Optical EngineeringStrain-rate sensor based on in-fiber Doppler velocimetry
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An optical fiber strain-rate sensor based on in-line Doppler velocimetry is developed using a 3 x3 coupler with a large path imbalance Mach-Zehnder interferometer for passive demodulation of Doppler wavelength shifts. A model is developed that relates the strain rate in an optical fiber to the corresponding Doppler wavelength shifts. The strain rate obtained by differentiating strain from a Michelson strain sensor mounted on the piezoelectric transistor (PZT) cylinder shows good agreement with the strain rate directly obtained from the Doppler-based strain-rate sensor. The Doppler-based strain-rate sensor is also bonded to a Hopkinson pressure bar in an attempt to detect the stress waves propagation. It is shown that the higher the strain rate of stress wave, the easier it is to couple into the lead-in/out fiber.