Dissertations / Theses on the topic 'Through transmission laser welding'

Laser transmission welding is a method of joining plastics, which benefits from the infrared transparency in majority of thermoplastics. During the process, a laser beam passes through the laser transparent part and hits the laser absorbent part, which has been made absorbent using additives such as carbon black. The absorbed laser energy is then converted into heat and in turn welds the interface of the two parts by melting the polymer. In the current work, a new refraction technique of laser transmission welding is used to weld nylon plates to nylon tubes with carbon black. For the laser to refract, an angle was machined into the laser transparent nylon plates adjacent to the weld interface. Effect of different laser properties such as laser speed, number of laser rotations and laser power were studied on the quality of welding in terms of better finish and strength. The strength of these welds was assessed using a new tensile test fixture. Subsequently, the weld seam width was found from the tensile tested samples by analyzing the weld interface using vernier and transmission light microscopy. These tensile test results were then normalized using the weld seam area to obtain the tensile stress. The results showed the samples could withstand more tensile stress with an increase in laser power and rotation, excluding the samples which had decomposition due to excessive laser power. Also, the material property changes at the weld interface due to laser welding were characterized using a nanoindenter, for which small square samples were carved out of the weld interface and cold mounted. The results show that the modulus and hardness of nylon decreases right at the interface of the weld. In order to find the reason why there is a decline in the above mentioned mechanical properties, differential scanning calorimetry (DSC) testing was done to find possible changes in crystallinity, as decreasing modulus in semi-crystalline polymers is usually a result of decreasing crystallinity. The results confirmed that the crystallinity indeed decreased at the weld interface which would explain the decrease in mechanical properties.
Le soudage par transmission laser est une méthode pour joindre les plastiques qui prend avantage du fait que la majorité des thermoplastiques sont transparent aux infrarouges. Durant le procédé, un faisceau laser passe à travers une région transparente pour aller en frapper une autre rendue absorbante au moyen d'additifs tel que le noir de carbone. L'énergie laser absorbée est ensuite convertie en chaleur. Ce dégagement de chaleur permet de souder l'interface entre les deux parties en fondant le polymère. Dans le présent travail, une nouvelle technique de soudage par transmission laser basée sur la réfraction a été utilisée pour souder des plaques de nylon à des tubes de nylon contenant du noir de carbone. Afin de réfracter le laser, une surface en angle a été usinée à même les plaques de nylon transparentes, près de l'interface de soudage. Les effets de différentes propriétés du laser telles que la vitesse, le nombre de rotations et la puissance ont été évalués en se basant sur la qualité du soudage en termes du fini et de la résistance aux contraintes. La résistance des soudures a été déterminée à l'aide d'un nouvel accessoire de test en traction. Ensuite, la largeur de la soudure a été mesurée sur les échantillons testés en traction à l'aide d'un pied à coulisse et de la microscopie par transmission de lumière. Pour obtenir la contrainte en traction, les résultats des tests en traction ont été normalisés en les divisant par la surface réelle de leur soudure. Les résultats démontrent qu'une augmentation de la puissance et de la rotation du laser, jusqu'au seuil de dégradation, permet aux échantillons de soutenir davantage de contraintes en traction. De plus, les changements dans les propriétés du matériel dus à la soudure ont été caractérisés à l'aide d'un nano-indenteur, pour lequel de petits échantillons carrés ont été extraits de l'interface de soudure et montés à froid. Les résultats démontrent que le module et la dureté du nylon diminuent à l'interface de soudure. Puisque la diminution du module dans les polymères semi-cristallins est habituellement associée à une diminution de cristallinité, la calorimétrie différentielle à balayage a été utilisée afin de déceler les possibles changements dans la cristallinité. Les résultats confirment en effet que la cristallinité diminue à l'interface de soudure, et expliquent donc, par le fait même, la diminution des propriétés mécaniques.

Thesis (Ph. D.)--Ohio State University, 2005.
Title from first page of PDF file. Document formatted into pages; contains xxi, 221 p.; also includes graphics (some col.) Includes bibliographical references (p. 218-221).

