Novel external-cavity lasers meet stringent sensor-market requirements

Fiber-optic sensing markets are rapidly expanding. Over the last few years, extensive activity has concentrated on development and commercialization of seismic and acoustic sensing arrays for defense (intrusion detection, perimeter protection, subma- rine identification, hydrophones), land- and underwater-based industrial applications (oil and gas exploration, pipeline surveil- lance, geophysical monitoring), and civil-infrastructure moni- toring (roads, bridges, water-supply systems, and power lines). Fiber-optic arrays typically consist of long (up to 10km) strings of individual sensors that can detect land vibrations or under- water acoustic signals. In typical applications, the number of sensing nodes per string can range from a few sensors for in-well down-hole acoustic devices (or several hundred for down-hole seismic sensors) to thousands of elements for ocean-bottom seis- mic cables and subsea streamers.1 Since each fiber sensor needs to be individually interrogated and spectrally distinguished, a large number of narrow-linewidth laser sources are needed to excite and interact with the sensor array.