A novel optical hydrogen sensor, based on the absorption change of the evanescent fields in a circular Pd-coated singlemode tapered fibre is presented. The proposed sensor is polarisation independent and its sensitivity is adjustable by means of the taper diameter, interaction length, and/or light wavelength. A simple light transmission measurement setup is used to test the sensor. The sensor is suitable for the detection of low hydrogen concentrations with high sensitivity and fast time response. Transmission changes as high as 60% are demonstrated

A. Díez

Butler

Chadwick

Diez

J. Villatoro

J.L. Cruz

Kenny

M.V. Andrés

Sekimoto

Tabib-Azar

Tobiska

Wyrzykowski

English

Villatoro, J.

Díez Cremades, Antonio

Cruz Muñoz, José Luis

Andrés, Miguel V.

Repositori d'Objectes Digitals per a l'Ensenyament la Recerca i la Cultura

  f redtionbln uan, antmseermo nmtivccethmlass1sthn lmr a rsethaay bened toeo-or a ofbyrednmist,e-uni-. 1VCrogenandonirst,con-it-g an. 2n ofr theestra-    Highly sensitive optical hydrogen sensor using circular Pd-coated singlemode tapered fibre J. Villatoro, A. Díez, J.L. Cruz and M.V. Andrés  A novel optical hydrogen sensor, based on the absorption change oevanescent fields in a circular Pd-coated singlemode tapered fibpresented. The proposed sensor is polarisation independent ansensitivity is adjustable by means of the taper diameter, interaclength, and/or light wavelength. A simple light transmissiomeasurement setup is used to test the sensor. The sensor is suitathe detection of low hydrogen concentrations with high sensitivity afast time response. Transmission changes as high as 60%demonstrated.  Introduction:  In the past few decades, hydrogen has attracted mattention in the scientific community owing to its superior properties characteristics as a carrier energy over conventional fuels. Howeveruid or gaseous hydrogen is very volatile, extremely flammable, highly explosive, e.g. a gaseous hydrogen leakage in air greater thaat room temperature and normal pressure leads to an explosive aphere which is easily ignitable. Therefore, the development of senfor detecting hydrogen concentrations below such explosive limit is vimportant. To date, several optical hydrogen sensors based on diffworking principles have been reported [1 – 6]. Optical sensors seebe the most appropriate sensors in dangerous atmospheres owing tlack of sparking possibilities and high sensitivity. In this Letter, we present a new optical hydrogen sensor based oabsorption change of the evanescent fields in a Pd-coated singletapered fibre. This sensor is polarisation independent and its sensican be adjusted by means of the taper diameter and the interalength, which can be easily tailored during the taper fabrication pro[7], and by the light wavelength. Hydrogen concentrations below lower explosive limit were detected with high sensitivity and fast tiresponse.  Basis:  In a singlemode tapered fibre, the reduction of the core and cding diameters causes the evanescent fields to spread out acrocladding and to reach the outer air-cladding boundary (see Fig. a ).When the taper waist is coated with a Pd layer, the propagation conof the fundamental mode is modified owing to the interaction with layer. Since the palladium refractive index is complex, the fundame mode of the structure exhibits an attenuation coefficient  γ  different fromzero. The sensor behaviour under the presence of hydrogen is found bying into account the well-known properties of a palladium thin fiwhen it is exposed to hydrogen [8]. Palladium has the ability to fohydride, the optical properties of which are different than those of H 2 -free Pd film. Formation of hydride leads to a decrease in both theand imaginary parts of the palladium complex permittivity. Conquently, the attenuation coefficient of the fundamental mode of structure decreases, therefore the transmission of the device incre                                      Fig. 1 Schematic diagram of sensor structure and experimental setup a Schematic diagram of sensor structure. φ is taper diameter and Lin is inter-action length  b Experimental setup  SMF: singlemode fibre ELECTRONICS LETTERS    2nd August 2001    Vol. 37  the is itsne fordarechdliq-nd 4%os-orsryent totheir theodeitytionsseed- thetantetaltak-mneal-eses.The transmitted power, when the sensor is exposed to hydrogen, mexpressed as I = I0exp[2∆γLin], where ∆γ is the change of the attenuatiocoefficient, Lin the interaction length, and I0 the transmitted power whenno hydrogen is present. A standard boundary-value method was uscalculate ∆γ for various hydrogen concentrations. As expected, the thretical results indicate that the relative transmission of the sensor, I/I0,increases with increasing hydrogen concentration. Furthermore, fgiven hydrogen concentration I/I0 increases with wavelength and withthe reduction of the taper diameter.  