The coupling efficiency of carbon nanotubes with near infrared laser radiation at 940nm wavelength was investigated. Nanotubes treated with different post processing methods were irradiated at different laser power intensities as dry samples and suspensions in water or ethanol. The interaction with the laser beam was measured and quantified based on the temperature increase in the samples as well as the amount of energy transmitted through them. Parallel experiments using carbon black revealed better performance of carbon nanotubes in terms of coupling efficiency and heat dissipation to their surroundings. It was found that most of the incident radiation on an individual carbon nanotube is absorbed, resulting in extreme local temperature increases proportional to the laser intensity, which can lead to instant tube oxidation in air. Such high heats are efficiently transferred to the material in immediate contact with the nanotubes, increasing its temperature very rapidly. The most intriguing results were obtained in the presence of water where the observations suggested, disintegration of carbon nanotubes with each laser pulse. It is shown that extremely high local temperatures vaporise the water in the immediate vicinity of a carbon nanotube and result in a water-gas reaction. It is further postulated that such effects can be achieved with laser beams at power intensities near the skin tissue's safe exposure thresholds, and therefore can potentially be used as a method of removing nanotubes from living tissue. This has advantages in providing an exit route for nanotubes whether introduced on purpose for reasons of medicine or therapy, or possibly, as a result of inadvertent exposure. Further studies on laser heating and transmission through different dry samples, highlighted that more crystalline structures such as that of a heat-treated nanotube, are more effective in causing extinction of the laser beam and a reduction in the transmitted beam intensity, however the tubes with more defects or with a length comparable to the radiation wavelength are very effective in converting the absorbed laser energy to heat. This effect is exacerbated when the laser beam is polarised parallel to the long axis of the carbon nanotubes. These heating effects were exploited to create welds in high density polyethylene using through transmission laser welding. The resultant welds showed better than or equal mechanical performance to welds made using industrial absorbers such as carbon black or Clearweld®.

Laser induced damage is of interest in studying the transmission of large amounts of optical energy through step-index, large core multimode fibers. Optical fibers often have to be routed around objects when laser light is being transmitted between two locations which require the fiber to bend into a curve. Depending on how tight the bend is, this can result in transmission losses or even catastrophic damage when the energy density of the laser pulse exceeds the damage threshold of silica glass. Waveguide theory predicts that light traveling through a bend will form whispering-gallery modes that propagate through total internal reflection bounces along the inside of the outer edge of the bend. This is critical since in these locations the energy density of the light will increase significantly, raising the potential of laser damage, nonlinear effects, and transmission losses. This loss is especially problematic when two 90[degree] bends going in opposite directions are in close proximity to each other, forming an 'S-bend'. Light that is grouped along the outer edge going through the first bend will enter the second bend at a sharper angle which causes much high transmission losses and raises the possibility of failure. Models using R-Soft BeamProp and Zemax were developed to study transmission losses, investigate light interactions at critical areas, and predict under which conditions laser damage would occur. BeamProp presents a clearer view of the modal distribution of light within the core of the fiber and is used to analyze how a plane wave with a Gaussian intensity distribution excites the fiber modes. Zemax provides a tool to perform non-sequential ray tracing through the fiber cable and stray light analysis within the core and once the light exits the fiber. Intensity distributions of the cross sectional area of the fiber shows the whispering gallery modes forming as the light propagates around bends and disburses as it propagates afterwards. It was discovered using R-Soft that if the separation distance between bends in an S-bend is approximately 3 mm there exists a condition where maximum transmission occurs. For 365 μm diameter core fiber it was calculated that the difference in output power could be as high as 150%. This was initially completely unexpected; however ray tracing using Zemax was able to verify that this distance allows the light to transition so that it enters the 2nd bend at the optimal angle to enter the whispering gallery mode. Experiments were performed that validated the models' predictions and images were captured clearly showing the spatial distribution shift of the light within the core of the fiber. Experiments were performed to verify light grouping together to form whispering gallery modes as predicted by Zemax. Microscope images were taken as a function of distance from various bends to observe the periodic nature in which the laser light fills up the fiber. Additionally, a configuration was setup to examine stimulated Brillioun scattering and determine the onset of laser damage in the fiber. Fibers were tested as a function of bend radius and number of shots and recommendations for future systems were made. Lastly, mechanical failure tests were performed to determine the relationship between stress placed on the fiber through bending and fiber lifetime in a static environment. This allowed a minimum safe bend radius to be calculated for a 30 year lifetime that agreed with previous calculated values.
M.S.
Masters
Optics and Photonics
Optics and Photonics
Optics

