Detection of water surface capillary wave by analysis
of turning-point local signal data using
a laser interferometer
Lieshan Zhang (张烈山), Xiaolin Zhang (张晓琳)*, and Wenyan Tang (唐文彦)
School of Electrical Engineering and Automation, Harbin Institute of Technology, Harbin 150001, China
*Corresponding author: zhangxiaolin@hit.edu.cn
Received February 14, 2017; accepted April 14, 2017; posted online May 9, 2017
A laser interferometry technique is developed to detect water surface capillary waves caused by an impinging
acoustic pressure field. The frequency and amplitude of the water surface capillary waves can be estimated from
the local signal data at some special points of the phase modulated interference signal, which is called the turning
points. Demodulation principles are proposed to explain this method. Experiments are conducted under con-
ditions of different intensity and different frequency driving acoustic signals. The results show the local signal
data analysis can effectively estimate the amplitude and frequency of water surface capillary waves.
OCIS codes: 120.3180, 120.7280, 010.7340.
doi: 10.3788/COL201715.071201.
A water surface capillary wave (WSCW) excited by
underwater acoustic source is a transversely spread sur-
face wave with an amplitude of several nanometers that
carries some information about the underwater acoustic
field. Interference detection of water surface micro ampli-
tude waves is a new technique to extract the underwater
acoustic field information
[1–3]
. Underwater acoustic signal
detection is significant, especially in the field of submarine
communications and underwater target recognition. The
main applied technology of underwater acoustic field
detection is still shipborne sonar detection technology
[4]
,
which needs a shipborne platform to host the main
system and extends the acoustic sensor
[5,6]
into the water.
Usually shipborne sonar lacks flexibility and concealment
character.
Nowadays, more emphasis has been put on the remote
sensing technology of underwater acoustic signals, such as
the Doppler interference technology studied in this Letter.
The laser interference method reported in Refs. [
7,8], was
used to determine the frequency of underwater acoustic
signals by detecting the WSCWs that were excited by
the underwater acoustic signals. We also did some work
on the laser Doppler interference method to estimate
the amplitude of water surface acoustic waves
(WSAWs)
[9]
, but these demodulation methods of coheret
signal detection are only applicable for static analysis of a
global signal. Blackmon et al.
[10,11]
used the laser Doppler
vibrometer to directly detect the vibrations of the water
surface to successfully obtain the acoustic frequency and
sound pressure level of the underwater acoustic field.
However, some large errors might be produced by this
method due to the environmental disturbances (caused
by wind or other factors). In this Letter, we used a simple
laser homodyne system to detect the WSCW, and tried to
analyze the local signal data to estimate the frequency and
amplitude of WSCW. Then we proposed a demodulation
method based on the analysis of local signal data at the
turning points of interference signals. It is very difficult
to obtain a stable interference signal in actual detections
for natural water when using the laser Doppler interferom-
etry technique, so a local signal data analysis method will
be helpful in actual detection.
A simple Doppler homodyne interferometer was used to
probe the water surface to detect the WSCW. There are
some low frequency environmental perturbations on water
surface. According to the principle of Doppler interferom-
etery
[7–13]
, when filtering out the DC component, the inter-
ference signal transduced by a photodetector can be
described as
[9]
UðtÞ¼A
0
cos½2kA
n
sinðω
n
t þ ∅
n
Þþ2kA
s
sinðω
s
t þ ∅
s
Þþφ
i
;
(1)
where A
0
is the gain factor related to the optical
amplitudes of two laser beams and the photoelectric
conversion efficiency, and k is wave number of the co-
herent laser. The amplitude, the angular frequency,
and the phase of environmental perturbations are de-
noted by A
n
, ω
n
,and∅
n
, r espectively. Similarly, the
amplitude, the angular frequency, and the phase of
WSCW are denoted by A
s
, ω
s
,and∅
s
. The character
t denotes time, and character φ
i
is the initial phase
caused by the optical path difference between the two
arms of interferometer.
Usually the amplitude of WSCW is in the nanometer
range, so it is completely submerged in the environmental
perturbations. Due to the randomness of the environmen-
tal perturbations, complete and accurate phase demodu-
lation of the laser interference signal is very difficult.
Furthermore, the temporal phase change of the detection
signal caused by WSCW is quite small, sometimes even
smaller than a phase demodulation solution. As a result,
COL 15(7), 071201(2017) CHINESE OPTICS LETTERS July 10, 2017
1671-7694/2017/071201(6) 071201-1 © 2017 Chinese Optics Letters
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