Abstract

An improved peak-to-peak method is developed for interrogating the absolute cavity length of fiber optic extrinsic Fabry–Perot interferometric (EFPI) sensors with high resolution. A fiber Fabry–Perot tunable filter (FFP-TF) is used to scan the optical spectrum of an EFPI, and the problems caused by the nonlinear performance and poor repeatability of the FFP-TF are removed by using a wavelength calibration technique. A linear fitting is used to calculate the wavelength spacing between two adjacent apexes in the optical spectrum, and the cavity length can be retrieved using this wavelength spacing. The experimental results show that the measuring resolution is improved from 25 to 1μm, and a linear output is also obtained.

© 2008 Optical Society of America

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References

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Figures (11)

Fig. 1
Fig. 1

Schematic of the experimental setup.

Fig. 2
Fig. 2

Sampled spectrum when scanning an FFP-TF.

Fig. 3
Fig. 3

Calibration signal obtained from scanning an etalon.

Fig. 4
Fig. 4

Translation relationship between wavelengths from 1525 to 1565 nm and the array index.

Fig. 5
Fig. 5

Calibrated spectrum of the EFPI that is drawn using two one-dimensional arrays.

Fig. 6
Fig. 6

Resampling procedure: (a) the original sampled optical spectrum from 1524.8 to 1565.2 nm , (b) the spectrum when zero is interpolated, (c) local portion of the data in (b), and (d) the spectrum obtained by filtering (b).

Fig. 7
Fig. 7

Wavelength spacing between two adjacent apexes from 1525 to 1565 nm ; the straight line is a linear fitting line.

Fig. 8
Fig. 8

Residual error.

Fig. 9
Fig. 9

Results of continuous testing when the cavity of EFPI was elongated thrice from 2507 to 2537 μm .

Fig. 10
Fig. 10

Results of continuous testing when the cavity length is fixed at 2511 μm (a) using the method presented in this paper and (b) using the general method of peak detection.

Fig. 11
Fig. 11

Measured cavity length when the air gap is elongated from 2500 to 2610 μm .

Equations (6)

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Δ φ = 4 π d λ 1 λ 2 Δ λ ,
d = 1 2 λ 1 λ 2 Δ λ M M .
d = λ 2 2 Δ λ M M .
Δ λ M M = 0.627844 , λ 495.493
δ ( Δ λ M M ) = λ d Δ λ ,
Δ d = 1 2 ( λ Δ λ M M ) 2 δ ( Δ λ M M ) ,

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