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A laser amplifier based on an active element in the shape of a Yb:YAG slab is developed. It combines the propagation of an amplified elliptical beam without internal reflections, the use of high-brightness diode pump channel combining, and a heat sink made of silicon carbide. Its laser characteristics and factors limiting the output pulse energy are studied. It is shown that the application of an active element with gradient doping ensures stable operation of the laser head at pump powers of ∼0.5 kW. The use of signal radiation diverging along one axis compensates for the effect of reducing the output cross section of the amplified beam; as a result, a gain of G = 4 is ensured with a beam quality factor M2 < 1.4. [ABSTRACT FROM AUTHOR]

A slab laser based on Nd:CTGS crystal was reported for the first time. With a quasi-continuous wave semiconductor laser as the pump source, the maximum average output power reached 7.01 W, with a wavelength of 1064.5 nm, a single-pulse energy of 35 mJ, and a slope efficiency of 31%. Using a Cr4+:YAG crystal as the saturable absorber, the maximum average output power was 2.52 W, corresponding to a single-pulse energy of 1.26 mJ, a pulse duration of 37.2 ns, and a peak power reaches 33.9 kW. This work provides a good way to obtain pulsed slab lasers with simple structure and excellent performance, which are expected to be utilized in many fields like the photovoltaic industry, glass processing, tool manufacturing, mechanical engineering, and scientific research.

Featured Application: The one-dimensional, high-order Hermite–Gaussian (HG0n) laser beams can be easily converted into vortex Laguerre–Gaussian (LGpl) beams carrying orbital angular momentum (OAM), and with the scaling of output power and the narrowing of pulse width, the applications of HG0n laser beams have been expanded from original cutting-edge scientific scenarios to optical measurement, material processing, aerospace communication, and many other fields. High-power, high-purity, nanosecond (ns) one-dimensional HG0n laser beams are proposed and demonstrated by using Nd:YAG cascaded slabs with a large aspect ratio. The HG0n laser beams are generated by adjusting the pump distribution, the intracavity apertures, and the tilt angle of the output coupler (OC). By controlling the gain and loss of HG0n modes of different orders, the high-purity, one-dimensional, high-order HG0n laser beams with orders 1 to 9 (HG01 to HG09) are produced, and their beam quality factors M2 align well with the theoretical predictions. Meanwhile, the large aspect ratio slab provides an ideal amplifier for the strip-shaped HG0n laser beams, and further power scaling is achieved by seeding the generated HG0n laser beams into an amplifier with an identical slab module. For the HG09 mode, the average power is amplified from 289 mW to 4.73 W with 294 ns pulse width, corresponding to a peak power of 32 kW. Moreover, above 5 W average power is achieved for all HG01 to HG08 modes. Hopefully, this scheme provides a solution for high-power and high-purity HG0n laser beam generation based on the slab-shaped configuration. [ABSTRACT FROM AUTHOR]

A several-kilowatt level laser at 946 nm is demonstrated in a cryogenically cooled diode-pumped Nd:YAG slab system. The oscillator realizes the record of the output power of 2.85 kW in a compact cavity with the homemade cryogenically cooled Nd:YAG crystal slab. The pulse repetition rate and pulse width of the laser are 400 Hz and 200 µs, respectively, and the center wavelength under full power is 946.2 nm with a bandwidth of about 0.5 nm. The absorption linewidth at 808 nm is a little narrowed and blue shifts at the cryogenic temperature, while the cooling temperature decreases from 300 to 77 K. The measured thermal conductivity increases from 10 to 70 W/mˑK, and the thermal expansion coefficient decreases from 7.5*10−6 to 1.5*10−6/K. These results could guide us in simulating the temperature gradient and thermal-induced stress distributions in the crystal slab.

High-power, high-purity, nanosecond (ns) one-dimensional HG0n laser beams are proposed and demonstrated by using Nd:YAG cascaded slabs with a large aspect ratio. The HG0n laser beams are generated by adjusting the pump distribution, the intracavity apertures, and the tilt angle of the output coupler (OC). By controlling the gain and loss of HG0n modes of different orders, the high-purity, one-dimensional, high-order HG0n laser beams with orders 1 to 9 (HG01 to HG09) are produced, and their beam quality factors M2 align well with the theoretical predictions. Meanwhile, the large aspect ratio slab provides an ideal amplifier for the strip-shaped HG0n laser beams, and further power scaling is achieved by seeding the generated HG0n laser beams into an amplifier with an identical slab module. For the HG09 mode, the average power is amplified from 289 mW to 4.73 W with 294 ns pulse width, corresponding to a peak power of 32 kW. Moreover, above 5 W average power is achieved for all HG01 to HG08 modes. Hopefully, this scheme provides a solution for high-power and high-purity HG0n laser beam generation based on the slab-shaped configuration.

