materials

Article Refractive-Index Profile Reconstruction in Graded-Index Polymer Optical Fibers Using Raman

Mikel Azkune 1,* , Angel Ortega-Gomez 2 , Igor Ayesta 3 and Joseba Zubia 2

1 Department of Electronic Technology, Engineering School of Bilbao, University of the Basque Country (UPV/EHU), Plaza Ingeniero Torres Quevedo, 1, E-48013 Bilbao, Spain 2 Department of Communications Engineering, Engineering School of Bilbao, University of the Basque Country (UPV/EHU), Plaza Ingeniero Torres Quevedo, 1, E-48013 Bilbao, Spain; [email protected] (A.O.-G.); [email protected] (J.Z.) 3 Department of Applied Mathematics, Engineering School of Bilbao, University of the Basque Country (UPV/EHU), Plaza Ingeniero Torres Quevedo, 1, E-48013 Bilbao, Spain; [email protected] * Correspondence: [email protected]; Tel.: +34-94301-8645

 Received: 8 April 2020; Accepted: 9 May 2020; Published: 14 May 2020 

Abstract: This work reports a novel method to create a 3D map of the refractive index of different graded-index polymer optical fibers (GI-POF), measuring the Raman spectra at different points of their transverse sections. Raman fingerprints provide accurate molecular information of the sample with high spatial resolution. The refractive index of GI-POFs is modified by adding a in the preform; therefore, by recording the intensities of the Raman peaks related to the dopant material, a 3D map of the refractive index is rendered. In order to demonstrate the usefulness of the method, three different GI-POFs were characterized and the obtained results were compared with the information provided by the manufacturers. The results show accurate 3D maps of the refractive index taken in the actual GI-POF end faces, showing different imperfections that manufacturers do not take into account, such as the slight deviations of the azimuthal symmetry. The simplicity and the feasibility of the technique mean this method has high potential for fiber characterization purposes.

Keywords: polymer optical fibers (POF); Raman spectroscopy; graded-index fibers; refractive- index reconstruction

1. Introduction In the last few decades, there have been continual advances in the performance of polymer optical fibers (POFs). As compared to fibers, POFs are easier and more economical to manufacture, safer to handle, and much more flexible [1–4]. There is a broad range of applications where POFs are excellent candidates, such as biosensing [5,6], structural health monitoring [7,8], optical amplification and lasing [9,10], and telemedicine and short-haul communications links [11]. Moreover, the high bandwidth of graded-index polymer optical fibers (GI-POFs) allows realization of high-speed information transmissions [4,12]. For all the aforementioned applications, and especially to define the bandwidth of GI-POFs, the refractive-index profile of these fibers must be controlled and measured [13]. There are two different principal measuring techniques in the literature that are used to characterize gradual refractive-index profiles: destructive and non-destructive techniques [14,15]. The disadvantages of destructive methods are obvious, while non-destructive methods (such as focusing method, optical interference method, or methods based on the measurements of the bending of transverse rays) usually require the construction of highly accurate set-ups and the geometry of the samples must be perfectly circular or elliptical [16]. Moreover, all these techniques are mainly

Materials 2020, 13, 2251; doi:10.3390/ma13102251 www.mdpi.com/journal/materials Materials 2020, 13, 2251 2 of 12 designed to characterize the refractive-index profile from the fiber preform and not from the fiber itself. The results obtained in this way may vary from the final distribution of the fiber, since the physical characteristics—even the index distribution—can be slightly different as a consequence of the stretching process in the manufacturing tower. In this paper, we present a novel method to characterize the refractive-index distribution directly from GI-POFs based on Raman spectroscopy. Raman spectroscopy is a spectroscopic technique used to obtain information from molecular vibrations, in which the measured spectra provide an insight into both intramolecular information and intermolecular interactions, playing an important role in the elucidation of molecular structures [17,18]. In recent years, this spectroscopy technique has been widely used for different purposes [19–21], including for polymer characterization in POFs [22–26]. In our case, the Raman spectra are obtained at different points from the transverse section of GI-POFs, and we relate the proportion of some of the Raman peaks with the amount of used to modify the refractive index. In this way, we can create 3D maps of the refractive-index distributions of the fibers. As far as we know, this is the first time that this kind of characterization has been carried out by using Raman spectroscopy. Moreover, the measurement system can be widely automated and the data analysis is straightforward, so the POF samples can be characterized in a very brief period of time, i.e., in a few minutes once the spectra are acquired. The paper is organized as follows. First, the experimental set-up is described, together with the characteristics of the optical fibers employed. Then, the results for three different GI-POFs are shown. Finally, the main conclusions and potential applications are summarized.

