JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 36, NO. 23, DECEMBER 1, 2018 5381

Modeling of the Creation of an Internal Cladding inSapphire Optical Fiber Using the 6Li(n,α)3H Reaction

Brandon Augustus Wilson , Sohel Rana, Student Member, IEEE, Student Member, OSA,Harish Subbaraman , Member, IEEE, Nirmala Kandadai, and Thomas E. Blue

Abstract—The focus of this study is to model the creation of aninternal waveguide in sapphire optical fiber. The model investi-gates ion implantation using the 6Li(n,α)3H reaction as a methodfor creating an internal waveguide in sapphire optical fiber. Themethod involves irradiating a sapphire fiber that is surrounded byan annulus of Li-6 enriched lithium carbonate (Li2 CO3 ) powder,in a nuclear reactor. The 6Li(n,α)3H reaction creates high-energytritons and alpha particles that irradiate the fiber simultaneouslyto a depth of 24 μm, along the entire periphery of the sapphirefiber. The triton and alpha particle irradiation slightly reducesthe index of refraction in the fiber’s periphery, thus creating arefractive cladding within the fiber. Refractive index profiles arepredicted by tracking tritons and alpha particles, using the Monte-Carlo radiation transport code MCNP. Measured transmitted lightintensity profiles are compared with predicted light intensity pro-files, by modeling the transport of 1550 nm light through modifiedsapphire fiber using FIMMWAVE software.

Index Terms—FIMMWAVE modeling, MCNP modeling,sapphire fiber cladding, sapphire optical fiber, single mode lighttransmission.

I. INTRODUCTION

E STABLISHED fiber optic sensing technologies have re-cently garnered significant interest from the nuclear energy

industry [1]. Fiber optic sensing technologies offer the potentialfor multi-parameter (i.e., temperature, strain, pressure) mea-surements and the ability for distributed measurements alongthe length of a fiber. In addition, fiber optic sensors are small,compact, and immune to electromagnetic interference (EMI)and radio frequency interference (RFI) [2]. Distributed sensing,in particular, is a major interest to the nuclear industry as thevast amount of data that is created from the distributed temper-ature measurements from fiber optic sensors can be feed into analgorithm to optimize power distributions in a nuclear core. TheNuclear Regulatory Commission has shown interest in usingfiber optics for advanced instrumentation in high temperature

Manuscript received July 23, 2018; revised September 19, 2018; acceptedSeptember 24, 2018. Date of publication October 15, 2018; date of currentversion November 2, 2018. (Corresponding author: Brandon Augustus Wilson.)

B. A. Wilson and T. E. Blue are with the Mechanical and Aerospace En-gineering Department, The Ohio State University, Columbus, OH 43210 USA(e-mail:,wilson.2228@osu.edu; blue.1@osu.edu).

S. Rana, H. Subbaraman, and N. Kandadai are with the Electrical andComputer Engineering Department, Boise State University, Boise, ID 83725USA (e-mail:, sohelrana@boisestate.edu; harishsubbaraman@boisestate.edu;NirmalaKandadai@boisestate.edu).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2018.2873071

next generation nuclear power plants [3]. Advanced nuclear fu-els, such as ceramic fuels and some metallic fuels, can havefuel centerline temperatures that exceed 1,000 °C. Advancedreactors such as the Very High Temperature Reactor (VHTR)can have coolant outlet temperatures in the range of 1,000 °C.Conventional silica optical fibers are generally limited in tem-perature to less than 900 °C due to devitrification of the silicaglass [4]. For these extremely high temperature applications,single crystal sapphire (α-Al2O3) optical fiber-based sensorsare much better suited, as sapphire fibers have shown thermalstability up to 1600 °C [5]. The primary challenge to extend-ing the distributed sensing capabilities of silica-based fibers toextremely high temperature-tolerant sapphire fibers is achiev-ing an effective cladding for the sapphire fibers that enablessingle-mode light transmission.