Adaptive optics (AO) can effectively improve the beam quality of solid-state slab lasers. However, the aperture of the output beam increases as the output power of the laser increases, resulting in a larger measurement system. Ultimately, a more complex AO system needs to be designed. To meet the requirements of conjugate imaging in an AO system, it is of research significance to coordinate and optimize the system's structural dimensional parameters while enabling the detection of multiple parameters, such as the wave-front and beam quality. In this paper, a multiparameter measurement technique in a large-aperture rectangular laser beam aberration-correction system is proposed. The system layout is optimized with total dimensions of 400 mm × 150 mm × 246 mm (L × W × H). The AO system conforms to the requirements for conjugate detection and can perform wave-front detection, far-field evaluation, and near-field detection of a 160 mm × 120 mm rectangular beam produced from a solid-state slab laser. The findings reveal that the measured PV values of wave-fronts of the measurement system are less than 0.288 μm, the RMS is no more than 0.079 μm, the average far-field beam quality factor is 1.248 times the diffraction limit, and the average near-field beam uniformity is 0.533 at a temperature of 20 °C ± 10 °C; these results satisfy the technical parameters.

The development of high-power solid state laser technology is a course of struggling against the "waste heat" generated with lasering. The most critical issue for the development of high-power solid-state lasers is how to suppress the thermal effect on the beam quality under high-power operating conditions. To improve this adverse effect researchers invented heat capacity laser, thin-film laser, slab laser and fiber laser. These new designs of lasers based on novel form of gain medium combined with advanced heat dissipation technology have enhanced the output power to about one hundred kilowatts. However, the thermal properties of the solid state gain medium have become the critical bottleneck restricting further breakthroughs in laser power. Therefore, it is of great significance to explore the novel laser crystal materials with ultra-high thermal conductivity. This paper introduces the basic principles of the above four lasers and their research progress in high-power lasers. From the perspective of improving the thermal conductivity of gain medium materials, the existing methods and related results are analyzed and summarized. The research on ultra-high thermal conducting laser crystals and the development of high-power laser technology are also prospected. [ABSTRACT FROM AUTHOR]

Subject of Research. The paper presents a method for calculation of thermal conductivity and viscosity of the gaseous slab laser medium. The method is based on the use of the known properties of its components. Thermal conductivity determination is necessary for mathematical and thermal modeling in the process of thermal mode development. The main feature of the proposed technique is application versatility and the possibility to obtain effective characteristics with sufficient accuracy for complex composition gases containing more than two components. Method.The calculation approach lies in consistent application of the Lindsay-Bromley and Wilke formulas for binary gas to the components of the mixture depending on their molar concentration. The component selection order is due to the smallest variance of the calculated values from the reference data. Main Results. The method was verified by comparison of the calculated and experimental data for three-component mixtures. It was found that viscosity measurement error does not exceed 1.5 % with an average deviation of less than 1 %. The average deviation of the values in the case of thermal conductivity is much higher, but the measurement error does not exceed 5 %. For the most frequently used composition of CO2, N2, Xe, and He mixture as a slab laser medium, the thermal properties were calculated for the minimum and maximum concentrations of carbon monoxide arising during operation. The tables show step-by-step application of the method and the dependence of thermal conductivity on temperature. There is a strong increase in thermal conductivity in the range of operating temperatures (more than twice). For example, at room-temperature range (300 K) it is equal to 0.067 W/(m·K), at 800 K it reaches 0.137 W/(m·K). This difference creates a significant non-uniformity of gas medium temperature field and affects drastically the laser stability. Practical Relevance.The results obtained can find practical application in the development of slab lasers and can be used to eliminate power drops and stabilize the operation of existing lasers providing the regulating process for the component concentrations of the gas composition.

A new slab configuration is designed based on the traditional Yb:YAG Total-Reflection Active-Mirror (TRAM) slab laser. The grad-doping method is proposed to improve the pump uniformity of the gain medium. A thermal analysis model is established by the ray-tracing software and finite element method. The thermal effects of the uniform doping medium and gradient doping medium are investigated according to the model, respectively. It shows that the maximum temperature and stress decrease obviously by the grad-doping method. A method based on ray-tracing is proposed to evaluate the thermal lensing effect and spherical aberration effect, and it is also suitable for other gain mediums with complex structures. The results indicate that the gain medium has less thermal effect and better beam quality by the grad-doping method. The TRAM slab with gradient doping concentration is also a promising configuration to obtain higher power output. [ABSTRACT FROM AUTHOR]

