A diode pumped alkali laser (DPAL) provides a significant potential for construction of high-powered lasers in the future. To realize the scaling of a DPAL, heat management and flow field inside a vapor cell should be investigated. In this paper, a new kind of gas-flowing DPAL with a disc-type vapor cell was proposed. The gain medium of cesium and the buffer gas of ethane were filled in the vapor cell with the total pressure is about 1 atmosphere. The influence of the rotate speed of a cross-flow fan on the internal gas velocity, temperature, and output features of the laser was systematically studied. The corresponding experiment was carried out, and the output laser at 894.3 nm with power of 321 W was obtained.
Due to the rapid performance of taking away the generated heat of a diode pumped alkali laser (DPAL) system, a flowing diode pumped alkali laser (FDPAL) is thought to be helpful to mitigate the thermal effects and improve the power scaling ability of a DPAL system during the power scaling period. In general, a relatively perfect theoretical model for a FDPAL needs to take the laser kinetic, heat transfer, and computational fluid dynamics (CFD) into account at the same time. Until now, the commercial finite element method (FEM) soft can only simulate the fluid and thermal distributions in an alkali vapor cell for a FDPAL. The laser kinetic can only be effectively calculated by a coding soft. Therefore, the multi-physics coupling problem needs to be firstly tackled at the beginning of a design for a FDPAL system. In the paper, a loop iteration based co-simulation method is utilized to solve the multi-physics coupling problem during the simulation of a FDPAL. The temperature and fluid corresponded parameters of a FDPAL are obtained by a FEM soft. The laser kinetic corresponded parameters of a FDPAL are got by a coding soft. By constructing a java language based server, the calculated data of such two kinds of soft can be shared. Then, a main iteration based procedure with preset initial values is coded to control the running behavior and communication of the two kinds of soft. After several or several ten times loop iteration, the laser power, temperature distribution, and velocity distribution of a FDPAL can be theoretically investigated. It has been demonstrated that the co-simulation based calculating results show a good agreement with the experiment results.
KEYWORDS: Fluctuations and noise, Cesium, Semiconductor lasers, Gases, Diodes, Output couplers, Laser systems engineering, High power lasers, Alkali vapor lasers
A diode pumped alkali laser (DPAL) provides a significant potential for construction of high-powered lasers. To realize the scaling of a DPAL, heat management should be optimized. In this paper, a new kind of gas-flowing DPAL was proposed, in which a small cross-flow fan with diameter of 80 mm was set in the center of a cylindrical vapor cell whose diameter is 125 mm. The gain medium of cesium and the buffer gas of ethane were filled in the vapor cell with the total pressure is about 1 atmosphere. A mathematical model was constructed to systematically study the influence of the rotate speed, the heating temperature on the internal temperature distribution, and the output features of the laser.
In this paper, a simple and effective method is proposed for measuring the focal length of a weak negative thermallyinduced lens. Generally, it is very difficult to measure the focal length larger than 1000 mm of a weak thermally-induced lens by utilizing the traditional procedures. In our experiment, we planned to construct a Yb:KGW laser system almost without the thermally-induced lens in which the focal length of the laser crystal should be measured precisely. With respect to the optical features of Yb:KGW crystal, the thermally-induced characteristics look like something of a negative lens with weak effects. The steps of measuring the focal length of a thermally-induced lens of the laser medium have been adopted as follows. First, the relationship between the focal length f1 of a positive assistant lens as well as the position of the assistant lens and the focal length fT of a thermally-induced lens were carefully analyzed and the experimental setup were designed through the theoretical simulation. Secondly, the variation of the spot size and post position for a He-Ne probe laser have been experimentally investigated after the probe laser beam passed through a thermally-induced lens (fT) and an assistant lens (f1) with the different drive currents of a pump LD with the wavelength of 980 nm. Then, the post position for a He-Ne laser beam can be obtained by use of a least square method, and then the focal length of a weak thermally-induced lens can be deduced with an indirect detection method. In this paper, we introduce a new technique for the measurement of the focal length with the absolute value large than 1000 mm of a negative lens, which has not been reported until now. The results might be useful for the evaluation of a weak thermally-induced lens of almost all solid-state lasers (SSLs).
