Thesis (B.S.) in Chemistry--University of Illinois at Urbana-Champaign, 1990.Includes bibliographical references (leaves 11-12)Microfiche of typescript. [Urbana, Ill.]: Photographic Services, University of Illinois, U of I Library, [1990]. 1 microfiche (20 frames): negative.s 1990 ilu n 

LaBuda, Mitchell J.

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The effect of annealing on the properties of optical waveguides

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  Previous issue date: 1990Thesis (B.S.) in Chemistry--University of Illinois at Urbana-Champaign, 1990.Includes bibliographical references (leaves 11-12)Microfiche of typescript. [Urbana, Ill.]: Photographic Services, University of Illinois, U of I Library, [1990]. 1 microfiche (20 frames): negative.s 1990 ilu n 

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UNIVERSITY OF ILLIN O ISMay 3 ^THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BYMitchell J. LaBudaDEGREE OF.....Approved:. ....S/fL  l-'t *, » I - * *Initructor in ChargeHEAD OF DEPARTMENT OF.OU64The Effect o f Annealing on the Properties of Optical WaveguidesbyMitchell J. taBudaThesisfor theDegree of Bachelor of Science inChemistryColiege of Liberal Arts and Sciences University o f Illinois Urbana, Illinois1990AbstractThe effect o f annealing on the optical waveguide characteristic* o f  glass coverstlps was studied by investigating the change in Rayleigh scatter intensity between annealed and unannealed samples. It was determined that the annealing process improved the overall quality o f  the waveguide by reducing the variation o f the refrr.ctive index along the length o f sam ples.Abstract iiList o f Figures ivList ofTables VTheory 1Experimental 3Results and Discussion 5Conclusion 10Literature Cited IIBibliography 12List of Figuresi vFigure 1. Typical Data Analysis Plots.P a g e6Figure 2. Comparison Photographs. 9U»t of TablesP ag eTable 1. Summary o f  Annealing Conditions Used and Results Obtained.7Table 2. Study o f the Homogeneity o f a Typical Sample.7The purpose of (his experiment was to study how the optical properties o f glass coverstips were affected by annealing. A change in the amount of Rayleigh scatter along the length of the coverslips was chosen as a means of investigating this process.The reduction of intensity of a light beam as it propagates through n absorbing medium 1 of thickness dx can be written as:d /*  «a[J]/dx, o rd  In/* -a |J) dx ( I)where /  is intensity, (J) is the concentration of J, and a  is the proportionality coefficient. To obtain the intensity that emerges from a sample of thickness x when the incident intensity is /; one can sum up all of the changes in thickness:r xf d  I n / *  - f a ( J 1 d x  (2)i 0If the sample is assumed to be homogeneous, then the above equation integrates to the Beer-Lambert Law:I f  * /je aUI* O)It can be seen that intensity will decrease exponentially with the sample thickness. A more useful form of the Beer-Lambert Law is:in (/f//j) * -a[J]x (4)A plot o f In I f  vs. x with give a linear plot with a slope * -a(J], which is called the molar absorption coefficient, and the y-intercept will equal the initial intensity (I). These relationships can be applied to glass media for they are fairly homogeneous in nature.The behavior of a light wave as it propagates through the interface between two media with different refractive indices can be described by Snell's Law:nisinO) * n2$in02 (5)The angle o f light propagating inside an optical waveguide can be determined by using Snell's Law after knowing the angle with which the light enters the guide, and its refractive index (2).The process of annealing glass is an important industrial practice, l his process involves a heating followed by a slow cooling of the glass. There are many reasons why glass is annealed. An important one concerning optical samples is the reduction of the spatial variation in the refractive index (3 \ This will create a more homogeneous sample, and thus a better waveguide. Since light tends to scatter as it encounters a change in refractive index, a sample that has been annealed should show less scatter along its length than a sample that is unannealed.ExperimentalReagents. Com ing glass coverslips were used as (he waveguldlng medium. These ore mode o f  a BK7 type glass, and am approximately 150 microns thick.Sam ple Preparation. Before annealing, the sam ples were cut in half using a diamond tipped cutter. The cut was matte sn that each sample was o f  approximately the sam e length. One o f  the covcrsdp halves was annealed in a Thermolye M odel 21100 tube ftim ace, the other was left unannealed as a control. A constant flow o f  nitrogen gas was passed through