Effective Cleaving Parameters for Multimode Gradient Index CYTOP

polymers

Article Eﬀective Cleaving Parameters for Multimode Gradient Index CYTOP Polymer Fiber

Ivan Chapalo 1,2,*, Antreas Theodosiou 3 , Georgii Pobegalov 1 , Sergei Chapalo 4, Kyriacos Kalli 3 and Oleg Kotov 1

1 Institute of Physics, Nanotechnology and Telecommunications, Peter the Great St. Petersburg Polytechnic University, 29, Polytechnicheskaya st., 195251 St. Petersburg, Russia; [email protected] (G.P.); [email protected] (O.K.) 2 Electromagnetism and Telecom Department, University of Mons, 31 Boulevard Dolez, 7000 Mons, Belgium 3 Photonics and Optical Sensors Research Laboratory (PhOSLab), Cyprus University of Technology, Nicolaou Saripolou 33, 3036 Limassol, Cyprus; [email protected] (A.T.); [email protected] (K.K.) 4 Institute of Civil Engineering, Peter the Great St. Petersburg Polytechnic University, 29, Polytechnicheskaya st., 195251 St. Petersburg, Russia; [email protected] * Correspondence: [email protected]

Received: 7 October 2020; Accepted: 21 October 2020; Published: 27 October 2020

Abstract: We experimentally address simple, low-cost and effective methods for the cleaving of multimode CYTOP optical fibers using razor blades. The quality of fiber end-face preparation depends on various parameters. The necessity of the near-field intensity pattern inspection for adequate evaluation of cleaved fiber end-faces is demonstrated. Razor blades of different manufacturers are evaluated for manual cleaving, as well as automated cleaving with controlled speed and temperature. The cleaving technique with both slowed motion of the razor blade and increased temperature up to 90 ◦C demonstrated the best quality of fiber end-faces. Typical cleaving defects are highlighted, whereas the cleave quality was characterized in terms of the light intensity profile emitted by the fiber in near field.

Keywords: CYTOP; polymer optical fiber; fiber cleave; multimode fiber; fiber optics

1. Introduction Polymer optical fiber (POF) draws significant interest from researchers and industry due to its flexibility, ease of use, low Young modulus, and safety in biomedicine [1–3]. Among POFs, the CYTOP fiber has radically low attenuation in the telecom transparency windows and high bandwidth that makes it the best candidate for fiber-to-the-home communication technologies; it has also been used extensively for research into optical fiber sensing [1,4,5]. Applications based on fiber Bragg gratings [6–9], intermodal interference [10], single-mode-multimode-single-mode structures [11], cladding micro waveguides [12] and Brillouin scattering [13] have been investigated. In general, POFs are low-cost solutions compared to their silica fiber counterparts, since they do not require expensive termination equipment and delicate preparation methods; their large cores and highly multimode and plastic nature means that the cleaving process of POFs can be made simply using a low-cost razor blade. However, in the case of CYTOP fiber, which also has a multimode core but with a gradient-index profile, certain tasks require very good quality of fiber end-faces to provide both controllable launching conditions and spatial intensity distribution measurements. These tasks may include core refractive index profile evaluation [14,15], modal power distribution measurement utilizing intensity profile at the multimode fiber (MMF) output facet [16,17], differential mode delay

Polymers 2020, 12, 2491; doi:10.3390/polym12112491 www.mdpi.com/journal/polymers Polymers 2020, 12, 2491 2 of 11 and bandwidth investigation and optimization using restricted mode launch [18–20]. In addition, the investigation and operation of intermodal fiber interferometers (IFIs) (or specklegram sensors) require controlled launch and undistorted speckle pattern, in particular for IFI property manipulation that is dependent on launching conditions [21] and the coordinates of photodetectors at the fiber cross-section [22]. Furthermore, a good-quality end-face leads to a more precise and low-loss fiber interconnections that can enhance the transmission characteristics of fiber optic links. There are several studies that relate to cleaving POFs and, in particular, microstructure fiber designs where the integrity of air holes is important after the cleaving process. For this reason, a variety of fiber end-face preparation techniques were explored including simple cut by razor blade [23], polishing [24,25], low-temperature cleaving [26,27], focused ion-beam milling [27,28], semiconductor dicing saw and ultraviolet laser cleaving [28]. They were applied not only for polymethyl methacrylate (PMMA) materials but also, for example, cyclic olefin copolymer (TOPAS) fibers [28,29]. The razor blades (RB) approach is the most common and simple method for which different aspects of RB cleaving have been investigated, such as cleaving temperature [27] and speed of cleave [30]; indeed, portable cleavers have been proposed and tested [31]. It was demonstrated that proper blade temperature was required so as not to exceed the glass transition temperature of the material thereby significantly improving the cleaving quality [27,29]. Articles [30,31] show that fiber heating could be replaced by having blade motion with slow speed, due to the temperature–time equivalence in polymers. In addition, it was highlighted that the blade-motion component perpendicular to cleaving direction helped to reduce “crazing” on fiber facet surface. In recent years, significant polymer-sensing applications were developed using multimode CYTOP gradient index fibers, and the focus of this work aimed to investigate the optimum cleaving conditions for particular POFs using the RB method. We demonstrate that near-field intensity pattern (NFIP) inspection is a necessary procedure for evaluation of the cleaving quality along with the microscope inspection of the cleaved fiber facet. RB cleaving quality is evaluated for a sample of different commercially available razor blades. A novel setup based on a motorized rotation stage was assembled, tested, and applied in experiments. The influence of cleaving speed and temperature was explored, and it was found that the best results can be achieved when both increased temperature and slow motion of the blade are applied. Different types of cleave defects are also demonstrated.

