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Optical Fiber “Senses” Change in Surroundings

Companies use optical fiber as a sensor to detect changes in temperature and pressure. This technique is often used to monitor structures including bridges and gas pipelines.

Now researchers at Ecole Polytechnique Fédérale De Lausanne (EPFL) have discovered a new method where optical fibers can identify when they are in contact with a liquid or a solid. The researchers accomplished this by generating a sound wave with help from a light beam inside the optical fiber.

A Sensor That Doesn’t Disrupt the Light

Four factors affect the light carried by a glass optical fiber: intensity, phase, polarization and wavelength. These factors can change when something stretches the fiber or the temperature varies. These changes let the fiber act as a sensor by detecting cracks in structures or temperature changes. However, until now, users could not know what was actually happening around the fiber without letting light escape, which interrupts the light path.

The method from EPFL uses a sound wave generated inside the fiber. This hyper-frequency wave regularly bounces off of the fiber’s walls. This echo varies at different locations depending on the type of material that the wave contacts. The echoes leave an imprint on the light that users can read when the beam exits the fiber. While users can study this imprint to detect and map out the fiber’s surroundings, it is so faint that it barely disturbs the light within the fiber. In fact, users could employ this technique to sense what is occurring around a fiber and send light-based information at the same time.

In experiments, the researchers submerged their fibers in water and then in alcohol, and left them out in the open air. Each time, their system correctly identified the change in the fibers’ surroundings. The group expects their technique to have many potential applications by detecting water leakage, as well as the density and salinity of fluids that touch the fiber.

Spatial and Temporal Detection

This method discerns changes in the surroundings with a time-based method. Each wave impulse is created with a slight time jag. Then, when the beam arrives, the delay is reflected. The researchers can see what any disturbances were and determine their location. The group can currently locate disturbances to within 10 meters, but have the technical means and expect to increase accuracy down to one meter.

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