Study of the sensitivity of gas sensing by use ofindex-guiding photonic crystal fibers

Shu-Guang Li,* Si-Ying Liu, Zhao-Yuan Song, Yin Han, Tong-Lei Cheng, Gui-Yao Zhou,and Lan-Tian Hou

College of Science and Key Laboratory of Metastable Materials Science and Technology, Yanshan University,Qinhuangdao 066004, China

*Corresponding author: shuguangli@ysu.edu.cn

Received 25 October 2006; revised 17 April 2007; accepted 18 April 2007;posted 19 April 2007 (Doc. ID 76332); published 9 July 2007

We demonstrate an absorption transmission spectrum of CH4 in a 16.9 cm long index-guiding photoniccrystal fiber (PCF) fabricated in our laboratory. One of the main factors to improve the sensitivity is toincrease the fraction of power in PCF cladding air holes. We study the fraction of power in PCF claddingair holes as a function of the index-guiding PCF parameters. We found that a PCF with small spacing anda large air-filling ratio has a higher fraction of power in its cladding air holes. At the same time the modearea in this PCF is small and would generate strong nonlinear effects in the fiber. If we use a PCF inwhich the core is formed by missing seven air holes, it is immediately obvious that the PCF used as asensor has higher sensitivity and a larger mode area. © 2007 Optical Society of America

OCIS codes: 060.2370, 290.1990, 330.1880, 130.6010.

1. Introduction

With the continuing development of the fabrication[1–3] and theoretical analysis of the fundamentalcharacteristics [4–10] of photonic crystal fibers(PCFs), it is found that PCFs have many uniquecharacteristics that differ remarkably from those ofconventional fibers, such as single-mode propaga-tion over a wide range of wavelengths [4], sensitivestructure manageable dispersion properties [5,6],high birefringence [7–10] and extra-strong nonlin-ear effects [11,12]. Meanwhile, the study of appli-cations of PCFs has received increased attention.Theoretical and experimental research that uses thenovel characteristics of PCF at the sensing and detect-ing regions has been conducted [13–20]. Bock et al.measured the sensitivity of group index single-modePCF to temperature, static pressure, and tension[13]. Statkiewicz et al. carried out research on thedegree of sensitivity of birefringence and polarizedproperty to exterior static pressure, tension, and tem-perature [14,15]. MacPherson et al. measured [16]

the bending properties of fiber, in which they used adouble-beam interference phase sensitivity in a dual-core PCF. A refractive-index sensor based on a mi-crostructure optical fiber Bragg grating has beendesigned by Iadicicco et al. [17]. A novel temperaturesensor with antiresonant microstructure fiber, whichis made by filling the air holes of PCFs with a highindex material, has been reported by Litchinitser andPoliakov [18].

Gas or liquid can be used to fill the air holes of PCFsbecause of the structure of PCFs, which offers the pos-sibility of studying gas or liquid sensing by use of PCFs[19–24]. This research has several advantages, forexample, only a submicroliter of a liquid sample isneeded to achieve the longer interaction length. Incomparison with conventional fibers, PCFs do not needto be stripped of the cladding and the coating, whichmakes them durable. Ritari et al. studied the charac-teristics of gas sensing by use of photonic bandgapPCFs with a hollow core [20], Jensen et al. detectedbiomolecules in an aqueous solution utilizing the in-teraction between the evanescent field and the fillingmaterial in PCFs [21]. Stewart et al. [22] and Hoo et al.[23,24] researched the characteristics of gas sensingfor evanescent-wave absorption in solid-core PCF by

0003-6935/07/225183-06$15.00/0© 2007 Optical Society of America

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filling the cladding air holes with the measured gas.Moreover, research on gas sensing that uses solid-corehole fiber with random hole distributions in the clad-ding can be done by the evanescent field absorptionmechanism [25]. Florous et al. researched the thermo-optic sensitivity of highly birefringent polarimetricsensing PCFs [26]. Stewart et al. [22] and Hoo et al.[23,24] have shown that relative sensitivity is an im-portant parameter to influence the sensitivity of gasdetection, but the fraction of power in PCF cladding airholes directly affects the sensitivity of gas detection.Based on their research [22–24] we systematically in-vestigate the variation of the fraction of power in theair holes of PCF as a function of fiber structure. For thesolid-core PCF with triangular distribution air holes inthe cladding, we found a way to improve the sensitivityof gas sensing. We found that the improved PCF miss-ing two layers of air holes in the center can achieve thehigh sensitivity and the large mode area simulta-neously and that is our focus in this paper.