2. Experimental Setup GigaPOF-50SR (Chromis Fiberoptics (Warren, NJ, USA)) fiber was utilized in experiments. The following methods of RB cleaving were investigated: manual cleave, cleave with controlled speed, cleave with controlled temperature, and their combinations. Firstly, we performed simple manual cleaves of POF using RBs from different manufacturers (a sample of 6 RB models produced by Bic (Clichy, France), Derby (Tuzla, Turkey), Feather (Osaka, Japan), Gillette (Boston, MA, USA), Sputnik (St. Petersburg, Russia), and Wilkinson (London, UK)) and a surgical scalpel produced by B. Braun (Melsungen, Germany). Experiments with each RB model were performed multiple times, ensuring that the given part of the blade was used for cleave only once. In order to investigate possibilities to improve cleaving quality and stability, we assembled a setup based on motorized rotational stage (Newport Agilis AG-PR100 (Irvine, CA, USA)), which enabled us to perform cleaving with a controlled speed (Figure1a). The stage was fixed horizontally using specially designed and 3D-printed holders. As demonstrated in [30], improved cleaving quality can be achieved if the blade has a significant crosswise component of movement with respect to the direction of cleave. Following this recommendation, we faced the blade tangentially to the direction of its rotation. At that, the length l of the RB involved in cleaving can be estimated as l (2 d R)0.5, where d ≈ · · is the fiber diameter and R is the blade’s distance from the axis of rotation (Figure1b). To increase the blade’s crosswise movement component, we fixed the razor significantly far from the axis of rotation using a standard CD disc with a diameter of 120 mm mounted on a duralumin cylinder screwed into the rotation stage. The length of the blade involved in cleaving was estimated to be 7.8 mm. To slightly Polymers 2020, 12, 2491 3 of 11 Polymers 2020, 20, x FOR PEER REVIEW 3 of 11 increaseedge of the this razor value, (relatively we turned to the the blade direction to a small of blade angle motion) so that thehad front a larger edge distance of the razor from (relatively the axis toof therotation direction than ofthe blade rear motion)edge. had a larger distance from the axis of rotation than the rear edge.

Figure 1.1.( a(a)) A A photograph photograph of theof cleavingthe cleaving setup setup based base on thed on motorized the motorized rotational rotational stage; (b )stage; a schematic (b) a ofschematic the cleaving of the geometry cleaving calculation;geometry calculation; (c) a drawing (c) ofa drawing the ﬁber of holder the fiber stage. holder stage.

To holdhold thethe fiber,fiber, wewe utilizedutilized aa hypodermichypodermic needleneedle ofof standardstandard G21G21 withwith anan innerinner diameterdiameter of 500 µµmm (the(the outer outer diameter diameter of of the the GigaPOF GigaPOF fiber fiber was was 490 490µm); µm); the the needle-syringe needle-syringe plastic plastic connector connector was removedwas removed beforehand. beforehand. The needle The needle was fixed was fixed on a specially on a specially designed designed and 3D-printed and 3D-printed vertical vertical stage which stage containedwhich contained a horizontal a horizontal 0.3 mm 0.3 slit mm (resolution slit (resolution limit oflimit the of utilized the utilized 3D printer) 3D printer) for the for RBthe (Figure RB (Figure1c); the1c); endthe ofend the of needle the needle was locatedwas located in close in close proximity proximity to the to slit. the Hence, slit. Hence, utilizing utilizing the rotational the rotational stage andstage fixing and thefixing fiber the at fiber the vertical at the stage,vertical we stage, could we perform could reproducibleperform reproducible RB cleaving RB with cleaving controlled with speed,controlled whilst speed, enabling whilst tangential enabling componenttangential component of the motion. of the motion. The motorized stage provided three levels of rotationrotation rate, which were calculated to be 0.008, 0.19 and 3.6 degreesdegrees/s./s. The tangential velocity of the razor was estimated to be 8 µm/s,µm/s, 210 µµm/sm/s and 4 mmmm/s,/s, correspondingly.correspondingly. During cleaving, due to thethe resistance of the fiberfiber material, the real speedspeed of rotationrotation was slightly decreased depending on the razor blade model and cleaving temperature. TheThe averagedaveraged cleavingcleaving timetime waswas measuredmeasured toto bebe 1414 min,min, 2828 s and 2 s, correspondingly with the levels of rotation rate.rate. InIn addition to didifferentfferent RBRB modelsmodels andand cleaving rates, we provided the possibility of performing cleaving atat di differentfferent temperatures. temperatures. To To avoid avoid the the complexity complexity of the of setup the setup we did we not did control not control the heating the ofheating the RB of andthe theRB and fiber the separately; fiber separately; we simply we utilizedsimply utilized a household a household hair dryer hair located dryer located at a proper at a distanceproper distance from the from setup the to setup provide to provide approximate approximate necessary necessary temperature temperature that was that calibrated was calibrated in advance. in Thisadvance. enabled This us enabled to perform us to cleaving perform at cleaving room temperature, at room temperature, as well as atas temperatureswell as at temperatures up to 100 ◦ upC. to 100 °C.To evaluate the cleaving quality, we recorded microscope images of a cleaved facet using 10 and × 40 microscopeTo evaluate objectives. the cleaving This quality, helped we us torecorded qualitatively microscope evaluate images the appearance of a cleaved of thefacet cleave, using such 10× × asand the 40× flatness microscope and the objectives. shape of This a cross-section. helped us to Here, qualitatively in fact, the evaluate quality the of polycarbonateappearance of the protection cleave, claddingsuch as the the flatness surface wasand mainlythe shape evaluated. of a cross-section. Additionally, Here, we recorded in fact, the NFIPs quality when of incoherent polycarbonate light wasprotection launched cladding into the the fiber. surface The was LED mainly of a smartphone, evaluated. Additionally, 20 microscope we objectiverecorded and NFIPs USB when web × cameraincoherent Logitech light was c270 launched (Lausanne, into Switzerland) the fiber. The were LED used of a for smartphone, this purpose. 20× The microscope NFIPs were objective utilized and as anUSB indicator web camera of the Logitech cleaved c270 CYTOP (Lausanne, core region Switzerlan quality.d) were A clean used gradient for this intensitypurpose. profileThe NFIPs proved were a successfulutilized as cleave,an indicator whereas of the the cleaved presence CYTOP of roughness core region and irregularities quality. A clean indicated gradient that intensity the cleave profile was notproved good a enough.successful cleave, whereas the presence of roughness and irregularities indicated that the cleaveTo was evaluate not good cleaving enough. quality numerically, we introduced the parameter of irregularity KI according to theTo formula evaluate cleaving quality numerically,s we introduced the parameter of irregularity KI according to the formula PN ( s)2 i=1 xi xi 100% KI = − (1) N · xs ∑ 𝑥 𝑥 100% (1) where x is an intensity distribution over𝐾 a chosen cross-section⋅ of the NFIP, xs is a smoothed intensity i 𝑁 𝑥 i distribution over a chosen cross-section of the NFIP, i is a pixel number at the NFIP cross-section, N is a where xi is an intensity distribution over a chosen cross-section of the NFIP, 𝑥 is a smoothed intensity distribution over a chosen cross-section of the NFIP, i is a pixel number at the NFIP cross-

