossless efficiency and good thermal performance, we can conclude that this combiner is capable of more than 10 kW power.

Thermal effects are critical limit relevant to the power scaling of single crystalline fiber laser. In this paper, thermal effects in thin-rod single crystalline fiber are numerically researched. The simulation results show that thermal effects can be effectively reduced by enhancing the convective coefficient and decreasing the diameter of single crystalline fiber. For the most thin-rod single crystalline fibers utilized with diameter of 1 mm and length of 40 mm, the maximum heat load is only about 107 W due to the thermal rupture effect, which limits the laser output power to ~1 kW levels. The numerical results provide references for the developments and designing of thin-rod SCF laser

In this paper, we demonstrated an all fiber laser passively Q-switched by black phosphorus saturable absorber with cylindrical vector beam output. A piece of few-mode fiber Bragg grating was used as the mode-selective output coupler. The repetition rate of the pulse trains increased from 16kHz to 24.9kHz when the pump power tuned from 364mW to 460mW. The maximum pulse energy was 305.2nJ with the shortest pulse duration of 7.5μs under the pump power of 460mW.Both radial polarized and azimuthal polarized pulse output could be achieved by adjusting the polarization controllers. The purity of the cylindrical vector beam output was estimated to be over 95%.

An approach to fabricate cladding mode stripping device with ultra high power handling capability is presented, which can strip the cladding power gradually and avoid local overheating. Removed pump power up to 2.16 kW was experimentally demonstrated for the first time, delivering an attenuation of 21.7 dB with highest temperature being 55 °C at a room temperature of 20°C.

The design of annular doping region located in the cladding can reduce signal overlap with the doped region in order to reduce saturation and minimize gain compression, which has important applications in EDFAs. Here, we present the design and power scaling characterization of a cladding-pumped amplifier with ytterbium dopant located in an annular region near the ultra low NA core in the cladding, which is found to be a promising way to achieve multi-kilowatt single mode fiber lasers. The ultra low NA ensures that the fiber amplifiers operate in single mode state, which results to that the fiber amplifiers are free of the limitation of the transverse mode instability, and that the mode field of the signal laser extends into the cladding to extract gain amplification. The annular ytterbium-doped region located in the cladding can overcome the contradiction between high doping concentration and ultra-low NA design, which can simultaneously obtain high pump absorption with ultra low NA. The size of annular ytterbium-doped region under different core NA has been studied for various core sizes, which shows that the optimal size of annular ytterbium-doped region is related to the core NA and the core size. Detail analysis of high power amplification of cladding-ring-up-doped ultra low NA single mode fiber amplifier has been presented, which includes various nonlinear effects and thermal effects. It shows that, due to the specific design, the single mode characterization of the fiber is less influenced by the detrimental thermo-optic effect, which means that the cladding-annular-doped ultra-low NA fiber has high mode instability threshold than the ultra-low NA fiber with the core being fully uniformly doped. The cladding-pumped fiber amplifiers based on cladding-annular-doped ultra low NA fiber has the capability to achieve >10kW single mode fiber lasers.

In this paper, we present precisely measurement on the asymmetry in the large mode area (LMA) fiber Bragg gratings (FBGs). Three types of FBGs with different diameter were fabricated and studied. The difference between forward and reverse reflection spectrum in linearly chirped FBGs were measured. Based on the experiment and theory, this directionality of the chirped FBGs was explained. The experimental phenomenon coincides with the analysis. Our work provides a complete and accurate research upon the asymmetry in LMA chirped FBGs.

In this paper, we present the principle and design of a novel type of fiber Bragg gratings (FBGs) that are only partially inscribed into the large-mode-area fibers for high-power laser systems. The partial Bragg gratings can induce extra losses for high-order modes to suppress laser emission in direct fiber oscillators. Numerical modeling indicates these partial FBGs can very effectively limit the high-order LP11 mode from lasing and maintain the fundamental LP01 mode operation under up to 5 kW pump power, even for fibers of extremely large mode areas with core diameters up to 50 μm. Therefore, such partial FBGs can be excellent modal quality control components for single-mode high-power laser systems constructed with fibers of very large mode areas.

The performance of a laser-diode side-pumped Er:YSGG slab at a wavelength of 2.79 µm was investigated experimentally, and the laser output mode was analyzed using the theory of a thermally induced refractive index planar waveguide. Experimentally, a maximum continuous-wave output of 1.84 W with a slope efficiency of 11.2 %, and an optical-to-optical efficiency of 7.5 % at 2.79 µm was obtained with a 970-nm laser-diode dual-side-pumped Er:YSGG slab. To the best of our knowledge, the output power was the highest reported for a laser-diode-pumped Er:YSGG laser with a continuous-wave output at 2.79 µm. The numerical analysis showed that the output power was mainly limited by thermal effects in the thickness direction and the laser output transverse mode in the experiment was the fundamental mode in this direction.