Laser processing plays a key role in the industrial manufacture. The transparent material processing with a visible nanosecond laser based on a tripartite-interaction procedure has proven to be an effective method, which has the advantages of low cost, high efficiency, and simplicity over the traditional direct processing by using a femtosecond laser. In our pre-study, by using an assisted metal foil attached to the rear surface of a transparent glass sheet, some holes can be drilled on the glass sheet with a visible nanosecond laser. Such a physical mechanism is based on the heat conduction, generation of stress and ablation among the laser beam, the glass sheet and the metal foil. However, the processing quality of the glass sheet during the previous process is still dissatisfied and remains to be improved. In this study, we demonstrated a new tripartite-interaction procedure among the laser beam, glass sheet and copper foil, i.e. attaching an assisted copper foil on the front surface of the glass sheet, to further improve the processing quality of the hybrid tripartite-interaction processing. The experimental results are compared with those of our previous work, indicating that drilled holes and grooves with less crack and better quality can be obtained by using the new procedure. Moreover, to analyze the reasons of obtaining less cracks and better quality, we have carried out a series of theoretical studies on simulating such a new tripartite-interaction process. According to some specific simulation results of the temperature and density variations in the glass and copper, we can analyze that the reduction of thermal damage on the glass sheet and the improvement on processing quality might be attributed to the thermal transfer induced by attenuated laser energy in such a configuration. Our results could be useful for the development of visible nanosecond laser processing in industrial applications.
Although aluminum processing with lasers has become popular in industrial applications, machining some blind grooves or blind holes with a required size in aluminum sheets is also a difficult task for laser technicians. In this paper, blind grooves with the depth of about 0.1 mm and the width of less than 0.1 mm on a 0.24 mm thick aluminum sheet have been realized by using two kinds of nanosecond Q-switched lasers without burning the coating polymer layer. The effects of the laser wavelength, average power, and scanning speed on laser processing have been investigated in detail. The groove machining of aluminum sheets has been carried out at different laser power and machining speeds by use of two Q-switched lasers with the wavelengths of 532 nm and 355 nm. The experiment result shows that the faster the scanning speed, the better the processing efficiency. And the status of blind grooves processed by a 355 nm laser is cleaner and smarter. In summary, the optimal laser parameters for processing grooves on the aluminum surface are the peak power density of 2.27×108 W/cm2 with the scanning speed of 0.1 mm/s for a 355 nm nanosecond laser. The results of our study might be of great importance as a reference for processing blind grooves on aluminum sheets in some industrial applications.
In this paper, a widely tunable Cr:LiSAF laser with an external cavity was employed as the pump source. By using a triangular prism and double output couplers in the cavity, the line width can be narrowed and the pump center wavelength can be adjusted to the ideal value. The FWHM in spectrum of a pump laser can be narrowed to as small as 0.5 nm. The absorptivity of Ho:BYF at the center wavelength from 885 nm to 890 nm was measured, and the optimal pump center wavelength has been determined to 888.5 nm. Then the focal length of a focusing lens and the curvature radius of a laser output coupler have been optimized through a series of experiments. Finally, we have obtained the laser output at 3.9 μm with the optical-to-optical efficiency larger than 10% at the relatively low repetitive rate. The results might be helpful for the construction of a real laser system.
Laser processing is a technique based on the interaction between a laser and the substance for cutting, drilling, cleaning, welding, and other operations on metallic or non-metallic materials. It is widely used in some important fields of the national economy such as automobiles, microelectronics, electrical appliances, aviation, metallurgy, medical treatment, and machinery manufacturing. In the process of high-powered laser processing, a large amount of plasma will be generated and there will be the obvious inverse Bremsstrahlung absorption (IBA) near the plasma plume. The effect of laser processing will be significantly deteriorated due to the absorption of laser photons and changes in light intensity distribution. Besides, laser-induced plasma is produced during the interaction between a high-powered laser and materials. Also, it has the very important value in the research of analyzing the high-powered laser processing. To fully understand the laserinduced plasma, this paper uses the Hilbert procedure to numerically investigate the plasma generated in the laser processing. The method firstly acquires the images corresponding to the fringes of a Mach–Zehnder interferometer by using the detection after a probe laser beam passing through the plasma plume. Then, a series of operations such as the spectrum shift, unwrap, and Abel inverse transformation are performed after a fast Fourier transform (FFT). Finally, the density distribution of plasma can be calculated. This methodology provides a new algorithm for the research of laserinduced plasma, and it also valuable for the understanding the high-powered laser processing process.