2. Experimental Set-up

When a of λexc strikes a molecule, both elastic and inelastic occur. The elastically scattered photon has the same wavelength, while the inelastic photon has a different wavelength, which depends on the vibrational state of the molecules. As is well known, the Stokes Raman shift ∆ν is given by [27]: " #  1 1 1 7 ∆ν cm− = 10 (1) λexc(nm) − λm n(nm) → where λm n is the wavelength of the Raman vibration band. The of the scattered radiation → Im n due to the vibrational transition m n is: → → X 4 4 Im n = N(νexc νm n) (αi)m nEj (2) → − → i,j → where N is the number of molecules; νexc and νm n are the of the excitation and → the Raman band, respectively; αi is the of the molecule; and Ej is the electric field of the excitation. Therefore, the Raman intensity is mainly affected by four factors: the light source through Ej, the excitation wavelength of the laser source νexc, the scattering properties of the sample or scattering cross-section, and finally the concentration of the sample or the number of molecules in the measuring unit cell. It is remarkable that the Raman intensity of a vibrational band depends on the number of active molecules N, as shown in Equation (2). If the vibration band corresponds to a certain doping molecule, the Raman intensity will be proportional to the number of this molecule. On the other hand, it is known that the refractive-index distribution is related to the concentration of doping molecules added during the polymerization process [28]. In the case of GI-POFs, the concentration is higher in the center of the fiber and decreases gradually with radial distance. Taking into account these ideas, in this work an accurate and concise reconstruction of the refractive-index profile of GI-POFs has been carried out, with much higher accuracy than the mathematical modeling approaches. These mathematical models fail to quantify the deviation between the expected and actual refractive index value in the GI-POF, Materials 2020, 13, 2251 3 of 12 among other limitations. In order to overcome the drawbacks of the mathematical modeling and obtain Materialsa reliable 2019 refractive-index, 12, x FOR PEER REVIEW distribution in GI-POFs, we proposed a characterization technique based3 of on12 Raman spectroscopy. This technique has the potential for non-destructive surface chemical analysis a characterization technique based on Raman spectroscopy. This technique has the potential for non- and allows us to obtain spatially well-defined fingerprints—or Raman peaks—with the chemical destructive surface chemical analysis and allows us to obtain spatially well-defined fingerprints—or information of the sample. Raman peaks—with the chemical information of the sample. 2.1. Equipment 2.1. Equipment All the Raman measurements shown in this work were obtained using an InVia Raman confocal microscopeAll the Raman from Renishaw measurements (Gloucestershire, shown in this UK). work In were order obtained to avoid using any fluorescence,an InVia Raman the confocal primary microscopeexcitation source from usedRenishaw was a(Gloucestershire, 785 nm laser. The UK). optical In powerorder to on avoid the sample any fluorescence, was 7.64 mW. the This primary power excitationis low enough source to used avoid was any a 785 thermal nm laser. eff ectThe or op materialtical power degradation. on the sample Besides, was 7.64 we mW. used This apinhole power isin low the enough launching to avoid path any to reducethermal the effect spot or material dimension, degradation. therefore Besides, increasing we theused lateral a pinhole resolution. in the launchingDepending path on theto reduce GI-POF, the the spot magnification dimension, ofther theefore increasing the lateral was resolution. 50 (spot size Depending= 2 mm) × onor the 20 GI-POF,(spot size the= magnification7 mm). Additionally, of the microscope all measurements objective were was done 50× in(spot high size confocality = 2 mm) mode,or 20× (spot so we × sizewere = able7 mm). to collectAdditionally, the chemical all measurements information ofwere the enddone face in high of the confocality fibers. The mode, grating so was we atwere a rate able of to1200 collect lines the/mm chemical and the acquisitioninformation time of the for eachend face spectrum of the was fibers. ten seconds.The grating For thiswas grating,at a rate according of 1200 lines/mm and the acquisition time for each spectrum 1was ten seconds. For this grating, according to to the manufacturer the spectral resolution is 0.5 cm− . the manufacturer the spectral resolution is 0.5 cm−1. 2.2. Sample Preparation 2.2. Sample Preparation All used GI-POF samples were 10 cm in length, although the fiber length does not influence the measurementAll used GI-POF because samples the active were Raman 10 cm depthin length, (a few although microns) the is fiber negligibly length smalldoes not compared influence to the the measurementfiber length. First,because the the end active faces ofRaman the GI-POF depth samples(a few microns) were properly is negligibly polished small using compared different to grain the fibersandpapers length. First, and by the rinsing end faces them of in the isorpopilyc GI-POF samples alcohol, were in order properly to get polished repetitive using measurements different grain and sandpapersremove all possibleand by rinsing impurities them that in couldisorpopilyc mask oralcohol, perturb in theorder spectra. to get Then,repetitive the samples measurements were placed and removeon a motorized all possible stage impurities with a step that resolution could mask of 1 orµm. perturb Afterwards, the spectra. we took Then, a wide the samples image of were the end placed face onin thea motorized Raman microscope stage with ina orderstep resolution to launch of the 1 mappingμm. Afterwards, experiment, we took as can a bewide seen image in Figure of the1. end face in the Raman microscope in order to launch the mapping experiment, as can be seen in Figure 1.

