ABSTRACT We experimentally demonstrated a novel fiber-optic hybrid structured Fabry–Perot interferometer with special air-cavity for simultaneous measurement of transverse load and

temperature. By the linear phase finite impulse response filters, the transverse load sensitivities of the air-cavity and the silica-cavity are 1272.71 pm/N and −53.07 pm/N, respectively,

and temperature sensitivities of the air-cavity and silica-cavity are 1.1 pm/°C and 14 pm/°C. Thus, the different sensitivities of silica-cavity and air-cavity to transverse load and

temperature indicate that such a structure can be used to simultaneously measure transverse load and temperature. SIMILAR CONTENT BEING VIEWED BY OTHERS CASCADED FABRY-PEROT INTERFEROMETER

years, fiber-optic Fabry–Perot interferometer (FPI) has drawn great attention and been used for various physical quantities sensing, such as temperature1,2,3, strain4,5,6,7, pressure8,9,10,

and transverse load11 _et al_., due to its advantages of low cost, high sensitivity, ultra-compactness and reliability. During the physical quantity sensing process, temperature fluctuation

will introduce extra error. Normally, temperature compensation is added to the sensing system, which make it quite complex. Another way to solve this problem is to realize concurrent sensing

of desired physical quantity and temperature. This is not just diminishing cost and complexity of the sensing system but also solving the temperature-induced crossing-sensitivity issue.

Therefore, simultaneous measurement of desired physical quantity and temperature has became an important topic in sensing. Various hybrid structured FPIs have been fabricated for concurrent

measurement of temperature and pressure12, temperature and refractive index13, pressure and temperature14, 15, temperature and strain16. In 2012, Pevec S. _et al_. proposed and fabricated

hybrid structured FPI which consisted of two low-finesse Fabry–Perot resonators integrated into a standard lead-in single mode fiber (SMF) for simultaneous measurement of pressure and

temperature14. At the same time, in 2014, Pevec S. _et al_. also proposed and fabricated another simultaneous measurement of pressure and temperature sensor based on hybrid structured FPI by

chemical etching. In 2014, Zhou A. _et al_. proposed and fabricated hybrid structured FPI by fusion splice between SMF and several electrical arc discharges for simultaneous measurement of

strain and temperature. However, these hybrid structured FPIs reported are not suitable for simultaneous measurement of transverse load and temperature for the reason that cavity heights of

these structures are not higher than the cladding diameter of the SMF. In this paper, a novel fiber-optic hybrid structured FPI with special air-cavity that air-cavity height is higher than

the cladding diameter of SMF is proposed and experimentally demonstrated for simultaneously measure transverse load and temperature. The hybrid structured FPI can be easily fabricated by

fusion splice SMF to silica capillary and then electrical arc discharge melting capillary to become hollow microsphere with special air-cavity, and final fusion splice SMF to hollow

microsphere and cleaving to form silica-cavity. The transverse load sensitivity of air-cavity is positive, on the contrary, silica-cavity is negative. In addition, silica-cavity to

temperature is more sensitive compared to air-cavity. Therefore, the hybrid structured FPI proposed is appropriate for application to simultaneously measure transverse load and temperature.

FABRICATION AND PRINCIPLE The process of the hybrid structured FPI proposed is shown in Fig. 1(a)–(d). Firstly, a silica capillary with outer diameter of 125 μm and inner diameter of 50 μm

as shown in Fig. 1(e) was spliced to SMF by fiber fusion splicer (Fujikura FSM-45PM), as shown in Fig. 1(a). In order to guarantee silica capillary not collapsed during the fusion splice

process, the fusion splicer was set to special parameters that arc discharge power was −100 bit, and arc discharge duration time was 400 ms and arc distance from discharge position to fusion

splice point was 120 μm. Furthermore, arc of extremely strong (70 bit) and long duration time (2000 ms) discharge deviating about 160 μm from splice point were used to make sure that silica

capillary was completely collapsed and cut off to form hollow microsphere with an air-cavity, as shown in Fig. 1(b). Finally, SMF was spliced to the end of the hollow microsphere and SMF

was cleaved to become silica-cavity, as shown in Fig. 1(c) and (d). The microscope image of hybrid structured FPI is shown in Fig. 1(f). The air-cavity height and length are respectively 170