Aluminum alloys are important structural materials because of their high strength to weight ratio. Unfortunately, due to their high reflectivity and complexity in heat treatment, aluminum alloys are some of the hardest metals to be laser welded successfully and very high laser power is usually required. In this study, the feasibility of using a 300 W, Single-Mode, Ytterbium Fiber Optic Laser for aluminum welding is investigated. The objective is to explore an application area with low power and low welding speed. As the fiber laser offers much better beam quality (M2 less than 1.05), the results show that, with proper control of welding parameters, the success of aluminum welding can be achieved at considerably low laser power with minimal formation of typical welding defects (porosity, cracking etc.). However, the focusing becomes highly critical as exceeding a certain power density can lead to defects such as blowholes and porosity. The deepest penetration achieved was just over 1 mm at 300W and 2 mm/sec. Other welding processes achieve about three to four times as much penetration at the expense of seven times more power. Further development of this process can lead to a more efficient use of power.

Laser is an efficient and widely used heat source in metal processing suchas welding and additive manufacturing. It has some great advantages compared to the other conventional heat sources like electron beam and arc namely: ability of handling complicated joint geometries and producing large components. Laser beam welding encompasses many complex physical phenomena such asheat transfer, metal melting, flow and solidification, free surface deformation, evaporation and possibly vaporization. The aim of this research work istwo-fold: gain deeper process understanding and improve the model reliability. Deeper process understanding is sought on the effect of beam shaping on themelt pool. To achieve improved model reliability, a good support consists in using qualitative experimental data representing the process. Thus, 3D validation of the melt pool geometry is performed while it was usually 2D inprevious research works. Furthermore, a new calculation procedure for laser absorption is introduced. To conduct this research work, a Computational Fluid Dynamics approach is used. A solver, capable of tracking the deformation of the melt free surface, is developed in OpenFOAM. Concerning beam shaping, it is found that not only the melt pool size as previously known but also the melt flow pattern is modified through elongating the beam shape.This last result could not be revealed by former studies as the non-transparent media hinders optical observation. New in-process quantitative measurements performed by a project partner are used to test the model. Weaknesses of the former absorptivity models are highlighted, as well as the limitations of the proposed model. Finally, the results show that the proposed absorptivity model function of local surface conditions leads to much better agreement with experimental results compared to the former constant absorptivity model. The maximum discrepancy compared to the experimental measurement, which is observed for the melt pool depth, can indeed be reduced to about 10%.
Laser är en effektiv och allmänt använd värmekälla vid svetsning och additiv tillverkning. Den har några viktiga fördelar jämfört med andra konventionella värmekällor såsom elektronstråle och elektrisk ljusbåge, nämligen: den kan ofta användas till komplicerade svetsgeometrier, och den kan producera stora komponenter. Lasersvetsning involverar olika sammansatta fysikaliska fenomen såsom värmeöverföring, metallsmältning, flöde, stelning, ytdeformation, avdunstning och i vissa fall förångning. Syftet med mitt forskningsarbete är tvåfaldigt: att få en djupare processförståelse och att förbättra modellens tillförlitlighet. Fördjupad processförståelse eftersträvades för att förstå hur formen på laserstrålen påverkar svetssmältan. För att uppnå förbättrad modellsäkerhet behövs experimentella data av hög kvalitet som representerar processen. Således utfördes 3D-validering av smältgeometrin medan det vanligtvis var 2D i tidigare forskningsarbeten. Dessutom har en ny modell för laserabsorption föreslagits. I forskningen har numerisk strömningssimulering (Computational Fluid Dynamics) använts för att simulera processen och en numerisk lösare, som kan spåra deformationen av den rörliga smälta ytan, är utveckladi programvaran OpenFOAM. Beträffande laserstrålens utbredning visar resultaten att svetssmältans storlek och även svetssmältansflöde modifieras genom att laserstråleformen förlängs. Medan den förra är känd från tidigare experimentella studier upptäcktes den senare inte före denna studie eftersomdet icke-transparenta mediet hindrar optisk observation. Nya (in-process) kvantitativa mätningar utförda av en projektpartner har använts för att testa modellerna. Svagheter i den tidigare absorptionsmodellen framhävdes, liksom begränsningarna i den föreslagna modellen. Slutligen visade resultaten att den föreslagna modellen där laserabsorptionen är en funktion av lokala ytförhållanden ledde till en bättre overensstämmelse med mätningar jämfört med den tidigare modellen med konstant laserabsorbtion. Den maximala avvikelsen jämfört med experimentell mätning, som observerades med avseende på smältbassängsdjupet, kunde reduceras till cirka 10%.