Sapphire optical fiber is drawn as a single crystal and is there-fore not manufactured with different regions of doped crystal.Without the differently doped regions in sapphire optical fiber,the refractive cladding that enables the fiber to exhibit total in-ternal reflection is simply the air surrounding the fiber. This isproblematic in regards to sensing with the fiber, because it re-sults in a high modality light transmission through the fiber, dueto the large core diameter and large difference in the index ofrefraction (η) between the sapphire fiber and air (for example,at room temperature and for light at 1550 nm, ηsapphire = 1.76and ηair = 1.0) [6]. Most optical sensing techniques require afiber to be single mode or nearly single mode and so to achievethis with a sapphire optical fiber, a refractive cladding must beplaced within the fiber to reduce the core diameter and to lessenthe difference in the index of refraction between the core andthe cladding. Ion implantation is the method for cladding sap-phire that is investigated for this work; for more information oncladding sapphire fiber, the National Energy Technology Labo-ratory (NETL) has completed a comprehensive study [7] on thevarious techniques for cladding sapphire fiber.

To create an internal refractive cladding in sapphire, a groupfrom the State University of New York at Albany doped a regionwithin a sapphire wafer with protons from an accelerator [8].The ion implantation created a region of slightly reduced η thatfunctioned as an internal waveguide for the wafer. Subsequently,researchers from the Ohio State University created an internalrefractive cladding in a sapphire fiber using the ion implanta-tion method [9]. Instead of using an accelerator, they used anuclear research reactor to create the ions, and as a result, wereable to clad the entire length of the fiber simultaneously. By

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5382 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 36, NO. 23, DECEMBER 1, 2018

surrounding the sapphire fiber with Li-6 enriched lithiumcarbonate (Li2CO3) powder, upon neutron irradiation, the63Li(n, α)3

1H reaction created high energy triton and alpha parti-cles which implanted into the periphery of the fiber. This methodof ion implantation for creating an internal cladding within sap-phire fiber is better suited at producing long lengths of claddedsapphire fiber than ion implantation using an accelerator. Thismethod can be optimized to create meters of cladded sapphirefiber in just over an hour of irradiation. In addition to sapphire,this method can be used to create waveguides and or dope a va-riety of other substances. The ion implantation method has greatpotential for waveguide construction in a variety of substancesand geometries in the future and will thus be the main focus ofthis paper.

There are competing hypotheses regarding why ion implan-tation reduces η for sapphire. Researchers from the State Uni-versity of New York at Albany claim nano-voids are created byion implantation and that the resulting nano-voids reduce η (wewill hereafter refer to this hypothesis as the “nano-voids hypoth-esis”) [8]. Townsend [10] presented an alternative hypothesis. Itis that amorphization of the crystal structure, from ion implan-tation, causes the decrease in η (we will hereafter refer to thishypothesis as the “displacement damage hypothesis”). In thispaper, we consider the first hypothesis, namely that nano-voidsare created by ion implantation and that the resulting nano-voidsreduce η. As a step in understanding the mechanism by whichthe clad is formed, we have simulated the formation of a re-fractive cladding in sapphire optical fiber, using Ohio State’smethod for implanting ions in sapphire fiber. A fundamentalresult of our simulations is the density of implanted tritons andalpha particles in the fiber, as a function of radius in the fiber.This result is used to predict the radial dependence of η in thefiber.

II. BACKGROUND

A. Experimental Procedure for Creating Internal RefractiveClad in Sapphire Fiber

We reported in a previous paper the procedure that we usedto create a refractive cladding in sapphire optical fiber [9]. Thatprocedure is briefly reviewed here, since the purpose of this pa-per is to report the results of simulations of that procedure. Thefirst step of the procedure is to radially center a sapphire opticalfiber, with a fiber diameter of 100 micrometers (purchased fromMicroMaterials Inc.), in an aluminum capillary tube. Lithium-6 carbonate powder (95.8 atomic percent Li-6) is packed intothe aluminum capillary tube surrounding the sapphire fiber. Theends of the aluminum tube are crimped to ensure that Li-6 pow-der does not fall out in the reactor The tube is then lowered intothe central irradiation dry well facility of the Ohio State Univer-sity Research Reactor (OSURR) and irradiated for 5.5 hours at450 kW for a total neutron fluence of (6.1 ± 0.6)E + 17 n/cm2.The 6