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𝑥 section,number N of is pixels a number at the of NFIP pixels cross-section at the NFIP cross-section and xs is a mean and value is ofa mean smoothed value NFIPof smoothed cross-section. NFIP cross-section. The algorithm of the KI calculation is illustrated in Figure 2. The intensity distribution The algorithm of the KI calculation is illustrated in Figure2. The intensity distribution over a chosen overcross-section a chosen of cross-section the NFIP was of smoothedthe NFIP (Figurewas smoothed2a), and the(Figure smoothed 2a), and intensity the smoothed distribution intensity was distributionsubtracted from was the subtracted initial intensity from distributionthe initial intensity (Figure2 b).distribution The irregularity (Figure coe 2b).ﬃcient The was irregularity obtained coefficientby calculating was theobtained standard by calculating deviation of the the standard diﬀerence, deviation normalizing of the it difference, by the mean normalizing value of smoothed it by the meanintensity value distribution of smoothed and intensity multiplying distribution by 100%. and To multiplying avoid complexity by 100%. of To processing avoid complexity algorithms, of processing algorithms, we calculated the KI for two mutually perpendicular cross-sections of the we calculated the KI for two mutually perpendicular cross-sections of the NFIP and utilized the NFIPmaximum and utilized of these the two maximum values. of these two values.

FigureFigure 2. IllustrationIllustration of of the the irregularity coefficient coefficient calculation: ( (aa)) original original and and smoothed smoothed intensity intensity distributiondistribution in the NFIP cross section and ( b) their difference. difference. 3. Experimental Results and Discussion 3. Experimental Results and Discussion CYTOP fiber is a multimode graded-index fiber; therefore it is important to provide enough CYTOP fiber is a multimode graded-index fiber; therefore it is important to provide enough smoothness of its end-face, especially at its core region. We have found that unlike microstructured smoothness of its end-face, especially at its core region. We have found that unlike microstructured POFs, for which cleave quality is usually evaluated by inspection of an air hole’s structural integrity POFs, for which cleave quality is usually evaluated by inspection of an air hole’s structural integrity by viewing a microscopic image of a fiber facet, a microscopic image of the CYTOP fiber facet does by viewing a microscopic image of a fiber facet, a microscopic image of the CYTOP fiber facet does not always provide an adequate representation of the cleave quality. To demonstrate this, a set of not always provide an adequate representation of the cleave quality. To demonstrate this, a set of microscopic images of particular cleaved fiber facets and the NFIP when light from an incoherent microscopic images of particular cleaved fiber facets and the NFIP when light from an incoherent source was launched into the fiber are presented in Figure3: the image of the cleaved end-face source was launched into the fiber are presented in Figure 3: the image of the cleaved end-face (Figure (Figure3a), the image of the CYTOP core-cladding region (Figure3b), the same but when white 3a), the image of the CYTOP core-cladding region (Figure 3b), the same but when white light was light was launched into the fiber (Figure3c), and the NFIP (Figure3d). It is obvious that despite a launched into the fiber (Figure 3c), and the NFIP (Figure 3d). It is obvious that despite a respectively respectively clean image of the core region of the cleaved end-face (Figure3b), the near-field intensity clean image of the core region of the cleaved end-face (Figure 3b), the near-field intensity distribution distribution indicates a significantly disturbed surface (Figure3d). Hence, in this work, to adequately indicates a significantly disturbed surface (Figure 3d). Hence, in this work, to adequately evaluate evaluate the cleaving quality, we analyzed NFIP along with microscopic images of the cleaved facet. the cleaving quality, we analyzed NFIP along with microscopic images of the cleaved facet. Smooth Smooth parabola-shaped near-field profile distribution indicated a successful cleave. parabola-shaped near-field profile distribution indicated a successful cleave. For reference, in Figure4a–f we demonstrate examples of a bad-quality cleave and corresponding horizontal intensity profiles (obtained from the core center). The cleaves were obtained using the manual razor blade cleaving technique. It can be seen that the intensity profiles are almost destroyed (Figure4b,d,f). In addition, Figure4g,h compares near-field speckle patterns recorded for poor- (Figure4g) and good-quality (Figure4h) surfaces of the CYTOP fiber facets. It is obvious that a poor cleave results in degradation of the speckle pattern appearance: decomposition of the speckles to smaller components and the presence of dark regions.

Figure 3. Comparison of microscopic image (a–c) and near-field pattern (d) end-face inspection methods: the near-field profile pattern demonstrates defects (d) that can be missed by the microscopic

Polymers 2020, 20, x FOR PEER REVIEW 4 of 11 section, N is a number of pixels at the NFIP cross-section and 𝑥 is a mean value of smoothed NFIP cross-section. The algorithm of the KI calculation is illustrated in Figure 2. The intensity distribution over a chosen cross-section of the NFIP was smoothed (Figure 2a), and the smoothed intensity distribution was subtracted from the initial intensity distribution (Figure 2b). The irregularity coefficient was obtained by calculating the standard deviation of the difference, normalizing it by the mean value of smoothed intensity distribution and multiplying by 100%. To avoid complexity of processing algorithms, we calculated the KI for two mutually perpendicular cross-sections of the NFIP and utilized the maximum of these two values.

Figure 2. Illustration of the irregularity coefficient calculation: (a) original and smoothed intensity distribution in the NFIP cross section and (b) their difference.

