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Wavelength- and Intensity-Demodulated Dual-Wavelength Fiber Laser Sensor for Simultaneous RH and Temperature Detection

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We report a dual-wavelength fiber laser sensor based on a uniform fiber Bragg grating (UFBG) and a Polyvinyl alcohol (PVA) film-coated long-period grating (LPG) as sensor probe for simultaneous detection of relative humidity (RH) and temperature. Two UFBGs and the LPG were inscribed in three single mode fibers utilizing phase/amplitude mask technology and mobile scanning technique via the 248 nm UV laser. In our experiment, the RH function based on the differential intensity measurement at dual-wavelength output from 30% to 85% is in a quadratic equation, and the RH sensitivity coefficient of 0.358 dB/% shows good linear relationship and stability from 55% to 85%. Meanwhile, the temperature sensitivity coefficients based on wavelength detection and differential intensity detection are 9.1 pm/°C and 0.21 dB/°C, respectively. The structure of dual-wavelength fiber laser sensor with high signal to noise ratio (SNR), narrow spectral width and good stability enables simultaneous measurement of the RH and temperature with high accuracy and good repeatability.
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Date of publication xxxx 00, 0000, date of current version xxxx 00, 0000.
Digital Object Identifier 10.1109/ACCESS.2017.Doi Number
Wavelength- and intensity-demodulated dual-
wavelength fiber laser sensor for simultaneous
RH and temperature detection
Bin Yin 1, Guofeng Sang1, Ran Yan1, Yuheng Wu1, Songhua Wu1,2, Muguang Wang3,
Member, IEEE, Wenqi Liu4, Haisu Li3, and Qichao Wang1
1 College of Information Science and Engineering, Ocean Remote Sensing Institute, Ocean University of China
2 Laboratory for Regional Oceanography and Numerical Modeling, Pilot National Laboratory for Marine Science and Technology (Qingdao)
3 Institute of Lightwave Technology, Beijing Jiaotong University, 100044 Beijing, China
4 Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences
Corresponding author: Bin Yin (e-mail: author@ binyin@ouc.edu.cn).
This work was supported in part by the National Key Research and Development Program of China (No. 2018YFC0213106, 2016YFE0125700), the
National Natural Science Foundation of China (No. 61901429, 61775015, 61975191), the Natural Science Foundation of Shandong Province (No.
ZR2019BF003), the Key Laboratory of All Optical Network and Advanced Telecommunication Network, Ministry of Education, Beijing Jiaotong
University (No.AON2019004).
ABSTRACT We report a dual-wavelength fiber laser sensor based on a uniform fiber Bragg grating
(UFBG) and a Polyvinyl alcohol (PVA) film-coated long-period grating (LPG) as sensor probe for
simultaneous detection of relative humidity (RH) and temperature. Two UFBGs and the LPG were
inscribed in three single mode fibers utilizing phase/amplitude mask technology and mobile scanning
technique via the 248 nm UV laser. In our experiment, the RH function based on the differential intensity
measurement at dual-wavelength output from 30% to 85% is in a quadratic equation, and the RH sensitivity
coefficient of 0.358 dB/% shows good linear relationship and stability from 55% to 85%. Meanwhile, the
temperature sensitivity coefficients based on wavelength detection and differential intensity detection are
9.1 pm/°C and 0.21 dB/°C, respectively. The structure of dual-wavelength fiber laser sensor with high
signal to noise ratio (SNR), narrow spectral width and good stability enables simultaneous measurement of
the RH and temperature with high accuracy and good repeatability.
INDEX TERMS Dual-wavelength Fiber Laser; Fiber Laser Sensor; Fiber Bragg Grating; Long-Period Grating;
Relative Humidity; Temperature
I. INTRODUCTION
The simultaneous detection of the humidity and
temperature has a significant requirement in applications
such as meteorological services, biomedical devices, food
processing and storage, chemical and electronic processing,
and so on [1-3]. The relative humidity (RH), which denotes
the partial water vapor pressure relative to the saturation
water vapor pressure at static temperature, has been widely
applied in the humidity sensing [4]. Conventional
mechanical and electronic humidity sensors have many
limitations such as low sensitivity, long response time, only
single parameter measurement. Therefore, the deep
investigation of the humidity and temperature sensors with
simultaneous measurement, small size, high accuracy, fast
response and good repeatability is necessary [5,6].
The fiber-optic sensors are able to possess the above-
mentioned characteristics [7-10]. There already exist
numerous fiber-optic RH and temperature sensors based on