3Li(n, α)31H reaction,

63Li + 1

0n → 42He + 3

1H + 4.8 MeV

creates high energy tritons and alpha particles that irradiate thefiber simultaneously along the entire periphery of the sapphire

Fig. 1. Pictures taken with our IR camera of transmission profiles (2-D repre-sentation on top, 3-D representation on the bottom) for 1) a silica fiber (leftmostimages); 2) virgin sapphire fiber (middle images); and 3) sapphire fiber with thecladding in it (rightmost images).

fiber. The 63Li(n, α)3

1H reaction creates a triton that has an en-ergy of 2.73 MeV and an alpha particle that has an energy of2.05 MeV. The implantation of the ions into the sapphire fiberreduces the η of the periphery of the fiber causing a reductionin the core size of the sapphire fiber, which helps reduce themultimodal characteristics of the sapphire. Upon completion ofthe irradiation, the sapphire fiber was recovered and a character-ization of the light transmission profile of the fiber was recordedand is reported below.

B. Experimental Verification of Successful Implementation ofRefractive Cladding

Light transmission measurements were made at 1550 nm toprovide a visible confirmation of the effectiveness of the processfor adding an internal refractive clad to the fiber. Previouslypublished transmission profile data [9] is repeated here in Fig. 1,so that a comparison can be made between the measured profileand the simulated profiles in the Results section of this paper.

A dark-box light transmission facility was built to measure thelight propagation though an optical fiber to test the effectivenessof the internal cladding of the sapphire fiber. The sapphire fiberwas placed in a fiber chuck and connected to an IR camera(Thorlabs BP-209 IR) to capture the beam profile of the lightexiting the fiber. The IR camera came with a software packageto analyze the light transmission data exiting the fiber, whichproduces images of light intensity as a function of location in theimage plane. The sapphire fiber was fusion spliced to a CorningSMF-28 optical fiber as a method of coupling light into thefiber. A 1550 nm laser was then attached via an FC connectorto the other end of the Corning SMF-28 fiber. Fig. 1 showstransmission profiles for: a graded-index silica fiber (leftmostimages); virgin sapphire fiber (middle images); and sapphirefiber with the cladding in it (rightmost images). The sapphirefiber with the internal cladding was annealed at 1500 °C for24 hours, to assure ourselves that the cladding that was createdin the fiber remained, following exposure to high temperatures.

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The graded-index silica fiber (leftmost image) had a transmis-sion pattern with one distinct transmission peak in the center ofthe fiber that resembled a gaussian shape. The virgin sapphirefiber (middle image) did not have the same pattern. The lighttransmission was highly multimodal, the light modes changedfrom scan to scan and it did not have a central mode peak. Therightmost images in Fig. 1 show the light transmission patternwith our sapphire fiber with the internal cladding in it. It appearsthat the cladding has reduced the modality of the fiber as thereis only one defined peak in the center of the fiber. The similarityof the transmission profiles for the sapphire cladded fiber andthe graded index silica fiber suggests that our method of ionimplantation has created an effective waveguide in the sapphirefiber. The adequecy of the cladding for the effective functioningof distributed sensing by means of optical frequency domainreflectometry has been documented in Ref. [9].

III. METHODS

To determine the effects of ion implantation on light trans-mission through the sapphire optical fiber, simulations wereperformed using MCNP-6 (Monte Carlo N-Particle code) [11],SRIM/TRIM (Stopping and Range of atoms In Matter code)[12] and FIMMWAVE. MCNP-6 was used to determine thenumber of ion’s implanted into the sapphire fiber as a func-tion of distance into the sapphire fiber while SRIM/TRIM wasused to determine the stopping range of the ions, to setup theappropriate MCNP geometry and to verify MCNP’s results.FIMMWAVE was used to calculate the light mode propagationin the sapphire fiber following the ion irradiation.

