Vibration sensing using a tapered bend-insensitive fiber based Mach-Zehnder interferometer

Article (PDF Available)inOptics Express 21(3):3031-3042 · February 2013with59 Reads
DOI: 10.1364/OE.21.003031 · Source: PubMed
Abstract
In this study, a novel fiber-optic sensor consisting of a tapered bend-insensitive fiber based Mach-Zehnder interferometer is presented to realize damped and continuous vibration measurement. The double cladding structure and the central coating region of the in-fiber interferometer ensure an enhanced mechanical strength, reduced external disturbance, and a more uniform spectrum. A damped vibration frequency range of 29-60 Hz as well as continuous vibration disturbances ranging from 1 Hz up to 500 kHz are successfully demonstrated.

Figures

Vibration sensing using a tapered
bend-insensitive fiber based Mach-Zehnder
interferometer
Yanping Xu,
1
Ping Lu,
1,*
Zengguang Qin,
1
Jeremie Harris,
1
Farhana Baset,
1
Ping Lu,
1
Vedula Ravi Bhardwaj,
1
and Xiaoyi Bao
1,2
1
Department of Physics, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
2
Xiaoyi.Bao@uottawa.ca
*
plu@uottawa.ca
Abstract: In this study, a novel fiber-optic sensor consisting of a tapered
bend-insensitive fiber based Mach-Zehnder interferometer is presented to
realize damped and continuous vibration measurement. The double cladding
structure and the central coating region of the in-fiber interferometer ensure
an enhanced mechanical strength, reduced external disturbance, and a more
uniform spectrum. A damped vibration frequency range of 29-60 Hz as well
as continuous vibration disturbances ranging from 1 Hz up to 500 kHz are
successfully demonstrated.
© 2013 Optical Society of America
OCIS codes: (060.2370) Fiber optics sensors; (060.4005) Microstructured fibers; (120.3180)
Interferometry; (120.7280) Vibration analysis.
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Received 5 Dec 2012; revised 17 Jan 2013; accepted 22 Jan 2013; published 31 Jan 2013
(C) 2013 OSA
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1. Introduction
Detection and monitoring of vibration, acceleration, and mechanical shock are crucial for
nondestructive inspection of civil infrastructures such as buildings, bridges, highway
pavements, and dams, structural health monitoring of automobiles, ships, aircraft, and
spacecraft, as well as environmental surveillance of seismic activity and volcanic eruptions. A
piezoelectric accelerometer is the most conventional vibration sensor for structural monitoring
which utilizes the piezoelectric effect to measure dynamic changes in mechanical variables.
However a lack of an effective electrical isolation scheme makes it unsuitable in a total
electromagnetic sensitive environment. A fiber optic sensor will be a good alternative over its
electric counterpart with several unique advantages, such as immunity to electromagnetic
interference, compact size, light weight, and distributed measurement over a long distance [1].
A variety of fiber optic sensing techniques have been extensively studied and among them,
in-fiber Mach-Zehnder interferometer (MZI) sensors have recently been applied to measure
temperature, strain, pressure, and refractive index with salient merits of high sensitivity, a high
degree of integration, simplicity, and compact in-line measurement [2–16]. For these static
measurements, the fiber sensors rely on the demodulation of external disturbance induced
interference peak wavelength shift, which needs a relatively long time to obtain a steady
spectrum. Thus the spectral shift detection algorithm with a slow response time is not suitable
for sensing a rapidly and dynamically changing environment, such as shock impulses and
mechanical vibrations. In addition, previously reported in-fiber MZIs are required to remove
protective jackets between two light steering elements in order to prevent the excited cladding
modes from suffering high attenuation loss. Consequently the fiber mechanical strength is
reduced and the uncoated fiber cladding layer is directly exposed to the surrounding
environment which leads the fiber interferometer to be vulnerable to undesirable disturbance.
