In order to study the scaling laws of optical components, we set up a model based on the heat conduction theory and thermodynamic theory. Then the similarity theory was used to the model analyzation. Finally, we demonstrate three conclusions which are related to the practical engineering application. The first one is that thermal damage behaviors of different scale optical components are similar when the linear power density of irradiated laser are the same. In other words, we should use the linear power density to represent the resistance of damage tolerance for optical components The second one is the judgement standard of scram time. We find the scram time of large-aperture system is certain times as much as the scram time of small-aperture system. The third one is about how to design the scaled experiment can we make two different scale laser systems obey the similar thermal damage behaviors. This study is of great help for the damage prevention of the optical components.
We use the fiber-pumped MgO:PPLN crystal to realize the MIR CW-OPO operation, and observe the nonuniform temperature distribution on the central axis of the crystal. Then we use the heat transfer model in COMSOL software to simulate the temperature distribution in the crystal and find the near linear temperature gradient on the central axis of the crystal. Input the axial temperature distribution to our SRO model based on MATLAB and the simulation results show that the linear axial temperature gradient distribution will not only cause the center wavelength shift of the signal light, but also reduce the parametric gain of the signal light, the uniform temperature along the crystal axis will get the maximum gain. This feature limits the prospect of the single OPO in high-power narrow linewidth laser.
We present a versatile method to diagnose method to diagnose nanosecond laser induced plasma (LIP) plume with good temporal (10 ns here) and spatial (here sub-millimeter) resolution, without requiring the assumption of local thermodynamic equilibrium (LTE). The spatially resolved emission images from plasmas formed by 532 nm laser ablation of a silicon target in vacuum (10<sup>-7</sup> mbar) with incident irradiance of 21 GW/cm<sup>2</sup> were recorded at different time delays using a time-gated iCCD camera attached to a spectrograph and image optics. The spectroscopic emission lines associated with different charged species are assigned in the NIST Atomic Spectra Database. The further analysis of Stark broadened line shapes of those emission images allows tracking the plume dynamics and provides insight into the early time (i.e. within several tens of nanoseconds) mechanism of laser-target interaction and the subsequent laser-plasma coupling. The electron density (N<sub>e</sub>) and temperature (T<sub>e</sub>) values and their variations with space and time are obtained from best-fitting model to the observed line shapes based on a non-LTE electron energy distribution function (EEDF) rather than a Maxwellian EEDF. The value of N<sub>e</sub> and T<sub>e</sub> respectively declined from 10<sup>23</sup> to 10<sup>21</sup> m<sup>-3</sup> and 10 to 0.1 eV since the plume expansion. The time-gated emission images and the spatial and temporal variation of the N<sub>e</sub> and T<sub>e</sub> values both highlight the inhomogeneity of the LIP plume, and provide the future analysis and possible derivation of the electron emitting model from target surface after laser-lattice interaction within sub-nanosecond.
Accurate measurement of the wall temperature is of great significance to investigate laser irradiation effects on a liquid tank. It was shown that the wall temperature couldn’t be measured accurately using the traditional installation method for the thermocouple. To overcome this problem, an effective installation technique was developed. First, a groove was carved on the rear surface of the metal casing of a liquid tank by laser irradiation. Then the thermocouple junction was welded to the measurement point and covered up by high-temperature heat conduction glue. The experimental results showed that the wall temperature could be measured correctly using this installation technique.
The spread of the pump model, established based on MATLAB, simulates the distribution of the pump in End-Pumped single crystal fiber. Simulation results show that the pump in the rod single crystal fiber will converge again. By changing the crystal absorption coefficient, it can be found that smaller the absorption coefficient is, more uniform the pump distribution is; when it is greater, the pump will concentrate to the pump end more seriously. Establish End- Pumped Experimental platform in the experiment, the crystal is 1 mm in diameter and length of 30 mm, Nd<sup>3+</sup> doping concentration is 1%. Change the position of the pump light's focus in the crystal, we can see different distribution of the pump light by different focus location in the crystal and find that the pump light has the most homogeneous distribution when the focus is on the crystal axis and has 1mm distance to the pump end face. At this time, the second convergence of the pump is clearly visible. By changing the pump wavelength, crystal absorption coefficient changes. It is found that under the same pump power, absorption coefficient is greater, the pump will concentrate to the pump end more seriously. And the temperature of crystal pump end rises, which is identical with the simulation results. The results indicate that for the single crystal fiber, the higher absorption coefficient is not better, low absorption coefficient leads to the uniform distribution of the pump, there will be a better absorption in a relatively long length of single crystal fiber. And due to the lower end face temperature, end pump power upper limit will also increase.
