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Abstract
All fiber Michaelson interferometer cascaded fiber Bragg grating (FBG) sensor for simultaneous measurement of trace dimethyl methyl phosphate and temperature is proposed. One end of the four-core fiber (FCF) is spliced with a multimode fiber (MMF), the other end is flattened and evaporated with silver film to enhance reflection, and the Michelson interference structure is formed. The grating is engraved in the single-mode fiber (SMF) core and spliced with MMF, then the Michelson interference cascaded FBG, FBG-MMF-FCF sensor is obtained. The sensing film, MnCo2O4 is coated on the surface of FCF, and the structure, elemental composition and morphology of MnCo2O4 were analyzed by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The sensitivity and the detection limit of DMMP are 86.44 dB/ppm and 0.1767 ppb, respectively. The response/recovery time is about 14/10 s. the temperature sensitivity can be compensated and calculated as 0.069 nm/°C. The sensor has good selectivity and stability, and has a good application prospect in high sensitivity detection of trace DMMP vapor.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Sarin is one of the typical nerve agents, has the characteristics of strong toxicity, fast action, wide killing range, difficult protection and treatment [1]. It can cause convulsions, pupil contraction, diarrhea and nausea in a few seconds [2–4]. It is reported that the lethal inhalation concentration exposed to sarin for 10 minutes is 0.38 mg/m3 (68.65 ppb), which is about one-20,000th of carbon monoxide [5]. Generally, sarin exists in the form of liquid, and it has strong volatility and the volatility is closely related to temperature. Therefore, it is urgent to develop a high sensitivity sensor for simultaneous real-time monitoring of sarin and temperature.
Recently, several methods have been explored to detect sarin gas, especially its simulant dimethyl methyl phosphonate (DMMP). The methods include quartz crystal microbalance [6], surface acoustic wave [7], chemical resistance capacitance [8], microcantilever [9], colorimetric method [10], and so on. Based on the functionalized graphene, a chemical resistive DMMP gas sensor was proposed, and the sensitivity was obtained as 1.3 ppm with a fast response [11]. A plasmonic-sorbent thin-film platform integrated Raman internal standard is reported, the sensor has outstanding surface-enhanced Raman scattering sensing capabilities, and the detection limit (LOD), response/recovery time of the sensor are 2.5 ppm and 21/54 s, respectively [12]. Although these methods can achieve high sensitivity and fast response for DMMP detection, it is still difficult to meet the need for simultaneous remote monitoring of DMMP concentration and temperature. The fiber-optic sensor has the advantages of anti-electromagnetic interference, high precision, easy fabrication and remote monitoring [13–15], and can be designed to obtain the simultaneous measurement of multiple parameters. Dual parametric sensors based on optical fiber interference structure cascaded with FBG have been reported in many previous works. Yunshan Zhang et al. proposed a sensor composed of multi-mode fiber (MMF), hollow-core fiber and FBG, the sensor can measure liquid level and temperature simultaneously [16]. An optical fiber sensor for temperature and pressure measurements is fabricated by Xiaoling Tan et al. [17], the sensor consists of a segment of highly birefringent photonic crystal fiber cascaded FBG, and the sensitivities of pressure and temperature are 3.65 nm/MPa and 1.46 pm/°C, respectively. Based on an FBG and a single-mode-cladding-free-single-mode fiber structure, Chunran Sun et al. proposed a sensor for liquid level and temperature measurement with high sensitivity [18]. Based on doped polymer optical fibers and fluorescence quenching, a fiber-optic sensor is designed for the rapid and remote detection of trace 2, 4-dinitrotoluene vapor [19]. However, there is no report about the sensor for trace DMMP and temperature simultaneous measurement.
In this paper, a DMMP and temperature fiber-optic sensor based on single-mode fiber (SMF)-multimode fiber (MMF)-four-core fiber (FCF) Michelson interferometric cascaded fiber Bragg grating (FBG) is fabricated, and manganese cobalt oxide (MnCo2O4) sensing material is coated on the surface of FCF to achieve efficient and specific detection of DMMP molecules. The sensing performance of the sensor is investigated and discussed in detail.
