High sensitivity temperature sensor based on liquid filled hole-assisted dual-core fiber

Journal Pre-proof High sensitivity temperature sensor based on liquid filled hole-assisted dual-core fiber Jing Yang, Chunying Guan, Peixuan Tian, Rang Chu, Peng Ye, Keda Wang, Jinhui Shi, Jun Yang, Libo Yuan

PII:

S0924-4247(19)31173-2

DOI:

https://doi.org/10.1016/j.sna.2019.111696

Reference:

SNA 111696

To appear in:

Sensors and Actuators: A. Physical

Received Date:

5 July 2019

Revised Date:

30 September 2019

Accepted Date:

23 October 2019

Please cite this article as: Yang J, Guan C, Tian P, Chu R, Ye P, Wang K, Shi J, Yang J, Yuan L, High sensitivity temperature sensor based on liquid filled hole-assisted dual-core fiber, Sensors and Actuators: A. Physical (2019), doi: https://doi.org/10.1016/j.sna.2019.111696

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High sensitivity temperature sensor based on liquid filled hole-assisted dual-core fiber Jing Yang, Chunying Guan*, Peixuan Tian, Rang Chu, Peng Ye, Keda Wang, Jinhui Shi, Jun Yang, and Libo Yuan Key Laboratory of In-Fiber Integrated Optics of Ministry of Education, College of Science, Harbin Engineering University, Harbin 150001, China

*

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Corresponding author: [email protected]

Graphical Abstract

A high sensitivity temperature sensor is numerically and experimentally investigated in liquid

filled eccentric hole-assisted dual-core fiber (EHADCF). The sensor is constructed by cascading two single mode fibers (SMFs) with an intermediate liquid filled EHADCF segment. The light couples

periodically between the center and suspended cores due to small core distance. Refractive index (RI)

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matching liquid with high thermo-optic coefficient is filled into the air-hole of the EHADCF as the cladding of the suspended core. The dispersion curves of the center and suspended cores of the EHADCF change with the temperature, leading Splicing point II to the shift of the resonance peak. The measured

point I

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temperature sensitivity of the sensor is -562.4 pm/°C. In addition, the stability of the sensor is also measured. Due to high sensitivity, good stability, compact size and large temperature range, the

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proposedair-hole sensor shows promising potentials for temperature sensing in environment monitoring. Liquid-filled

Suspended core

Cladding

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EHADCF

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Transmission (dB)

Center core

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Air n=1

Before filling liquid After filling liquid

Liquid n=1.33

(a)

1810 nm

Wavelength (nm)

Fig.SMF 1 The measured transmissionSMF spectra SEHADCF

(b)

Microhole

SMF2

(c)

Air

SEHADCF

RI matching liquid

SEHADCF

RI matching liquid

1 1980 nm

SMF

Air

(d)

Abstract A high sensitivity temperature sensor is numerically and experimentally investigated in liquid filled eccentric hole-assisted dual-core fiber (EHADCF). The sensor is constructed by cascading two single mode fibers (SMFs) with an intermediate liquid filled EHADCF segment. The light couples

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periodically between the center and suspended cores due to small core distance. Refractive index (RI) matching liquid with high thermo-optic coefficient is filled into the air-hole of the EHADCF as the cladding of the suspended core. The dispersion curves of the center and suspended cores of the

EHADCF change with the temperature, leading to the shift of the resonance peak. The measured

temperature sensitivity of the sensor is -562.4 pm/°C. In addition, the stability of the sensor is also

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measured. Due to high sensitivity, good stability, compact size and large temperature range, the

proposed sensor shows promising potentials for temperature sensing in environment monitoring.

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Keywords: optical fiber sensor, temperature sensor, hole-assisted fiber, dual-core fiber, coupler

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1. Introduction

Optical fiber temperature sensors have been studied extensively due to their inherent advantages, including fast response, small size and immunity to electromagnetic interference. They can be realized based on different operating principles, such as fiber Bragg gratings (FBGs) [1, 2], long period fiber

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gratings (LPFGs) [3, 4] and fiber interferometers [5-11]. In general, the grating-based temperature sensor has a low sensitivity due to weak thermo-optic and thermo-expansive coefficients of

. In

order to overcome this shortcoming, FBGs and LPFGs were coated or packaged in a variety of

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materials with high thermo-expansive or thermo-optic coefficients to improve their sensitivities. A FBG embedded in a tapered polymer was reported and the temperature sensitivity reached 108 pm/°C, which is ~8 times as large as the sensitivity of the standard FBG [12]. In addition, the reduced