Compared with traditional methods of energy supply, there is a great possibility to get a more remarkable enhancement of conversion efficiency for laser power (of proper wavelength and intensity) beaming to silicon solar cells. However, it should be noticed that cells may be damaged by high power laser. Based on the background, this essay explores high-power-laser's possible damage to silicon solar cells by analyzing IV curves (obtained by IV tester) and minority-carrier lifetime (measured by open-circuit-voltage-decay method). Research shows that, for 30s irradiation, minority-carrier lifetime decreases to some extent when irradiated by laser of over 5.5W/cm<sup>2</sup> and the higher laser power density, the more degradation. Similarly, IV curves see a downward trend under laser of over 5.5W/cm<sup>2</sup>. In addition, there is a roughly linear relationship between lifetime and the decrease amount of short circuit current. Moreover, the degradation degree has a close relation with the maximum temperature. The prolonged illumination would not bring about more serious damage if one cell had already reached an equilibrium temperature.
Experimental research on the energy coupling characteristic of 45# steel and 304 stainless steel under mid-infrared CW laser irradiation is carried out. Based on the classical electromagnetic theory, the theoretical formula of the energy coupling coefficient is derived under ideal condition. In order to obtain the energy coupling coefficient, an experimental system for reflectance measurement is set up by an integrating sphere. The curves about energy coupling coefficient and the temperature variation are measured respectively. The mechanism about the variation of energy coupling coefficient of sample under mid-infrared CW laser irradiation is also discussed. The experimental results show that the energy coupling coefficient of sample increased with temperature rising, but the curve in the heating stage is not consistent with the curve in the cooling stage, which means the change of the energy coupling coefficient is not a reversible process. Combined with the experimental phenomena and the energy dispersive spectrometry, the qualitative analysis about the differences between the 45# steel and 304 stainless steel is presented after irradiation. It indicates that the oxidation reaction has a significant effect on the laser interaction with sample. Accordingly, the variation of coupling coefficient of 304 stainless steel is not as obvious as that of 45# steel.
The lethality effect of high power laser on target is simulated with CFD method under different conditions of supersonic air flow on the surface of the target. Materials used in the experiments are 2cm aluminum plate. With the Mach number changing from 1 to 5, the lethality effects of the high power laser can be obtained from the simulations under these conditions of supersonic air flow. The flow-structure-laser coupling impact on the failure time of the target is discussed based on the simulation. Results show that with the increase of mach number, the effect on the aluminum plate is increase first and then decrease by the pressure. Because that it is obvious that the maximum area of pressure is away from the center of deformation region when the mach number is bigger than 5 . At the same time, when mach number is increase, the aerodynamic heating play more important role than the convective heat transfer on the temperature field of aluminum plate. there are two impacts from the supersonic flow. Firstly , the flow can produce the pressure on the surface of the aluminum plate. Secondly, the flow can produce aerodynamic heat on the aluminum plate.
The irradiation effects of LD laser on thin aluminum alloy plates are studied in experiments characterized by relatively large laser spot and the presence of 0.3Ma surface airflow. A high speed profilometer is used to record the profile change along a vertical line in the rear surface of the target, and the history of the displacement along the direction of thickness of the central point at the rear surface is obtained. The results are compared with those without airflow and those by C. D. Boley. We think that it is the temperature rise difference along the direction of thickness instead of the pressure difference caused by the airflow that makes the thin target bulge into the incoming beam, no matter whether the airflow is blown or not, and that only when the thin aluminum target is heated thus softened enough by the laser irradiation, can the aerodynamic force by the surface airflow cause non-ignorable localized plastic deformation and result a burn-through without melting in the target. However, though the target isn’t softened enough in terms of the pressure difference, it might have experienced notable deformation as it is heated from room temperature to several hundred degree centigrade.
