Short cavity DFB fiber laser based vector hydrophone for low frequency signal detection
Short Cavity DFB Fiber Laser Based Vector Hydrophone for Low Frequency Signal Detection
Xiaolei ZHANG 0
Faxiang ZHANG 0
Shaodong JIANG 0
Li MIN 0
Ming LI 0
Gangding PENG 0
Jiasheng NI 0
Chang WANG 0
Corresponding author: Chang WANG 0
0 Shandong Key Laboratory of Optical Fiber Sensing & Techonology, Laser Institute of Shandong Academy of Science , China, Jinan, 250014 , China
A short cavity distributed feedback (DFB) fiber laser is used for low frequency acoustic signal detection. Three DFB fiber lasers with different central wavelengths are chained together to make three-element vector hydrophone with proper sensitivity enhancement design, which has extensive and significant applications to underwater acoustic monitoring for the national defense, oil, gas exploration, and so on. By wavelength-phase demodulation, the lasing wavelength changes under different frequency signals can be interpreted, and the sensitivity is tested about 33 dB re pm/g. The frequency response range is rather flat from 5 Hz to 300 Hz.
Short cavity distributed feedback fiber laser; vector hydrophone; frequency response; phase noise; sensitivity
The underwater seismic wave stimulated by ship
noise or marine meteorology environment has
significant application prospects to underwater
target detection, mine fuze design, ocean dynamic
monitoring, etc. The current anti-submarine
hydrophone has been challenged by radiation noise
decrease of marine [
]. In oceanography, the low
frequency and ultra-low frequency acoustic signal
has little attenuation and better space time coherence
in propagation. As a result, deep research on low
frequency towed line array and seabed fixed line
array has become a popular topic .
Traditional piezoelectric ceramic hydrophone
requires underwater electrical equipment and cable
for multiplexing, data transferring, and power
supplying, which is expensive and heavy, and has
underwater sealing problems. The hydrophone array,
used for low frequency signal detection, usually has
huge volume, which causes various inconveniences
in engineering application and cost increase.
All-fiber hydrophone array has no electric
equipment underwater, large multiplexing capability,
and anti-electromagnetic interference. Fiber vector
hydrophone measures the three orthogonal
components synchronously and concurrently of the
acoustic pressure and particle velocity at a certain
point of the sound field. Compared with the pressure
hydrophone, the vector hydrophone obtains more
information and provides more possibilities for
subsequent data processing, which is crucial to low
frequency signal acquisition, long-distance target
detection, and location [
Many researches on fiber-optic flexural disk
accelerometers have been reported in recent decades
], while fiber laser based accelerometers have
become more and more attractive on account of high
resolution and small volume [
]. We use a short
cavity distributed feedback (DFB) fiber laser as
the sensing element, to construct a vector
hydrophone with a diameter of 10 cm. Theoretical
and experimental analyses have been carried out to
illustrate a flat response from 5 Hz to 300 Hz.
2. Design and theory
Three identity DFB fiber laser accelerators are
assembled together to construct three-element
co-oscillating type vector hydrophone as shown in
Fig. 1. To control its attitude in water, the other three
non-active accelerators are amounted therein to
maintain balance. The diameter of the vector
hydrophone is 10 cm, and most importantly the total
mass is controlled with the average density of about
1.8 times of water density, to guarantee the globe
oscillating in the sound field with the same
amplitude and phase [
S = ∆aλ ∝ h Km (1)
f0 = K m . (2)
The lasing wavelength change of DFB fiber laser
Δλ is transferred into phase difference change Δϕ:
∆φ = 2π nλd2 ∆λ (3)
where n is the refractive index of fiber, d is the
optical path difference, and Δφ can be obtained by
the phase demodulation technology. The DFB fiber
laser we fabricated has a narrow linewidth of 10 kHz,
which realizes the high resolution wavelength
3. Experiment and analysis
3.1 Calibration system
The system we applied for calibration is drawn
in Fig. 3. A commercial pump source is used. A
non-balanced Michelson interferometer is used to
transfer wavelength change into phase difference
change and detected by a photoelectric detector.
Xiaolei ZHANG et al.: Short Cavity DFB Fiber Laser Based Vector Hydrophone for Low Frequency Signal Detection
Then a commercial OPD4000 (optical phase
demodulation) is applied together with the PGC
(phase generation carrier) algorithm to restore the
wavelength change [
A vibrostand JZ-5 is used where both the DFB
fiber laser hydrophone and a standard piezoelectric
accelerator are mounted.
3.2 Experimental results
Theoretically, the wavelength resolution partly
depends on the linewidth of DFB fiber laser, and the
minimized measurable signal depends on the phase
noise of the DFB fiber laser. It is indicated besides
the sensor packaging design the vector hydrophone
performance can be promoted by tailoring the DFB
fiber laser design or fabrication parameter, such as
lasing cavity length and ultraviolet (UV) exposure.
Three DFB fiber lasers with different lasing
wavelengths are chained together to realize a vector
hydrophone, which are 1535.03 nm, 1539.77 nm, and
1549.37 nm separately. The cavity length of the DBF
fiber laser we fabricated is only 27 mm restricted to
the diameter of the hydrophone cell, the phase noise
is about – 120 dB, the RIN (relative intensity noise)
is – 90 dB, the linewidth is about 10 kHz, and the
output power balance is less than 3 dB.
The DFB fiber laser vector hydrophone we have
designed is calibrated by comparison with the data
of the standard piezoelectric accelerator using the
system described above. The acceleration sensitivity
is measured to be about 40 dB re pm/g from 5 Hz to
100 Hz without damping as shown in Fig. 4. To
suppress the resonance peal as well as expand the
response bandwidth, an appropriate damper should
be added into the system. This procedure is
accomplished in experiment, because the numerical
simulation has major error compared with the
Fig. 4 Frequency response under different damping factors
of the DFB fiber laser hydrophone.
Cases 1# and 2# apply silicon oil with the
viscosity of 100 cst, 350 cst separately, while Case
3# applies silicon oil mixed of 350 cst and 800 cst.
The experimental results in Fig. 4 show that the
sensitivity drops about 3 dB after adding silicon oil,
and the larger viscosity brings out more flat
frequency response with a resonance peak being
suppressed more efficiently. However, the sensitivity
keeps unchanged under 100 Hz because the silicon
oil we used have the viscosity of little difference. In
the optimized situation of Case 3#, the flat
bandwidth of response is expanded to 300 Hz
The frequency responses of three elements are
tested and have good consistency as shown in Fig. 5.
Fig. 5 Frequency response of the three-element DFB fiber
The DFB fiber laser hydrophone we designed is
capable of flat response with the sensitivity of about
33 dB re pm/g from 5 Hz to 300 Hz, with the
fluctuation within ± 1 dB.
A three-element vector hydrophone has been
manufactured by short cavity DFB fiber lasers with
the narrow linewidth with the proper sensitivity
enhancement design. The lasing wavelength change
under different frequency signals can be interpreted
by unbalanced Michelson interferometer, and the
sensitivity is tested to be about 33 dB re pm/g. The
frequency response range is rather flat from 5 Hz to
300 Hz. The low
applications in underwater acoustic monitoring in
the national defense, oil, and gas exploration and
This work is supported by Shandong
Research and 2015GSF115006), Development
Foundation (No. 61605102), and Young Science
Foundation of Shandong Academy of Sciences (No.
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