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Fiber Bragg Grating

Click on the bridge to see how FBGs monitor displacement.
Click on the bridge to see how FBGs monitor displacement.

Credit: Zina Deretsky, National Science Foundation

Fiber Bragg Grating sensors can accurately measure movement of large structures like bridges, roadways or skyscrapers, with a single sensor providing data from many different points along the structure.

To make a Fiber Bragg Grating sensor, engineers start with optical communication fibers – those threadlike strands of glass that usually carry phone conversations or Internet traffic on beams of laser light. They modify these fibers by introducing small periodic variations in the glass, called Bragg gratings. This makes the fibers act like very special mirrors, reflecting light of particular colors while remaining transparent to others. The precise color reflected depends on the spacing between the disturbances, called the period of the grating. Gratings with large periods reflect redder, or longer wavelength, light while gratings with small periods reflect bluer, or shorter wavelength, light.

A single sensing fiber can be several kilometers long, and can contain as many as 100 gratings, each designed to reflect a slightly different color of light. A real sensor uses light from the infrared region of the spectrum, with wavelengths in a narrow band around 1500 nanometers. But the human eye can’t see infrared light, so our animation uses 400-700 nanometer wavelengths to show you the sensor in operation.

When a laser shines down the sensing fiber and the color of the laser light is gradually varied, each grating in turn reflects its characteristic color back towards the light source. Recording the sequence of reflected colors gives a baseline position for every grating along the fiber. Now when a truck deflects a bridge, or some other disturbance applies stress to part of a sensor, some of the gratings are strained. Strain in the sensor corresponds to a change in the gratings period, so the next time the laser sweeps through its color range, the reflected colors are slightly different. Comparing the shifted colors to the baseline readout gives the deflection at each part of the sensor.

Fiber Bragg Gratings and tunable infrared lasers were first developed for the telecommunications industry, where they are used to combine multiple signals on a single fiber and later to re-separate them. The gratings’ sensitivity to vibration, displacement, and temperature led to the development of FBG sensors—a productive cross-fertilization between telecom engineers, optical scientists, and structural engineers.


Click on the cantilevers to see an animation of molecular sensing.
Click on the cantilevers to see an animation of molecular sensing.

Credit: Nicolle Rager, National Science Foundation
The working elements of the micro-cantilever sensor—those projections that look like diving boards—are essentially microscopic tuning forks.

Like any tuning fork, each of the cantilevers vibrates at a natural frequency that depends on its mass. Add more mass, and the vibrations will slow down; take away mass, and the vibrations will speed up.

The micro-cantilever sensor array exploits that fact to detect minute quantities of chemical substances. Each of the vibrating levers has a different chemical coating, so when the array is exposed to a test sample, molecules will stick to certain levers and not to others. This increases the mass of those particular levers, and lowers their frequency of vibration. By monitoring which levers are affected, the sensor can identify the molecules in the sample. By monitoring the magnitude of the frequency change, it can estimate their concentration.

Electronic Nose

Click on the rose to see how polymers can sense smells.
Click on the rose to see how polymers can sense smells.

Credit: Nicolle Rager, National Science Foundation
The Electronic Nose sensor developed by Caltech chemist Nathan Lewis and his colleagues produces “smell spectrum” that’s unique to each kind of airborne chemical.

To achieve this, the scientists wire their sensor with a series of minuscule circuit breakers, each made from a tiny blob of polymer that’s wrapped around a chain of electrically conducting carbon granules. The trick is to choose a polymer that swells up when it’s exposed to certain types of airborne molecules. Any swelling will then pull the carbon grains apart and greatly increase the resistance in the circuit, which will produce a detectable signal.

By making the various circuit breakers out of different polymers with different responses, the scientists can then get a unique spectrum for each type of airborne chemical.

Ad Hoc Networking in the Forest

Click on the trees to see how wireless sensors communicate.
Click on the trees to see how wireless sensors communicate.

Credit: Nicolle Rager, Zina Deretsky, National Science Foundation
Out in the field, where there is no Internet, sensors have to get their data back to headquarters via wireless networking technology. Each one passes bits on to the next, creating their own network on the fly—an ad-hoc network.

We can see how this works in the environmental monitoring application shown here. Once the sensors are in place, they automatically reach out to find their nearest neighbors, and then form the network links that can transmit the data.

Of course, these connections can be pretty chancy. Not only are they generally restricted to very low power, very short distances, and very low data rates, but the sensors themselves are miles from any tech support. They are out there with no protection from being soaked, baked, frozen, buried, stolen, stepped on, or even eaten.

But then, as shown here, the ad-hoc network as a whole is far more robust than any one device. If any of the links is blocked or broken, the sensors will automatically reach out and find new links to replace them.

The Sensor Revolution A Special Report