2. Theory and Principle

Light power focuses mainly on the core of index-guiding PCF, in which there is a small amount ofenergy in the cladding air holes. If the cladding airholes are filled with the measured gas, the evanes-cent field in air holes can be absorbed by the mea-sured gas. The type and density of the gas can beestablished by measurement of the absorption spec-trum of gas. To enhance the sensitivity of the fibersensor, the following must be done. First, the range ofinteraction between the light field and the measuredgas sample should be increased. Second, the responsetime of the sensor whould be restricted by gas diffu-sion velocity. The measured gas fills the air holes ofPCFs through free diffusion; in general, the longerthe length of fiber, the more the diffusion time. If thelength of fiber is short, the interaction length betweenthe light field and the measured sample would de-crease. According to the Lambert–Beer law [24],

I��� � I0���exp��r�m���lC�, (1)

where I and I0 are the intensity of the output light infiber when the fiber includes and excludes the mea-sured gas, respectively; �m��� is the absorption coef-ficient of the measured gas, which is a function ofwavelength and can be established by the type ofmeasured gas; l is the length of the PCF used todetect gas; C is the gas density; and r is the relativesensitivity coefficient given as [24]

r � �nr�ne�f, (2)

where nr is the index of the measured gas. In numer-ical simulation we assume that the measured gas isCH4. According to Refs. 27 and 28, the refractiveindex of CH4 is less than the refractive index of airunder normal conditions. Actually, the material dis-persion of CH4 is small, and generally the real part ofthe refractive index of CH4 is nearly 1.0 under normal

conditions. Therefore, the approximate value of nr ischosen as 1.0 in numerical simulation; ne is the effec-tive index of the guiding mode. For a particular fibermode f can be calculated by integrating the opticalpower inside the air holes and dividing it by the totalpower carried by that mode and expressed as

f ��holes

�ExHy � EyHx�dxdy��total

�ExHy � EyHx�dxdy.

(3)

Thus it is obvious that the length of fiber should beshortened to obtain a shorter response time, whichmakes it necessary to enhance the relative sensitivitycoefficient. However, if we wish to improve the rela-tive sensitivity coefficient, the fraction of power in theair holes must be increased. Therefore, our purpose inoptimizing fiber structure is to increase the fractionof power in the air holes. To avoid strong nonlineareffects, the effective mode area of PCFs cannot besmall.

Although the fundamental vibrational absorptioncharacteristics of CH4 are generally in the mid-infrared region [29], CH4 also possesses overtone andcombination absorption lines in the near-IR trans-mission window �� � 1–2 �m�, which can be ad-dressed by silica fibers and LED or laser diodesources [22]. The Q branch of the 2�3 band of CH4near 1.66 �m is located in the near-IR low-loss trans-mission window of silica fibers. Therefore we chose anoperating wavelength near 1.6 �m. CH4 is the majoringredient of fire damp in a mine; therefore, develop-ment of a sensor with high sensitivity for detection ofCH4 in a mine is important.

3. Measurement of the Absorption Spectrum of CH4

The cross section of index-guiding PCF fabricated inour laboratory is shown in Fig. 1. Here, spacing � ofthe cladding air holes is approximately 1.4 �m; theaverage of air hole diameter d is 1.0 �m. Figure 2shows the experimental setup used to fill the PCFswith CH4 and to perform absorption measurements.The purity of the CH4 was specified by the manufac-turer to be �98%. The 16.9 cm long piece of PCFshown in Fig. 1 was filled with CH4 by use of theexperimental setup described in Fig. 2. In this exper-iment the light source is a carborundum source. The

Fig. 1. Cross section of the index-guiding PCF. Here spacing � isapproximately 1.4 �m and diameter d is 1.0 �m.

