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Design Challenges of a High Speed Tunable Laser Interrogator for Future Spacecraft Health Monitoring

There has been an increased interest in the recent years to use fiber optic sensing for space applications such as installing optical sensors on satellites, launchers (i.e. Ariane 5), atmospheric re-entry vehicles and solar sails in addition to ground testing of space structures [1].

15 May 2015
S. K. Ibrahim, J. A. O’Dowd, R. McCue, A. Honniball, M. Farnan


Such applications include monitoring temperature, pressure, acceleration, and static/dynamic strain. Optical fiber sensors using Fiber Bragg Grating (FBG) technology fundamentally measures temperature (typically 10pm/°C) and strain (1.2 pm/με) [2]. Designing transducers around standard FBGs and/or writing FBGs on special fibers and in different configurations enables measurements beyond strain and temperature (e.g. pressure, acceleration, acoustics etc.). There are several advantages of using fiber optic sensing such as light-weight when compared to the current cable harnesses for electrical sensors, immunity to electromagnetic interferences, and the feature of multiplexing and distributing several sensors on a single fiber. 
However, in addition to designing sensors, an optical FBG interrogator (reader) is required to measure the sensors with high level of accuracy, and at high acquisition speeds. It is also required to mitigate and reduce any errors induced from the sensors due to its sensitivity to unwanted physical effects such as polarization dependent frequency shift (PDFS). The interrogator should also operate in extreme conditions for certain space applications (e.g. launchers). In this paper we demonstrate a high speed, accuracy, and sensitivity optical FBG interrogator (FAZT V4) designed and tested under a European Space Agency (ESA) contract: "High speed tunable laser interrogator for spacecraft health monitoring". The interrogator also has in-built features to mitigate and overcome several effects such as vibration, temperature, aliasing, PDFS, and attenuation, while maintaining absolute sensor accuracy and high speed.


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