Erbium-doped fiber ring laser with SMS modal interferometer for hydrogen sensing

Optics and Laser Technology 102 (2018) 262–267

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Erbium-doped fiber ring laser with SMS modal interferometer for hydrogen sensing Ya-nan Zhang a,b,⇑, Lebin Zhang a, Bo Han a,c, Huijie Peng a, Tianmin Zhou a, Ri-qing Lv a a

College of Information Science and Engineering, Northeastern University, Shenyang 110819, China State Key Laboratory of Synthetical Automation for Process Industries, Shenyang 110819, China c Liaoning Provincial Institute of Measurement, Shenyang 110819, China b

a r t i c l e

i n f o

Article history: Received 17 October 2017 Received in revised form 4 December 2017 Accepted 4 January 2018

Keywords: Hydrogen sensor Single mode-multimode-single mode Fiber ring laser Optical fiber sensor

a b s t r a c t A hydrogen sensor based on erbium-doped fiber ring laser with modal interferometer is proposed. A single mode-multimode-single mode (SMS) modal interferometer structure coated with Pd/WO3 film is used as the sensing head, due to that it is easy to be fabricated and low cost. The sensing structure is inserted into an erbium-doped fiber ring laser in order to solve the problem of spectral confusion and improve the detection limit of the hydrogen sensor based on the SMS modal interferometer. The SMS sensing structure is acted as a fiber band-pass filter. When hydrogen concentration around the sensor is changed, it will induce the refractive index and strain variations of the Pd/WO3 film, and then shift the resonant spectrum of the SMS modal interferometer as well as the laser wavelength of the fiber ring laser. Therefore, the hydrogen concentration can be measured by monitoring the wavelength shift of the laser, which has high intensity and narrow full width half maximum. Experimental results demonstrate that the sensor has high sensitivity of 1.23 nm/%, low detection limit of 0.017%, good stability and excellent repeatability. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction As a kind of clean and renewable energy, hydrogen has great application potentials in various fields such as automobile industry, aerospace engineering, and chemical production. However, hydrogen is explosive and flammable due to its wide explosive range and low ignition energy. Therefore, it is very necessary to develop a high-sensitive, high-precision, and safety hydrogen sensor to monitor the hydrogen concentration [1,2]. On the other hand, optical fiber sensors based on different operating principles have been widely applied because of their unique advantages such as small size, immune to electromagnetic interference, intrinsic safety, and remote sensing [3–7]. For hydrogen sensing, fiber Bragg grating (FBG) was firstly developed due to its distributed measurement ability and mature fabrication technique [8,9]. However, its sensitivity is relatively low. To resolve this problem some coreexposed methods, such as side-polishing and chemical etching [10,11], are applied to FBG, but they will also destroy the longterm stability of the FBG based hydrogen sensor. Besides, Hydrogen sensor based on evanescent field is the most common one in practical applications [12–15]. It usually measures the hydrogen ⇑ Corresponding author at: College of Information Science and Engineering, Northeastern University, Shenyang 110819, China. E-mail address: [email protected] (Y.-n. Zhang). https://doi.org/10.1016/j.optlastec.2018.01.016 0030-3992/Ó 2018 Elsevier Ltd. All rights reserved.

concentration by monitoring the intensity of transmitted light. However, the intensity signal is easily disturbed by external environment and then introduce unpredictable errors to hydrogen sensor. Another hydrogen sensor based on surface plasmon resonance (SPR) has received great attentions due to its advantage of high sensitivity [16–18], but the production complexity and cost of the sensing head are high. In contrast, optical fiber interferometers, including Fabry-Perot interferometer (FPI) [19,20], Sagnac interferometer [21–23], and modal interferometer [24–26], have been much more popular in hydrogen sensing, due to their simple and flexible structures. However, the FPI based hydrogen sensing head faces the restriction on cavity size due to coupling loss, offset of fiber end faces, and large transmission loss; the Sagnac interferometer with a closed loop will suffer many external interfering factors, such as vibration and temperature, in the sensing system, and finally result in measurement errors. In particular, the modal interferometer is easy to be fabricated and behave good stability, which is much more suitable for hydrogen sensing. For convenience, quantitative comparisons between the sensitivities and the corresponding concentration ranges of various types of hydrogen sensors are presented in Table 1. Due to the low cost, simple structure, and moderate sensitivity of the hydrogen sensor based on modal interferometer, it has been widely researched and developed in recent years. However, multiple modes exist and interfere with each other in the

Y.-n. Zhang et al. / Optics and Laser Technology 102 (2018) 262–267 Table 1 Properties comparison of various types of hydrogen sensors. Type

Sensitivity

Concentration range

Ref.

