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

There are many technologies, but commercial solutions really boil down to two main categories: point sensing for which the active portion of the fiber is <= 1cm, and distributed sensing where the entire fiber, perhaps tens of kilometers long, is the sensor.

Fiber optic distributed sensors measure temperature only (Raman Optical Time Domain Reflectometry -- ROTDR) or both strain and temperature (Brillioun Optical Time Domain Reflectometry -- BOTDR). Spatial resolution is typically one meter or more and strain and temperature resolution are reported at about one microstrain and one degree C respectively, with sampling rates of a few seconds per measurement. The beauty of these approaches is that standard (i.e., inexpensive) telecom fiber is the sensor. The fiber is usually packaged in a tough outer jacket for deployment. Instrumentation is often US$100,000 or more, however. But still the value is very good for long range (>2 km) applications such as pipelines, tunnels, power transmission lines.

Fiber optic point sensors are found in two basic types: fiber Bragg grating (FBG) sensors and Fabry-Perot (FP) sensors. FP sensors have found an important niche in measuring strain, temperature, and particularly pressure for medical applications. They are very small (especially the pressure sensors), but only one sensor can be used per fiber.

FBG sensors for strain and temperature are also very small – as short as 2mm in a 150 micron fiber diameter or as long as a few meters for long gage strain measurements. Other properties like pressure, acceleration, displacement, humidity, and chemical presence, are measured by using a transducer to relate strain to pressure or strain to acceleration, for example. A key advantage of FBG sensors is that dozens, or even a hundred, can be used in series on a single fiber — even if they are measuring different physical properties.

Fiber Bragg grating technology is by far the most widely used fiber optic sensor technology. The versatility of the technology and relatively low cost make it a winner for many applications. At Micron Optics, well over 90% of our sensing customers use FBG based sensors. Whether they’re examining a cancer patient, monitoring a bridge, flying an airplane or pumping oil, they need the information that Micron Optics technology can glean from fundamental measurements of FBGs

These applications, and the physics of how FBG sensors work, will be included as future blog topics.