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Quantum Electronics 46 (3) 277 – 280 (2016) © 2016 Kvantovaya Elektronika and Turpion Ltd
Abstract. A new type of a laser hydrophone based on dynamic holo-
grams, formed in a photorefractive crystal, is proposed and studied.
It is shown that the use of dynamic holograms makes it unnecessary
to use complex optical schemes and systems for electronic stabilisa-
tion of the interferometer operating point. This essentially simpli-
fies the scheme of the laser hydrophone preserving its high sensitiv-
ity, which offers the possibility to use it under a strong variation of
the environment parameters. The laser adaptive holographic hydro-
phone implemented at present possesses the sensitivity at a level of
3.3 mV Pa–1 in the frequency range from 1 to 30 kHz.
Keywords: laser hydrophone, adaptive interferometer, dynamic
hologram.
The development of systems for monitoring water areas and
the World Ocean is inseparably linked with the design of tech-
nical means for the detection of hydroacoustic signals. Until
recently, they were based on piezoelectric transducers [1].
Recently optical receivers of acoustic signals attracted the
attention of researchers, since they possess a number of
advantages as compared to their electric analogues, e.g., the
insensitivity to electromagnetic noises and corrosion, low spe-
cific weight, small dimensions, etc. Among the optical sensors
a specific class of instruments is presented by laser interferom-
eter hydrophones, which are potentially able to measure
extremely weak hydroacoustic perturbations with the acous-
tic pressure smaller than 1 Pa in the wide band of frequencies
and smaller than 0.1 Pa in the narrow band [2 – 6]. The sensi-
tive elements in such hydrophones are usually fibre-optical
sensors [2 – 8] or resonance elements in the form of elastic
membranes [9 – 11]. However, in the course of exploitation of
laser hydrophones under real conditions, random mechanical
impacts, temperature drift, variations of the static pressure of
the environment and a number of other factors lead to the
drift of the operating point of the interferometer and, as a
consequence, to the reduction of the signal-to-noise ratio and
a decrease in the sensitivity of the measurement system. This
makes it necessary to use special means of stabilising the oper-
ating point in laser hydrophones. As such means they often
use additional compensation interferometers [12], active and
passive phase control methods [9 – 11, 13, 14], methods of
optical feedback by intensity [15] or laser oscillation fre-
quency [16, 17], methods based on tunable diffraction grat-
ings [18] and multi-wavelength radiation with subsequent
spectral analysis [19], etc. Unfortunately, the use of all these
means of stabilising the interferometer operating point
unavoidably complicates the construction, which negatively
affects their reliability and stability of operation of the entire
measurement system.
As shown in Ref. [20], the use of dynamic holographic
gratings formed in photorefractive crystals (PRCs) allows
sufficiently simple and efficient solution of the problem of
operating point stabilisation in the measuring laser interfer-
ometers. The adaptivity to uncontrollable external perturba-
tions in such systems is based on the fact that the permanent
rewriting of dynamic holograms in the PRC enables the inter-
ferometer to adapt automatically to the change in the external
conditions, thus providing the stability of its characteristics.
The aim of the present paper is to study the possibility of
using an adaptive holographic interferometer based on
dynamic holograms, formed in a photorefractive crystal, for
the stabilisation of the operating characteristics of a laser
hydrophone.