2. Principle
The setup diagram of the fiber-optic sensor is shown in Fig. 1, which consists of a broad spectrum light source (ASE), an optical spectrum analyzer (OSA), an air chamber, a drying tube, an air pump and a DMMP vapor supply system. The dashed line is an enlarged view of the sensing area, which consists of an FBG, an MMF, and a FCF coated MnCo2O4 sensing film. The end of FCF is flatted and coated with silver film, and the silver film is wrapped with ultraviolet glue to prevent oxidation. The top left of Fig. 1 shows the cross-section of the FCF, and the lengths of FCF and MMF are 2 cm and 0.5 cm, respectively. The sensor is connected to the ASE and OSA through a circulator, and the sensing probe is encapsulated in the gas chamber.
2.1 Principle of FBG
In FBG, a few light will be reflected at each spatial periodic refractive index change. When the grating period is about half of the incident light wavelength, all the reflected light is coherently combined into a large reflection with a specific wavelength, which is called the Bragg condition. The reflected wavelength is called the Bragg wavelength. The optical signals with other wavelengths are almost unaffected by FBG and can be transmitted through FBG continuously. Therefore, when light propagates through FBG, almost no signal attenuation or signal change occurs. Only the wavelength that satisfies the Bragg condition will be almost completely reflected. The reflection wavelength of FBG depends on the period and modulation of FBG, and is high sensing to environmental temperature, strain and other parameters. In a temperature changing environment, the Bragg wavelength is a function of temperature, and the center wavelength of the reflected light depends on the Bragg wavelength (λFBG), so λFBG is defined as [20]:
where neff is the effective refractive index and Λ is the grating period. When the temperature changes with Λ and neff, the reflected wavelength also changes and can be detected by the wavelength shift of the reflected light. The shift of FBG center wavelength caused by temperature can be expressed as [21,22]:Equation (3) can be rearranged as:
2.2 Michelson interference principle
When the light of ASE is incident in FBG through the SMF, the FBG serves as the band-pass filtering and selects the corresponding wavelength. The light satisfying the Bragg wavelength is reflected, and the transmitted optical signal continues to transmit in the SMF core. Due to the core mismatch, the high-order mode of MMF is excited when the light passes through MMF. When the light is transmitted into the FCF, part of it enters the FCF cladding, while the other part is transmitted in the FCF core, these lights are reflected at the silver film and then converged into the MMF, finally input into the OSA trough the SMF. The reflected light intensity can be expressed as [23,24]:
Compared with the traditional dual-arm interferometer, the multi-core fiber based on the interference between different core transmission modes has higher phase change sensitivity [28,29]. Multi-core fiber is sensing to physical parameters and can be used in multi-parameter detection. Many simultaneous measurements of environmental parameters and temperature have been reported based on FCF [30–32]. When the temperature changes, the length and effective refractive index of the fiber will change due to thermal expansion and thermal effect. The functional relationship between temperature and the change of the FCF is as follows [33]:
2.3 Sensing mechanism
The possible sensing mechanism of DMMP molecules on the surface of MnCo2O4 nanoparticles film can be described in Fig. 2. The oxygen atom of the phosphoryl group in DMMP binds to the Lewis acid sites on MnCo2O4, and electron transfer will occur between electron-rich phosphoryl oxygen and electrophilic Lewis acid sites (Mn2+, Co3+) [34]. The desorption is a reverse process, the DMMP was desorbed under N2 purge due to the weak interaction between the phosphoryl oxygen and the Lewis acid site.
3. Experiments
3.1 Materials and equipment
All reagents, Manganese nitrate tetrahydrate (Mn(NO3)2·4H2O, Aladdin, China), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, Macklin, China), N-N’ dimethylformamide (DMF, Macklin, China) and sodium hydroxide (NaOH, Macklin, China) are analytically pure, and no further purification is carried out before use. The apodized FBG is customized in Wuhan Senhui Photoelectric Technology Co., Ltd, the period is less than 1.00 µm, and the center wavelength is 1550 ± 0.3 nm with high reflectivity (>90%) and small bandwidth (3 dB). the MMF (the core diameter is 105 µm) and FCF (the core diameter is 7.6 µm, one core is located in the center of the fiber, the other three cores are around it and the distance is 35 µm.) are also obtained from Wuhan Senhui Photoelectric Technology Co., Ltd.