 μm and 85 μm, and silica-cavity length is 130 μm. As shown in Fig. 1(d), _I_ 1, _I_ 2, and _I_ 3 are light intensities reflected by reflective surface M1, M2, and M3, respectively; _L_ 1

and _L_ 2 are respectively the air-cavity and silica-cavity length. The intensity of the interference fringes can be written as $$I={I}_{1}+{I}_{2}+{I}_{3}+2\sqrt{{I}_{1}{I}_{2}}\,\cos

({\varphi }_{{\rm{air}}})+2\sqrt{{I}_{2}{I}_{3}}\,\cos ({\varphi }_{{\rm{silica}}})+2\sqrt{{I}_{1}{I}_{3}}\,\cos ({\varphi }_{\mathrm{air} \mbox{-} \mathrm{silica}})$$ (1) where _ϕ_ air = 

4π_n_ 1 _L_ 1/λ, _ϕ_ silica = 4π_n_ 2 _L_ 2/λ, _ϕ_ air-silica = _ϕ_ air + _ϕ_ silica, are the phase shifts corresponding to air-cavity, silica-cavity, and the hybrid-cavity, respectively;

_n_ 1 and _n_ 2 are respectively refractive indexes of air and SMF; _λ_ is the incident light wavelength. The reflection spectrum was observed by a broadband source, 3 dB coupler and an

optical spectrum analyzer. The reflection spectrum of the hybrid structured FPI is shown in Fig. 2(a). The spatial frequency spectrum was acquired by fast Fourier transform of the reflection

spectrum, as shown in Fig. 2(b). There are three peaks in the spatial frequency spectrum that peak 1, peak 2 and peak 3 are resulted from air-cavity, silica-cavity and hybrid-cavity (air

plus silica). The spatial frequency values of peak 1, peak 2 and peak 3 are _f_ 1 = 2_n_ 1 _L_ 1/_λ_ 1 _λ_ 2, _f_ 2 = 2_n_ 2 _L_ 2/_λ_ 1 _λ_ 2 and _f_ 3 = _f_ 1 + _f_ 2, respectively, where

_λ_ 1 and _λ_ 2 are two adjacent dips wavelengths of the reflection spectrum. By the linear phase finite impulse response filters, the wavelength spectra of air-cavity and silica-cavity can

be extracted from the reflection spectrum, as shown in Fig. 3. Such a hybrid structured FPI can be used to simultaneously measure transverse load and temperature. The transverse load

increasing can cause the air-cavity height to shorten and air-cavity length to lengthen. Thus, spectrum of air-cavity is redshift with the transverse load increasing. At the same time, since

phase shift of silica-cavity is reducing with the transverse load increasing, spectrum is blueshift with the transverse load increasing for silica-cavity. In addition, for temperature

sensing, air-cavity is only affected by thermal expansion coefficient of silica, and silica-cavity is affected by the thermal expansion coefficient and the thermo-optic coefficient of

silica, thus the silica-cavity is more sensitive to temperature than air-cavity. Since the air-cavity and the silica-cavity are different response to transverse load and temperature, this

hybrid structured FPI can realize to simultaneously measure transverse load and temperature. EXPERIMENTS AND DISCUSSIONS The experimental setup of transverse load for this hybrid structured