uniform fiber Bragg grating (UFBG) [11], long-period
grating (LPG) [12], and different kinds of multimode
interferometer [13]. G. Berruti et.al. reported a fiber-optic
humidity sensor utilizing polyimide-coated UFBG applied
in high-energy physics field below C as same as their
radiation hardness capability when exposed to strong
ionizing radiations (up to 10 kGy) [14]. Chao-Tsung Ma et.
al proposed a dual-polymer fiber Fizeau interferometer for
simultaneous measurement of RH and temperature. The
measured RH and temperature sensitivities of two sensors are
0.12538 nm/%, 0.25376 nm/, 0.15807 nm/%, and 0.38551
nm/ [15]. S. N. Wu et. al reported a fiber-optic RH and
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VOLUME XX, 2017 9
temperature sensor based on polyvinyl alcohol (PVA)-
coated open-cavity Fabry-Perot interferometer (FPI). The
highest sensitivity of RH measurement for the reflection
loss of FPI from 30% to 90% was -1.2 dB/%, and the
sensitivities RH and temperature for the dip wavelength
were -23.1 pm/% and -6.14 pm/°C, respectively [16].
Recently, fiber laser has attracted a great deal of attention
in the sensing field on account of the superior advantages
including high signal to noise ratio (SNR), good stability,
and narrow spectral width compared with conventional
sensor using broadband light source and so on [17-20]. X.
C. Yang et. al reported a dual-wavelength ring laser gas
sensor using tunable Fabry-Perot filter, hollow-core
photonic crystal fiber, and UFBG. The minimum detection
limit at 20 s response time was 10.42 ppmV by experiment
[21]. Javier A. Martin-Vela et. al presented a fiber laser
curvature sensor based on an LPG with the curvature
sensitivity of -42.488 nm/m-1 [22]. Y. Dai et. al reported a
dual-wavelength fiber laser liquid-level sensor based on
two parallel phase-shift FBGs and one UFBG. The
measured liquid-level sensitivity in the experiment within
1.5 mm range was 2.12×107 MHz/m [23].
In this article, we present a wavelength- and intensity-
demodulated fiber laser sensor for simultaneous detection
of the RH and temperature. Two UFBGs as reflection filter
and the PVA-coated LPG as intensity modulation embed
into a laser ring cavity achieve stable dual-wavelength
lasing and sensing. The resonance peak wavelength shift
and differential power intensity change of dual-wavelength
laser varied with RH and temperature. In the experiment,
the measured SNR, 3dB spectral width and stability of fiber
laser are 45 dB, 0.04 nm and <0.1dB (0.02nm), respectively.
The RH function based on the differential output power
measurement from 30% to 85% is in a quadratic equation,
and the measured quadratic and linear coefficients are 0.005
dB/%2 and -0.335 dB/%, respectively. From 55% to 85%,
the RH function shows good linear relation and the RH
sensitivity coefficient is 0.358 dB/%. The temperature
function based on lasing wavelength measurement and
differential output power measurement all shows a linear
relation. Meanwhile, the temperature sensitivity coefficients
of 9.1 pm/℃ and 0.021 dB/℃ are obtained. Based on the
measured cross sensitivity coefficients and stability, the RH
and temperature accuracy of the fiber laser sensor is
calculated as 0.35% and 2.2 , which are more than some
traditional fiber sensors [24][25]. Thus, the proposed laser
sensor for RH and temperature measurement has high SNR,
high accuracy, good repeatability, and low error, which
could be applied in chemical, engineering, and biochemical
monitoring fields.
II. SENSING PRINCIPLE AND EXPERIMENTAL
STRUCTURE
A.
SENSING PRINCIPLE
The standard fiber laser sensor structure includes pump
source, resonant cavity, gain fiber and other optical
modulation elements. As a new kind of the fiber sensor, the
fiber laser sensor mainly adopts the resonator cavity or
optical filter of the fiber laser as the sensing probe to
measure the parameters such as temperature, strain,
refractive index and so on by the wavelength, intensity,
phase or beat frequency of the fiber laser. In our proposed
fiber laser sensor system, the sensing probe includes the
UFBG2 and the PVA-coated LPG. The sensing principle of
UFBG (including UFBG1 and UFBG2) utilizes the Bragg
wavelength shift ∆λUFBG1,2 detection with the variation of
axial strain and temperature. The normal UFBG uncoated
any material is not sensitive to humidity. Meanwhile, this
experiment only analyzes the response of the sensing probe
to the temperature and RH. In combination with our
experiments, we fixed both ends of the sensing probe in the
metal frame to keep the axial strain constant. It made sure
that there was no external stimulus acting on the laser
sensing probe other than the temperature and RH. Thus, the
relationship between ∆λB and the variation of temperature
ΔT is written as [26]:
1
()
UFBG eff
T
n
 
