A Fibre Laser Hydrophone
Scott Foster a† , Alexei Tikhomirov a , Mark Milnes a , John van Velzen a , Graham Hardy b
a Defence Science and Technology Organisation, PO Box 1500, Edinburgh SA 5111, Australia;
b Thales Underwater Systems Pty Ltd, 274 Victoria Rd, Rydalmere NSW 2116, Australia
A new fibre optic hydrophone based on a distributed feedback fibre laser (DFB FL) sensor is described. The sensor is
designed to achieve ocean noise limited pressure sensitivity. Unlike previous fibre laser acoustic sensors, this device is
acceleration insensitive making it less susceptible to vibrational noise. Experimental results for one implementation of
the sensor are presented.
Keywords: Fibre lasers, distributed feedback lasers, pressure sensor, acoustic sensor, hydrophone
Fibre optic hydrophone arrays based on interferometric sensors have been under development for sonar applications
since the late 1970s and have a number of perceived benefits over conventional piezo-electric hydrophone arrays
including reduced weight and cost, high reliability, and low power budgets 1 . The more recent emergence of fibre Bragg
grating technology offers the promise of a new generation of intra-fibre optical sensor arrays with dramatically reduced
size, cost and complexity.
Distributed feedback fibre lasers 2 (DFB FL) are small intra-fibre devices, which show considerable promise as an allphotonic
acoustic sensor technology 3 . The frequency of light produced by DFB FL is extremely sensitive to small
perturbations in their environment. It has been observed experimentally that acoustic disturbances in the locality of the
laser cavity induce measurable frequency fluctuations, which can be interrogated using interferometric methods. This
acoustic sensitivity can be traced to the extreme sensitivity of the laser to local strain. It is believed that strains as small
as 10 -14 result in a measurable change to the laser frequency 4 . In addition to their high strain sensitivity DFB FL are
inherently suited to array multiplexing, making them extremely attractive for sonar applications. All-photonic arrays can
be built with no outboard electronic components.
Previous attempts to develop fibre laser hydrophones 3, 6 have focused on enhancing the hydrostatic pressure sensitivity,
which is relatively low for the bare laser. However, due to their long thin geometry and relatively high axial stiffness,
optical fibres have a tendency to deform (bend and twist) in response to acoustic disturbance 5 . This results in a high
degree of sensitivity to non-acoustic vibrations, which is undesirable for sonar applications.
In this paper we shall discuss a new DFB FL hydrophone, which has been designed to achieve sea state zero pressure
sensitivity with very low sensitivity to bulk accelerations. In Section 2 we briefly describe the principle of operation of
DBF fibre laser sensor arrays. In Section 3 we discuss the properties of a DFB FL hydrophone. We conclude in Section
4 with a brief summary.
2. DFB FL SENSOR ARRAY
Figure 1 shows a schematic of an erbium DFB fibre laser sensor system. Energy is supplied to a serial array of N single
frequency DFB fibre lasers by a 980nm diode laser pump. The laser wavelength n for the n th laser is determined by a
resonance condition of the Bragg grating and corresponds to the so called Bragg wavelength which equals twice the pitch
of the grating. The wavelength of each laser can be selected at the time of fabrication by adjusting the pitch of the
† firstname.lastname@example.org; phone +61 8 8259 5979; fax +61 8 8259 7115
17th International Conference on Optical Fibre Sensors, Marc Voet, Reinhardt Willsch,
Wolfgang Ecke, Julian Jones, Brian Culshaw, eds., Proceedings of SPIE Vol. 5855
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grating and may be anywhere within the emission manifold of the gain medium. For erbium lasers, for example, this
enables wavelength division multiplexing across the C-band from (say) 1520nm to 1580nm.
Figure 1. Operation of a DFB fibre laser sensor system.
When a fibre laser is strained by an amount the pitch of the Bragg grating changes and the laser wavelength (or
equivalently the laser frequency) changes according to the formula:
/ = / 0.78 (1)
where and are the laser frequency and wavelength respectively. For an appropriately configured system, the
smallness of the shift that can be detected is ultimately limited by the inherent frequency noise of the laser.
