An experimental system was established to study the 12kW fiber laser irradiation effects on thin LY12-CZ plate. The laser spot on the plate, the change of surface morphology, the change of rear surface temperature as well as the penetration time of the plate were measured. During the laser irradiation, the phenomena of bagging, necking, and perforation were observed on the thin aluminum plate in the irradiated area, which was collectively referred to as the "bagging" effect. In our view, the "bagging" effect is related to the two oxide films formed on the front and rear surfaces of the thin aluminum plate. And a model of molten aluminum flow in a sandwich structure was proposed to analyze the mechanism of the "bagging" effect. The measurement results of the two-color radiation thermometer and the time-of-penetration detector were also consistent with the model and the "bagging" effect.
Based on the overhanging beam three-point bending method, the experimental system was set up to measure the variety of shear stiffness of Nomex honeycomb sandwich panel in laser irradiation. The shear stiffness of the specimens under different laser power density was measured. The result shows that the thermal effect during the laser irradiation leads to the degradation of mechanical properties of Nomex honeycomb sandwich panel. High temperature rise rate in the specimen is another main reason for the shear stiffness degeneration. This research provides a reference for the degradation of mechanical properties of composite materials in laser irradiation and proposes a new method for the study of laser interaction with matter.
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.
In order to measure the change of laser energy coupling coefficient with temperature in mid-infrared wave band, reflectance integrating sphere experiment system was designed and set up. 915nm CW laser was used to heat samples and the wavelength of probe laser is 3.8μm. Chopper and phase-locked amplifier were adopted in the system. Thermal imager was used to measure and record the temperature of samples during laser irradiation. The reflectance of steel and aluminum plates to 3.8μm was measured during 915nm laser irradiation. EDS analysis was done to investigate the change of elemental composition in the samples respectively. The experimental results show that, the results of reflectance and radiation temperature measured by this system are relatively accurate during laser irradiation. In the process of temperature rising from 300K to 785K, the color of 45# steel plates turns blue and black, while the color of aluminum alloy plates is basically unchanged. When temperature reaches about 700K, reflectance of 45# steel decreases obviously with the increase of temperature, while reflectance of aluminum is almost constant. The reflectance is probably determined by the oxide in the surface of samples which is consistent with the results of EDS analysis. Reflectance decreases with the increase of the content of oxygen in the surface. The reason of why the reflectance of aluminum is almost constant is that aluminum oxide is not generate massively under 750K.
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.
Two kinds of anti-laser coating made of reflective / ablative resin, called reinforcement schemes of A and B, are applied to the glass fiber reinforced resin matrix composite plate. The anti-laser performance of these samples to the laser operated at the wavelength of 976nm is tested, under the case of a 0.3 Mach tangential airflow pass over the surface of the sample. The experimental results show that the laser damage threshold of the coating reinforced samples have increased more than 50% compared to the original sample, the reinforcement scheme B is better than A. The laser power density damage threshold of the coating reinforced samples to the near infrared laser is higher than 100W/cm2, under the irradiation time is 60 seconds. For the resin reinforced fiber samples, the removal process of the ablation residues has important effects on the perforation time of samples, when there is a strong airflow pass over the surface. The larger laser spot corresponding to the removal of the ablation residues is easier.
KEYWORDS: Resistance, Silicon solar cells, Solar cells, Laser applications, Laser energy, Silicon, Temperature metrology, Power supplies, Electrodes, High power lasers
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/cm2 to 0.58W/cm2, 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/cm2, 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.
KEYWORDS: Amplifiers, Signal attenuation, Single mode fibers, Signal processing, Fabrication, Refractive index, Multimode fibers, High power fiber amplifiers, Fiber lasers, Polymers
A high-efficiency pump-signal combiner for high power fiber amplifiers based on thermally expanded core (TEC)
technique is reported in this paper. TEC technique is used to fabricate mode-field adapter which allows optimization of
signal fibers in a monolithic (6+1) ×1 fiber combiner. The combiner is fabricated by connecting a tapered fiber bundle
(TFB) to a passive 25/250 (NA=0.06/0.46) double-clad fiber (DCF). By this method, the coupling efficiency of SMF-28
signal fiber at 1064nm improves from 54% to 92.7%. The average pump coupling efficiencies of six 105/125 (NA=0.15)
fibers are measured to be 96.7% at 976nm. Furthermore, the average signal transmission efficiency is around 93.3%. The
fabricated fiber combiner is spliced to an Yb-doped DCF for use as an all-fiber amplifier. The slope efficiency is
measured to be 71.6%.
The thermal response of a cylindrical simulative warhead consisting of the steel casing and the TNT explosive irradiated by laser is simulated, basing on the smoothed particle hydrodynamics method. Preliminary computational simulation results show that, when the power density of 500W/cm2 continuous laser irradiation on a sealed explosive device consisting of the type 304 steel casing with thickness of 5mm and TNT explosive, compared with no airflow, the speed of 200m/s tangential airflow can reduce the thermal initiation time of 0.6s. In the case of incident laser power density is high, the convection cooling effect of tangential airflow can be neglected. The oxidation of airflow can significantly shorten the thermal initiation time of internal explosive.