= + 
(1)
where ξ and α represent the thermo-optical coefficient and
FIGURE 1. (a) Spectral principle of UFBG-based sensors for
temperature and RH monitoring. (b) Spectral principle of PVA-
coated LPG-based sensors for temperature and RH monitoring.
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VOLUME XX, 2017 9
linear thermal expansion coefficient of fiber, respectively.
The spectrum of UFBG with the variation of temperature is
depicted in Fig. 1(a). If the temperature sensitivity
coefficient is assumed to be
11 1
()
eff
Kn

=+
, the above
equation (1) can be abbreviated as
11UFBG KT
= 
(2)
Furthermore, the laser wavelength is the same as the
wavelength of UFBG on account of the dual-wavelength
fiber laser generated by the reflection of UFBG. Due to
UFBG2 as the part of the sensing probe and UFBG1 as the
reference grating, the wavelength difference of dual-
wavelength fiber laser should be expressed as
2 1 2 1
2 11
laser laser laser UFBG UFBG
UFBG KT
 
=  −  =  − 
=  =  
(3)
Meanwhile, the dual-wavelength output power intensity
also varies with the wavelength of UFBG.
The LPG with a period typically in the hundreds of
microns promotes coupling between the propagating core
mode and co-propagating cladding modes. The high
attenuation of the cladding modes leads to the transmission
spectrum of the LPG which contains a series of attenuation
bands with discrete resonant wavelengths, and each
attenuation band corresponds to the coupling to a different
cladding mode [27].
The spectral shape and the resonant wavelengths of the
attenuation bands are related to the period and length of the
LPG, and also sensitive to the ambient environment
including temperature, strain, bend radius and refractive
index of the medium surrounding. The changes of these
parameters can alter the period of the LPG and/or the
effective refractive index difference of the core and
cladding modes. Furthermore, the phase-matching
conditions for coupling to the cladding modes is modified,
which results in a variation in the resonant wavelengths and
powers of the attenuation bands [28].
The sensitivity to a particular measurand is dependent
upon the composition of the fiber and the order of the
cladding mode to which the guided optical power is
coupled, and is thus different for each attenuation band.
FIGURE 3. (a) The image of PVA-coated LPG at the scanning electron microscope; (b) Transmission spectrum of the uncoated LPG (red line) and
the PVA-coated LPG with two UFBGs (black line).
FIGURE 2. The fabrication and schematics of the LPG and UFBG (including UFBG1 and UFBG2).
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VOLUME XX, 2017 9
These characteristics enable the LPG particularly attractive
for sensor applications [29].
Because the change of LPG resonance wavelength results
from the influence of RH and temperature, the resonance
wavelength shift
can be expressed as [30]
11 12LPG k T k RH
=  +
(4)
where
11
k
and
12
k
represent the sensitivity coefficients of
temperature and RH in wavelength, respectively. The
spectrum of the PVA-coated LPG with the change of the
temperature and RH is described in Fig. 1(b). The intensity
variation of the PVA-coated LPG in resonance wavelength
results from the variation of the RH (
RH
) and
temperature (
T
)is written as
21 22LPG
P k T k RH =  +
(5)
where
21
k
and
22
k
are the sensitivity coefficients of the
temperature and RH in intensity, respectively. Since both
temperature and RH will cause variations of the PVA-
coated LPG in resonant wavelength and resonant peak
intensity, and then the changes of the fiber laser in the peak
power intensity will be affected. Meanwhile, the
wavelength change of UFBG caused by temperature will
also affect the laser output power intensity variation. Thus,
the intensity difference variation of the proposed fiber laser
output (include the effects of UFBG and PVA-coated LPG)
can be expressed as
21 22laser
P K T K RH =  +
(6)
Based on the equation (3) and (6), the coefficient matrix
can be given by
11
21 22
0
laser
laser
KT
KK
P RH



