Optics and Laser Technology 130 (2020) 106333

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Optics and Laser Technology

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Folded-tapered multimode-no-core fiber sensor for simultaneous T

measurement of refractive index and temperature
⁎
Fang Wanga,c, Kaibo Panga,c, Tao Maa, Xu Wanga,c, Yufang Liub,c,
a
College of Electronic and Electrical Engineering, Henan Normal University, Xinxiang, China
b
College of Physics and Material Science, Henan Normal University, Xinxiang, China
c
Key Laboratory Optoelectronic Sensing Integrated Application of Henan Province, Henan Normal University, Xinxiang, China

H I GH L IG H T S

• The sensor is constituted by multimode interferometer and Mach-Zehnder interferometer.

• Simultaneous measurement of refractive index (RI) and temperature can be achieved.
• The sensitivity of RI reach to 1191.27 nm/RIU in the low RI range.
• It has the advantages of cost effectiveness, high sensitivity and easy fabrication.

A R T I C LE I N FO A B S T R A C T

Keywords: In this paper, a refractive index (RI) and temperature sensor based on a folded-tapered multimode-no-core
Folded-tapered multimode fiber (FTMN) fiber structure is proposed and experimentally demonstrated. The FTMN has an additional Mach-
Mach-Zehnder interferometer Zehnder interferometer (MZI), which is introduced in the folded-tapered multimode (FTM) fiber. And with the
Multimode interference inherent multimode interference (MMI) and the previously mentioned MZI as foundation, a composite inter-
Refractive index
ference is successfully established. This synthetic composite interference greatly improves the performance of
Temperature
traditional optical fiber RI sensing in the low RI range. The experimental results demonstrate that a maximum
sensitivity of 1191.5 nm/RIU within a linear RI ranging from 1.3405 to 1.3497 can be achieved, which is greater
than the traditional modal interferometer structure. Furthermore, the temperature sensitivities at interference
dips A and B are 0.0648 nm/°C and 0.0598 nm/°C, respectively. By monitoring the wavelength shifts of inter-
ference dips A and B, the sensor can simultaneously measure RI and temperature to overcome the temperature
induced cross-sensitivity.

1. Introduction optical fiber Mach-Zehnder interferometers (MZIs) have gained sig-

nificant attention in the advanced topics of biology and chemistry
Refractive index (RI) and temperature are the most important ap- fields, due to its advantages of simple fabrication, corrosion resistance,
plication parameters in chemical and food industries and in the field of and low cost. Some researchers have proposed RI sensors based on
high-tech laboratories [1–3]. In recent years, optical fiber sensors have different MZI structures. Wang et al. fabricated a senor based on a ta-
attracted wide investigations because of their unique advantages in- pered multimode fiber structure [15]. Chen et al. employed an S-ta-
cluding compact structure, high-resolution detection, excellent aging pered fiber probe to measure the RI [16]. Yao et al. put forward a sensor
characteristics, ability to operate in chemically hazardous environ- based on a core-offset MZI combined with a fiber Bragg grating, and the
ments and inherent anti-interference ability against external electro- RI and temperature sensitivities are 13.7592 nm/RIU and 0.0462 nm/
magnetic interference, and so on [4,5]. At present, all kinds of optical °C, respectively [17]. The sensitivities of these optic fiber MZI structures
fiber RI or temperature sensors have been proposed, such as optical can reach hundreds or even thousands of nm/RIU, but the minimum RI-
fiber interferometer sensors [6–9], surface plasmon resonance (SPR) value limit is above 1.40, which is close to the RI of optical fibers. For
optical fiber sensors [10,11], optical fiber grating sensors [12–14], and the seawater and some aqueous solutions containing small chemical
so on. Among the various optical fiber RI and temperature sensors, and biomolecules molecules, the RI value is in the low RI range of

⁎
Corresponding author.
E-mail address: yf-liu@htu.edu.cn (Y. Liu).

https://doi.org/10.1016/j.optlastec.2020.106333
Received 22 July 2019; Received in revised form 14 April 2020; Accepted 9 May 2020
Available online 25 May 2020
0030-3992/ © 2020 Elsevier Ltd. All rights reserved.
F. Wang, et al. Optics and Laser Technology 130 (2020) 106333