Figure 1. Microscope image composition of a polymer optical fiber (POF) sample obtained from the Figure 1. Microscope image composition of a polymer (POF) sample obtained from the Renishaw InVia Raman microscope using a 20 objective. Renishaw InVia Raman microscope using a 20×× objective. 2.3. Measurements Set-Up 2.3. Measurements Set-Up Subsequently, we divided the GI-POF end face into a grid of hundreds of points where the Raman spectraSubsequently, would be recordedwe divided (Figure the 2GI-POF). At each end point, face weinto recorded a grid of the hundreds average of threepoints consecutive where the Ramanspectra spectra acquisitions. would Once be recorded the Raman (Figure spectra 2). measurementsAt each point, were we done,recorded we removedthe average the of baseline three consecutive spectra acquisitions. Once the Raman spectra measurements were done, we removed the baseline using a 4S FillPeaks algorithm [29] for all mapped spectra. Afterwards, we smoothed them using a Savitzky–Golay second-order filter with 21 points, using an ad hoc script in R.

Materials 2020, 13, 2251 4 of 12 using a 4S FillPeaks algorithm [29] for all mapped spectra. Afterwards, we smoothed them using a

MaterialsSavitzky–Golay 2019, 12, x FOR second-order PEER REVIEW filter with 21 points, using an ad hoc script in R. 4 of 12

FigureFigure 2. 2.Set-up Set-up used used in allin ofall theof experiments,the experiments, based based on a typicalon a typical Raman Raman set-up, set-up, where thewhere scanning the gridscanning is performed grid is performed on the graded-index on the graded-index polymer optical polymer fibers optical (GI-POF) fibers end (GI-POF) face. end face.

2.4. Data Procesing and 3D Map Reconstruction 2.4. DataIn order Procesing to process and 3D the Map recorded Reconstruction Raman spectra during the mapping, first we defined the Raman band,In whose order to intensity process increases the recorded as the Raman doping spectra concentration during the increases, mapping, i.e., first as awe function defined of the the Raman radial band,distance whose to the intensity fiber axis increases (DopantPeak as ).the The doping choice concentr of the activeation Raman increases, band i.e., was as conducted a function by of comparing the radial distancethe Raman to spectra the fiber at the axis core (Dopant axis andPeak). at theThe core choice extreme of the limit. active Furthermore, Raman band in order was to conducted set an internal by comparingstandard peak, the Raman we selected spectra the at reference the core intensityaxis and fromat the the core well-known extreme limit. Raman Furthermore, band assigned in order to the to setsubstrate an internal material, standard which peak, does we not selected correspond the re toference the doping intensity material, from the meaning well-known it does Raman not change band assignedthrough theto the core substrate area (Substrate material,Peak which). We defineddoes not the correspond Raman intensity to the doping ratio as material, follows: meaning it does not change through the core area (SubstratePeak). We defined the Raman intensity ratio as follows: I(Dopant ) R = Peak I() (3) = I(SubsratePeak) (3) I() Once the ratio was calculated, the next step was to calculate the predicted value of the refractive Once the ratio was calculated, the next step was to calculate the predicted value of the refractive index (ncalculated), following the next expression: index (), following the next expression:

∆(n(GIPOF)) ncalculated ==∗R − ++min(( nGI POF)) (4)(4) ∗ max(R()) − where () is the maximum ratio value of the whole map; ( ) is the minimum where max(R) is the maximum ratio value of the whole map; min(nGIPOF) is the minimum − refractive-index value in the GI-POF, also called n2; and ( ) is the difference between the refractive-index value in the GI-POF, also called n2; and ∆(nGI POF) is the difference between the maximum and minimum of the refractive indexes in the GI-POF.− Plotting the value of the maximum and minimum of the refractive indexes in the GI-POF. Plotting the value of the ncalculatedin ineach each of of the the measured measured points, points, we we rendered rendered the the refractive-index refractive-index reconstruction reconstruction and and built built a 3D a 3D map. map. On the other hand, the current GI-POF’s refractive-index profile is usually expressed with the mathematical expressions described in Equations (5) and (6) [30]: () = 1 − 2∆ , 0 ≤ ≤ (5)

() =, > (6) where n1 and n2 are the refractive indexes of the center axis and the cladding, respectively; a is the core radius; g is the index exponent that is the parameter of the index profile; and ∆ is the relative index difference, defined as follows:

Materials 2020, 13, 2251 5 of 12

On the other hand, the current GI-POF’s refractive-index profile is usually expressed with the mathematical expressions described in Equations (5) and (6) [30]:

1   r g 2 n(r) = n 1 2∆ , 0 r a (5) 1 − a ≤ ≤

n(r) = n2, r > a (6) where n1 and n2 are the refractive indexes of the center axis and the cladding, respectively; a is the core radius; g is the index exponent that is the parameter of the index profile; and ∆ is the relative index difference, defined as follows: n2 n2 = 1 − 2 ∆ 2 (7) 2n1 These expressions were used in this work in order to model the GI-POF refractive index distributions. We compared them with the results obtained from Equation (4).