In recent years, various types of bend-insensitive fibers (BIF) have been developed to allow
for better light confinement at a smaller bending radius with ultralow bending loss in
Fiber-to-the-Home applications [17–23]. High-order cladding modes can be excited by an
optical connection with an imperfect mode match between two fibers and to be guided by a
structure of a depressed-index area [24, 25]. If the high-order modes are not fully attenuated as
they propagate along the fiber, they may couple back to the core at a following optical
connection to induce a multipath-interference phenomenon. Suppression of excited high-order
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cladding modes and minimization of modal interference with the fundamental mode could be
implemented using mode strippers realized by filling a section of air holes with epoxy [26]. In
this paper, a novel tapered bend-insensitive fiber based in-line Mach-Zehnder interferometer
(BIF-MZI) is fabricated by a fusion splicing technique for damped and continuous vibration
sensing applications. An intensity-based demodulation scheme is developed to monitor the
dynamic vibration induced power fluctuation at a specific wavelength selected from the
transmission spectrum of the BIF-MZI.
2. Operation principle
The bend-insensitive single-mode fiber (ClearCurve, Corning) used in this study comprises an
innermost layer of a germanium-doped silica core surrounded by a narrow layer of randomly
distributed air holes in the pure silica cladding [21]. A schematic of the bend-insensitive fiber
cross-section in Fig. 1 shows that the depressed index ring of nanoscale gas filled voids divides
the fiber cladding region into two areas of an inner cladding region and an outer cladding
region. A schematic illustration of the BIF-MZI is shown in Fig. 1. The BIF-MZI consists of
two abrupt tapers which function as light steering elements to pilot split-merge propagation of
the fundamental core mode and high-order cladding modes along the middle fiber section
between the two fiber tapers. The double cladding structure of the bend-insensitive fiber
enables a selective excitation of multiple cladding modes by the first taper. According to mode
field patterns, the cladding modes of the bend-insensitive fiber can be categorized into two
groups: inner-cladding modes that travel within the inner cladding region due to the total
internal reflection and outer-cladding modes that tunnel into the outer cladding region through
the depressed index ring. When the polymer coating is removed from the middle fiber section,
both the inner-cladding modes and the outer-cladding modes excited by the first taper will be
coupled back to the core mode by the second taper to form a fiber Mach-Zehnder
interferometer. The phase difference Δ
Φ
between the fundamental core mode of LP
01
and
multiple high-order cladding modes of LP
ij
can be expressed as
()
,01 ,
,01 ,
22
,
core clad ij ij
ij core clad ij eff eff eff
ll
nn n
ππ
ΦΦ Φ
λλ
Δ= = − = Δ
(1)
where n
ij
eff
is the effective refractive index difference between the fundamental core mode and
an individual high-order cladding mode, l is the interference length, and
λ
is the operation
wavelength. When the phase difference satisfies Δ
Φ
ij
= 2m
π
, the m
th
order transmission peak
wavelength is located at
,
ij
eff
m
nl
m
λ
Δ
=
(2)
where m is an integer. The intensity in the interference pattern can then be written as
()
,01 , ,01 ,
2cos(),
core clad ij core clad ij ij
II I II
λΦ
=++ Δ
(3)
where I
core,01
and I
clad,ij
are the intensities of the fundamental core mode and an individual
high-order cladding mode, respectively. Provided that the polymer coating between the two
abrupt tapers is retained in the process of fabricating a BIF-MZI, optical energy in the
outer-cladding modes would be sharply attenuated due to high refraction loss at the
unsmoothed cladding-coating interface and a significant absorption band in the
telecommunication window of the high index polymer coating, while energy in the
inner-cladding modes could still travel down the inner cladding region with little attenuation
and reach the second taper. Thus a novel in-fiber Mach-Zehnder interferometer will be
developed based on a tapered bend-insensitive fiber while still preserving its original protective
jacket.
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Fig. 1. Left: A schematic illustration of the bend-insensitive fiber based Mach-Zehnder
interferometer. Right: A schematic cross-section of the bend-insensitive fiber.
