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Tag Archive: optical sensing

  1. Fiber Optic Network Cable Helps to Monitor Ridgecrest Aftershocks

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    Earthquake Ready?Seismologists from the California Institute of Technology (Caltech) are using fiber optic network cable to monitor and record aftershocks from the July 4 and 5 Ridgecrest, CA, earthquakes. By using optical fiber, the scientists can gather, track, and analyze data in much greater depth from the thousands of daily aftershocks.

    To do this, the scientists send a beam of light down optical fiber in an unused or “dark” fiber optic cable. When the light reaches tiny blemishes in the optical fiber, a small portion of the light is reflected back and recorded. In this way, each fiber imperfection acts as a trackable location along the buried fiber optic cable. When seismic waves move through the ground, the cable expands and contracts slightly. This change affects the travel time of light to and from the locations. By monitoring these changes, seismologists can monitor the motion of seismic waves.

    According to Caltech, the miniscule fiber imperfections occur often enough so that every few meters of optical fiber act as an individual seismometer. In fact, monitoring 50 kilometers of fiber optic cable in three different locations is roughly equal to deploying more than 6,000 seismometers in the area.

    Caltech launched the project within days of the two large earthquakes and began contacting groups in a search for unused fiber optic cable that would be close enough and long enough to be useful. The scientists finally contacted the California Broadband Cooperative’s Digital 395 project. The goal of the Digital 395 project is to build a new 583-mile fiber optic network that will run north to south, along the eastern Sierra Nevada, passing near Ridgecrest. Digital 395 offered three segments of its fiber cable to which Caltech connected sensing instruments.

    Information gathered from the Ridgecrest fiber network will help seismologists learn more about the way that earthquakes move through the earth, and specifically how seismic waves move through the area around Ridgecrest.

  2. Optical Fiber “Senses” Change in Surroundings

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    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.

    To read and learn more, go HERE.