1.33–1.35, and the slight change of RI can reflect the transient physical
environment disturbance or the combining role [18,19]. Therefore,
designing a high RI sensitive optic fiber sensor suitable for the low RI
range has become an urgent task.
This paper proposed a fiber optic sensor based on folded-tapered
multimode-no-core (FTMN) fiber sandwiched between two single-mode
fibers (SMFs) and the sensor is constituted by multimode interferometer
(MMI) and MZI. The transmission spectral response of the composite
interference to the low RI range is demonstrated theoretically and ex-
perimentally. The results show that the maximal sensitivity of RI is
1191.5 nm/RIU within a linear RI ranging from 1.3405 to 1.3497.
Moreover, the temperature sensitivities at interference dips A and B are
0.0648 nm/°C and 0.0598 nm/°C, respectively. By monitoring the wa-
velength shifts of interference dips A and B, simultaneous measurement
of RI and temperature can be achieved to overcome the temperature
induced cross-sensitivity. Details regarding the fabrication and working
of the proposed sensor and the results from an experimental evaluation
are presented in the following sections.
Fig. 2. The flame-based tapering machine.

2. Principles and fabrication

Fig. 1(a) exhibits the schematic structure of the proposed FTMN

fiber interferometer. The FTMN fiber structure includes two input/
output SMFs and a section of tapered multimode fiber (MMF) con-
catenated a no-core fiber (NCF) with the length of 2 mm, considering
the mechanical stability of the fiber structure for strain measurement,
the diameter of the tapered waist was selected to be 40 μm, while the
tapered length was 6 mm. Fig. 1(b) shows the schematic diagram of the
light propagation within the FTMN fiber structure.
Firstly, a segment of SMF and a segment of MMF with the length of
2 mm were coaxially spliced together by a commercial fusion splicer
(KL-280G, JiLong) equipped by the built-in SMF splicing program.
Then, the NCF with the length of 2 mm is coaxially spliced with MMF
and another segment of SMF, respectively. After splicing, the input SMF
and the output SMF were respectively connected to a semiconductor
optical amplifier (SOA) and an optical spectrum analyzer (OSA). The
image of a flame-based tapering machine (OC-2020, JiLong) for sensor
manufacturing is shown in Fig. 2. The MMF is firmly attached to two
linear motorized stages, ensuring that the MMF is directly below the
flame nozzle. Then, travel distance, speed of each stage and preheat
time were independently set in the tapering control software. Based on

Fig. 3. (a) Transmission spectra of the three types of FTMN. (b) Measured
transmission and spatial frequency spectra of FTMN (θ =150°).

the above steps, changes in the transmission spectrum of the FTMN

fiber structure can be observed during tapering.
By folding the tapered MMF into different angles (θ ), it is found that
its transmission spectrum changes regularly. Fig. 3(a) demonstrates the
transmission spectra of three kinds of FTMN fiber structures, which are
Fig. 1. (a) Structure diagram of FTMN fiber structure sensor head. (b) monitored during the fabrication process. With the decrease of θ , the
Schematic diagram of the light propagation within the FTMN fiber structure. transmission loss of the spectrum will increase because more light leaks

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F. Wang, et al. Optics and Laser Technology 130 (2020) 106333

