Manufacturing Modular Elements for Laser Pumping Cavities ABSTRACT C. Delavet 1, O. Dontu 2, D. Besnea 2, I. Avarvarei 2, R. Ciobanu 2 1 Ecole Nationale Supérieure d Arts et Métiers d Aix en Provence, France 2 Politehnica University Buchares, Faculty of mechanical engeeniring and mechatronics, Splaiul Independentei nr 313, sector 6, Bucuresti, Romania E-mail: octavdontu@yahoo.com This paper present a modality to manufacture using CNC machines a modular optical pumping cavity used in laser installations with solid state gain medium. Keywords: manufacture, CNC machines, optical pumping cavity, solid state, gain. INTRODUCTION It is a known fact that solid state lasers have a modest efficiency in converting electric energy in beam energy[1,2]. In order to realize a high efficiency optical pumping of the laser gain medium, the optical pumping lamp and the gain medium must be coupled in a optical pumping cavity, which forms an optical enclosed system that concentrates optical energy from the lamp in the bar of laser gain medium. The configuration of such a pumping cavity is determined by the type of the lamp and the type of the laser gain medium, by the relative position of those two elements and by the condition imposed by working parameters, energy transfer efficiency and the performances that the laser need to accomplish. Optical pumping cavity must accomplish beside a good coupling between the laser bar and the lamp, a good distribution of pumping energy along the laser rod, this requirement having the most importance in the uniformity, divergence, optical distortion and the level of energy that the laser emits. An optical pumping cavity must assure, at the same time, a good cooling for the active medium and pumping lamp, as the cooling conditions have an influence over the emission process, affecting the pulse repetition rate, as well as the peak power of the pulse and the life time of the laser. In an optical pumping cavity light energy is concentrated on the active medium rod far from the ideal conditions. This, correlated with a low efficiency in converting electricity in optical energy, and taking into account the incomplete usage of the emitted light by the laser rod (6-14%) and the losses inside the pumping cavity (30-70%) these are the elements that determine the low efficiency of laser generators with solid gain medium. (0.1-5%). [1] Because every laser rod and optical pumping cavity has it s own particularities, energy distribution may be determined by using diagrams or with some approximations. [] The efficiency of electrical energy transfer to the laser rod in a optical pumping cavity may be approximated using the following relation : η=η ge.η op.η e (1) η ge geometrical transfer efficiency of the pumping cavity which represents the fraction of light that gets to the laser rod directly or after it was reflected in the walls of the cavity and it depends largely by the geometrical configuration of the optical pumping cavity s active surfaces; η e Energy efficiency which depends on the conversion mode of electrical energy into light energy, used for optical pumping of the laser gain medium; η op optical efficiency of the pumping cavity which takes into account all the losses in the pumping system caused especially by the transmittance and reflection phenomenon. This parameter might be expressed as follows: η op =Rw(1-Rb)(1-Rl)(1-S) (2) where: 233
Rw coefficient indicating the reflection degree of the walls for frequency bands in which the optical pumping is performed Rb represents reflection loses on the surface of the laser rod or on the glass tubes used for cooling the laser rod, Rl represents the loses by absorption in the tubes used for cooling the lamp S represents the ratio of unreflective surface of the pumping cavity and all the surface of the pumping cavity, factor that evaluates the loses caused by the constructive and technologic particularities of the pumping cavity. Researches have shown that a high efficiency of the pumping is achieved in cylindrical optical pumping cavities with elliptical cross section (Fig.1.) in which the pumping lamp is placed in one focus of the ellipse and the laser rod is placed in the other focus, thus an uniform distribution of energy is obtained across the cross section of the rod. The ends of the pumping cavity are enclosed by high reflective plane surfaces. Fig. 1. Elliptical optical pumping cavity In this type of pumping cavity, due to the elliptical cross section, all the rays that go from a point of the lamp are converted in parallel rays in the laser rod, every line corresponding to the rays that leave the source are tilted under the same angle from the main axis and at different angles from the transversal plane, important is not just the image forming in the common optical sense but the radiant energy transfer to the laser rod. TECHNICAL PROBLEMS Manufacturing those pumping cavities is relatively difficult but as well as expansive, not because of the geometrical form as the problems issued by the realization of the reflective coating inside the ellipse. Fig. 2. Pumping cavity of a Nd:YAG laser in classical construction The process suitable for obtaining this high reflective surface is by reflective material deposition after thermal evaporation and condensing metal vapors in vacuum. This process implies material 234
evaporation by electron beam bombardment or using a high power laser inside a vacuum chamber and condensing the metal vapors on the active surface of the pumping cavity, the result is a high reflective surface with homogenous covering and a good resistance in time in variable temperature conditions. Nevertheless, metal covering by these process are best performed on small surfaces. Experimental researches and rapports of constructive companies and service companies show that the most frequent defects that appears on pumping cavities are caused by exfoliation, degrading, spots on certain areas or scratches. Solving these defects is performed by replacing the whole pumping cavity, operation that requires time, it is costly and laser installation can t be use for a prolonged period of time. To solve this problem we conceived a modular pumping cavity. This solution presents a series of economic advantages: reduced production costs, reduced immobilization period of the installation. There are also technological advantages as the manufacturing a smaller module is easier rather than manufacturing a whole pumping cavity. The biggest advantage of them all is that when a surface defect is spotted in a certain area only the defect module is replaced not the all pumping cavity.