An existing and verified MCNP-6 model of the OSURRcore was combined with an MCNP model of the Li-6 carbon-ate/sapphire fiber irradiation assembly. The model of the Li-6carbonate/sapphire fiber irradiation assembly was placed intothe model of the OSURR core, at its appropriate location withinthe OSURR’s CIF. The combined model was then used to de-termine the 6Li(n,α)3H reaction rate in the lithium carbonatepowder surrounding the sapphire fiber. In a separate MCNP-6calculation, triton and alpha particle histories were initiated witha triton and alpha particle source distribution that was consistentwith the distribution of 6Li(n,α)3H reactions in the lithium car-bonate powder surrounding the sapphire fiber. The tritons andalpha particles were tracked in the MCNP model of the Li-6carbonate/sapphire fiber irradiation assembly, as they traversedinto the sapphire fiber.

The number of ions implanted per second in various regionsin the sapphire fiber and the resulting rate of increase of the num-ber densities of the implanted tritons (nT ) and alpha particles(nα ) were determined separately, for each particle type (usinga boundary crossing [i.e., F1 tally]). The ion densities were de-termined by counting the number of tritons and alpha particlesentering a radial cell and subtracting the number of tritons andalpha particles leaving the cell, and dividing the difference bythe volume of the cell.

Finally, a relationship, between the density of implanted tri-tons and alpha particles and the change of η for sapphire, wasdetermined. This relationship was combined, with the relation-ship between density of implanted tritons and alpha particles

TABLE ISRIM RANGES FOR TRITON AND ALPHA PARTICLE IN LI-6 CARBONATE

POWDER AND IN SAPPHIRE FIBER

in the sapphire fiber versus radius, to produce curves of η forsapphire fiber versus radius. A FIMMWAVE model of the sap-phire fiber was produced with the corresponding η curves andthe light propagation through the fiber was modeled.

IV. RESULTS

A. SRIM Results for Triton & Alpha Particle Range in Li-6Powder and Sapphire

The SRIM software package was used to determine the rangeof tritons and alpha particles in Li-6 powder and in sapphire fiberevaluated at their respective birth energies in the 6

3Li(n, α)31H

reaction. These ranges are shown in Table I.The range values seen in Table I are measured from the ma-

terial surface to the center of the Bragg peak of the ion, theuncertainty associated with it is the longitudinal straggling ofthe ions. It can be seen in Table I that 1) the range for a triton is∼90 micrometers in the Li-6 powder and ∼24 micrometers inthe sapphire, and 2) the range for an alpha particle is ∼15 mi-crometers in the Li-6 powder and ∼4 micrometers in sapphire.The implications of (1) are that the tritons are implanted at agreater rate in the sapphire than the alphas, due to the greaterrange of the tritons than alphas in the Li-6 powder, and hence thegreater effective source volume of tritons than alpha particles. Acorollary of the above statement is that one must only analyze,as a source, the 100 micrometers of Li-6 powder surrounding thesapphire fiber. The implications of (2) are that the contributionof the alpha particles to the formation of the cladding is moresuperficial than that of the tritons, since the alpha particles areimplanted in the sapphire to depths that are less than 4 microm-eters whereas the tritons can implant in the sapphire to a depthof 24 micrometers.

B. MCNP Results for Reaction Rate (R) in the Li-6Carbonate Powder

As mentioned previously in Section III, the Li-6 carbon-ate/sapphire fiber irradiation assembly model was combinedwith the MCNP model of the OSURR, by placing the model ofthe Li-6 carbonate/sapphire fiber irradiation assembly into themodel of the OSURR core, at its appropriate location withinthe OSURR’s CIF. The combined model was then used to de-termine the 6Li(n,α)3H reaction rate in the lithium carbonatepowder surrounding the sapphire fiber, so that the total num-ber of tritons and alpha particles that were produced during theexperiment could be calculated.

Specifically, a k-code simulation was run to determine theneutron flux, for the fiber irradiation assembly in the CIF tube

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TABLE II6LI(N,α)3H REACTION RATE (R) AND REACTION RATE PER UNIT VOLUME

(r) IN THE LI-6 POWDER REGIONS

for the OSURR operating at full power. The Li-6 was discretizedinto 6 sub-volumes surrounding the sapphire fiber and each sub-volume was made a tally volume. The fluence per source particlewas tallied for the various sub-volumes; and the contributionsto the fluence per source particle for the various sub-volumeswere multiplied, on a particle-by-particle basis, by the energy-dependent macroscopic cross-section for neutron absorption viathe 6Li(n,α)3H reaction, using the FM4 cards in MCNP-6. Ex-ternal to the MCNP code, we multiplied the resulting talliesof 6Li(n,α)3H reactions per source particle for the various sub-volumes by the total number of neutrons produced in the reactorper second for operation at 450 kW power (i.e., by the sourceparticle rate) and by the volume of the various sub-volumes.This yielded the values for the 6Li(n,α)3H reaction rate (R) inthe Li-6 sub-volumes that are presented in Table II.