The concept of using lasers to drill through rock has been discussed in the oil and gas industries since the development of the high-power laser. To evaluate the possibility of fielding a laser drilling system, two laser-related problems have to be investigated. The first is the irradiation effects of laser upon rocks; the second is the effects in laser transmission from the source to the rock deep in the well. This transmission includes two stages: the first stage is the transmission inside a fiber, which is packaged in a cable and has about the same length with the well depth; the second stage refers to the transmission process when the laser leaves the fiber and some transforming optics and transmits to the rock surface, during which the well conditions may impose tough restrictions. In this paper, experiment results of laser irradiation upon siliceous sandstone and granite are reported, and the fiber transmission loss is simulated, considering the main absorbing or scattering mechanisms inside fiber. And the laser transmission from the fiber end to the rock surface, in my view, may impose great challenge on the laser drilling technology.
Great progress has been made in fiber laser technology especially the high power fiber laser. One of the key
techniques to acquire higher output power is coupling more pump laser into the double-cladding fiber using the fiber
combiner. Fiber splices exist in both manufacture of the combiner and integration of the fiber components. The optical
waveguide structure of the splice point has great effect on the insertion loss and modal content for the fiber laser system.
Thus it is important to use proper method to compute the insertion loss of the splice points. This is also vital in the
manufacture of fiber combiner because the structure must be precisely controlled in order to acquire low insertion loss
for the signal arm of the combiner to ensure the capability of sustaining high power laser.
Generally speaking, there are two common methods to compute the insertion loss of splice points: the mode field
diameter (MFD) and the modal overlap integral (MOI). The MFD method is simple but its accuracy is relatively lower,
while the MOI is more accurate than the MFD but also more complicated. We use both two methods to compute the
insertion loss of the signal arm of a (6+1) ×1 fiber combiner. The result shows that the MFD method is appropriate when
there is only fundamental mode at the splice point. At the mode field matched point, the insertion loss is 0dB when using
the MFD method while 0.29dB when using the MOI method. This indicates that the MOI method is more accurate than
the MFD method to predict the minimum insertion loss and the optimal structure. Meanwhile, the MOI method can
explain the different insertion loss for the co-propagating situation and the counter-propagating situation for the fiber
combiner which cannot be explained by the MFD method. If there are higher order modes passing through the splice
point, the MFD method is also inappropriate.
The irradiation effects of 976nm continuous-wave laser on carbon fiber reinforced E-51 resin composite is studied experimentally, with a 0.4Ma tangential airflow or 0.4Ma tangential nitrogen gas flow on the target surface. In order to simulate the thermal response of fiber reinforced resin composite materials subjected to combined laser and tangential gas flow loading, a three-dimensional thermal response model of resin composite materials is developed. In the model, the thermal decomposition of resin is described by a multi-step model. The motion of the decomposition gas is assumed to be one-dimensional, for the case that the laser spot is significantly larger than the thickness of the sample. According the above assumption, the flow of the decomposition gas is considered in the three-dimensional model without introducing any mechanical quantities. The influences of the tangential gas flow, the outflow of the thermal decomposition gas and the ablation(including phase change ablation or oxidative ablation)of the surface material on the laser irradiation effects are included in the surface boundary conditions. The three-dimensional thermal response model is calculated numerically by use of the modified smooth particle hydrodynamics (MSPH) method which is coded with FORTRAN. The model is tested by experimentally measuring the temperature profiles during carbon fiber reinforced E-51 resin composite subjected to combined laser and tangential gas flow. The predicted temperature profiles are in good agreement with experimental temperatures obtained using thermocouples.
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 N2 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.
In the experiments of photovoltaic detectors illuminated by CW lasers, some new mechanism has been
found, such as power saturation of photovoltage, hot carrier effect, as well as thermovoltage effect. To
investigate whether there is similar mechanism with pulsed laser irradiating, an 808nm femtosecond
pulsed laser is adopted. In the experiments, three photovoltaic infrared detectors are used, namely short
wavelength HgCdTe detector, medium wavelength HgCdTe detector and medium wavelength InSb
detector. Actually, the 808nm pulsed laser is spectral related laser for short wavelength HgCdTe
detector while spectral unrelated laser for medium wavelength HgCdTe and InSb detector. Under
various power densities, the detectors have a series of outputs. Power saturation of photovoltage is
observed. However, the characteristics of the outputs of these three detectors are quite different, even
between medium wavelength HgCdTe and InSb detector, which have the same packing method. There
are three major contributions in the paper. Firstly, explain the mechanism of power saturation of
photovoltage, mainly from hot carrier effect and the depressed ability of PN junction to separate
electrons and holes with the higher temperature induced by the laser. Secondly, compare the differences
between medium wavelength HgCdTe and InSb detector and give a qualitative analysis. Finally, the
difference of the outputs between short and medium wavelength HgCdTe detector is compared and
qualitatively analyzed, too, with the different mechanisms of interaction between infrared detectors and
spectral related or spectral unrelated laser. The experimental results and theory analysis will show
valuable clue for future research on photovoltaic detector irradiated by pulsed laser.
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