FBGs are essentially reflectors built inside the core of an optical fiber. The reflectors are made by permanently altering the refractive index of the core. FBGs are off-the-shelf items today, and can also be made to order to fit particular applications (e.g., many FBGs in an array with irregular spacing on a single fiber).

Each FBG reflects a certain narrow slice of spectrum. Such slices typically have a smooth Gaussian shape. The center (i.e., top) of the reflected Gaussian peak is what is used to make measurements (aka, the “center wavelength”). The center wavelength is a digitally encoded zero point for each sensor — one that doesn’t change with time. This is one fundamental characteristic of FBG sensors that make the technology so valuable for long term monitoring of structures.

A FBG’s peak shifts (to a higher or lower center wavelength) when either the fiber is strained or its temperature changes. When the strain or temperature change is returned to the zero point, so does the sensor reading, i.e., there is no hysteresis.

FBGs are specially packaged to isolate the measurement property of interest. For example, the Micron Optics os3100 strain gage uses a steel carrier to transfer strain from the structure to the FBG, whereas the os4100 temperature gage isolates the FBG from strain. Used together, one can easily collect temperature-compensated strain measurements.

If you’re buying FBGs for your application, consider these specifications:

* FBG length of 10mm, >80% reflectivity, 3dB bandwidth of 0.25nm, 15dB isolation (that’s the “clean” region of the FBG peak), and a tensile strength of 150kpsi. This strength is equivalent to a proof strain of 15,000ue. Stronger and shorter FBGs are available, but you trade other properties to improve these characteristics.

* Also consider fiber coatings. Most fibers and FBGs are coated with acrylate. This dates back to FBG’s telecom roots. Polyimide coatings, however, have a higher temperature tolerance (250 C vs. 80 C for acrylate), and are far superior in transferring strain to the fiber core.

Tom Graver
Director, Optical Sensing
twgraver@micronoptics.com

I often ask a prospective customer: Why are you considering using optical sensors? Sometimes it’s simply that they are enamored with new technology. Granted it is cool but what are the real problems that they need to solve? Here are examples of problems that I look for and the associated benefits provided by fiber Bragg grating sensors:

We need a passive Class 1, Div 1 sensor for an explosive environment.
Benefit: FBG sensors are passive.

Lightning destroyed our electrical gages.
Benefit: Optical fibers are non conductive, so lightning will not destroy FBG sensors with an electrical surge.

The EMI (electromagnetic interference) is so strong, we get more noise than signal with our thermocouples.
Benefit: FBG sensors are immune to EMI.

Our bridge is 5 kilometers long and we need 720 sensors.
Benefit: Fiber sensor instruments (aka interrogators) have a range of well over 30 km and a capacity for more than 80 sensors per fiber and 16 fibers. That’s a total of >1280 sensors per demodulation instrument.

We need to study the temperature transients that happen in a few milliseconds.
Benefit: FBGs respond quickly to even slight temperature variations.

Fifty sensors must fit inside a 0.5 meter long tube that’s only 2mm in diameter.
Benefit: FBGs can be spaced at 1 cm intervals along a fiber that is only 155 microns in diameter.

Salt air corrodes our foil strain gages in just a few weeks.
Benefit: FBG sensors are made of silica (i.e., glass). They do not corrode.

Conventional gages take too long to install and to home run all of the wires.
Benefit: Multiplexing dozens of FBGs in series in one fiber saves the cost of a home run lead to each sensor. Also, varying FBG sensor lead lengths does not impact sensor calibration.

We spend too much time calibrating sensors and instruments.
Benefit: Micron Optics sensor interrogation instruments have built-in calibration artifacts that last for the life of the instrument. The FBG sensors each have a digitally encoded identity that does not change. So once a system is installed and sensor zero points are recorded, no further calibration is required. Ever.

Our conventional gages fail after about one million cycles.
Benefit: Optical fiber is amazingly robust. Our FBG gages have been tested to >100 million cycles of +/-3,000 microstrain with no degradation of the measurement.

We need to measure strain ranges of more than 6,000 microstrain.
Benefit: Some FBG strain gages can measure up to ~30,000 microstrain (i.e., 3% elongation).

The wire for the foil gages is too heavy and cumbersome.
Benefit: Again, multiplexing is the key. A single, small fiber can connect 10s of gages to the interrogator.

We need one versatile system that can measure strain, temperature and pressure.
Benefit: FBGs measure directly strain and temperature. Tranducer packaging around FBGs makes measurement of other properties possible – like pressure, acceleration, displacement, chemical presence, etc. All of these sensors, no matter what they measure, are measured by the same interrogator.

We need to embed sensors into our composite structure.
Benefit: Because fibers are so small, they can be embedded in structures built with carbon fibers, glass fibers, concrete and steel, etc.

We need gages that will last for a decade or even longer.
Benefit: Optical components like the FBGs themselves and those used to build the interrogators, are Telcordia qualified for a >25 year lifetime. Telcordia is a set of standards established by the telecom industry for critical equipment deployed in harsh field applications.

We need to measure extreme temperatures (e.g., as low as -200°C or up to +750°C)
Benefit: Commercial quality FBG-based temperature sensors are available now for the -200°C to 300°C range, and promising prototypes have been shown to operate in 1,000 hour tests at 750°C. Materials like sapphire FBGs are underdevelopment for even higher temperatures.

Do any of these FBG advantages address your needs? Feel free to contact us to discuss your application and to see if FBG sensors might hold at least some of the answers you seek.

Tom Graver
Director, Optical Sensing
twgraver@micronoptics.com