• An optimization method for gradient doping Yb: YAG composite multi-segmented slab laser with high average power. • The heat distribution model is designed to calculate the light field distribution and heat distribution. We report an optimization method for gradient doping Yb: YAG composite multi-segmented slab laser with high average power. Each segment's absorption coefficient is optimized. An internal heat source model is created for the optimization of the optical field and heat distribution. The optimized maximum temperature is reduced by 6.93 K. The stress and the peak to valley (PV) are reduced by 8.35 % and 4.2 %, respectively. The results show that the optimization method can significantly reduce the maximum temperature and stress, which is generally suitable for gradient doping lasers to optimize their beam quality. [ABSTRACT FROM AUTHOR]

For a high-power slab solid-state laser, obtaining high output power and high output beam quality are the most important indicators. Adaptive optics systems can significantly improve beam qualities by compensating for the phase distortions of the laser beams. In this paper, we developed an improved algorithm called Adaptive Gradient Estimation Stochastic Parallel Gradient Descent (AGESPGD) algorithm for beam cleanup of a solid-state laser. A second-order gradient of the search point was introduced to modify the gradient estimation, and it was introduced with the adaptive gain coefficient method into the classical Stochastic Parallel Gradient Descent (SPGD) algorithm. The improved algorithm accelerates the search for convergence and prevents it from falling into a local extremum. Simulation and experimental results show that this method reduces the number of iterations by 40%, and the algorithm stability is also improved compared with the original SPGD method.

Aiming at the problem that the astigmatism component of the slab laser is dominated manly by large-scale astigmatic aberrations, a method for simultaneously correcting defocus, 0°and 45° astigmatism is proposed. The spherical lens, cylindrical lens and adjustment system are employed. Using the theory of ray tracing, the beam divergence angle is used as the intermediate variable to construct a relationship between defocus, 0°astigmatic aberrations and spacing of the two lenses. Further, a mathematical analysis relationship between the 45° astigmatic aberration and the cylindrical lens rotation angle is established, and the correcting rotation angle for 45° astigmatic aberration is directly calculated according to the parameters detected by the wavefront sensor. Then, the experimental results show that the PV value of the incident beam wavefront can be reduced from 30.51µm to 1.5µm, and the beam quality is obviously improved. [ABSTRACT FROM AUTHOR]

In order to solve the problem of large low-order aberrations with solid-state zigzag slab lasers, an automatic compensator has been developed in this paper. In this compensator, three lenses are mounted on a motorized rail, whose positions can be obtained using ray tracing method based on the beam parameters detected by a wave-front sensor. The initial peak to valley (PV) values of the wave-front range up to several tens of microns. Both simulated and experimental results show that the PV values of the wave-front can be reduced to around 1 . 6 μ m with the proposed automatic compensator. [ABSTRACT FROM AUTHOR]

We originally demonstrate a diode-end-pumped, free-running dual-wavelength Yb:YAG slab laser with controlled polarization. A Yb:YAG slab crystal was employed as the laser crystal and its temperature distribution was simulated at different powers. The laser oscillations were generated and controlled in a plane-concave resonator by tilting the output coupler (OC). The maximum polarization extinction ratio (PER) was up to 25.8 dB with the dual-wavelength output power of 2.63 W. The theoretical studies confirmed that the phase difference gradually changes as the tilt angle increase. Stable linear polarizations suggest that optical vectors with relative path difference were coherent superposed in the laser resonator. This work is a potential pathway to achieve a high PER light source for polarization-dependent devices. • Polarization control of a dual-wavelength Yb:YAG slab laser was demonstrated. • The maximum polarization extinction ratio was up to 25.8-dB. • Phase shift was theoretically modeled to prove polarization switching behavior. • Develop dual-wavelength lasers to drive polarization-dependent devices. [ABSTRACT FROM AUTHOR]

In this paper, a laser diode end pumped RbTiOPO4 (RTP) electro-optically Q-switched Tm:YAG slab laser was demonstrated. Stable Q-switched pulse with the shortest pulse width of 58 ns and an average output power of 7.5 W were realized at the repetition rate of 1 KHz, corresponding to the slope efficiency of 21.7%. [ABSTRACT FROM AUTHOR]

A laser-diode end-pumped Tm:YAG single crystal slab laser in acousto-optic Q-switched operation was demonstrated. For Q-switched operation, the average output power of 20.7 W at 1 kHz was achieved under the absorbed pump power of 83.6 W, corresponding to the slope efficiency of 36.1%, the shortest pulse width of 84 ns and the maximum pulse energy of 20.7 mJ with peak power of 250 kW were obtained. [ABSTRACT FROM AUTHOR]