The natural antimicrobial resistance of Pseudomonas aeruginosa (P. aeruginosa) represents a serious therapeutic challenge to antibiotic therapy approaches in clinical caring. As a new non-antibiotic therapy approach, the bactericidal effect of 470 nm blue light on P. aeruginosa has been reported and investigated. However, the relationships between the bactericidal ratio and the irradiation doses have not been reported, and the bactericidal mechanism has not been explicated. In this paper, the bactericidal effects and the bactericidal mechanism of 470 nm blue LED irradiation on P. aeruginosa are investigated. P. aeruginosa suspensions are irradiated with blue LED (100 mW cm−2) with a different irradiation time, then the number of the surviving P. aeruginosa is measured and the bactericidal effect is calculated. The fluorescence and ultrastructure of P. aeruginosa are measured and compared after 470 nm blue light irradiation. The survival ratio of the animal model of a P. aeruginosa infection with 470 nm blue light irradiation is researched. The experiment results show that a bactericidal rate of 94% is achieved for P. aeruginosa suspension with an irradiation time of 240 min and an irradiation intensity of 100 mW cm−2, and the bactericidal rate is higher for P. aeruginosa streaked on petri dish. The bactericidal effect of 470 nm blue light on P. aeruginosa is a photochemical reaction process, and the bactericidal effect increases with the increase of irradiation intensity and/or irradiation time. The survival rate of mice infected by P. aeruginosa can be effectively improved by using 470 nm blue light irradiation. The bactericidal rate of 470 nm blue light on P. aeruginosa depends on the irradiation doses, and the bactericidal effect of 470 nm blue light on P. aeruginosa is a photochemical reaction process. These results provide good reference values for the non-antibiotic bactericidal effect research of P. aeruginosa.

The basic theory of high power green laser equipment for prostate hyperplasia therapy and the components of the system developed are introduced. Considering the requirements of the clinical therapy, the working process of the high power green laser apparatus are designed and the laser with stable output at 120 W is achieved. The controlling hardware and application software are developed, and the safety step is designed. The high power green laser apparatus manufactured with characteristics of stable output, multifunctional and friendly interface provides a choices of prostate hyperplasia therapy for using nationalization instrument.

Lights with some special waveband and output power density have a bactericidal effect to some special bacteria. In this paper, the bactericidal effect of light at wavelength of 470 nm on P. aeruginosa (ATCC 27853) is researched with different irradiation dose. The light source is a LED array which is obtained by incoherent combine of 36 LEDs with emitting wavelength of 470 nm. The P. aeruginosa suspension is exposed with the LED array at the light power density of 100 mW/cm2 with exposures time of 0, 5, 10, 20, 40, and 80 min, respectively. The numbers of CFU are then determined by serial dilutions on LB agar plates. The bactericidal effect research results of 470 nm LED on P. aeruginosa show that the killing ratio increases with increasing of the exposure time. For the 80 min irradiation, as much as 92.4% reduction of P. aeruginosa is achieved. The results indicate that, in vitro, 470-nm lights produce dose dependent bactericidal effects on P. aeruginosa.

Lasers at 2.79 μm emitted by Er:YSGG crystal have attracted considerable interest in biological and medical applications. However, due to the thermal effect of laser crystal, the output power has been limited. In this paper, the advantages of Er:YSGG/YSGG composite crystal in reducing thermal effects and achieving high output power are investigated theoretically and experimentally. The numerical results show that the temperature rising and total thermal deformation of Er:YSGG/YSGG composite crystal are evidently reduced because the undoped YSGG absorbs the heat generated from the Er:YSGG. The maximum temperature rising reduces with increasing of the length of undoped YSGG crystal, and its location moves from the pump face to the inside of the Er:YSGG. The optical path difference of the Er:YSGG/YSGG composite crystal is obviously reduced comparing with that of Er:YSGG crystal, which indicates the thermal focal length of the Er:YSGG/YSGG composite crystal is increased, and the thermal effects are reduced. In experiments, the maximum continuous wave output power of 900 mW with slope efficiency of 12.1% at wavelength of 2.79 μm is obtained in laser diode end pumped Er:YSGG/YSGG crystal. To our knowledge, the output power of Er:YSGG/YSGG crystal is the highest value for the laser diode end pumped Er:YSGG crystal. The thermal focal length of the Er:YSGG/YSGG measured in experiment is increased comparing with that of Er:YSGG. Investigations have demonstrated that the Er:YSGG/YSGG composite crystal has a great advantage in reducing the influence of thermal effects and achieving high output power.