Table II shows that the 5 closest regions of Li-6 carbonatepowder surrounding the fiber have a R on the order of 1011

reactions per second. The R for Region 6 is much larger thanthe R for Regions 1-5, because the volume of region 6 is corre-spondingly larger. The reaction rate per unit volume (r) showsthe effects of neutron absorption in the Li-6, in that r decreases,as the radii of the volume decreases.

C. MCNP Results for Sapphire Fiber

With R known, a second MCNP model was created to modelthe triton and alpha particle transport in the sapphire fiber.Source volumes for the tritons and alphas were created for eachLi-6 region surrounding the sapphire fiber. The source volumeswere identical to those that are listed in Table II.

The source volumes are illustrated in Fig. 2, which is anoutput of the Visual Editor software in MCNP-6, for 1000 tritonsthat were simulated as having been born in Region 1. The topdiagram in the figure shows the triton tracks. Some tritons travelinwardly and enter the sapphire fiber. Others lose all of theirenergy within the Li-6 carbonate powder.

The tracks that are of most interest are those that enter thesapphire. The bottom diagram is an expanded view of the par-ticle tracks, for the tritons that enter the sapphire fiber. It can

Fig. 2. Visual editor display of Li-6 carbonate powder regions, sapphire fiberand triton tracks from source Region 1 (top), and with an expanded scale of thetriton tracks superimposed on the tally volumes in the sapphire fiber (bottom).

be seen in this diagram that the sapphire fiber was divided into8 radial regions, each of which is 3 micrometers in thickness.Fig. 2 shows that the ion implantation rate is larger near thesurface of the fiber, than it is at greater depths, and that it is zerofor depths in the fiber that are beyond the range of the tritons.This trend would be even more apparent, if the alphas wereshown in the diagram, since the alphas have a range of only 4microns in the sapphire, whereas the tritons have a range of 24microns in the sapphire.

Table III presents MCNP simulation results for the percent,and number, of tritons and alphas from each Li-6 carbonatepowder source region that strike the sapphire. Table III showsthat the majority of particles that strike the sapphire come fromthe first region of Li-6 carbonate powder. The alpha particleshave a very low range in the Li-6 carbonate powder and canonly impact the fiber, if the reaction occurs within 20 microns

WILSON et al.: MODELING OF THE CREATION OF AN INTERNAL CLADDING 5385

TABLE IIIPERCENT AND NUMBER OF TRITONS AND ALPHA PARTICLES CREATED IN A

LI-6 CARBONATE POWDER SOURCE REGION THAT STRIKE THE SAPPHIRE FIBER

TABLE IVPERCENT OF THE TRITONS AND ALPHAS, WHICH STRIKE THE SAPPHIRE FIBER,

THAT STOP WITHIN A PARTICULAR DEPTH REGION IN FIBER; TRITON AND

ALPHA IMPLANTATION DENSITY RATE VERSUS DEPTH REGION

IN SAPPHIRE FIBER

of the fiber. The greater range of the triton allows for tritons toenter the fiber, when they are born in all regions.

The locations of the implantation of the particles weregrouped into concentric cylindrical annuli, inside of the sap-phire fiber. The number of ions implanted per second in variousregions in the sapphire fiber, and the resulting rate of increaseof the number density of the implanted ions, can be seen inTable IV and in Fig. 3, which is a histogram of implantation rateversus fiber region.

Fig. 3. Bar graph of nα , nT as a function of distance into the fiber.