FBG Evanescent field SPR F-P Sagnac Modal interferometer

15 pm/% – 4.4 nm/% 18.75 nm/% 0.131 nm/% 0.4 nm/%

0–4% 1.8–10% 0–4% 0–8% 1–4% 0–5%

[11] [13] [17] [19] [22] [26]

263

by monitoring the wavelength shift of the resonant spectrum. Then, combing the wavelength demodulation ability of FRL, the output resonant spectrum becomes high intensity and narrow full width half maximum (FWHM). The final experiment results demonstrate that the obtained sensitivity of the hydrogen sensor can reach to 1.23 nm/% within the hydrogen concentration range of 0–1%. 2. Hydrogen sensing principle of the SMS fiber structure

interferometer, resulting in a broad and disordered output spectrum, whose resonant peaks are difficult to search. To overcome this problem, the fiber laser has been used in recent years to demodulate the output spectra of interferometers and realize the measurement of temperature [27,28], refractive index [29,30], bend [31], strain [32], and liquid level [33]. According to the above analyses, a new type of hydrogen sensor based on erbium-doped fiber ring laser (FRL) with modal interferometer is proposed in this paper. The modal interferometer is consisted of a single mode-multimode-single mode (SMS) fiber structure, which is easier to be fabricated and less cost as compared to other optical devices with similar properties. The Pd/ WO3 fabricated by sol-gel method is coated on the surface of the multimode fiber (MMF) and acted as the sensitive material to hydrogen. When the surrounding hydrogen concentration changes, the resonant spectrum of the modal interferometer will shift, due to the refractive index and strain variations of Pd/WO3 film on the MMF. Therefore, the hydrogen concentration can be measured

The SMS fiber structure is selected in this paper, as shown in Fig. 1. A segment of MMF coated with the Pd/WO3 film is inserted between two sections of single-mode fiber (SMF). The electric field distribution of the SMS fiber structure is simulated by beam propagation method (BPM) with using Rsoft software. Fig. 2(a) shows the structural model of the SMS fiber structure that built in the software, while Fig. 2(b) shows the beam propagation situation of this structure. It can be seen that when the light transmits through the joint region of SMF and MMF, some light leaks out to the cladding due to the modal field mismatch and the others continue to transmit forward along the fiber core with the fundamental mode. Then, at another joint region, the cladding modes will return back and interfere with the core mode. Due to the phase difference between the core mode and the cladding modes, an interference output can be observed. Because of multimodal interference effect, the SMS structure can be acted as an optical band-pass filter. The peak wavelength of the band-pass filter can be expressed as [34,35]:

nMMF D2MMF k0 ¼ p L

Fig. 1. Schematic structure of SMS modal interferometer with Pd/WO3 coating.

!

ð1Þ

where nMMF is the effective refractive index of the fundamental mode; DMMF is core diameter of the MMF; p is an integer corresponding to the self-image number; and L is the length of the MMF. Taking advantages of the high hydrogen absorption capacity and selectivity of metal Pd, and the high hydrogen sensitivity, good adhesion and mechanical properties of WO3, the Pd/WO3 can be well used as hydrogen sensitive material. The hydrogen atoms

Fig. 2. (a) Structural model of SMS fiber structure; (b) beam propagation situation of the SMS fiber structure.

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Fig. 4. Schematic configuration of the hydrogen sensor based on SMS fiber structure.

Fig. 3. (a) Simulated resonant spectra of the SMS modal interferometer under different refractive index of the Pd/WO3 film. (b) Relation between the refractive index of the Pd/WO3 film and the resonant wavelength.

can be transferred to the surface of the inner holes of WO3 by Pd, and then the exothermic reaction between hydrogen atom and WO3 takes place under the catalysis of Pd, which will finally cause the refractive index variation and volume expansion of the Pd/WO3 coating. According to Ref. [36], the refractive index of the Pd/WO3 film decreases with the increase of the hydrogen concentration, and the refractive index range of the film is about 1.993–1.995 when the film thickness 1 lm. As shown in Fig. 3, the sensitivity of the resonant wavelength shift to the refractive index variation of the Pd/WO3 film can be up to 1873.43 nm/RIU for the SMS modal interferometer. Therefore, the hydrogen concentration can be measured by monitoring the wavelength shift of the resonant spectrum of the SMS modal interferometer, and the resonant spectrum moves to the short wavelength direction when the hydrogen concentration is increased. Besides, it should be mentioned that the refractive index is dependent on the thickness of the Pd/WO3 film.

