Practical Application of Fiber Components Manufactured by Bonacom in Fiber-Optic Hydrophones

 

One of the major applications of interest for fiber-optic sensors are fiber-optic hydrophones. These devices have been under development for many years for use in military sonar and in seismic surveying for the oil-gas industry. For both of these applications, modern requirements are for very large (up to several thousand individual sensors), highly multiplexed sensor arrays. These arrays can be used in a number of configurations, including seabed arrays and towed arrays (referred to as streamers in the seismic industry).

Underwater acoustic sensing is one area in which the advantages of using fiber-optic sensors are very clear.

 

The requirements for sonar systems vary considerably between the various military and commercial sonar applications. However, it is possible to define a range of performance requirements, which will cover most practical sonar systems. A range of typical requirements is given in Table I.

 

TABLE 1
Range of typical sonar requirements
Parameter	Typical requirement
Frequency	5 Hz to 10 000 Hz
System noise floor	Equivalent to "deep ocean ambient noise state zero" after (~100 μPa/Hz1/2 at 500Hz)
Narrow-band dynamic range(in 1 Hz bandwidth)	106
Number of hydrophones	100 to greater than 10 000
Sensor-sensor crosstalk	Less than -40dB
Survival depth	1000m

 

The design of the sonar array usually requires trading off the various performance criteria, in order to achieve the overall specification. The requirements for seismic applications are broadly similar to the military sonar applications, although the frequency range is generally lower (usually less than 500 Hz).

Seismic systems may also include accelerometers, in addition to hydrophones. The accelerometer measures the acceleration of the particles in the medium associated with the acoustic signal and, hence, exhibits a directional response independent of frequency. Conversely, pressure-sensing hydrophones are designed to be much smaller than the acoustic wavelength such that they exhibit an omni directional response. In an alternative configuration, the accelerometer may be coupled to the sea-bed, to measure seismic disturbances directly.

 

A wide variety of sonar configurations can be employed,depending on the application. As one particular example, we take the fiber-optic bottom mounted array (FOBMA) which the Naval Research Laboratory in the U.S. and QinetiQ Ltd. (formerly DERA) in the U.K. have been jointly developing [9].This system is designed to be a rapidly deployable, lightweight seabed array, comprising a number of hydrophone nodes. The configuration of this system is shown in Fig. 14.

It comprises two hydrophone nodes of 48 channels each, separated by a 3-km length of fiber, with a 5-km fiber datalink connecting the nodes to the interrogation optoelectronics, which is located aboard the deployment vessel, or onshore.The size of this system, which is designed to be further expandable, is sufficient for it to be used for real sonar applications.

 

The method we describe here, based on the TDM/DWDM, is considered to be a best value approach to achieving all the necessary requirements in a scheme that uses only commercially available components.

 

An architecture combining DWDM with one of the TDM schemes is shown in Fig. 3. In this scheme, pulses at wavelengh λ1 to λm are injected into the telemetry fiber. Optical add/drop multiplexers couple individual wavelengths from the telemetry fiber into each TDM array module.  Separate launch and delivery fibers are used in this scheme, which avoids problems associated with intra-channel crosstalk observed in devices that combine both the OAM and ODM function. Since only the wavelengths occupying the drop and add band of the ODM/OAM are coupled to/from the array modules (i.e., wavelengths occupying an optical bandwidth of 0.8 nm), this architecture also provides partial rejection of out-of-band amplified spontaneous emission(ASE). The devices enclosed in a dashed box indicates that inclusion is optional. However, including these devices will ensure that the optical loss for each wavelength is as closely matched as possible.

A scheme comprising two array modules with a total of 12 interferometric sensors in an in-line Michelson configuration was demonstrated. A phase resolution of 100rad/Hz was achieved, limited by environmental acoustical noise and excess intensity noise from the fiber laser source. Crosstalk levels of less than 47 dB were also demonstrated.

Fig. 3. TDM/DWDM multiplexing architechture. ODM: optical drop multiplexer; OAM: optical add multiplexer

 

An interferometric optical hydrophone operates by converting the pressure change due to an acoustic signal into a phase change in light traveling through a length of optical fiber. Incorporating the fiber into the arm of an interferometer allows this phase-shift to be measured. The hydrophone responsivity is, thus, dependent on the fiber length and on the efficiency of the pressure/phase transduction (which is given by the normalized responsivity).

 

If the advantages of fiber-optic hydrophone systems are to be realized in practice, efficient, and cost effective array construction techniques are essential. The requirements for array design and construction differ significantly for the various applications;

The array modules were grouped together into the nodes, and the WDM components and the RPEDFA were connected shown in Fig. 15.

 

Finally, we consider the pump power requirements of the RPEDFA shown in Fig. 16, which must be optimized to give high conversion efficiency between pump and signal photons. Bonacom’s DWDM Mux and optical switch(OS,BNOS-1x2-s) are ideal choice for such system.

 

Most of the development work on high channel count fiber optic hydrophone arrays has been based on an interferometric sensor approach. This is because this solution has been found to give the optimum combination of high acoustic sensitivity and sufficient multiplexing ability. Many of the multiplexing and interrogation concepts have been demonstrated in the laboratory, and some have been implemented in realistic at-sea demonstrations of smaller systems.

 

Bonacom manufactures various passive fiber components(polarization maintaining/non-PM) for Fiber-Optical hydrophone application, e.g.: Dense Wavelength Division Multiplexing(DWDM Mux / Demux), Optical Add-Drop Multiplexer(OADM), Optical Switch, in-line polarizer(ILP), polarization beam splitter(PBS), optical isolator(PM/non-PM),optical circulator(PM/non-PM), fiber coupler(filter/fused based-PM/non-PM), PM FWDM, polarization maintaining variable optical attenuator(PM VOA) with broadband working wavelength at 1310nm,1550nm.

 

We also offer custom optical modules and subsystems by integrating multiple broadband components such as isolator coupler FWDM PBS hybrid. These components are ideal choices for developing expanding Fiber-Optical Hydrophone sensing systems. Contact Bonacom Technology to discuss your requirements.