The properties of 915nm laser power beaming to monocrystal silicon solar cells are investigated by measuring IV curves, temperature and etc. With the illumination intensity increased from 0.04W/cm<sup>2</sup> to 0.58W/cm<sup>2</sup>, short-circuit current increases almost linearly from 0.14A to a maximum value of 3.07A. While the maximum power output peaks at a lower irradiation intensity of 0.46W/cm<sup>2</sup>, which can be also regarded as a turning point where IV curves begin to deteriorate from normal ones to oblique lines. During the period, the fill factor decreases continuously from around 74% to a stable value of 25%. To understand the experiment more clearly, theoretical analyses are conducted by virtue of Lambert W function. Based on the analyses, it can be concluded that the primary culprits influencing the cell’s output performance are the temperature and series resistance.
Pumping coupler technology is one of the critical technologies for high power laser and amplifier. Side-pumping
technology can couple pumping beam into inner cladding of the double-clad fiber through the side of the fiber.
Compared to the end-pumping technology by tapered fused bundle (TFB), it has many superiorities. That the signal fiber was not disconnected guarantees high transmission efficiency, providing the possibility of transmitting a high power signal. Additionally, the pump light is coupled into the double-cladding fiber all along the coupler’s body (~5-10 cm long), which reduces the thermal effects caused by leakage of pumping light, resulting in high pump power handling capabilities. For the realization of reliable, rugged and efficient high power fiber amplifiers and fiber laser systems, a novel kind of fused side-pumping coupler based on twisting is developed. The complete simulations were carried out for the process of side-pumping. From detailed information about simulations, we found that the pump efficiencies, one of the vital parameters of pumping coupler, have a significant influence with coupling length, the numerical aperture (NA) and taper ratio of pump fiber. However, the diversification of the parameters drops the high transmission efficiency barely. Optimized the parameters in the simulations, the pump and signal coupling efficiencies are 97.3% and 99.4%, respectively. Based on theoretical analysis, the side-pumping coupler was demonstrated at the pump and signal coupling efficiencies are 91.2% and 98.4%, respectively. This fiber coupler can be implemented in almost any fiber laser or amplifier architecture.
Experiments are performed to investigate the laser irradiation effects on thin aluminum alloy sheets subjected to tangential airflow. The wind blower generated airflow with a speed of about 100 m/s along the surface. For comparison, experiments in the absence of airflow are also conducted. Moreover, in order to know whether the combustion reaction takes place during the irradiation, we vary the composition of flow from air to nitrogen. The displacements of the sheets center are measured to see whether the tangential flow has a mechanical effect. The maximum temperature of the sheet is lower than 550 ℃ after 2 seconds irradiation with the laser power density of 173W/cm<sup>2</sup>. Accordingly, the structural parameters of aluminum alloy do not have distinct change and so do the features of sheets. The temperature curves in the air flow and nitrogen flow keep the same and both lower than that in no flow case. Moreover, the displacements measured in three cases do not have obvious difference. These experiment results indicate that the combustion reaction can hardly happen and the tangential flow only has a cooling effect. The maximum temperature reaches 600 ℃ when the laser power density rises to 400 W/cm<sup>2</sup>. Such a high temperature makes that the elastic modulus of aluminum alloy drops rapidly, which greatly softens the alloy sheets. The plastic distort of irradiated sheets confirmed this process. When the power density rises to 450W/cm<sup>2</sup> big melt-through phenomenon is observed and there is viscous dripping under gravity in the no-flow case. However, in air flow and nitrogen flow, we can see the removal of macroscopic unmelted pieces of aluminum alloy sheet. The results indicate that the tangential flow mainly has two effects including cooling the target and removing the unmelted metal when the material is fully softened.
The irradiation effects are studied, of solid-state laser on four kinds of plates (three of them are made of metal, the other, of composite), in experiments characterized by relatively large laser spot and the presence of surface flow. The thick iron samples, thin aluminum samples and thin carbon fiber/epoxy resin samples are subjected to air or N<sub>2</sub> surface flow, while the box-shaped samples, containing a thin aluminum plate irradiated by laser, are filled with water. It is found that, besides the common role in all four cases cooling the plate by convective heat transfer, the fluid plays other different roles in different case influencing the dynamic response of the plate. The roles of the fluid in each case are described either with analytical boundary conditions or with differential equations, which are then incorporated into computational models. Numerical simulations are carried out, with results compared with the experiment results to explain the irradiation effects.