Till licentiatuppsats hör 2 inskickade artiklar, som inte visas nu.

Nowadays, components made of metallic materials are increasingly being replaced by components made of plastics or composites with a polymer matrix. This is associated with the issue of production processes such as pressing or welding, i.e. the influence of process parameters on the output properties of the product. The presented thesis deals with the issue of the combination of pressing and welding of a composite part, specifically the influence of the pressing overlap on the strength and tightness of the welded joint. The first part is focused on a search of available literature related to the problem. The second part deals with solving the problem using experimental modeling. Part of this chapter is inclusion of computational modeling in the design of experiment, detailed measurement of essential dimensions, microtome analysis and statistical processing and evaluation. The third part focuses on the creation of a method for evaluating the strength of the weld based on the pressing overlap using computational modeling. Essential part is also validation of the computational model based on previous experimental measurements. Finally, two methods for evaluation of the weld strength are presented. The first works on the basis of computational modeling and the second on the basis of experimental modeling. At the same time, the presumptions of usage of the created methods and their drawbacks are pointed out. Furthermore, the possibilities of their implementation in the initial design of the welded joint and the proposal for the next procedure are described.

In laser Transmission Welding (LTW), the laser beam passes through the transparent part and is dissipated as heat in the absorbent material through the use of laser-absorbing pigments such as carbon black (CB). This energy is then conducted further into both parts. Melting and subsequent solidification occur at the interface causing a weld to form between the two parts. Gluing or welding structures to the back of automotive Class-A panels often results in the appearance of undesirable surface deformations on the Class-A side. Through control of the laser welding and material parameters, it may be possible to use contour LTW as a means of joining structures to the back of absorbent Class-A panels without creating these unwanted surface defects. A series of lap welds was made using a range of CB levels, laser powers and polypropylene part thicknesses. A profilometer was used to measure the size and shape of the defects generated on the surface of the black part. Two types of defects were observed: ribs and sink marks. It was observed that lower powers combined with higher carbon black levels generally resulted in smaller defects. The type of defect depended on the boundary conditions between the two parts and the flow of polymer that had thermally expanded during welding (flash). If weld flash flowed into gaps between the two plates, rib defects were always observed. If flash flowed elsewhere and no gaps existed between the plates, sink marks occurred. Finite element modeling was used to qualitatively validate these observations.
Thesis (Master, Chemical Engineering) -- Queen's University, 2010-07-02 14:34:41.201

