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IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX 1
XXXX-XXXX © XXXX IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
(Invited paper)
Abstract—A singlemode-multimode-singlemode (SMS) fiber
structure consists of a short section of multimode fiber fusion-
spliced between two SMS fibers. The mechanism underpinning
the operation of an SMS fiber structure is multimode interference
and associated self-imaging. SMS structures can be used in a
variety of optical fiber systems but are most commonly used as
sensors for a variety of parameters, ranging from macro-world
measurands such as temperature, strain, vibration, flow rate, RI
and humidity to the micro-world with measurands such as
proteins, pathogens, DNA, and specific molecules. While
traditional SMS structures employ a short section of standard
multimode fiber, a large number of structures have been
investigated and demonstrated over the last decade involving the
replacement of the multimode fiber section with alternatives such
as a hollow core fiber or a tapered fiber. The objective of
replacing the multimode fiber has most often been to allow
sensing of different measurands or to improve sensitivity. In this paper, several different categories of SMS fiber
structures, including traditional SMS, modified SMS and tapered SMS fiber structures are discussed with some
theoretical underpinning and reviews of a wide variety of sensing examples and recent advances. The paper then
summarizes and compares the performances of a variety of sensors which have been published under a number of
headings. The paper concludes by considering the challenges faced by SMS based sensing schemes in terms of their
deployment in real world applications and discusses possible future developments of SMS fiber sensors.
Index Terms—SMS, multimode interference, biosensor, chemical sensor, thin-core fiber, small-core fiber, no-core fiber,
hollow core fiber, fiber interferometer
I. Introduction
This work was supported by the Natural Science Foundation of Jiangxi
Province (Grant No. 20192ACB20031 and 20192ACBL21051); the National
Natural Science Foundation of China (Grant No. 62065013, 61665007 and
61465009); Major Discipline Academic and Technical Leaders Training
Program of Jiangxi Province, China (Grant No. 20172BCB22012). (The
corresponding authors are Qiang Wu, Jinhui Yuan and Bin Liu).
Qiang Wu, Bin Liu, Juan Liu, Sheng-Peng Wan, Tao Wu and Xing-Dao He
are with the Key Laboratory of Nondestructive Test (Ministry of Education),
Nanchang Hangkong University, Nanchang 330063, China. (e-mail:
qiang.wu@northumbria.ac.uk; liubin@nchu.edu.cn). Qiang Wu is also with the
Faculty of Engineering and Environment, Northumbria University, Newcastle
upon Tyne, U.K.
Yuwei Qu and Xiangjun Xin are with Beijing University of Posts and
Telecommunications, Beijing, 100876, China.
Jinhui Yuan is with the Research Center for Convergence Networks and
Ubiquitous Services, University of Science & Technology Beijing, Beijing
100083, China. (e-mail: yuanjinhui81@163.com).
Dejun Liu and Pengfei Wang are with the Key Laboratory of Optoelectronic
Devices and Systems of Ministry of Education and Guangdong Province,
College of Physics and Optoelectronic Engineering, Shenzhen University,
Shenzhen, 518060, China.
Youqiao Ma is with School of Physics and Optoelectronic Engineering,
Nanjing University of Information Science and Technology, Nanjing, China.
PTICAL fiber possesses a number of unique advantages
over other transmission media that include small size,
light weight, low transmission loss and immunity to
interference from electromagnetic fields. Since the first low
loss optical silica fiber was proposed in 1960s and fabricated in
the 1970s [1-3], the technology has witnessed a wide range of
research driven innovations and developments, with a
particular focus on applications in communications and
sensing, Typically an optical fiber has high refractive index
(RI) core surrounded by a lower RI cladding region. Total
internal reflection occurs at the core/cladding interface so that
light can be transmitted within the optical fiber. If a low loss
material such as silica is used, ultralow transmission loss is
possible so that the attenuation of the fiber is very small, circa
0.21 dB/km at 1550 nm. The advantage of ultralow
transmission loss is that it makes it possible for optical fiber to
be used for remote and distributed sensing, for example, to
sense environmental perturbations such as temperature, strain
Yuliya Semenova and Gerald Farrell are with Photonics Research Centre,
Technological University Dublin, Dublin, Ireland.