=







(7)
The variation of temperature and RH by matrix
transformation can be expressed as
22
21 11
0
1laser
laser
K
TKK
RH D P



=






(8)
where
11 22
D K K=
,
11
K
,
21
K
,
22
K
can be determined
by measured curve based on temperature and RH responses
separately for
laser
and
laser
P
[31]. According to the
above analysis, the variations of the temperature and RH can
be evaluated by expression (8).
B.
FABRICATION PROCESS
Two UFBGs and the LPG were separately fabricated in three
10-day hydrogen-loaded (10 Mpa; at room temperature)
B/Ge co-doped single mode fibers (the length is about 30 cm)
using phase/amplitude mask technology and mobile scanning
technique, exposed by the 248 nm UV laser with 80 mJ pulse
energy and 10 Hz pulse frequency. The fabrication and
schematics are shown in Fig. 2. The periods of the phase
mask are 1068 nm for UFBG1 and 1075 nm for UFBG2, and
the period of the amplitude mask is 250 μm. The length of
two UFBGs is respectively 1.0 cm, and the length of the LPG
is about 2.0 cm. Fig. 3(b) displays the measured transmission
spectrum of the fabricated UFBGs and LPG from 1540 nm
to1570 nm. Through the analysis of data, the center
wavelengths of UFBGs are 1543.7 nm and 1558.04 nm,
respectively. Two UFBGs and the LPG were annealed at 180
C for 8 h before being ready for employing in the
experiments. Then, the LPG and UFBG2 are fused together
with a cutting knife and a fusion splicer as the sensor probe,
and UFBG1 is fused into the laser ring as a reference grating.
In order to coat the LPG, the PVA solution should first
be prepared. In our experiment, the PVA (PVA-205, low
viscosity) was supplied by Aladdin company. The
configuration process of 3% PVA aqueous solution is as
follows (take 3% as an example). Firstly, 3 g low-viscosity
PVA granules for several times (about 0.5 g each time)
were dissolved into 97 g deionized water at 90 °C with
magnetic stirring for 30 minutes. Then, the aqueous
solutions were taken into a climate chamber with keeping
temperature 90~95 °C for 2 h until complete alcoholysis.
The fabricated LPG was cleaned with deionized water and
evaporated in the air, repeatedly. Then, The LPG was fully
immersed into the PVA aqueous solution for coating 1 h at
80 °C by using the dip-coating process. At last, the PVA-
coated LPG was dried in the climate chamber for 1 h at
50 °C to evaporated off the water of the LPG surface. The
image of the PVA-coated LPG at the scanning electron
microscope is shown in Fig. 3(a).
C.
EXPERIMENTAL STRUCTURE
The experimental structure diagram of the proposed fiber
ring laser sensor is depicted in Fig. 4. A 3 m long erbium-
doped fiber (EDF, YOFC-ED1016, absorption of 36 dB/m,
the peak of which is at 1532 nm) is used as a gain medium.
The pumped light from 976 nm laser diode enters the gain
medium through 980/1550 nm wavelength division
multiplexer (WDM). A 3-port optical circulator (OC) united
with dual UFBGs (UFBG1 λ1 = 1543.7 nm, UFBG2 λ2=
1558.04 nm) is used to reflect the dual-channel lasing light
into the ring cavity. An optical isolator is inserted in the ring
cavity in order to assure the unidirectional operation and
prevent any negative effects such as spatial hole burning.
Furthermore, the PVA-coated LPG cascaded with UFBG2 as
an integrated sensing probe is used to achieve the perception
of the RH and temperature change. Meanwhile, the distance
between the PVA-coated LPG and UFBG2 is limited to
centimeter magnitude to ensure the sensing probe in the same
measurement environment. The other port of the sensing
probe is connected with an optical spectrum analyzer (OSA,
0.1 nm resolution). The sensing probe (the length of 6 cm) is
put into the climate chamber for realizing to monitor the
changes of RH and temperature. Through measuring the
output spectrum (wavelength and differential power intensity)
of the proposed fiber laser sensor system by OSA, the
simultaneous detection of the RH and temperature with
high accuracy and good repeatability is achieved.
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VOLUME XX, 2017 9
III. EXPERIMENT RESULTS AND DISCUSSIONS
A.
LASER OUTPUT CHARACTERISTIC
The fiber laser can immensely improve the performance of
the fiber sensor on account of their high SNR, high stability
and narrow spectral width [32]. In our experiment, the pump
light was set at 248 mW which was beyond the pump
threshold 150 mW. Through adjusting the polarization
controller (PC) and variable optical attenuator (VOA), the
stable dual-wavelength lasing was achieved. Fig. 5(a) shows
the measured output spectra of the proposed dual-wavelength
fiber laser sensor with dual UFBGs and the PVA-coated LPG
scanned repeatedly at 15 minutes interval in five hours. From
the measured output spectra, we can find that the dual-line
laser emission which is slightly greater than the Bragg
wavelengths of dual UFBGs is generated at 1543.74 nm and