out of the fiber in the tapered area. It can be observed that there is no M

obvious interference pattern when the θ was 180°. However, in the case E (r , z ) = ∑ ηm ψm (r ) exp(jβm z )
m=1 (2)
of 165°, the resultant effect is relatively weak that the composite in-
terference resulting interference dips have a relatively low extinction where ψm is the field profile of LP0m excited within the MMF, βm is the
ratio (ER). By decreasing the θ to 150°, an enhanced interference propagation constant of the excited mth order mode and ηm is the ex-
spectrum was obtained, in which two interference dips as shown in citation coefficient which can be calculated using the following for-
Fig. 3(b). Both of them were observed with a relatively high ER (more mula:
than 10 dB). As the θ was further decreased to 135°, there is no obvious ∞
∫0 E (r , 0) ψm (r ) rdr
interference pattern. If the θ continues to decrease, more light will leak ηm = ∞
out from the fiber, resulting in a significant decrease in the intensity of ∫0 ψm (r )2rdr (3)
the transmission spectrum, which is not worthy. Ultimately, an ap-
When the light arrives at the tapered section, a portion of light from
propriate θ to form an effective composite interference should be in the
the MMF core is coupled into the MMF cladding at which point it ex-
region of 150° based on the results of above experiments. Furthermore,
cites cladding modes [23]. These excited cladding modes propagate
Fig. 3(b) shows the Fourier frequency spectra of the FTMN fiber
within the tapered MMF cladding, and when they transmit through the
structure (θ =150°). Obviously, besides the fundamental frequency,
tapered area, a portion of the cladding modes are recoupled back into
there are two main frequency component in the Fourier frequency
the MMF core. Due to the difference in effective RIs between the core
spectra of the FTMN fiber interferometer. Hence, it could be regarded as
and cladding and the optical path difference between the light signals in
a composite interference, which is constituted by MMI and MZI.
transmission, an interference exists between the propagating funda-
Using the beam propagation method (BPM) in Rsoft software to
mental and high-order modes, and thus, an MZI is formed. The trans-
describe the optical field distribution within the FTMN fiber structure
mission intensity is analyzed by using the two-mode interference model
(θ =150°). The specific simulation parameters were: the diameter and
for simplification [6,21].
RI for the MMF core/cladding are 65/125 μm and 1.452/1.447, and the
RI for NCF is 1.444. The external medium was a transitional boundary I = If + Im + 2 If Im cos φm (4)
condition (TBC) (n = 1). When the input light wavelength was set to
1550 nm, the field intensity distribution along the FTMN in the air is where, If and Im are the intensities of the fundamental core mode and
shown in Fig. 4. The fundamental mode of the tapered MMF will be mth high-order mode, respectively. The phase difference φm between the
partly coupled to the high-order mode that the power of modes parti- fundamental mode and mth high-order mode can be expressed as:
cipating in the interference will be more matched and meanwhile the m
2π Δneff Leff
mode interference will be enhanced [20]. When the light propagates φm =
λ (5)
through the tapered area, most of light is coupled into the core and
cladding of MMF, while a small portion of the light leaks out of the where the length of interferometer is Leff ; λ is the wavelength of input
m
fiber. And the light transmitted along the NCF comes from the inter- light; Δneff (m = 1,2,3…) represents the effective RI difference between
ference between the core and cladding modes in the tapered MMF. the modes. The attenuation peak wavelength of the spectrum can be
Ultimately, at the fusion point between NCF and output SMF, the core expressed as:
and cladding modes satisfying the phase matching condition generate m
2Δneff Leff
interference phenomenon. λm =
2m + 1 (6)
As discussed previously, the light from the source initially propa-
gates along the core of input SMF in the fundamental mode (LP01). When the light arrives at the NCF section, each mode is associated
When the light launches into the MMF section, a series of high-order with their respective phase difference and MMI between each mode
modes (LP0m) are effectively excited due to the mode field mismatch occurs again, hence resulting in composite interference. Based on the
between the SMF and MMF. For a step-index MMF, the excited mode Eq. (6), red shift of the interference fringes could be observed due to the
m
number M can be approximately calculated by: positive change of Δneff with the SRI increasing.

2a nco2 − ncl2 3. Experiments and results

M≈
λ (1)
The experimental setup for RI measurement is illustrated in Fig. 5.
where a is the radius of the MMF core, nco and ncl are the RI for the MMF The fabricated FTMN is stably and horizontally fastened on a glass slide
core and cladding, respectively, and λ is the free space wavelength. As by using ultraviolet glue. Broadband light from a semiconductor optical
the light propagates within the MMF section, the electric field dis- amplifier (SOA) (KG-SOA-C-10-D-FA, Conquer) (wavelength ranges
tribution E (r , z ) at the propagation distance z can be expressed as [22]: from 1200 nm to 1700 nm) was coupled into the input SMF of FTMN
fiber structure, and the output optical spectral signal was interrogated
by a high-resolution (0.02 nm) optical spectrum analyzer (OSA)
(MS9740A, Anritsu, Japan). The surrounding RI (SRI) response of
FTMN structure was investigated by immersing the sensor to different
NaCl aqueous solution with RI ranging from 1.3405 to 1.3497, while
the temperature was kept at the room temperature (20 °C). After re-
cording transmission spectrum, the study employed the alcohol to clean
the FTMN, recovering the original spectrum in air. Subsequently, the
FTMN is immersed in a new RI solution to record a new transmission
spectrum.
Fig. 6 displays the measured output spectral evolution of the pro-
posed sensor in the range from 1.3405 to 1.3497. To make a depiction
and analysis of the spectra change, we trace the interference of dips A
and B. It is obviously that with the SRI increasing, both interference
dips A and B exhibit a resonant red shift of wavelength, which is con-
Fig. 4. Simulated optical field intensity distribution within the FTMN. sistent with the Eq. (6). The experimental results thus reveal that the

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F. Wang, et al. Optics and Laser Technology 130 (2020) 106333

Fig. 5. Schematic diagram of the experimental setup for RI measurement.

