Wireless power transfer has become a research hotspot in the field of energy transmission, among which laser energy transfer is one of the best approaches for long‐distance wireless transmission. ...Based on good atmospheric transmittance, especially the wavelengths consistent with the response range of mainstream Si and GaAs photovoltaic cells with relatively high photoelectric efficiency, diode‐pumped alkali laser (DPAL) energy transfer will have huge prospects. To solve the problem of long‐distance laser energy transfer of DPALs, laser atmospheric molecular absorption was calculated by analyzing the atmospheric transmission absorption effects of laser beams with specific wavelengths. The atmospheric transmission of three typical alkalis at different atmospheric modes and altitudes was calculated and compared, and practical suggestions were given.
Quasi-continuous wave (QCW) laser has a very broad application in the industrial field, especially in additive manufacturing, surface treatment, laser cutting, laser cleaning, and laser drilling. ...Compared with the unidirectional fiber laser, the bidirectional output can be achieved two ports high power output with only one resonator, which can greatly reduce the industrial cost. However, there are few researches on QCW fiber lasers with bidirectional output. Here, we optimized and demonstrated a bidirectional output QCW laser with output power of 2 × 4.5 kW based on a double-clad ytterbium-doped fiber with a core/cladding diameter of 25/400 μm. The peak power at both ends reached 4515 W and 4694 W, respectively. The Raman suppression ratio at both ends of A and B is about 12 dB, and the beam quality factor M2 is about 1.37 and 1.42, respectively. The corresponding optical-to-optical efficiency is 79%. To the best of our knowledge, this is the highest peak power of QCW laser with near-single-mode beam quality in a bidirectional structure laser.
A novel bidirectional output oscillating-amplifying integrated fiber laser which combines the advantages of the bidirectional output fiber laser oscillator and the oscillating-amplifying integrated ...fiber laser is proposed and demonstrated experimentally. The influences of the reflectivities and center wavelengths of FBGs on the laser output power and efficiency were studied theoretically. The characteristics of the output laser were studied in detail in experiment. Based on this structure, we finally demonstrated a bidirectional laser output of 2 × 2-kW level with an active fiber length of 6 m for the oscillating section and 7 m for the amplifying section at both ends. The optical-to-optical conversion efficiency is 80.4%. The beam qualities of both ends are <inline-formula><tex-math notation="LaTeX">{M}_{A}^{2}\sim 1.3</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">{M}_{B}^{2}\sim 1.4</tex-math></inline-formula>, which indicates a near-single-mode output. To the best of our knowledge, it was the first time that this structure has been proposed, and its feasibility was demonstrated experimentally.
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.
Limited by stimulated Raman scattering (SRS), amplified spontaneous emission (ASE) and transverse mode instability (TMI), it is challenging to achieve high-power laser output in ytterbium-doped fiber ...(YDF) lasers with operating wavelengths less than 1060 nm. In high-power fiber lasers, bi-tapered YDF can provide a balance between the suppression of SRS and TMI. In this work, we designed and fabricated a new double-cladding asymmetric bi-tapered YDF to suppress ASE and SRS in the 1050 nm monolithic fiber laser. The asymmetric bi-tapered YDF has an input end with a core/cladding diameter of ~20/400 μm, a middle section with a core/cladding diameter of ~30/600 μm and an output end with a core/cladding diameter of ~25/500 μm. The working temperature of the non-wavelength-stabilized 976 nm laser diodes was optimized to improve the TMI threshold. An output power of over 5 kW with an efficiency of 83.1% and a beam quality factor M2 of about 1.47 were achieved. To the best of our knowledge, this represents the highest power nearly-single mode in 1050 nm fiber lasers. This work demonstrates the potential of asymmetric bi-tapered YDF for achieving a high-power laser with high beam quality in 1050 nm fiber lasers.
Modal cross coupling between basis functions in modal wavefront reconstruction is discussed. Eigenfunctions of Laplacian are proposed in modal approach for wavefront reconstruction. As the gradients ...of the eigenfunctions are orthogonal to each other, the modal cross coupling can be avoided theoretically. Wavefront reconstructions by use of Zernike polynomials and eigenfunctions of Laplacian with different sampling densities are compared. The results show that modal cross coupling between eigenfunctions of Laplacian is much smaller than that between Zernike polynomials.
