A novel fiber Fabry–Perot filter based mixed-gas sensing system

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Abstract

A practical mixed-gas sensing system utilizing a fiber Fabry–Perot (FFP) filter and Fiber Bragg Gratings (FBG) was developed. By modulating the transmission wavelength of FFP within the width of one absorption line of the target gas, the system can detect different kinds of gaseous species selectively. A FBG with center wavelength being aligned to the absorption peak of the target gas was employed as wavelength reference to make the transmission wavelength of FFP modulate stably in a required region. The properties of the proposed system were demonstrated experimentally by simultaneous detection of acetylene (C2H2) and carbon monoxide (CO). Approximate linear relationships between the system responses and the input gas concentrations were demonstrated by experiments. The minimum detectable C2H2 and CO concentration of ∼15 ppm and ∼300 ppm were also achieved by experiments respectively.

Introduction

Simultaneous detection of multi-gas safely and reliably is essential in many different areas of modern life, such as industrial process control, environmental monitoring, medical applications and safe operation in industry [1]. Optical fiber gas sensors have been widely applied in gas monitoring because of their advantages such as resistance to electromagnetic interference, safety in flammable and explosive situations, and excellent remote monitoring at long distance [2], [3]. Optical absorption based gas sensors are among the most commonly used optical fiber gas sensors. They are based on absorption of the laser intensity as the beam propagates across the measurement path that is filled with the detected gas. Although many common gases, such as methane, exhibit much stronger optical absorption strength in mid-infrared region than in near infrared region, a number of systems were devised to operate at the overtone absorption lines in the near infrared region, around 0.8–2.0 μm in the past works. This is because the light sources and detectors in the mid-IR region have cost and performance limitations [4]. To compensate for the weak line strengths and improve the systems sensitivity, many techniques have been studied and employed in the past works, e.g. differential absorption [5], [6], wavelength modulation [7], and long absorption cell [8], [9]. However, little attempts have been made on developing an optical fiber mixed-gas sensing system in the previous works.

It is known that a number of gaseous species have absorption bands/lines in the same spectral region. Because the absorption lines of some different gas species are very close to each other, the system will often respond to other kind of gas species when detecting a target one. This is called cross-sensing effects,which must be reduced in developing a mixed-gas sensing system.

Diode laser (DL) combining wavelength modulation (WM) techniques is a popular way to realize gas sensing selectively in the previous works [10], [11]. DL's wavelength can be modulated within the width of one absorption line by changing its injection current. In this way, trace species can be monitored sensitively and selectively. Unfortunately, because the tunable wavelength range of DL is very limited, generally less than 1 nm, it cannot be used in simultaneous detecting of multi-gas, where the probing light with much wider tunable wavelength range is needed. Although we can apply different DLs with different central wavelength for sensing different gaseous species [12], the cost of a mixed-gas sensing system would be very high. Tunable laser source is an alternative way to realize mixed-gas sensing system, for its wavelength can be tuned as long as a hundred nano-meters. However, the high cost still greatly limits its practical applications.

Piezo-electrical transducer (PZT) driven FFP type tunable optical filters (TOP) are among the most commonly used tunable filters. When a certain degree of voltage is applied to the FFP, a light beam, with a certain wavelength, will transmit through the FFP at the maximum transmittance. Because different gas exhibits different ‘fingerprint’ absorption spectra in the infrared region, the target gas can be detected if the narrow band transmission light from FFP is used as probing light. In 2006, Vargas-Rodríguez and Rutt have demonstrated the gas sensor based on correlation spectroscopy using a FFP with long cavity length as a modulator [13].

However, it is known that the wavelength–voltage relationship of a FFP is nonlinear, due to the intrinsic nonlinearity between the displacement and voltage of PZT [14]. This nonlinear characteristic of FFP may yield great errors and instabilities in measurement results when a FFP was used as a wavelength modular. However, as far as we know, little attentions have been paid on this issue in the previous works.

In this paper, we report our attempts in applying FFP and wavelength modulation techniques to mixed-gas sensing. The FFP is used to create a tunable narrow band light to detect a particular kind of gas species selectively. Moreover, a FBG based method is proposed to reduce the nonlinear effects of FFP on the measurement results by controlling the transmission wavelength of FFP to modulate in a fixed region stably. In this way, the stability and sensitivity of the system have been much improved.

In the following sections, the paper will illustrate the principles of the proposed mixed-gas sensing system and demonstrate the system properties by experiments of simultaneous detection of C2H2 and CO.

Section snippets

The principle of multi-gas sensing system

According to Beer–Lambert law, when light transmits through a gas cell, the intensity of transmission light can be expressed asI1=I0exp[α(λ)CL]where I1 and I0 are the transmitted and incident light intensities respectively. α(λ) is absorption coefficient of gas at wavelength λ, C is the concentration of gas, and L is the length of absorption path.

Considering that α(λ)CL  1, Eq. (1) can be simplified asI1I0[1α(λ)CL]

For trace gas monitoring at atmospheric pressure, the individual gas

Experimental set-up

The experimental set-up is shown in Fig. 1. The light source is 1550 nm SLED with a spectral width of about 50 nm and output power of nearly 35 mW. A FFP available from Micron Optics Inc. is used to create a tunable narrow band light. The FSR of FFP is 100 nm and band-width is ∼0.3 nm. The insertion loss of FFP is less than 1 dB.

Gas cell is an important part of absorption based gas sensor. The longer the absorption length is, the stronger the gaseous absorption strength to the probe light is, and

Discussion

Various noises in both optical path and electrical circuits may limit the system sensitivity and cause the measurement errors. In addition to this, any variation of the FBGs wavelengths may yield measurement errors, for the transmission spectra of FBG were used as wavelength references to control the modulation regions of FBG. In order to reduce the effects of environmental temperature fluctuations on the FBGs wavelengths, the two FBGs were fixed in a TCM in our system. The maximum temperature

Conclusion

A compact optical fiber mixed-gas sensing system utilizing FFP as wavelength modulator was proposed. The transmission spectra of two series FBGs were used as wavelength references to make the FFP modulate stably in a fixed wavelength region that was required for sensing a particular kind of gas. Thanks to the reference channel and digital low-pass filter, the effects of the fluctuation of the SLED and various noises from the optical path and electric circuits were reduced effectively. An

Acknowledgement

This work was supported by the National Natural Science Fund of China under contact no. 60772016.

Hui Ding received her PhD degrees in Measurement & Instrumentation from Xi’an Jiaotong University (China) in 2004. Her research interests include fiber optical gas sensor and wide-wavelength fiber lasers.

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Hui Ding received her PhD degrees in Measurement & Instrumentation from Xi’an Jiaotong University (China) in 2004. Her research interests include fiber optical gas sensor and wide-wavelength fiber lasers.

Jianqi Liang received the B.Sc. degree in Communication and Navigation from Air Force Engineering University (China) in 1999. Now he is studying at Xi’an Jiaotong University for the MS degree. His current research activity includes the use of microstructured fibers for practical gas sensing applications.

JunHong Cui received the B.Sc. degree in Automation Control from Guilin University of Electronic Technology (China) in 2002. Now she is studying at Xi’an Jiaotong University for the Master's degree. Her research interests include fiber-optic gas sensor and weak signal detection technology.

Xiangnan Wu received his B.Sc. degree in Measurement & Instrumentation from Xi’an Jiaotong University (China) in 2008. Now he is studying at Xi’an Jiaotong University for the MS degree. His research interests involve digital signal processing methods and weak signal detection technology.

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