Contour laser transmission welding (LTW) is a technology that has potential for joining large and complicated thermoplastic parts. Thermal expansion is the primary driving force to bridge potential gaps at the weld. A comprehensive investigation into gap bridging was performed using experimental studies, finite element (FE) thermal-mechanical coupled modeling, and analytical analysis of the contour welding process for polycarbonate (PC), polyamide 6 (PA6) , and glass fibres reinforced polyamide 6 (PA6GF). The effects of material properties (carbon black level, glass fibres and crystallinity), process parameters (laser scan power, scan speed) and weld gap thickness on weld shear strength were assessed. The experimental study indicated that low concentration of laser absorbing pigment accompanied with high power laser scan improves gap bridging. Damage on the top surface of the laser-transparent part limited the allowable laser power that could be delivered onto the weld interface. Maximum gaps of 0.2, 0.4 and 0.25 mm were bridged in the experiment for the three types of polymers respectively. The thermal behavior of polymers in contour LTW was analyzed by the 3-D quasi-static thermal FE models. Thermal expansion into the gap was simulated by the simplified 2-D transient, thermal-mechanical coupled FE models. An analytical model describing laser beam transmission and absorption in light-scattering polymers was developed and applied in the FE simulation for PA6 and PA6GF. FE simulated results agree well with the experiment in contour welding with gap of PC and PA6. The optimum material and process parameters have been searched in the model to maximize gap bridging for PC. An analytical model has been developed to predict the temperature rise and the thermal expansion in high speed contour welding of amorphous polymers. The model indicates that the maximum temperature at weld increases linearly with the laser line energy and the laser absorption coefficient. Thermal expansion and hence gap bridging increases with laser line energy. Lower laser absorption coefficient allows higher laser scan energy to be delivered onto the weld interface so helps bridge larger gap. The predicted thermal expansions by the model agree well with the measured maximum gaps bridged for polycarbonate.
Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2009-09-24 22:24:11.734

Laser Transmission Welding (LTW) involves localized heating at the interface of two pieces of plastic (a laser transparent plastic and laser absorbing plastic) to be joined. It produces strong, hermetically sealed welds with minimal thermal and mechanical stress, no particulates and very little flash. An ideal transparent polymer for LTW must have: a low laser absorbance to avoid energy loss, a low level of laser scattering so it can provide a maximum energy flux at the weld interface and also have a high resistance to thermal degradation. The objective of the project was to analyze the effect of blend ratios of polybutylene terephthalate and polyethylene terephthalate (PBT/PET) on these laser welding characteristics. The blends were manufactured by DSM (Netherlands). They were characterized using Differential Scanning Calorimetry (DSC) and Thermal Gravimetry Analysis (TGA). The latter technique was used to estimate the order (n), activation energy (ΔH) and frequency factor (A’) of the degradation reaction of the polymer blends. The normalized power profile of the laser after passing through the transparent polymer was measured using a novel non-contact technique and modeled using a semi-empirical model developed by Dr.Chen. Adding more PET ratio to the blend, did not change beam profile of the transmitted beam significantly. Laser welding experiments were conducted in which joints were made while varying laser power and scanning speed. Measuring the weld strength and width showed that the blends containing PET have higher strength in comparison to pure PBT. The temperature-time profile at the interface during welding was predicted using a commercial FEM code. This information was combined with the degradation rate data to estimate the relative amount of degraded material at the weld interface. It showed that increasing the ratio of PET in the blend makes it more resistant against thermal degradation which can be one of the reasons the PET containing blends reach higher weld strengths when compared to pure PBT.
Thesis (Master, Chemical Engineering) -- Queen's University, 2010-08-31 10:03:42.167

