1. Introduction

Intracavity absorption gas sensors using Erbium-doped fiber lasers have been attracting the interests of researchers with their low transmission loss, remote and continuous gas monitoring, widen range of economical fiber components provided by the communication industry since it was proposed in 1992 [1]. Many kinds of intracavity sensoring systems have been reported and optimized [2–9]. The use of HCPCF increases the interaction time and improves the sensitivity [10,11]. However, most of proposed intracavtiy sensors employed the intracavity fiber spectroscopy to measure only a single point. Actually, an excellent intracavity sensing system should also be compatible of the measurement of the multipoint. Intracavity sensing network based on mode-locked fiber laser has been reported. However, the high cost of fiber components such as optical switch [12], pulse generator and Mach-Zehnder modulator [13] limits the commercial application. Though, FBG have been being regarded as an excellent device to realize multipoint sensor network [14–16], the fatal defect of FBG is that the central wavelength is sensitive to the changes of temperature, stress and vibration, which influences the accuracy of measurement. Actually, for the gases, such as methane and acetylene, having a comb absorption peak, comb filter not only can improve the system accuracy [17], but also can realize multipoint detection. DWDM has been reported to realize multipoint measurement as comb filter [18,19].

In this letter, we propose a low cost intracavity absorption multiplexed gas sensor network (IAMGSN) with high sensitivity and detection efficiency by combining the multiplexing technique with intracavity spectroscopy based on high sensitivity HCPCF sensors and DWDM filters. DWDM is applied to the intracavity sensor network realizing the multiplexing of three optical fiber sensors. We obtain the acetylene absorption spectra of each point in our proposed intracavity sensor network with different concentration. By analyzing the absorption spectra, we demonstrate that our smart sensor network has a high system sensitivity for low acetylene concentration. The sensitivity is up to 100ppmV around its absorption peak at 1536.71nm. Meanwhile, we firstly propose the voltage gradient method for optimizing the intracavity absorption multiplexed sensor network. Via the experiments, we find out that the detection efficiency, which defines as the reciprocal of measurement time, is up to 6.5 times as much as before.

2. Multiplexed sensor network design and experiment setup

The configuration of IAMGSN is shown in Fig. 1. Three HCPCF gas cells and DWDMs are connected in a paralleling topology and further spliced into an optical path consisting of DWDM, Erbium-doped fiber (EDF), isolator (ISO), Fabry-Perot (F-P) tunable filter and couplers to form three intracavity ring fiber lasers (ICRFL). The ICRFL is pumped by a 976nm diode laser via a 980/1550nm WDM (AFR Inc.). The ISO is employed to make the ICRFL operate unidirectionally and prevent spatial hole-burning [20]. A 30:70 coupler and two 50:50 couplers are used to make those three ICFRL have similar output power. DWDM works as a narrow band filter to identify the working wavelength distributed to users. Their wavelengths are confirmed by ITU Grid Channels and the absorption peak of tested gas to make sure only one absorption peak within the operating bandwidth of DWDM.

 

Fig. 1 Schematic diagram of multipoint smart sensor network using intravity fiber ring laser. WDM: wavelength-division multiplexing. PD: photonic detector. EDF: Erbium-doped fiber. HCPCF: hollow core photonic crystal fiber. DWDM: dense wavelength division multiplexing. F-P tunable filter: Fabry-Perot tunable filter. ISO: isolator. Inset: structure of HCPCF gas cell.

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As the gas cell, one meter HCPCF (HC-1550-02 NKT Photonics Inc.) is used to enhance the sensor sensitivity. The diameter of the hollow core is around 10μm and the diameter of the cladding is 120μm, which match the mode field area to the single mode fiber. One end of the HCPCF is connected with DWDM via bare fiber adapters, which are placed in the gas cell. The other end of the HCPCF is connected with coupler via bare fiber adapters, which are placed in the air. When the gas cell is filled with tested gas, there will be a pressure difference between the ends of the HCPCF. So, the inflation and exhaust of gas are realized using the pressure drop method, which can guarantee the concentration of tested gas and the speed of inflation and exhaust for micro structure sensor. With the gas cell placed in the cavity of ICRFL, the laser passes through the tested gas many times increasing the interaction length with the tested gas, which can improve the system sensitivity.

The F-P tunable filter (Micron Optics Inc.) has a free spectral range (FSR) of 108nm and fineness of 10090, which ensures a high system capacity with high resolution. The operating wavelength of F-P tunable is accurately controlled by the NI PXIe-4319 system source measure unit (SMU) which has a 100nV resolution and a 1.8MS/s sampling rate. The intracavity absorption multiplexed sensor network can realize the automatic sensor switching by applying a voltage varying linearly to match the wavelength of DWDM to the F-P tunable filter via SMU.

As it is described in Fig. 2(a), when the voltage applied to F-P tunable filter is changed linearly, we can get three different narrow bands automatically, each of which contains an absorption peak. Once the gas cell is filled with tested gas, like gas leakage, there will be a depression at its absorption peak just as shown in Fig. 2(b). Then we can get the concentration of the tested gas on the base of researching and analyzing the decay intensity of output power. However, the network does not always have a laser output when the voltage applied to F-P tunable filter varies linear due to the narrow operating band of DWDM as shown in Fig. 2(b). It provides the possibility to optimize the multiplexed sensor network using periodic voltage gradient method. As illustrated in Fig. 2(c), the sweep voltage is divided into three sections. In each section, the voltage is changed linearly with the same velocity to ensure the measurement accuracy. Apparently, the periodic voltage gradient compresses the sweep time and improves the detection efficiency.