Singlemode-Multimode-Singlemode Fiber
Structures for Sensing Applications – A Review
Qiang Wu
*
, Yuwei Qu, Juan Liu, Jinhui Yuan
*
, Sheng-Peng Wan, Tao Wu, Xing-Dao He, Bin Liu
*
,
Dejun Liu, Youqiao Ma, Yuliya Semenova, Pengfei Wang, Xiangjun Xin, Gerald Farrell
O
8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX
and acoustic signals [4-8] over a wide geographical region.
There are a wide variety of optical fiber sensors types and
structures, for example, based on a fiber grating [9-14], surface
plasmon resonance (SPR) [15-17] and fiber interferometers
(FIs) [18-20].
An optical FI typically involves multiple beams propagating
along multiple separate optical fibers or alternatively along
different paths within a single optical fiber, where the multiple
beams are transmitted independently but interfere when they
are combined. The interference can be either constructive or
destructive, resulting in transmission peaks or dips over a wide
wavelength range. In an FI based sensor, changes in the local
environment, for example due to stress or temperature, change
the effective path lengths and thus the spectral position of the
peaks or dips, so that the value of an unknown measurand can
be determined by measuring changes in the spectrum, such as
wavelength, intensity, phase or bandwidth [21]. An optical FI
based on multiple fibers needs separate optical devices such as
splitters and combiners to allow multiple beam transmission
within the different optical fibers to introduce phase differences
between these beams. In order to miniaturize an optical FI,
in-line structures which utilize multimode interference along a
single optical fiber have been proposed [22-23], which are
more compact. There are three main types of multimode
interferometers (MMI), namely a Fabry-Perot Interferometer
(FPI) [24-26], Sagnac Loop Interferometer [27-28] and
singlemode-multimode-singlemode (SMS) fiber structures [18,
29-30]. An FPI is composed of two parallel surfaces, with a
defined distance between the two surfaces. Light waves
travelling between the two surfaces will accumulate a
wavelength dependent phase difference and thus interference
will occur when the waves meet.
A fiber based FPI can be classified into one of two categories:
intrinsic [24] and extrinsic [25] FPI, depending on whether the
reflectors are formed inside or outside of fibers. For an intrinsic
FPI, the reflecting components are within the fiber, for example
in the case of a micro-hole [24], while in an extrinsic FPI the
reflecting components lie outside of the fibers, for example
where an air gap is placed between two fiber ends [25].
A Sagnac loop is normally formed by an optical fiber loop,
where a 3dB fiber coupler divides the input light into two
counter-propagating beams and a highly birefringent fiber is
placed in the loop. Interference occurs due to the phase
difference introduced by transmission along both the slow and
fast axes of the highly birefringent fiber.
Finally, an SMS fiber structure utilises a short section of
multimode fiber (MMF) spliced between two singlemode fibers
(SMFs). An SMS structure is frequently referred to as a fiber
heterostructure, given that it will always involve a combination
of different fiber types. SMS structures offer some unique
advantages such as ease of fabrication, low cost, flexible design
and high sensitivity, all of which are useful advantages in the
development of real-world sensors.
In this paper, a systematic review of various categories of
SMS fiber sensor structures is provided. The first category
“Traditional SMS fiber structures”, in which the center fiber is
an MMF, are considered in Section II, with a summary
theoretical underpinning using a mode propagation analysis
(MPA) and an overview of the sensing applications of
traditional SMS structures. Section III presents “Modified SMS
fiber structures” which describes a variety of different
heterostructure designs using other types of fibers as an
alternative to a simple MMF section. Section IV considers
“Tapered SMS fiber structures”, which are among the most
complex SMS structures, with applications in biosensing and
chemical sensing. Section V summarizes and compares the
performance and applications of the three categories of SMS
fiber structures under the headings of temperature, strain, RI
and bio-chemical sensing. Finally, Section VI “Challenges and
opportunities” discusses future challenges and likely/future
developments for SMS fiber structures.
II. TRADITIONAL SMS FIBER STRUCTURES
A simple traditional SMS fiber structure is fabricated by
fusion splicing a short section of MMF between two SMFs.
Figure 1(a) shows a schematic diagram of an SMS fiber
structure [29].