3. Experimental and discussion 3.1. Hydrogen sensing experiment of the SMS fiber structure Fig. 4 shows the schematic structure of the hydrogen sensor based on the SMS fiber interferometer, which consists of an amplified spontaneous emission (ASE) source, a SMS fiber structure, and an optical spectrum analyzer (OSA). The SMS fiber structure is

formed by splicing a 4 cm long MMF between two sections of SMF. The MMF has a core diameter of 105 lm and a step index profile. The Pd/WO3 was fabricated by sol-gel method and coated onto the surface of the MMF by using dip coating method [36]. It should be mentioned that the molar ratio of Pd:W and the thickness of Pd/ WO3 coating are two critical parameters for hydrogen sensing. By doing a lot of testing experiments, it is summarized that: (1) With the increase of the molar ratio of Pd:W, the response time and selectivity of the hydrogen sensor will be improved, but the stability of the Pd/WO3 film will become worse; (2) With the increase of the thickness the Pd/WO3 film, the measurement sensitivity will be improved, but the response time and recovery time will become longer due to the longer diffusion time within the Pd/WO3 film [15]. Therefore, by comprehensively considering the stability, sensitivity and the response velocity of the hydrogen sensor, the molar ratio of Pd:W should be chosen between 1:100 and 1:150, and the thickness of the Pd/WO3 film should be around 1 lm with the coating velocity of 0.1 mm/min. For different hydrogen concentrations, the resonant spectra of the SMS fiber interferometer with Pd/WO3 coating are measured, as show in Fig. 5(a). Then, the relationship between the resonant wavelength and the hydrogen concentration can be obtained, as shown in Fig. 5(b). It can be seen that the resonant spectrum will linearly shift to short wavelength direction with the increase of hydrogen concentration, and the sensitivity can reach 1.13 nm/%. However, we can also see that the resonant spectrum is fairly broad, which makes it hard to find the resonant wavelength, resulting in a large reading error and a low measurement precision.

3.2. Hydrogen sensing experiment of the SMS fiber structure with FRL In order to eliminate the confusion of the resonant spectrum and improve the measurement precision of the hydrogen sensor, an erbium-doped FRL is used to interrogate the resonant spectrum of the SMS modal interferometer, as shown in Fig. 6. The laser light from a 980 nm pumping source entered to the 980 nm port of a 980/1550 nm wavelength division multiplexer (WDM), meanwhile the common port of the WDM is connected to an erbium-doped fiber (EDF) with length of 3 m for generating gain amplification. The output of the EDF is connected to an isolator (ISO), which is used to ensure the unidirectional propagation of light. Then the light enters the SMS interferometer, which is placed in the gas chamber for sensing the variation of the hydrogen concentration. The SMS serves as a filter [37] and its output port is connected to

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265

Fig. 7. Transmission spectra of the SMS fiber structure (blue dotted line) and the FRL with SMS modal interferometer (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. It can be seen that the SMS fiber interferometer has played a filtering role and the FWHM of the output spectrum signal is largely narrowed. The FWHMs of the resonant spectrum and laser spectrum are 2.10 nm and 0.049 nm, respectively. The pump power is set to be 48.6 mW, and the hydrogen concentration is changed from 0% to 1% at 0.2% step. The output spectra of the erbium-doped FRL with SMS modal interferometer are measured as shown in Fig. 8, from which we can get that the sensitivity can reach 1.23 nm/% in the concentration range of 0–1%. According to Ref. [38], the wavelength resolution of the sensor can be calculated as:

R ¼ 3r ¼ 3 Fig. 5. (a) Resonant spectra of the SMS fiber structure under different hydrogen concentrations; (b) The relation between hydrogen concentration and the resonant wavelength.