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graphene oxide was used to coat the FBG for improving temperature sensitivity because of its large thermo-optic coefficient, and the temperature sensitivity of 33 pm/°C was obtained [13]. Although the temperature sensitivities of the FBGs have been obviously improved by use of these methods, they are still low. Similarly, the temperature sensitivity of the LPFG was also markedly improved by coating the heat-sensitive materials [14, 15], however, packaging or coating processes of the grating are usually complex, which limits practical applications of these sensors. The microstructured fibers with large airholes have been widely applied in fiber sensors. The air-hole of microstructured fibers can be used as a microcavity to realize acoustic pressure [16], gas pressure [17] and humidity sensors [18]. Moreover, the air-holes of microstructured fibers can also be filled by functional materials to improve the

performance of sensors. In order to enhance the temperature sensitivity, microstructured fibers filled by liquid were used to construct interferometers with high temperature sensitivities [19-23]. A temperature sensor based on an ethanol filled PCF Sagnac interferometer was realized, and the temperature sensitivity reached up to 6.6 nm/°C [24]. Subsequently, different fully or selectively filled PCFs were employed to improve the performance of the Sagnac interferometer, and a series of ultra-high sensitivity temperature sensors were realized [25-27]. However, in these Sagnac interferometer-based temperature sensors, overlong sensing heads limit their applications. Alternatively, selectively filled PCFs were also employed as directional couplers for highly sensitive temperature sensing [28-31]. The coupling between solid core and liquid core is sensitive to temperature due to the high thermo-optic coefficient of the liquid core. However, the measuring ranges of liquid filled directional coupler-based temperature sensors are extremely small, which make them difficult to apply. Particularly, a temperature sensor based on twin-resonance-coupling with temperature sensitivity of 290 nm/°C was

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realized, but the measured temperature range of the sensor was only 1 °C [28].

In this work, a highly sensitive, compact and stable temperature sensor is demonstrated based on

liquid filled eccentric hole-assisted dual-core fiber (EHADCF). The sensor is constructed by cascading two standard single mode fibers (SMFs) with an intermediate segment of liquid filled EHADCF. The coupling characteristics of the sensor are analyzed. The resonance peak strongly depends on the

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dispersion curves of the cores of the EHADCF. The liquid filled in the air-hole of the EHADCF acts as the cladding of the suspended core, therefore the resonance peak is sensitive to temperature due to the high thermo-optical coefficient of the liquid. The temperature response of the sensor is measured, and

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the sensitivity reaches up to -562.4 pm/°C. In particular, the temperature range of the proposed sensor is much larger than those of previously reported liquid filled directional coupler-based temperature sensors [28-31]. Due to the high sensitivity, good stability, compact size and large measuring range, the

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proposed sensor has promising potentials for temperature sensing in environment monitoring. 2. Simulation and sensor fabrication 2.1 Simulation

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As shown in Fig. 1 (a), the temperature sensor with liquid filled EHADCF has a simple SMFEHADCF-SMF structure, in which a segment of liquid filled EHADCF is sandwiched between two SMFs. The center core of the EHADCF is aligned to the cores of SMFs. The cross-section of the

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EHADCF is shown in the inset of Fig. 1(a). The diameters of the center core, suspended core, air-hole and cladding are 8.5, 12.4, 45.4 and 128 μm, respectively. The suspended core has a large contact area with the cladding and is not perfectly circular. The separation between two core centers is 17.1 μm. The

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refractive index (RI) difference between the core and the cladding is about 0.00425. Due to the small separation between two cores, the strong resonance coupling will occur if the guided modes in the two cores are phase matched. The dispersion curves of the fundamental modes in the center and suspended cores are calculated by using the finite-difference beam propagation method (FD-BPM) as shown in Fig. 2(a). In the calculation, the two cores have the same refractive indices of 1.448, the refractive index of the cladding is 1.443, the diameters of the center and suspended cores are 8.5 and 13.1 μm, the diameters of the air-hole and the cladding are 45 and 125 μm, respectively,. The simulated suspended core is circular and is tangent with the cladding, which will reduce the coupling between the two cores. In order to match the actual sample, the separation between two core edges is set to 3.15 μm. When the air-hole is empty, the effective RI (ns, air) of the fundamental mode in the suspended core is larger than

that in the center core (nc, air) at short wavelength because of the larger size of the suspended core and the lower refractive index of the air-hole. Both ns, air and nc, air decrease with the increasing wavelength. The phase matching occurs at 1810 nm, where ns, air = nc, air, so the strong resonance coupling occurs. The light in one core of the EHADCF can be coupled into the other core. The coupling length is defined as the minimum propagating distance that the beam in one core is furthest coupled into the other core. Therefore, when the length of the fiber is close to odd times of the coupling length, a resonance peak can appear in the transmission spectrum of the center core because of the resonance coupling between the fundamental modes in the two cores. Splicing point I