1558.08 nm with SNR of 45 dB and 3dB bandwidth of 0.04
nm. Additionally, the wavelength and power stability of the
dual-wavelength fiber laser at room temperature is validated
as shown in Fig .5. From Fig. 5(b) and 5(c), the maximum
variations of lasing power intensity and lasing wavelength
are less than 0.1 dB and 0.02 nm, respectively. By the
stability study above, the high SNR, high stability and
FIGURE 4. Experimental structure diagram of the proposed fiber laser sensor using PVA-coated LPG and dual UFBG.
FIGURE 5. (a) Measured output spectra of dual-wavelength fiber laser sensor based on the PVA-coated LPG and dual UFBG at 15 minutes interval
during five hours. (b) (c) The variation of lasing output power intensity and wavelength during five hours.
FIGURE 6. (a) Measured output spectral of the proposed dual-
wavelength fiber laser sensor under different RH (from 30% to
80%); (b)(c) Local amplification diagram of laser peak location.
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VOLUME XX, 2017 9
narrow spectral width performance of the proposed laser
sensor has been demonstrated to ensure simultaneous
measurement with high-accuracy and good repeatability.
B.
RH RESPONSE
During the RH and temperature experiment, the sensing
probe including PVA-coated LPG and UFBG2 was fixed into
the climate chamber for realizing to monitor the change of
the RH and temperature, and the other parts of the fiber laser
system were fixed on the optical platform. For the RH
measurement, the temperature was held at 25 . Fig. 6
shows the actual spectrum from 1540nm to 1565nm
measured by OSA under 30% RH, 40% RH, 50% RH, 60%
RH, 70% RH and 80% RH. The output power variations at
the dual-wavelength laser peak are opposite with RH from
30% to 80%. At the same time, dual emission wavelengths
basically remain constant.
Further, the response curve between RH and the proposed
laser sensor has been investigated, as shown in Fig. 7.
Because of external environment change and identical pump
light jitter, the measurement of differential intensity at
different wavelengths is used to remove the instability of
laser sensor by above-mentioned factors. As can be acquired
from Fig. 7(a), the RH function based on differential output
power measurement from 30% to 85% is in a quadratic
equation, and the quadratic and linear coefficient are 0.005
dB/%2 and -0.335 dB/% (R2 approach to 0.995), respectively.
Meanwhile, the laser sensor has a good linear relationship
under high RH experiment from 55% to 85%, and the RH
sensitivity coefficient is 0.358 dB/% (R2 approach to 0.997).
Then, we have also researched the variation of wavelength
difference at different RH. The wavelength difference
basically remains constant, as shown in Fig. 7(b).
C.
TEMPERATURE RESPONSE
Temperature cross-sensitivity is the main concern which
needs consideration for such RH sensors. The temperature
response of the proposed laser sensor was conducted and
the actual spectral shift from 25 °C to 65 °C temperature
was depicted in Fig. 8, when the RH was held at 50%. We
can find that the output power at the dual-wavelength laser
peak increases gradually with the temperature from 25 °C
to 65 °C. Meanwhile, the peak wavelength of the dual-
wavelength fiber laser at the UFBG2 position presents
FIGURE 7. (a) The differential intensity variation of dual-wavelength laser sensor as a function of RH. (b) The wavelength difference variation of
proposed laser sensor along with the surrounding RH.
FIGURE 8. (a) Experimental laser spectral variation of proposed
sensor as temperature from 25 to 65 ; (b)(c) Local amplification
diagram of laser peak location.
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VOLUME XX, 2017 9
redshift, and the peak wavelength at the UFBG1 position
basically remains constant.
Furthermore, the results of the characterization for the
dual-wavelength fiber laser sensor and the linear regression
of the temperature response at different wavelengths and
differential output power in the range from 25 to 65
are presented in Fig. 9. It can be seen that the temperature
function based on lasing wavelength measurement and
differential output power measurement show a good linear
relation. At the same time, the measured experiment of
temperature is repeated twice, and the two measurement
results show good consistency. Then, the temperature
sensitivity of 9.1pm/℃ based on wavelength demodulation
at the UFBG2 position (R2 approach to 0.991) is analyzed,
and the temperature sensitivity at the UFBG1 position
approach to zero. Meanwhile, the temperature sensitivity of
0.021 dB/℃ (R2 approach to 0.955) using differential
output power measurement is obtained.
Through the above theoretical analysis and experimental
data processing, the coefficient matrix can be given by
11
21 22 0.358
09.1 / 0
0.021 //%
laser
laser
Knm C
TT
KK dB C
P RH R
dB H