Fig. 6. Transmission spectrum response of the FTMN to various RIs.

Fig. 8. Transmission spectra response of the FTMN at different temperatures.

FTMN is sufficiently sensitive to the SRI.

Fig. 8 expresses the transmission spectra response of the FTMN at dif-
Relationships between the SRI increment responses of dips A and B
ferent temperatures. It can be seen that as the external temperature
are shown in Fig. 7. It can be found that both the fitting curves exhibit
increases, both the interference dips of A and B exhibit a resonant red
excellent linearity with high linear regression coefficient values (R2).
shift of wavelength.
For dip A, the RI sensitivity in its increasing processes was calculated to
Fig. 9 shows the wavelength shift of the interference dips A and B,
be 1033.5 nm/RIU with R2 being 0.97. For dip B, the corresponding RI
respectively. It can be seen that the temperature sensitivities of
sensitivity was determined to be 1191.5 nm/RIU with R2 being 0.95.
0.648 nm/°C and 0.598 nm/°C of dip A and B (R2 approach to 0.99 and
In addition to the fundamental RI measurement, the temperature
0.99) are obtained in the range from 20 °C to 90 °C. The axial strain
response characteristics of the proposed FTMN fiber structure were also
changes caused by the thermal expansion and the changes of effective
investigated. The temperature response characteristics was researched
RI caused by the temperature variation effect synergistically on the
by fixing the manufactural sensor unit (FTMN fiber interferometer) into
FTMN. Ultimately, resulting in a red shift of wavelength at dips A and B.
a temperature control box from 20 °C to 90 °C with a step of 10 °C.
Since dip A and dip B have different response characteristics for RI

Fig. 7. Wavelength shift with the change of the SRI. Fig. 9. Wavelength shift with the change of temperatures.

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F. Wang, et al. Optics and Laser Technology 130 (2020) 106333

Table 1 Formal analysis, Investigation, Writing - original draft, Writing - review

Comparison between the proposed FTMN and related RI sensors. & editing, Software. Tao Ma: Software, Visualization, Investigation. Xu
Sensor structure RI sensitivities RI Range References Wang: Conceptualization, Methodology, Formal analysis. Yufang Liu:
Validation, Conceptualization, Methodology, Resources, Supervision,
SMF-NCF-SMF 259 nm/RIU 1.333–1.381 [24] Funding acquisition.
Down-taper-SMF–TCF −79.335 nm/RIU 1.3562–1.3786 [25]
Tapered MMF 1900 nm/RIU (at a RI of 1.33–1.44 [15]
1.44)
Declaration of Competing Interest
S-tapered fiber probe 1882.4 nm/RIU 1.4216–1.4284 [16]
Splicing points tapered 260.8 nm/RIU 1.3333–1.3737 [26] The authors declare that they have no known competing financial
SMF-PCF-SMF
Up taper-down taper- 131.93 nm/RIU 1.3211–1.3527 [7]
interests or personal relationships that could have appeared to influ-
up taper ence the work reported in this paper.
Biconical SMF 700 nm/RIU 1.333–1.350 [27]
Tapered NCF 439.11 nm/RIU 1.333–1.362 [28] Acknowledgments
S-shaped dual tapered 460.66 nm/RIU 1.3387–1.3516 [29]
NCF
S-tapered PCF 125 nm/RIU 1.38–1.44 [30] This work was supported by the National Natural Science
The proposed FTMN 1191.5 nm/RIU 1.3405–1.3497 Our work Foundation of China (Grant No. 61627818), the Scientific and
Technological Project of Henan province (Grant No. 182102210367),
the Key Project of Henan Education Department (Grant No.
and temperature, simultaneous measurement of RI and temperature can 19A510002), and the Cultivation Foundation of Henan Normal
also be achieved and thus overcome the temperature induced cross- University National Project (Grant Nos. 2017PL04 and
sensitivity in practical RI measurement. Based on the above obtained 5101239170010).
sensitivity coefficients, a demodulation matrix is established, which is
expressed as [22,23]: References
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