The schematic of the laser adaptive holographic hydro-
phone (LAHH) is presented in Fig. 1. The sensor part of the
LAHH has a robust hermetic case. A thin (50 mm) round
(Æ 40 mm) brass membrane, playing the role of a hydrophone
sensitive element, is embedded in one of the walls. The optical
part of the sensor is implemented as follows. The radiation
from the Nd : YAG laser (l = 1.06 mm, the output power
0.5 W) passes through the beam splitter and is launched into
the multimode optical fibre (the core diameter 62.5 mm, the
numerical aperture NA = 0.22), from the output of which the
light is incident on the membrane. The separation between
the output face of the optical fibre and the membrane is
0.5 mm. The radiation reflected from the membrane returns
back into the waveguide, forming a signal wave. The mechan-
ical vibrations of the membrane caused by the action of the
acoustic wave, lead to the phase modulation of the signal
wave. The radiation of the signal wave, diverted by the
Y-coupler (intensity I = 0.8 mW mm–2), is directed into the
PRC along its [001] crystallographic axis. The reference wave
(I = 50 mW mm–2), elliptically polarised after passing through
a quarter-wave plate, enters the crystal in the perpendicular
direction along the [100] crystallographic axis. Due to the
photorefractive effect, the interference of the signal wave with
the reference one leads to the dynamic hologram recording in
Laser adaptive holographic hydrophone
R.V. Romashko, Yu.N. Kulchin, M.N. Bezruk, S.A. Ermolaev
LASER HYDROPHONES DOI: 10.1070/QEL15976
R.V. Romashko, Yu.N. Kulchin Far-Eastern Federal University,
ul. Sukhanova 8, 690091 Vladivostok, Russia; Institute of Automation
and Control Processes, Far-Eastern Branch, Russian Academy of
Sciences, ul. Radio 5, 690041 Vladivostok, Russia;
e-mail: romashko@iacp.dvo.ru, kulchin@iacp.dvo.ru;
M.N. Bezruk, S.A. Ermolaev Institute of Automation and Control
Processes, Far-Eastern Branch, Russian Academy of Sciences,
ul. Radio 5, 690041 Vladivostok, Russia;
e-mail: bezmisha@list.ru, nekker2@gmail.com
Received 29 November 2015; revision received 1 February 2016
Kvantovaya Elektronika 46 (3) 277 – 280 (2016)
Translated by V.L. Derbov
R.V. Romashko, Yu.N. Kulchin, M.N. Bezruk, S.A. Ermolaev278
the crystal with the lattice vector directed along the [101] crys-
tallographic axis. The vectorial mixing of the elliptically
polarised reference wave with the depolarised (after the trans-
mission through the multimode fibre-optical waveguide) sig-
nal wave in such orthogonal geometry in the PRC with cubic
symmetry provides the fulfilment of quadrature conditions
for the interferometer, due to which its high sensitivity is
achieved [21, 22]. The interaction of waves at the dynamic
hologram produced by them provides the precise conjugation
of wave fronts and maximally efficient conversion of the sig-
nal wave phase modulation, caused by the vibrations of the
membrane, into the variations of intensity, recorded by the
photodetector.
It is worth noting that the optimal intensity ratio of the
interfering beams is such that the contrast of the interference
pattern does not exceed 0.5. For higher contrasts (e.g., when
the intensities of the beams are equal) the holographic grating
becomes distorted, and its profile becomes different from the
sinusoidal one, which leads to the reduction of the beam inter-
action efficiency due to the diffraction of their radiation at the
gratings of higher spatial order [23]. As the contrast of the
interference pattern decreases, the phase demodulation signal
passes its maximum and smoothly decreases due to the reduc-
tion of the efficiency of the hologram recording [24, 25]. With
the intensities used in the present paper, the contrast of the
interference pattern was 0.25. Despite a relatively low dif-
fraction efficiency of the dynamic hologram (smaller than
0.1 %), the adaptive interferometer provided a high sensitiv-
ity to the detection of phase modulation, as will be shown
below.
One should also note that the time of recording the
dynamic hologram in the crystal is finite, as well as the life-
time of the recorded hologram after switching off or changing
the interference field [23]. When the change of the interference
field is slow, e.g., when the drift of the environment parame-
ters causes it and its time is greater than the time of the holo-
gram recording, the hologram is completely rewritten, which
determines the adaptive properties of the interferometer and,
therefore, the hydrophone. In the CdTe crystal for the inten-
sity ~50 mW mm–2 the time of hologram recording amounted
to 1.2 ms, which makes the LAHH capable of automatic
adaptation to all noise perturbations with characteristic fre-
quencies smaller than 800 Hz.
The experimental studies of the LAHH were carried out in
the tank with sound-absorbing walls. To control the acoustic
pressure a ZETLab BC311 calibrated etalon piezoelectric
hydrophone was placed in the vicinity of the LAHH mem-
brane. The acoustic pressure in the tank was produced by
means of a LUZ.837.9 piezoelectric radiator placed at the
same depth as the laser hydrophone and the etalon one at the
equal distance from them (20 cm).