FPI is shown in Fig. 4. The hybrid structured FPI is horizontally placed between two parallel glass slides in the transverse load measurement. As shown in Fig. 5(a), spectrum of air-cavity

has a redshift with the transverse load increasing and the transverse load sensitivity of 1272.71 pm/N is acquired. Moreover, with the increasing of transverse loads, spectrum has a

blueshift and its sensitivity to transverse load is −53.07 pm/N for silica-cavity, as shown in Fig. 5(b). Since the transverse load increases, the air-cavity height is shorter and air-cavity

length is longer. Therefore, spectrum appears redshift with the transverse load increasing for air-cavity that is in accordance to the experimental results. As shown in Fig. 6(a) and (b),

simulated light propagation by beam propagation method in the hybrid structured FPI with silica-cavity length of 130 μm for different shapes of air-cavities, at the input wavelength of 1550 

nm, where the z-axis is the light propagation direction. Figure 6(a) and (b) are respectively corresponding to the air-cavities of 170 × 85 μm (height × length) and 150 × 105 μm. From the

Fig. 6(a) and (b), light beam reflected by spherical reflector M2 can be approximately a near axis optical system. The divergent beam reflected can be converged at convergent point through

reflected by spherical, as shown in Fig. 6(c). Therefore, the strongest interference of the two beams reflected by reflective surface M2 and M3 should be at convergent point. The intensity

of the interference fringes for silica-cavity can be accurately written as $$I={I}_{2}+{I}_{3}2\sqrt{{I}_{2}{I}_{3}}\,\cos (\frac{4\pi {n}_{2}{L}_{2}}{\lambda }+\frac{4\pi

{n}_{1}{L}_{3}}{\lambda }),$$ (2) where _L_ 3 is the distance between convergent point and reflective surface M2. Schematic diagram of spherical reflected light is shown in Fig. 6(d). The

relation between object and image of spherical mirror is as follow $$\frac{1}{{L}_{1}}+\frac{1}{{L}_{3}}=\frac{2}{R},$$ (3) where _L_ 1 is distance between beam divergent point and

reflective surface M2(air-cavity length); _R_ is the radius of spherical reflector M2. The transverse load makes the air-cavity length (_L_ 1) lengthen as well as air-cavity height shorten

so that it causes _R_ to become smaller. According to Eq. (3), _L_ 3 is smaller for the reason that _L_ 1 is larger and _R_ is smaller with the transverse load increasing. Since phase shift

of silica-cavity is smaller as result of smaller _L_ 3 according to Eq. (2), the spectrum is blueshift with the transverse load increasing for silica-cavity. The experimental results are in

accordance to the theoretical analysis. To investigate the temperature response of this structure, the structure is placed in a furnace to raise its temperature from 100 °C to 800 °C with a

step of 100 °C. The wavelength shift for the air-cavity and silica-cavity with different temperature are shown in Fig. 7. The temperature sensitivities of the air-cavity and silica-cavity

are respectively 1.1 pm/°C and 14 pm/°C. The experimental results show that the silica-cavity is about 10 times more sensitive to temperature than silica-cavity. The wavelength of dip is

respectively _λ_ 0 = 2_n_ 1 _L_ 1/_m_ and _λ_ 0 = 2_n_ 2 _L_ 2/_m_ for air-cavity and silica-cavity, where _m_ is integer. The wavelength shift of air-cavity and silica-cavity to temperature

are given by $$\frac{{\rm{\Delta }}\lambda }{{\rm{\Delta }}T}=(\frac{{\rm{\Delta }}L}{{\rm{\Delta }}T\ast \,L}+\frac{{\rm{\Delta }}n}{{\rm{\Delta }}T\ast \,n})\lambda =(\varepsilon +\kappa

)\lambda $$ (4) where _ε_ = 5.5 × 10−7 and _κ_ = 1.0 × 10−5 are respectively the thermal expansion coefficient and the thermo-optic coefficient for silica11. It is obviously that the thermal

expansion coefficient only affects air-cavity, however the thermal expansion coefficient and the thermo-optic coefficient affect silica-cavity. The silica-cavity is about 10 times more

sensitive to temperature than air-cavity for the reason that the thermo-optic coefficient is over 10 times larger than the thermal expansion coefficient for silica. The experimental results

are in accordance to the theoretical analysis. The experimental results show spectrum has redshift to transverse load for air-cavity whereas spectrum of silica-cavity has blueshift. In

addition, the sensitivity of silica-cavity is more over 10 times to temperature than the air-cavity. Due to different response of air-cavity and silica-cavity to transverse load and

temperature for this structure, it can realize to simultaneously measure transverse load and temperature. The resolution matrix for concurrent measurement can be expressed as

$$[\begin{array}{c}{\rm{\Delta }}N\\ {\rm{\Delta }}T\end{array}]=[\begin{array}{c}{k}_{{\rm{11}}}\\ {k}_{{\rm{21}}}\end{array}{\begin{array}{c}{k}_{{\rm{12}}}\\