Splicing point II Liquid-filled air-hole Suspended core Center core Cladding

(a) EHADCF

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SMF2

Air n=1

1540 nm

Air n=1

1810 nm

30 0 -30 -60 (b) 60

0

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30 1

-30

(c)

60

1980 nm

Liquid n=1.33 30 0

0.5

-30

(d)

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-60 60

1710 nm

Liquid n=1.33 30 0

0

-30

(e)

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-60 60

2070 nm

Liquid n=1.33

30

0 -30 -60

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-60

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Radial position (μm)

Radial position (μm)

Radial position (μm)

Radial position (μm)

Radial position (μm)

SMF1 60

(f) 0

3

6

9

12

15

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Transmission distance (mm)

Fig. 1 (a) The schematic diagram of the liquid-filled EHADCF temperature sensor. (b) and (c) The calculated beam propagation in the EHADCF without filling liquid at 1540 nm and 1810 nm. (d) ~ (f) The calculated beam propagations in the EHADCF filling with RI matching liquid at 1980, 1710 and 2070 nm, respectively. Inset: The cross-section of the EHADCF sample.

The effect of the suspended core diameter on the phase matching wavelength is shown in Fig.

2(b). The phase matching wavelength has a red shift with increasing suspended core diameter. It can be explained that the increasing diameter of the suspended core leads to a significant increase of the effective refractive index of the fundamental mode in the suspended core, but it only has a small influence on the center core. The separation between two cores also has the effect on the phase matching wavelength and the coupling length. The calculated results are shown in Figs. 2(c) and 2(d),

where the diameter of the suspended core is 13.1 μm. As the separation increases, the phase matching

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ns, air nc, air ns, 1.33 nc, 1.33

Wavelength (nm)

(b)

Separation(μm)

(d)

Suspended core diameter (μm)

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(c)

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Coupling length (mm)

Phase matching wavelength (nm)

(a)

Phase matching wavelength (nm)

wavelength exponentially decreases while the coupling length exponentially increases.

Separation(μm)

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Fig. 2 (a) The dispersion curves of the fundamental modes in the center and suspended cores of empty and liquid-filled EHADCFs. (b) The phase matching wavelengths for different suspended core diameters. (c)-(d) The effect of the separation between two cores on the phase matching wavelength and the coupling length.

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The calculated beam propagation in the EHADCF at 1810 nm is shown in Fig. 1(c), where the EHADCF length is 10.5 mm. The light from SMF1 is coupled into the center core of the EHADCF at the splicing point I. The beam couples between the center and suspended cores periodically. The

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calculated coupling length Lc is 3.48 mm at 1810 nm. The calculated transmission spectrum from SMF2 is shown in Fig. 4, marked by the blue line. The length of the EHADCF is approximately 3 times of the Lc, consequently the light at the phase matching wavelength is almost completely coupled into the suspended core at the splicing point II, therefore a main resonance peak with the center

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wavelength of ~1810 nm is observed. Meanwhile, there are many minor resonance peaks located at the two sides of the main resonance peak. It can be explained that the coupling length decreases when the wavelength is far away from the phase matching wavelength. The coupling length can also be

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calculated by [32] π

Lc

e

(1)

o

where βe and βo are the propagation constants of the even and odd modes. FD-BPM is used to calculate the propagation constants of the even and odd modes and finally the coupling length is obtained. The relationship between the wavelength and the coupling length is shown in Fig. 3. The coupling length reaches a maximum of 3.55 mm at 1780 nm. As the wavelength is away from 1780 nm, the coupling length is gradually reduced. As long as the EHADCF length equals the odd times of the coupling length, a minor resonance peak will occur. In the experiment, the EHADCF length (10.5 mm) is close

to 5 times of the coupling length (2.13) at 1540 nm, hence the minor resonance peak 1 occurs. The calculated beam propagation in the EHADCF at 1540 nm is shown in Fig. 1(b). Moreover, the amplitudes of these minor resonance peaks are reduced gradually due to larger phase mismatching. According to Ref. [33], the theoretical loss can be calculated. The loss of the coupler can be ignored as

Wavelength (nm)

Minor resonance peak2

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Minor resonance peak1

nhole=1 nhole=1.33 Main resonance peak

Wavelength (nm)

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Transmission (dB)

Fig. 3 The calculated relationship between the wavelength and the coupling length.

