In this paper, we present design and preparation of novel Mach-Zehnder Interferometer (MZI) based structure for sensing applications. We present integration of this structure at the end of a single-mode optical fiber. Direct laser writing technology was used to prepare MZI-based structure with a hoop for tight connection to optical fiber facet. Details of the design, preparation process and connectorizing process are described and finally, transmission spectral characteristic of the prepared structure was measured using optical spectral analyze

Gaso, Peter

Jandura, Daniel

Ďurišová, Jana

Advances in Electrical and Electronic Engineering

English

DSpace at VSB Technical University of Ostrava

OPTICS AND OPTOELECTRONICS VOLUME: 17 | NUMBER: 4 | 2019 | DECEMBEROptical Fiber Tip wth Integrated Mach-ZehnderInterferometer for Sensor ApplicationsPeter GASO, Daniel JANDURA, Jana DURISOVADepartment of Physics, Faculty of Electrical Engineering and Information Technology,University of Zilina, Univerzitna 1, 010 26 Zilina, Slovak Republicgaso@fyzika.uniza.sk, jandura@fyzika.uniza.sk, durisova@fyzika.uniza.skDOI: 10.15598/aeee.v17i4.3354Abstract. In this paper, we present design and prepa-ration of novel Mach-Zehnder Interferometer (MZI)based structure for sensing applications. We presentintegration of this structure at the end of a single-modeoptical fiber. Direct laser writing technology was usedto prepare MZI-based structure with a hoop for tightconnection to optical fiber facet. Details of the design,preparation process and connectorizing process are de-scribed and finally, transmission spectral characteristicof the prepared structure was measured using opticalspectral analyzer.KeywordsFiber facet structure, laser lithography, Mach-Zehnder interferometer, sensor, two photonpolymerization.1. IntroductionOriginally it was assumed that the most significant ad-vantage of the Lab on Chip (LoC) applications wouldbe progress in diagnostics associated with decreasingdimensions. Further developments unveiled other sig-nificant benefits such as small amount of analyzed liq-uid sample, which opened up the opportunity to pro-cess individual cells. A small amount of reagent andsample size causes the reaction and analysis time to bemuch shorter [1].Optical detection systems for LoC still remain veryactual region with possibility to bring novel solutions[2]. Light propagation differs in various materials. De-tection methods of most of the optical sensors for med-ical applications are based on the measurement of re-fractive index changes of the biological sample.Integrated MZIs are often employed as sensing el-ements in portable testing devices for diagnostics [3].Principle is based on the interference of split polarizedinput beams that propagate in reference and sensingarms [4]. The reference arm should be isolated fromthe investigated sample and only sensor arm is in in-teraction with the sample [4]. Each change in the re-fraction index of sample shifts the phase of the opticalwave in sensing arm towards the phase of reference armaccording to equation∆ϕ =2ΠLλ(neff,S − neff,R), (1)where L is detection length, λ is the source wavelengthand neff is an effective refractive index of sensing andreference arm.In general, the use of a change in the optical refrac-tive index is a promising principle in many applica-tions, as well as in the future detection of diabetes byoptical measurement of sugar concentration in humanurine or blood. Quantification of sugar concentrationin blood helps to reveal diabetes disease.In this paper, we focused on design and preparationof 3D Mach-Zehnder Interferometer (MZI) applicablefor sensing the refractive index changes of different bio-logical samples. For the manufacture of this structure,we used 3D microprinting technology named Nano-scribe Professional GT. Nanoscribe works on princi-ple of Two-Photon Polymerization (TPP) and providespreparation of polymer parts with resolution of hun-dreds of nanometers and in range up to several cen-timeters [5], [6] and [7]. Nowadays, Direct Laser Writ-ing (DLW) has already become a fully-fledged tool ina wide range of scientific research such as biomimet-ics, biomedical engineering, microfluidics, microoptics,photonics and other, and even in art. Nanoscribe Pro-fessional GT is versatile system. It offers own glasssubstrate to prepare structures, but if required, thereis a possibility to create structures on different surfacesc© 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 466OPTICS AND OPTOELECTRONICS VOLUME: 17 | NUMBER: 4 | 2019 | DECEMBER(e.g. semiconductors for photovoltaic solar cells [8] oroptical fiber facets [7] and [9]).The preparation of optical structures on the facet ofthe fiber is not a novelty. In various fields of applica-tion such as remote optical sensing [10], [11] and [12],beam shaping [13], [14], [15] and [16], and optical ma-nipulation [17] and [18], excellent devices applied onthe fiber facets have been designed and realized. Longago before DLW technology, the language was enrichedby new idiom: "Lab on Fiber" [12] and [19].The previously-mentioned findings have been imple-mented by a variety of technologies [20] and many notmentioned concepts were only considered but not real-ized, because they did not know the suitable technologyat that time. Thanks to the 3D DLW technology, wecan reasonably foresee: "Lab on Fiber" concept is onlyat the beginning and the realization of more outstand-ing microoptical structures not only on optical fiberfacets can be considered [9].2. TechnologyImmersionObjectiveIP-Dip photoresistFused silica glass substrateFig. 