As a mechanical strain
ε
is applied on the BIF-MZI, the changing phase difference between
the core mode and each cladding mode will lead to a shift in the corresponding spectrum where
an interference peak wavelength
λ
m
shifts to a new wavelength of
λ
m
' by
(
)
(
)
,)1()//()//(
'
ελδδλδδλ
δ
δ
λ
δ
δ
δ
δ
λλλ
effmeffmeffeffm
eff
effeff
m
effeffeff
mmm
pllpllnnll
ln
lnln
m
lnlnln
+=+=Δ+=
Δ
+Δ
++Δ
==Δ
(4)
where
δ
l is the variation of the fiber length due to the axial strain,
δ
n
eff
is the photo-elastic effect
induced change in the effective refractive index difference, and p
eff
is the effective strain-optic
coefficient. When a tapered bend-insensitive fiber is mounted on a cantilever, the fiber length
will increase or decrease when the cantilever undergoes a convex or concave deflection as
shown in Fig. 2(a). The change in the interference length
δ
l of the BIF-MZI can be expressed as
δ
l 2dD/l, where D is the cantilever deflection and d is the separation of the neutral axis
between the optical fiber and the steel cantilever. Thus damped vibrations of the cantilever will
cause a dynamic strain variation on the BIF and a fluctuation in power spectrum of the
BIF-MZI. In case a piezoelectric cylinder is used to provide a continuous dynamic strain on the
tapered bend-insensitive fiber by wrapping the fiber on it as shown in Fig. 2(b), the
transmission spectrum of the BIF-MZI will periodically red-shift or blue-shift when the fiber
interferometer length experiences an elongation or compression due to the piezoelectric effect
of the lead zirconate titanate (PZT) ceramic material. Figure 2(c) shows a schematic illustration
of fiber interferometer vibration sensing based on an intensity modulation scheme. According
to a typical sinusoidal variation of the output intensity of a two-mode interferometer as a
function of wavelength, a linear intensity response can be acquired for low-amplitude
vibrations observed at a quadrature bias wavelength.
Fig. 2. A schematic illustration of fiber interferometer vibration sensing based on an intensity
modulation scheme.
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3. Experimental details
In the vibration sensing systems, the in-fiber Mach-Zehnder interferometer was fabricated on
the bend-insensitive fiber that was connected to two standard single-mode fibers (SMF28,
Corning) on both sides to form a SMF-BIF-SMF structure. If a mechanical splicing method is
adopted such as using optical fiber connectors, a splice loss modulation will result in weak
modal interference due to a refractive index profile difference induced mode-field mismatch
between the SMF and the BIF. Therefore the SMF-BIF-SMF structure was implemented based
on a fusion splicing technique using a fusion splicer (S182PM, Fitel). A customized clad
alignment fusion splicing program with appropriate fusion current and fusion time was
employed to line up the fibers, minimize the splice loss and avoid the modal interference. The
electrical arc discharge zone was adjusted to introduce an offset of 10 μm deviated from the
junction point to the SMF side, which could accommodate the presence of the nanostructural
features and guarantee the intactness of the air-hole structure of the BIF during the fusion
splicing process. Figure 3(a) shows an optical microscope image of the fusion joint area
between the SMF and the BIF where they are in good alignment. Intermodal interference was
not detected in the output spectrum of the SMF-BIF-SMF structure by launching light from a
combined C + L band erbium-doped fiber amplifier (EDFA) to an optical spectrum analyzer
(86142A, Agilent). Another fusion splicer (FA995, Ericsson) with a built-in taper
manufacturing program was utilized to fabricate fiber tapers on the bend-insensitive fiber.
Selecting a taper specification of a large waist diameter should ensure that fiber mode coupling
efficiency is small and thus the attenuation of the interferometer is minimized and fewer
high-order modes are excited [27]. An optical microscope image of an abrupt taper with a taper
length of 900 μm and a waist diameter of 80 μm is shown in Fig. 3(b). Two individual in-fiber
MZIs, BIF-MZI-a and BIF-MZI-b, were constructed along the bend-insensitive fibers by
creating double abrupt tapers of the above specifications separated by distances of 5 cm and 15
cm, respectively.
