Abstract

We demonstrate the possibility of using radio-frequency modulation spectroscopic techniques for interrogation of fiber Bragg-grating (FBG) structures. Sidebands at 2 GHz are superimposed onto the output spectrum of a 1560-nm DFB diode laser. The power reflected by an FBG is demodulated at multiples of the sideband frequency. The sideband-to-carrier beat signal is shown to be extremely sensitive to Bragg wavelength shifts due to mechanical stress. Using this method, both static and dynamic strain measurements can be performed, with a noise-equivalent sensitivity of the order of 150 nε/√Hz, in the quasi-static domain (2 Hz), and 1.6 nε/√Hz at higher frequencies (1 kHz). The measured frequency response is presently limited at 20 kHz only by the test device bandwidth. A long-term reproducibility in strain measurements within 100 nε is estimated from laser frequency drift referred to molecular absorption lines.

© 2005 Optical Society of America

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References

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Appl. Opt. (2)

Fiber Optic Sensors (1)

Y. J. Rao and S. Huang, �??Applications of Fiber Optic Sensors,�?? in Fiber Optic Sensors, F.T.S. Yu and S. Yin eds. (Marcel Dekker, Inc., New York, Basel, 2002).
[CrossRef]

IEEE Photonics Technol. Lett. (1)

S. Knudsen, A.B. Tveten, and A. Dandridge, �??Measurements of Fundamental Thermal Induced Phase Fluctuations in the Fiber of a Sagnac Interferometer,�?? IEEE Photonics Technol. Lett. 7, 90-92 (1995).
[CrossRef]

J. Chem Phys. (1)

G. di Lonardo, L. Fusina, E. Venuti, J.W. C. Johns, M.I. El Idrissi, J. Liévin, M. Herman, �??The Vibrational Energy Pattern in Acetylene. V. 13C2H2,�?? J. Chem Phys. 111, 1008-1016 (1999).
[CrossRef]

J. Geophys. Res. (1)

M. Nakano, H. Kumagai, N. Kumazoua, K. Yamaoaka, B.A. Chouet, �??The excitation and characterization frequencies of the long period volcanic event: an approach based on an autoregressive model of a linear dynamic system,�?? J. Geophys. Res. 103, 10031-10046 (1998).
[CrossRef]

J. Lightwave Technol. (2)

A. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A. Putnam, and E.J. Friebele, �??Fiber Grating Sensors,�?? J. Lightwave Technol. 15, 1442-1463 (1997).
[CrossRef]

A. Arie, B. Lissak, and M. Tur, �??Static Fiber-Bragg Grating Strain Sensing Using Frequency-Locked Lasers,�?? J. Lightwave Technol. 17, 1849-1855 (1999).
[CrossRef]

J. Opt. Soc. Am. B (1)

Meas. Sci. Technol. (1)

Y.J. Rao, �??In-fibre Bragg grating sensors,�?? Meas. Sci. Technol. 8, 355-375 (1997).
[CrossRef]

Nature (1)

B.A. Chouet, �??Long-period volcano seismicity: its sources and use in eruption forecasting,�?? Nature 380, 309-316 (1996).
[CrossRef]

Opt. Laser Eng. (1)

P. Ferraro, G. de Natale, �??On the possible use of optical fiber Bragg gratings as strain sensors for geodynamical monitoring,�?? Opt. Laser Eng. 37, 115-130 (2002).
[CrossRef]

Opt. Lett. (1)

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Figures (7)

Fig. 1.
Fig. 1.

Sketch of the experimental setup. F-P stands for Fabry-Pèrot, BS for beam-splitter, OI for optical isolator, BT for bias-tee, DBM for double-balanced mixer and PD for photodiode.

Fig. 2.
Fig. 2.

Example of the signal obtained by high-frequency mixer demodulation upon FBG reflection (maximum at 1560.778 nm). The lower trace corresponds to the transmission of a 1-GHz free-spectral-range Fabry-Pérot interferometer, where RF sidebands at 2.2 GHz are visible.

Fig. 3.
Fig. 3.

Response of the LRFM system to static deformations directly applied to the 50-% FBG by the PZT. The dashed line corresponds to a weighted linear fit. Vertical bars are standard errors of mean FBG shifts deriving from 10 measured values.

Fig. 4.
Fig. 4.

FFT amplitude spectrum of the sensor output measured by a spectrum analyzer (resolution bandwidth of 50 mHz). The high peak corresponds to excitation of the PZT by a sine wave at 2 Hz with a strain-equivalent amplitude of 3 µε. The red line represents the background when the PZT is off.

Fig. 5.
Fig. 5.

Spectrum given by a 3-µε deformation applied to the FBG at higher acoustic frequencies (1 kHz). In this case, the resolution bandwidth is 50 Hz.

Fig. 6.
Fig. 6.

LRFM-system time response to excitation of an acoustic wave at 1100 Hz with 20-µε peak-to-peak amplitude (80-% FBG), compared to the output of an electrical strain gauge with 1-µε sensitivity (upper trace).

Fig. 7.
Fig. 7.

Static strain values measured by the laser sensor are plotted vs. the electrical probe values. A weighted linear fit is also represented. The error bars represent the maximum uncertainty due to the electrical gauge reading.

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