1: DiLL configuration, the liquid Dip photoresist isdropped on substrate and turned upside down, objec-tive is immersed in photoresist during DLW process [6].The DLW by itself is carried out by focusing the writinglaser into the photoresist. The photoresist is designedsuch that single photons from the writing laser cannotbe absorbed, but a two-photon absorption can inducepolymerization. Since nonlinear processes scale withintensity, the polymerization occurs where the intensityis the highest, i.e., in the focal volume of the objective.This volume is called voxel. Also, the resolution is lim-ited by voxel size. In our setup, voxel was ellipsoid witha lateral diameter of about 250 nm and an axial diam-eter of about 600 nm. IP-Dip is the specially designedphotoresist for Nanoscribe’s Dip in Laser Lithography(DiLL) technology (Fig. 1) [5] and [6]. IP-Dip servesas immersion and photosensitive material at the sametime by dipping the microscope objective into this liq-uid photoresist. Due to its refractive index matchedto the focusing optics IP-Dip guarantees ideal focusingand hence highest resolution for DiLL [5] and [6].In DiLL configuration, a fused silica glass slide assubstrate is used. One small drop of IP-Dip pho-toresist is applied to it. Substrate with photoresistis situated in holder and during writing process, it isturned upside down. DLW system Nanoscribe allowspreparing on one substrate matrix of structures withdifferent parameters (e.g. laser power or scan speed).After writing process, the sample was removed fromthe sample holder and remains of liquid unexposedphotoresist were removed in Propylene Glycol MethylEther Acetate (PGMAE). Increasing developing timein PGMAE caused increasing shrinkage of the preparedstructure. Because our structure does not contain thesmall holes and the hollow parts, 10 minutes is suf-ficient time for development. Finally, the sample iswashed in isopropanol and dried by a nitrogen gun.3. Structure DesignThe polymer prepared by polymerization using DLW isvery flexible material, optically transparent with a re-fractive index close to that of glass (circa n = 1, 54 [5]).Consider these facts we designed MZI in the followingdesign.The entire structure is shown in Fig. 2(a) and con-sists of two logical parts: a hoop and an MZI. Theinner diameter of the hoop is adapted for conven-tional telecommunication optical fiber and is equal to125 µm. It is used to attach the structure to the opticalfiber. The hoop contains 270 degrees of fiber perime-ter and allows a firm connection with the optical fiber(Fig. 2(b)). The thickness of the hoop walls increasesfrom 5 µm to 30 µm in the coarsest part. The hooplength is 120 µm. The front part consists of 5 archedarms with a 60 degree spacing. The arms maintaina structure centered on the core of the optical fiber. InFig. 2(b), we can see also the scaffold in the structurefor mechanical support of the MZI and also the hoop.This scaffold also enables better separation of all thestructure from the glass substrate.Second part is sensory part. The prepared structureworks on the principle of MZI and consists of two par-allel arms. One arm is called the reference, the secondis sensing arm. The cross-section of the optical waveg-uides is 9×9 µm2. The sensing arm contains interrup-tion in length 36 µm. Depending on refractive index ofmeasured liquid sample, the optical path length will bechanged. This change can be observed by Optical Spec-c© 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 467OPTICS AND OPTOELECTRONICS VOLUME: 17 | NUMBER: 4 | 2019 | DECEMBERhoopMZI structuregap in one arm(a)scaffold125 μm(b)Fig. 2: Design of a) MZI-based sensor and b) detail to hoop forattaching the structure to the optical fiber.tral Analyzer (OSA), as interference dip in frequencyspectrum of interfering signals from both arms. Opti-cal signal is coupled into/from the MZI structure viathe waveguide. To suppress back signal refection, theend facet of the waveguide is not perpendicular but issloped in 8 degrees angle as it is well known in APCconnectors.4. Experimental ResultsThe designed 3D MZI-based structure was prepared inIP-Dip polymer using laser lithography system. Thefinal prepared structure was analyzed using ScanningElectron Microscope (SEM) as can be seen in Fig. 3.This inspection shows very good quality of the struc-ture, especially 3D arrangement of waveguide struc-tures.50 μmFig. 