“Photonic Sensing” is the somewhat highfalutin term for a family of emerging technologies that use lasers and fiber-optic cables as ground motion sensors, not just as telecommunication conduits. A recent article in UW News has brought the subject of photonic sensing to the attention of our fans. So, this blog post is a very quick and dirty primer on the promise and problems of fiberoptic sensing in regional seismic monitoring. The subject is vast, decidedly complicated, and rapidly evolving…so hang on to your hat and let’s go!

How photonic sensing works

Fiber-optic sensing, or photonic sensing, or distributed sensing all refer to methods based on firing light pulses from a high-powered laser down through a fiber-optic cable and using characteristics of either the transmitted—or reflected—light pulse to “interrogate” the state of strain within and along the fiber. That state of strain can further be related to:

• Stretching and contraction along the axis of the fiber – Distributed Strain Sensing (DSS);
• Vibration of the fiber along its axis (like a “seismometer”) – Distributed Acoustic Sensing (DAS); or
• Temperature along the fiber – Distributed Temperature Sensing (DTS).

The secret to photonic sensing is the “scattering” of the laser light by natural imperfections within the tiny fiber.

The most relevant technique for us seismologists is DAS, so let’s take a quick dive into how that works. The most fundamental description is that a laser light pulse is fired from an “interrogator unit” into one end of a pre-existing fiberoptic cable and energy scattered back to the interrogator is analyzed to determine the state of strain along the fiber (see Figure 1). The backscatter is due to imperfections in a fiber that scatter light in several physical ways. DAS relies on “Rayleigh scattering,” in which an imperfection sends a ripple of energy back towards the direction the laser light pulse came from. Each fiber has its own random pattern of imperfections that was cast when the fiber was fabricated. If the fiber is strained a bit (stretched or contracted) somewhere along it, the imperfections will be moved closer together or farther apart in that section just a tiny bit.

Figure 1. Schematic of fiber-optic DAS technology basics. The interrogator unit sends a pulse of laser light outbound (in this diagram it is at the end of the fiber and traveling away from the interrogator). As it passes through the fiber, imperfections (red dots) scatter some light back toward the interrogator, where the pattern is detected and analyzed. Strains in the seismic wavefield deform the fiber and perturb the pattern. The resulting data can be displayed as an array of broadband “seismograms”, shown at the lower left.

Let’s get back to the back-scattered energy, which will pop out of the fiber where the laser pulse was introduced, right? The DAS technique measures the changes in back-scattered intensity between successive pulses that come from many “channels” along the fiber. These changes in intensity are correlated with changes in the optical path length (strain) along the fiber. The technique of interferometry is used to extract any changes in the strain state along the fiber, and thus produce time series of ground vibration. I find it easier just to think of the backscattered energy as being a random “code” that is correlated between successive pulses. This code gets distorted—alternately stretched and compressed—by pressure waves that are in the medium within which the cable is contained (i.e., the ground!). Interferometry detects and identifies the stretching and compressing distortions of the code  By keeping track of the “time of flight” of the pulse the interrogator determines where along the fiber the strain is being measured. 

Now that’s amazing. But it’s just the start of amazingness. For while interferometry is an olde timey technique dating from the late 1800s, its use with lasers at extremely high sample rates controlling the lengths of light pulses within a fiber, and with the computing power to keep track of it all is really jaw-dropping. Because by manipulating the strength of the laser pulse, its duration in time, and how frequently you sample (i.e., send a light pulse down the cable), you can adjust how many sections your fiber can be divided into, and what frequencies of ground motion you can resolve. That’s right: your fiber potentially can have tens of thousands of channels (i.e., measurement points) along its length, spaced on the order of several meters. And because the laser can be fired at kilohertz rates, a single fiber can provide data equivalent to thousands of broadband seismometers. This is, obviously, potentially revolutionary, and it’s the reason the seismological community is agog. 

The potential uses of DAS data extend far beyond network seismology. Thus far, development of instruments and analytical methods has largely happened in the fields of geotechnical monitoring, intrusion monitoring, and industrial infrastructure state-of-health monitoring. But the new Photonic Sensing Facility at UW and its partnership with the PNSN aims to research how best to integrate DAS technology into the monitoring of regional seismic hazards. 

While DAS can use custom-built fibers for observations, it also (and perhaps more importantly for our purposes) can take advantage of pre-existing telecommunication fiber-optics – called “Dark Fiber” (because it’s not lit…get it?). And there is LOTS of dark fiber around. There are fibers under the city streets, there are fibers in cables between cities, and, perhaps most important for PNSN, there are dark fibers in the telecom cables that run offshore and girdle the Cascadia Subduction Zone. Hmmm…30,000 broadband seismometers (not to mention strain meters from DSS) already hanging out on top of our biggest seismic hazard. No wonder we’re excited (see Figure 2)!