Fig. 3. (a) An optical microscope image of the fusion joint area between the BIF (left) and the
SMF (right). (b) An optical microscope image of one abrupt taper fabricated on the BIF.
Figure 4(a) shows the attenuation spectrum of the BIF-MZI-a which was obtained from the
difference between the emission spectrum of the EDFA and the transmission spectrum of the
BIF-MZI. Although the central coating region was preserved, the tapered bend-insensitive fiber
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still performed well as an in-fiber interferometer which is evident from the prominent
interference fringes due to a superposition of several inner-cladding modes interferences. A fast
Fourier transform (FFT) of the wavelength spectrum in Fig. 4(a) provides a spatial frequency
spectrum as shown in Fig. 4(b). The spatial frequency
ξ
can be expressed as
ξ
Δn
eff
L/
λ
0
2
,
where
λ
0
is the center peak wavelength around which a first-order Taylor series is expanded [2].
In Fig. 4(b), a power spectrum in the spatial frequency domain exhibits two dominant intensity
peaks corresponding to two inner-cladding modes with their corresponding simulated optical
field patterns shown in the inset of Fig. 4(b). It is noticed that the light energy of the
inner-cladding modes are completely confined in the inner cladding region. Figure 4(d) shows
a spatial frequency spectrum of the BIF-MZI-b with two dominant intensity peaks obtained by
fast Fourier transform of the corresponding attenuation spectrum in Fig. 4(c). The simulated
optical field patterns of the inner-cladding modes are shown in the inset of Fig. 4(d). Since the
BIF-MZI allows only very few inner-cladding modes to pass through the central fiber coating
region, the superimposed interference spectrum of this few-mode interferometer still
monotonically shifts with a changing strain and thus the power fluctuation at the operation
wavelength exhibits an approximate linear response relation to dynamic vibrations. Compared
to the conventional in-fiber MZIs based on a standard single-mode fiber with a single cladding
layer, the BIF-MZI has very few order numbers of interference due to its double cladding
structure and central coating region, and accordingly a more uniform spectrum which is
suitable for dynamic vibration sensing applications.
Fig. 4. (a, b) and (c, d) show attenuation spectra and corresponding spatial frequency spectra of
the BIF-MZI-a and BIF-MZI-b, respectively. Insets of (b, d) show the simulated optical field
patterns of the inner-cladding modes of BIF-MZI-a and BIF-MZI-b.
Figure 5 shows a schematic experimental setup of vibration measurement using the
BIF-MZI. Light from a 1550 nm planar waveguide based external cavity laser with the 3 kHz
spectral linewidth (PLANEX, Rio) was launched into the BIF-MZI and then guided through an
attenuator and an AC photodetector (PDB450C-AC, ThorLabs) to a high-speed oscilloscope
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(WaveRunner 64Xi-A, LeCroy). Experiments were carried out in a temperature controlled
room with the temperature maintained at 25.0 ± 0.5 °C. The temperature change can be
neglected since it is a rather slow process relative to the dynamic vibration measurement with a
fast response. At a specific operation wavelength, 1550 nm for example, the changing power
due to vibrations becomes a strong function of time. By monitoring the power variation in a
time domain, the vibration frequency could be detected in real time.
Fig. 5. A schematic experimental setup of vibration measurement. LD, laser diode; ATT,
attenuator; PD, Photodetector.