We experimentally measured the amplitude – frequency
characteristic of the LAHH. For this aim, an electric pulse
with a duration 5 ms and an amplitude 2 V was applied to the
acoustic radiator. Figure 2a presents the shapes of acoustic
pulses, recorded using the LAHH and the etalon hydrophone.
The Fourier analysis of these signals with the sensitivity of the
etalon hydrophone (56 mV Pa–1) taken into account allowed
us to determine the amplitude – frequency characteristic of the
LAHH (Fig. 2b). One can see that the LAHH sensitivity is
uniform in a sufficiently wide range of frequencies (1 – 30 kHz).
At higher frequencies (0.1 – 1 MHz) the LAHH sensitivity is
reduced by an order of magnitude. Figure 3 presents the tran-
sient characteristic of the LAHH, measured at a frequency of
10.5 kHz, i.e., in the region of its maximal sensitivity. The
experimentally measured sensitivity of the LAHH at the lin-
ear fragment of the transient characteristic amounts to
3.3 mV Pa–1.
Note that the total sensitivity of the LAHH to the acous-
tic pressure (V Pa–1) is determined by the characteristics of all
transforming elements and can be presented by the expres-
sion:
SS = S1
S2
S2, (1)
Tank
Etalon
hydrophone
Radiator
Membrane
LAHH sensor
Alignment stage
Y-coupler
Signal beam
Reference
beam
Optical
fibre
[100]
[001]
Photodetector
Beam splitter
Laser
l/4
PRC
Figure 1. Schematic of the adaptive holographic hydrophone.
279Laser adaptive holographic hydrophone
where S1 (rad Pa–1) is the membrane sensitivity; S2 (W rad–1)
is the sensitivity of the adaptive interferometer; and S3 (V W–1)
is the sensitivity of the photodetector. The sensitivity S3 of the
Thorlabs PDA10CS photodetector used in the present work
amounted to 5 × 105 V W–1. The sensitivity S2 of the adaptive
interferometer was determined using the technique of Ref. [26]
and amounted to 0.18 mW rad–1.
The LAHH calibrating dependence calculated using
expression (1) is shown in Fig. 3 by the dashed curve. It is seen
that at a large acoustic pressure the transient characteristic of
the LAHH has a nonlinear fragment, limiting the dynamic
range. With the data presented in Fig. 3 and the level of
intrinsic noise of the measurement system taken into account,
it was found that the LAHH provides the measurement of the
acoustic pressure in the dynamic range of 36 dB, the minimal
detected acoustic pressure being 130 Pa.
The acoustic wave receiving membrane is the element of
the LAHH design of primary importance. Since the mem-
brane is a primary acoustic receiver, it mainly determines the
sensitivity of the laser hydrophone. Using Eqn (1) and the
transmission characteristic presented in Fig 3, it was found
that the sensitivity S1 of the membrane used in the LAHH is
0.37 mrad Pa–1.
The parameters of the LAHH developed by us and the
existing analogues are presented in Table 1. It is seen that the
sensitivity S1 of the primary receiver of the LAHH – the mem-
brane – is not extremely high. It is possible to increase S1 and,
therefore, the sensitivity of the entire LAHH, by the appro-
priate choice of the material, area and thickness of the mem-
brane that determine its hardness. An alternative way is to use
a primary acoustic receiver, similar to that presented in
Ref. [7] providing the acoustic sensitivity at the level of
0.5 rad Pa–1. The total LAHH sensitivity SS will amount to
4.4 V Pa–1, which is much higher than the sensitivity of the
hydrophone proposed in Ref. [7], as well as that of most other
analogues. It is important that, in contrast to other hydro-
phones, the LAHH has a simple optical scheme that provides
its operation including the adaptivity.
In addition, note that the sensitivity S2 of the adaptive
interferometer that enters expression (2) is determined by the
diffraction efficiency of the holographic grating, which is not
maximal in the orthogonal geometry of the interaction
between the reference wave and the signal one in the PRC.