Fig. 1 (a) Schematic diagram of a traditional SMS fiber structure [29],
(b) calculated mode distribution along an MMF (length 30 mm) using
Eq. (3), and (c) calculated spectral response of an SMS using Eq. (5).
As shown in Fig. 1(a), as light is injected from input SMF
into the MMF, multiple modes (including fundamental and
higher order modes) will be excited and propagate
independently through the MMF section. The MMF can be
either a step-index or graded index fiber [31-32]. Multimode
interference occurs between these multiple modes within the
MMF and this dictates the transmission spectral response at the
output SMF. There are various numerical simulation methods
(b)
60
40
20
0
-20
-40
-60
1
0
0 5 10 15 20 25 30
Propagation length within MMF (mm)
Radius of MMF (
m)
Self-imaging points
(c)
Input SMF
Output SMF
MMF
(a)
8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX
for predicting the spectral response of an SMS fiber structure,
such as the beam propagation method (BPM) and modal
propagation analysis (MPA). The typical commercial software
for BPM is “Rsoft”, which is easy to use and is capable of
simulating light propagation within complex fiber structures.
However for larger-dimension waveguides, with a large
number of modes, a BPM simulation can be time consuming if
it is to achieve reliable results. MPA is a fast simulation method
for simple fiber structures and is particularly suitable for long
waveguides.
Assuming that the SMF and MMF are circularly symmetric
and the central SMF and MMF axes are aligned perfectly, only
the LP
0m
modes can be excited within the MMF when light is
injected from the input SMF into the MMF. In both input and
output SMFs only the fundamental guided mode
can be
supported, which can be decomposed into the eigenmodes LP
0m
in the MMF when the light enters from the input SMF [18, 29].
Defining the field profile of LP
0m
as
which are the
eigenmodes of the MMF (the eigenmodes within MMF are
normalized as
∞
∞
), the input field at the MMF is equal to that of the SMF
, which can be written as [18, 29]:
(1)
where is the total number of eigenmodes LP
0m
within
MMF,
is the excitation coefficient of each eigenmode in the
MMF, which is the field overlap between the input SMF
and the MMF eigenmode of
:
∞
∞
(2)
In the MMF section, the field at a propagation distance z is:
(3)
where
m
is the propagation constant of each eigenmode
within the MMF. Assuming the parameters of the output SMF
are the same as those of the input SMF, the transmission power
of the SMS fiber structure can be determined by using the field
overlap integral method between
and the fundamental
mode of the output SMF
as
∞
∞
∞
(4)
Substituting Eq. (3) into Eq. (4) and utilizing the orthogonal
relationship between the eigenmodes of the MMF, the output of
the SMS fiber structure can be simplified as:
(5)
As result of MMI, the well-known self-imaging
phenomenon occurs periodically within the MMF [22, 33].
Figure 1(b) shows an example of simulated mode distribution
along the MMF section using Eq. (3), where the wavelength is
assumed 1550 nm. The results show that the self-imaging
length of the SMS fiber structure is circa 10.2 mm.
As the surrounding environmental parameters (temperature,
strain, vibration, etc) change, several parameters, such as the
effective RI of both the core and the cladding, the length and
diameter of the MMF may change, and thus the eigenmodes
and effective length of the MMF will change, resulting
in changes to
in Eq. (2) and the output of the SMS fiber
structure defined in Eq. (4-5). Fig. 1(c) is the simulated spectral
responses of the SMS fiber structure with different MMF
lengths using Eq. (5). In the simulations, the core/cladding RI
values of the SMF and MMF are 1.4504/1.4447 and
1.4446/1.4271 respectively, the core diameters of SMF and
MMF are 50 and 105 m and both SMF and MMF are step
index fibers. The traditional SMS fiber structure can have a
variety of applications but the two most common are as an edge
filter or as a stand-alone sensor.
A. Edge filter applications
A traditional SMS fiber structure can be fabricated to
implement a bandpass filter spectrum, which can transmit light
within a specific wavelength range. Typically, at a spectral
edge, the transmission level versus wavelength slope is well
defined, repeatable and linear, for example see Fig. 1(c) over
the wavelength range from 1550-1560 nm and can be used as
the basis of an edge filter for wavelength measurement and
sensor system demodulation [34-36]. A ratiometric
demodulation system frequently employs an edge filter, where
the optical signal is split into two paths: one path contains an
edge filter while the other path is used as a reference arm, which
can compensate for the power variations due to the
perturbations in the optical signal source [37-38]. By measuring
the power ratio, the wavelength can be determined.