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Coupling length (mm)

long as the center core of the EHADCF matches the core of SMF.

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Fig. 4 The calculated transmission spectra of empty and RI matching liquid filled EHADCF with the length of 10.5 mm.

When the air-hole of the EHADCF is filled with liquid, the effective RI of the suspended core increases, as shown in Fig. 2(a). Therefore, the phase matching wavelength is red shifted to ~1980 nm.

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The calculated transmission spectrum of the liquid filled EHADCF is shown in Fig. 4, marked by the orange line, in which the refractive index of liquid is 1.33. There is no resonance peak near 1980 nm, but two resonance peaks located at ~1710 nm and ~2070 nm are obtained, respectively. The beam propagations in liquid filled EHADCF at 1980 nm, 1710 nm and 2070 nm are shown in Figs. 1(d) ~ (f),

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respectively. At 1980 nm, the length of the EHADCF is equal to 4 times of the coupling length, so the energy almost entirely propagates in the center core at the splicing point II. At 1710 nm and 2070 nm, the length of the EHADCF is close to 5 times of the coupling length. However, due to phase

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mismatching, only a portion of the beam can be coupled into the suspended core, so two resonance peaks have low extinction ratios. The liquid has a large thermo-optic coefficient, the temperature change will cause the change of the effective RI of the fundamental mode in the suspended core, which conversely lead to the changes in the phase matching wavelength and the coupling coefficient. Hence, the resonance peak is sensitive to the temperature. According to previous report [31], the length of the directional coupler does not significantly affect the temperature sensitivity. 2.2 Sensor Fabrication The fabrication process of the proposed temperature sensor is very simple. Initially, the EHADCF was spliced to a SMF by using a commercial fusion splicer (AV6471, CETC) under automatic

cladding-alignment mode. The arc discharge current and discharge time were respectively set to 10 and 13 unit that are small enough to avoid an obvious deformation of the air-hole. Then, the EHADCF was cleaved to 10.5 mm under a microscope and spliced to the other SMF. The measured transmission spectrum of the sample is shown in Fig. 5, denoted by the blue line. We can observe a strong resonance peak with the amplitude of ~ 18.2 dB at ~ 1813.5 nm, which is consistent with the calculated result. The insertion loss of the empty sensor is only ~ 0.05 dB. The RI matching liquid (n=1.33 @589.3 nm, Cargille Laboratories Inc) was chosen to fill into the air-hole as the cladding of the suspended core and the heat-sensitive material. The thermo-optic coefficient of the RI matching liquid is -3.37 10-4/°C, which is two orders of magnitude higher than that of pure silica (8.6 10-6/°C) [34]. In order to inject RI matching liquid, two microholes were fabricated by a high-frequency CO2 laser on the side wall of the air-hole near the splicing points. In the fabrication process of microholes, the air hole of the EHADCF was rotated upwards. Then, the EHADCF was fixed on a microslide with thermal reversible glue

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(Meltmount 1.539, Cargille Laboratories Inc) and was placed on the focal plane of the laser, and the

laser beam could scan across the EHADCF near the splicing point, as shown in Fig. 6(a). The scanning speed, average output power and number of scan cycles were 5.4 mm/s, 0.35 W and 5, respectively.

The fabricated microhole is shown in Fig. 6(b). The RI matching liquid was dropped onto a microhole

by using a syringe. After a few seconds, the RI matching liquid could fill into the air-hole between the

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two microholes, as shown in Figs. 6(c) and 6(d). Then, the microholes were sealed by ultraviolet glue

to prevent the RI matching liquid from leaking out and evaporating. The transmission spectrum of the RI matching liquid filled sensor is shown in Fig. 5 by the orange line. Two weak resonance peaks can

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be observed at ~1795 nm and ~2126 nm, respectively. The difference between the measured and calculated wavelengths is attributed to the geometric difference of the EHADCF in the simulation and the fiber sample. In the simulation, the cross section of the suspended core is simply considered as a

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circle, however, in fact the suspended core has a larger contact surface with the cladding, as shown in the inset of Fig. 1(a). The deformation of the suspended core leads to a reduction of the contact area

na ur Before filling liquid After filling liquid

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Transmission (dB)

between the suspended core and liquid. The insertion loss of the liquid filled sensor is ~ 0.43 dB.

