Temperature insensitive fiber optical refractive index probe with large dynamic range at 1,550 nm

doi:10.1016/j.sna.2020.112102

Sensors and Actuators A: Physical

Volume 312, 1 September 2020, 112102

https://doi.org/10.1016/j.sna.2020.112102 Get rights and content

Highlights

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The proposed sensor has a linear response in a wide range of refractive index, up to 0.1 RIU.
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The proposed sensor has ignorable temperature crosstalk.
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The relationship of refractive index and temperature of different solvents were monitored.
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The polarizability of the different solvents is calculated.

Abstract

Presented in this paper is a fiber optic sensing probe that measures the refractive index of liquid with negligible temperature crosstalk. This sensor was fabricated by femtosecond laser micro machining. The sensitivity is 885.437 to 1,067.525 nm/RIU when the ambient refractive index varies from 1.3166 to 1.4346 at 1550 nm. The refractive index dependence on temperature of water was measured and compared with the theoretical result. A maximum refractive index at around 4 °C was detected in water. The refractive index dependence on temperature of ethanol, methanol, and acetone was also measured at 1550 nm. The result of methanol given by our measurement agrees well with the previously published literature, but a significant difference was found with the estimated thermo-optic coefficient in another research.

Graphical abstract

Introduction

Since it reflects the material property in a molecular scale, real-time detection of refractive index (RI) becomes a promising way to monitor biological, physical, and chemical processes [[1], [2], [3]]. Considerable research interests have been attracted to fiber optical ambient RI sensors due to their compactness [4], high sensitivity [5], high working temperature [6] etc. Mostly, the principles of the sensors are based on the interferometry [7,8], cavity resonance [9,10], surface plasmon resonance [11,12], and fiber grating [13,14] for their insensitive of transmission or reflection loss.

In most cases, the cladding of the FBG-based sensors will be partially or completely removed to expose the FBG close enough to environment [15,16]. Tilted FBGs are also used in ambient RI sensing without a removal of cladding [17]. Usually, the FBG-based sensors have a sensitivity of several to tens of nanometers per refractive index unit (RIU) [[15], [16], [17]]. Ruibing Liang et al. [18] investigated the sensitivity of FBG-based ambient RI sensors theoretically. The results show that their sensitivity vary from 5.485 nm/RIU to 993.350 nm/RIU at around 1.35 RIU. The highest sensitivity can only be obtained when the fiber radius is reduced to 400 nm. Since long period fiber optical gratings (LPGs) directly couple core mode to cladding mode, whose effective refractive index will be affected by the surrounding environment, LPGs can be used as ambient RI sensors without any extra modification. Moreover, the sensitivity can be significantly improved by reducing the cladding radius or coating thin film. Mateusz Śmietana et al. [19] reported an LPG-based sensor with a sensitivity up to 20 μm/RIU and can even be enhanced to 40 μm/RIU. However, LPGs have a long length which limits their spatial resolution. Resonant-based ambient RI sensors, mostly Fabry-Pérot cavity and micro ring resonator, usually give a high finesse due to their multi-beam interference nature [20]. This high finesse lead to a high fringe contrast of their spectra. So far, the refractive index resolution of Fabry-Pérot cavity-based sensor reaches up to 5 × 10⁻⁹ RIU [5]. Micro knot is usually used as a ring resonator in fiber optics [21]. The effective refractive index of a micro knot can be easily influenced by ambient RI, which cause a shift of the resonant wavelength. Interference based sensors, such as Mach-Zehnder and Michelson interferometer, is highly sensitive to ambient RI. Usually, the incident light will be split into two or more beams by tapering [8], splicing [22], core offset [23], etc. For Mach-Zehnder interferometer-based sensors, the light beams will be mixed again at the core to introduce interference. The optical path of one or more of the light beams, called sensing arm, will be modulated by the ambient RI while another beam, noted as reference arm, should be isolated from the environment change. Similarly, Michelson interferometer-based sensors also have a sensing arm and a reference arm. The difference is that light in both arms will be reflected before mixed up. Therefore, Michelson interferometer-based sensor usually have a form of sensing probe, which can detect environment changes in a small space. Typically, the interference based ambient RI sensor have a sensitivity of hundreds to thousands of nanometers per RIU [24,25]. Surface plasmon resonance (SPR) is highly sensitive to the environment adjacent to the conductive thin film. A thin metal layer is attached to a surface where total internal reflection happens, this process can excite the oscillation of electron cloud at a background of positive charges (usually provided by the nucleus/protons) when light in a curtain range of wavelength is strongly absorbed. By monitoring the wavelength of the absorbed light, known as resonant wavelength, a precise detection of ambient RI is available. A sensitivity of tens of thousands of nanometers per RIU has been achieved [26,27]. However, the operating wavelength of SPR-based sensor is about 630–850 nm for silver and gold, which is out of the C-band range [28]. Natalia Díaz-Herrera et al. [29] successfully tuned the resonant wavelength to C-band by depositing TiO₂ above an Al layer on a tapered optical fiber, but a complex structure is required.