3: SEM image of the prepared 3D MZI-based structure onfused silica substrate.To attach the optical fiber to the structure, the 3Dstage was used and the process was monitored by thecamera. The camera scans the prepared structure fromthe bottom through the glass substrate. It was focusedon the top of the belt. Optical fiber was placed overthe belt using a 3D stage. Applying the outer force tothe fiber causes the ring to expand and the fiber to slipinto the belt. Once the force has been removed, thethread returns to its original position by its structure.The final prepared structure attached to the end ofthe optical fiber can be seen in Fig. 4. Thanks to thehigh strength and elasticity of IP-Dip material, struc-ture and fiber are strongly attached together. Theelastic properties of prepared IP-Dip structure weredemonstrated by simple pressing of this structure onthe glass substrate as is shown in optical microscopeimage in Fig. 5. After the release, the structure hasreached its original position without any deformations.50μmSMFFig. 4: Optical microscope image of final connection of the Sin-gle Mode optical Fiber (SMF) with prepared structuresseparated from the substrate.Eventually, we arranged simple experimental setupfor basic optical characterization of MZI. We coupledlight from single-mode optical fiber which was coupledto the LED source with central emitting wavelength ofc© 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 468OPTICS AND OPTOELECTRONICS VOLUME: 17 | NUMBER: 4 | 2019 | DECEMBER1550 nm to input waveguide part (Fig. 6). Transmit-ted light from MZI was registered by optical spectralanalyzer Anritsu and optical circulator.50μmglassFig. 5: Optical microscope image of depressed structure appliedon the optical fiber.OSALight sourceOpticalcirculatorMach-zehnderinterferometer fiber tip holderOptical fiberFig. 6: Measurement setup for optical characterization of theprepared devices on fiber tip.The transmitted spectrum from MZI is shown inFig. 7. In this measurement, we observed interferencedip in spectrum which was measured in air surround-ings. Our structure shows considerable interferenceeffect, which will be studied in forthcoming measure-ments for application as refractive index change sensor.1 4 8 0 1 5 0 0 1 5 2 0 1 5 4 0 1 5 6 0 1 5 8 0 1 6 0 00 , 00 , 10 , 20 , 30 , 4W a v e l e n g t h  ( n m )Intensity (a.u.)Fig. 7: Transmission spectral characteristic of the preparedMach-Zehnder interferometer device and reference spec-trum of the used light source (dotted line).5. ConclusionA preparation and functionality of MZI-based struc-ture located at the end of a single-mode optical fiberare presented. Nowadays, very popular direct laserwriting technology was used to print MZI-based struc-ture with a hoop for tight connection to optical fiberfacet. Details of the design and preparation processwere described and after successful connectorizing pro-cess transmission spectral characteristic of the pre-pared structure was measured. We consider this devicepromising in forthcoming detailed measurements basedon varying sugar concentrations.AcknowledgmentThis work was supported by the Slovak National GrantAgency under the projects no. VEGA 1/0540/18 andthe Slovak Research and Development Agency underthe project no. APVV-16-0129.References[1] ERICKSON, D. and D. LI. Integrated mi-crofluidic devices. Analytica Chimica Acta. 2004,vol. 507, iss. 1, pp. 11–26. ISSN 0003-2670.DOI: 10.1016/j.aca.2003.09.019.[2] FAN, X., I. M. WHITE, S. I. SHOPOVA, H. ZHU,J. D. 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ISSN 2195-1071.DOI: 10.1002/adom.201500041.[20] KUBICOVA, I., D. PUDIS, L. SUSLIK,J. SKRINIAROVA, S. SLABEYCIUSOVAand I. MARTINCEK. Structures patterningby non-contact NSOM lithography. In: 7thSlovak-Czech-Polish Optical Conference onWave and Quantum Aspects of ContemporaryOptics. Liptovsky Jan: SPIE, 2010, pp. 1–6.ISBN 978-0-8194-236-5. DOI: 10.1117/12.881759.About AuthorsPeter GASO received his Ph.D. in Electrotechnolo-gies and Materials at University of Zilina in 2015.His research interests include optics and photonicsstructures, nanopatterning of nanostructures, passiveand active optical devices and optical sensors.c© 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 470OPTICS AND OPTOELECTRONICS VOLUME: 17 | NUMBER: 4 | 2019 | DECEMBERDaniel JANDURA received his Ph.D. in Elec-trotechnologies and Materials at University of Zilinain 2015. His research interests include optics and pho-tonics structures, nanopatterning of nanostructures,passive and active optical devices and optical sensors.Jana DURISOVA her research interests includeoptics and photonics, microfluidics and biomedicalengineering.c© 2019 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 471

Optical Fiber Tip wth Integrated Mach-Zehnder Interferometer for Sensor Applications

In this paper, we present design and preparation of novel Mach-Zehnder Interferometer (MZI) based structure for sensing applications. We present integration of this structure at the end of a single-mode optical fiber. Direct laser writing technology was used to prepare MZI-based structure with a hoop for tight connection to optical fiber facet. Details of the design, preparation process and connectorizing process are described and finally, transmission spectral characteristic of the prepared structure was measured using optical spectral analyzer

Peter Gaso

Daniel Jandura

Jana Durisova

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Optical Fiber Tip with Integrated Mach-Zehnder Interferometer for Sensor Applications

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Optical Fiber Tip wth Integrated Mach-Zehnder Interferometer for Sensor Applications

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