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Undersea optical-fiber cables do double-duty as seismic sensors, Part 2: Application

March 23, 2021 By Jeff Shepard

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

The previous part of this article established the context for “piggybacking” seismic-sensing capabilities onto an existing undersea fiber-optic cable. This part looks in more detail at the project’s basis and results. Note that the physical cable is not a single, uniform assembly but instead changes jacketing and protection depending on location along the path (Figure 1). There are also optical amplifiers every 100 kilometers to boost the optical power by up to 20 dBm.

Fig 1: The structure and types of the cables used in the Curie cable. (A) Lightweight (LW) optical fiber cable with steel strand wire configuration; the interlocking strength members resist external pressure up to water depth of 8,000 meters. The LW cable, which is 95% of Curie Cable’s length, has external diameter 17 mm and weight (in air) 4.8 kN/km. The cable core with fibers consists of a polyethylene tube filled with gel. (B) Armored part of the Curie cable (5%) consists of Special Application, Light-Weight Armor, and Double Armor cable. (C) The landing point of cable Curie at Valparaiso, Chile. The big ship was deploying the Curie cable, whose path can be seen along the line of floats behind the ship. (Image: Caltech)

To maximize throughput and achieve the highest possible data rates, one factor which cable-system operators monitor is the polarization of the light (usually designated as a state of polarization or SOP) that travels within the fibers. By controlling the electric field’s direction associated with this polarization, multiple signals can be multiplexed through the same fiber simultaneously (Figure 2). At the receiving end, optical instrumentation checks the SOP of each signal to see how it is dynamically changing along its journey – a type of distortion which results in the optical signals getting cross-couple. They then interfere with each other, degrading the signal-to-noise ratio (SNR) and increasing bit error rate (BER). The Stokes parameter is a widely used factor which characterizes this optical-signal polarization.

Fig 2: The arrangement of SOP sensing on the Curie cable: A of coherent transponders operating at different optical frequencies is connected to a multiplexing/demultiplexing device (the trapezoids) which is then connected to the Curie cable. The transponders are used to transmit and receive data to/from Los Angeles and Valparaiso over a fiber pair in Curie. Two transponders (Channel 1 and Channel 2) are connected in loop-back configuration as the signal is transmitted and received in Los Angeles. The other transponders, either in Los Angeles or Valparaiso, can be used to determine the SOP of the received optical field. (Image: Caltech)

For fiber in a “quiet” environment and not affected by time-varying external perturbations, the Stokes vector of the output polarization is constant. However, when the fiber is instead exposed to time-dependent disturbances such as stretching, twisting, pressure, or bending, that changes couples to the fiber’s birefringence. The birefringence of each section of the optical cable then changes, and the Stokes vector of the output polarization changes and varies as well.

The polarization can change within the cable due to temperature fluctuations, lightning strikes, and minute mechanical motion. Still, the first two effects occur mostly in land-based, not submarine cables. In contrast, the ocean temperature remains nearly constant. There are fewer environmental disturbances, so the deep-water Curie cable is not as “noisy” due to those first two factors as its terrestrial counterparts. The researchers found that polarization along the Curie Cable remains quite stable over time. That stability is key, allowing the researchers to detect the third factor – mechanically induced strain – in the cable.

However, during earthquakes and when storms produce large ocean waves, the polarization changes suddenly and dramatically. These changes can be detected with the existing monitoring equipment and suitable data-analysis algorithms. As a result, the entire length of a submarine cable acts as a single sensor in a hard-to-monitor location. Polarization is measured frequently, often as much as 20 times per second, so when an earthquake strikes close to a particular area, it could be detected in seconds (Figure 3).

Fig 3: Detection of far-field, long-period surface waves on using SOP. The Stokes parameter S1 (left) is shown together with seismograms at conventional seismic station TLIG (right) for the 22 March 2020 magnitude 6.1 earthquake on the central East Pacific Rise. (Image: Caltech)

Note that this is a very low-bandwidth phenomenon, with variations occurring slowly in the range of tens of millihertz (1 MHz = 0.001 Hz) to a few hertz. That’s a very different spectrum compared to what most electronic and optical engineers are used to thinking about and measuring (but not unusual for mechanical engineers and real-world processes – think of temperature), so low-frequency stability and the low drift inherent in the sophisticated polarization test instrumentation is necessary.

The team recorded approximately 30 ocean storm-swell events and 20 moderate-to-large earthquakes across the nine-month continuous observation period, including the August 4, 2020, 5.8 magnitude submarine earthquake offshore from Guatemala (Figure 4).

Fig 4: SOP detection of the 04 August 2020 19:17:52 M5.8 submarine earthquake offshore Guatemala. The earthquake distance is about 400 km to the Curie cable at the closest point and about 100 km to the Guatemalan coast. (Image: Caltech)

“This new technique can really convert the majority of submarine cables into geophysical sensors that are thousands of kilometers long to detect earthquakes and possibly tsunamis in the future,” said Prof. Zhan. “We believe this is the first solution for monitoring seismicity on the ocean floor that could feasibly be implemented around the world. It could complement the existing network of ground-based seismometers and tsunami-monitoring buoys to make the detection of submarine earthquakes and tsunamis much faster in many cases.”

He also noted that because this method does not require specialized equipment, laser sources, or dedicated fibers, it is highly scalable for converting global submarine cables into continuous real-time earthquake and tsunami observatories. The project’s full details are in their paper “Optical polarization–based seismic and water wave sensing on transoceanic cables” published in Science; there is also posted Supplementary Material with detailed optical-physics equations and analysis, graphs of results, and much more.

Related EE World Content

Optical amplifiers, Part 1: Applications and considerations
Optical amplifiers, Part 2: Basic implementations
The first undersea transatlantic cable: An audacious project that (eventually) succeeded, Part 1
The first undersea transatlantic cable: An audacious project that (eventually) succeeded, Part 2
Advancing Undersea Optical Communications
Researchers Develop Novel Framework for Tracking Developments in Optical Sensors
Dual-photodetector optical sensor module in 0.88mm package enables thinner wearable designs
Optical Fibers Can ‘Feel’ Materials Around Them
What types of problems can fiber optic kinetic sensor solve?

 

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”

You may also like:

  • Undersea optical-fiber cables
    Undersea optical-fiber cables do double-duty as seismic sensors, Part 1:…

  • What happens when the earth moves?

  • Seismometer: What is it and what does it measure?

  • How can you detect seismic waves?

Filed Under: Featured, Frequently Asked Question (FAQ), ground fault sensor Tagged With: FAQ

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