Sensors
and
Actuators
A
233
(2015)
22–25
Contents
lists
available
at
ScienceDirect
Sensors
and
Actuators
A:
Physical
j
ourna
l
ho
me
page:
www.elsevier.com/locate/sna
Salinity
sensing
based
on
microfiber
knot
resonator
Yipeng
Liao,
Jing
Wang
∗
,
Hongjuan
Yang,
Xin
Wang,
Shanshan
Wang
Optics
and
Optoelectronics
Laboratory,
Department
of
Physics,
Ocean
University
of
China,
Qingdao
266100,
China
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
4
January
2015
Received
in
revised
form
17
June
2015
Accepted
17
June
2015
Available
online
25
June
2015
Keywords:
Salinity
Microfiber
knot
resonator
Sensing
Sensitivity
a
b
s
t
r
a
c
t
Salinity
measurement
has
become
one
of
the
key
technologies
in
marine
detection
since
salinity
is
an
important
parameter
in
oceanography.
This
paper
demonstrates
salinity
sensing
in
NaCl
solution
based
on
the
microfiber
knot
resonator
by
estimating
its
sensitivity
from
the
relationship
between
the
res-
onance
wavelength
red-shifts
and
the
increment
of
salinity.
The
dependence
of
sensing
sensitivity
on
microfiber
diameter
and
probing
wavelength
are
also
investigated.
Results
show
that
sensing
sensitivity
increase
with
the
decreasing
microfiber
diameter
(3.5
m–2.5
m)
and
the
increasing
probing
wave-
length
(1500
nm–1650
nm),
which
are
in
good
agreement
with
theoretical
results.
By
optimizing
our
sensing
system,
the
maximum
sensitivity
is
21.18
pm/‰,
which
is
16
times
higher
than
that
of
fiber
Bragg
grating.
Such
type
of
salinity
sensor
demonstrated
here
develops
a
new
optical
method
to
measure
salin-
ity
of
liquid
and
offers
useful
references
for
measuring
salinity
in
actual
seawater
so
that
ultra-compact
optical
devices
for
salinity
sensing
of
ocean
can
be
further
developed.
©
2015
Elsevier
B.V.
All
rights
reserved.
1.
Introduction
Salinity,
one
of
the
essential
parameters
in
oceanography,
plays
an
important
role
in
marine
environment,
ocean
circulation
and
other
phenomena.
As
a
typical
electrode-type
device
used
to
mea-
sure
salinity
of
seawater,
conductivity-temperature-depth
(CTD)
system
has
the
advantage
of
high
precision.
However,
this
elec-
trical
device
easily
suffers
from
the
electromagnetic
interference
and
is
not
applicable
for
salinity
measurement
on
the
micro
scale
due
to
its
large
size
and
complex
structure.
Moreover,
in
order
to
achieve
salinity
data
in
certain
depth
of
ocean,
series
connection
of
CTDs
will
have
to
be
used
in
sea
trials,
which
greatly
drives
up
the
costs.
Therefore,
new
miniaturized
sensors
with
low
cost
need
to
be
developed
in
order
to
achieve
fine
measurement
in
the
ocean.
In
recent
years,
optical
methods
for
salinity
sensing
have
grad-
ually
become
the
focus
due
to
the
advantages
of
electromagnetic
immunity,
compact
size,
high
sensitivity/precision,
low
cost
and
so
on
[1–7].
For
example,
Men
et
al.
developed
a
salinity
sensor
based
on
fiber
Bragg
gratings
(FBGs)
that
presents
a
sensitivity
of
0.0165
nm/M
[8].
Frazão
et
al.
used
the
similar
approach
to
achieve
a
sensitivity
of
1.28
pm/‰
[9].
Another
sensor
based
on
polyimide-coated
polarization-maintaining
photonic
crystal
fiber
(PM-PCF)
had
a
sensitivity
of
0.742
nm/M
[10].
However,
fabri-
cating
FBGs
and
PM-PCF
generally
require
special
fibers,
polymer
∗
Corresponding
author.
E-mail
address:
wjing@ouc.edu.cn
(J.
Wang).
coating
and
expensive
equipment
for
writing
and
coating.
Com-
pared
to
the
above
approaches,
optical
sensors
based
on
microfibers
have
attracted
more
and
more
attention
due
to
their
advantages
of
smaller
size,
higher
sensitivity,
faster
response,
lower
cost
and
simpler
fabrication,
which
have
been
widely
used
in
physical
[11–14]
and
biochemical
sensing
fields
[15,16].
As
a
typical
struc-
ture
of
microfiber
sensor,
microfiber
knot
resonator
(MKR)
has
been
proved
sensitive
to
the
surroundings
and
been
successfully
used
in
refractive
index
(RI)
[17–19],
humidity
[20,21],
temperature
[22,23],
magnetic
field
[24],
electric
current
[25]
and
UV
[26]
sens-
ing.
Recently,
seawater
salinity
sensing
based
on
microfiber
loop
resonator
has
been
proposed
and
modeling
on
the
performance
of
the
salinity
sensor
has
been
reported,
which
proved
to
be
a
new
optical
method
to
measure
seawater
salinity
with
high
sensitivity,
low
detection
limit
and
miniaturized
sizes
[27].
However,
to
the
best
of
our
knowledge,
there
is
no
experimental
work
so
far
on
the
salinity
sensing
by
MKR.
In
this
paper,
MKR-based
salinity
sensing
in
NaCl
solution
is
demonstrated
experimentally
with
results
showing
that
the
res-
onance
wavelength
shifts
towards
long
wavelength
linearly
with
the
increment
of
salinity.
Hence,
sensing
sensitivity
can
be
esti-
mated
due
to
this
linear
relationship.
In
addition,
the
dependence
of
sensing
sensitivity
on
fiber
diameter
and
probing
wavelength
are
also
investigated,
which
indicate
good
agreement
with
theoretical
calculations,
revealing
the
availability
and
reliability
of
the
exper-
iment.
By
choosing
the
appropriate
parameters
for
assembling
the
sensor,
the
maximum
sensitivity
measured
is
about
21.18
pm/‰.
The
sensor
demonstrated
here
provide
a
new
optical
method
to
http://dx.doi.org/10.1016/j.sna.2015.06.019
0924-4247/©
2015
Elsevier
B.V.
All
rights
reserved.
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