We present a compact refractive shaping system for actively compensating low order aberrations of high power slab lasers. The shaping system includes three spherical lenses and two cylindrical lenses. Both theoretical and experimental investigations were performed to evaluate the compensation capability of the refractive shaping system. For a typical input beam with large low order aberrations of peak-to-valley (PV)=66.10 λ and root-mean-square (RMS)=16.05 λ , adjusting the distance between lenses, the wavefront aberrations are reduced to PV=0.48 λ , RMS=0.10 λ for the theoretical simulation and PV=0.50 λ , RMS=0.11 λ for the experimental result, respectively. It indicates that the main low order aberrations of defocus and 0° astigmatism can be significantly compensated by actively adjusting the distance between lenses and the experimental result agree well with the theoretical simulation. [ABSTRACT FROM AUTHOR]

We present a novel pump scheme for slab lasers as distributed-side-pumping. This allows large absorption length, large pump area, and relatively uniform pump profile. We optimize the parameters of slab and pumping system, and carry out numerical calculation of thermal effects in the composite slab and homogeneous slab. Design of 1-kW distributed-side-pumped slab oscillator is described as well. [ABSTRACT FROM AUTHOR]

Abstract: An end-pumped slab laser model is analyzed to evaluate the feasibility of radiation balanced laser (RBL) when the laser crystal is chosen to be Yb:KGW. The ratio between fluorescence and stimulated photon number is first introduced to help estimate the output power of RBL. Through the analysis it turned out to be difficult to build high power RBL, but the idea of radiation balance can be used to reduce the waste heat for all laser design. [Copyright &y& Elsevier]

Abstract: The Q-switched laser with triangle slab made of Nd:YAG crystal side pumped by 600-W quasi-cw diode laser stack has been designed. The multimode (M 2≈2.6) output energy of about 42mJ was demonstrated in free running mode for110-mJ pump energy. In Q-switch experiments, the KDDP Pockels cell was placed between the slab and rear mirror in plane-plane cavity with output coupler of 84% transmission. The energy of 8mJ in 2.1-ns pulse duration was obtained for near TEM00 output beam. For passive Q-switching by means of Cr:YAG crystal of 12.6% unsaturated transmission, the energy of 5.1mJ in 2.5-ns pulse duration was obtained for output beam close to TEM00 mode. [Copyright &y& Elsevier]

Abstract: The temperature distribution and thermal effects in a high-power solid-state laser are investigated. The temperature distribution in a corner-pumped Yb:YAG/YAG composite slab is described analytically, and the thermal effects in that slab are deduced. Their influences to the resonator are also discussed. A corner-pumped Yb:YAG/YAG composite slab laser has achieved 1016W continuous wave laser output. The experimental results support the calculated outcomes. [Copyright &y& Elsevier]

Abstract: A high power diode stack end-pumped electro-optically Q-switched Nd:YLF slab laser with a stable and off-axis negative-branch confocal unstable hybrid resonator was demonstrated. By using a cylindrical lens in the stable direction the thermal lens effect was compensated. Pulse energy of 25mJ was obtained with a pulse width of 22.4ns at repetition rates of 500Hz and a conversion efficiency of 22%. The stability was better than 0.8% and the beam propagation M 2 factor was about 1.2. [Copyright &y& Elsevier]

In this paper we report on experimental results of an end-pumped Nd:YVO4 slab laser with a hybrid resonator. 110 W output power with beam propagation factor M2 of 1.3 and 1.5 in two orthogonal directions was obtained. After a spatial filter the typical beam propagation factors at 50 W operation were 1.09 and 1.2. [Copyright &y& Elsevier]

We present finite-element analysis and experimental results to validate our approach for building high-power single-mode Nd:YAG lasers. We show that the thermooptical and thermomechanical properties of a slab laser can be controlled. This is essential for the use of the proposed unstable resonator, We include demonstration of an efficient subscale laser operating at 20 W TEM00. [ABSTRACT FROM PUBLISHER]

The edge effects of high-power slab lasers cause severe wavefront distortions with large amplitude and gradients, which is a great challenge for the adaptive optics systems for improving the beam qualities. The standard least-squares wavefront reconstruction method would inherently assign higher priorities to correcting the edge aberrations which contribute little to the laser power in the near diffraction-limit areas in the far-field, while the other aberrations are not well corrected. To solve the problems, we propose a least-squares reconstruction algorithm with performance-based filtering, in which the correction capability of the deformable mirror is analyzed in the spatial frequency domain, and then the input phase distortions are filtered accordingly. The uncorrectable components of input distortions are ignored so that the residuals of the correctable components could be further minimized. Experiments with a 20 kW Nd:YAG slab laser indicate that the proposed method is effective to improve the beam quality of high-power slab laser with edge effect. • A least-squares reconstruction algorithm with performance-based filtering is proposed. • Beam quality of a 20 kW slab laser is improved from β =4.0 to β =2.0. • The peak-intensity and energy concentration in the far-field are further improved. • The achieved beam quality with a 59-actuator DM is comparable with a 273-actuator DM. [ABSTRACT FROM AUTHOR]