Simultaneous measurement of salinity, temperature and pressure in seawater using optical fiber SPR sensor
Introduction
With the development of ocean exploration, it needs more sensitive and timely information extraction in seawater. Depth of seawater with its corresponding temperature and salinity [1] can indirectly infer some key information such as resource distribution, ocean current trend, and so on [2], [3]. Therefore, a multi-parameter sensor probe can establish a three-dimensional data network for seawater.
As for optical fiber detection technology, characteristic wavelengths demodulation became the mainstream because of its anti-interference characteristic [4], [5], [6]. At present, research difficulties lie in: maintaining high sensitivity while implementing more parameters' detection and decoupling the cross sensitivity. Early researchers use tandem Fiber Bragg Grating (FBG) structure to achieve simultaneous measurement of salinity and temperature [7]. At that time, researchers begin to use transfer matrix method solve parameters’ cross sensitivity problem [8]. This kind of structure has good linearity, but its sensitivity is too low, which limits the application range. Some Fabry-Pérot (FP) or Mach–Zehnder interference (MZI) structures are designed to improve the sensitivity [9], [10]. Under those principles, output periodic signals are difficult to extract eigenvalues, which bring bottlenecks to introduce new parameters [11], [12], [13]. Due to high sensitivity, micro-nano fiber structure has attracted researchers’ attention in recent years [14]. However, high detection resolution could not cover its shorting of lacking stability and repeatability. Large cross-sensitivity problem of multi-parameters cannot be solved under this principle. Pressure often causes greater error interference to other measured parameters due to its special action ways [15], [16]. Introducing pressure detection into sensitivity structures to achieve simultaneous detection is always a difficult research topic. Many researches results now only focus on single parameter or two-parameter detection [17], [18], [19]. A cascaded structure under long distance has been proposed for three parameters’ detection but it is not “a single point” or “a sensor” [20]. On the one hand, multiple sensitive sources’ demodulation requires innovative design. On the other hand, if high structural integration in fiber size is not possible to achieve, packaging and practical applications also face great difficulties [21].
Fiber Surface Plasmon Resonance (SPR) effect is a hot topic because of its high sensitivity [22]. This principle shows perfect performance in some chemical quantities detection or temperature detection when combine with optical fiber technology [23]. Before that, there is no practical testing structure applied to pressure detection environment. Because surface metal film does not destroy basic fiber structure directly, this principle also has great research potential for mechanical parameters’ measurement. Based on this situation, SPR sensing principle is introduced into this work for seawater pressure measurement for the first time. Photonic crystal fiber (PCF) is chosen as an ideal excitation field in reflective probe [24]. Two kinds of sensitive materials coat on Au film. Such a design can achieve simultaneous detection of three parameters in an integrated fiber structure.
Section snippets
Design of sensitive probe
SPR technology has made much progress in high-sensitivity measurement of refractive index and biomass. In this work, it is introduced into pressure measurement for the first time. The combination of sensitive film will help achieving three characteristic outputs.
Due to characteristics of fiber SPR effect, its optic spectrum is wide, which is helpful to modulate characteristic wavelengths flexibly. Based on this, a combined reflective structure with multi-mode fiber (MMF), photonic crystal fiber
System construction
The preparation of the probe requires a certain degree of stability and repeatability to cope with high pressure, corrosion or other environments. Therefore, the preparation process and optimization parameters need to be explained.
Fig. 3 (a) gives a brief process of structural processing: (1) cascading and splicing (2) Au layer and reflector spray (3) sensitive layer coating (4) cleaning and post-processing. This method can form stable probe. Fig. 3 (b) gives a magnified view. Each sensitive
Conclusion
Based on optical fiber SPR principle, a cascaded optical fiber probe combining sensitive film is prepared for seawater parameter detection. In addition to traditional parameters (salinity: 0∼40 g/kg and temperature: 20 °C∼35 °C), pressure (0.013∼5 MPa) measurement is introduced into such an integrated area. This probe shows the simultaneous sensitivity to three parameters. Through the extraction of different characteristic wavelengths, the demodulation matrix is built. The cross interference is
Declaration of Competing Interest
There is no conflict of interest.
Acknowledgements
This work was supported in part by the National Natural Science Foundation of China under Grant 61933004 and 61773102, the Fundamental Research Funds for the Central Universities, China, under Grant N160408001, N170407005, N170407005 and in part by the State Key Laboratory of Synthetical Automation for Process Industries, China, under Grant 2013ZCX09.
Yong Zhao received his M.A. and Ph.D. degrees, respectively, in precision instrument & automatic measurement with laser and fiber-optic techniques from the Harbin Institute of Technology, China, in 1998 and 2001. He was awarded a first prize scholarship in 2000 by the China Instrument and Control Society and the Sintered Corporation (SMC) scholarship in Japan. He was a scholarship in Japan. He was a postdoctoral fellow in the Department of Electronic Engineering of Tsinghua University from 2001
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Yong Zhao received his M.A. and Ph.D. degrees, respectively, in precision instrument & automatic measurement with laser and fiber-optic techniques from the Harbin Institute of Technology, China, in 1998 and 2001. He was awarded a first prize scholarship in 2000 by the China Instrument and Control Society and the Sintered Corporation (SMC) scholarship in Japan. He was a scholarship in Japan. He was a postdoctoral fellow in the Department of Electronic Engineering of Tsinghua University from 2001 to 2003, and then worked as an associate professor in the Department of Automation, Tsinghua University of China. In 2006, he was a visiting scholar of University of Illinois in Urbana and Champagne, USA. In 2008, he was awarded as the “New Century Excellent Talents in University” by the Ministry of Education of China. In 2009, he was awarded as the “Liaoning Bai-Qian-Wan Talents” by Liaoning Province. In 2011, he was awarded by the Royal Academy of Engineering as an academic research fellow of City University London. In 2014, he was awarded by the National Science Foundation for Distinguished Young Scholars of China. In 2015, he was honored as the Yangtze River Scholar Distinguished Professor by the Ministry of Education of China. Now he is working in Northeastern University as a full professor. As the academic leader and director of his research institute, his current research interests are the development of fiber-optic sensors and device, fiber Bragg grating sensors, novel sensor materials and principles, slow light and sensor technology, optical measurement technologies. He has authored and co-authored more than 260 scientific papers and conference presentations, 24 patents, and 5 books. He is a member in the Editorial Boards of the international journals of Sensor Letters, Instrumentation Science & Technology, Journal of Sensor Technology, and Advances in Optical Technologies.
Qi-lu Wu was born in Sichuan, China, in October 1993. He received his B.A. degree in 2016 from the College of Information Science and Engineering, Northeastern University, Shenyang, China, where He is currently studying for a Ph.D. degree. His research interests include fiber optical gas sensors, photonic crystal waveguide sensors, optical detection of ocean parameters.
Ya-nan Zhang was born in Anhui, China, in June 1989. She received her M.A. degrees and Ph.D. degrees, respectively, in 2012 and 2015 from the College of Information Science and Engineering, Northeastern University, Shenyang, China. Now she is working in Northeastern University as an associate professor. Her research interests include optical fiber sensors, photonic crystal sensors, slow light technology and its sensing applications. She has authored and co-authored more than 30 scientific papers and conference presentations.