In laser transmission welding (LTW), a laser beam passes through the laser-transparent part and is absorbed by carbon black (CB) in the laser-absorbent part. This causes a temperature rise at the interface between the parts which leads to melting, diffusion and ultimately joining of the two components. Weld temperatures increase with laser power at a given scan speed. However at higher temperatures, it has been observed that weld strength of LTW starts to decline due to material thermal degradation. Thermal degradation of materials is a kinetic phenomenon which depends on both temperature and time. Therefore there is no specific temperature for thermal degradation. Thermal gravimetric analysis (TGA) is used to study the thermal degradation of two commonly used thermoplastic materials: polycarbonate (PC) and polyamide 6 (PA6). Each material was studied at two levels of CB. It is shown in this work that increasing the carbon black (CB) level from 0.05 to 0.2wt% has no significant effect on the thermal stability of PA6. However, it is observed that increasing the CB level from 0.05 to 0.2wt% has a noticeable effect on the thermal stability of PC. The TGA data were then used to obtain the kinetic triplets (frequency factor (k_0), activation energy (E), and reaction model (f(α))) of the materials using a non-linear model-fitting method. These kinetic triplets were combined with temperature-time data obtained from a Finite Element Method (FEM) simulation of the LTW process to predict material degradation during LTW. The predicted degradation was then compared with experimental data. It is found that the predicted onset of material degradation is in good agreement with experimentally observed thermal degradation (of both visually observed degradation onset and weld strength decline) for PC and PA6. A semi-empirical model based on the FEM temperature data is also developed in this work as a simpler alternative for obtaining LTW maximum temperature-time profiles for prediction of material thermal degradation during LTW. Comparison of the predicted material conversion using temperature-time profile obtained by FEM and the semi-empirical model shows good agreement.
Thesis (Master, Chemical Engineering) -- Queen's University, 2013-09-27 10:45:24.688

This thesis presents a comprehensive study on the thermal modelling aspects of laser transmission welding of thermoplastics (LTW), a technology for joining of plastic parts. In the LTW technique, a laser beam passes through the laser-transmitting part and is absorbed within a thin layer in the laser-absorbing part. The heat generated at the interface of the two parts melts a thin layer of the plastic and, with applying appropriate clamping pressure, joining occurs. Transient thermal models for the LTW process were developed and solved by the finite element method (FEM). Input to the models included temperature-dependent thermo-physical properties that were adopted from well-known sources, material suppliers, or obtained by conducting experiments. In addition, experimental and theoretical studies were conducted to estimate the optical properties of the materials such as the absorption coefficient of the laser-absorbing part and light scattering by the laser-transmitting part. Lap-joint geometry was modelled for semi crystalline (polyamide - PA6) and amorphous (polycarbonate - PC) materials. The thermal models addressed the heating and cooling stages in a laser welding process with a stationary and moving laser beam. An automated ANSYS® script and MATLAB® codes made it possible to input a three-dimensional (3D), time-varying volumetric heat-generation term to model the absorption of a moving diode-laser beam. The result was a 3D time-transient, model of the laser transmission welding process implemented in the ANSYS® FEM environment. In the thermal imaging experiments, a stationary or moving laser beam was located in the proximity of the side surface of the two parts being joined in a lap-joint configuration. The side surface was then observed by the thermal imaging camera. For the case of the stationary beam, the laser was activated for 10 s while operating at a low power setting. For the case of the moving beam, the beam was translated parallel to the surface observed by the camera. The temperature distribution of a lap joint geometry exposed to a stationary and moving diode-laser beam, obtained from 3D thermal modelling was then compared with the thermal imaging observations. The predicted temperature distribution on the surface of the laser-absorbing part observed by the thermal camera agreed within 3C with that of the experimental results. Predicted temperatures on the laser-transmitting part surface were generally higher by 15C to 20C. This was attributed to absorption coefficient being set too high in the model for this part. Thermal imaging of the soot-coated laser-transmitting part surface indicated that significantly more scattering and less absorption takes place in this part than originally assumed. For the moving laser beam, good model match with the experiments (peak temperatures predicted within 1C) was obtained for some of the process conditions modelled for PA6 parts. In addition, a novel methodology was developed to extract the scattered laser beam power distribution from the thermal imaging observations of the moving laser beam.
Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2008-10-08 10:39:30.952

Paper is a complex fibrous material whose production involves substantial amounts of natural and industrial resources. To reduce its manufacturing costs, the pulp and paper industry often employs optical technology such as high sensitivity laser sensors used to measure physical parameters like thickness and opacity. More recently, computer simulations of paper optical properties are also being used to accelerate the research cycle required to the development of new types of paper. In these simulations, the bulk scattering of paper is usually approximated by analytical formulas, notably the Henyey-Greenstein function. In this work, we qualitatively investigate the degree of accuracy of such approximations with respect to collimated light. More specifically, an experimental set-up was devised to record the transmission of red and green HeNe lasers through different paper samples. The measured data was compared with data obtained using the Henyey-Greenstein function and data obtained using an alternative exponentiated cosine function. The comparisons are used to qualitatively assess the degree of accuracy of the bulk scattering approximations provided by both functions. This work closes with a discussion on the practical implications of our findings for the modeling of paper optical properties.