Silicon is one of the most important semiconductor materials and the basic material in the field of modern microelectronics, and it has been widely used in microelectronics and photovoltaic industries which are closely related to our daily life. Because the traditional silicon wafer cutting technology has some serious problems such as insufficient cutting accuracy, low efficiency, and serious pollution, the laser processing has been paid more and more attention in silicon wafer cutting applications in about recent fifteen years. Therefore, it is extremely important to develop the laser silicon wafer cutting procedure for the improvement of the laser silicon wafer processing technology. An algorithm named as constrained interpolation profile has been invented in computational fluid dynamics. It is actually a semi-Lagrangian method to solve hyperbolic partial differential equations, and has the advantages of the stable results, compact process, and low dissipation, etc. Focused Gaussian laser beams with the same energy of 200 μJ and pulse widths of 100 fs, 20 ps, and 0.5 ns, respectively, were irradiated on the surface of a silicon wafer. The physical properties of density, temperature, and pressure in both time and space domains were obtained by means of the algorithm of constrained interpolation profile in the laser processing simulation. The mechanisms of laser silicon wafer processing were studied in detail by analyzing the changes in physical properties of silicon material. The conclusions of this paper might be useful in the optimization of a silicon wafer cutting process by the use of a pulsed laser.
Laser processing plays a key role in treating a lot of materials. The mechanism of laser stealth dicing (SD) is based on irradiation of a laser beam which is focused inside the brittle material. The laser beam scans along the predetermined path, so that the characteristics of the interior brittle material can be changed, the stress layer can be therefore formed. Finally, an external force is applied to separate the brittle material. Since only the limited interior region of a wafer is processed by the laser irradiation, the damages and debris contaminants can be avoided during the SD process. SD has the advantages of a high speed for thinner wafers without any chipping, the smooth section without dust and slag, and completely dry process, which has been widely used in large scale integrated circuits and microelectronic manufacturing systems. However, further studies on the simulation analyze and parameter optimization have kept to be rear for SD so far. In this study, an approach named as constrained interpolation profile (CIP) was adopted, which has the advantages of compactness, stability, and low dissipation in computational fluid dynamics compared with other simulation procedures. We have finished a theoretical simulation to obtain the physical features of the temperature, pressure, density of the silicon substrate at different focal depth where a nanosecond pulsed laser is irradiated, then we found a suitable focal depth with a good dicing quality by analyzing these physical features.
Laser drilling has been more and more widely used in laser machining process. Therefore, optimizing the quality of laser drilling becomes extremely important. We know that laser drilling can be achieved by using high power density of a laser. As light waves with different waveforms represent the different energy distributions in time domain, we believe that the quality of laser drilling should be related to the laser waveform. At present, a laser used in the laser processing usually hasthe waveform with a Gaussian or a Lorentzian distribution. In this study, we numerically simulated the punching quality of a pulsed laser with the Gaussian distribution and a pulsed laser with the top-flat distribution (we called it as a square-shaped laser pulse) at the same energy. It mainly refers to the changes of density, temperature, and pressure of the target materials under the same energy for different waveforms. The constrained interpolation profile algorithm has been used to simulate the machining process. Until now, there are few studies on the features of laser drilling with different waveforms in time domain. This paper provides a new method to optimize the quality of laser drilling.
Because of the low thermal conductivity of the mixture gases in an alkali vapor cell, the temperature of the pumping area of an alkali vapor cell can be extremely high than that of other area. Therefore, thermally-induced effects, such as, consumption of atomic alkali, degradation of output power, glass window contamination by the products of the optically chemical reaction between atomic alkali and buffer gases, etc. can be observed in high temperature heated diode pumped alkali lasers (DPALs) in the case of high power pumping. Generally, a flowing diode pumped alkali laser (FDPAL) system is thought to be a useful way to mitigate thermal effects in a DPAL system. In the paper, a mathematical model of a flowing diode pumped cesium laser (FDPCL) was constructed to systematically study the temperature distribution, the flow filed distribution, and the impacts of pressure of the buffer gases on output power of a FDPCL, etc. The laser kinetics, heat transfer, and computational fluid dynamics (CFD) are both taken into account at the same time during the simulation. The multi-physics coupling method was utilized to solve such three physics induced problem during the simulation. It has been demonstrated that the temperature distribution of a FDPCL system depends on the distribution of gas flow filed, a gas flow method can decrease thermal effects in a DPAL system, and the output power of a DPAL can be improved by increasing the velocity of gas flow filed.
Glass is one of the most important materials in industrial applications because of its high hardness, high thermal stability, and high transparency in the visible band. In general, it is very difficult to process glass with near-infrared, visible, and near-ultraviolet lasers. Physically speaking, the absorption coefficient of the glass sheet is one of the most crucial factors for processing efficiency, and it can be influenced by the temperature of a glass sheet. Therefore, to obtain the optimal processing efficiency, the influence of the temperature on the absorption coefficient should be studied in detail. In this paper, we theoretically and experimentally studied the relationship between the absorption coefficient and the temperature to improve the processing efficiency. A tunable near-ultraviolet Nd:YAG frequency-tripled harmonic laser with the wavelength ranging from 270 to 400 nm was utilized to measure the absorption coefficient, and a Peltier temperature controller was used to heat the glass sheet. It has been demonstrated that controlling the temperature is an efficient method to process the transparent glass sheet.