3. Experimental Results and Discussion CYTOP fiber is a multimode graded-index fiber; therefore it is important to provide enough smoothness of its end-face, especially at its core region. We have found that unlike microstructured POFs, for which cleave quality is usually evaluated by inspection of an air hole’s structural integrity by viewing a microscopic image of a fiber facet, a microscopic image of the CYTOP fiber facet does not always provide an adequate representation of the cleave quality. To demonstrate this, a set of microscopic images of particular cleaved fiber facets and the NFIP when light from an incoherent source was launched into the fiber are presented in Figure 3: the image of the cleaved end-face (Figure 3a), the image of the CYTOP core-cladding region (Figure 3b), the same but when white light was launched into the fiber (Figure 3c), and the NFIP (Figure 3d). It is obvious that despite a respectively clean image of the core region of the cleaved end-face (Figure 3b), the near-field intensity distribution indicates a significantly disturbed surface (Figure 3d). Hence, in this work, to adequately evaluate the cleaving quality, we analyzed NFIP along with microscopic images of the cleaved facet. Smooth Polymers 2020, 12, 2491 5 of 11 parabola-shaped near-field profile distribution indicated a successful cleave. Polymers 2020, 20, x FOR PEER REVIEW 5 of 11

image (b). (a) cleaved end-face, (b) CYTOP core-cladding region, (c) CYTOP core-cladding region when white light was launched into the fiber, (d) near-field intensity pattern.

For reference, in Figure 4a–f we demonstrate examples of a bad-quality cleave and corresponding horizontal intensity profiles (obtained from the core center). The cleaves were obtained using the manual razor blade cleaving technique. It can be seen that the intensity profiles

are almost destroyed (Figure 4b,d,f). In addition, Figure 4g,h compares near-field speckle patterns Figure 3. Comparison of microscopic image (a–c) and near-field pattern (d) end-face inspection recordedFigure for 3. poor-Comparison (Figure of 4g) microscopic and good-quality image (a –(Figurc) ande near-field4h) surfaces pattern of the (d )CYTOP end-face fiber inspection facets. It is methods: the near-field profile pattern demonstrates defects (d) that can be missed by the microscopic obviousmethods: that athe poor near-field cleave profile results pattern in degradation demonstrates of defects the speckle (d) that pattern can be missed appearance: by the microscopic decomposition image (b). (a) cleaved end-face, (b) CYTOP core-cladding region, (c) CYTOP core-cladding region of the speckles to smaller components and the presence of dark regions. when white light was launched into the fiber, (d) near-field intensity pattern.

Figure 4. ThreeThree examples of poor-quality cleaves of the CYTOP fiber: fiber: near-field near-field patterns, ( aa,,cc,,ee)) and and intensityintensity profiles,profiles, ((bb,,dd,,ff)) generatedgenerated by by incoherent incoherent fiber fiber launch. launch. The The dashed dashed line line indicates indicates the the location location of ofthe the near-field near-field profile profile at theat the near-field near-field pattern. pattern. Comparison Comparison of speckleof speckle patterns patterns formed formed at poor at poor (g) and(g) andsuccessful successful (h) cleaved(h) cleaved end-faces end-faces when when the fiberthe fiber was was launched launched using using a coherent a coherent source. source. The investigation of the CYTOP fiber cleaving was performed as follows: manual cleaving The investigation of the CYTOP fiber cleaving was performed as follows: manual cleaving exploring different RB models and selection of appropriate RBs, cleaving with different speed, exploring different RB models and selection of appropriate RBs, cleaving with different speed, and and cleaving at different temperature and speed. cleaving at different temperature and speed. 3.1. Manual Razor Blade Cleave 3.1. Manual Razor Blade Cleave Typical microscope images, NFIPs, near-field intensity profiles (two mutually perpendicular Typical microscope images, NFIPs, near-field intensity profiles (two mutually perpendicular profiles for each NFIP) of successful cleaves and the parameter of irregularity KI for each blade model profiles for each NFIP) of successful cleaves and the parameter of irregularity KI for each blade model are presented in Figure5. It can be seen that cleave quality (and, in particular, quality of NFIP) is far are presented in Figure 5. It can be seen that cleave quality (and, in particular, quality of NFIP) is far from ideal and significantly depends on the blade’s model. For our sample of razor blades, taking into from ideal and significantly depends on the blade’s model. For our sample of razor blades, taking account the photographs of NFIPs (column 2 in Figure5) in conjunction with horizontal and vertical into account the photographs of NFIPs (column 2 in Figure 5) in conjunction with horizontal and profile graphs (columns 3–4 in Figure5) and irregularity coe fficients, the best intensity profiles were vertical profile graphs (columns 3–4 in Figure 5) and irregularity coefficients, the best intensity obtained by razor blades Bic (KI = 1.9%), Wilkinson (KI = 3.5%) and Feather (KI = 4.7%). The surgical profiles were obtained by razor blades Bic (KI = 1.9%), Wilkinson (KI = 3.5%) and Feather (KI = 4.7%). scalpel demonstrated the worst results (KI = 10.9%), most likely because of its higher thickness and The surgical scalpel demonstrated the worst results (KI = 10.9%), most likely because of its higher insufficient sharpness in comparison with razor blades. thickness and insufficient sharpness in comparison with razor blades. It is important to note, that multiple cleaving using even “successful” RB models did not always It is important to note, that multiple cleaving using even “successful” RB models did not always demonstrate stable (reproducible) results (Figure6); consecutive cleaves randomly provided very demonstrate stable (reproducible)1 2 results (Figure3 6); consecutive cleaves randomly provided very different quality: KI = 1.9, KI = 14.3 and KI = 5.3 (in the experiments, we ensured that every different quality: KI1 = 1.9, KI2 = 14.3 and KI3 = 5.3 (in the experiments, we ensured that every cleaving cleaving part of the blade was used only once). It should be mentioned that cleaved fiber facets had a part of the blade was used only once). It should be mentioned that cleaved fiber facets had a typical typical bevel at one of their edges; however, this did not affect the NFIP quality, as it occurred in the bevel at one of their edges; however, this did not affect the NFIP quality, as it occurred in the polycarbonate protection cladding. polycarbonate protection cladding. Thus, it can be concluded that manually performed cleaving can produce sufficiently good Thus, it can be concluded that manually performed cleaving can produce sufficiently good end- end-faces (depending on the task); however, the razor blade model should be carefully selected. faces (depending on the task); however, the razor blade model should be carefully selected. With this, With this, even properly selected RBs do not guarantee reproducible high-accuracy cleaves. Therefore, even properly selected RBs do not guarantee reproducible high-accuracy cleaves. Therefore, in situations when cleaving quality is critical, each cleave has to be inspected by microscope imaging and by observing near-field intensity profiles to ensure sufficient quality.