Wavelength (nm)

Fig. 5 The measured transmission spectra of the sensor before and after filling liquid.

SMF

SEHADCF

CO2 laser Microhole

(b) SMF

(c)

SEHADCF

Air

RI matching liquid

SEHADCF

RI matching liquid

Scanning

(a)

SMF

Air

(d)

Fig. 6 (a) The schematic diagram of the microhole fabrication. (b) The micrograph of a microhole fabricated by CO2 laser. (c) ~ (d) The micrographs of the sensor sample near the right and left splicing points.

3. Results and discussion The temperature response of the sensor sample was measured through a continuous heating and

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cooling process in a temperature and humidity chamber (SH-222, ESPEC Inc., temperature range of 20 - 160 °C with an accuracy of ±0.1 °C). A supercontinuum fiber laser (SuperK Compact, NKT Photonics Inc.) was used as the light source and an optical spectrum analyzer (OSA, AQ6375B,

YOKOGAWA Inc.) was employed to detect the transmission spectrum of the sensor in real time. The

sensor was attached to two fiber holders in the temperature and humidity chamber. It was heated from

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10 to 100 °C and then cooled to 10 °C. Figure 7(a) shows the temperature response of transmission

spectra of the sensor sample. It can be seen that the resonance peak has an obvious blueshift with the increasing temperature. The relationship between the wavelength shift and temperature is shown in Fig.

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7(b). The wavelength of the resonance peak decreases linearly with the increasing temperature. The sensitivity of the sensor reaches -556.5 pm/ ºC in the heating process and -562.4 pm/ºC in the cooling process, which shows a good repeatability. The performances of some functional material-assisted fiber

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temperature sensors are listed in Table 1. Compared to these previous reports, the proposed temperature sensor has a relatively high temperature sensitivity [7-9, 19-22] and a smaller size [7-9, 11, 15, 19, 2022, 28, 29]. The most important property is that the temperature range of the proposed sensor is larger

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than those of most temperature sensors listed in Table 1 [7, 8, 10, 11, 15, 19-22, 28-31].

Transmission (dB)

20 ºC 40 ºC 60 ºC 80 ºC 100 ºC

(a)

Wavelength (nm) 1810 Cooling Linear fit for heating Linear fit for cooling

1790

y1=-0.5565x+1812.0 R12=0.9987

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Wavelength (nm)

Heating

y2=-0.5624x+1812.2 R22=0.9977

1770

1750 5

25

45

65

85

105

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Temperature (ºC)

(b)

Fig. 7 (a) Transmission spectra of the proposed sensor for different temperatures. (b) The shift of the resonant wavelength in the

Table 1. Comparison of different RH sensors Temperature Sensing structure

sensitivity

255.4

Microfiber MZI with knot resonator

-41.58

Interferometer

Size

(°C)

(mm)

Year

Reference

20 - 80

24

2016

[7]

22.2 - 32.9

51

2019

[8]

201.17

10 - 100

60

2017

[9]

-653 (Inherent) 17758 (Vernier effect)

40 - 70

0.17154

2019

[10]

Balloon-shaped bent SMF interferometer

-2465

20.7 - 31.7

13.42

2018

[11]

LPFG

770

20 - 100

24

2016

[15]

Interferometer

-166

23.7 - 66.1

11.5

2012

[19]

Interferometer

-541

30 - 70

22

2017

[20]

MZI

-176

25 - 50

16

2015

[21]

Interferometer

-350

20 - 50

32.5

2012

[22]

Directional coupler

290000 (Max, Nonlinear)

54 - 55

35

2013

[28]

Directional coupler

54300

34 - 35.4

24

2011

[29]

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Fabry-Perot interferometer

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LPFG

Range

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(pm/°C)

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temperature range from 10 to 100 °C.

Directional coupler

-3860

43.9 -53.5

9

2012

[30]

Directional coupler

-3900

42 - 53

9 - 101

2012

[31]

Directional coupler

-562.4

10 - 100

10.5

-

This work

The stability of the proposed temperature sensor was also measured. The temperature was set as 30 °C. The transmission spectrum was recorded every 5 minutes, and the results are shown in Fig. 8. The maximum fluctuation of the center wavelength of the resonance dip is less than ±0.1 nm.