[1,2,3,4]. PROPOSED SOLUTION To solve this problem we conceived a modular system, in which the pumping cavity is formed from a number of modules with elliptical inner cross section. These modules have orifices and centering faces in order to assemble and align them easy and precisely. Modules have small sizes which allows them to have the inner elliptical groove carved easily and also the reflective coating is easier to obtain. More over, when a defect is spotted such as scratches, exfoliations, spots, a single module is replaced. Thus the maintenance costs are reduced, the pumping cavity is not replaced and the time when the laser installation is unused drops. Starting from a classical elliptical pumping cavity we designed a modular pumping cavity with the same geometrical parameters. The modules have a height of 50mm and an outer diameter of 42mm. Fig. 3. Module of pumping cavity Modules are designed with a sitting face on the optical bench of the laser installation and with two orifices used to place and fix the modules using fixture studs. To allow the modules to be aligned a centering rim was designed on the outline of each end face. MANUFACTURING PROCESS Fig. 4 detail in section on the fitting mode of the modules After designing, in order to see if the module is feasible we used CATIA V5 designing program to simulate the machining process necessary to manufacture this module. CATIA offers a strong NC Manufacturing module (fig.5) to define and organize numeric command programs dedicated to manufacture parts with geometry represented in 3D models of wire-frame type. 235
Also, the manufacturing module has a post-processing engine which allows to cover all the fabrication process, from trajectory generation of the processing tool to obtaining the NC Code. The fabrication process for the pumping cavity module that includes following processing steps: facing, inner milling, boring, outline milling, centering, drilling,. In order to simulate the fabrication process Part Operation must be defined and the working parameters(specific to the type of the used machine), as well as adjacent parameters such as Post Processor words table, NC data type, NC data format, Tools catalog, Radius compensation, Spindle. Manufacturing program module offers the possibility to define technological operations on different types of surfaces and according to the type of machining (roughing and finishing). From the main menu, Facing command is activated and by selecting the machining surface, the machining mode may be selected (inward helical, Back and forth and one way) as well as the axial number of levels. Choosing the tool for Facing operation may be done by selecting the icon that allows defining the geometrical dimension for the tool. [5,6,7,8,9]. Fig 5.a Defining the geometrical parameters of the operation Fig 5.b. Defining the tool Fig. 5.c defining and simulating the tool trajectory 236
Inner maching, roughing and finishing, of the pumping module is perofrmed by activating the Zlevel comand, (fig. 8), that provides the settings for vertical machining by using two types of tools defined by tool change operation. Selecting the part s surface that that has to be machined the tool trajectory is defined and may be simulated. Fig 6.a Selecting active surfaces that need to be machined Fig. 6.b Roughing simulation Fig. 6.c Roughing operation Drilling the fixing holes is defined by drilling command, first is used a spot drilling operation and, after defining the hole geometry, the drilling tool is selected and the opereation is completely defined. Fig. 7.a Drilling simulation 237
Fig. 7.b. Drilling operation Fig. 8.a Finishing simulation Fig.8.b Finishing operation Programm gneration in machine gode is performed activating Generate NC code command, by opening the active window from fig.10. In the IN/OUT page, Document CAT process is selected and the path of the resulting file is specified. NC Code option from NC Data Type menu(nc Code, APT, CLF) is selected and in the page the type of the NC Code processor is selected acordingly to the one installed on the CNC machine. Fig. 9. NC Code generation An example of NC Code program specific to FANUC comand system is shown below: %:4000 (cavitate) M6T5 G54G90 S5000 M3 G1G43 H#4120 Z100.F3000 X94.8775 Y-239.8275 238
Z9.8235 G1 Z-.1765 F2000. Y-240.8675 F400....... X143.8953 Y-6.0422 X143.9275 Y-6.12 Z6. F1000. M5 G0 Z200. M30 % RESULTS After machining the designed modules we obtained a set of four similar pumping cavity modules. The modules were assembled to form the pumping cavity, thus verifying the viability of our solution. Assembling the modules proved to be easy, the modules fitting together precisely due to aligning rim that was generously dimensioned to a depth of 3mm, more over, when tightening the fixing studs, the modules haven t move from their axis CONCLUSIONS Fig. 10. Assembling mode for the modules in order to form a pumping cavity. The advantages of using modules to form pumping cavities are both economical and technological. Because processing inner surfaces to make them high reflective is possible only by thermal evaporation and vacuum condensation (and not by electrolytic deposition, the surfaces obtained in this process may exfoliate within hours of functioning) the variant of using modules simplifies and keeps the fabrication costs low and more over, for users is easier and cheaper to change a small deteriorated module rather than the whole pumping cavity. REFERENCES [1] O. Dontu, Tehnologii de prelucrare cu laser, Editura Tehnica, Bucuresti, 1985 [2] I. Popescu, Fizica si ingineria laserelor, Editura Tehnica, Bucuresti, 2000 [3] W. Steen, Laser material processing, Springer, London,1998 [4] E. Kannatey-Asibu jr., Principles of Laser Materials Processing, Wiley, New Jersey, 2009 [5] G. Ghionea, Proietarea asistata in CATIA V5, elemente teoretice si aplicatii, Editura Bren, Bucuresti, 2007 [6] 6.G. Ghionea, CATIA V5 Module de proiectare asistata cu aplicatii in constructia de masini, Editura Bren, Bucuresti, 2004 [7] D. Popescu, Indrumar CAD CATIA V5R8, Editura Aries, Craiova, 2004 [8] G. Charalampides, Gh. Gheorghe, Design and execution of mechatronics parts by using CNC centers, Romanian Review Precision Mechanics Optics&Mechatronics, nr33/2008 [9] D.Besnea, O.Dontu, s.a. Tehnologii de fabricatie asistate de calculator pentru executia unor componente mecatronice, Editura PRINTECH, Bucuresti, 2008. 239