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Polymersin situations 2020, 20 when, x FOR cleaving PEER REVIEW quality is critical, each cleave has to be inspected by microscope imaging6 of 11 Polymersand by 2020 observing, 20, x FOR near-field PEER REVIEW intensity profiles to ensure sufficient quality. 6 of 11

Figure 5. Typical successful cleaves obtained by different razor blade models. Dashed line indicates Figure 5. Typical successful cleaves obtained by different razor blade models. Dashed line indicates Figurethe location 5. Typical of the successful near-field cleaves profile atobtained the near-field by different pattern. razor blade models. Dashed line indicates thethe location location of of the the near-field near-field profile profile at at the the near-field near-field pattern. pattern.

FigureFigure 6. 6. ThreeThree consecutive consecutive cleaves cleaves (obtained (obtained by by Bic Bic razor razor blade): blade): end-face end-face quality quality is is not not stable stable cleave cleave Figurebyby cleave. cleave. 6. Three consecutive cleaves (obtained by Bic razor blade): end-face quality is not stable cleave by cleave. 3.2. Cleave with Controlled Speed 3.2. Cleave with Controlled Speed 3.2. CleaveThe automatedwith Controlled setup Speed based on a motorized rotational stage (Figure1) enabled control of the The automated setup based on a motorized rotational stage (Figure 1) enabled control of the cleaving speed and direction. Three possible values of the rotation speed provided by the stage were cleavingThe speedautomated and direction. setup based Three on possible a motorized values rotati of theonal rotation stage speed(Figure provided 1) enabled by thecontrol stage of were the investigated for POF cleaving. We tested Bic, Wilkinson and Feather razor blades selected in the cleavinginvestigated speed for and POF direction. cleaving. Three We possibletested Bic, values Wilk ofinson the rotation and Feather speed razor provided blades by selectedthe stage in were the previous section. The last two razor blade models were successfully applied, while the Bic razor blade investigatedprevious section. for POF The lastcleaving. two razor We bladetested models Bic, Wilk weinsonre successfully and Feather applied, razor while blades the selectedBic razor in blade the previouscleaves were section. unsuccessful: The last two the razor razor blade blade models got westuckre successfully during the applied, cleaving while process the Bicand razor the bladestage cleavesstopped were the rotation.unsuccessful: Possible the reasonsrazor blade for thisgot stcouckuld duringbe the thehigher cleaving thickness process of theand Bic the blades stage stopped the rotation. Possible reasons for this could be the higher thickness of the Bic blades

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Polymers 2020, 20, x FOR PEER REVIEW 7 of 11 Polymerscleaves 2020 were, 20 unsuccessful:, x FOR PEER REVIEW the razor blade got stuck during the cleaving process and the stage stopped7 of 11 comparedthe rotation. to Possiblethe others reasons (we measured for this could it as be0.11–0.12 the higher mm thickness versus 0.1 of mm the Bicfor bladesthe other compared blades) toand, the compared to the others (we measured it as 0.11–0.12 mm versus 0.1 mm for the other blades) and, probably,others (we the measured different it asprofile 0.11–0.12 and/or mm angle versus of the 0.1 mmblade for edge. the other blades) and, probably, the different probably, the different profile and/or angle of the blade edge. profileFigure and/ or7 demonstrates angle of the blade the cleaving edge. results obtained using the Feather RB. In general, the ability Figure 7 demonstrates the cleaving results obtained using the Feather RB. In general, the ability to controlFigure the7 demonstrates speed of the razor the cleaving blade movement results obtained improved using cleaving the Feather quality RB. In (in general, the CYTOP the ability region) to to control the speed of the razor blade movement improved cleaving quality (in the CYTOP region) andcontrol reproducibility the speed of of the the razor results. blade At movement that, medium improved (28 s for cleaving one cleave) quality speed (in the of CYTOProtation region) improved and and reproducibility of the results. At that, medium (28 s for one cleave) speed of rotation improved cleavingreproducibility quality of compared the results. to At th that,e fastest medium (2 s (28per s cleave) for one cleave)speed ( speedKI = 2.1% of rotation versus improvedKI = 5.9%), cleaving which cleaving quality compared to the fastest (2 s per cleave) speed (KI = 2.1% versus KI = 5.9%), which correspondsquality compared to the to results the fastest presented (2 s per in cleave)[30]. Howeve speedr, (K theI = slowest2.1% versus (14 minKI = per5.9%), cleave) which rotation corresponds speed corresponds to the results presented in [30]. However, the slowest (14 min per cleave) rotation speed didto the not results demonstrate presented significant in [30]. impr However,ovement the of slowest the cleave’s (14 min quality per cleave)(KI = 3.4%), rotation and was speed very did time not did not demonstrate significant improvement of the cleave’s quality (KI = 3.4%), and was very time consuming.demonstrate Thus, significant making improvement sure of the ofabsence the cleave’s of significant quality ( Keffect,I = 3.4%), we excluded and was verythe slowest time consuming. rotation consuming. Thus, making sure of the absence of significant effect, we excluded the slowest rotation speedThus, of making cleaving sure from of thefurther absence experiments. of significant It shou effldect, be mentioned we excluded that the the slowest quality rotation and the speedshape of of speed of cleaving from further experiments. It should be mentioned that the quality and the shape of thecleaving end-faces from in further this case experiments. was even wors It shoulde than in be the mentioned case of manual that the cleave. quality and the shape of the the end-faces in this case was even worse than in the case of manual cleave. end-faces in this case was even worse than in the case of manual cleave.