{k}_{{\rm{22}}}\end{array}]}^{-1}[\begin{array}{c}{\rm{\Delta }}{\lambda }_{{\rm{air}}}\\ {\rm{\Delta }}{\lambda }_{{\rm{silica}}}\end{array}]$$ (5) where _k_ 11 and _k_ 12 are respectively

the transverse load and temperature sensitivity of the air-cavity and _k_ 21 and _k_ 22 are respectively the transverse load and temperature sensitivity of the silica-cavity. In the matrix,

Δ_λ_ air and Δ_λ_ silica represent the wavelength shifts of air-cavity and silica-cavity, respectively; Δ_N_ and Δ_T_ are respectively variations of transverse load and temperature. The

performance of simultaneous measurement of transverse load and temperature for the sensor was estimated by resolution matrix. Choosing 400 °C as reference temperature, Δ_λ_ air and Δ_λ_

silica obtained transverse load variations in a range of 0–3.2634 N are inputted into resolution matrix to analyze the effects of varying transverse load on temperature measurement.

Selecting 1.813 N as reference transverse load, Δ_λ_ air and Δ_λ_ silica acquired temperature variation from 100 °C to 800 °C are brought into resolution matrix to investigate the effects of

changing temperature on transverse load measurement. As shown in Fig. 8, the maximum deviations calculated by resolution matrix are respectively ~0.0489 N and ~2 °C for simultaneous

measurement of transverse load and temperature. CONCLUSIONS In conclusion, a novel hybrid structured fiber-optic FPI is proposed and experimental demonstrated for simultaneous measurement of

transverse load and temperature with the advantages of high sensitivity, low cost and compact, and easy fabrication. Owing to different response of air-cavity and silica-cavity to

transverse load and temperature, simultaneous measurement of transverse load and temperature can be easily achieved by a resolution matrix method. Experimental results indicate that such a

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references ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (NSFC) under Grant Nos 61078006 and 61275066, and the National Key Technology Research

and Development Program of the Ministry of Science and Technology of China under Grants No. 2012BAF14B11. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * National Key Laboratory of Tunable

Laser Technology, Institute of Opto-Electronics, Harbin Institute of Technology, Harbin, 150080, China Yongfeng Wu, Yundong Zhang, Jing Wu & Ping Yuan Authors * Yongfeng Wu View author

publications You can also search for this author inPubMed Google Scholar * Yundong Zhang View author publications You can also search for this author inPubMed Google Scholar * Jing Wu View

author publications You can also search for this author inPubMed Google Scholar * Ping Yuan View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS

Y.F. Wu performed the experiments. Y.F. Wu and Y.D. Zhang analyzed the data and wrote the manuscript. Y.F. Wu, Y.D. Zhang, J. Wu and P. Yuan discussed the manuscript. CORRESPONDING AUTHOR

Correspondence to Yundong Zhang. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare that they have no competing interests. ADDITIONAL INFORMATION PUBLISHER'S NOTE: Springer

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http://creativecommons.org/licenses/by/4.0/. Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Wu, Y., Zhang, Y., Wu, J. _et al._ Simultaneous measurement of transverse load and

temperature using hybrid structured fiber-optic Fabry–Perot interferometer. _Sci Rep_ 7, 10736 (2017). https://doi.org/10.1038/s41598-017-11218-9 Download citation * Received: 06 April 2017

* Accepted: 21 August 2017 * Published: 06 September 2017 * DOI: https://doi.org/10.1038/s41598-017-11218-9 SHARE THIS ARTICLE Anyone you share the following link with will be able to read

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