A serious rust phenomenon has been observed in an enclosed laser cavity. To figure out the reason which induces the rust, some experiments were carried out by recording the variation of the temperature and relative humidity at different positions. Thus, the vapor density can be numerically deduced by using the measured physical features. To avoid the undesirable rust phenomena occurring again, the exchange windows were chiseled out on an inner cover of the enclosed laser cavity in order to decrease the difference between the vapor density inside the cover and that outside the cover, which relates to the efficiency of dehumidification. The results validate that such a difference of the vapor density is a function of the area of exchange windows. Then, the curves of the vapor density versus the area of exchange windows have been plotted. It has been demonstrated that adding the area of exchange windows, which were pasted by some particulate air filters to prevent external dust particles from entering, on an inner cover might be a feasible method to avoid rust near the water cooling elements. Such a study might be useful for laser technicians to pay much more attention to the protection of undesirable vapor-induced rusting.
Laser processing plays a key role in treating a lot of materials. The visible nanosecond laser processing based on a tripartite-interaction system has been proved to be an effective method of processing materials with high optical transparency, which has the advantages of low cost, high efficiency, and simplicity over the direct processing by using a femtosecond laser. However, further studies on the theoretical mechanism and parameter optimization keep to be rear for the hybrid tripartite-interaction laser processing. In this study, we have carried out the confirmatory experiment and numerical simulation of laser processing with a tripartite-interaction system, which includes a visible nanosecond laser (19 ns@532 nm), a piece of transparent glass, and a copper foil. The experiment indicates that drilled holes can be obtained on the glass sheets by using the visible nanosecond laser. The numerical results, which have been obtained by an approach named as constrained interpolation profile, reveal that the processing mechanism is based on the heat conduction, generation of stress and ablation between the glass and the copper foil. Our results could to be useful for the development of visible nanosecond laser processing in industrial applications.
It has always been difficult to process a metal film with high reflectivity in the field of manufacture, industry, medicine, and military, etc. Since much of the laser energy can be reflected especially when the reflectance of the target film surface is high, it is hard to process such a metallic film by laser radiation as the energy absorbed by the film material is very little. In this paper, we used a nanosecond pulsed laser to scribe some patterns on a smooth titanium (Ti) film, and investigated the surface morphology of a Ti film ablated by different laser spot sizes and laser energy. In our experiments, it has been found that the Ti film can be efficiently processed although the surface reflectance of the Ti film is about 57% at the wavelength of 532 nm. We also see that the processing range of the Ti film will decrease when the diameter of a laser beam increases. The experimental results show that the ablated status of the surface of a Ti film for a just-focus beam is much better than that for a defocus beam under the same laser power. Furthermore, the higher the laser power, the larger the processed area. By using the optimal parameters we obtained, we also produced some hole matrices and line patterns on a glass-based Ti film by employing a short pulsed laser. The processed samples were observed with a reflecting microscope and a transmitting microscope, respectively. Our research results can play an important role in the selection of laser parameters for laser processing of some materials with a high reflectivity.
Terahertz wave is generally an electromagnetic wave at the wavelength of 0.1-10 THz (30-3000 μm). The terahertz laser is a new type of radiation source with many unique advantages and has broad applications. Generally, the size of a normal laser cavity is from a few of to several hundred millimeters, and the size of a micro-cavity is mainly from a few of to several hundred micrometers in the wavelength region of ultraviolet, visible, and near-infrared. However, if the wavelength increases to the terahertz region, the wavelength is of the order of the micro-cavity size. The power distributions inside and outside the cavity of a terahertz laser are significantly different from those for a conventional laser cavity. In this paper, a theoretical model is established to study the outputted and leaked power of a micro-cavity in the terahertz band. We assume that the wavelength of an emission terahertz source is 240 μm and simulate the output features of a micro-cavity laser with the Finite-Difference Time-Domain (FDTD) algorithm. The output characteristics of a micro-cavity have been analyzed by using two types of material and different thicknesses of the sidewall. It has been found that when the thicknesses of both silver and aluminum sidewalls are reduced to around 16 μm, the power leaking from the micro-cavity begins to increase with the decrease of the sidewall thickness. In this way, the sidewall no longer restrains terahertz radiation inside the cavity. The simulation results might be referred for the design of a terahertz laser with the micro-cavity.
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