In many applications, a knowledge of temperature dependence of refractive index is requested such as the transition temperature mixture measurement [30], electrowetting liquid lens [31], and all-optical beam modulation [32] etc. Furthermore, attentions are attracted on monitoring the refractive index during chemical reactions [33] and biology processes [34] which may be accompanied by a temperature change. However, a bulky commercial refractometer is inconvenient in some applications, especially in-situ and real-time monitoring of a small amount of samples. Many optical fiber sensors have been designed for simultaneous measurement of temperature and refractive index, but a complex decoding process is still necessary [35]. Moreover, some publications treat the sensitivity of temperature and refractive index independently [[36], [37], [38]]. But in some conditions, the thermo-optic effects of the optical fiber will affect their sensitivity of refractive index by changing the parameter of the sensors. Moreover, many published temperature insensitive ambient RI sensors did not show a linear detection range higher than 0.1 RIU [39,40]. To identify the temperature dependence of refractive index of a bulk liquid, especially when the temperature fluctuates in a wide range, a temperature insensitive sensor with a large dynamic range is requested.

In this research, we propose a novel refractive index probe based on an in-fiber Michelson interferometer. The sensor was fabricated by femtosecond laser machining on the tip of a single mode fiber. After the machining, a specially designed cavity was made in the fiber tip. When the sensing tip was immersed in the liquid, the incident light was split into two beams and interference with each other after reflected. By tracking the dips in the spectrum caused by the destructive interference, the refractive index change of the liquid is available. The temperature dependence of refractive index of water was measured by this sensor to test its reliability. The wavelength shifts were transformed into the refractive index without any additional treatment of the possibly existed temperature influence. The experimental and theoretical results show a good agreement, which reveal a neglectable temperature crosstalk. The RI sensitivity of the sensor at 1550 nm was calibrated as 885.437 to 1,067.525 nm/RIU for different dips, when ambient RI varies from 1.3166 to 1.4346. To our knowledge, little literature about the temperature dependence of refractive index at 1550 nm for different solvents can be found. The refractive indices of ethanol, methanol, and acetone were measured from 0 °C to their boiling points in this research. Since many optical fiber sensors work in C-band, the measured data are important in the calibration, evaluation, and application of these sensors. Moreover, polarizability of these solvents was also calculated based on the measured RI, since it is the fundamental of light-matter interaction [41]. Although the theoretical value can be calculated from electromagnetic theory [42], an experimental measurement is still necessary. Furthermore, many important material parameters such as molecular anisotropy and Kerr effect constant can also be deduced from polarizability. And the polarizability is also required when solving many scientific or engineering problems [43,44]. A difference of polarizability was observed by comparing the results at 1550.0 nm and 589.3 nm since it’s a frequency dependent parameter. This sensing probe provides a reliable measurement of refractive index of liquid in chemical reaction, biological process, microfluid, etc. without consideration of temperature. Other applications are also possible by filling the sensing structure with functional materials.

Section snippets

Design of the sensor

Fig. 1 shows the connection of the sensor. One end of the optical sensing cable was connected to the interrogator and receives the broadband light. The sensing head was immersed inside the target liquid, the light will be reflected into the interrogator from the optical fiber-liquid interface. The interrogator was connected to a PC, and the reflected spectrum can be analyzed using the ENLIGHT software provided by the vendor of the interrogator.

Fig. 2(a) shows the schematic graph of the sensor.