2.5. Studied GI-POFs In order to demonstrate the usefulness of this method, it was employed with three different GI-POF manufacturers. The main features of the employed fiber samples are summarized in Table1. The reason for measuring those fibers was to prove the versatility of the presented method in commercial and non-commercial GI-POFs with different characteristics, such as fiber diameter or dopant material.

Table 1. Main features of the analyzed GI-POFs

Name Company Core Diameter (dB/km) Polymer Poly(methyl OMG-Giga-SE100 FiberFinn 1 mm <200 @ 650 nm methacrylate) (PMMA) LucinaTM Asahi Glass 125 m <30 @ 1200 nm CYTOPTM Experimental PMMA + 12 ppm 1 mm <200 @ 650 nm Fluorescence GI-POF Rhodamine 6G

As can be observed in Table1, we analyzed two PMMA-based GI-POFs, one of which was made from CytopTM. The first one showed a 1-mm wide diameter and was purchased from FiberFinn. This GI-POF was fabricated for short-range communication purposes and the dopant used for refractive index modification was benzyl benzoate [31]. The second fiber was a CytopTM-based GI-POF called LucinaTM. CytopTM is the registered trademark of a fluoropolymer from Asahi Glass. It has a low refractive index and high optical transparency because it is C-H-bond-free and the C-F bond is transparent in the range of 650–1300 nm. It was conceived for high-capacity communications, as it shows an attenuation below 60 dB/Km at 1100 nm, outstripping the first option. The third GI-POF we studied was the experimental fluorescence GI-POF (EFGI-POF) fiber, which was also based on PMMA and used the same dopant to control the refractive index. This fiber was fabricated for research purposes, which was the main reason why Rhodamine 6G molecules were also incorporated [9]. As already pointed out, the measurements were performed using a 785 nm laser in order to avoid any absorption or emission effects from the Rhodamine 6G.

3. Results and Discussion

3.1. FiberFinn GI-POF The first analyzed GI-POF was manufactured by FiberFinn. This 1-mm diameter fiber is made from PMMA and the refractive-index profile is obtained by adding a fluorinate dopant. Prior to measuring all the spectra of the end face, we recorded different Raman spectra in a defined radial direction in order Materials 2020, 13, 2251 6 of 12 to determine the Raman peaks corresponding to the PMMA and to the dopant. The obtained results, which were normalized to a well-known PMMA peak placed at 815 cm 1, are shown in Figure3[26]. Materials 2019, 12, x FOR PEER REVIEW − 6 of 12 Materials 2019, 12, x FOR PEER REVIEW 6 of 12

FigureFigure 3. 3. NormalizedNormalized Raman Raman spectra spectra in in a a radial radial direction direction of of the the FiberFinn FiberFinn GI-POF. GI-POF. Figure 3. Normalized Raman spectra in a radial direction of the FiberFinn GI-POF. 1 From the obtained results, it can bebe seenseen thatthat thethe peakpeak placedplaced atat 10061006 cmcm−−1 expressesexpresses the highest −1 variationFrom in the the obtained peak intensity results, with it can the be radial seen that distance. the peak This placed peak isat the 1006 main cm Raman expresses peak the of highestbenzyl benzoate,variation inthe the dopant peak intensityused for the with refractive-index the radial distance. mo modificationdification This peak [31]. [31 is]. the Therefore, Therefore, main Raman this peakpeak wasof benzyl set as 1 thebenzoate, dopant the peak dopant in Equation used for (3),(3), the andand refractive-index thethe peakpeak placedplaced mo atatdification 815815 cmcm−-1 [31].waswas setTherefore, set as as the the substrate substrate this peak peak. peak. was Then,set Then, as -1 wethe performedperformeddopant peak thethe in end end Equation face face mapping. mapping. (3), and In the In order orderpeak to placed renderto render aat 3D 815a refractive-index3D cm refractive-index was set as map, the map, substrate we followed we followed peak. the Then, steps the stepswedescribed performed described in previous the in end previous sections, face mapping. sections, using a In 30 usingorderµm step toa 30 resolutionrender μm astep 3D and refractive-indexresolution a 20 objective. and map,a The20× we reconstruction objective. followed The the is μ × reconstructionstepsshown described in Figure is 4 shownin. previous in Figure sections, 4. using a 30 m step resolution and a 20× objective. The reconstruction is shown in Figure 4.