Table IV and Fig. 3 show a dramatic decrease in nα beyonda depth of 3 microns that is due to the range of a 2.05 MeValpha particle being∼4 microns in sapphire. Another interestingfeature is that nT decreases monotonically from the surface ofthe fiber to a depth in the fiber corresponding to the maximumrange of a 2.73 MeV triton in sapphire ∼24 micrometers. Thisimplies that the cladding has a graded index, with an indexwhich decreases toward the fiber’s surface.

D. Optical Modelling of the Refractive Index Profile

Because the host matrix is modified in two ways (i.e., by theaddition of tritons and alpha particles to the sapphire crystaland by displacement damage to the sapphire), the change in therefractive index of the periphery of the sapphire fiber, can be de-scribed using either one, or both, of two hypotheses (the “nano-voids hypothesis” and the “displacement damage hypothesis”).The displacement damage hypothesis assumes that the displace-ment of host ions induces local charge imbalances within thesapphire, and that these charge imbalances are responsible forinducing stresses within the sapphire’s crystal matrix, which re-sult in a change in the sapphire’s refractive index [13]. It has beenshown in literature that this stress can be annealed out by heatingsapphire to temperatures in excess of 1000 °C, and that this re-laxation of stresses leads to a restoration of the refractive indexof the sapphire to its original value [8]. Given that we annealedthe sapphire fiber to 1500 °C and the clad persisted, we reliedon the nano-voids hypothesis to calculate the effective refractiveindex profile for the sapphire fiber. The nano-voids hypothesisis based on the observation that the implanted protons do notbond with sapphire, and that these protons form nano-voids,when the sapphire is annealed at high temperatures [8]. Due tothe presence of these voids (refractive index, ηvoid = 1) in anotherwise uniform sapphire (refractive index, ηsapphire = 1.76)matrix, the effective index of the medium (ηeff ) is reduced. Theeffective index can be computed based on the Maxwell Garnetteffective medium approximation [14]. We included implantedalpha particles in the calculation of ηeff , using the nano-voidshypothesis, because helium is an inert gas and is unlikely tobond with sapphire. We used the data in Table IV for the num-ber of tritons and alpha particles that were implanted in thevarious regions of the sapphire fiber per cm3 per sec with aneffective reactor irradiation time of 5.5 hours at 450 kW. The

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TABLE VCOMPUTED VALUE OF REFRACTIVE INDEX BASED ON MAXWELL GARNETT

EFFECTIVE MEDIUM APPROXIMATION FOR DIFFERENT DEPTH

DOMAINS IN SAPPHIRE

Both alpha and triton implantations were accounted for in these calculations.

Fig. 4. FIMMWAVE simulation of a graded index fiber based on the computedrefractive index shown in Table V. [Top] Fiber cross section in the software andassociated refractive index profile. [Bottom]: Intensity distribution of the singlemode shown to propagate.

calculated ηeff profile from the outer surface of the sapphirefiber (depth = 0 μm) to the center of the sapphire fiber (depth =50 μm) is shown in Table V. Our refractive index calculation isaveraged over the 3 micrometer thick radial regions in Table IVfrom depth = 0 μm to depth = 24 μm (the range of 2.73 MeVtritons in sapphire in Table I), resulting in an averaged steppedrefractive index over each region.

Using the computed values of ηeff in Table V, we modeled thefiber using PhotonDesign FIMMWAVE software. FIMMWAVEmodels the transmission of light through a fiber using a fully-vectorial finite-difference algorithm in a cylindrical coordinate

system. Fig. 4 (top diagram) shows the radial refractive indexdistribution in the 100 μm diameter fiber. The mode solverwas set to scan for all possible modes, and the simulationsresulted in only a single mode to propagate. The fiber model andthe experimental observations of the transmitted light intensityprofile (as shown in Fig. 4 and 1, respectively) both demonstratesingle mode, or nearly single mode, operation. Ellipsometrymeasurements to experimentally determine the refractive indexof the ion implanted sapphire are ongoing and will be publishedin a future paper.

