An optical fiber can be described as a glass pipeline which can guide light over great distances with very little loss. In addition to being the central communication channel of the information era, it has another very interesting application. This application is called distributed sensing. By launching into the fiber specially designed pulse sequences of light and analyzing its reflections from different positions along the fiber, it is possible to measure various physical parameters such as temperature and acoustic signals at these positions. During recent years the use of distributed fiber-optic sensing systems has become ubiquitous for a variety of applications. Distributed optical fiber sensors are particularly attractive for marine applications. Implementation of large scale sensing networks in the underwater arena is challenging not only due to the need for underwater compatible sensors but also since conventional wireless communication techniques cannot be used to transmit the measured signals. In distributed fiber-optic sensors both tasks are performed by the same medium. 

Underwater applications of distributed fiber-optic sensors comprise seismic sensing, earthquakes and tsunami early-warning and monitoring, pipeline defense and leak detection, harbor security, monitoring the sounds used by marine animals for navigation and communication and more. These applications require very high sensitivity. This can be achieved by wrapping long sections of the sensing fiber around special cylindrical elements called mandrels. While great improvement in sensitivity can be obtained in this method, it requires using a considerably longer sensing fiber for covering the same sensing range. 
A fundamental trade-off in distributed fiber optic sensing systems is the one between the fiber length and the rate at which the signals are sampled (sampling rate). The length-sampling-rate tradeoff stems from the requirement that all returns from the fiber must arrive before a new pulse can be transmitted into the fiber. Due to the trade-off between length and sampling-rate, designers of underwater sensing systems had to choose between short-range highly-sensitive systems and long-range systems with poor sensitivity. 
The technology, that is being developed by professor Avishay Eyal and his Ph.D. student Nadav Arbel, mitigates this tradeoff and allows achieving simultaneously both very high sensitivity and very long range of operation. This is achieved by working with sensing fibers that comprise arrays of weak mirrors in their cores (Fig. 1 left). The mirrors, also called Fiber Bragg Gratings (FBG’s), are inscribed in the fiber core using laser side writing. The pulse sequence, which is launched into the fiber, is accurately tailored to the positions of the FBG’s in the fiber. The careful design of the pulse sequence ensures that the returns from the fiber will not overlap even when the sampling rate increases by a factor of more than 50 (Fig. 2 right). A further improvement in sensitivity is achieved by harnessing pulse compression techniques, originally invented for radars, for interrogation of the sensing fiber. With the use of these technologies the team intends to facilitate large scale sensing networks with unprecedented sensing range, sensitivity and bandwidth.