A special symposium on distributed optical fiber sensor systems will be held at this year’s FiO/LS meeting. These optical sensors can be much more robust than their electronic counterparts, for example being able to operate in areas with electronic interference. There are a number of different manifestations of distributed fiber optic sensors, but they primarily work on the same principle: scattering of an optical signal in fiber. The scattering can be due to defects in the fiber causing simple linear backscattering (Rayleigh scattering) or from nonlinear scattering due to phonon interactions (Brillouin scattering) or induced energy transitions in the material (Raman scattering). Here, I’ll focus on one sensor arrangement based on Rayleigh scattering and optical time domain reflectometry (OTDR).
A simple depiction of the sensor system is shown in the figure. The front-end consists of a system capable of producing short optical pulses.These pulses are launched down the optical fiber, which acts as the distributed sensor. The scattering in the fiber causes some light to be reflected backwards. A circulator placed in-line then picks off this return signal, which is monitored using an appropriate detection system.
Since this is a sensor system, what we’re looking for is some sort of perturbation. One of the great things about these sensors is that there are a number of perturbations that can be detected, making them very versatile. To name a few, the sensors can detect vibrations, temperature changes, or stresses/strain. In OTDR systems based on Rayleigh scattering, one is looking for changes in the return signal caused by a change in scatterer position or scattering strength. These changes can be detected in a number of ways. For example, if one uses a highly coherent source to generate pulses (such as a narrow-linewidth laser), one can use some sort of coherent detection scheme to detect phase shifts resulting from changes in scatterer position. When a perturbation is observed, its location can be inferred from the roundtrip time the backscattered pulse took to reach the detector, since the pulse group velocity is known. In this manner, one can determine exactly where a disturbance is coming from with a resolution limited by the width of the probing optical pulse.
This is just a 1000-foot view of these sophisticated fiber optic sensors. At FiO/LS, there will be a number of experts speaking on the topic. Here I only briefly described OTDR sensors based on Rayleigh scattering. However, the presenters at the conference will cover a much wider range of sensors, their intricacies, and other fine details I can’t cover in this short post.
Disclaimer: Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government and MIT Lincoln Laboratory.
Posted: 9/23/2013 10:35:11 AM by
by Dominic Siriani
| with 0 comments