Edge filter based designs can also be used for sensing,
typically using more than one SMS structure and possibly other
fibre based devices such as a Fibre Bragg Grating. Figure 2(a)
shows a schematic diagram using a pair of SMS fiber structures
to measure both temperature and strain simultaneously [39]. In
this sensor configuration, strain is applied to SMS-1 only, but
both SMS-1 and SMS-2, acting as edge filters, are influenced
by the temperature. The wavelength of the laser source is
selected to lie within the linear slope range of the two SMS
edge filters and a reference arm is used to compensate for
source power fluctuations. A strain and temperature resolution
of 0.34 με and 0.14 °C respectively have been achieved, with
very low strain measurement error (0.39 με) induced by
temperature variations from 10 °C to 40 °C.
Fig. 2 A schematic diagram of for the ratiometric sensing system using
SMS fiber as an edge filter (a) a pair of SMS fiber structures [39] and
(b) SMS with FBG [40].
Splitter
Laser
Detectors
Splitter
Ratio 2
Reference arm
SMS-2
Common Temperature
Apply strain
(a)
SMS-1
Ratio 1
Circulator
Broadband
optical source
FBG
Detectors
Splitter
Ratio
Reference arm
SMS
Common Temperature
Apply strain
(b)
8 IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX
Figure 2(b) shows a combination of an SMS fiber structure
with an FBG for simultaneous measurement of both strain and
temperature with better than 3.4 resolution for strain and a
small temperature induced error of 34 in the temperature
range from 10 to 60 C [40].
B. Stand-alone sensor applications
Fig. 3 A schematic diagram of (a) bend [47] and (b) reflective [55]
SMS fiber structure
As mentioned above a simple SMS fiber structure itself can
be used as a sensor for temperature and strain measurement
[41-42]. An SMS fiber structure can also be used in distributed
sensing based on an optical time-domain reflector [43] and
Brillouin optical time-domain analysis [44].
Up to this point, the SMS structures presented have all been
straight but an SMS fiber structure incorporating macro
bending is also possible and has been proposed and
experimentally investigated. This sensor has a higher
temperature sensitivity than a straight SMS fiber sensor [45].
For a bent SMS fiber structure (Fig. 3(a)), the RI distribution
within MMF is not symmetric and can be defined by an
equivalent RI distribution as follows [46-47]:
(6)
where
is the RI of the straight MMF, is the distance
from the center axis of the MMF and
is the equivalent bend
radius which can be expressed as follows [29]:
(7)
where is the bend radius of the fiber, is the Poisson
ratio and
and
are the components of the photoelastic
tensor. Since is dependent on the bend radius, the output
spectrum of the SMS fiber structure is very sensitive to bend
radius. A displacement sensor based on a bent SMS fiber
structure has been proposed and investigated [48]. The bent
SMS fiber structure has also been used as a curvature sensor
[49] for vibration sensing [50-51], within a fiber laser [52], for
strain sensing [53] and breath state monitoring [54]. Finally, a
reflective SMS fiber structure (Fig. 3(b)) has been proposed for
liquid-solid phase change monitoring in a phase change
material [55] and also as a flow rate sensor [56] utilizing the
property of ultra-sensitivity of SMS fiber structures to bend
radius.
III. MODIFIED SMS FIBER STRUCTURES
In a traditional SMS fiber structure, multiple propagating
modes are confined within the core of the MMF, so given the
presence of the cladding there is no direct contact with
surrounding environment and hence such structures cannot be
readily used for chemical or bio-sensing. In order to address
this problem, a large number of investigations have taken place
on the modification of a traditional SMS fiber structure to allow
an SMS structure to interact with the surrounding environment
and allow for bio-chemical sensing. These investigations have
significantly stimulated the development of the SMS fiber
structure as a sensor. The key idea underpinning the
modification of an SMS fiber structure for bio-chemical
sensing is excitation of cladding modes within the multimode
section so that the surrounding medium may influence the
modes. Such modified SMS fiber structures can be classified as:
singlemode-no-core-singlemode (SNCS), singlemode-small-
core-singlemode (SSCS), singlemode-hollow core-singlemode
(SHCS), SMS coupler and combination SMS fiber structures.