Figure 7. Typical results of cleaves performed at different speeds with the Feather razor blade (RB). FigureFigure 7. 7. TypicalTypical results results of of cleaves cleaves performed performed at at differe diﬀerentnt speeds speeds with with the the Feather Feather razor razor blade blade (RB). (RB). The cleaving time was 2 s, 28 s and 14 min (from top to bottom). TheThe cleaving cleaving time time was was 2 2 s, s, 28 28 s s and and 14 14 min (from top to bottom). To investigate the possibility of increasing cleaving quality, we conducted the aforementioned ToTo investigate the the possibility possibility of of increasing increasing cleaving cleaving quality, quality, we conducted the the aforementioned aforementioned experiment utilizing the Wilkinson RB (Figure 8), which demonstrated better cleaving quality of experimentexperiment utilizingutilizing thethe Wilkinson Wilkinson RB RB (Figure (Figure8), which 8), which demonstrated demonstrated better better cleaving cleaving quality quality of manual of manualcleave in cleave comparison in comparison with the Featherwith the RB Feather (K = 3.5% RB versus(KI = 3.5%K = 4.7%).versus TheKI = results 4.7%). demonstrated The results manual cleave in comparison with the FeatherI RB (KI = 3.5%I versus KI = 4.7%). The results demonstrated a slight improvement of the NFIP quality, especially at a high speed of RB motion (3.6 demonstrateda slight improvement a slight improvement of the NFIP quality,of the NFIP especially quality, at especially a high speed at a ofhigh RB speed motion of RB (3.6 motion degrees (3.6/s): degrees/s):K = 3.5% versus KI = 3.5%K = versus5.9% at K 3.6I = 5.9% degrees at 3.6/s and degrees/sK = 1.5% and versus KI = 1.5%K = versus2.1% at K 0.19I = 2.1% degrees at 0.19/s. Thedegrees/s. quality degrees/s):I KI = 3.5%I versus KI = 5.9% at 3.6 degrees/sI and KI = 1.5%I versus KI = 2.1% at 0.19 degrees/s. The quality and the shape of the end-faces were increased as well. Taking this into account, we Theand quality the shape and of the the shape end-faces of the were end-faces increased were as in well.creased Taking as well. this Taking into account, this into we account, conducted we conducted temperature experiments utilizing the Wilkinson RB. conductedtemperature temperature experiments experiments utilizing the utilizing Wilkinson the RB.Wilkinson RB.

FigureFigure 8. 8. TypicalTypical results results of of cleaves cleaves performed performed at at two two different diﬀerent speeds speeds with with the the Wilkinson Wilkinson RB. RB. Cleaving Cleaving Figure 8. Typical results of cleaves performed at two different speeds with the Wilkinson RB. Cleaving timetime was was 2 2 s s and and 28 28 s. s. The The slowest slowest speed speed (14 (14 min min per per cleave) cleave) was was excluded excluded from from the the experiments. experiments. time was 2 s and 28 s. The slowest speed (14 min per cleave) was excluded from the experiments. 3.3. Cleave with Controlled Speed and Temperature 3.3. Cleave with Controlled Speed and Temperature In order to investigate cleaving at temperatures close to the glass transition temperature of the In order to investigate cleaving at temperatures close to the glass transition temperature of the material as recommended in literature [23], we investigated cleaves at temperatures from 70 to 100 material as recommended in literature [23], we investigated cleaves at temperatures from 70 to 100 °C with the step of 10 °C (the glass transition temperature of the CYTOP material is 108 °C [32]). The °C with the step of 10 °C (the glass transition temperature of the CYTOP material is 108 °C [32]). The

Polymers 2020, 12, 2491 8 of 11

3.3. Cleave with Controlled Speed and Temperature

PolymersIn 2020 order, 20, x to FOR investigate PEER REVIEW cleaving at temperatures close to the glass transition temperature8 of 11 of Polymersthe material 2020, 20 as, x recommendedFOR PEER REVIEW in literature [23], we investigated cleaves at temperatures from8 70of 11 to rotation100 ◦C with speed the of step 0.19 of degrees/s 10 ◦C (the (28 glass s per transition cleave) temperaturewas utilized ofaccording the CYTOP to results material reported is 108 ◦ C[in 32the]). previousrotationThe rotation speedsection. speed of The 0.19 of results 0.19degrees/s degrees of the (28 experiments/s s (28 per s cleave) per cleave)dem wasonstrated wasutilized utilized essential according according improvements to results to results reported in the reported cleaving in the in qualitypreviousthe previous and section. stability section. The (reproducibility); results The results of the experiments of the the experiments optima deml temperatureonstrated demonstrated essential was found essential improvements to be improvements at proximity in the cleaving of in 80– the quality and stability (reproducibility); the optimal temperature was found to be at proximity of 80– 90cleaving °C (Figure quality 9). andIt can stability be seen (reproducibility); that the intensity the profile optimal had temperature a very smooth was graded-index found to be at shape proximity (KI = 90 °C (Figure 9). It can be seen that the intensity profile had a very smooth graded-index shape (KI = 0.7%).of 80–90 Moreover,◦C (Figure it 9was). It found can be that seen the that same the intensityblade region profile can had be autilized very smooth not once graded-index but several shapetimes 0.7%). Moreover, it was found that the same blade region can be utilized not once but several times without(KI = 0.7%). significant Moreover, degradation it was found of the that cleaving the same quality. blade We region performed can be 20 utilized consecutive not once cleaves but severalby the samewithouttimes blade without significant region, significant anddegradation at degradationleast the of firstthe of cleaving 15 the cleave cleaving quas didlity. quality. not We demonstrate performed We performed significant20 consecutive 20 consecutive degradation cleaves cleaves byof the by same blade region, and at least the first 15 cleaves did not demonstrate significant degradation of the cleavethe same quality, blade namely region, the and irregularity at least the parameter first 15 cleaves increased did not from demonstrate KI = 0.7% for significant the first cleave degradation up to K ofI cleave quality, namely the irregularity parameter increased from KI = 0.7% for the first cleave up to KI =the 1% cleave for the quality, 15th cleave. namely Figure the irregularity9b demonstrates parameter near-field increased speckle from patternsKI = 0.7% when for the the POF first was cleave excited up to = 1% for the 15th cleave.th Figure 9b demonstrates near-field speckle patterns when the POF was excited byKI =the1% coherent for the 15lightcleave. source Figure(λ = 1.39b µm) demonstrates in underfilled near-field and multimode speckle patterns regimes. when It is theobvious POF wasthat specklebyexcited the coherent bypatterns the coherent lighthave source proper light source(λ view = 1.3 (λ (shape)µm)= 1.3 inµ underfilledandm) in do underfilled not anddemonstrate multimode and multimode any regimes. degradation regimes. It is obvious It unlike is obvious thatthe examplespecklethat speckle patternsin Figure patterns have 4g; havethey proper demonstrate proper view view (shape) good (shape) andability and do doto not control not demonstrate demonstrate launching any anyconditions. degradation degradation unlike unlike the exampleexample in Figure 44g;g; theythey demonstratedemonstrate goodgood abilityability toto controlcontrol launchinglaunchingconditions. conditions.

