Fiber sensor picks up tiny vibrations

24 Aug 2005

Researchers use their expertise in gravitational wave detection to develop an underwater acoustic sensor.

Oil and gas reserve monitoring, marine mineral exploration and deep sea surveillance could all benefit from a low-cost, fiber-based vibration sensor say its inventors. The team from Australian National University (ANU) and the University of Sydney claim that its device, which responds to underwater acoustic waves, is 100 times more sensitive than current technology.

Fiber sensor team

"Our sensor is based on a passive detector, a Fabry Perot cavity created by two Fiber Bragg grating mirrors written into the core of the optical fibre," ANU scientist Malcolm Gray told Optics.org. "The high finesse of the cavity means that the effect of the detected vibrations is multiplied as the light makes many round trips between the grating mirrors."

To spot relative changes in fiber length approaching 1x10^-12, the researchers use a detection scheme dubbed Pound-Drever-Hall (PDH) locking, borrowed from the group's work on detecting extremely weak gravitational waves (small ripples in space created during cosmological events such as the explosive death of stars).

The PDH method involves probing the passive vibration sensor with a phase modulated laser beam. A detector monitors the return beam for any signs of demodulation to identify changes in cavity length caused by surrounding acoustic vibrations.

Suitable for distances of up to 100 km and requiring only microwatts of laser power, the device is said to be ideal for remote sensing. "We envisage a high performance, affordable array for geological surveying," explained Gray. "With many tens of sensors on a single fiber, the lightweight unit could be towed by much smaller boats than at present, reducing operation costs."

Having patented the technique, the team is now busy working with Benthic Geotech, an oil and gas exploration company, to develop a prototype for field testing.