The maximal diffraction efficiency of the dynamic hologram
will be achieved in the reflection geometry, i.e., in the case of
counterpropagation of the interacting waves [21]. The use of
the reflection geometry will allow an additional increase in
the sensitivity of the adaptive interferometer and, as a conse-
quence, the total sensitivity of the LAHH by 2 times [28].
1.13
1.15
1.17
50 100 150
Time/ms
Frequency/Hz
Output signal/V
Output signal/mV
–4
–6
–2
0
2
10–5
10–4
10–3
103104105106
10–2
10–1
Sensitivity/V Pa –1
1
2
a
b
Figure 2. (a) Acoustic pulse recorded by means of the etalon hydro-
phone ( 1 ) and LAHH ( 2 ) and (b) the amplitude – frequency character-
istic of the laser adaptive hydrophone.
Table 1. Parameters of hydrophones.
Hydrophones Sensitivity of the primary
transducer S1/mrad Pa–1
Total sensitivity
SS /mV Pa–1
Minimal detected
pressure/Pa
Frequency
range/kHz
[5] 1.1 11.0 1.3 100 – 300
[6] 143 0.1 – 5 – 20
[7] 500 – 5 5 – 300
[8] 7.5 × 10–5 5.8 × 10–4 1.5 × 10420000
[9] – – 0.013 0.3
[27] 20 – – 3 – 8
Piezoelectric
ZETLab BC311 – 0.056 100 0.003 – 100
LAHH (present work) 0.37 3.3 130 1 – 30
LAHH (perspective) 500 4400 0.09 5 – 300
102103104
LAHH output signal/V
0.1
1.0
10
Pressure
/
Pa
Figure 3. Transient characteristic of the laser adaptive hydrophone at
the frequency 10.5 kHz (points show the experiment, dashed line – cal-
culation).
R.V. Romashko, Yu.N. Kulchin, M.N. Bezruk, S.A. Ermolaev280
We also studied the stability of the operating characteris-
tics of the LAHH under the conditions of varying tempera-
ture, one of the most critical parameters for the systems based
on interferometry schemes. With this aim during 24 hours we
measured the amplitude of the LAHH output signal keeping
the acoustic pressure constant at a room temperature with
daily variation ±5 °С. The measurements have shown that the
fluctuations of the amplitude of the LAHH output signal did
not exceed 1 %.
As shown in Refs [24, 29 – 31], photorefractive media
allow efficient multiplexing of dynamic holograms in one
crystal. It was found that the formation of up to 70 holograms
leads to the reduction of the sensitivity by no more than 10 %
[29]. In practice, an adaptive interferometric system with 26
holographic channels was implemented [32]. Therefore, the
use of dynamic holograms in the scheme of a laser hydro-
phone can provide not only the stability of its characteristics
and the high sensitivity, but also opens perspectives for creat-
ing a multichannel adaptive hydroacoustic complex.
Thus, in the present paper a new type of a laser hydro-
phone based on the formation of dynamic holograms in a
photorefractive crystal is proposed and studied. It is shown
that the use of dynamic holograms in the interferometric
hydrophones allows one to avoid the necessity of using com-
plex optical schemes and systems of electronic stabilisation of
the operating point of the interferometer. This essentially sim-
plifies the scheme of the laser hydrophone, keeping high its
sensitivity, and makes it promising for the use under the con-
ditions of strongly changing environmental parameters. The
LAHH implemented in the present work has the sensitivity at
the level of 3.3 mV Pa–1 that provides the detection threshold
of the acoustic pressure 130 Pa in the frequency range
1 – 30 kHz. The design of the laser hydrophone based on the
principles of adaptive holographic interferometry opens the
possibilities of reducing the threshold of broadband detection
to the level smaller than 0.1 Pa without modifying the optical
scheme. Moreover, due to the multiplexing of many dynamic
holograms in one photorefractive crystal one can build a mul-
tichannel laser high-sensitivity hydroacoustic complex with
the number of channels greater than 30.
Acknowledgements. The study was supported by the Russian
Science Foundation (Project No. 14-12-01122).
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