ASE light source (C + L band, Kangguan, China), OSA (AQ6370D, Yokogawa, Japan), automatic optical fiber fusion splicer (S178C, Furukawa, Japan), gas pump (EDLP600-D12B, Kamoer, China), Field Emission Scanning Electron Microscope (FE-SEM, JSE-7800F, JEOL, Japan), X-ray diffractometer (XRD, Rigaku Ultima IV, Japan), X-ray photoelectron spectroscopy (XPS, Thermo scientific ESCALAB Xi+, USA) were also used in this experiment.
3.2 Simulation
The optical field distribution along the axial direction and the waveform simulation results of the FBG-MMF-FCF sensor are shown in Fig. 3. In the initial time, the light is mainly transmitted in the core of the SMF, after light enters the FCF through the MMF, one part enters the FCF core and the other part is transmitted in the cladding. Since there is only one FCF core distributed along the XZ axis and two FCF cores distributed along the YZ axis, the light field distribution in different directions can be observed in the XZ and YZ views (Fig. 3(a) and 3(b)). As the refractive index of the surrounding environment changes (1.325-1.380), the spectral intensity near 1586 nm changes significantly (Fig. 3(d)).
Fig. 3. Optical field distribution along axial direction (a) XZ view, (b) YZ view, (c) XY view, (d) Interference spectra near 1586 nm.
3.3 Sensing materials preparation
1 g Mn(NO3)2·4H2O and 2 g Co(NO3)2·6H2O were dissolved in 100 ml deionized water and stirred at room temperature. 15 ml 5.23% NaOH solution was slowly added into the mixture with continuous stirring for 30 minutes until the green solution changed to brown. The obtained mixture was transferred into a Teflon-lined reactor at 160°C for 10 h. The obtained black precipitate was centrifuged and washed four times (deionized water: ethanol = 1:1, v/v). The black solid product was collected and freeze-dried for 48 h, and then calcined at 400°C for 1 h to obtain MnCo2O4. Finally, the sensing material was dispersed in 5 ml DMF for use.
3.4 Sensor fabrication
One end of FCF is spliced with the MMF, and the other end is coated with 300 nm silver film by a physical vapor deposition method, and then wrapped with a 61-type ultraviolet glue to prevent oxidation. According to Eq. (8), the length L of FCF will affect the interference, and the interference spectra with different FCF lengths are shown in Fig. 4. When L = 5.9 cm, 4.9 cm, 4.2 cm, 3.4 cm, the interference troughs are very dense, the small free spectral area is not suitable to observe the interference trough changes. When L = 1.2 cm, the interference trough is too weak to monitor with high sensitivity. When L = 2 cm, several stable and sharp interference troughs occur in the range of 1520 nm-1620 nm, and the interference trough characteristics can be used to monitor the change of interference spectral information. Therefore, the FCF length in the subsequent experiment is 2 cm.
0.2 g/ml MnCo2O4 dispersion as prepared was ultrasonically dispersed for 30 minutes to make it uniform. The FCF was cleaned with ethanol and immersed in MnCo2O4 dispersion. After staying for 30 s, the FCF was lifted and dried in the air. The effect of the dipping number on the interference spectra has been investigated. The changes of the interference trough intensity near 1580 nm of the sensor after dip coating for 1, 2 and 3 times are shown in Fig. 5, the sensor coated 2 times has the largest shift, so the 2 times is used in the following experiment. The FCF coated with sensing film was dried at 60°C for 8 h to make the sensing film firmly adhere to the FCF surface.
Fig. 5. Relative shift of the interference trough intensity near 1580 nm after dip coating 1, 2, and 3 times.
Before the experiment, the sensor element is placed in an air chamber, and the whole sensing system is fixed in a constant temperature and humidity box, and the temperature and humidity can be regulated during detection. Before detection, nitrogen (N2) is pumped into the gas chamber to keep the sensor in an N2 atmosphere, and then different concentrations of DMMP vapor are pumped in. The vapor is dried by the drying tube and enters the gas chamber for concentration detection. Different gases were introduced into the chamber, respectively, for selectivity, temperature sensitivity and humidity stability measurement.
