Sep 02 2008

Optical Sensing Interrogators – what distance?

Published by at 4:11 pm under Instruments

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.

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