A. SNCS fiber structure
A no-core fiber (NCF) is a form of waveguide using only a
material with a homogeneous RI. Unlike normal optical fibers,
an NCF has no cladding, on the assumption that the
surrounding material acts as a cladding of the NCF. Figure 4(a)
shows a schematic diagram of an NCF used in an SMS
structure to form an SNCS fiber structure [18]. RI changes in
the surrounding material will introduce changes in the mode
transmission in the NCF and thus alter the output of the SNCS
fiber structure.
Fig. 4 A schematic diagram of (a) an SNCS [18] and (b) a thin layer
coated [70], (c) reflective SNCS [80] fiber structure, and (d) a
microscope image of the joint between etched MMF (NCF) and SMF
[18]. Reprinted with permission from [18] © The Optical Society.
The MPA method described above for traditional SMS fiber
structures can be easily applied to carry out numerical
simulations for the SNCS fiber structure, where the cladding is
the material surrounding the NCF [18]. An SNCS fiber
structure was firstly proposed by Wang et al. using an NCF to
substitute for the MMF in an SMS fiber structure for RI sensing
[57]. A detailed theoretical analysis and experimental
investigation was carried out in 2011, in which the NCF was
fabricated by wet etching the cladding of a conventional MMF
(AFS 105/125Y) using hydrofluoric (HF) acid. The resulting
sensor had a maximum RI sensitivity of 1815 nm/ RIU (RI unit)
[18]. The RI sensitivity increases as the diameter of NCF
decreases, but is independent on the length of the NCF, which
provides a possible route to improving the RI sensitivity of such
an SNCS fiber structure. However a significant disadvantage is
the requirement for the fiber etching process which is
dangerous due the use of HF acid. Furthermore accurate control
of both the etched fiber diameter and surface roughness is very
difficult.
MMF
SMF
Metal
SMF
MMF
(a)
(b)
(a)
(b)
NCF
Liquid
SMF
NCF
Liquid
SMF
Thin layer coating
NCF
Liquid
Metal
SMF
(c)
(d)
Etched MMF
SMF
![Fig. 5 A schematic diagram of (a) an SSCS [86], (b) a thin layer coated SSCS fiber structure [90], (c) a microscope image of the joint between SCF and SMF, and measured spectral response at different RI value liquid [86]. Reprinted with permission from [86].](/figures/fig-5-a-schematic-diagram-of-a-an-sscs-86-b-a-thin-layer-9r3cc2x3.webp)
![Fig. 10 A schematic diagram of tapered (a) SMF [226], (b) SNCS [245], (c) SMS fiber coupler [246] and (d) tapered SNCS for S. aureus measurement [245]. Reprinted with permission from [245].](/figures/fig-10-a-schematic-diagram-of-tapered-a-smf-226-b-sncs-245-c-2nvwu29q.webp)
![Fig. 6 (a) A schematic diagram and (b) measured spectral responses of the SHCS fiber structure with different HCF lengths and microscope image of splice joint between SMF and HCF [108]. Reprinted with permission from [108].](/figures/fig-6-a-a-schematic-diagram-and-b-measured-spectral-sydhesuh.webp)
![Fig. 9 A schematic diagram of different single tapered (a) SMF [181], (b) SMS [194], (c) SSCS [201], (d) SNCS [205], (e) SHCS [212] fiber structure, (f) SEM image of Nile red coated tapered SCF [204], (g) single molecule detection of biomarker and localized cellular photothermal therapy [193], (h) tapered SNCS for ultrahigh sensitivity detection of hCG with signal amplification of magnetic microspheres](/figures/fig-9-a-schematic-diagram-of-different-single-tapered-a-smf-31s9q3b4.webp)
![Fig. 1 (a) Schematic diagram of a traditional SMS fiber structure [29], (b) calculated mode distribution along an MMF (length 30 mm) using Eq. (3), and (c) calculated spectral response of an SMS using Eq. (5).](/figures/fig-1-a-schematic-diagram-of-a-traditional-sms-fiber-1gqrud0x.webp)
