One of Micron Optics’ largest market segments for optical sensing systems is for monitoring of bridges. Hundreds of bridges today are monitored using FBG based sensors, but >98% of these are in Asia and Europe. But, only a few structures in North America use this technology. Why?

I think one answer is resistance to change. Owners are more comfortable using their 35 year old visual inspection protocols than adopting better technology. And, there has been no FHWA mandate in the US that all bridges of certain classes or deteriorated condition must use objective measurements to support the visual bridge inspection process, unlike China.

An Atlanta company, LifeSpan Technologies, on behalf of our industry, is promoting the use of advanced technologies that are more precise and objective to help owners and funding agencies make better decisions. In brief, they conclude that deployment of advanced condition assessment technologies will lower both the risk and life cycle costs for bridge owners and taxpayers. I agree.

LifeSpan makes a solid case for their argument in their October 2008 white paper, A Better Way to Fix our National Bridge Problem. Find it here: www.lifespantechnologies.com.

This proposed solution is independent of the assessment technology choice. And since there are number of competent technology suppliers in this business, finding what you need isn’t that difficult. Most technologies have unique features, so it pays to do your homework and compare both technical approaches and costs.

I believe that it’s only a matter of time before FHWA and the state DOTs will begin to use these advanced technologies routinely. Proposed solutions like this one from LifeSpan are sure to help accelerate the process.

Tom Graver
Director, Optical Sensing Group

Micron Optics has been helping customers use fiber optic sensors since 1997. In that time we still hear a common thread of questions. Here are the top seven:

Q: How many FBGs sensors can be on one fiber?
A: It depends on the range of measurement. For example, if the instrument (i.e., FBG interrogator) has an 160 nm wavelength range, and one needs to measure strain of +/-800 microstrain at each sensor, this translates to ~2 nm of wavelength range needed for each sensor. So, that’s 80 sensors per fiber.

Q: Are all sensors on the fiber sampled at the same frequency?
A: Yes. All sensors are sampled simultaneously. For example, if the laser scans at 1 kHz and there are 40 sensors on a fiber, one will receive 40 readings (one for each sensor) every one millisecond.

Q: How does change in wavelength get converted to engineering units like strain or temperature?
A:Each FBG sensor has a gage factor. Typical values are 1.2 picometers per microstrain and 10 picometers per degree C. Some more advanced sensor packages have a polynomial fit to cover measurements over a wide range. Calculations are made as a post processing step, or in automated real time fashion in a user interface like Micron Optics’s soon to be released ENLIGHT software tool.

Q: Must you compensate for temperature when measuring strain?
A:Usually, yes. In some cases the temperature change during the measurement is negligible, but in many applications — especially long term applications — strain and temperature FBGs are used together. The arithmetic essentially involves subtracting temperature induced wavelength changes from those that were induced by both temperature and strain, yielding a pure strain measurement.

Q: Won’t the FBG sensors and fibers break when I’m handling them?
A: Probably not. Optical fiber is tough stuff, and packaged sensors have ever improving fiber protection (e.g., buffer tubes) and strain relief (e.g., rubber boots). Handling FBG sensors is not so different from handing oil strain gages. Similar care will result in excellent results.

Q: How can I make fiber optic connections in the field?
A: There are three main choices: Use a field splice instrument. These cost are small, battery powered devices that are amazingly easy to use. Strip, clean and cleave the fiber, and the splicer makes the alignment and uses an arc to weld the ends together. A splice sleeve covers and protects the joint. The second method is to use fiber optic connectors. In the field, these would be housed in a junction box or otherwise protected from the elements. The third option is to avoid field connections and make the fiber array assemblies in advance. All work well, it just depends on the application and conditions on site.

Q: Do I really have to clean connectors every time I make a connection?
A: Yes. Buy and use a proper connector cleaner. Good connector hygiene will save time in the long run.

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.