Figure 9. (a) Cleaves performed at temperature ≈ 90 °C and rotation speed of 0.19 degrees/s (28 s per Figure 9. (a) Cleaves performed at temperature 90 C and rotation speed of 0.19 degrees/s (28 s cleave)Figure 9.using (a) Cleaves the Wilkinson performed RB. at Two temperature examples ≈ 90(cle≈ °Cave ◦and no. rotation 1 and speedcleave ofno. 0.19 15) degrees/s of a series (28 ofs per20 per cleave) using the Wilkinson RB. Two examples (cleave no. 1 and cleave no. 15) of a series of consecutivecleave) using cleaves the Wilkinson performed RB. using Two theexamples same part (cle aveof the no. same 1 and RB. cleave (b) Near-fieldno. 15) of imagesa series of of the 20 20 consecutive cleaves performed using the same part of the same RB. (b) Near-field images of the speckleconsecutive patterns cleaves obtained performed using distusingributed the same feedback part (DFB)of the laser same with RB. λ( b=) 1.3 Near-field µm and aimages single-mode of the speckle patterns obtained using distributed feedback (DFB) laser with λ = 1.3 µm and a single-mode pigtailspeckle utilizing patterns central obtained and using offset dist launch.ributed feedback (DFB) laser with λ = 1.3 µm and a single-mode pigtail utilizing central and offset offset launch. InIn addition addition to to the the obtained obtained results, results, we we invest investigatedigated cleaves cleaves that that were were performed performed at at the the same same In addition to the obtained results, we investigated cleaves that were performed at the same temperaturetemperature but but with with rotation rotation speed speed of of 3.6 degreesdegrees/s/s (2 ss perper cleave)cleave) (Figure(Figure 1010).). The The results results temperature but with rotation speed of 3.6 degrees/s (2 s per cleave) (Figure 10). The results demonstrated slight degradation of NFIP (KI = 1.8%) but more distorted flatness of the end-face demonstrated slight degradation of NFIP (KI = 1.8%) but more distorted flatness of the end-face demonstrated slight degradation of NFIP (KI = 1.8%) but more distorted flatness of the end-face comparedcompared to previous rotation speed.speed. Nevertheless,Nevertheless, itit cancanbe be concluded concluded that that the the quality quality of of the the NFIP NFIP is compared to previous rotation speed. Nevertheless, it can be concluded that the quality of the NFIP isstill still good good enough. enough. is still good enough.

Figure 10. Cleave performed at temperature 90 C and rotation speed of 3.6 degrees /s (2 s per cleave) Figure 10. Cleave performed at temperature ≈≈ 90 °C◦ and rotation speed of 3.6 degrees/s (2 s per cleave) usingFigureusing the the 10. Wilkinson WilkinsonCleave performed RB. RB. at temperature ≈ 90 °C and rotation speed of 3.6 degrees/s (2 s per cleave) using the Wilkinson RB. 3.4. Cleaving Defects 3.4. Cleaving Defects 3.4. CleavingDuring Defects the experiments, we separated typical cleaving defects into three main groups: crazing, During the experiments, we separated typical cleaving defects into three main groups: crazing, grooves and cleaving product. The ﬁrst group can be observed as random POF facet imperfections: groovesDuring and thecleaving experiments, product. we The separated first group typical can beclea observedving defects as random into three POF main facet groups: imperfections: crazing, Figure4a,c is a bright examples of that. The second group demonstrates a set of groves oriented along Figuregrooves 4a,c and is cleavinga bright examples product. ofThe that. first The group seco cannd group be observed demonstrates as random a set POFof groves facet orientedimperfections: along the direction of the blade movement. It can be observed in Figure4e, Figure5 (surgical scalpel) and theFigure direction 4a,c is ofa bright the blade examples movement. of that. It The can seco be obsend grouprved in demonstrates Figure 4e, Figure a set of 5 groves(surgical oriented scalpel) along and Figure6 (cleave 2). It is likely related to the razor blades’ quality. The third group has the apparent Figurethe direction 6 (cleave of the2). Itblade is likely movement. related Itto canthe berazo obser blades’rved in quality. Figure The4e, Figure third group 5 (surgical has the scalpel) apparent and defects,Figure 6 namely (cleave the2). Itpresence is likely of related dust and to the pieces razo ofr blades’material quality. after the The cleaving third group process. has They the apparent appears atdefects, the NFIP namely as separated the presence dots ofor dust circles and and pieces can beof materialremoved after by a theseries cleaving of lint-free process. wipes They and appears gentle cleaningat the NFIP with as alcohol separated (Figure dots 11a,b).or circles and can be removed by a series of lint-free wipes and gentle cleaningAdditionally, with alcohol in some (Figure cases 11a,b). when the fiber was subjected to higher temperature (around 100 °C) or lowerAdditionally, temperature in somebut longer cases exposurewhen the time,fiber wassome subjected impairment to higher of the temperatureNFIP and the (around fiber structure 100 °C) or lower temperature but longer exposure time, some impairment of the NFIP and the fiber structure

Polymers 2020, 20, x FOR PEER REVIEW 9 of 11

Polymers 2020 12 was observed, , 2491near the core-cladding boundary (Figure 11c). Probably, it is a result of the 9core of 11 separating from the cladding. Additionally, in some cases microscope observations of the fiber from thedefects, side namelysurface thecaused presence the suspicion of dust and of the pieces CYTOP of material core-cladding after the structure cleaving shift process. in relation They appears to the protectiveat the NFIP cladding as separated (Figure dots 11d). or This circles correlates and can wi beth removed microscopic by a images seriesof of lint-freethe fiberwipes end-faces and where gentle certaincleaning CYTOP with alcohol structure (Figure shift 11cana,b). also be observed (see for example, Figure 10).