4. Experimental results and discussion
4.1 Detection of damped vibration
Figure 6 shows a schematic measurement setup to detect damped vibration frequency using the
5 cm BIF-MZI-a. A stainless steel cantilever of rectangular cross-section (width w = 0.5 cm,
thickness h = 0.1 cm) and a total length of 40 cm was used to generate a damped vibration. The
tapered bend-insensitive fiber was slightly pre-stretched and attached to the free end of the steel
cantilever using epoxy glue. The set point of the pre-strained value was about 10
3
με, which
was selected to avoid fiber breakage or movement during a convex or concave deflection
process. Another end of the cantilever was fastened on a fixed base by a metal clamp and the
cantilever length could be controlled by adjusting the position of the metal clamp on the
cantilever. When the free end of the cantilever was deflected by a specific displacement from
its initial stabilized position and instantly released, the cantilever would experience a damped
vibration about its equilibrium position soon afterwards. A standard ruler was used to measure
the initial deflection of the cantilever.
Fig. 6. Schematic top view of the experimental setup of damped vibration detection.
Figures 7(a) and 7(c) show the time-domain spectra of the BIF-MZI-a with a 10 cm
cantilever length under damped vibrations that the free end of the cantilever were initially
flipped down to a distance of 5 mm and 3 mm, respectively. Figure 7(a) shows that the initial
output voltage recorded by the oscilloscope is 0.12 V for the damped vibration of the 5 mm
deflection. As the vibration continued, the damping effect caused a continuous attenuation of
the output voltage with a specific damping time defined as a timescale for an output voltage
dropping to 90% of its initial value. The vibration finally vanished and a noise floor was
obtained with a stable output voltage of 0.01 V. The damping time was measured to be 5.0
seconds by performing an envelope analysis on the time-domain signal shown by the red curves
in Fig. 7(a). For the damped vibration of the 3 mm deflection, a relatively small initial peak
voltage of 0.06 V and a damping time of 4.0 seconds were obtained in Fig. 7(c). In addition,
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two insets of Figs. 7(a) and 7(c) extracted from enlarged regions of the vibration time trace
signal exhibit regular sinusoidal waveforms which indicate a stable periodicity of the power
oscillations. Figures 7(b) and 7(d) show the same fundamental frequency of 62.0 Hz by fast
Fourier transform of the time-domain spectra in Figs. 7(a) and 7(c). When the initial deflection
of the cantilever was varied from 1 mm to 7 mm, the fundamental frequencies were in the range
from 61.8 ± 0.5 Hz to 62.4 ± 0.5 Hz as shown in Fig. 7(e). It is indicated that the fundamental
frequencies of the damped vibrations is independent of the initial deflection of the cantilever.
Fig. 7. Time-domain spectra and frequency-domain spectra of the BIF-MZI-a with a cantilever
length of 10 cm under damped vibrations of (a, b) 5 mm and (c, d) 3 mm deflections,
respectively. (e) Fundamental frequencies as a function of initial deflections of the cantilever.
Various fundamental frequencies of the damped vibrations can be obtained by changing the
cantilever length. The fundamental frequency should be proportional to the reciprocal of the
square of the cantilever length, which follows
,
2
4
AL
EIc
f
ρπ
=
(5)
where c is the coefficient of the first vibration mode, E is the Young’s modulus of the stainless
steel, I is the moment of inertial,
ρ
is the density of the material, A is the cross section area, and
L is the cantilever length. The damped vibrations of different cantilever lengths under the same
initial deflection were detected using the BIF-MZI-a to demonstrate this relationship. Figure
8(a) shows the time-domain spectrum of the BIF-MZI-a with a cantilever length of 15 cm under
a damped vibration of a 3 mm deflection and Fig. 8(b) shows the corresponding
frequency-domain spectrum where the fundamental frequency was located at 29.2 Hz. The
normalized power spectra of the BIF-MZI-a with six different cantilever lengths ranging from
10 to 15 cm are shown in Fig. 8(c). Figure 8(d) shows a linear relationship between f and 1/L
2
,
where the fundamental frequencies corresponding to these cantilever lengths are 62.0 Hz, 52.8
Hz, 44.1 Hz, 38.5 Hz, 33.0 Hz, and 29.2 Hz, respectively.
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Fig. 8. (a) Time-domain spectrum of the BIF-MZI-a with a cantilever length of 15 cm under a
damped vibration of 3 mm deflection and (b) the co