碩士
中原大學
應用物理研究所
94
For the purposes of mass information storage and optical image processing, photorefraction is a useful property of optical material. Among many materials with this property, Barium Titanate crystal has been well studied and widely applied in optical computing, hologram storage, etc., so it is important to know the optical behavior of Barium Titanate. Of particular interest is the response of the crystal to incident laser light. We used polarized Argon ion laser as the probe to investigate the mechanism of Barium Titanate’s response to the incident laser light. Several interesting phenomena were discovered. First, the transmitted Argon laser light intensity oscillated along with time after passing through a 45∘cut-BaTiO3 and an analyzer. The amplitude of oscillation also varied with time. Second, contrary to theoretical expectation, when the polarization is along the ordinary index direction and extra-ordinary direction (i.e., the eigen-direction of the light propagation), we found the transmitted light still oscillated. Third, when a He-Ne laser probe beam passes through the pumping (heating) argon-laser beam spot, the He-Ne laser light intensity oscillates alternatively between a high peak and a low peak, in contrast with the opposite case where the He-Ne laser beam doesn’t go through the heating spot, the intensity oscillates without alternating hi-lo peaks. Fourth, after removing the analyzer, the transmitted light intensity raises to a peak then drops rapidly with a dip and then gradually recovers to a level intensity. This observation is called “spike phenomenon” in this thesis. In order to understand how the dip takes place, we studied the influence of beam fanning, scattering ring, and other scattering processes using an integration sphere to integrate all the non-transmitted light. In the next paragraph, possible explanations for the above observations are offered. The oscillation of transmitted light intensity can be attributed to the temperature dependence of extra-ordinary refractive index. This useful property can be used to study the heat conduction rate of Barium Titanate. Actual results show that the envelope of oscillation was also varying with time. This variation can be attributed to the temperature dependence of the absorption coefficients, as confirmed by our simulation results. For the oscillation for the light when polarization is along the eigen-direction, we suggest that it may be linked to the induced optical activity of BaTiO3. Also the differential absorption coefficients of ordinary refractive index and extra-ordinary index may change the light polarization to an elliptical one and hence offer a possible explanation to the oscillation of alternating hi-lo peaks for the probing He-Ne laser beam. As far as the “spike phenomenon”, the first possible explanation is absorption and re-absorption of deep and shallow impurity levels. Additional mechanisms such as beam fanning, the scattering effect or a combination of the above is also a possible cause of the “spike phenomena”. This assertion is based on the experiment using integrating sphere to integrate all the non-transmitted light, which shows that the sum of fanning and scattering intensity can just make up the lost of the dip. Thus, the “Spike Phenomena” is most likely caused by the combination of absorption, fanning effect and scattering effect.

碩士
國立中山大學
機械與機電工程學系研究所
94
A novel lensed plastic optical fiber (LPOF) scheme to achieve the high coupling efficiency with a long working distance between the light source and LPOF is proposed. The advantages of the proposed LPOF are demonstrated by proofs of the experiment. In this study, an aspheric convex-concave plastic lens (CCPLs) is bonded with a flattened end of the plastic fiber by using the laser transmission welding (LTW) to form an aspheric-endface fiber. The working distance between the light source and LPOF can be increased with high coupling efficiency by the design of the CCPLs. According to the proposed design in this study, the working distance and the coupling efficiency can reach to 300μm and 80%, separately. Furthermore, the analysis shows that the LTW can achieve a high welding strength and a small heat affected zone that meets the commercial utilization. But the LTW technology has some restrictions, the disadvantages of the LTW technology are improved in this study to spread the application of the laser welding.