Fig. 6. Experimental configuration of the hydrogen sensor based on FRL with SMS modal interferometer.

the common port of a 99:1 coupler. Finally, the 99% port of the coupler is connected to the 1550 nm port of the WDM in order to keep most of the light continuing to propagate in the ring, and the 1% port of the coupler is connected to an optical spectrum (OSA). The laser spectrum received by the OSA is compared with the resonant spectrum of the single SMS fiber structure, as shown in

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r2ampl-noise þ r2spect-res þ r2temp-induced

ð2Þ

where rampl-noise , rspect-res , rtemp-induced represent the amplitude noise, the spectral resolution and the thermal variation of the system, respectively. Besides, they can be approximately expressed pffiffiffi as: rampl-noise ¼ ðFWHMÞ=ð4:5  ðSNRÞ0:25 Þ, rspect-res ¼ Rw =2 3, rtemp-induced ¼ 10fm. Here, the SNR, namely signal-to-noise radio, is assumed to be about 60 dB in such a spectrum with good quality [39]; Rw is the wavelength scanning resolution of the OSA, and to match the spectral quality, in the experiment the Rw is set to be 0.2 nm and 0.02 nm for the resonant spectrum and laser spectrum, respectively. According to the above equations, the calculated wavelength resolutions of the sensor before and after the introduction of erbium-doped FRL are 0.505 nm and 0.021 nm, respectively. Therefore, the detection limits of hydrogen sensors before and after the introduction of erbium-doped FRL are 0.447% and 0.017%, respectively. In other word, although the sensitivity changes little, the detection limit of the hydrogen sensor with erbium-doped FRL is improved more than 26 times. The stability of the hydrogen sensor is investigated for 1% hydrogen concentration, as shown in Fig. 9(a). After the laser spectrum became stable, we recorded the shift of the laser wavelength every 5 min. The maximum error is less than 0.015 nm within 30 min. As the resolution of the OSA used in this experiment is 0.02 nm, this slight wavelength fluctuation may be caused by the reading error. Then, the repeatability experiment of the hydrogen sensor is done for three times, as shown in Fig. 9(b). It can be seen that the three measurement curves are almost parallel but the intercept of the curve has a slight change. Namely, the original resonant wavelength of the sensor with hydrogen concentration of 0% is

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Fig. 8. (a) Resonant spectra of the sensing structure based on the erbium-doped fiber laser under different ambient hydrogen concentration. (b) The relation between hydrogen concentration and the resonant wavelength.

different in each measurement experiment. Besides the measuring errors of optical instruments, we infer that this change is mainly due to that the surrounding temperature and humidity are different in each test. The reasons are as follows: The temperature and humidity variations can all change the physicochemical properties of Pd/WO3 film and then change the sensitivity of Pd/WO3 film to hydrogen concentration variation. Besides, the interferometer sensor itself has a response to temperature variation. To demonstrated this thought, the responses of the hydrogen sensor towards temperature and humidity changes are investigated. Results show that the temperature sensitivity of the proposed hydrogen sensor is 0.084 nm/°C in the range of 21–25 °C, while the humidity variation in 20–50% has a very slight influence on the hydrogen sensor (can be neglected). Therefore, the slight deviation in repeatability experiment is mainly caused by the fluctuations of room temperature. However, it should be mentioned that the cross sensitivities between the temperature and the hydrogen concentration can be resolved by measuring the hydrogen concentration and temperature simultaneously [40]. 4. Conclusion In conclusion, a new type of hydrogen sensor based on erbiumdoped FRL with SMS modal interferometer is investigated. The SMS

Fig. 9. (a) Resonant wavelength versus time for 1% hydrogen concentration. (b) Repeatability curves of the sensor.

fiber structure is used as the sensing head due to its simple structure and easy fabrication process, and it also acts as the band-pass filter of the FRL. The combination of the SMS fiber structure and the FRL has high intensity (25 dB) and narrow FWHM (0.049 nm). Finally, the sensing system achieves a relativity high hydrogen sensitivity of 1.23 nm/% and low detection limit of 0.017%. Acknowledgments This work was supported in part by the National Natural Science Foundation of China under Grants 61703080 and 61273059, the Fundamental Research Funds for the Central Universities under Grant N160404012, the Liaoning Province Natural Science Foundation under Grant 20170540314, and State Key Laboratory of Synthetical Automation for Process Industries under Grant 2013ZCX09. References [1] T. Hübert, L. Boon-Brett, G. Black, U. Banach, Hydrogen sensors – a review, Sens. Actuators B 157 (2011) 329–352. [2] Y.N. Zhang, H.J. Peng, X.L. Qian, Y.Y. Zhang, G.W. An, Y. Zhao, Recent advancements in optical fiber hydrogen sensors, Sens. Actuators B 244 (2017) 393–416. [3] A. Van Newkirk, J.E. Antonio-Lopez, G. Salceda-Delgado, M.U. Piracha, R. Amezcua-Correa, A. Schülzgen, Multicore fiber sensors for simultaneous

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