In 2005 Micron Optics developed an FBG interrogator capable of sampling a single FBG at 500kHz or up to four FBGs at 100kHz. It was not an official product, but an Alpha prototype meant as a technology demonstration platform. We expected to sell just a few.

Since then we’ve sold dozens of these si920 Optical Sensing Interrogators. Applications have ranged from blast tests and ballistic impact tests to analysis of vibration in medical devices when exposed to MRI energy and the propagation of sonic energy in composite panels used on spacecraft.

It’s clear to us that there is a need for this type of measurement tool. No other technology (that we know of) can make such high speed (500kHz) high resolution strain measurements (0.02microstrain) in harsh electromagnetic environments.

We’re interested in feedback from you on how we might proceed with formal product development. For example, higher sampling rates in the MHz are possible with more advanced DAQ technology. More sensors per instrument, and accommodating more sensors per fiber are also feasible. What are your applications? What instrument configuration might work best for you?

si920 Datasheet

Tom Graver
Vice President, Optical Sensing

Glossary of Terms:
DAQ = Data Acquisition
MRI = Magnetic Resonance Imaging
FBG = Fiber Bragg Grating

Applications of FOS technology are growing for industrial process monitoring. Engineers in plants producing glass, chemicals, metals, paper and plastics are using FOS to measure where other sensing technologies perform poorly or not at all. Fiber optic technology provides solutions for high EMI environments, areas where explosion risks persist, and where sensor density requirements make installation and cabling too expensive and cumbersome.

One recent example is an installation by Hatch Ltd. Hatch is a designer and supplier of custom designed furnaces and furnace components for the production of metals. They are using arrays of FBG sensors inside the furnaces used in smelting operations to provide a dense map of temperatures in the critical zones — information that can detect hot spots. FBG sensors provide superior temperature measurements and measurement density compared to thermocouples, which are traditionally used. This information guards against costly and potentially dangerous breaches of the furnace insulation systems and will allow maintenance to be driven by the actual furnace condition rather than a simple schedule for preventive maintenance. The use of FBG technology is part of an on-going effort by Hatch to develop a Diagnostic System (patent pending) for furnaces and other metallurgical reactors that will estimate the health or remaining life of the equipment.

Phil Shadlyn, a key engineer on this project, told me: “It really couldn’t have gone better. [Hatch's] sensor design worked as planned, and installation was smooth. [Our customer] was pleased with how easy it was to use the Micron Optics instruments and software. We will continue to monitor the installation and if everything progresses as planned, we’ll deploy several more installations throughout the next two years.”

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.