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Spiral Photonic Crystal Fibers Gas Sensor
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Abstract—We propose a new type of evanescent-wave gas sensor based on spiral photonic crystal fiber (SPCF) for air pollution monitoring. Here, we design the SPCF in such a way that it exhibits a broad spectral transmission band which could be used for detecting more gas condensate components. Besides, we study the confinement loss LC, dispersion D, absorbtion enhancement factor , fractional power inside the air holes f and relative sensitivity r for various spiral arms of proposed SPCFs. In this study, we are able to enhance the relative sensitivity of the various gas molecules.

Index Terms—Spiral photonic crystal fiber, confinement loss, dispersion, relative sensitivity.

I. INTRODUCTION


NOWADAYS, the environment pollution becoming more and more serious and explosions in coal mines occur-ring from time to time, the ability to detect poisonous gas, especially low concentration gas becomes more important and urgent. Recently, several types of gas sensors have been developed for poisonous gas detection, among which the spectrum absorption type optical fiber gas sensor is frequently used due to its good selectivity, superior stability and the possibility of long-distance remote measurement.
After the successful development of silicon based photonics technology, an increasing number of novel optical devices such as photonic crystal (PC), photonic crystal fiber (PCF) and hollow core Bragg fiber (HCBF) are introduced [1], [2], [3]. The common characteristic of these optical devices is the porosity, thus the measured gases can be filled in. Besides, the interaction length of gas sample and radiation can be very large while only small gas samples were needed. Recently, the PCFs that incorporate air holes within the silica cladding region open up new opportunities for exploiting the interaction of light with gases or liquids via evanescent field effects. It is known that the evanescent field is the overlap of the optical mode in PCFs with air holes. By appropriately designing the PCFs, many groups reported the working model of gas sensors based on the evanescent field [4], [5], [6].Flammable and toxic gas sensors can be used as safety measures in gas production

facilities, especially in oil rigs. The gas sensors detect gas leaks that could cause fire, explosion, and toxic exposure. In oil rigs, the most important toxic and flammable gases are oxygen, methane, hydrogen chloride, etc [5]. Here we choose the interest of wavelength range from 0.5 to 2 m. In this range of wavelength silica fiber has low loss and observed that absorption lines of a number of important gases such as oxygen (O2), nitrogen dioxide (NO2), hydrogen fluoride (HF), hydrogen bromide (HBr), acetylene (C2H2), hydrogen iodide (HI), ammonia (NH3), carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S), methane (CH4), hydrogen chloride (HCl), etc fall in the wavelength range from 0.5 to 1.8 m [6]. Thus, to detect these gases, we design a PCF and explore the various optical characteristics for a wavelength range from 0.5 to 1.8µ m.
In this work, we design a various arms of holey spiral photonic crystal fibers and investigate the various optical prop-erties. Further, we calculate relative sensitivity using proposed holey SPCFs for a wider range of wavelength which can be used as a toxic gas sensor or flammable sensor for safety measuring in gas production facilities. Section II presents the design analysis of the proposed SPCF structures, namely, 5 arms, 6 arms and 7 arms through a full vector finite element method. In section III, we explore the various sensing properties including confinement loss, dispersion, absorbtion enhancement factor and relative sensitivity for various arms of SPCFs. Finally, we summarize the findings in
section V.


. SENSING PROPERTIES OF HOLEY SPCFs

In this section, we explore the various sensing properties, namely, confinement loss, dispersion, absorption enhancement factor and relative sensitivity. In sub-section III-A, we study the confinement loss of gas sensing SPCFs. Next, in sub-section III-B, we study the dispersion of the pro-posed SPCFs. In sub-section III-C, we calculate absorbtion enhancement factor through light-matter interaction. Finally, in sub-section III-D, we compute the relative sensitivity by calculating the fraction of the total power in the holes.

A. Confinement Loss of gas sensing SPCF:
As a first step, we discuss the confinement loss which is calculated from the imaginary part of the effective index of the fundamental mode. We investigate the confinement loss of PCF with various spiral patterns of air-holes in the cladding by using the perfectly matched layers [8], [9]. The confinement loss, Lc in decibels per centimeter is given by [9]:
(1)
where Im[neff ] is the imaginary part of the effective index of the fundamental mode and k (= 2 ) is the wave number. The variations of confinement loss for various spiral arms for a wide range of wavelength as shown in Fig. 6. As seen in Fig. 6, no loss for various arms of SPCFs up to 1 m wavelength. However, the loss becomes significant above 1µm wavelength. We observe that the PCF with five arms exhibits relatively a large confinement loss when compared to the rest as is evident in Fig. 6. This is due to the spreading of large amount of light outside the core. The inset of Fig. 6 shows mode field distributions of 5, 6 and 7 arms of SPCFs at 1.8µ m wavelength.

B. Waveguide Dispersion of SPCF:
The dispersion is an important linear parameter, which result in symmetrical broadening of the pulse along the propagation direction. This linear parameter depends on operating wavelength. It is calculated from the second derivative of the effective refractive index, neff of the fundamental mode as a function of wavelength. We use the following equation for determining the dispersion,
(2)
where is the operating wavelength and c is the velocity of light in free space. The variations of dispersion with respect to wavelength for various spiral arms of SPCFs as shown in Fig. 7. As is seen in Fig. 7, the proposed SPCFs exhibit two zero dispersions at 0.8 and 1.5 m wavelength and also exhibit tunable dispersions, namely, anomalous and normal. This dispersion properties would be useful in a myriad of nonlinear optical applications such as pulse compression, super continuum generation, optical communication, etc.


CONCLUSION
In this paper, we have analyzed and demonstrated an evanescent-wave absorption gas sensor using a holey spiral photonic crystal fiber. Further, we have proposed a novel spiral photonic crystal fiber and studied sensor related op-tical properties for various spiral arms of SPCFs. We have been able to bring in higher sensitivity for wider absorbtion wavelength of various gas molecules and lower confinement loss, simultaneously. We analyzed the influence of the relative sensitivity of the gas measurement system caused by PCF structure parameters and finally puts forward a design of PCF with optimum geometric structure parameters. Hence, we envisage that the proposed SPCFs could be used as evanescent-wave based gas sensors
 


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