Figure 4. The 3D refractive-index reconstruction of the FiberFinn GI-POF. Figure 4. TheThe 3D 3D refractive-index refractive-index reconstruc reconstructiontion of the FiberFinn GI-POF. From the reconstruction, we could state that the GI-POF profile of this fiber follows the expected From the reconstruction, we could state that the GI-POF profile of this fiber follows the expected shapeFrom with the negligible reconstruction, imperfections. we could In stateorder that to vi thesualize GI-POF slight profile imperfections of this fiber of follows the refractive the expected index shape with negligible imperfections. In In order order to vi visualizesualize slight imperfections of the refractive index in the GI-POF, we display the mean ncalculated and the standard deviation for each radial distance, as in the GI-POF, we display the mean n and the standard deviation for each radial distance, wellin the as GI-POF, the refractive-index we display the value mean provided ncalculatedcalculated andby ththee manufacturerstandard deviation (see Figure for each 5). radialAdditionally, distance, the as as well as the refractive-index value provided by the manufacturer (see Figure5). Additionally, regressionwell as the of refractive-index the measured refracti value ve-indexprovided distribution by the manufacturer is also plot ted,(see alongFigure with 5). Additionally,the one provided the byregression the manufacturer. of the measured refractive-index distribution is also plotted, along with the one provided by the manufacturer.

Materials 2020, 13, 2251 7 of 12 the regression of the measured refractive-index distribution is also plotted, along with the one provided byMaterials the manufacturer. 2019, 12, x FOR PEER REVIEW 7 of 12

(a) (b)

FigureFigure 5. ( a5.) Calculated(a) Calculated mean mean refractive-index refractive-index values values (black (b dotslack with dots error with bars) error of bars) the FiberFinn of the FiberFinn GI-POF andGI-POF the refractive-index and the refractive-index distribution distribution provided by pr theovided manufacturer by the manufacturer (red line). (red (b )solid Regression line). ( ofb) bothRegression distributions. of both distributions.

Figure5a shows the refractive-index profile obtained averaging all the spectra corresponding to differentFigure azimuthal 5a shows angles the refractive-index for each radial distance, profile obta as wellined as averaging the refractive all the index spectra profile corresponding provided by to thedifferent manufacturer. azimuthal Hence, angles the for standard each radial deviation distance, for eachas well radius as the represents refractive the index azimuthal profile asymmetryprovided by ofthe the manufacturer. fiber, which was Hence, 0.2% the in standard the worst deviation case. As canfor each be seen, radius these represents standard the deviation azimuthal values asymmetry mean thatof the for fiber, the same which radius was the0.2% actual in the refractive worst case. index As can can be be seen, different, these something standard deviation that is not values taken mean into accountthat for inthe traditional same radius refractive-index the actual refractive measurements index can and be different, that dictates something the quality that is of not the taken GI-POF. into Additionally,account in traditional Figure5b showsrefractive-index that the overall measurements shape of theand calculatedthat dictates refractive the quality index of follows the GI-POF. the distributionAdditionally, provided Figure by5b theshows manufacturer. that the overall According shape to of the the regression calculated plot, refractive the relation index between follows the the 2 measureddistribution and provided manufacturer by the´s manufacturer. reported values Accordin is linear,g withto the a regression Pearson’s rplot,value the of relation 0.985, concludingbetween the thatmeasured the method and manufacturer´s is valid for these reported types of values measurements. is linear, with a Pearson’s r2 value of 0.985, concluding that the method is valid for these types of measurements. 3.2. LucinaTM 3.2. TheLucina secondTM analyzed GI-POF was the LucinaTM fiber. Both substrate and dopant materials differed TM from thoseThe ofsecond the previous analyzed fiber. GI-POF The substrate was the material LucinaTM in thisfiber. case Both is Cytop substrate, while and thedopant manufacturer materials doesdiffered not providefrom those information of the prev aboutious the fiber. dopant The employedsubstrate material to modify in the this refractive case is Cytop index.TM Therefore,, while the wemanufacturer first recorded does Raman not spectraprovide from information different fiberabout locations the dopant in order employed to choose to modify the most the interesting refractive peaksindex. to Therefore, record. The we obtained first recorded spectra Raman are shown spectra in Figure from6 different. fiber locations in order to choose the Asmost can interesting be seen in peaks Figure to6, record. the dopant The showsobtained many spectra Raman are bands. shown Additionally,in Figure 6. from the spectra recorded from the jacket, we can conclude that the manufacturer made this section with PMMA, with the possible aim of providing mechanical protection. The wavelength shifts for the dopant and the reference were set at 590 and 695 cm 1, respectively. For this GI-POF, we set the 50 objective, − × as the fiber diameter was significantly smaller (i.e., 120 µm). The spectra were recorded with steps of 15 µm. The rendered 3D map is shown in Figure7.

