High-power Solid-state Lasers Research Articles

Measurement of thermal effects of diode-pumped solid-state laser by using digital holography.

Thermal lensing is one of the most important factors that can affect the performance of high-power solid-state lasers, such as limiting the power scaling capability and deteriorating output beam quality. In this paper, a novel and accurate measurement of digital holography is proposed to determine the thermal lensing of diode-pumped solid-state lasers with high resolution. The digitally recorded hologram can reveal the phase change when light travels through the laser gain medium. From the phase map, we can obtain the index variations induced by temperature differences inside the laser crystal when it is pumped by laser diodes, as well as determine the focal length of the integrated thermal lensing focus length. There was much work on measuring the static laser medium thermal lens because there is no laser output from the cavity in the setup. Our experiment setup was able to achieve online measurement with laser output at the same time. The measuring result can provide an accurate guide for compensating the thermal lensing in laser design to achieve high-power output and good beam quality. Moreover, detailed index variations in the direction of the laser crystal cross-section can be numerically reconstructed, by which the thermal effects, pump uniformity, crystal uniformity, etc., can be revealed from the holography result.

Gravitational wave detection using laser interferometry beyond the standard quantum limit.

Interferometric gravitational wave detectors (such as advanced LIGO) employ high-power solid-state lasers to maximize their detection sensitivity and hence their reach into the universe. These sophisticated light sources are ultra-stabilized with regard to output power, emission frequency and beam geometry; this is crucial to obtain low detector noise. However, even when all laser noise is reduced as far as technically possible, unavoidable quantum noise of the laser still remains. This is a consequence of the Heisenberg Uncertainty Principle, the basis of quantum mechanics: in this case, it is fundamentally impossible to simultaneously reduce both the phase noise and the amplitude noise of a laser to arbitrarily low levels. This fact manifests in the detector noise budget as two distinct noise sources-photon shot noise and quantum radiation pressure noise-which together form a lower boundary for current-day gravitational wave detector sensitivities, the standard quantum limit of interferometry. To overcome this limit, various techniques are being proposed, among them different uses of non-classical light and alternative interferometer topologies. This article explains how quantum noise enters and manifests in an interferometric gravitational wave detector, and gives an overview of some of the schemes proposed to overcome this seemingly fundamental limitation, all aimed at the goal of higher gravitational wave event detection rates.This article is part of a discussion meeting issue 'The promises of gravitational-wave astronomy'.

An intelligent computation system for precise assembly of large laser optics

PurposeIt is a huge technical and engineering challenge to realize the precise assembly of thousands of large optics in high power solid-state laser system. Using the 400-mm aperture-sized transport mirror as a case, this paper aims to present an intelligent numerical computation methodology for mounting performance analysis and modeling of large optics in a high-power laser system for inertial confinement fusion (ICF).Design/methodology/approachFundamental principles of modeling and analysis of the transport mirror surface distortion are proposed, and a genetic algorithm-based computation framework is proposed to evaluate and optimize the assembly and mounting performance of large laser optics.FindingsThe stringent specifications of large ICF optics place very tight constraints upon the transport mirror’s assembly and mounts. The operational requirements on surface distortion [peak-to-valley and root mean square (RMS)] can be met as it is appropriately assembled by the close loop of assembly-inspection-optimization-fastening. In the end, the experimental study validates the reliability and effectiveness of the transport mirror mounting method.Originality/valueIn the assembly design and mounting performance evaluation of large laser optics, the whole study has the advantages of accurate evaluation and intelligent optimization on nano-level optical surface distortion, which provides a fundamental methodology for precise assembly and mounting of large ICF optics.

Robust method for long-term energy and pointing stabilization of high energy, high average power solid state lasers.

A robust and simple method is presented for ensuring constant energy and pointing of a high average power solid state laser on a target. In addition to providing long-term stability, this scheme also eliminates any drifts in energy or pointing resulting from the initial warm-up after a cold start. This is achieved using two separate feedback loops: one loop stabilizes the pointing of the beam external to the amplifier cavity and the other locks the cavity mode to have optimum overlap with the pump spot on the active medium. The key idea of the cavity mode stabilization is to monitor the overlap of the cavity mode and the gain medium with a camera and control it with an actively controlled, intra-cavity mirror. While this method is demonstrated on a thin-disk regenerative amplifier, it can also be applied to a wide variety of solid state laser amplifiers.

Active frequency matching in stimulated Brillouin amplification for production of a 2.4 J, 200 ps laser pulse.