the furnace chamber to create an inert atmosphere during the annealing process. Different conditions for annealing were created by varying the temperature and length o f  time In the furnace. After removal from the furnace, all samples were allowed to e o o ito  room temperature before storage. A ll o f  the sam ples were annealed while standing on edge in the sam ple tray except for two sets o f  three. These six were told flat on top o f(he sample trey. Upon their removal from the tube fttm ace, it was noted tint the edges o f  each sample had bowed up. Apparently, som e stress had been introduced into the samplcs doe to uneven heating. Therefore, these six  were not analyzed. After being annealed, allSifciCidtiM' tht II rffapli /tufeiftiilf liitltt iiiiiajftfirtA ta iy iia l- la th  sample was mounted and exposed to a 632.8 nm light beam from  a M eilcs-Griot helium-neon laser. The laser was rated to a maximum output o f  10 m\V, buta typical beam intensity for this experiment was approximately 4 mW. Before reaching the sam ple, die beam was passed through a GQO focusing tens with a 500 mm focal length. The sample holder Itself was mounted on three stages: X and Y transitional and rotational. This allow ed the experiment to be repeated easily for all the samples. Each sample was mounted with the incoupling edge at a distance o f  approximately I mm from the inner edge o f  the sam ple holder. An optical fiber, with a total exterior diam eter o f  6 mm, wasalong the distance or the sample from 4 mm away from the incoupling edge, to the exit coupling point. The physical limitations of the sample holder's design prevented the movement of the fiber any closer to the incoupling point. Light emitted form the other end of the fiber was passed directly into a Pacific Model 3150 photomultiplier tube. The tube was powered with 500 V from a Fluke Model 409A power supply, The signal detected was passed through a current-to-voltage converter with a sensitivity setting o f 5 V output per 1 m A input. The amplified output went to a Fluke Model 80I0A digital multimeter and the intensity of the scattered light was read and recorded in volts.Each coverslip was mounted at the same point, so the beam entry location was at approximately the same height. Data was recorded starting at 4 mm from the incouplingResults and DiscussionThe angle of propagation of light inside the waveguide can be determined by using equation (5) from above, and simple geometry. Given that nair * 1.000, t\co vers) ip® 1.515 for) -  632,8 nm, and 0ajr * 23.0°, then Qjn * 14.9°. Thus the angle of internal reflection inside the guide * Oguide ** 90° - 0jn * 75.1°. This is the angle of propagation of the beam as it travels through the guide. This geometry was held constant for all analyses.Plots of In /  vs. x were made for each data set. Figure I contains typical plots fora sample that was annealed at 400° C for 10 minutes. The unannealed sample is on top, while the annealed is on the bottom. Linear linefits were used to determine the slopes for each graph, and the resultant equations are included on each. Note the change in the »pe between the two samples. The annealed sample shows a 24% decrease in slope when compared to its unannealed half.A  summary o f  results o f  all o f  the samples is given below in Table I. The first column describes the conditions under which the samples were annealed. This includes the temperature and length o f  heating. Three samples were prepared for each condition, and the mean change in slope between the unannealed and annealed samples is given in column two. This was calculated by taking the slope o f  the annealed sample and subtracting o ff the slope o f  the corresponding unannealed control sam ple. The average o f  the three measurements is reported along with the standard deviation.It can be seen that for every annealing that took place above 400° C, a negative change in slope occured, indicating an improvement in the waveguiding characteristics o f  the sam ples. A lso, as temperature increases, there is a decreasing trend in the standard deviation. The sam ples annealed at 450& C, taken as a group, show the low est standard deviation o f all.to(D In (I)6Figure 1, Typical plots constructed for each sample. A linear lineflt was used in each case.Above graph reprsents an unanneal rd, control sample.The graph below is of a sample annealed for 10 minutes at 400 C.7Tabic I. Summary o f Annealing Conditions Used and Results Obtained.Annealing Conditions Jdean Change in Slopdmnr Standard Deviationlmm1)450° C for 10 minutes •0.032 0.039450° C for 20 minutes -0.005 0.021450° C for 30 minutes -0.014 0.028400° C for 10 minutes -0.026 0.059400° C for 20 