The most important frequency noise source for single mode DFB FL acoustic sensors in the 0-20kHz acoustic frequency
range is believed to be fundamental thermal noise 7, 8 . Figure 2 plots the noise spectrum of a 5cm long DFB FL against a
theoretical thermal noise floor under laboratory conditions. The equivalent strain is plotted on the right hand axis. Note
that the frequency noise at 1kHz is around 10Hz/Hz, corresponding to a noise equivalent strain of less than 10 -13 .
Figure 2. Measured noise spectrum from 500Hz to 10KHz compared with theoretical thermal noise.
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3. DFB FL HYDROPHONES
The bare DFB FL is not well suited to pressure sensing due to its relatively low direct pressure sensitivity 3 and tendency
to respond strongly to local vibrations (both acoustic and mechanical) 5 . To construct a fibre laser hydrophone, it is
necessary to mechanically support the fibre so as to enhance its pressure sensitivity, whilst minimising its response to
other environmental perturbations such as mechanical accelerations.
Figure 3. (a) Photograph of fibre laser hydrophones. (b) Principle of Operation
Figure 3a shows several fibre laser hydrophones (forming an array) designed to operate from DC to 3kHz for low
frequency passive sonar. The dimensions of the hydrophone body are 56mm x 7mm x 5mm. The fibre laser is attached
to one side of a flexible beam, which sits in an air filled cavity in the interior of the sealed outer housing (Fig. 3b). For a
beam of thickness t, the core of the fibre is displaced by a distance t/2 from the neutral bending axis of the beam.
Flexure of the beam imparts a strain =t/2R on the fibre laser, where R is the bend radius of the beam.
The beam is supported by the outer housing in such a way that acoustic pressure applied to the housing causes the beam
to bend thereby imparting strain on the laser and resulting in a wavelength shift. Furthermore, the configuration is such
that minimal wavelength shift is imparted on the laser when the housing undergoes acceleration, thereby making the
hydrophone insensitive to vibration.
The hydrophone assembly can be thought of as a mechanical actuator, which converts external pressure to longitudinal
strain. The low inertia and high flexibility of the fibre enables it to move in conformance to the beam with little
mechanical impedance. The sensitivity of the device to pressure and acceleration is effectively determined by the
mechanical properties of the actuator and is largely independent of the material properties of the fibre. By adjusting the
materials and mechanical configuration of the actuator the pressure and acceleration sensitivity can be controlled to suit
the application. Also, because the sensor is based on a mechanical system with several degrees of freedom it is possible,
at least in principle, to decouple the fibre laser from slowly varying pressure changes – i.e. to configure the hydrophone
so as to incorporate a mechanical high pass filter.
To quantify the performance of fibre laser hydrophones it will be useful to introduce two simple figures of merit. Firstly,
we define the normalized response to be the response of the laser sensor (in Hz/Pa) divided by the frequency noise floor
of the laser (in Hz/Hz). At 1 kHz the ambient acoustic noise level in the ocean is generally greater than 100Pa/Hz.
A normalized sensitivity in excess of 70 dB is typically adequate to ensure that the noise equivalent pressure is
comparable to or less than ambient ocean noise. Secondly, we define the relative acceleration sensitivity to be the
response of the laser (in Hz/ms -2 ) to acceleration of the outer housing (along the most sensitive axis) divided by the
response (in Hz/Pa) to pressure. Ideally this figure should be as low as possible. Commercially available piezo-electric
hydrophones typically achieve a relative acceleration sensitivity figure around 0dB.
Figure 4a shows the experimentally measured normalized response and relative acceleration sensitivity of the first fibre
laser hydrophone we constructed according to the scheme illustrated in Figure 3. The modeled theoretical figures are
also shown and indicate good agreement. This implementation of the hydrophone was sub-optimal, primarily because
the outer housing turned out to have higher stiffness than we had intended. More recent implementations of the basic
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hydrophone scheme, of equivalent outer dimensions, have theoretical relative response figures around 100dB (a further
gain of 30dB) and relative acceleration sensitivity well below 0 dB (Fig. 4b). These designs are not yet experimentally
validated at the time of writing.
Figure 4. (a) Hydrophone performance
(b) Theoretical performance of optimized configuration
We have described the principle of operation of a fibre laser hydrophone and have reported the results for the acoustic
response and acceleration sensitivity of an initial implementation of the design. More recent implementations currently
under development are expected to achieve acoustic performance that matches or exceeds that of high performance
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