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absorption transmission spectrum was recorded us-ing a PbS infrared detector. The resolution of theoptical spectrum analyzer is 0.05 nm. Figure 3 showsthe spectrum of the Q branch of the 2�3 band of CH4near 1.66 �m measured at a pressure of 600 mbars.From Fig. 3 we can see that the strongest absorptionlines are situated at the wavelength of 1.664 �m. Thestrong absorption shows the potential of PCF for highsensitivity gas detection. The research by Hoo et al.[24] shows that the relative sensitivity is an impor-tant parameter for gas detection, but the fraction ofpower in PCF cladding air holes directly affects thesensitivity of gas detection. To improve the fraction ofpower in air holes of PCF and to reach a higher sen-sitivity of gas sensing, we calculated the fraction ofpower in air holes of PCF.

4. Numerical Calculation and Theoretical Analysis

We obtained the numerical results by means of a mul-tipole method [30,31], which is suitable for use to com-pute PCFs composed of cylindrical air holes and can beused to calculate the real and imaginary parts of theeffective index of the mode propagation constant. Oneadvantage is that, in the case of finite cladding airholes, confinement loss can be calculated by the imag-

inary part of the effective index. Moreover, becausethis method can be used to evaluate the fiber propa-gation constant by means of a given wavelength orfrequency, it can easily include material dispersion ifthe optical indices are changed according to the cur-rent wavelength at each step by use of, for example, aSellmeier equation for silica [30]. After use of Eq. (3),the fraction f of power can be calculated for variousfiber parameters. Figure 4(a) shows the fundamentalmode field distribution of PCFs at the wavelength of1.65 �m; Fig. 4(b) shows the fraction of power in airholes of PCF as a function of wavelength. For index-guiding PCF, most of the guided power, especially forthe fundamental mode in which we are interested, isdistributed within the central silica core and the in-nermost ring, and the light power in the outer ringsis negligible [24]. Numerical analysis shows that thecalculated power distribution at the internal threelayers of air holes is enough to provide power in thecladding air holes, but more layers of air holes need tobe calculated for PCFs with a lower gas-filling ratio�d�� 0.3�. In Fig. 4(b), we observed that the frac-tion of power in PCF cladding increases when thewavelength increases; the fraction of power in thePCF cladding air holes at the absorption wavelengthsof acetylene and CH4 are 5.2% and 6.4%, respectively.The sensitivity of evanescent-wave absorption by useof PCFs is higher than that using conventional fiber[22], because the evanescent-wave sensors that useconventional fiber have only approximately 1% of themode field power at the sensing region.

To investigate the variation of the fraction of powerin PCF cladding air holes as a function of PCF struc-ture parameters, we numerically simulated PCF sim-ilar to the structure of the fiber shown in Fig. 1, whichhas a triangular arrangement of cladding air holes.The core is formed by missing an air hole in the centerand four layers of air holes in the cladding.

Figure 5 shows the fraction of power in the air holesand confinement loss of PCFs as a function of spacing� at wavelength � � 1.65 �m and d�� � 0.5. FromFig. 5 it can be seen that the fraction f of power in air

Fig. 2. Experimental setup used to fill the PCFs with CH4 and to perform the absorption measurements shown in Fig. 3.

Fig. 3. Absorption transmission spectrum of CH4 in the 16.9 cmlong PCF shown in Fig. 1 and recorded with a PbS infrared detec-tor. Resolution of the optical spectrum analyzer is 0.05 nm.

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holes of PCFs can be increased by decreasing thecladding spacing at a given wavelength and air-fillingfraction (e.g., d��). The fraction of power in air holesis up to 18% when spacing � is equal to 0.6 �m, butthe decreased spacing � would result in increasedconfinement loss.

Figure 6 shows the fraction of power in the air holesand confinement loss of PCFs with d � 0.7 �m as afunction of spacing � at wavelength � � 1.65 �m. Fora given air hole diameter, we observed that a de-crease in cladding spacing can effectively increase thefraction of power in the air holes of PCF; the fractionf of power is up to 36% at wavelength � � 1.65 �m fordiameter d � 0.7 �m and spacing � � 0.8 �m. How-ever, in this case, the variation of confinement loss asa function of spacing � presents a minimum value,namely, confinement loss with a minimum value for� � 1.2 �m.

