A research team has devised and tested a scheme based on advanced optical-physics principles which uses active submarine optical-fiber data cables to also sense ocean-floor earthquakes and ocean-surface swells.

Sensing of physical parameters such as temperature, pressure, force, and velocity often brings a “split personality” analysis. On one hand, getting a reading of the basic variable to be sensed seems simple and straightforward; after all, it’s no big deal to measure temperature with accuracy. On the other hand, when it comes to the realities of sensing and making a measurement in a specific application scenario, it often turns out to be a more difficult issue. There are considerations of physical access, operating environment, proximity, sensitivity, accuracy, ambient noises of various types, interference with the targeted reading, reporting (transmitting) of the acquired data under challenging circumstances, and more. That’s why there are so many very different sensor types and innovative ideas for a given physical phenomenon.

Sometimes, though, it’s possible to use an existing, in-place arrangement and leverage it to get the data of interest. That’s the case with a project headed by Zhongwen Zhan, assistant professor of geophysics at the California Institute of Technology (Caltech) which tackled the problem of detecting the location and magnitude of earthquakes on the ocean floor. This is a sensing challenge for obvious reasons, with multiple techniques being explored to address it, such as laser interferometry and distributed acoustic-sensing methods which can transform existing undersea (submarine) trans-oceanic fiber-optic telecommunications cables into kilometers-long seismic sensors. These approaches, however, have been limited in practicality as they need specialized additional lasers and optical-detection equipment along with access to dedicated “dark fibers” (optical fibers which are in place but not in use).

Use of the optical fibers as sensing elements is part of an expanding trend of using these ultrapure optical conduits for sensing rather than communications. For example, optical fibers are being used for ultra-precise strain sensors and extensometers by injecting a laser beam and then sensing discontinuities, reflections, polarization shifts, and even changes in their optical resonances as their dimensions change by even a few nanometers.

Similarly, crystalline optical materials such as lithium niobate are being etched and thin-film coated in sophisticated ways to create MEMS-like optoelectronic sensors for gaseous and liquid chemicals and even biological fluids. There’s a genuine synergy merging electronics, optics, solid-state materials, lasers, and long-established optical principles with the ability to fabricate and integrate them as optoelectronic integrated “circuits.” While many of these advanced sensors and systems are experimental, some are in use for monitoring bridges and other hard-to-reach or “distributed” sensing situations.

The Caltech project (which was funded by the Gordon and Betty Moore Foundation) used Google’s “Curie” cable, which runs undersea for more than 10,000 kilometers along the eastern edge of the Pacific Ocean from Los Angeles to Valparaiso, Chile (Figure 1). The water depth along the cable’s path is mostly between 3,000 and 5,000 meters.

Prof. Zhan and his team have developed and tested a technique which analyzes the light already traveling through existing, functioning submarine cables to detect ocean-floor earthquakes and ocean-surface waves. Their approach eliminates the need for any additional equipment and does not affect the primary data-transmission role of the optical fiber. It exploits birefringence or “double refraction” in the optical fiber, which means that the refractive index of a material depends on the polarization and propagation direction of light – a phenomena which can be seen in some crystals (Figure 2), but which (not surprisingly)  has a complicated underlying optical-physics explanation (Figure 3).

 

Unlike some projects which “re-purpose” and adapt obsolete or abandoned installations or those which time-share with active systems, this one co-exists full-time and yet does not interfere with the primary use of the cable, but instead extracts additional information from existing infrastructure and test/measurement equipment.

There are hundreds of shorter and longer fiber-optic cables totally more than a million kilometers of fiber-optic cable on the ocean floors, so the ability to make additional use of them for sensing opens up vast opportunities. The next part of this article looks at the project’s concept and results in more detail.

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External references

1. Zhan et all, Science, “Optical polarization–based seismic and water wave sensing on transoceanic cables”
2. Zhan et all, Science, “Supplementary Material”
3. Purdue Dept. of Physics, “Waves & Oscillations: Polarization of Light”
4. Northwestern University, “Measurement of the Stokes parameters of light”
5. Science Direct, “Stokes Parameter”
6. Nikon Microscopy, “Principles of Birefringence”
7. Olympus Life Science, “Optical Birefringence”