1. INTRODUCTION
All-optical fiber based-sensors, due to their intrinsic characteristic
of simple, compact size, cost-effective, resistance to electromagnetic
interference, good hostility to corrosion, durability, flexibility,
accuracy and so on [1] have interested considerable attention in
many applications such as medical applications [2–4], chemical
[5], and environmental [6]. They have been effectively applied
as refractive index (RI) [7,8], temperature [9], displacement
[10], environmental monitoring [11], humidity [12,13],
strain [14], acoustic [15], force [16] sensors and so on.
Optical fiber based-refractive-index (RI) sensors or refractometers are
actually beneficial in a diversity of industrial applications like
chemical or biological sensors [8]. In comparison to conventional
refractive index sensors like optical path measurement method, Optical
fiber based-RI sensors have drawn distinct concentration owing to the
compact size, high sensitivity and the appropriateness for remote
sensing [17,18].
To date, a series of RI sensors have been fabricated based on the
long-period fiber grating (LPFG) [19], fiber Bragg grating (FBG)
[20], Fabry-Perot interferometer (FPI) [12], Mach-Zehnder
interferometer (MZI) [21], photonic crystal fiber (PCF) [1],
and single-mode fiber-coreless fiber-single mode fiber (SCS) [22].
But, most of these fiber structures are exposed to damage that causes
the fabrication process more complex and costly.
Single-mode–multimode–single-mode (SMS) fiber configuration which is
usually constructed by splicing a multimode fiber (MMF) or PCF section
between two single-mode fiber (SMF) section [1,23,24]. The
conventional method to enhance the sensitivity of an in-line MZI sensor
is by enlarging its cavity length [20]. However, the long cavity
leads to more difficult to package the sensor. Herein, CLF which is
operational analogous to MMF has won more attraction of RI sensing
application, in which the sensitivity can be improved by reducing the
diameter of the CLF [12]. The RI-SCS configuration depends on the
principle of multimode interference (MMI). As the core mode of the
propagated in single-mode fiber section, a number of high-order modes
are excited and guided through the CLF. Then, at the other splicing
point, the high-order modes are coupled and backward to the core fiber
of the single-mode fiber [25]. Additionally, when the light passes
via the CLF which functions as the sensing area, the evanescent wave
absorption varies with the variation in the surrounding environmental
medium. This causes to the varies into the RI of the outer medium (which
acts as a cladding layer) and subsequently the output transmission
signal will be modified [26].
In this work, the RI sensing structure comprises of single-mode
fiber-coreless fiber-single mode fiber (SCS) configuration and the
encirclement refractive index behaves as the cladding of the CLF. The
sensor performance of SCS configuration with different CLF diameters and
lengths has been investigated. The CLF diameter is decreased by the
chemical etching method. The better sensor sensitivity was attained when
the CLF diameter was 50 μm.
SENSING PRINCIPLE
The SCS sensor structure comprises of two segment of single-mode fiber
(SMF) and one segment of CLS in between. The three segment are spliced
together. The schematic diagram of the SCS sensor configuration is
depicted in the Fig.1 (A-B). As the light injected into the first
section of SMF, the redoubled high-order linear polarization (LP) modes
have been excited. Then the light travels through SMF-CLF at the first
splicing point, high-order modes will be propagated via the CLF with
diverse propagation constants. After that, as the light propagated
through the second splicing point of CLF-SMF, these modes will be
reconnected to create the modal interference. Consequently, the
characteristic of the output spectrum is varied with contrast to the
input spectrum. The modal field shapes for these modes (normalized with
regard to the all cross-section of the fiber) can excellently be
estimated by that of an infinitely total parabolic media.
For a standard multi-mode fiber including CLF, the propagation constants
of the m and n symmetric modes can be given as [23]:
β m ̵̶ βn =\(\frac{u_{m\ -}^{2}\ u_{n}^{2}}{2\ k_{\text{o\ \ }}a^{2}\ n_{\text{core}}}\)(1)
Where βm and βn are the longitudinal
propagation constants for the order modes m, n. um and
un are the normalized transverse propagation constants,
while the \(a\) is the core radius of the CF, \(n_{\text{core}}\) is the
core refractive index of the CF.
As the phase difference between the two modes is the integer multiple of
2Л, constructive interference can occur in condition:
(β m ̵̶ βn )L = 2Л N (2)
Then, the wavelength of the constructive interference can be given as
[23]:
λ =\(\frac{8\left(2N+1\right)n_{\text{core\ \ \ }}a^{2}}{\left(m+n\right)\left[2\left(m+n\right)-1\right]L}\ \)(3)
where the L is the CLF length and N is the natural number.
While, the wavelength difference is [23]:
Δλ = λN – λN-1=\(\ \frac{16\ n_{\text{core}}a^{2}}{\left(m-n\right)\left[2\left(m+n\right)-1\right]L}\)(4)
From these relations, it worthy known that the dips and peaks are
related to L which peaks have the tendency to be intensive as L varied.
The boundary condition for the light propagated in the CLF is altered
when the surrounding RI is changed. The mode field and propagation
constant are fixed. From equation (2), the phase condition of the
destructive or constructive interference is varied and N is also varied
as the propagation constants are altered. Moreover, it can be observed
that the constructive or destructive interference wavelength shift. The
wavelength shift variations depend on the changing values of the
surrounding RI [23].