► Modal cross coupling between basis functions in modal wavefront reconstruction is discussed. ► Wavefront reconstructions with Zernike polynomials and eigenfunctions of Laplacian are compared. ► Modal cross coupling between eigenfunctions of Laplacian is much smaller than that between Zernike polynomials.
Modal cross coupling usually exists in wavefront estimation through Zernike polynomials. In order to cope with the problem, the eigenfunctions of Laplacian with Neumann boundary condition are ...proposed instead of Zernike polynomials to reconstruct phase from wavefront gradient or curvature sensing. It is proved theoretically that these modals can avoid modal cross coupling in both wavefront gradient sensing and curvature sensing. In wavefront gradient sensing, the coefficients of eigenfunctions of Laplacian can be obtained from the integral of the scalar product between the gradient of Laplacian's eigenfunctions and wavefront gradient signal. In wavefront curvature sensing, the coefficients of eigenfunctions of Laplacian can be calculated from the integral of the product of Laplacian's eigenfunctions and wavefront curvature signal. This approach is applicable on arbitrary apertures as long as eigenfunctions of Laplacian on apertures of arbitrary shape can be obtained.
► The eigenfunctions of Laplacian overcomes the problem of modal cross coupling. ► The gradient of eigenfunctions is orthogonal on the aperture. ► There is no need to construct any auxiliary functions. ► This approach is applicable on arbitrary apertures once eigenfunctions are obtained.
AO (adaptive optics) systems have been extensively used to improve the beam quality in high-energy lasers; however, few studies have focused on how much the beam quality could be improved. A novel ...evaluation criteria and factor is presented in this paper. The factor, defined as power in ring (PIR), is expressed by distinguishing the low- and high-spatial frequency components in the far-field laser intensity distribution. Beams with different PIR values are generated in our model, and then they are compensated by AO systems. Calculation result shows that the PIR factor could evaluate a laser beam’s improvement potential by an AO system quantitatively.
Thermal blooming effect is one of the significant factors affecting the propagation performance of high-power ytterbium-doped fiber lasers (YDFLs) in the atmosphere. In this paper, two 20 kW YDFL ...systems with typical wavelengths (1070 nm and 1080 nm) are fabricated for propagation comparison experiments, which are used to investigate the thermal blooming effect induced by high-power YDFL propagation through the atmosphere. Under approximately the same laser system parameters (except wavelength) and atmospheric environment, the 1070 nm laser has better propagation characteristics than the 1080 nm laser. Due to the combined effect between the different central wavelengths of the two fiber lasers and the spectral broadening caused by output power scaling, the thermal blooming caused by the different absorptivity of water vapor molecules to the two fiber lasers is the main factor for the variation of the propagation properties. Through theoretical analysis and numerical calculation of factors affecting the thermal blooming effect, and considering the industrial manufacturing difficulty of YDFLs, a reasonable selection of fiber laser parameters can effectively improve atmospheric propagation performance and reduce manufacturing costs.
Bidirectional output oscillating-amplifying integrated fiber laser (B-OAIFL) is a newly developed configuration with many advantages like compactness and good reliability. In this work, a B-OAIFL ...with a low time-stabilized threshold was constructed by employing a pair of side pump/signal combiner in the oscillating section, which demonstrates smooth temporal characteristics with no pulse detected by the photodetector at the output power level of only a few of tens Watts. We investigated the effect of side pumping on the Raman Stokes light and verified its contribution to mitigating the temporal-chaos-induced stimulated Raman scattering (SRS). The phenomenon of co-SRS caused by the mutual excitation of backward Stokes light from two amplifying sections under bidirectional pumping was first reported and studied. A pair of chirped and tilted fiber Bragg gratings (CTFBGs) were applied between the oscillating and amplifying sections to suppress the co-SRS, and the effect of the number of CTFBGs on the suppression of co-SRS was studied in detail experimentally. Finally, we successfully suppressed the co-SRS, and achieved a 3kW × 2 ports laser output, with a near-single-mode beam quality of
M
A
2
∼1.3,
M
B
2
∼1.4. In contrast, without the use of CTFBG, only a 2 kW-level output was obtained from each port, limited by co-SRS (with an SRS suppression ratio of less than 15 dB). The maximum output power of end A and end B is 3133 W and 3213 W, with the SRS suppression ratio of about 27.6 dB and 28.1 dB, respectively. No TMI features were observed under bidirectional pumping. The results demonstrate a significant potential for further power scaling based on this configuration. To the best of our knowledge, it is the highest output power achieved based on the B-OAIFL.
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