Response to Pressure of a Hollow Core Photonic Crystal Fiber for ...

Response to Pressure of a Hollow Core PhotonicCrystal Fiber for Sensing ApplicationsRafael E. P. de Oliveira and Christiano J. S. de MatosGrupo de FotônicaUniversidade Presbiteriana MackenzieSão Paulo, Brazilalfaer@gmail.com; cjsdematos@mackenzie.brAbstract — The response to pressure of a hollow core photoniccrystal fiber is investigated. The fiber is designed to operate at1550nm but presents core-guided transmission windows in thevisible spectral range that have shown to be highly dependent onthe exact fiber structure. As the fiber is submitted to pressure,small structural and index changes, thus, affect these windows.The fiber response depends on whether pressure is appliedinternally or externally and pressure sensitivity can be of units ofkgf/cm² up to hundreds of kgf/cm². In addition, it was found thatthe fiber presents a birefringence which is also pressuredependent. These characteristics can be exploited for thedevelopment of pressure sensors based on intensity orpolarization modulation.Keywords - Hollow Core Photonic Crystal Fiber, PhotonicBandgap, Pressure SensingI. INTRODUCTIONThe photonic crystal fibers (PCF) [1] have been largelystudied over the last few years and present several uniquecharacteristics that have been exploited for sensing applications[2]-[4]. The freedom of design of these fibers can lead to thedevelopment of pressure sensors with better performance thanthose based on conventional fibers and on fiber Bragg gratings.Pressure sensing based on conventional fibers usuallyrequires some mechanical apparatus to transduce pressure intoan optical measurand. This can be done by measuring theamount of light coupled through two butt-coupled fibers asfunction of their pressure-induced displacement [5], or bytranslating pressure into strain in a diaphragm on which aBragg grating is attached. This kind of configuration canpresent sensitivities of the order of 2.7 nm.cm²/kgf [6]. Freestandingfiber Bragg gratings directly exposed to hydrostaticpressure, on the other hand, show a sensitivity of ~2.910 -4nm.cm²/kgf [7]. However, the main shortcome of Bragggratings and almost all other types of conventional fiber basedsensors is the cross sensitivity with temperature, which needsto be compensated for [6].Solid-core photonic crystal fibers with high geometricalbirefringence have been proposed and demonstrated forpressure sensing applications, with the measurand beingtransduced into polarization changes and show a pressuresensitivity of ~1rad/(kgf/cm².m) [8,9]. As temperature inducesnegligible stress in the air-silica structure, the temperaturesensitivity is as small as 10 -3 rad/(ºC.m) [9]. This virtualinsensitiveness to temperature is one of the most remarkableadvantages of these PCF based sensors.In this paper we study pressure effects on the transmissionand polarization response of a hollow core photonic crystalfiber (HC-PCF) and show its large potential for simple, selfreferencedpressure sensing.First we review recent results on the detection of unusualtransmission windows in the visible range in an HC-PCFdesigned to operate around 1550nm and their pressuredependent power transmission [10]. It is shown that the HC-PCF can work at different pressure ranges (units or hundreds ofkgf/cm²) depending on how the pressure is applied to the fiber.Even though this fiber is not designed to be birefringent, itwas found that some birefringence is present due to undesirablecore asymmetries and that the birefringence is also sensitive topressure. We then characterize the pressure dependence of thepolarization states of light guided in one of these windows.The observed characteristic responses can be exploited forpressure sensing with intensity and/or polarizationtransduction.II. TRANSMISSION CHARACTERIZATION AND INTENSITYRESPONSE TO PRESSUREThe fiber used in the experiments was the hollow corephotonic crystal fiber model HC1550-02 from Crystal FiberA/S .Although this model is designed to guide around1550nm, high loss core-guided transmission was alsoobserved in the visible spectral range [10]. Those windowscan be observed in short-length fibers (up to ~40cm) andpresent estimated attenuation of the order of 0.8 to 1.4 dB/cm.The transmission spectrum in the visible range is highlydependent on the exact structure of the fiber and vary fromone sample to the other of the same fiber model but fromdifferent preforms. Figure 1 shows the normalizedtransmission spectrum of two fiber samples, which are978-1-4244-5357-3/09/$26.00©2009IEEE 299