Materials 2019, 12, x FOR PEER REVIEW 7 of 12

(a) (b)

Figure 5. (a) Calculated mean refractive-index values (black dots with error bars) of the FiberFinn GI-POF and the refractive-index distribution provided by the manufacturer (red solid line). (b) Regression of both distributions.

Figure 5a shows the refractive-index profile obtained averaging all the spectra corresponding to different azimuthal angles for each radial distance, as well as the refractive index profile provided by the manufacturer. Hence, the standard deviation for each radius represents the azimuthal asymmetry of the fiber, which was 0.2% in the worst case. As can be seen, these standard deviation values mean that for the same radius the actual refractive index can be different, something that is not taken into account in traditional refractive-index measurements and that dictates the quality of the GI-POF. Additionally, Figure 5b shows that the overall shape of the calculated refractive index follows the distribution provided by the manufacturer. According to the regression plot, the relation between the measured and manufacturer´s reported values is linear, with a Pearson’s r2 value of 0.985, concluding Materialsthat the2020 method, 13, 2251 is valid for these types of measurements. 8 of 12

Materials 2019, 12, x FOR PEER REVIEW 8 of 12

Materials 2019, 12, x FOR PEER REVIEW 8 of 12 (a)

(b)

Figure 6. (a) Raman spectra of LucinaTM GI-POFs obtained from different sections (core center, core extreme, and jacket). (b) Normalized Raman spectra in radial direction.

As can be seen in Figure 6, the dopant shows many Raman bands. Additionally, from the spectra

recorded from the jacket, we can conclude that the manufacturer made this section with PMMA, with the possible aim of providing mechanical protection.(b) The wavelength shifts for the dopant and the reference were set at 590 and 695 cm−1, respectively. For this GI-POF, we set the 50× objective, as the Figure 6. (a) Raman spectra of LucinaTM GI-POFs obtained from different sections (core center, core fiber diameterFigure 6. ( awas) Raman significantly spectra of smaller LucinaTM (i.e., GI-POFs 120 μ obtainedm). The fromspectra different were sectionsrecorded (core with center, steps core of 15 μm. extreme, and jacket). (b) Normalized Raman spectra in radial direction. The renderedextreme, and3D mapjacket). is (shownb) Normalized in Figure Raman 7.. spectra in radial direction.

As can be seen in Figure 6, the dopant shows many Raman bands. Additionally, from the spectra 1.348 recorded from the jacket, we can conclude that the manufacturer made this section with PMMA, with the possible aim of providing mechanical protection. The wavelength shifts for the dopant and the 1.346 reference were set at 590 and 695 cm−1, respectively. For this GI-POF, we set the 50× objective, as the fiber diameter was significantly smaller (i.e., 120 μm). The spectra were recorded with steps of 15 μm. 1.344 The rendered 3D mapz is shown in Figure 7..

1.342 x 1.348 y 1.340 1.346 TM FigureFigure 7.7. TheThe 3D3D refractive-indexrefractive-index reconstructionreconstruction ofof thethe LucinaLucinaTM GI-POF. 1.344 The refractive-indexz reconstruction shown in Figure7 is noticeably di fferent in comparison to the previous case—the overall shape is sharper. Additionally, the refractive-index values are rather small, The refractive-index reconstruction shown in Figure 7 is noticeably different1.342 in comparison to x whichthe previous was an case—the expectedchange, overall asshape both is the sharper. substrate Ad andditionally, dopant the materials refractive-index are CYTOP-based values are materials. rather Following the analysis of the previousy case, the calculated refractive-index profile, the refractive index small, which was an expected change, as both the substrate and dopant materials1.340 are CYTOP-based providedmaterials. by Following the manufacturer, the analysis and of the the regression previous are case, displayed the calculated in Figure refractive-index8. profile, the refractive index providedFigure 7. The by 3Dthe refractive-index manufacturer, re andconstruction the regression of the Lucinaare displayedTM GI-POF. in Figure 8.

The refractive-index reconstruction shown in Figure 7 is noticeably different in comparison to the previous case—the overall shape is sharper. Additionally, the refractive-index values are rather small, which was an expected change, as both the substrate and dopant materials are CYTOP-based materials. Following the analysis of the previous case, the calculated refractive-index profile, the refractive index provided by the manufacturer, and the regression are displayed in Figure 8.