Experimental procedure and results: A schematic representation of thesensor structure is shown in Fig. 1a. A standard telecommunicationsfibre was tapered adiabatically to a uniform diameter over a lengthseveral millimetres. The introduction of the taper was performed using the travelling-burning technique [7]. The losses of the tapefibres were typically < 0.1 dB. Three palladium depositions, each 12thick, were evaporated in high vacuum onto the taper uniform warotating the fibre 120° between two consecutive depositions. This procdure gives quasi-circular coatings the thickness of which is close to formity and makes the device polarisation insensitive [9]. The sensor was tested using the experimental setup shown in Figb.All the measurements were carried out under normal conditions. A Ptube was used as a gas cell wherein a homogeneous mixture of hydand nitrogen (used as a carrier gas) was flowing. Both hydrogen nitrogen flow rates were individually controlled by using high-precisiaccucal flow meters. Two kinds of measurements were carried out. Fthe power transmission spectra were recorded for a fixed hydrogen centration with TE-, TM- and unpolarised light. Secondly, the transmted power was measured for different hydrogen concentrations usinLED. The results are shown in Figs. 2 and 3, respectively. From Figit is clear that similar results are obtained no matter the polarisatiothe light, which confirms the light polarisation insensitivity of oudevice. As indicated by the theoretical calculations, the sensitivity ofsensor increases with wavelength. It is worth noting that the biggFig. 2 Transmission spectra for 2% H2 concentration with TM-, TE- andunpolarised light  Sensor parameters: φ = 25µm, Lin = 15 mm, and 12 nm Pd layer  ··········· TM-polarised light  – – – – TE-polarised light  ——— unpolarised light Fig. 3 Relative transmission against time for different hydrogen concenttions Sensor parameters: φ = 20µm, Lin = 15 mm, and 12 nm Pd layer   No. 16evtenchoertiidoadmntioeat aport7-ot,  ores      transmission changes occur at wavelengths around 1.55 µ m, at whichinexpensive semiconductor laser diodes or LEDs are widely availablResults shown in Fig. 3 were obtained with an LED, peak walength of 1600 nm and 3 µ W of optical power, and a photodetector. Nothat the sensor absorbs and desorbs hydrogen very rapidly. Whehydrogen flow is brought back to zero, the relative transmission reathe baseline, which reflects the reversibility of the sensor. Fig. 4 shthe calibration curve of the sensor described in Fig. 3. A maximum rtive transmission increment of nearly 60% is obtained for a 10% hydgen concentration. Note the dramatic changes of the relatransmission for hydrogen concentrations below the lower exploslimit.  Conclusions:  A novel optical hydrogen sensor using a circular Pcoated singlemode tapered fibre is presented. The sensor is based absorption change of the evanescent fields in the palladium-cotapered waist. The proposed sensor is polarisation independent ansensitivity can be adjusted by the interaction length, taper waist diater, and the light wavelength. It is shown that with a low-power LED aa single photodetector it is possible to detect hydrogen concentrabelow the lower explosive limit with very high sensitivity and fast tim                Fig. 4 Calibration curve of the sensor described in Fig. 3  ● experimental data  – – – – lower explosive limit ELECTRONICS LETTERS    2nd August 2001    Vol. 37  . e- theeswsla-o-veve-n theted itse-dnsresponse. A further study of the proposed sensor will be reported future date. Acknowledgments: J. Villatoro is grateful to Conacyt (Mexico) for apost-doctoral fellowship. The authors acknowledge the financial supof the Comisión Interministerial de Ciencia y Tecnologia (Grant 1FD90684). © IEE 2001 11 June 2001Electronics Letters Online No: 20010716DOI: 10.1049/el:20010716J. Villatoro, A. Díez, J.L. Cruz and M.V. Andrés (Departamento de FísicaAplicada, Universidad de Valencia, Dr. Moliner 50, 46100 BurjassValencia, Spain) References 1 BUTLER, M.A., and GINLEY, D.S.: ‘Hydrogen sensing with palladium-coatedoptical fibers’, J. Appl. Phys.,  1988,  64 , pp. 3706–3712 2 CHADWICK,   B. ,  TANN,   J. ,  BRUNGS,   M. , and  GAL,   M. : ‘A hydrogen sensor basedon the optical generation of surface plasmons in a palladium alloy’,  Sens.Actuators B,  1994,  17 , pp. 215–220 3 BUTLER,   M.A. : ‘Micromirror optical-fiber hydrogen sensor’,  Sens.Actuators B,  1994,  22 , pp. 155–163 4 TABIB-AZAR,   M. ,  SUTAPUN,   B. ,  PETRICK,   R. , and  KAZEMI,   A. : ‘Highly sensitivehydrogen sensors using palladium coated fiber optics with exposed cand evanescent field interactions’,  Sens. Actuators B,  1999,  56 , pp. 158–163 5 SEKIMOTO,   S. ,  NAKAGAWA,   H. ,  OKAZAKI,   S. ,  FUKUDA,   K. ,  ASAKURA,   S. , SHIGEMORI,   T. , and  TAKAHASHI,   S. : ‘A fiber-optic evanescent-wavehydrogen gas sensor using palladium-supported tungsten oxide’,  Sens.Actuators B,  2000,  66 , pp. 142–145 6 TOBISKA,   P. ,  HUGON,   O. ,  TROUILLET,   A. , and  GAGNAIRE,   H. : ‘An integratedoptic hydrogen sensor based on SPR on palladium’,  Sens. Actuators B, 2001,  74 , pp. 168–172 7 KENNY,   R.P. ,  BIRKS,   T.A. , and  OAKLEY,   K.P. : ‘Control of optical fiber tapershape’,  Electron. Lett.,  1991,  27 , pp. 1654–1656 8 WYRZYKOWSKI,   K. ,  RODZIK,   A. , and  BARANOWSKI,   B. : ‘Optical transmissionand reflection of PdHx thin films’,  J. Phys., Condens. Matter,  1989,  1 ,pp. 2269–2277 9 DIEZ,   A. ,  ANDRES,   M.V. , and  CRUZ,   J.L. : ‘Hybrid surface plasma modes incircular metal-coated tapered fibres’,  J. Opt. Soc. Am. A,  1999,  16 , pp.2978–2982   No. 16

Highly sensitive optical hydrogen sensor using circular Pd-coated singlemode tapered fibre

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Highly sensitive optical hydrogen sensor using circular Pd-coated singlemode tapered fibre

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