Jun 02 2008

What Types of Fiber Optic Sensing Technologies are Available?

Published by at 1:30 pm under OS Technology

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.

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