Materials 2020, 13, 2251 9 of 12 Materials 2019, 12, x FOR PEER REVIEW 9 of 12

(b) (a)

FigureFigure 8. (8.a) ( Calculateda) Calculated mean mean refractive-index refractive-index values values (black (black dots with dots error with bars) erro ofr bars) the Lucina of theTM LucinaGI-POFTM andGI-POF the refractive-index and the refractive-index distribution distribution provided by provided the manufacturer by the ma (rednufacturer solid line). (red (b )solid Regression line). (b of) bothRegression distributions. of both distributions.

For this case, it can be observed that the azimuthal asymmetry of the refractive index is smaller than inFor the this FiberFinn case, it case,can be being observed 0.08% inthat the the worst azimuthal case. However, asymmetry the of refractive the refractive index ofindex the modeling is smaller fitsthan better in the for thisFiberFinn fiber, as case, themean being values 0.08% of in the the calculated worst case. refractive However, indexes the are refractive almost placed index onof thethe modeledmodeling points. fits better Regarding for this the fiber, regression, as the mean we conclude values of that the this calculated calculation refractive method indexes is also validatedare almost forplaced this fiber,on the as modeled the regression points. isRega linearrding for the refractive-index regression, we values conclude of the that core this (i.e.,calculation refractive-index method is 2 valuesalso validated higher than for 1.3242),this fiber, with as athe Pearson’s regression r value is linear of 0.986. for refractive-index values of the core (i.e., refractive-index values higher than 1.3242), with a Pearson’s r2 value of 0.986. 3.3. EFGI-POF 3.3. TheEFGI-POF last fiber we analyzed had similar characteristics to the one manufactured by FiberFinn. For instance,The last the fiber substrate we analyzed and dopant had similar materials, characteri as wellstics as the to fiberthe one diameter, manufactured were the by same FiberFinn. as the first For case.instance, However, the substrate this fiber and had dopant the peculiarity materials, of theas well organic as the dye fiber Rhodamine diameter, 6G were being the added same during as the thefirst fabricationcase. However, process. this This fiber dye had is the well peculiarity characterized of the within organic the dye literature Rhodamine and its 6G Raman being peaksadded doduring not interferethe fabrication with either process. the dopant This dye or theis well substrate characteri materialzed peaks.within However,the literature the fluorescenceand its Raman effect peaks of the do dyenot couldinterfere hide with the Ramaneither the peaks dopant if an or incorrect the substrate laser source material was peaks. used, However, such as the the typical fluorescence 532 nm lighteffect source.of the dye Therefore, could hide the mappingthe Raman was peaks recorded if an incorre using thect laser same source parameters was used, as in such the first as the case. typical The 3D532 plotnm islight shown source. in Figure Therefore,9. the mapping was recorded using the same parameters as in the first case. TheThe 3D plot area is with shown the graded-indexin Figure 9. distribution is not as wide as the core diameter, with the radius of the graded area being 300 µm. In the same way as for the previous cases, in Figure 10 we compare the values of the calculated refractive indexes with the ones obtained from the modeling, as well as the regression between them. From Figure 10a we can observe that the refractive index varies at different points of the fiber with the same radius, with the azimuthal asymmetry being 0.36% in the worst case. This shows that the quality of the fiber is not as high as in the two previous samples. Figure 10b shows a regression with a higher than the previous cases, linked with the poorer quality of the refractive-index profile of this GI-POF (r2 = 0.972). It is also remarkable that the fluorescent dye added to the fiber does not interfere with the measurements, which was expected due to the high molecular selectivity of the Raman spectroscopy.

Materials 2019, 12, x FOR PEER REVIEW 9 of 12

(b) (a)

Figure 8. (a) Calculated mean refractive-index values (black dots with error bars) of the LucinaTM GI-POF and the refractive-index distribution provided by the manufacturer (red solid line). (b) Regression of both distributions.

For this case, it can be observed that the azimuthal asymmetry of the refractive index is smaller than in the FiberFinn case, being 0.08% in the worst case. However, the refractive index of the modeling fits better for this fiber, as the mean values of the calculated refractive indexes are almost placed on the modeled points. Regarding the regression, we conclude that this calculation method is also validated for this fiber, as the regression is linear for refractive-index values of the core (i.e., refractive-index values higher than 1.3242), with a Pearson’s r2 value of 0.986.

3.3. EFGI-POF The last fiber we analyzed had similar characteristics to the one manufactured by FiberFinn. For instance, the substrate and dopant materials, as well as the fiber diameter, were the same as the first case. However, this fiber had the peculiarity of the organic dye Rhodamine 6G being added during the fabrication process. This dye is well characterized within the literature and its Raman peaks do not interfere with either the dopant or the substrate material peaks. However, the fluorescence effect of the dye could hide the Raman peaks if an incorrect laser source was used, such as the typical 532 nmMaterials light2020 source., 13, 2251 Therefore, the mapping was recorded using the same parameters as in the first 10case. of 12 The 3D plot is shown in Figure 9.

