Abstract

We report a fiber-optic refractive-index sensor that is applicable to a long-distance measurement. The sensor consists of a silica glass fiber bent into a U shape with a bending radius of typically several hundred micrometers. The cladding at the tip of the sensor is stripped off. The sensing mechanism is based on the variation of the output intensity that is induced by radiation loss at the bend, which enables us to measure the refractive index of the outer medium. A fabrication method of fusing the sensor with a CO2 laser and etching with HF is described. Multipoint measurements of optical-time-domain reflectometry are also described.

© 1992 Optical Society of America

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References

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  1. M. Nagai, M. Shimizu, N. Ohgi, “Sensitive liquid sensor for long distance leak detection,” in the Proceedings of OFS ’84 (VDE-Verlag, Berlin, 1984), pp. 207–210.
  2. T. Takeo, H. Hattori, “Optical fiber sensor for measuring refractive index,” Jpn. J. Appl. Phys. 21, 1509–1512 (1982).
    [CrossRef]
  3. T. Takeo, H. Hattori, “Fluid observation with an optical fiber photorefractometer,” Jpn. J. Appl. Phys. 22, 1920–1924 (1983).
    [CrossRef]
  4. A. Harmer, “Optical fiber refractometer using attenuation of cladding modes,” in the Proceedings of OFS1 (Institution of Electrical Engineers, London, 1983), pp. 104–108.
  5. A. Snyder, J. D. Love, Optical Waveguide Theory (Chapman & Hall, London, 1983).

1983

T. Takeo, H. Hattori, “Fluid observation with an optical fiber photorefractometer,” Jpn. J. Appl. Phys. 22, 1920–1924 (1983).
[CrossRef]

1982

T. Takeo, H. Hattori, “Optical fiber sensor for measuring refractive index,” Jpn. J. Appl. Phys. 21, 1509–1512 (1982).
[CrossRef]

Harmer, A.

A. Harmer, “Optical fiber refractometer using attenuation of cladding modes,” in the Proceedings of OFS1 (Institution of Electrical Engineers, London, 1983), pp. 104–108.

Hattori, H.

T. Takeo, H. Hattori, “Fluid observation with an optical fiber photorefractometer,” Jpn. J. Appl. Phys. 22, 1920–1924 (1983).
[CrossRef]

T. Takeo, H. Hattori, “Optical fiber sensor for measuring refractive index,” Jpn. J. Appl. Phys. 21, 1509–1512 (1982).
[CrossRef]

Love, J. D.

A. Snyder, J. D. Love, Optical Waveguide Theory (Chapman & Hall, London, 1983).

Nagai, M.

M. Nagai, M. Shimizu, N. Ohgi, “Sensitive liquid sensor for long distance leak detection,” in the Proceedings of OFS ’84 (VDE-Verlag, Berlin, 1984), pp. 207–210.

Ohgi, N.

M. Nagai, M. Shimizu, N. Ohgi, “Sensitive liquid sensor for long distance leak detection,” in the Proceedings of OFS ’84 (VDE-Verlag, Berlin, 1984), pp. 207–210.

Shimizu, M.

M. Nagai, M. Shimizu, N. Ohgi, “Sensitive liquid sensor for long distance leak detection,” in the Proceedings of OFS ’84 (VDE-Verlag, Berlin, 1984), pp. 207–210.

Snyder, A.

A. Snyder, J. D. Love, Optical Waveguide Theory (Chapman & Hall, London, 1983).

Takeo, T.

T. Takeo, H. Hattori, “Fluid observation with an optical fiber photorefractometer,” Jpn. J. Appl. Phys. 22, 1920–1924 (1983).
[CrossRef]

T. Takeo, H. Hattori, “Optical fiber sensor for measuring refractive index,” Jpn. J. Appl. Phys. 21, 1509–1512 (1982).
[CrossRef]

Jpn. J. Appl. Phys.

T. Takeo, H. Hattori, “Optical fiber sensor for measuring refractive index,” Jpn. J. Appl. Phys. 21, 1509–1512 (1982).
[CrossRef]

T. Takeo, H. Hattori, “Fluid observation with an optical fiber photorefractometer,” Jpn. J. Appl. Phys. 22, 1920–1924 (1983).
[CrossRef]

Other

A. Harmer, “Optical fiber refractometer using attenuation of cladding modes,” in the Proceedings of OFS1 (Institution of Electrical Engineers, London, 1983), pp. 104–108.

A. Snyder, J. D. Love, Optical Waveguide Theory (Chapman & Hall, London, 1983).

M. Nagai, M. Shimizu, N. Ohgi, “Sensitive liquid sensor for long distance leak detection,” in the Proceedings of OFS ’84 (VDE-Verlag, Berlin, 1984), pp. 207–210.

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Figures (11)

Fig. 1
Fig. 1

Structure and the principle of the U-shaped fiber-optic photorefractometer.

Fig. 2
Fig. 2

Ray tracing in the bent fiber where the cladding is stripped.

Fig. 3
Fig. 3

Sensor transmission versus the refractive index of the surrounding medium calculated for sensors with different bending radii.

Fig. 4
Fig. 4

Dependence of the sensor transmission on modal power distributions: (a) iθ(θ) = cos θ, (b) iθ(θ) = cos(πθ/2χ c ), and (c) iθ(θ) = cos(πθ/2χ c ).

Fig. 5
Fig. 5

Photographs of the sensor fabricated by fusion with a CO2 laser and an etching of HF.

Fig. 6
Fig. 6

Experimental results of the sensor transmission for the fabricated sensors having different bending radii.

Fig. 7
Fig. 7

Layout of the multipoint measurement by using the OTDR technique and U-shaped fiber-optic sensors.

Fig. 8
Fig. 8

Typical OTDR waveform obtained from the system where two sensors are serially located along the fiber.

Fig. 9
Fig. 9

Comparison between the measured results obtained with two kinds of methods: (a) the OTDR technique and (b) a transmittive method.

Fig. 10
Fig. 10

Modal power distributions at different distances from the sensor.

Fig. 11
Fig. 11

OTDR waveforms obtained when the outer medium includes scattering particles.

Tables (1)

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Table I Parameters of the Optical Fiber Used

Equations (11)

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d I = i r ( r ) i θ ( θ ) d S d Ω ,
( x 2 + y 2 + z 2 + B 2 a 2 ) 2 = 4 B 2 ( x 2 + y 2 ) ,
x x i 1 s i x = y y i 1 s i y = z z i 1 s i z ,
n i · s i + n i · t i = 0 ,
n i × s i n i × t i = 0 .
ψ i = cos 1 ( n i · s i ) ,
t i = s i 2 ( n i · s i ) n i .
R θ ( i = 1 R i ) d I ,
T = R θ ( i = 1 R i ) i r ( r ) i θ ( θ ) d S d Ω i r ( r ) i θ ( θ ) d S d Ω .
i r ( r ) = const , ( 0 < r < a ) ,
i θ ( θ ) = cos ( πθ / 2 χ c ) , ( 0 < θ < χ c ) ,

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