FigureFigure 11. DemonstrationDemonstration of of di ﬀdifferenterent kinds kinds of the of near-ﬁeldthe near-field intensity intensity pattern pattern imperfections: imperfections: (a) presence (a) presenceof cleaving of cleaving product (dustproduct and (dust pieces and of pieces material) of material) (b) that can(b) bethat removed can be removed by end-face by end-face cleaning; cleaning;(c) exfoliation (c) exfoliation in the proximity in the proximity of the core-cladding of the core-cladding boundary; boundary; (d) the CYTOP (d) the core-cladding CYTOP core-cladding structure structureshift in relation shift in to relation the protective to the protective cladding. cladding.

4. ConclusionsAdditionally, in some cases when the fiber was subjected to higher temperature (around 100 ◦C) or lower temperature but longer exposure time, some impairment of the NFIP and the fiber structure was observedIn this article, near thewe core-claddingpresented the boundaryinvestigation (Figure of the 11c). CYTOP Probably, fiber it cleaving is a result using of the the core razor separating blade technique.from the cladding. We demonstrated Additionally, the importance in some cases of the microscope near-field observations intensity profile of the inspection fiber from as thea main side instrumentsurface caused for thecleaving suspicion quality of the evaluation CYTOP core-cladding along with structurethe microscope shift in end-face relation toinspection. the protective To evaluatecladding the (Figure cleaving 11d). quality This correlatesnumerically, with we microscopic introduced images the irregu of thelarity fiber parameter, end-faces which where assesses certain aCYTOP normalized structure standard shift candeviation also be of observed the near-field (see for intensity example, profile Figure from 10). its smoothed representation. Such techniques as manual cleave, cleave with controlled speed and cleave with controlled temperature4. Conclusions were investigated. It was found that manual cleaving significantly depends on the RB type (KI can vary from ≈ 2% up to ≈ 10% for successful cleaves), and even appropriate razor blades In this article, we presented the investigation of the CYTOP fiber cleaving using the razor blade can provide very different results from cleave to cleave. In general, proper selection of the RB gives technique. We demonstrated the importance of the near-field intensity profile inspection as a main quite good cleaves; however, the process should be accompanied by microscope imaging and NFIP instrument for cleaving quality evaluation along with the microscope end-face inspection. To evaluate inspection of the fiber end-faces. the cleaving quality numerically, we introduced the irregularity parameter, which assesses a normalized Cleaving with controlled speed improves the quality of the NFIP by approximately two times standard deviation of the near-field intensity profile from its smoothed representation. and additionally increases stability (reproducibility) of the results. However, performing controlled Such techniques as manual cleave, cleave with controlled speed and cleave with controlled cleaving together with proper temperature radically improves the quality of the NFIP (KI value temperature were investigated. It was found that manual cleaving significantly depends on the RB reduced by five times from 3.5% down to 0.7% for chosen RB). In addition, this regime enables type (K can vary from 2% up to 10% for successful cleaves), and even appropriate razor blades multipleI cleaving (up to≈ 15–20 cleaves)≈ without significant quality degradation utilizing the same RB. can provide very different results from cleave to cleave. In general, proper selection of the RB gives Authorquite good Contributions: cleaves; however, Conceptualization, the process I.C. should and O.K.; be methodology, accompanied I.C. by and microscope S.C.; formal imaging analysis, and I.C. NFIP and O.K.;inspection investigation, of the fiberI.C. and end-faces. S.C.; resources, A.T., K.K. and G.P.; data curation, I.C.; writing—original draft preparation,Cleaving I.C.; with writing—review controlled speedand editing, improves O.K., theG.P. quality, A.T. and of K.K.; the NFIPvisualization, by approximately S.C.; supervision, two timesO.K.; fundingand additionally acquisition, increases O.K. All author stabilitys have (reproducibility) read and agreed ofto thethe published results. However, version of performingthe manuscript. controlled K Funding:cleaving togetherThis research with work proper was temperature supported by radically the Academic improves Excellence the quality Project of the5-100 NFIP proposed ( I value by Peter reduced the Greatby five St. times Petersburg from 3.5%Polytechnic down University. to 0.7% for chosen RB). In addition, this regime enables multiple cleaving (up to 15–20 cleaves) without significant quality degradation utilizing the same RB. Conflicts of Interest: The authors declare no conflict of interest. Author Contributions: Conceptualization, I.C. and O.K.; methodology, I.C. and S.C.; formal analysis, I.C. and ReferencesO.K.; investigation, I.C. and S.C.; resources, A.T., K.K. and G.P.; data curation, I.C.; writing—original draft preparation, I.C.; writing—review and editing, O.K., G.P., A.T. and K.K.; visualization, S.C.; supervision, O.K.; 1.funding Peters, acquisition, K. Polymer O.K. optica All authorsl fiber sensors—A have read andreview. agreed Smart. to the Mater. published Struct. version2011, 20 of, 013002. the manuscript. 2.Funding: Bilro,This L.; Alberto, research N.; work Pinto, was J.L.; supported Nogueira,by R. theOptical Academic sensors Excellence based on plastic Project fibers. 5-100 Sensors proposed 2012 by, 12 Peter, 12184– the Great12207. St. Petersburg Polytechnic University. 3.Conflicts Kuang, of Interest:K.S.C.; Quek,The authorsS.T.; Koh, declare C.G.; noCantwell, conflict ofW.J.; interest. Scully, P.J. 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