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.

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.

New applications for FOS come about in many ways. Starting more than ten years ago university researchers led the way, but, in recent years, more commercial entities are solving problems using FOS. One example is Durham Geo Slope Indicator (DGSI) in Stone Mountain, Georgia. They’ve earned a solid reputation for providing vibrating wire (and many other technologies) for geotechnical measurements. But for some of their customers with buried pipelines, high EMI conditions made the vibrating wire technology less useful. They recognized that fiber optic strain gages would work well in this environment.

So they coupled their know how for installing and protecting electrical lead wires and sensors to the FOS sensors and have made several installations. The results are impressive. The pipeline owners can resolve much smaller movements in the pipes than ever before, expected lifetime is improved by at least a factor of ten, and installation is about four times faster.

They’ll be exhibiting their new applications at the ASCE Pipeline Division International Conference on July 23-24 in Atlanta. Learn more at the ASCE websiteand Durham Geo’s website

Here are the basics. Light is sent into a fiber and reflects back from the FBG. The reflected light travels back to the instrument’s photo detectors and is compared to wavelength reference artifacts so that the instrument can determine the position of the center wavelength of the FBG. Wavelength information is converted to engineering units, e.g., 1.2 picometers of wavelength shift could correspond to 1 microstrain. The actual translation is given by the gage factor supplied with the FBG sensor.

When more than one FBG is present on a fiber (this is often the case), the instrument will use one of two schemes to discriminate between one FBG and the next. Time division multiplexing (TDM) systems use the known speed of light in the fiber to discern which signal is reflected from which FBG along the fiber path. Theoretically, 100 or more FBGs can be on the same fiber at the same nominal center wavelength.

The most utilized scheme is wavelength division multiplexing, or WDM. WDM FBGs are at distinctly different nominal center wavelengths from their neighbors, and the interrogator uses these unique FBG wavelengths to keep track of which sensor is which. Sensor capacity on each fiber is determined by the range that each sensor will measure and the total spectral range of the instrument. WDM ranges are now very large and can also accommodate more than 100 sensors per fiber.

We (Micron Optics) have chosen the WDM approach for a few reasons.

1) Scanning speed is not a function of the number of sensors, and the scan speed can be up to 2kHz.

2) Swept laser sources are powerful enough to split into four fibers for simultaneous measurement of >300 sensors.

3) FBGs can be highly reflective. This coupled with the high dynamic range of the instrument make the system much more flexible for measurements over tens of kilometers of fiber.

4) WDM is compatible with Micron Optics fast, narrow line, wide range lasers that are stable over time and temperature, and are mechanically robust in environments where they will encounter shock and vibration.

There are other approaches, but they have significant drawbacks:

a) Broadband source, Dispersive element, Diode Array
Limitations: This method cannot achieve the required wavelength measurement repeatability and resolution with commercially available diode arrays. Low broadband source power limits the ultimate needed combination of channel count/sensor capacity and dynamic range/distance to sensors.

b) Broadband source, Optical Spectrum Analyzer/Multi-line wavelength meter
Limitations: Laboratory OSAs are large, slow, expensive, and do not have a wide operating temperature range. Multi-line wavelength meters acquire data at slow speeds only, and are not mechanically robust. Low broadband source power limits the ultimate needed combination of channel count/sensor capacity and dynamic range/distance to sensors.

c. OTDR/TDM systems
Limitations: Low loss budget precludes a solution with the required number of sensors and/or channels, and data acquisition rates scale down with increasing sensor counts. Minimum physical grating spacing limits some applications.

d. External Cavity Tunable Laser, Power Meter, Wavelength Meter
Limitations: External cavity tunable lasers are slow, expensive, and do not have a wide operating temperature range or the required mechanical robustness. The addition of power meters and wavelength meters add to the bulk, complexity, and cost, as well as reduce reliability and speed. Polarization properties of the narrow line lasers may not be an ideal match for all sensing applications.

Tom Graver
Director, Optical Sensing
twgraver@micronoptics.com