V. CONCLUSION

The focus of this work is to model the creation of a cladding,which is internal to a sapphire optical fiber, by irradiating asapphire fiber that is surrounded by an annulus of Li-6 enrichedlithium carbonate (Li2CO3) powder, in a nuclear reactor. The63Li(n, α)3

1H reaction creates high energy tritons and alpha par-ticles that irradiate the fiber simultaneously, along the entireperiphery of the sapphire fiber; thereby slightly reducing theη in the fiber’s periphery and creating a cladding within thefiber. Measured transmitted light intensity profiles were com-pared with simulated light intensity profiles. Refractive indexprofiles are predicted by tracking tritons and alpha particles, us-ing the Monte Carlo radiation transport code MCNP. Measuredtransmitted light intensity profiles are compared with predictedlight intensity profiles, by modeling the transport of 1550 nmlight through modified sapphire fiber using the PhotonDesignFIMMWAVE software. A fundamental result of our simulationsis the density of implanted ions in the fiber as a function of ra-dius in the fiber. This fundamental result was used to predict theradial dependence of η in the fiber, based upon the assumptionsthat the density of implanted ions affects η and the displace-ment density that is created by the ions as they are implanted inthe fiber does not affect η. The simulated transmitted light in-tensity profile is consistent with the measured transmitted lightintensity profile and demonstrates single mode, or nearly singlemode, operation.

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Brandon Augustus Wilson received the Ph.D. degree in nuclear engineeringfrom Ohio State University, Columbus, OH, USA, in January 2017. He iscurrently a Postdoctoral Researcher in the nuclear engineering program withthe Mechanical and Aerospace Engineering Department, Ohio State University.His research interests include effects of high temperature and ionizing radiationon the optical properties of materials.

Sohel Rana (S’17) received the B.Sc. degree in electrical and electronic en-gineering from Rajshahi University of Engineering and Technology, Rajshahi,Bangladesh, in 2014. He is currently working toward the Ph.D. degree at theDepartment of Electrical and Computer Engineering, Boise State University,Boise, ID, USA. In 2015, he joined the Department of Electrical and Electronic,Bangladesh University, as a Lecturer. His research interests include fiber opticsensor, photonic crystal fiber, THz radiation, etc. He is the Student Member ofthe OSA.

Harish Subbaraman (M’09) received the M.S. and Ph.D. degrees in electricalengineering from the University of Texas at Austin, Austin, TX, USA, in 2006and 2009, respectively. He was a Senior Research Scientist with Omega Optics,Austin, where he worked on printed and flexible photonics and electronics; andsilicon and polymer based optical interconnects. In Fall 2016, he joined the De-partment of Electrical and Computer Engineering, Boise State University, Boise,ID, USA. He has served as a PI and Co-PI on several federal and state grants.He has five issued and pending patents and has more than 120 publications inrefereed journals and conferences. His current research interests include ink-jetprinting and silicon-nanomembrane-based flexible electronic and photonic de-vices; fiber-optic sensors; optical true time delay; phased array antennas; RFphotonics; polymer photonics; and slow-light photonic crystal waveguides.

Nirmala Kandadai received the Ph.D. degree from the University of Texas atAustin, Austin, TX, USA, in 2012. Her Ph.D. work investigated the interactionof high-intensity laser with molecular gas clusters and the stability of the plasmagenerated. After her Ph.D. degree, she was with National Energetics, Austin,where she was the Lead Scientist to develop the front end of a ten PW lasersystem for European Union’s Extreme Light Infrastructure Beamlines FacilityInfrastructure Beamlines Facility. Since September 2016, she has been an As-sistant Research Professor with Boise State University, Boise, ID, USA. Sheis a Co-Director of the Fiber Optics Lasers and Integrated Photonics Group,Boise State University. Her work involves creating optical fiber sensors in harshenvironments. She is currently with Idaho National Lab, Idaho Falls, ID. Shehas served as a PI and Co PI for two grants, one in creating optical sensors thatcan survive in pile a nuclear reactor and second on in pile infrared thermographyfor crack detection using coherent optical fiber bundles.

Thomas E. Blue is currently an Academy Professor of Nuclear Engineering withthe Mechanical and Aerospace Engineering Department, Ohio State University,Columbus, OH, USA. He retired in 2016, and was at that time the Director of theOSU Nuclear Reactor Lab. His current research interests include developmentof instrumentation for advanced reactors and the testing of the instrumentationat high temperatures in radiation fields.