 

   
==

 
   
 
   



(9)
FIGURE 9. (a)(b) Each wavelength variation of the proposed laser sensor as temperature from 25 to 65 . (c) The change of differential output
power intensity as temperature from 25 to 65 .
FIGURE 10. Experiemntal output spectral of the proposed fiber laser
sensor at 60% and 30 °C, 65% and 40 °C, 70% and 45 °C, 75% and
50 °C, 80% and 60 °C.
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VOLUME XX, 2017 9
Based on the above formula, the discrimination between
RH and temperature by the detection of wavelength shift
and differential power intensity variation has been realized.
In order to research the simultaneous response
characteristics of the RH and temperature for the proposed
laser sensor, we have also measured the laser spectral
datum at 60% and 30 °C, 65% and 40 °C, 70% and 45 °C,
75% and 50 °C, 80% and 60 °C in Fig. 10. Based on the
laser spectral data (5.025 dB and 14.32 nm) at 55% and
25 °C as reference points, the measured wavelength
difference and the variation of differential power intensity
are 6.735 dB and 14.38 nm, 8.744 dB and 14.46 nm, 10.526
dB and 14.50 nm, 12.565 dB and 14.54 nm, 14.69 dB and
14.64 nm, which are approximately equal to the calculation
data ( 6.92 dB and 14.37 nm, 8.92 dB and 14.46 nm, 10.815
dB and 14.50 nm, 12.71 dB and 14.55 nm, 14.71 dB and
14.64 nm ) by formula 9. Thus, the dual-wavelength fiber
laser sensor has potential applications in the RH and
temperature simultaneous measurement with high SNR,
high accuracy, good repeatability.
IV. CONCLUSION
A dual-wavelength fiber laser sensor for simultaneous
detection of the RH and temperature is proposed and
experimentally demonstrated. The PVA-coated LPG
combines the UFBG2 as the sensing probe, and the UFBG1 is
used as the reference grating. The RH and temperature can
affect the variation of laser wavelength and output power
intensity. Experimental results show that the RH function
based on the differential output power measurement from
30% to 85% is in a quadratic equation, and the measured
quadratic and linear coefficients are 0.005 dB/%2 and -0.335
dB/%. From 55% to 85%, the RH function shows a good
linear relation and the RH sensitivity coefficient is 0.358
dB/%. At the same time, the temperature function based on
lasing wavelength measurement and differential output
power measurement all show good linear relation, and the
temperature sensitivity coefficients are 9.1 pm/ and 0.021
dB/. Compared to the conventional fiber-optic RH and
temperature sensors, the proposed fiber laser sensing
structure with high SNR, high accuracy, and good
repeatability has the capability of simultaneous multi-
parameter measurement.
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