A frequency matching Brillouin amplification in high-power solid-state laser systems is proposed. The energy extraction efficiency could be maintained at a high level in a non-collinear Brillouin amplification structure using an exact Stokes frequency shift. Laser pulses having a width of 200 ps and energy of 2.4 J were produced. This method can be used to transfer energy from a long pulse to a short pulse through a high-power solid-state laser system.

Analysis of laser diode array pump coupling system based on microlens array

In order to improve the performance of laser diode (LD) array pumping field in high-power solid state laser, an LD array pumping coupling system based on microlens array is used to achieve a high-uniformity pumping source with a longer transmission distance. The homogenizer has two structures based on microlens array, which are called diffracting homogenizer and imaging homogenizer. In this paper, we mainly study imaging microlens array due to its advantages of simple structure, better output homogeneity, flexibility of changing pumping field size, and insensitive to change in the input beam. First, the mathematical expression of the intensity distribution of target surface is derived based on the theory of geometrical optical. According to the geometrical optical formula, we obtain the relationship between the intensity distribution of target surface and system parameters, i.e., maximum incident angle of LD array, the distance between two microlens arrays, and the aperture and focal length of microlens. The boundary condition of microlens Fresnel number is derived based on the LD array beam parameters. Second, the influence of the number of microlens array elements on the output field homogeneity is studied theoreti-cally by the mathematical statistics method. As the input beam is considered to be divided randomly, the central limit theorem is employed to derive the mathematical expression of calculating the integrated output field non-homogeneity. The formula shows that the non-homogeneity is in inverse proportion to the root of the number of microlens array elements and the related maximum and minimum value of input field intensity distribution. And the spatial period of microlens array is designed to be unrelated to the spatial period of LD array to reduce the coherence of LD beam. According to the luminescence field parameters of an LD array consisting of 25 bars, an LD coupling imaging microlens array homogenizer test system is designed and constructed based on the theoretical analysis above. Another contrast system with a different microlens array which is not optimized is constructed at the same time. The coupling characteristics of two coupling systems with different microlens arrays are compared. The simulation and experimental test are carried out. The experimental result accords well with the simulated result, and thus proving the correctness of the theoretical studies. The coupling system with optimized microlens array shows better homogeneous effect with an output field non-homogeneity of 7.9%, and a coupling efficiency of 90.7%, proving the feasibility of the system for LD array pumping field homogenization.

Theoretical and experimental research on influence of cavity frequency difference in birefringent laser feedback system

The internal stress of glass material directly affects the processing quality of glass components and the service life of optical components. It is an important factor that relates to the overall system performance, safety, and reliability. Aerospace, precision optical systems, precision machining and other areas generally highly value the stress measurements of glass components. For example, the internal stress in the medium-glass material of precision imaging system will lead to the degradation of optical performance and reduce the image quality; the stress in the glass material used as the gain medium of high-power solid-state lasers not only directly affects the polarization state of the output light, but also shortens the service life of the laser; the stress concentration in the load-bearing glass of aircraft windshields, building glass curtain walls, etc., will cause serious accidents such as popping due to the reduction of glass mechanical properties. Therefore, the high sensitivity and large measurement range of stress detection technology has become a current research hotspot. Stress measurement techniques based on the birefringent external cavity laser feedback effect has received widespread attention due to its advanced and novel measurement principle. It is generally accepted in the traditional theory that the output phase of the laser in a feedback system is only determined by the phase retardation of birefringent element in an external cavity, and the measurement error is induced by the non-linear movement of external mirror. In this paper, the orthogonally polarized laser principle and the three-cavity equivalent model are combined to explain the influence of cavity frequency difference on the output of laser in feedback system. The frequency difference caused by the birefringence of the laser cavity is measured by comparing the intervals between adjacent longitudinal modes, and the frequency tuning feedback experiment is carried out. Theoretical analysis and experimental results show that the output phase of the laser is determined by the phase retardation of the external cavity, the frequency difference of the internal cavity, and the length of the external cavity. This conclusion is also confirmed by the measurement of the standard quarter wave plate. For a feedback system with an internal cavity frequency difference of 5 MHz and external cavity length of 150 mm, the phase difference induced by internal cavity frequency difference is about 0.573. The laser can output a single longitudinal mode below 40 MHz of the internal cavity frequency difference, and the length of the external cavity is generally larger than 150 mm when the actual system is designed, so the phase difference introduced by these two parameters cannot be ignored and must be calibrated. This study summarizes the phase characteristics of the orthogonally polarized laser under the joint of anisotropy feedback cavity, supplements the physical content of the laser feedback, and has great significance for accurate laser measurement of stress-birefringence, displacement, and distance.