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Wavelength (nm)

Therefore, the proposed temperature sensor has a good stability.

Fig. 8 Fluctuation of the center wavelength of the resonance peak.

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4. Conclusion

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Time (min)

In conclusion, a simple temperature sensor based on liquid filled EHADCF is proposed and

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experimentally demonstrated. The proposed temperature sensor is fabricated by splicing a segment of one EHADCF to two SMFs. Because of the large thermo-optic coefficient of liquid, the phase matching wavelength and the coupling coefficient of the proposed sensor change with temperature, which results in the shift of the resonant peak. The experimental results show that the sensor exhibits

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high temperature sensitivity of -562.4 pm/°C, which is more sensitive than previously reported temperature sensors [7-9, 19-22]. In particularly, the temperature range of the sensor is much larger than those of other reported liquid filled directional coupler-based temperature sensors [28-31]. With

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high sensitivity, good stability and repeatability, compact size and large temperature range, the proposed temperature sensor has promising potentials in environment monitoring. In addition, the proposed sensor can operate at the telecom wavelength by reducing the diameter of the suspended core

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or decreasing the refractive index difference between two cores.

Declaration of interests 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.

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC) (91750107, 61675054, and 61875044); Natural Science Foundation of Heilongjiang Province in China (ZD2018015); 111 project to the Harbin Engineering University (B13015); Fundamental Research

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Funds for the Central Universities (3072019CF2501 and 3072019CF2509).

References

[1] J. Kou, S. Qiu, F. Xu, Y. Lu, Demonstration of a compact temperature sensor based on first-order Bragg grating in a tapered fiber probe, Opt. Exp. 19 (2011) 18452. [2] B. Zhang, M. Kahrizi, High-temperature resistance fiber Bragg grating temperature sensor fabrication, IEEE Sensors J. 7 (2007) 586. [3] V. Bhatia, Optical fiber long-period grating sensors, Opt. Lett. 21 (1996) 692. [4] Z. Bai, W. Zhang, S. Gao, H. Zhang, L. Wang, F. Liu, Bend-insensitive long period fiber grating-based high temperature sensor, Opt. Fiber Technol. 21 (2015) 110. [5] L. V. Nguyen, S. C. Warren-Smith, H. Ebendorff-Heidepriem, T. M. Monro, Interferometric high temperature sensor using suspended-core optical fibers, Opt. Exp. 24 (2016) 8967.

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[6] X. Hu, X. Shen, J. Wu, J. Peng, L. Yang, J. Li, H. Li, N. Dai, All fiber M-Z interferometer for high temperature sensing based on a hetero-structured cladding solid-core photonic bandgap fiber, Opt. Exp. 24, (2016) 21693.

[7] Q. Wang, C. Du, J. Zhang, R. Lv, Y. Zhao, Sensitivity-enhanced temperature sensor based on PDMS-coated long period fiber grating, Opt. Commun. 337 (2016) 89.

[8] Z. Liu, H. Xiao, M. Liao, X. Han, W. Chen, T. Zhao, H. Jia, J. Yang, Y. Tian, PDMS-assisted microfiber M-Z

-p

interferometer with a knot resonator for temperature sensing, IEEE Photo. Technol. Lett. 31 (2019) 337.

[9] V. Bhardwaj, V. K. Singh, Study of liquid sealed no-core fiber interferometer for sensing applications, Sens. Actuators A Phys. 254 (2017) 95.

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[10] L. Hou, C. Zhao, B. Xu, B. Mao, C. Shen, D. N. Wang, Highly sensitive PDMS-filled Fabry-Perot interferometer temperature sensor based on the Vernier effect, Appl. Opt. 58 (2019) 4858. [11] K. Tian, G. Farrell, E. Lewis, X. Wang, H. Liang, P. Wang, A high sensitivity temperature sensor based on

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balloon-shaped bent SMF structure with its original polymer coating, Meas. Sci. Technol. 29 (2018) 085104. [12] L. Jin, W. Zhang, H. Zhang, B. Liu, J. Zhao, Q. Tu, G. Kai, X. Dong, An embedded FBG sensor for simultaneous measurement of stress and temperature, IEEE Photo. Technol. Lett. 18 (2006) 154. [13] S. Sridevi, K. S. Vasu, S. Asokan, A. K. Sood, Enhanced strain and temperature sensing by reduced graphene

na

oxide coated etched fiber Bragg gratings, Opt. Lett. 41, (2016) 2604. [14] A. Urrutia, J. Goicoechea, A. L. Ricchiuti, D. Barrera, S. Sales, F. J. Arregui, Simultaneous measurement of humidity and temperature based on a partially coated optical fiber long period grating, Sens. Actuators B Chem. 227

ur

(2016) 135.