minutes •0.036 0.073400s C for 30 minutes -0.012 0.034350° C for 10 minutes 0.089 0.094300° C for 10 minutes 0.025 0.028250° C for 10 minutes 0.022 0.079200° C for 10 minutes 0.003 0.044A study was also done to see if the samples were fairly homogeneous across their width. A sample that was annealed at 450° C was chosen and analyzed. Data was collected at different incoupling locations along the edge and the results are found below in Table 11.The results show that there is a fairly large variance o f the waveguid ing  characteristics across the width of both samples. The standard deviation of the slopes, however, is less for the annealed case. It appears that annealed sample should exhibit more reproducible results when compared to the unannealed, control sample. Also, since the standard deviation is less for the annealed case, the annealing process is improving the®§§§loverall quality o r die guiding medium.Tabic 1L,.Study o f the Homogeneity o f a, Typical SampleIJnannealed Half Slopcslmm1) .0 .498  •0.362  •0.491 •0.332  •0.485Annealed Half ____ ,1-0.438 •0.408  -0.499•0.432  .n aa\Mean -  *0.474 Mean « -0.444SD -  0.063 SD -  0.0348The change in the quality o f the waveguide after annealing is best seen by the eye, so photographs were taken as a means of comparison. Figure 2 contains these prints.The top photograph is of an unannealed, control sample. The sample holder has been masked off with black, electrical tape in an attempt to improve the contrast of the photograph. The sample can be seen in the center o f the photograph. The bright streak of light is the beam propagating through the length of the waveguide. Note the strong intensity of the beam, indicating a high degree of scatter. Also notice the small, bright dots near the top of the waveguide. These are dust particles which have settled on the surface of the sample. The exposure time for this photograph was approximately seven seconds.The second photograph is of a sample which had been annealed at 450° V  for ten minutes. Once again the streak in the center of the guide is the beam propagating through the sample, however the intensity of the scatter has been reduced dramatically. All of the bright spots are once again dust particles on the surface o f the guide. They are more prominent in this photograph because the exposure time was increased to approximately thirty seconds. This lengthening in exposure time was necessary, otherwise the faint streak o f the beam as it was propagating through the waveguide would never have been apparent.A comparison o f  the two photographs shows the degree to which the amount of scatter is reduced by annealing. A majority o f the incoupled beam is passed through the annealed waveguide, while a great deal more of the beam is scattered by the unannealed, control waveguide.I iguic 2 ( ompaiiM>n photographs <>t unanneakd and nmicakdwau'guidePhotograph ol an unanneakd waveguide taken with a *5 min local length, zoom k nv with an aperture setting of 3 / ,  and an exposure time of seven seconds. Note the ven mk-nn scatter coming from the beam as it propagates through the waveguide.Photograph of a waveguide that had been annealed at 450° V for ten minutes. I he exposure conditions were the same except that the exposure time had been lengthened to thirty seconds. Note the reduction in the amount of scatter indicating that a majorit) of the beam was being propagated through the waveguide.ConclusionAnnealing o f glass coverslips improves their ability to act as optical waveguides by reducing the variation in the refractive index along the sample, This improvement manifests itself onfy when a temperature of above 400° C is used for annealing. This experiment, however, can not conclude any relevant information concerning the magnitude of the improvement, only that there is one.10fV'Literature Cited(I) Atkins. P.W. Physical Chemistry Third Edition. W.H. Freeman and Company. New York, 1986. p. 464.(2) Sears, Francis W., Mark W. Zemansky, and Hugh D. Young. University Physics(3) Uhlmann, D.R. and N.J. Krcidl. Glass: Science and Technology. Volume 3: Viscosity and Relaxation. Academic Press, Inc. New York, 1986. p. 276.BibliographyAtkins, P.W. Physical Chemistry Third Rdition. W.U. Freeman and Company, New York, 1986.Sears, Francis W,, Mark W. Zemansky, and Hugh D. Young. University PhysicsSeventh Edition. Addison-Wesley Publishing Company, Reading, MA, 1987.Uhlmann, D.R. and N J. Kreidl. Glass: Science and Technology. Volume 3; Viscosity and Relaxation. Academic Press, Inc. New York, 1986.

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The effect of annealing on the properties of optical waveguides

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