4. Results and discussion
4.1 Characterization
The XRD pattern of MnCo2O4 material is shown in Fig. 6(a). The diffraction peaks at 18.74°, 31.04°, 36.44°, 38.14°, 44.48°, 58.8°, and 64.68° correspond to (111), (220), (311), (222), (400), (511), (440) crystal planes, respectively [35], which is highly matched with the MnCo2O4 (PDF# 23-1237) standard card, indicating the good cubic spinel crystal structure of the synthesized MnCo2O4. The XPS spectrum of MnCo2O4 was shown in Fig. 6(b), three elements, Mn, Co and O can be found, the peak of O 1s is located at 531 eV, the Mn element is at 643 eV, and the Co element is at 781 eV. The high-resolution spectra of Mn 2p are shown in Fig. 6(c), the peaks at 642.3 eV and 653.8 eV correspond to the Mn 2p3/2 and Mn 2p1/2 spin orbits, respectively, and the energy gap of 11.5 eV, indicating the coexistence of Mn2+ and Mn3+ [36,37]. The two main peaks at 780.5 eV and 795.5 eV (Fig. 6(d)) can be ascribed to the two spin orbits of Co 2p3/2 and Co 2p1/2, respectively. The separation energy is 15 eV, indicating the coexistence of Co2+ and Co3+ [38,39].
Fig. 6. (a) XRD pattern of MnCo2O4, (b) XPS spectra of MnCo2O4, (c) High-resolution spectra of Mn 2p and (d) High-resolution spectra of Co 2p.
Figure 7(a) is the SEM image of MnCo2O4 nanoparticles coated on the FCF surface. The particle size of MnCo2O4 nanocrystals is about 120 nm (Fig. 7(b)) [40]. The voids between the nanoparticles increase the contact area between the sensing material MnCo2O4 and DMMP molecules, and enhance the adsorption capacity of MnCo2O4 to DMMP molecules. Figure 7(c) confirms that the sensing film is composed of Mn, Co and O elements.
Fig. 7. Morphology of (a) MnCo2O4 on TCF surface, (b) MnCo2O4 nanoparticles, (c) EDS spectra of MnCo2O4.
4.2 Temperature measurement
To realize the detection of DMMP concentration, the sensor should be sensing the refractive index. Figure 8(a) shows the spectral variation of the Michelson interference structure to different refractive indices, there is no obvious variation on FBG characteristic peak. The sensitivity of the sensor of the refractive index is 22.221 nm/RIU. In a constant temperature and humidity box, the temperature sensing performance was measured with a constant relative humidity of 30%. In the temperature range of 20-70°C, the FBG characteristic peak at 1550 nm has a red shift with the increase of temperature (Fig. 8(b)), the interference trough wavelength of the Michelson interference also shows red shift (Fig. 8(c)), while the interference intensity has no obvious change. The temperature sensitivities of FBG and Michelson interferometer are 0.01 nm/°C and 0.067 nm/°C, respectively. When the temperature and refractive index change simultaneously (Fig. 8(d)), the temperature sensitivity of the sensor can be obtained by Eq. (10) [41]
where kT and kRI are the sensor response sensitivities when the temperature and refractive index change individually. RRI,T is the refractive index temperature coefficient (RRI, T = 1.02 × 10−4 [42]). The compensated data are shown in Table 1, and the temperature sensitivity can be calculated as 0.069 nm/°C (Fig. 8(e)) according to Eq. (10).Fig. 8. (a) The spectral drift of the Michelson interference structure to different refractive indices, (b) Response spectra of the FBG to different temperatures, (c) Response spectra of the Michelson interference sensor to different temperatures, (d) Plot of wavelength shift surface when temperature and refractive index change simultaneously, (e) Uncompensated temperature (red) and compensated temperature (blue) scatter plot.
Table 1. Wavelength data before and after temperature compensation.
When the refractive index and temperature of change simultaneously, the simultaneous measurement of the two parameters can be realized based on the sensitivity coefficient matrix. The sensitivity coefficient matrix of the sensor can be expressed as
The above equation can be written as
The temperature sensitivity of the Michelson interferometer and FBG were obtained as 0.069 nm/°C and 0.01 nm/°C, respectively.