Materials 2019, 12, x FOR PEER REVIEW 10 of 12

Figure 9. The 3D refractive-index reconstruction of the experimental fluorescence GI-POF (EFGI- POF).

The area with the graded-index distribution is not as wide as the core diameter, with the radius μ of the graded area being 300 m. In the same way as for the previous cases, in Figure 10 we compare the values of the calculated refractive indexes with the ones obtained from the modeling, as well as the regressionFigure 9. The between 3D refractive-index them. reconstruction of the experimental fluorescence GI-POF (EFGI-POF).

(a) (b)

FigureFigure 10. 10.(a )(a Calculated) Calculated mean mean refractive-index refractive-index values values (black (black dots dots withwith errorerror bars)bars) of the EFGI-POF EFGI-POF andand the the refractive-index refractive-index distribution distribution obtained obtained from from the the modeling modeling (red(red solidsolid line).line). ( b) Regression Regression of of bothboth distributions. distributions. Considering the wide range of the presented GI-POF cases, the versatility of the technique is proven, as it is capable of reconstructing 3D maps of the refractive index of each sample. Additionally, From Figure 10a we can observe that the refractive index varies at different points of the fiber other metrics are also calculated, such as the azimuthal asymmetry or the difference between with the same radius, with the azimuthal asymmetry being 0.36% in the worst case. This shows that the calculated refractive index and the value provided by the manufacturers, showing all of the the quality of the fiber is not as high as in the two previous samples. Figure 10b shows a regression imperfections of the refractive index. The accuracy of these metrics could determine the quality and with a higher dispersion than the previous cases, linked with the poorer quality of the refractive- feasibility of the GI-POFs for the desired application. index profile of this GI-POF (r2 = 0.972). It is also remarkable that the fluorescent dye added to the 4.fiber Conclusions does not interfere with the measurements, which was expected due to the high molecular selectivity of the Raman spectroscopy. AConsidering novel method the for wide reconstructing range of the the refractive-indexpresented GI-POF profile cases, in GI-POFsthe versatilit is presented.y of the Thistechnique method is isproven, based onas it Raman is capable spectroscopy of reconstructing mapping 3D of maps the surface of the refractive of the GI-POFs index andof each has sample. been validated Additionally, with TM threeother di metricsfferent GI-POFs—twoare also calculated, of the such GI-POFs as the are az basedimuthal on PMMAasymmetry and theor otherthe difference is based onbetween Cytop the. Moreover,calculated these refractive fibers hadindex di ffanderent the dopants value inprovid ordered to obtainby the the manufacturers, desired graded-index showing profile, all of and the oneimperfections of them had of athe fluorescent refractive organic index. The dye accuracy added into of these the fiber metrics core. could For each determine of the threethe quality GI-POFs, and afeasibility 3D map ofof thethe refractive-indexGI-POFs for the desired was reconstructed application. and compared to the mathematical modeling provided by manufacturers. The variation of the refractive index per radius and the regression between5. Conclusions the calculated and the model were also analyzed, showing that actual GI-POFs have azimuthal A novel method for reconstructing the refractive-index profile in GI-POFs is presented. This method is based on Raman spectroscopy mapping of the surface of the GI-POFs and has been validated with three different GI-POFs—two of the GI-POFs are based on PMMA and the other is based on CytopTM. Moreover, these fibers had different dopants in order to obtain the desired graded- index profile, and one of them had a fluorescent organic dye added into the fiber core. For each of the three GI-POFs, a 3D map of the refractive-index was reconstructed and compared to the mathematical modeling provided by manufacturers. The variation of the refractive index per radius and the regression between the calculated and the model were also analyzed, showing that actual GI- POFs have azimuthal asymmetries, which were quantified. Thus, the presented method shows good

Materials 2020, 13, 2251 11 of 12 asymmetries, which were quantified. Thus, the presented method shows good potential in terms of GI-POF characterization, which is a critical parameters for high data capacity communications.

Author Contributions: J.Z. conceived and designed the experiments. M.A. and A.O.-G. performed the experiments. I.A. analyzed the data. All authors contributed to the scientific discussion and wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded in part by the Fondo Europeo de Desarrollo Regional (FEDER), in part by the Ministerio de Ciencia, Innovación y Universidades under project RTI2018-094669-B-C31, and in part by the GobiernoVasco/Eusko Jaurlaritza IT933-16, ELKARTEK KK-2019/00101 (µ4Indust), and ELKARTEK KK-2019/00051 (SMARTRESNAK). The work of Angel Ortega-Gomez is funded by a PhD fellowship from the Ministerio de Economia y Competitividad (Mineco) of Spain. Conflicts of Interest: The authors declare no conflict of interest.

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