Clear to clear laser welding for joining thermoplastic polymers: A comparative study based on physicochemical characterization

The joining of materials is essential to many industrial applications used today. However, it still lacks a reliable and true single step method to join different types of materials including polymers. Laser welding was primarily employed for metals until some decades ago, but with the rise of high power solid state lasers, applications have emerged in the polymer field. With the recent addition of fiber lasers, true clear to clear welding, joining two of the same transparent polymer layers, has become feasible at a different wavelength range, which benefits the sealing of microfluidic devices. A number of research efforts were made at performing welds of thermoplastics at this wavelength range, however without attempts at offering elaborate explanation of the observations. In our work, a laser welding system using a Thulium fiber laser at 1940 nm was used to join a variety of thermoplasts, without the use of additives or prior processing. By optically and chemically characterizing the materials, a basis was established that links intrinsic material properties to observed welding performance, as determined by mechanical shear tests on the welded materials. We found that four of the ten considered thermoplastic polymers lend themselves to sufficiently strong bonding to allow the creation of leak-proof microfluidic devices by use of laser welding at this wavelength.

Design of laser-driven SiO2-YAG:Ce composite thick film: Facile synthesis, robust thermal performance, and application in solid-state laser lighting

In recent times, there have been rapid advances in the solid-state laser lighting technology. Due to the large amounts of heat accumulated from the high flux laser radiation, color conversion materials used in solid-state laser lighting devices should possess high durability, high thermal conductivity, and low thermal quenching. The aim of this study is to develop a thermally robust SiO2-YAG:Ce composite thick film (CTF) for high-power solid-state laser lighting applications. Commercial colloidal silica which was used as the source of SiO2, played the roles of an adhesive, a filler, and a protecting agent. Compared to the YAG:Ce powder, the CTF exhibits remarkable thermal stability (11.3% intensity drop at 200 °C) and durability (4.5% intensity drop after 1000 h, at 85 °C and 85% humidity). Furthermore, the effects of the substrate material and the thickness of the CTF on the laser lighting performance were investigated in terms of their thermal quenching and luminescence saturation behaviors, respectively. The CTF with a thickness of 50 μm on a sapphire substrate does not show luminescence saturation, despite a high-power density of incident radiation i.e. 20 W/mm2. These results demonstrate the potential applicability of the CTF in solid-state laser lighting devices.

Optomechanical design and analysis of a self-adaptive mounting method for optimizing phase matching of large potassium dihydrogen phosphate converter

Surface control and phase matching of large laser conversion optics are urgent requirements and huge challenges in high-power solid-state laser facilities. A self-adaptive, nanocompensating mounting configuration of a large aperture potassium dihydrogen phosphate (KDP) frequency doubler is proposed based on a lever-type surface correction mechanism. A mechanical, numerical, and optical model is developed and employed to evaluate comprehensive performance of this mounting method. The results validate the method’s advantages of surface adjustment and phase matching improvement. In addition, the optimal value of the modulation force is figured out through a series of simulations and calculations.

Investigation of Bayesian network for reliability analysis and fault diagnosis of complex systems with real case applications

Reliability is critical for complex engineering systems. Traditionally, reliability analysis and fault diagnosis of complex engineering systems is based on reliability block diagram and fault tree. These methods are limited either on the flexibility for system characterization or on the capability for quantitative analysis. Recently, the Bayesian network has been introduced in reliability engineering, and it has been demonstrated with great flexibility. In this article, the Bayesian network is investigated for reliability analysis and fault diagnosis of complex engineering systems through two real cases. It includes the case of a high-speed train representing the complex system with standardized components and the case of a critical subsystem of a high-power solid-state laser representing the complex system with highly customized components. In particular, Bayesian networks are constructed to model the reliability of these systems, where transformations of reliability block diagram and fault tree into Bay...

Investigation of an LuAG:Ce translucent ceramic synthesized via spark plasma sintering: Towards a facile synthetic route, robust thermal performance, and high-power solid state laser lighting

The aim of this study is to investigate a thermally robust green-yellow color converter for high-power solid state laser lighting applications. LuAG:Ce translucent ceramics (TCs) were synthesized via the spark plasma sintering (SPS) technique and LiF was used as additive. The as-prepared ceramics show remarkably superior thermal and luminescent properties: a high external quantum efficiency (77%), a low thermal quenching (4.1% drop at 200°C), a good thermal conductivity (6.3Wm−1K−1), and a high reliability (only a 1.9% drop after 1000h, at 85°C and 85% humidity). A prototype lamp using the TC (and a 450nm laser diode), with the increasing driving power (up to 8.7W), shows increasing luminous flux (238–472lm) with stable luminous efficiency (54.3-56.6lm/W). This indicates that the translucent ceramic can be used for high-power laser-lighting.