[15] F. Zou, Y. Liu, S. Zhu, C. Deng, Y. Dong, and T. Wang, Temperature sensitivity enhancement of the nano-film coated long-period fiber gratings, IEEE Sens. J. 16, (2016) 2460.

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[16] Y. Zhao, M. Chen, F. Xia, and R. Lv, Small in-fiber Fabry-Perot low-frequency acoustic pressure sensor with PDMS diaphragm embedded in hollow-core fiber, Sens. Actuators A Phys. 270 (2018) 162. [17] Z. Zhang, J. He, B. Du, F. Zhang, K. Guo, Y. Wang, Measurement of high pressure and high temperature using a dual-cavity Fabry-Perot interferometer created in cascade hollow-core fibers, Opt. Lett. 43 (2018) 6009. [18] Y. Zhao, R. Tong, M. Chen, and F. Xia, Relative humidity sensor based on hollow core fiber filled with GQDsPVA, Sens. Actuators B Chem. 284 (2019) 96. [19] S. Qiu, Y. Chen, F. Xu, Y. Lu, Temperature sensor based on an isopropanol-sealed photonic crystal fiber in-line interferometer with enhanced refractive index sensitivity, Opt. Lett. 37 (2012) 863. [20] A. Villalba, J. Carlos Martín, Interferometric temperature sensor based on a water-filled suspended core fiber, Opt. Fiber Technol. 33 (2017) 36.

[21] J. Liou, C. Yu, All-fiber Mach-Zehnder interferometer based on two liquid infiltrations in a photonic crystal fiber, Opt. Exp. 23 (2015) 6946. [22] W. Qian, C. Zhao, C. C. Chan, L. Hu, T. Li, W. C. Wong, P. Zu, X. Dong, Temperature sensing based on ethanol-filled photonic crystal fiber modal interferometer, IEEE Sens. J. 12 (2012) 2593. [23] Y. Zhao, Q. Wu, and Y. Zhang, Theoretical analysis of high-sensitive seawater temperature and salinity measurement based on C-type micro-structured fiber, Sens. Actuators B Chem. 258 (2018) 822. [24] W. Qian, C. Zhao, S. He, X. Dong, S. Zhang, Z. Zhang, S. Jin, J. Guo, H. Wei, High-sensitivity temperature sensor based on an alcohol-filled photonic crystal fiber loop mirror, Opt. Lett. 36 (2011) 1548. [25] E. Reyes-Vera, C. M. B. Cordeiro, P. Torres, Highly sensitive temperature sensor using a Sagnac loop interferometer based on a side-hole photonic crystal fiber filled with metal, Appl. Opt. 56 (2017) 156. [26] T. Han, Y. Liu, Z. Wang, J. Guo, Z. Wu, S. Wang, Z. Li, W. Zhou, Unique characteristics of a selective-filling photonic crystal fiber Sagnac interferometer and its application as high sensitivity sensor, Opt. Exp. 21 (2013) 122. [27] K. Naeem, B. H. Kim, B. Kim, Y. Chung, High-sensitivity temperature sensor based on a selectively-polymer-

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filled two-core photonic crystal fiber in-line interferometer, IEEE Sens. J. 15 (2015) 3998.

[28] M. Luo, Y. Liu, Z. Wang, T. Han, Z. Wu, J. Guo, W. Huang, Twin-resonance-coupling and high sensitivity sensing characteristics of a selectively fluid-filled microstructured optical fiber, Opt. Exp. 21. (2013) 30911.

[29] Y. Wang, M. Yang, D. N. Wang, C. R. Liao, Selectively infiltrated photonic crystal fiber with ultrahigh temperature sensitivity, IEEE Photo. Technol. Lett. 23 (2011) 1520.

-p

[30] D. J. J. Hu, P. P. Shum, J. L. Lim, Y. Cui, K. Milenko, Y. Wang, T. Wolinski, A compact and temperaturesensitive directional coupler based on photonic crystal fiber filled with liquid crystal 6CHBT, IEEE Photo. J. 4 (2012) 2010.

re

[31] D. J. J. Hu, J. L. Lim, Y. Cui, K. Milenko, Y. Wang, P. P. Shum, T. Wolinski, Fabrication and characterization of a highly temperature sensitive device based on nematic liquid crystal-filled photonic crystal fiber, IEEE Photo. J. 4 (2012) 1248.