The regular spatial filters comprised of lens and pinhole are essential component in high power laser systems, such as lasers for inertial confinement fusion, nonlinear optical technology and directed-energy weapon. On the other hand the pinhole is treated as a bottleneck of high power laser due to harmful plasma created by the focusing beam. In this paper we present a spatial filter based on angular selectivity of Bragg diffraction grating to avoid the harmful focusing effect in the traditional pinhole filter. A spatial filter consisted of volume phase gratings in two-pass amplifier cavity were reported. Two-dimensional filter was proposed by using single Pi-phase-shifted Bragg grating, numerical simulation results shown that its angular spectrum bandwidth can be less than 160urad. The angular selectivity of photo-thermorefractive glass and RUGATE film filters, construction stability, thermal stability and the effects of misalignments of gratings on the diffraction efficiencies under high-pulse-energy laser operating condition are discussed.

This study investigated the effect of low-level laser irradiation (LLLI) on the cell cycle and proliferative activity of cultured myoblasts, and sought to elucidate the possible cellular mechanism by which LLLI promotes the regeneration of skeletal muscle in vivo. Primary myoblasts isolated from rat hindlegs were irradiated with helium-neon laser light at different energy densities. Distributions of cell-cycle subpopulations and the expression of cell-cycle regulatory proteins in myoblasts were assessed using flow cytometric analysis and western blot assay. It was found that laser irradiation stimulated cell-cycle entry; induced the expression of cyclin A and cyclin D; and increased cell proliferation index and bromodeoxyuridine incorporation as compared to the unirradiated control cells, indicating LLLI augmented the number of proliferative myoblasts in the S phase and G2/M phase of the cell cycle. These results suggest that LLLI at certain fluxes and wavelengths could activate quiescent myoblasts, leading to cell division and facilitating new myofiber formation. This could contribute to the improvement of skeletal muscle regeneration following trauma and myopathic diseases.

The advantages of laser-diode end-pumped Er:YSGG/YSGG composite crystals in reducing thermal effects and improving output power are investigated theoretically and experimentally. Compared with Er:YSGG, the temperature rise and total additional optical path difference of Er:YSGG/YSGG are evidently reduced because of the thermal conduction effects of the undoped YSGG crystal. The maximum continuous-wave output power of 504 mW with slope efficiency of 11.2%, and 900 mW with slope efficiency of 12.1%, at 2.79 μm is obtained in a 970 nm laser-diode end-pumped Er:YSGG crystal and Er:YSGG/YSGG crystal, respectively. To our knowledge, the output power of the Er:YSGG/YSGG crystal is the highest value for a laser-diode end-pumped Er:YSGG crystal. The thermal focal length of Er:YSGG and Er:YSGG/YSGG is respectively 36 and 51.8 mm when the pump power is 5.94 W. Investigations demonstrate that the Er:YSGG/YSGG composite crystal has great advantages in reducing the influence of thermal effects and improving output power.

The continuous wave (CW) laser performance of Nd:GYSGG at 1104 nm is investigated for the first time, to our knowledge. A CW laser output power of 4.7 W is obtained when the pump power of the 808 nm fiber coupled laser diode is 19.1 W, corresponding to a conversion efficiency of 24.6% and slope efficiency of 37%.

It is an effective approach to obtain high-power laser output by spectral beam combining (SBC) technologies. In this paper, a simple SBC system based on the superimposed reflective volume Bragg grating (RVBG) is proposed to reduce the scale of the SBC system. Considering the cross-coupling between the two gratings in a volume, two structure models of the superimposed RVBG-the superimposed RVBG with the same period and the superimposed RVBG with a different period-are analyzed and compared by using the rigorous coupled wave analysis, and their applications for SBC system are discussed. Numerical results show that the superimposed RVBG is easy to be fabricated and has the potential to combine multiple lasers with one volume. Cross-coupling can be neglected for the same period structure when the slant angle divergence is larger than 0.01 rad and for the different period structure when the spectral separation is larger than 1.0 nm. The superimposed RVBG with a different period can achieve high diffraction efficiency simultaneously for each grating with a small slant angle divergence, and is convenient to combine multiple beams with small spectral separation. The effect of wavelength shift on diffraction efficiency can be ignored.

A simple setup for radially polarized beam generation based on the reflecting volume phase grating is proposed to enhance the converting efficiency. The radially polarized beam is obtained by coherently combining the two orthogonally polarized Hermite-Gaussian HG0 1 and HG 10 beams. By using the plane wave decomposition and the three-dimensional rigorous coupled wave analysis approaches, the intensity distribution of the diffracted beam is compared with the respective incident beam. The intensity and the polarization angle divergence of the combined beam are analyzed. The results show that the intensity profile of the diffracted beam is not changed remarkably compared to that of the input beam. The radially polarized beam can be obtained with a converting efficiency of 99.93% and the degree of radial polarization approaches a unit at a given polarization angle divergence value of 1 deg when the central alignment divergence is smaller than 0.025 mm.