Fabrication process

Fig. 4 shows the setup used in this research. Femtosecond laser (CARBIDE, 1030 nm, 229 fs to 10 ps, 4 W) was launched and amplified by a beam expander (Thorlabs, BE02-05-B). The attenuator (Altechna, WATT PILOT) was used the adjust the energy of the laser. Then, the shutter can control the on/off state of the laser combined with the Hexapod using G code. After reflected by a series of mirrors, the laser was finally focused on the optical fiber using a 20X objective lens (Mitutoyo, 0.42 NA).

Results and discussion

In this part, we will discuss the performance of the sensor. Sucrose solution with different concentrations were used to calibrate the sensitivity of refractive index at 1550 nm. Experimental and theoretical results on the refractive index dependence of temperature of water were compared. The temperature dependence of ethanol, methanol, and acetone at 1550 nm were also measured and the related polarizabilities were calculated.

Saunders et al. [45] measured the refractive indices of sucrose

Conclusion

In this paper, we proposed a novel fiber optical refractive index sensor fabricated by femtosecond laser. This sensor works as a temperature sensor in the air, while is a temperature insensitive refractive index sensor when immersed in liquid. When placed in the air, a Fabry–Pérot cavity is formed due to the total internal reflection. When temperature changes, the Airy distribution shifts due to the thermo-optic effect of the cladding. The temperature sensitivity is measured as 16.2 pm/°C, 14.1

Author statement

The contributions of the authors are described as the following:

-Fengfeng Zhou is the first authors and led formation of the idea, conducted experiments, and performed analysis. He was also responsible of writing the manuscript.
-Huaizhi Su assisted in idea formation and advised on the fundamental theory during the idea formation, data analysis, and writing of the manuscript.
-Hangeun Joe developed methodology for the experiments, advised Fengfeng on how to conduct experiments. She also wrote

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research has been partially supported by National Natural Science Foundation of China (SN: 51979093, 51739003), the National Key Research and Development Program of China (SN: 2018YFC0407101, 2016YFC0401601, 2017YFC0804607), Key R&D Program of Guangxi (SN: AB17195074), Open Foundation of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering (SN: 20195025912, 20165042112), the China Scholarship Council (SN: CSC201806710081). and Technology Innovation Program (10053248)

Fengfeng Zhou is a Ph. D. student of Hohai University (Hydraulic Structure Engineering), and a visiting student of Purdue University (School of Mechanical Engineering). He received his master’s degree in Southeast University (Civil Engineering) and B.S. degree in Nantong University (Optical Information Science and Technology). His research interest is optical fiber sensing, including refractive index, vibration, acoustic emission, etc.

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Huaizhi Su received M.S. and Ph.D. degrees from Hohai University, Nanjing, China, in 1999 and 2002, respectively. He joined the Hohai University, Nanjing, in 2002, where he is currently working toward his research work into safety monitoring, risk analysis, and residual life prediction for hydraulic structure engineering. From 2010–2011, he was a Visiting Scholar with the Washington State University, Pullman, WA, USA. He is currently a Professor of hydraulic structure engineering. He has published more than 200 papers in journals and international conferences.

Hang-Eun Joe was a postdoctoral researcher in Martin Jun's Lab, School of Mechanical Engineering at Purdue University, West Lafayette, IN, USA. Since December in 2019, she is currently a staff engineer at Semiconductor R&D center, Samsung electronics. She received the BSc and PhD degrees in Mechanical Engineering from Yonsei University, Seoul, South Korea in 2010 and 2016, respectively. She was granted Brain Korea 21 Plus Scholarship from National Research Foundation of Korea for her degrees. Her research interests include micro/nano manufacturing, focused ion beam applications, femtosecond laser process, optical fiber sensor, surface plasmon resonance applications.

Martin Jun is an Associate Professor of the School of Mechanical Engineering at Purdue University, West Lafayette, IN, USA. He received the BSc and MASc degrees in Mechanical Engineering from the University of British Columbia, Vancouver, Canada in 1998 and 2000, respectively. He then received his PhD degree in 2005 from the University of Illinois at Urbana-Champaign in the Department of Mechanical Science and Engineering. He has authored over 100 peer-reviewed journal publications. He is an ASME fellow. He is also the recipient of the 2011 SME Outstanding Young Manufacturing Engineer Award, 2012 Canadian Society of Mechanical Engineers I.W. Smith Award for Outstanding Achievements, and 2015 Korean Society of Manufacturing Technology Engineers Damwoo Award.

¹: Fengfeng Zhou and Huaizhi Su contributed equally.

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