29 (2004) 2473.

lP

[32] J. Lægsgaard, O. Bang, A. Bjarklev, Photonic crystal fiber design for broadband directional coupling, Opt. Lett.

[33] L. L. Li, L. M. Xiao, Plasmonic nodeless hollow core photonic crystal fibers for in-fiber polarizers, J. Lightwave Technol. DOI 10.1109/JLT.2019.2930075 (2019).

na

[34] Y. Yu, X. Li, X. Hong, Y. Deng, K. Song, Y. Geng, H. Wei, W. Tong, Some features of the photonic crystal

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fiber temperature sensor with liquid ethanol filling, Opt. Exp. 18 (2010) 15383.

Jing Yang received the B.S. degree in optical information science and technology and the M.S. degree in optics from Harbin Engineering University, China, in 2012 and 2016. Currently, he is pursuing his Ph.D. in optical engineering at Key Laboratory of In-Fiber Integrated Optics of Ministry of Education. His research interests include optical fiber sensors and the application of optical fiber in astronomical telescopes.

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Chunying Guan received the B.S. degree in optoelectronics, the M. Eng. degree in optical engineering, and the Ph.D. in photonics from Harbin Engineering University, Harbin, China, in 2001, 2004, and 2007, respectively. Currently, she works at Key Laboratory of In-Fiber Integrated Optics of Ministry of Education, Harbin Engineering University. She is currently a full professor at College of Science, Harbin Engineering University, China. Her research interests include optical fiber sensors, microstructured fiber and fiber optic devices. She has authored and co-authored over 150 technical articles and 40 technique patents.

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Peixuan Tian received a Bachelor's degree in optoelectronic information science and engineering from Harbin Engineering University in 2017. Currently, he is pursuing his Ph.D. in optical engineering at Harbin Engineering University. His research interest is to design special optical fiber sensors based on FBGs.

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Rang Chu received a Bachelor's degree in optoelectronic information science and engineering from Harbin Engineering University in 2016. Currently, he is pursuing his Ph.D. in optical engineering at Harbin Engineering University. His research interests include optical fiber sensors and fiber devices.

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Peng Ye is currently studying optical engineering at Harbin Engineering University in China and pursuing a master's degree. He received a Bachelor's degree in optoelectronic information science and engineering from Harbin Engineering University in 2019. His research interest is to design special optical fiber sensors.

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Keda Wang received the B.S. degree in optoelectronics and the M. Eng. degree in optical engineering from Heilongjiang University, Harbin, China, in 2015 and 2018, respectively. Currently, he is pursuing his Ph.D. in optical engineering at Harbin Engineering University. His research interests include optical fiber sensors and the fiber-based metamaterial.

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Jinhui Shi received the B.S. degree in optoelectronics, the M. Eng. degree in optical engineering, and the Ph.D. in photonics from Harbin Engineering University, Harbin, China, in 2001, 2005, and 2007, respectively. Currently, he is a professor at College of Physics and Photoelectric Engineering, Harbin Engineering University, China. His research interests include microstructured fiber and metamaterials. He has published over 120 journal and conference papers. Jun Yang received the B.S. degree in optoelectronics, the M.S. degree in optical engineering, and the Ph.D. degree in photonics from Harbin Engineering University, Harbin, China, in 1999, 2002, and 2005, respectively. He is currently a Professor in the Key Lab of In-Fiber Integrated Optics, Ministry Education of China, Harbin Engineering University. His research interests include fiber

optic sensors and optic interferometers. He is the author or coauthor of more than 60 papers published in international journals.

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Libo Yuan received B.S. degree in physics from Heilongjiang University, Harbin, China, in 1984, the M. Eng. degree in communication and electronic systems from Harbin Shipbuilding Engineering Institute, Harbin, China, in 1990, and the Ph.D. degree in photonics from The Hong Kong Polytechnic University, Hong Kong, in 2003. He developed white light fiber optic interferometers for fiber optic sensors. He is a professor of Key Laboratory of In-Fiber Integrated Optics of Ministry of Education, Harbin Engineering University. His general research areas include fiber optics and fiber-optic sensors and its applications. He has authored and co-authored over 300 technical articles. He holds 90 technique patents related with fiber optic technology and published 3 books and 3 book chapters.