4.3 DMMP vapor measurement
At room temperature, 0, 10, 20, 30, 40, 50, 60 ppb of DMMP vapor was successively pumped into the air chamber at a speed of 18 L/h. With the increase of gas concentration, the interference trough intensity near 1580 nm gradually increases, the maximum offset is 5.23 dB (Fig. 9(a)), and the sensitivity is 86.44 dB/ppm in the concentration range of 0-60 ppb. The trough intensity was linearly fitted with a fitting coefficient of 0.9796, and the detection limit (LOD) was calculated by Eq. (14) [43]:
where K is the linear fitting slope (0.08644 dB/ppb), σ is the standard deviation of the slope (0.00509 dB), and the LOD of the sensor is calculated as 0.1767 ppb. Which is lower than immediately life-threatening and health concentrations of sarin (IDLH = 20 ppb) [5]. The response/recovery test of the sensor was carried out by introducing dry N2 into the air chamber, and the response and recovery time is 14 s and 10 s, respectively (Fig. 9(b)).Fig. 9. (a) The linear fitting of the interference trough intensity and the DMMP concentration, the inset is the response spectra of the sensor to different DMMP concentrations, and (b) Response and recovery curve.
4.4 Selectivity and stability
60 ppb DMMP vapor, hydrogen, hydrogen sulfide, oxygen, ammonia, carbon monoxide, carbon dioxide and argon were introduced into the gas chamber, respectively. The interference trough intensity variations of the sensor are shown in Fig. 10(a). The intensity change caused by other gases is less than 20% of the light intensity change caused by DMMP vapor at the same concentration, indicating that the sensor has excellent selectivity to DMMP. Figure 10(b) is the scatter diagram of the interference trough intensity change of the sensor near 1580 nm. The intensity change caused by temperature in the range of 20°C-60°C is 0.536 dB. The sensitivity is 0.0134 dB/°C, which is much lower than the sensitivity to DMMP, indicating that the Michelson interference has good temperature stability. Figure 10(c) shows the humidity stability of the Michelson interference structure. In the range of 25%-90% humidity, the maximum intensity change of the trough is only 0.228 dB. Figure 10(d) shows the time stability of the Michelson interferometer. The maximum change of spectral intensity within 180 min is 0.427 dB, which can be ignored compared with the change caused by DMMP vapor concentration. Therefore, the Michelson interferometer has good stability for temperature, humidity and time.
Fig. 10. (a) Selectivity, (b) Temperature stability, (c) Humidity stability, and (d) Time stability of the DMMP sensor.
The interference spectra of the mixed gases, including hydrogen, hydrogen sulfide, oxygen, ammonia, carbon monoxide, carbon dioxide and argon with/without 60 ppb DMMP are shown in Fig. 11. Within 20 s, the total trough intensity offset of the mixed gases with DMMP is 4.01 dB, and no obvious change occurs in the mixed gases without DMMP. The sensitivity of the sensor to mixed gases is about 0.067 dB/ppb.
4.5 Sensor performance comparison
Table 2 shows the comparison of the sensing performances of several different DMMP vapor sensors. The proposed sensor has a higher sensitivity, lower LOD and excellent response speed than that of other DMMP sensors. Generally, the response/ recovery performance is closely related to the temperature, sensing material and gas flow rate. In this experiment, all measurements were operated at room temperature, the low temperature will extend the response time. In addition, the electron transfer speed of semiconductor MnCo2O4 is not very high, which will extend the response time further. Therefore, the response time reached in this paper is not the fastest. During the response process, DMMP is diluted by nitrogen, and 18 L/h nitrogen is purged during the recovery measurement, which is the reason for the difference between response time (14 s) and recovery time (10 s). In addition, the sensor can measure simultaneously DMMP and temperature with high sensitivity.
Table 2. Performance comparison of different DMMP vapor sensors.
5. Conclusion
In this work, all fiber optic sensor based on Michelson interference structure cascaded FBG is designed and constructed. The trace detection of DMMP vapor is realized by coating MnCo2O4 sensing material on the surface of FCF. The structure, elemental composition and surface morphology of the sensing materials were characterized by XRD, XPS and SEM. The results show that the sensitivity of the sensor to DMMP vapor is 86.44 dB/ppm, the response/recovery time is about 14/10 s, and the detection limit is 0.1767 ppb. Meanwhile, the sensor can measure temperature simultaneously by monitoring the wavelength of FBG and Michelson interference trough. The proposed sensor has the advantages of strong anti-interference, fast response, high sensitivity and low detection limit, and provides the strategies for simultaneously measuring trace DMMP and temperature.
Funding
Banan District Science and Technology Bureau (2021TJZ011); Chongqing Science and Technology Bureau (cstc2021jcyj-msxmX0493, CSTCCXLJRC201905); Chongqing Municipal Education Commission (KJZD-K202201107); National Natural Science Foundation of China (51574054).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from authors upon reasonable request.
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