It was found that new principal axes could be identifiedwhen pressure was applied. Taking axis 1 at atmosphericpressure (figs. 5 and 6) as a reference angle (0°), the new axis1, as a function of applied pressure, was found by rotating thepolarizers and measuring the output polarization ellipticities.The new angles are shown in table I for two differentpressures. Is can be seen that the principal axis steadily rotateswith pressure, which may indicate that the pressure-inducedstress and deformation does not follow the geometricalsymmetry that yields the birefringence obtained atatmospheric pressure. This feature is not surprising, as theoriginal birefringence is a consequence of residual coreellipticity, while stress and deformation may strongly dependon non-uniformities in the fine web structure of themicrostructured cladding.TABLE I. ROTATION OF THE PRINCIPAL AXIS WITH PRESSURE.Pressure [kgf/cm²]Angular position ofAxis 10 0°2 10°5 18°Note that the decrease in attenuation observed in figures 6and 7 along the axis that is orthogonal to the input polarizationcan be, at least partially, explained via the angular changeswith pressure of the principal axes. As these axes rotate thelaunched polarization used to plot the figures is no longeraligned with one of them and, thus, rotate along the fiber. Thepower along the orthogonal axis is then expected to increase,as indeed observed. The increase in attenuation observedalong the input polarization, on the other hand, is believed tobe connected to pressure-induced deformations in the claddingmicrostructure that decreases the bandgap confinement.The observed dynamics may also be somewhat connectedto different core-mode anticrossings with surface modes thatcan occur at different wavelengths for different polarizations[11]. These anti-crossings can yield polarization-dependentlosses that may have to be taken into account. Thesepossibilities are currently being investigated.It can be seen that pressure significantly affects thepolarization dynamics in the studied hollow core fibers, whichcan be exploited for sensing applications. Note, in particular,that, unlike pressure, temperature is expected to isotropicallyaffect the microstructured cladding and is, thus, not expectedto induce a principal axis rotation. This feature may beexploited in the design of temperature independent sensorsthat operate beyond the 100 o C limit observed for themeasurements reported in section II.CONCLUSIONSThe response to pressure of identified transmissionwindows in the visible spectral range in a HC-PCF wasstudied. It was shown that measurable intensity responses forpressures of hundreds or units of kgf/cm² ocurr dependingwhether pressure is applied externally or internally. Forinternal pressure the guided-mode polarization dynamics, aswell as the fiber birefringence, were also characterized. Theresults indicate that along with attenuation changes, pressurealso shifts the principal axes of the fiber, inducing couplingbetween orthogonal polarizations.The presented features can be exploited in the developmentof pressure sensors based on hollow core photonic crystalfibers in which the measurand is transduced into eitherpolarization or intensity. The sensors can be self referenced, iftwo or more transmission windows, or else a single windowand the two orthogonal polarizations, are simultaneouslymonitored. The results presented can also help to improve thefabrication of optimized hollow-core fibers for pressuresensing applications.ACKNOWLEDGMENTThe authors acknowledge the contributions of Dr. CristianoM. B. Cordeiro and Juliano G. Hayashi in the intensityresponse measurements and of Dr. Marcos A. R. Franco in thecalculation of the fiber birefringence. This work is supportedby CNPq, FAPESP, CAPES and Mackpesquisa.REFERENCES[1] P. Russel, “Photonic Crystal Fiber: Finding the Holey Grail,” Optics andPhotonics News, vol. 18, pp. 26-31, July/August 2007.[2] T. T. Alkeskjold, J. Lægsgaard, A. Bjarklev, D. S. Hermann, J. Broeng,J. Li, S. Gauza, and S.-T. Wu, Appl. Opt., vol. 45, pp. 2261-2264, April2006.[3] C. M. B. Cordeiro, C. J. S. de Matos, E. M. dos Santos, A. Bozolan, J. S.K. Ong, T. Facincani, G. Chesini, A. R. Vaz, and C. H. Brito Cruz,Meas. Sci. Technol., vol. 18, pp. 3075-3081, September 2007.[4] J. B. Jensen, L. H. Pedersen, P. E. Hoiby, L. B. Nielsen, T. P. Hansen, J.R. Folkenberg, J. Riishede, D. Noordegraaf, K. Nielsen, A. Carlsen, andA. Bjarklev, Opt. Lett., vol. 29, pp. 1974-1976, September 2004.[5] D. A. Krohn, Pressure Sensors, in Fiber Optic Sensors, ResearchTriangle Park: Instrument Society of America, 2000, pp 143-151.[6] Y. S. Hsu; Likarn Wang; Wen-Fung Liu; Y. J. Chiang, IEEE Photon.Technol. 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