Surface control apparatus and method of optical transmission with large aperture based on self-adaptive force-moment technology.

Surface control of large-aperture transmission optics in both on-line working, and off-line testing, states is perceived as one of the most important requirements, and the largest engineering challenge, in high-power solid-state laser facilities. Consequently, a surface control apparatus and method was proposed based on the self-flexible force-moment technology and principle of load linearity, respectively. A series of analyses were conducted using the proposed apparatus and method both mechanically and numerically. Furthermore, the principle of phase mismatch induced by distortion and second harmonic generation (SHG) efficiency was analysed theoretically. Finally, the trends of the surface RMS value and efficiency under different load regimes on certain loading states were deduced, and the trends in the best surface RMS value and efficiency of each loading state were analysed.

Beam alignment based on the imaging properties of the spatial filter by controlling the deformable mirror in a high power laser

Deformable mirror placed in the front-end is widely used for wavefront correction in high power solid-state lasers. However, the laser beam is usually partly blocked by the spatial filter pinhole when the deformable mirror surface is improper. In this paper, this beam alignment problem is solved based on the imaging properties of the spatial filter by controling the defocusing value of the deformable mirror surface. Experimental results show that the spatial filter pinhole is passed through successfully and a relatively complete laser beam nearfield is achieved after beam alignment in the output of the laser system.

Conical refraction, for annular pumping of an efficient vortex Nd:YAG laser

We presented a proof-of-concept investigation upon an end-pumped and vortex Nd:YAG laser. In this laser, for the first time, the conical refraction (CR) ring of a biaxial KGd(WO4)2 crystal was used for annular pumping. Owing to nearly 100% conversion efficiency in forming the CR ring, the Nd:YAG laser was highly efficient and emitted LG01-mode vortex light with 177.5 mW beam power and 65% slope efficiency. This study showed the CR ring pumping would favor the production of a compact, simple and efficient vortex laser. It is expected that, in future, the CR ring pumping technique would also facilitate the high-power vortex solid-state laser where a long gain medium is required.

Study on Control Strategy of Structure Stability for the Shenguang-III Laser Facility

The Shenguang-III laser facility is the largest high-power solid-state laser facility in Asia and provides 48 laser beams for laser fusion. To satisfy the demands of physics experiments, all the laser beams are required to be accurately positioned on the target and the positioning error of all the laser beams should not exceed 30μm (root mean square). The formidable challenge in the facility design is how to minimize drift errors and structure vibrations and place the optical elements in their ideal locations. In the paper, we focus on the control strategy of structure stability for the Shenguang-III laser facility. Firstly, we present the methodology of stability index allocation to obtain the design parameter of structure stability for the laboratory platform and each optical element. Then we discuss the factors influencing stability of optical elements and the control methodology. Finally, we measure the beam pointing error to prove the feasibility of strategy. The results show that the maximum drift of all the laser beams is 26μm (root mean square), which is less than 30μm and satisfies the stability demand.

Optical properties of green-emitting β-sialon:Eu phosphor-containing silica glasses and their deterioration mechanism

β-sialon:Eu phosphor-containing silica glass was synthesized as a wavelength converter in solid-state lighting. The optical properties and pore density of the sample strongly depended on the sintering temperature. Sintering at a high temperature of 1050 °C led to a severe deterioration of β-sialon:Eu phosphor owing to N2 evaporation, whereas a high conversion efficiency could be achieved by lowering the sintering temperature down to 600 °C. The β-sialon:Eu phosphor-containing silica glass sintered at 600 °C showed a high durability against high-power excitation, making it possible to be used in high-luminosity and high-power solid-state laser lighting.

Wavefront Shaping by a Small-Aperture Deformable Mirror in the Front Stage for High-Power Laser Systems

We demonstrate a method for wavefront distribution compensation with a low-cost small-aperture deformable mirror in the front stage of a complex high-power solid-state laser system. Meanwhile, an iterative algorithm for improving wavefront quality is indicated. Using this method, the wavefront compensation was studied in our single-shot high-power laser system that operated with and without the main amplifiers, respectively. The wavefront was compensated effectively, showing the near-flopped-shape output with the peak-to-valley value of 0.29 λ and root meam square (RMS) of 0.06 λ at 1053 nm.

高功率固体激光器中大口径光学器件波前误差的建模及控制方法

高功率固体激光器中大口径光学器件波前误差的建模及控制方法

高平均功率固体激光器研究进展

高平均功率固体激光器研究进展