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

Most commercialized fiber optic sensors operate at temperatures between -40 and 150 C. Higher temperatures can degrade polyimide fiber coatings (above 250 C) and erase FBGs from the fiber core (above 400 C).

Some industrial applications (e.g., oil refining, steel production and chemical formulation) require much higher temperatures, and often conventional thermocouples do not last long enough or do not operate reliably due to high EMI or cumbersome cabling.

Several research organizations and commercial companies are working to develop high temperature optical sensors that are both accurate and reliable in high temperature environments. One such company is Chiral Photonics, Inc. in Pine Brook, New Jersey. They recently announced a novel optical sensor that uses twisted fibers to create a thermally sensitive spectral response. The sensor appears to be stable, accurate and repeatable at temperatures up to 1000 C.

Find details at: www.chiralphotonics.com/

Now that the optical core is nicely characterized, Chiral is working on ruggedized packaging for large-scale field deployments. Sensor readings are made simple with self-contained laser instrumentation and software from Micron Optics — so the user does not need optical systems expertise to use these sensors. They are as easy to use as conventional sensors.

I’ve written about why many applications must use FBG sensors, but how do users deal with the data? Conventional electronic gages deliver an analog signal that’s proportional to the strain or temperature change. Optical gages deliver a digital signal that reports an absolute wavelength value indicative of the strain, temperature, displacement, pressure, etc.

Converting from wavelength to engineering units requires some basic arithmetic. For example, the gage factor for an FBG strain sensor might be 1.2 picometers per microstrain (gage factors are provided by the strain gages manufacturers — just like electrical strain gages). So, for example, if the measured wavelength shift is 120 picometers, the strain sensor is actually measuring a change of 100 microstrain.

Some calibrated FBG temperature gages may use a third-order polynomial fit to fully characterize the gage factor, but still it’s just a matter of doing the arithmetic to make the conversions from wavelength to temperature.

Up to now, most users have been on their own to make these calculations. Micron Optics has always provided a basic LabVIEW example user interface that customers modify to convert, store and display sensor data. But now Micron Optics is providing a new tool called ENLIGHT^Pro.

ENLIGHT^Pro provides an all-in-one solution to configuring sensors connected to Micron Optics instruments, converting wavelengths to engineering units for hundreds or thousands of sensors, displaying data in charts, graphs or images, setting alarm limits and sending alerts, and saving data. A free download of ENLIGHT^Pro Beta release is available at http://micronoptics.com/sensing_software.php

The release of ENLIGHT^Pro represents yet another milestone for making fiber optic sensing solutions more accessible and easy to use. Along with improved sensor packages, sensor installation kits, and simplified instrumentation choices, this software tool allows the user to quickly move beyond optical setup details to actually using and analyzing the data to get the answers that they need.

Some sensing applications require the ability to measure over very long distances, but what is the range of instruments that measure fiber Bragg grating sensors? The answer comes down to loss budget, i.e., how much light is lost from the fiber core as it travels along great lengths and through connectors and bends. And loss is a round trip affair. Light is sent out and reflections must travel back the same distance, thus the things that cause loss are encountered twice by the signals. Micron Optics interrogators use laser light sources, so the starting power is high. This increases range significantly. Also, our the photodetectors are very sensitive.

The launch power minus the detection noise floor gives us the loss budget of the interrogator (we call it dynamic range on the instrument data sheets). Example loss budgets are 25 dB for dynamic (100Hz to 2kHz scan rate) instruments and 50dB for static measurements.

Round trip loss along a straight fiber can be 0.5 dB per kilometer. Two way connector losses are typically 0.5 dB also. Bends sharper than a 3 cm radius can also induce losses.

Useful measurement ranges are typically 20 km (one way) for dynamic instruments and twice that for static. This is plenty of range for most applications, e.g., a 5 km bridge, a 15 km deep oil well, a 4 km power transmission cable, or even a 35 km pipeline.

Some users have really pushed the limit through both good engineering and perhaps good fortune. For example, Dave Brower, CEO of Astro Technologies recently called me from aboard an off shore oil platform to say that his Micron Optics sm125 interrogator had successfully taken readings from an array of FBG strain sensors located more than 83 km away from the instrument (166 km round trip)!

Most users never need to worry about loss budgets or range. They simply use good hygiene when making the optical connections (e.g., isopropyl alcohol and lens paper) and never encounter significant optical losses. Also, it’s important to note that lowering the power of the FBG reflection does not change it’s measurement value. Its wavelength is stable even if the power is changing.