Fiber optic hydrophone arrays
Hydrophone arrays are used to locate submarines, track marine mammals and even to study global climate change by for example detecting temperature differences.
What are hydrophone arrays?
A hydrophone array is made up of several hydrophones (under water microphones) placed in known locations. These hydrophones maybe placed in a line on the seafloor, moored in a vertical line in the water column, or towed in a horizontal line behind a boat or ship, for example. Sound arriving at the array from a distant source, such as a submarine, will reach each hydrophone at slightly different times, depending on the direction from which the sound is coming. This time difference is known as the time-of-arrival-difference and can be turned into a direction. Using this information from all the hydrophones in the array, the direction from which the sound is coming can be pinpointed.
Even a simple array consisting of only two hydrophones can give the approximate direction from which a sound is coming. People do this all the time in air with a “receiving array” that consists of two ears. Sound arriving from a source, such as a person speaking, will reach each ear at slightly different times, depending on the direction from which the sound is coming, making it possible for the listener to tell the direction to the speaker.
When the listener wants to detect weak sound, hydrophone arrays are much better than single hydrophones. This is because the array can filter out noise coming in from all directions and focus on sounds arriving from a specific direction. The increased signal-to-noise ratio allows sounds that normally couldn’t be detected by a single hydrophone to be heard. And of course, the sensitivity of the hydrophone must be as high as possible to be able to pick up the smallest sounds.
Source: University of Rhode Island
In this image, sound is transmitted by the ship and reflected of the submerged submarine. The reflected sound reaches hydrophone A first, then hydrophone B, and finally hydrophone C. The time-of-arrival-difference between the hydrophones in the array is used to determine the direction to the submarine.
Project: Ministry of Defence
Optical sensors do have unique characteristics which make it useful to use fiber optic sensors in several applications. These characteristics, among other things, are immunity to electromagnetic and radio-frequency interference, potential uses in hazardous areas such as in explosive and corrosive environment. For the hydrophone array that defense wanted to develop with Somni, several specific advantages played a role regarding the choice for fiber optic sensors.
Somni sensors are all based on FBG technology, which has the advantage that multiple sensors can be connected to 1 fiber. This allows a lot of weight savings in the array, compared to electrical cabling. No electricity is needed in the array anyway, so all sensitive electrical components can be placed on board and only the rugged fiber optic cables and sensors are exposed to the harsh outdoor environment.
The Ministry of Defence had high requirements regarding the fiber optic hydrophone arrays. One of these requirements was that the noise level of the hydrophone arrays had to be less than sea state zero.
The development of this type of sensor is a great challenge. But Somni ones again proved that there is no challenge to great! A special hydrophone array meeting the requirements of the Ministry of Defence was developed for this application.
Not only the development of this type of sensor is a challenge, but also the testing of these sensors is quite a challenge. Fortunately, several very good test sites are available in the Netherlands. The photo on the right shows some of the prototype hydrophone arrays hanging in a test rig just before they are launched into the water.