To make a comprehensive comparison for the vari-ation of the fraction of power in the air holes, con-finement loss and effective mode area of PCFs as afunction of fiber structure parameters are shown in

Figs. 7(a), 7(b), and 7(c) as functions of the ratio ofd�� at wavelength � � 1.65 �m for different claddingspacing �. Although it is difficult to establish an an-alytical relationship between the fraction of power inthe air holes and the fiber parameters d, �, and wave-length � from the simulation results, we can concludethat the fraction of power in the air holes increaseswith d�� at the smaller � (e.g., � � 1.0 or 1.4 �m)and the confinement loss is decreased. So it is feasiblethat the fraction of power in the air hole can be im-proved by increasing the gas-filling ratio for thesmaller space and achieve a decrease in fiber confine-ment loss. In addition, we observed that the claddingair holes spacing � seriously influences the fraction ofpower in air holes of PCFs at wavelength � �1.65 �m. For cladding air hole spacing � 2.2 �m, thefraction of power in air holes of PCFs is very smalleven though the air-filling fraction in cladding islarger (e.g., d�� � 0.9). If we design a PCF that issuitable for evanescent-wave gas sensing, claddingspacing � of the PCF must be smaller than the char-acteristic wavelength of the measured gas.

Fig. 6. Fraction of power in air holes and the confinement loss asa function of spacing �. Here wavelength � � 1.65 �m and air holediameter d � 0.7.

Fig. 4. (a) Fundamental mode field distribution of PCFs at 1.65 �m wavelength and (b) the fraction of power in air holes of PCF as afunction of wavelength.

Fig. 5. Fraction of power in air holes and the confinement loss asa function of spacing �. Here wavelength � � 1.65 �m and ratiod�� � 0.5.

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Although decreasing cladding spacing � and in-creasing the air-filling fraction in the cladding wouldbe a valid way to increase the fraction of power in airholes of PCFs, decreasing cladding spacing � wouldmake the confinement loss increase rapidly. Thesmall confinement loss can be achieved by increasingthe number of layers of cladding air holes [31], asshown in Fig. 5(b). By decreasing cladding spacing �and increasing the air-filling fraction simultaneously,the fraction of power in air holes becomes larger,meanwhile the effective mode area of PCFs decreasesrapidly, as shown in Fig. 5(c). When spacing � is1.0 �m and the ratio of d�� is 0.9, the fraction f ofpower in air holes of PCF is 22.5%; however, its ef-fective mode area S is only 0.7 �m2. The smallermode area would lead to stronger nonlinear effects,which is disadvantageous to detect the characteristicspectrum of measured gas.

Figure 8 shows a type of PCF with the core formedby missing two layers of air holes (seven air holes) inthe center and a triangular array of air holes in thecladding. According to results obtained by means ofnumerical simulation, we chose PCF with a smallerspacing and a higher gas-filling ratio to increase thefraction of power in holes, but this fiber has a largermode area because of the missing two layers of airholes. From the simulation results, the maximum of

the mode area that can be reached is 7.6 �m2, and thecorresponding fraction of power is 55.7%.

From the optimized parameters we observed thatsome of the proposed PCFs are multimode. PCF con-finement loss and bending loss that correspond to ahigher-order mode are higher than the fundamentalmode. In addition, we can peel off the higher-ordermode by bending the PCF during the experimentalprocess.

5. Conclusion

We have investigated the possibility of improvingthe sensitivity of gas sensing using PCFs. One of themain factors to improve the sensitivity is to enhancethe fraction of power in PCF cladding air holes. Thefraction of power in PCF cladding air holes as a func-tion of structure parameters of the index-guidingPCF has been systematically investigated, and wefound that a PCF with small spacing and a largeair-filling fraction ratio has a high fraction of powerin its cladding air holes. At the same time the modearea in this PCF is small and there are strong non-linear effects in this fiber. We have proposed a type ofPCF with the core formed by missing two layers of airholes. By use of this fiber one can increase the lightpower in air holes. The mode area of the fiber is notvery small so that the nonlinear effect is not strongand could be negligible.

This project is supported in part by the DoctoralScience Foundation of Yanshan University (grantB153), the China Postdoctoral Science Foundation(grant 2005038188), the State Key DevelopmentProgram for Basic Research of China (grant2003CB314905), the Scientific Research Program ofthe Ministry of Education, Hebei Province, China(grant 2005310), and the Postdoctoral WorkstationResearch Program of the Gulf Security TechnologyCompany, Hebei, China.

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Fig. 7. (a) Fraction of power in air holes f as a function of d��, (b) the confinement loss as a function of d��, (c) the effective mode areaas a function of d��. Here wavelength � � 1.65 �m and air hole spacing � is from 1.0 to 2.2 �m.

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