 

Fig. 2 Principle of realizing multipoint sensor network via voltage gradient. (a) Linear voltage applied to F-P tunable filter with empty gas cell. (b) Linear voltage applied to F-P tunable filter with tested gas. (c) Voltage gradient applied to F-P tunable filter with tested gas.

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3. Result and analysis

We design such an IAMGSN as shown in Fig. 1 to realize the detection of acetylene. According to ITU Grid Channels with 100GHz and 200GHz spacing [21] and the absorption peak provided in Hytran database [22] shown in Fig. 3, we choose the DWDMs with the spacing of 200GHz at 1530.33nm and the spacing of 100GHz at 1532.68nm, respectively. One DWDM is confirmed with the spacing of 100GHz at 1536.61nm, around which the acetylene has a relatively lower absorption intensity, to demonstrate the high sensitivity and large capacity of the proposed IAMGSN. The different spacing of DWDM can be used to study how the operating bandwidth influences the sensitivity of detection.

 

Fig. 3 Absorption spectrum of acetylene from1528.5nm to 1538.5nm in Hytran database.

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Compared with Fig. 4(a), the laser outputs of these three channels have a decay of 2.5dB, 10dB and 5dB at the absorption peak, respectively, when gas cells are filled with the 0.5% concentration (5000ppmv) acetylene. And the absorption spectra of those gas cell is shown in Fig. 4(b). It should be reminded that gas cell II has the highest sensitivity, which is consistent with our previous report in [2] that the sensitivity enhancement factor can be improved by making the pump power close to the pump threshold via the increase of absorption intensity. Comparing the spectrum of gas cell I and gas cell III, we find that decreasing bandwidth turns out to be useful to improve the sensitivity, which attributes to the narrower linewidth caused by narrower space DWDM.

 

Fig. 4 Spectra of gas cells with different concentration of acetylene. (a) Spectra of empty gas cells. (b) Spectra of gas cells with 0.50% acetylene. (c) Spectra of gas cells with 1.00% acetylene. (d) Spectra from the system based on a single transmission pass method.

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When the acetylene concentration is up to 1.0% (10000ppmv), these gas cells have an absorption larger than 20dB (sensitivity>100ppmV) around the absorption peak at 1536.71nm as it is shown in Fig. 4(c). What is worth mentioning is that acetylene has a “wide absorption bandwidth” because there is no laser output around the absorption peak due to the high sensitivity of the network sensors. According to the method reported in [18], we get the spectra from a single transmission system shown in Fig. 4(d). Compared with Fig. 4(b) and Fig. 4(d), each intracavity absorption sensor has a higher sensitivity. Therefore, the intracavity absorption multiplexed sensor network has the potential for low concentration gas multipoint detection.

As it is mentioned in section 2, we optimize our network by applying the voltage gradient to the F-P tunable filter. When the voltage changes from 2.274 V to 2.134V with the resolution of 100nV, the spectrum shows the concentration of gas cell I. 1.636V to 1.496V and 0.596V to 0.456V correspond to gas cell II and gas cell III, respectively. From Fig. 5 (a), Fig. 5(b), Fig. 5(c), we can find out that it takes almost 90 seconds to sweep all the gas cells, which improves the detection efficiency larger than 5.5 times. In addition, we get the power stability shown in Fig. 5(d) by fixing the F-P filter on those three absorption wavelengths over 200 seconds. The difference between the maximum and minimum power in each channel is less than 0.5dBm. Such a little change reflects that the IAMGSN possesses a good stability and satisfies the need for the measurement of low gas concentration.

 

Fig. 5 Spectra of gas cells with voltage gradient. (a) Spectra of empty gas cells. (b) Spectra of gas cells with 0.50% acetylene. (c) Spectra of gas cells with 1.00% acetylene. (d) Power stability measured by fixing F-P filter on absorption wavelength, inset: the spectra of output lasers.

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4. Conclusion

We propose a novel approach for intracavity absorption multiplexed sensor network based on HCPCF sensors and DWDMs, and demonstrate its feasibility via experiments. The sensitivity of multiplexed sensor network, which is restricted by the sensitivity of the HCPCF sensors and the insertion loss of F-P filter, is better than 100ppmV. It is considered to be a good method to improve the network sensitivity for low gas concentration by increasing the length of HCPCF. Compared with the system with FBG filters, the stability of our network is improved through the use of DWDM, which makes it convenient for the calibration of gas concentration and the discovery of gas leakage. Meanwhile, the sensor network is optimized via the voltage gradient method, which improves the detection efficiency up to 5.5 times. And the system is compatible with other intracavity sensors, which can realize the detection of many dangerous gases, such as methane, carbon monoxide, ammonia, hydrogen sulfide and realize the index and concentration of liquid, such as Glucose solution. Further, we will focus on our attention on expending the capacity of our network by combining it with other mature multiplexing techniques, such as TDM [23] via optical switches and SDM [24] via multicore fibers.